Thermal Spray Tips - Swinburne University of Technology

Compiled by Jo Ann Gan, Edited and advised by Christopher C. Berndt 

Swinburne University of Technology Thermal Spray Group (SwinTS) 

Please contact Prof. Christopher Berndt at cberndt @swin.edu.au for further enquiries 

Thermal Spray Tips 

by Thermal Spray Society (TSS) 

This document has been compiled from data and information acquired from ASM Thermal Spray Society 

(TSS) website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ for easy reference. 

This compilation serves as a contribution from Swinburne University of Technology (SUT) Thermal Spray 

Group (SwinTS). 

The tips have been categorized into several topics (thermal spray processes, feedstock materials, surface 

preparation, coating processing and operation, post coating operation, coating characterization and 

testing as well as applications) where individual articles can be accessed easily through the hyperlinked 

Table of Contents (TOC) or the overview of the thermal spray (TS) process. The overview highlights the 

different components of the TS process and related tips are attached to each segment. Users may return 

easily to the TOC or overview anytime by clicking the link located at bottom right of each page. 

Please note that a list of glossary for Thermal Spray Technology terminology can be accessed at the 

following website: http://www.asminternational.org/tss/glossary/tssgloss.htm 

Welcome to the wonderful world of Thermal Spray! 

Information and data acquired from ASM International Thermal Spray Society 

website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 

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Back to Overview 

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Table of Contents 

1. OVERVIEW OF THERMAL SPRAY PROCESS 5 

2. THERMAL SPRAY PROCESSES 6 

2.1. COMPARISON OF MAJOR COATING METHODS 6 

2.2. CLASSIFICATION OF THERMAL SPRAY PROCESSES 8 

2.3. CHARACTERISTICS OF THERMAL SPRAY 9 

2.4. THERMAL SPRAY PROCESSING CHARACTERISTICS 10 

2.5. THERMAL SPRAY PROCESS VARIATIONS 11 

2.6. EFFECT OF TORCH HARDWARE ON PARTICLE TEMPERATURE AND VELOCITY 12 

2.7. FLAME SPRAY PROCESS 13 

2.8. FLAME SPRAY GUNS 14 

2.9. FLAME SPRAY USES COMBUSTIBLE GAS AS HEAT SOURCE TO MELT COATING MATERIAL 16 

2.10. HYPERSONIC FLAME SPRAYING 18 

2.11. ARC AND FLAME SPRAYING PROCESSES 19 

2.12. ELECTRIC ARC WIRE SPRAYING 21 

2.13. OXYFUEL WIRE SPRAY 22 

2.14. REACTIVE PLASMA SPRAY FORMING 23 

2.15. REACTIVE PLASMA SPRAY 24 

2.16. TRANSFERRED PLASMA-ARC PROCESS 25 

2.17. HIGH-VELOCITY OXY FUEL SPRAY PROCESS 26 

2.18. DETONATION GUN THERMAL SPRAY PROCESS FOR CERAMICS 27 

2.19. SPRAY TABLES FOR THERMAL SPRAY PARAMETERS 28 

2.20. COLD-SPRAY PROCESS PARAMETERS 29 

3. FEEDSTOCK MATERIALS 30 

3.1. PRODUCTION OF THERMAL SPRAY POWDERS 30 

3.2. GAS-PHASE PRODUCTION METHODS FOR ULTRAFINE POWDERS 31 

3.3. ADVANTAGES AND DISADVANTAGES OF POWDER PRODUCTION PROCESSES 32 

3.4. WATER ATOMIZATION 33 

3.5. COMPOSITE MATERIALS 34 

3.6. POWDER TESTING AND CHARACTERIZATION: IMAGE ANALYSIS 36 

3.7. POWDER CHARACTERISTICS: MORPHOLOGY 37 

3.8. PARTICLE SIZE DISTRIBUTION PLOTS 38 

3.9. WIRE AND ROD FEEDERS 39 

3.10. FLUIDIZED-BED POWDER FEEDERS 40 



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3.11. GRAVITY-BASED POWDER FEEDING 41 

3.12. POWDER FEEDING SYSTEMS: ROTATING WHEEL DEVICES 42 

4. SURFACE PREPARATION 43 

4.1. DRY ABRASIVE GRIT BLASTING 43 

4.2. ALUMINA BLASTING ABRASIVES 44 

4.3. MACHINING AND MACROROUGHENING 45 

5. COATING PROCESSING AND OPERATION 46 

5.1. PART CONFIGURATION AND COATING LOCATION CONSIDERATIONS 46 

5.2. CONTACT AND SHADOW MASKING 47 

5.3. THERMAL SPRAY PATTERN 48 

5.4. RELATIONSHIP BETWEEN PARTICLE MELTING AND COATING STRUCTURE 49 

5.5. AIR-COOLING DEVICES TO MINIMIZE DEBRIS 51 

5.6. INFLUENCE OF TEMPERATURE CONTROL ON SUBSTRATE/COATING 52 

6. COATING STRUCTURES, PROPERTIES AND MATERIALS 53 

6.1. MICROSTRUCTURES OF THERMAL SPRAY COATINGS 53 

6.2. STRUCTURE OF THERMAL SPRAY COATINGS 54 

6.3. SOURCES OF COATING POROSITY 55 

6.4. INFLUENCE OF POROSITY ON COATING PROPERTIES 56 

6.5. COAT BONDING: MECHANICAL INTERLOCKING 57 

7. POST COATING OPERATION 58 

7.1. SEALING 58 

7.2. RECOMMENDED METALLOGRAPHIC PRACTICE: SECTIONING 59 

8. COATING CHARACTERIZATION AND TESTING 60 

8.1. MEASURING DEPOSITION EFFICIENCY 60 

8.2. TENSILE ADHESION TESTING OF THERMAL SPRAY COATINGS 62 

8.3. BOND PULL TESTING OF THERMAL SPRAY COATINGS 64 

8.4. ROLLING CONTACT FATIGUE FAILURE MODES IN THERMAL SPRAY COATINGS 65 

8.5. RESIDUAL STRESS DETERMINATION IN THERMAL SPRAY COATINGS 66 



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8.6. THERMAL FATIGUE TESTING OF TBCS 67 

9. APPLICATIONS 68 

9.1. COATINGS FOR CORROSIVE ENVIRONMENT: CONVENTIONAL COAL-FIRED BOILERS 68 

9.2. ELEVATED-TEMPERATURE CORROSION APPLICATIONS 69 

9.3. OXIDATION PROTECTION 70 

9.4. THERMAL BARRIER COATINGS 71 

9.5. FUNCTIONALLY GRADIENT MATERIALS 72 

9.6. WEAR AND ABRASION RESISTANCE OF THERMAL SPRAY COATINGS 73 

9.7. METALLOGRAPHY OF NICRAL/BENTONITE ABRADABLE COATINGS 74 

9.8. ABRADABLE SEALS 75 

9.9. PROCESSING ELECTRONIC DEVICES: BERYLLIA 77 

9.10. ALUMINUM COATINGS AND ZINC COATINGS 78 

9.11. THERMAL SPRAY POLYMER COATINGS 79 



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Tip 2.1 Tip 2.11 

Tip 2.2 Tip 2.12 

Tip 2.3 Tip 2.13 

Tip 2.4 Tip 2.14 

Tip 2.5 Tip 2.15 

Tip 2.6 Tip 2.16 

Tip 2.7 Tip 2.17 

Tip 2.8 Tip 2.18 

Tip 2.9 Tip 2.19 

Tip 2.10 Tip 2.20 

Image adapted from J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, 2004, p 43 

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1. Overview of Thermal Spray Process 

Tip 3.1 Tip 3.2 Tip 3.3 Tip 3.4 

Tip 3.5 Tip 3.6 Tip 3.7 Tip 3.8 

Tip 3.9 Tip 3.10 Tip 3.11 Tip 3.12 

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Tip 4.1 Tip 4.2 Tip 4.3 

Tip 5.1 Tip 5.2 

Tip 5.3 Tip 5.4 

Tip 5.5 Tip 5.6 

Tip 6.1 

Tip 6.2 

Tip 6.3 

Tip 6.4 

Tip 6.5 

Tip 7.1 

Tip 7.2 

Tip 8.1 Tip 8.2 

Tip 8.3 Tip 8.4 

Tip 8.5 Tip 8.6




2. Thermal Spray Processes 

2.1. Comparison of Major Coating Methods 

Characteristic Electro/electroless 

plating 

General Characteristics of Major Coating Methods 

Thermal spray Chemical vapor 

deposition 



Physical vapor 

deposition 

Equipment cost Low Low to moderate Moderate Moderate to high 

Operating cost Low Low to moderate Low to moderate Moderate to high 

Process 

environment 

Aqueous solution Atmospheric to 

soft vacuum 

Atmospheric to 

medium vacuum 

Hard vacuum 

Coating geometry Omnidirectional Line of sight Omnidirectional Line of sight 

Coating thickness Moderate to thick, 

10 μm-mm 

Substrate 

temperature 

Adherence Moderate 

mechanical bond to 

very good chemical 

bond 

Surface finish Moderately coarse 

to glossy 

Thick, 50 μm-cm Thin to thick, 0.1 

μm-mm 

Low Low to moderate Moderate to high Low 

Good mechanical 

bond 

Coating materials Metals Powder/wire, 

polymers, 

metals/ceramics 

Very good 

chemical bond to 

excellent diffusion 

bond 

Very thin to 

moderate 

Moderate 

mechanical bond 

to good chemical 

bond 

Coarse to smooth Smooth to glossy Smooth to high 

gloss 

Metals, ceramics, 

polymers 

Metals, ceramics, 

polymers 

Major coating methods include electro/electroless plating (EP/ElsP), chemical vapor deposition (CVD), 

physical vapor deposition (PVD), and thermal spray (TS). Both the initial equipment costs and the 

operating costs for EP/ElsP coatings are relatively low. However, the by-products are considered highly 

toxic and are subject to increasingly stricter government regulation, thereby making storage, reclamation, 

and disposal major economic concerns. Thermal spray equipment, and therefore its cost, varies widely 

from simple combustion devices to computer-controlled low-pressure plasma spray systems. 

The operating costs are very much dependent on the cost of consumables such as powder, wire, or rod 

materials as well as the quantities and types of gases used. Equipment costs for CVD are moderate, with 

the understanding that neutralization of the output gases is an integral part of any system. The operating 

costs of CVD are dominated by precursor gas costs and the frequent need to clean the systems. The cost 

of PVD coating equipment is very high, due to the need to maintain high vacuums in chambers of 

sufficient volume to make the process cost-effective. Operating costs are associated with the degree of 

surface cleanliness necessary for coating adhesion, as well as the target costs. 

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The EP/ElsP and CVD are omnidirectional coating processes, while TS and PVD are line-of-sight 

processes. Adherence for TS coatings, in general, is provided by mechanical bonding. Adherence of 

EP/ElsP and PVD coatings is, in general, provided by mechanical and/or chemical bonding. Adherence 

for CVD coatings is provided by either chemical and/or diffusion bonding. 

With the exception of EP/ElsP, which is limited to metals and some alloys, the TS, CVD, and PVD coating 

processes can apply a variety of metals, ceramics, cermets, and polymers. 

Source: Surface Science, Thomas Bernecki, BIRL, Northwestern University, as published in Handbook of 

Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 35, 2005. 



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2.2. Classification of Thermal Spray Processes 

Thermal spray processes and subsets 

Members of the thermal spray family of processes typically are grouped into three major categories: 

flame, plasma arc, and electric arc. (Kinetic energy-driven spray, or cold spray, is a recent addition to the 

family.) Each of these processes encompasses many more subsets, and each has its own characteristic 

range of temperature, enthalpy, and velocity. These attributes, in turn, develop coating characteristics that 

are unique to each process, which in simplest terms include coating bond strength, porosity, inclusions 

(usually oxides), and hardness. Selection of the appropriate thermal spray method typically is determined 

by: 

• Desired coating material 

• Coating performance requirements 

• Economics 

• Part size and portability 

Source: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, 2004, p 44 



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2.3. Characteristics of Thermal Spray 

Schematic of a typical thermal spray powder process 

Thermal spray is a generic term for a group of coating processes used to apply metallic or nonmetallic 

coatings. Three major categories of the processes are flame spray, electric arc spray, and plasma arc 

spray, with subsets under each category. (Cold spray is a recent addition that uses modest preheating, 

but is largely a kinetic energy process.) The energy sources heat the coating material (powder, wire, and 

rod form) to a molten or semimolten state. Heated particles are propelled toward a prepared surface via 

process gases or atomization jets, forming a bond with the surface upon impact. Subsequent particles 

cause thickness buildup and form a lamellar structure. Advantages of thermal spray processes include 

the ability to use a wide variety of coating materials, the ability of most thermal spray processes to apply 

coatings without significant heat input, and the ability, in most cases, to strip off and recoat worn or 

damaged coatings without changing part properties or dimensions. 

Source: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, p 4, 2004 



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2.4. Thermal Spray Processing Characteristics 

Typical thermal spray process parameters and variables 

From a simplistic point of view, the powder spray process can be viewed as high-speed heat treating 

(dealing with time, temperature, and mass) where the objective is to bring the powder mass to the 

required temperature in a given time period. The time a particle spends in the process jet is called dwell 

time, which is governed by gas velocity and powder particle characteristics. Gas velocity, in turn, is 

determined by the total gas flow through the nozzle, gas characteristics, and energy acting on or resulting 

from the process. Particle velocity is a function of jet velocity coupled with particle characteristics; i.e., 

particle size, morphology, and mass. Particle temperature is a function of enthalpy, velocity, trajectory, 

and its own physical and thermal properties. Gas temperatures in the spray stream vary greatly with the 

process. 




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2.5. Thermal Spray Process Variations 

Heat energy input and particle velocity for common thermal spray processes 

Process Input heat energy to 

particle 



Output particle velocity (a) 

High Low Highest High Medium high Medium Low 

Combustion wire X X 

Combustion powder X X 

Standard plasma X X 

High-velocity plasma X X 

Vacuum plasma X X 

Standard wire arc X X 

Vacuum arc X X 

High-velocity oxyfuel X X 

Detonation gun X X 

(a) Particle speed ranges from a high of approximately 1000 m/s (3000 ft/s) to a low of 25 m/s (80 ft/s). Further variations within 

each process depends on the particle size, material type and gas velocity 

Source: ASM Handbook: Metallography and Microstructures, Vol.9, p.1038, 2004 

The controllable parameters for each of the major thermal spray types are different, but all share some 

common fundamentals. Converting feedstock material (whether powder, wire, or rod) into a coating 

requires heat energy for melting and a gas flow for atomization and/or particle acceleration to propel the 

particles toward the substrate. Input heat energy and gas flow, in turn, directly influence process variables 

including flame/plasma/jet temperature and velocity; particle temperature, speed, and trajectory; and 

deposit temperature. Each of these variables affects spray-pattern formation and particle heating 

differently. 

In general, combustion-flame, combustion-wire, and twin-wire arc spray coatings are generated from high 

heat input plus low particle velocity, producing coatings having high oxide content, a large number of 

rounded particles, and relatively high porosity. Due to higher particle velocity, superheated particles in 

plasma spray coatings are flattened more than in combustion spray coatings, producing denser coatings 

with finer porosity. Very high-velocity systems such as detonation gun and HVOF, combined with lower 

heat inputs, typically produce very dense coatings not achievable by the other processes. 

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2.6. Effect of Torch Hardware on Particle Temperature and Velocity 

Gun Cooling, Pinch, and Flame Cooling air caps air caps were fitted onto nozzles, and the particle 

temperature (Tp) and particle velocity (Vp) were measured. The gun cooling air cap produced the highest 

Tp and lowest Vp because it minimized the interaction between the cooling air and the particulate loaded 

flame. The pinch air cap strongly directed the cooling air into the flame, and produced the highest Vp and 

lowest Tp values. The flame cooling air cap also directed air into the flame, but not as strongly as the 

pinch air cap, and as a result, the flame cooling air cap produced Tp and Vp values between those of the 

gun cooling and pinch air caps. 

Source: Journal of Thermal Spray Technology, Volume 19(4) June 2010, p. 824-827. 



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2.7. Flame Spray Process 

Powder flame spray system 

Conventional flame spray was the first thermal spray process developed (~1910) and is still in common 

use. Modern torches have changed little since the 1950s. Flame spray uses the chemical energy of 

combusting fuel gases to generate heat. Oxyacetylene torches are the most common, using acetylene as 

the main fuel in combustion with oxygen to generate the highest combustion temperatures. Powder, wire, 

or rod is introduced axially through the rear of the nozzle into the flame at the nozzle exit. The feedstock 

material is melted and the particles/droplets accelerated toward the surface by the expanding gas flow 

and air jets. An advantage of wire and rod over powder is that the degree of melting is significantly higher, 

producing denser coatings. In addition, the atomizing air produces finer droplets, which in turn produce 

finer, smoother coatings. In flame spray processes, fuel/oxygen ratio and total gas flow rates are adjusted 

to produce the desired thermal output. Optional air jets, downstream of the combustion zone, can further 

adjust the thermal profile of the flame. Flame spray is capable of depositing a wide range of materials, 

ranging from polymers to ceramics and refractory metals. 

Source: Thermal Spray Processes, revised by Daryl E. Crawmer, Thermal Spray Technologies Inc., as 

published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, 2005, p 55. 



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2.8. Flame Spray Guns 

Combustible gas serves as the heat source to melt the coating material in flame spray guns. Materials are 

in the form of rod, wire, or powder, most flame spray guns can be adapted to several combinations of 

gases to balance operating cost and coating properties. Acetylene, propane, methyl-acetylenepropadiene 

(MAPP) gas, and hydrogen, along with oxygen, are typical flame spray gases. 

In general, changing the nozzle and/or air cap is all that is required to adapt the gun to different alloys, 

wire sizes, or gases. The diagrams depict powder and wire flame spray guns. For all practical purposes, 

the rod and wire guns are similar. 

Cross sections of typical flame spray guns. (a) Wire or rod. (b) Powder. 



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Flame temperatures and characteristics depend on the oxygen-to-fuel gas ratio and pressure. The flame 

spray process is characterized by low capital investment, high deposition rates and efficiencies, and 

relative ease of operation and cost of equipment maintenance. 

In general, as-deposited (or cold spray) flame-sprayed coatings exhibit lower bond strengths, higher 

porosity, a narrower working temperature range, and higher heat transmittal to the substrate than most 

other thermal spray processes. The flame spray process is frequently selected for the reclamation of worn 

or out-of-tolerance parts, frequently with nickel-base alloys. Bronze alloys may be used for some bearings 

and seal areas. Blends of tungsten carbide and nickel-base alloys work well for wear resistance. Zinc is 

commonly sprayed for corrosion resistance on bridges and other structures. 

Source: R.C. Tucker, Jr., Thermal Spray Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM 

International, 1994, p 498. 



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2.9. Flame Spray Uses Combustible Gas as Heat Source to Melt Coating Material 

Flame spray uses combustible gas as a heat source to melt the coating material. Flame spray guns are 

available to spray materials in either rod, wire, or powder form. Most flame spray guns can be adapted to 

use several combinations of gases to balance operating cost and coating properties. Acetylene, propane, 

methyl-acetylene-propadiene (MAPP) gas, and hydrogen, along with oxygen, are commonly used flame 

spray gases. In general, changing the nozzle and/or air cap is all that is required to adapt the gun to 

different alloys, wire sizes, or gases. Figures 3(a) and 3(b) depict powder and wire flame spray guns. For 

all practical purposes, the rod and wire guns are similar. 

Cross sections of typical flame spray guns. (a) Wire or rod. (b) Powder. 

Source: R.C. Tucker, Jr., Thermal Spray Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM 




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Flame temperatures and characteristics depend on the oxygen-to-fuel gas ratio and pressure. The 

approximate temperatures for stoichiometric combustion at 1 atm for some oxyfuel combinations are 

shown in Table 1. 

The flame spray process is characterized by low capital investment, high deposition rates and 

efficiencies, and relative ease of operation and cost of equipment maintenance. In general, as-deposited 

(or cold spray) flame-sprayed coatings exhibit lower bond strengths, higher porosity, a narrower working 

temperature range, and higher heat transmittal to the substrate than most other thermal spray processes. 

The flame spray process is widely used for the reclamation of worn or out-of-tolerance parts, frequently 

using nickel-base alloys. Bronze alloys may be used for some bearings and seal areas. Blends of 

tungsten carbide and nickel-base alloys may be used for wear resistance. Zinc is commonly applied for 

corrosion resistance on bridges and other structures. 

Table 1: Maximum temperature of heat sources 

Heat source 

Approximate temperature 

(stoichiometric combustion) 

Propane-oxygen 2526 °C (4579 °F) 

Natural gas-oxygen 2538 °C (4600 °F) 

Hydrogen-oxygen 2660 °C (4820 °F) 

Propylene-oxygen 2843 °C (5240 °F) 

Methylacetylene/propadiene- 2927 °C (5301 °F) 

oxygen 

Acetylene-oxygen 3087 °C (5589 °F) 

Plasma arc 2200 to 28,000 °C 

(4000 to 50,000 °F) 

Source: Adapted Publication 1G191, National Association of Corrosion Engineers 



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2.10. Hypersonic Flame Spraying 

Cross-sectional view of a flame powder spraying system showing powder feed material being transported 

by the carrier gas and then melted by the oxyfuel mixture. 

In recent years, the field of thermal spraying has seen the introduction of hypersonic combustion flame 

spray (HCFS) guns. In the Jet-Kote method (the first gun of this type was commercially available in 1983), 

a high-velocity combustion process (chemical energy) accelerates and melts the particulates via 

convective heat transfer from the hot flame. 

The system consists basically of three parts: a main console, a pressurized powder feeder, and a 

portable torch. An internal combustion flame is produced in a water-cooled combustion chamber with a 

continuous flow of oxygen and fuel gases. 

This flame makes a right-angle bend and passes through four vortices that focus the hot gases into a 

narrow beam. This beam is accelerated through an extended-length water-cooled nozzle. The 

combustion process creates a high back pressure, thus requiring a pressurized powder feeder to propel 

particulates axially from the rear of the nozzle into the stream of hot gases. 

Source: Engineered Materials Handbook -> Densification and Sintering of Ceramics -> Nontraditional 

Densification Processes 



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2.11. Arc and Flame Spraying Processes 

Electric arc spraying and flame spraying are the most suitable methods for applying coatings of zinc, 

aluminum, and their alloys to steel structures. These methods are simple, they can be used on-site, and 

they offer relatively high coverage rates compared to other thermal spray processes. Wire consumables 

are also more economical and easier to handle than powders. 

Electric arc spraying involves feeding two electrically conducting metal wires toward 

each other. An electric arc is produced at the wire tips, melting the metal. A highpressure 

air line atomizes and then sprays the fine droplets onto the steel surface. 

In flame spraying, a metal wire is melted in a gas flame and then air sprayed onto the steel surface. 

Arc and flame spray coatings usually contain numerous pores, some of which are closed and the rest 

connected from the coating surface to the substrate. Natural sealing can be achieved by oxidation of the 

metallic coating under normal environmental exposure conditions, when the resulting oxides, hydroxides, 

and/or basic salts are not soluble in this environment. 



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It is often beneficial to apply an organic sealant to close these pores. However, penetration of sealant into 

the coating becomes more difficult with time after spraying. This is due to the adsorption of moisture on 

the pore surface, which can lead to the formation of corrosion product within the pores. As a 

consequence, it is often recommended that any post-treatment be completed within the same day as the 

coating application. 

Source: Thermal Spray Application Methods for TSA and TSZ Coatings, Thermal Spray Coatings for 

Corrosion Protection in Atmospheric and Aqueous Environments, Vol 13B, ASM Handbook, ASM 

International 



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2.12. Electric arc wire spraying 

Electric arc wire spraying applies coatings of selected metals in wire form. Push-pull motors feed two 

electrically charged wires through the arc gun to contact tips at the gun head. An arc is created that melts 

the wires at temperatures above 5500 °C (10,000 °F). Compressed air atomizes the molten metal and 

projects it onto a prepared surface. The diagram shows a schematic of the EAW spray process. 

Schematic of the electric arc wire spray process. 

The EAW process is excellent for applications that require heavy coating buildup or have large surfaces 

to be sprayed. The arc system can produce a spray pattern ranging from 50 to 300 mm (2 to 12 in.) and 

can spray at high speeds. It has built-in flexibility, allowing coating characteristics such as hardness or 

surface texture to be tailored to specific applications. 

The EAW method is characterized by strong coating adhesion because of the high particle temperatures 

produced. Because the process uses only electricity and compressed air, it allows equipment to be 

moved relatively easily from one installation to another, and it eliminates the need to stock oxygen and 

fuel gas supplies. Typical spray materials include austenitic and martensitic stainless steels, nickel 

aluminide, nickel-chromium alloys, bronze, Monel, babbitt, aluminum, zinc, and molybdenum. 

Source: ASM Handbooks Online, Volume 6, Welding, Brazing, and Soldering -> Hardfacing, Weld 

Cladding, and Dissimilar Metal Joining -> Thermal Spraying 



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2.13. Oxyfuel Wire Spray 

Schematic of the oxyfuel wire spray process 

The oxyfuel wire spray process (also called wire flame spraying or the combustion wire process) is the 

oldest of the thermal spray coating methods and among the lowest in capital investment. The process 

utilizes an oxygen-fuel gas flame as a heating source and coating material in wire form. Any material in 

the form of wire and capable of being melted below 2480°C (4500°F) can be flame-sprayed. 

During operation, the wire is drawn into the flame by drive rolls that are powered by an adjustable air 

turbine or electric motor. The tip of the wire is melted as it enters the flame, atomized into particles by a 

surrounding jet of compressed air, and propelled to the workpiece. The diagram shows a schematic of the 

OFW spray process. 

Spray rates for this process range from 0.5 to 10 kg/h (1 to 20 lb/h) and are dictated by the melting point 

of the material and the choice of fuel gas. Common fuel gases are acetylene, MAPP gas, propane, 

propylene, and natural gas, each combined with oxygen. 

Source: J.R. Davis, Hardfacing, Weld Cladding, and Dissimilar Metal Joining, ASM Handbook, Vol 6, 

Welding, Brazing, and Soldering, ASM International, 1993, p 805. 



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2.14. Reactive Plasma Spray Forming 

Reactive plasma spray process for the synthesis of composite materials 

Controlled-atmosphere plasma spraying, adapted for reactive plasma spraying, has recently been 

developed to combine controlled dissociation and reactions in thermal plasma jets, for the in situ forming 

of new materials or to produce new phases in sprayed deposits. Reactive plasma spray forming is 

consequently emerging as a viable method for producing a wide range of advanced materials. 

The process, a logical evolution from conventional plasma spraying, allows reactive precursors to be 

injected into the particulate and/or hot gas streams. These reactive precursors may be liquids, gases, or 

mixtures of solid reactants. On contact with the high-temperature plasma jet, they decompose or 

dissociate to form highly reactive and ionic species that can then react with other heated materials within 

the plasma jet to form new compounds. 

The diagram illustrates the reactive plasma spray concept. Chemical reactions rely on plasma-induced 

dissociation of the injected precursors. These species can react with other elements to produce carbides 

such as TiC and WC, or under certain conditions, diamond or diamondlike carbon (DLC) films. The 

primary requirements are that the precursors dissociate into reactive species and that the reaction times 

and temperatures are sufficiently long for the desired products or phases to form. 

Source: R. Knight and R.W. Smith, Thermal Spray Forming of Materials, ASM Handbook, Vol 7, Powder 

Metal Technologies and Applications, ASM International, p 415. 



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2.15. Reactive Plasma Spray 

Reactive plasma spray applications include the synthesis of composite materials, shaped brittle 

intermetallic alloys, reinforced or toughened ceramics, and tribological coatings with in situ-formed hard or 

lubricating phases. Reactive plasma spray forming enables a wide range of materials to be produced, 

such as aluminium with AlN, Al2O3, or SiC; NiCrTi alloys with TiC or TiN; intermetallics such as TiAl, Ti3Al, 

MoSi2, and other ceramics with oxides, nitrides, borides, and/or carbides. All of these have been 

produced in situ in reactive thermal plasma jets. 

The process allows "reactive" precursors to be injected into the particulate and/or hot gas streams. These 

reactive precursors may be liquids, gases, or mixtures of solid reactants which, on contact with the hightemperature 

plasma jet, decompose or dissociate to form highly reactive and ionic species that can then 

react with other heated materials within the plasma jet to form new compounds. 

Reactive plasma spray process for the synthesis of composite materials 

Chemical reactions rely on plasma-induced dissociation of the injected precursors, for example gaseous 

methane (CH4), which decomposes into elemental or ionic species such as CH3, C4- or even atomic 

carbon. These species can react with other elements to produce carbides such as TiC and WC; or, under 

certain conditions, diamond or diamond like carbon (DLC) films. The diagram shows the plasma heating 

of gases, using either a non-transferred electric arc or an inductively coupled plasma (ICP) radio 

frequency discharge; injection of reactive precursors into the plasma jet; and injection of powders in a 

carrier gas stream. 

Source: R. Knight and R.W. Smith, Thermal Spray Forming of Materials, ASM Handbook, vol 7, Powder 

Metal Technologies and Applications, ASM International, 1998, p 415 



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2.16. Transferred plasma-arc process 

The transferred plasma-arc process adds to plasma spray the capability of substrate surface 

heating and melting. The diagram shows a schematic representation of the process. 



A secondary arc current that controls surface 

melting and depth of penetration is established 

through the plasma and substrate. Several 

advantages result from this direct heating: 

metallurgical bonding, high density, high 

deposition rates, and high thickness per pass. 

Coating thickness of 0.50 to 6.35 mm (0.020 to 

0.250 in.) and widths up to 32 mm (1.25 in.) can 

be made in a single pass at powder feed rates of 

9 kg/h (20 lb/h). 

In addition, less electrical power is required than 

with nontransferred arc processes. For example, 

for an 88% tungsten carbide, 12% cobalt material, 

plasma spray deposition 0.30 mm (0.012 in.) thick 

and 9.50 mm (0.375 in.) in width might require 24 

passes at 40 to 60 kW to achieve maximum 

coating properties. However, the transferred 

plasma-arc process needs only one pass at approximately 2.5 kW for the same thickness. 

The method of heating and heat transfer in the transferred plasma-arc process eliminates many of the 

problems related to powders with wide particle size distributions or large particle sizes. Larger-particles, 

for example in the 50-mesh range, tend to be less expensive than closely classified 325-mesh powders. 

Some limitations of the process should be considered for any potential application. Because substrate 

heating is a part of the process, some alteration of its microstructure is inevitable. Applications are also 

limited to substrates that are electrically conductive and can withstand some melting. The transferred 

plasma-arc process is used in hardfacing applications such as valve seats, plowshares, oilfield 

components, and mining machinery. 

Source: ASM Handbooks Online, Volume 5, Surface Engineering -> Thermal Spray Coatings -> 

Processes 

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2.17. High-Velocity Oxy Fuel Spray Process 

Schematic of HVOF spray process. Photo courtesy of Sulzer-Metco Ltd. 

The HVOF (high velocity oxy-fuel) process efficiently uses high kinetic energy and controlled thermal 

output to produce dense, low-porosity coatings that exhibit high bond strengths (some of which exceed 83 

MPa, or 12,000 psi), low oxides, and extremely fine as-sprayed finishes. The coatings have low residual 

internal stresses, and therefore, can be sprayed to a thickness not normally associated with dense, 

thermal spray coatings. This process uses an oxygen-fuel mixture. Depending on user requirements, 

propylene, propane, hydrogen, or natural gas can be used as the fuel in gas-fueled spray systems and 

kerosene as the fuel in liquid-fueled systems. The coating material in powder form is fed axially through 

the gun, generally using nitrogen as a carrier gas. The fuel is thoroughly mixed with oxygen within the gun 

and the mixture is then ejected from a nozzle and ignited outside the gun. The ignited gases surround 

and uniformly heat the powder spray material as it exits the gun and is propelled to the workpiece 

surface. As a result of the high kinetic energy transferred to the particles through the HVOF process, the 

coating material generally does not need to be fully melted. Instead, the powder particles are in a molten 

state and flatten plastically as they impact the workpiece surface. The resulting coatings have very 

predictable chemistries that are homogeneous and have a fine granular structure. 



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2.18. Detonation Gun Thermal Spray Process for Ceramics 

The detonation gun, or D-gun, consists of a water-cooled barrel about 1 m (3 ft) in length with an inside 

diameter of approximately 25 mm (1 inch) and associated gas and powder metering equipment. 

Detonation gun coatings have some of the highest bond strengths and lowest porosities of the thermal 

spray coatings. 

Detonation gun process. Courtesy of Praxair Surface Technologies Inc. 

Initially, a mixture of oxygen and acetylene is fed into the barrel along with a charge of powder. The gas is 

then ignited. It accelerates the powder to a velocity of 800 m/s ( 2600 ft/s). The maximum free burning 

temperature of oxygen-acetylene mixtures occurs with 45 vol% C2H2 and is 3140 C ( 5680°F). It is 

estimated that under detonation conditions, the temperature reaches 4000°C (7230°F). Even higher 

maximum free burning temperatures are reached in the super D-gun. The great success of this process 

has been in decomposition-sensitive materials, such as cermets that require the metal matrix to be 

slightly melted to avoid carbide dilution. 

The extremely high velocities and consequent kinetic energy of the particles in the Super D-Gun process 

allow most of the coatings to be deposited with residual compressive stress, rather than tensile stress as 

is typical of most of the other thermal spray coatings. 

Source: Engineered Materials Handbook -> Densification and Sintering of Ceramics -> Nontraditional 

Densification Processes 



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2.19. Spray Tables for Thermal Spray Parameters 

Checklist for a plasma spray process 

Plasma processing equipment 

• Torch type (anode, cathode, injector ring), mm 

• Volts and amps = powder, kW 

• Primary gas and flow rate, slpm 

• Secondary gas and flow rate, slpm 

• Feed gas and flow rate, slpm 

• Standoff distance, mm 

• Powder injection, mm 

• Powder feed rate, g/min 

Hardware 

• Traverse speed of torch, m/s 

• Substrate cooling, m 3 /s 

• Spray footprint, mm 2 

Checklists, or spray tables, can be defined as a list of primary thermal spray parameters that need to be 

specified and controlled during a specific coating operation. All of these parameters influence process 

economics and coating quality. In many instances, manufacturers indicate the physical properties, such 

as surface roughness, microstructure porosity, and adhesion, that can be expected if coatings are 

produced using their recommended spray parameters. These values should be used only as a rough 

guideline for coating properties. The physical properties of the substrate also are very important, being 

essential to the production of an appropriate surface profile. 

Even with similar thermal spray processes, the spray parameters are likely to change from process to 

process and between different equipment. Powders that are presumably identical may exhibit different 

spray parameters for each set of equipment. 

Source: C.C. Berndt, Material Categories for Thermal Spray Coatings, Handbook of Thermal Spray 

Technology, J.R. Davis (Ed.), ASM International, p 143, 2005. 



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2.20. Cold-Spray Process Parameters 

Typical range of gas-jet parameters for cold spray coating 

Stagnation jet pressure, MPa (psi) 1 – 3 (145 – 435) 

Stagnation jet temperature, °C (°F) 0 – 700 (32 – 1290) 

Gas flow rate, m 3 /min (ft 3 /min) 1 – 2 (35 – 70) 

Powder feed rate, kg/h (lb/h) 2 – 8 (4 – 18) 

Spray distance, mm (in.) 10 – 50 (0.4 – 2) 

Power consumption, kW (for heating gas) 5 – 25 

Particle size, μm 1 - 50 

Process gases include nitrogen, helium, air, and mixtures of these gases. Nitrogen is a favored process 

gas because it can be used to spray some materials without promoting oxidation and because is much 

less expensive than helium. Helium is capable of providing the highest gas velocities, and therefore can 

be used in depositing the widest possible range of materials. Helium may also be diluted with nitrogen to 

improve the economics of the process, while providing particle velocities in excess of that achieved with 

nitrogen alone. 

Source: A. Papyrin, Cold Spray Technology, Adv. Mater. Process., Sept 2001, p 49-51; from Cold Spray 

Process, as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 

78, 2005. 



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3. Feedstock Materials 

3.1. Production of Thermal Spray Powders 

Particle size ranges for common industrial processes including flame spray and plasma 

spray thermal spray powders 

Thermal spray powders have evolved principally from powder metallurgy procedures. The most recent 

developments have arisen from ceramics and composites processing techniques. The production of 

polymeric powders that are suitable for thermal spray is an emerging engineering field. The powder 

production technique has a marked influence on the nature of the powder that is produced. The generic 

terms that describe the manufacture of powders by mechanical processes are comminution and attrition 

and denote the breaking up and size reduction of solid materials. The normal range of particle sizes 

required for thermal spray is between 5 µm to about 120 µm. 

Source: A. Bose, Advances in Particulate Materials, Butterworth-Heinemann, 1995, in Material 

Production Processes, Christopher C. Berndt, Stoney Brook University (at the time of publishing), as 

published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 147, 2005. 



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3.2. Gas-phase production methods for ultrafine powders 

Several different gas-phase methods are available for the production of ultrafine powders with submicron 

particle size. Four such methods are inert gas condensation, flame reduction, plasma reduction, and 

chemical vapor reaction. 

With inert gas condensation (IGC), high-quality powders can be produced with low chemical impurities 

from precursor materials, and low amounts of oxides or nitrides from the production process. 

Flame reduction and plasma reduction are based on the decomposition and reduction of metal salts in a 

gas flame or plasma. 

In chemical vapor reactions (CVR), metal chlorides and hydrogen are chemically reacted in a hot-wall 

reactor. 

Comparison of the most frequently used gas phase methods for production of metal 

nanopowders 

Method Advantage Disadvantage Typical capacity 

Flame reaction Large output Broad distribution of >1 tonne/day 

particle 

impurities 

size, ionic 

Plasma reaction Large output Broad distribution of >1 tonne/day 

particle 

impurities 

size, ionic 

Chemical vapour 

reaction 

Narrow distribution of 

particle size 



Ionic impurities 200 kg/day 

Inert gas condensation No impurities Small output Ultrafine 

and Nanophase Powders -> Metallic Nanopowders -> Production Methods 

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3.3. Advantages and Disadvantages of Powder Production Processes 

Advantages and disadvantages of selected powder preparation techniques 

Powder preparation technique Advantages Disadvantages 

Conventional method 

Conventional comminution Inexpensive 

Unaggregated powders 

Wide applicability 

Chemical preparation 

Solution methods 

High purity 

Solvent vaporization 

Small particles 

Simple evaporation 

Compositional control 

Spray drying 

Chemical homogeneity 

Spray roasting 

Fluid bed drying 

Emulsion drying 

Sol-gel or glass drying 

Freeze drying 

Precipitation or coprecipitation 

Vapour-phase methods 

High purity 

Vaporization-condensation Very small particles 

Vapor decomposition 

Vapor-vapor reaction 

Salt decomposition Used for solution techniques 

Simple apparatus 



Limited purity 

Limited homogeneity 

Large particles 

Expensive 

Aggregation of particles 

Poor for nonoxides 

Expensive 

Difficult for multi-component 

materials 

Aggregation of particles 

Thermal Spray powders have evolved principally from powder metallurgy procedures. The most recent 

developments have arisen from ceramics and composites processing techniques. The production of 

polymeric powders suitable for thermal spray is an emerging field. The powder-production technique has 

a marked influence on the nature of the powder that is produced. Feedstock production is done using 

mechanical and chemical routes. The generic terms that describe the manufacture of powders by 

mechanical processes are comminution and attrition and denote the breaking up and size reduction of 

solid materials. Atomization can be classified as a mechanical powder production method, but it differs 

from conventional comminution in that it involves the dispersion of melts rather than the dispersion of 

solids. The powder preparation subcategory that is used extensively for the production of ceramic 

feedstock lies within the solution methods of chemical preparation. The normal range of particle sizes 

required for thermal spray is 5 to about 120 µm. 

Source: Material Production Processes, Christopher C. Berndt, Stony Brook University (at the time of this 

publication), as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM 

International, p 147, 2005. 

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3.4. Water Atomization 

In general, water atomization is less expensive than the other methods of atomization because of the low 

cost of the medium (water), low energy use for pressurization compared with gas or air, and the very high 

productivity that can be achieved (up to 30 tons/hour or about 500 kg/min). The primary limitations of 

water atomization are powder purity and particle shape, particularly with more reactive metals and alloys. 

These generally give irregular powders of (relatively) high oxygen content. 

Schematic of water atomization process. The major components of a typical installation include 

a melting facility, an atomizing chamber, water pumping/recycling system, and powder 

dewatering and drying equipment. Melting of metals follows standard procedures. Air induction 

melting, arc melting, and fuel heating are suitable procedures. 



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3.5. Composite Materials 

Thermal spraying, either as a coating or as a bulk structural consolidation process, has clearly 

demonstrated advantages for the production of composites. Difficult-to-process composites can be readily 

produced by thermal spray forming, with vacuum plasma spray the process of choice for the most 

reactive matrix materials. Particulate-, fiber-, and whisker-reinforced composites have all been produced 

for various applications. Particulate-reinforced wear-resistant coatings such as WC/Co, Cr3C2/NiCr, and 

TiC/NiCr are the most common applications and comprise one of the largest single thermal spray 

application areas. 

Fig. 1: Schematic microstructures of possible thermally spray-formed composites. (a) Deposit with a 

particulate-reinforced second phase. (b) Deposit with a whisker-reinforced second phase. (c) Deposit with 

a continuous-fiber-reinforced second phase 

Figure 1 shows schematically the diverse forms of composites that can be thermally spray formed. 

Whiskers of particles can be incorporated using so-called "engineered" powders, mechanical blending, 

and by co-injecting different materials into a single spray jet. Mechanical blends and co-injection, although 

useful, have been found to result in segregation of the reinforcing phase and, in many cases, degradation 

of the second-phase whiskers or particles. Thermal spray composite materials can have reinforcingphase 

contents ranging from 10 to 90% by volume, where the metal matrix acts as a binder, supporting 

the reinforcing phase. The ability to consolidate such fine-grained, high reinforcing phase content 

materials is a major advantage of thermal spray over other methods. 

Thermal spraying of composite materials with discontinuous reinforcements, such as particulates or short 

fibers, is usually accomplished by spraying powders or powder blends. Investigators have developed 

techniques for the production of continuous fiber-reinforced materials that overcome the "line-of-sight" 

limitations of thermal spray processes. This includes "monotape" fabrication techniques, where 

continuous fibers are prewrapped around a mandrel and a thin layer of a metal, ceramic, or intermetallic 



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matrix material is sprayed. Plasma, HVOF, and wire arc spray have been used, although in the cases of 

intermetallics and high-temperature alloys, controlled atmosphere plasma spray (VPS) has generally 

been used. The fibers are thus encapsulated within thin monolayer tapes and are subsequently removed 

for consolidation to full density by hot pressing with preferred fiber orientations, producing continuously 

reinforced bulk composites. 

Powders for Sprayed Composites 

Discontinuously reinforced composites produced using thermal spray techniques use either composite 

powders or direct reactive synthesis approaches, as described below. Powders can be produced 

mechanically, chemically, thermomechanically, or by using high-temperature synthesis. "Engineered 

powders" defines powders in which different phases are incorporated to produce a "microcomposition" of 

the final desired structure. Typically these powders contain the desired sizes, size distributions, and 

morphologies of the equilibrium phases. These powders also permit the introduction of higher 

concentrations of phases than those normally achievable through conventional melt or reaction 

processing. 

Fig. 2: Schematic representations of typical "composite" thermal spray powders. 

Figure 2 shows two types of powder that could be produced in this way: short ceramic fibers within a 

metal matrix and an intermetallic-matrix material reinforced with more ductile phases. The latter has been 

found to be a viable approach for increasing the fracture toughness (KIc) of intermetallics. Varying 

reinforcing phase combinations, compositions, morphologies, and distributions can be produced. The 

rapid heating and cooling experienced by these powders during thermal spray forming limits dissolution 

and degradation of the phases, which remain relatively unchanged after consolidation, although some 

microstructural refinement and solutioning can take place. Generally, more significant changes occur 

during conventional powder consolidation processes because of the longer processing times. 

Source: Thermal Spray Forming of Materials, R. Knight and R.W. Smith, Powder Metal Technologies and 

Applications, Vol 7, ASM Handbook, ASM International, 1998, p 408-419. 



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3.6. Powder Testing and Characterization: Image Analysis 

Image analysis method of determining powder size and shape. (a) Original image. (b) Processed image, 

providing shapes that are less ambiguous and easier to measure. 

Image analysis techniques are used to digitize an image of the powder particles and, using image 

enhancement, thresholding, and binary processing routines, yield a size and shape distribution. While 

simple in theory, this method is difficult in practice due to the need to separate touching powder particles 

and to accumulate a significant number of images to properly statistically represent the specimen. 

Provided these issues can be overcome, image analysis is a precise method of measuring particle size 

and shape. 

Source: Powder Testing and Characterization, Walter Riggs, TubalCain Co., Inc., as published in 

Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 274, 2005. 



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3.7. Powder Characteristics: Morphology 

Micrograph of irregularly shaped water-atomized powders having high surface areas (top), and 

micrograph of spherical gas-atomized powder (bottom). 

Manufacturing process variations can effect changes in particle morphology, or shape. Morphology 

affects various parameters including feedability, apparent density, sprayability, unmelted particles in the 

coating, porosity, and vaporization. Water-atomized powders are one example of this phenomenon, 

where the morphology may range from almost perfect spheres to highly irregular, convoluted shapes, 

depending on the manufacturing process parameters. Irregularly shaped particles do not feed as easily 

as spherical or equiaxed (particles having the same dimension in every direction) powders, because they 

pack together more easily. Because a sphere has the lowest surface-to-volume ratio of any geometry, 

any nonspherical shape can adsorb more surface moisture than a sphere. Gas-atomized powders usually 

are more spherical, and, therefore, typically flow very well. 




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3.8. Particle Size Distribution Plots 

Particle classification curves, (a) Cumulative plot (b) Frequency plot 

A particle size distribution can be graphically represented as a cumulative plot or a frequency plot. These 

graphs are essentially identical since they can be derived from each other; the summation of the 

individual frequencies at each size enables the cumulative plot to be developed. The key features 

conveyed from particle size distribution data are: 

• The most commonly occurring particle size (i.e., the modal value) is indicated by the peak in the 

frequency plot. The mean value can be determined from the diameter at the 50 wt% value of the 

cumulative graph. 

• The sharpness of the plot gives a qualitative indication of the spread in particle size. A sharp peak 

indicates that the particles are of similar size, whereas a flat or broad curve indicates that the 

particle sizes are spread over a wide range of values. 

• The maximum and minimum values in measurable diameters allow the particle size range to be 

determined. 

• It is important to look for a distribution that does not fluctuate, because that indicates a bimodal 

distribution of particle sizes. Such powders do not feed reliably. 

Source: Particle Characterization, Christopher C. Berndt, Stony Brook University, as published in 

Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, 2005, p 164. 



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3.9. Wire and Rod Feeders 

Wire feeding principles and devices 

Wire and rod feeders are simple mechanical devices consisting of electrical or air-driven motors, drive 

rolls, and associated speed controls. Wires and rods need to be fed continuously and uniformly into the 

flame or arc. The injection point of the wire/rod and feed rate are extremely important in controlling the 

coating quality. Uniformity of feed depends on motor drives having sufficient torque to overcome friction 

within the system, and in the case of wires, from the wire spools. Drive roll design is another important 

aspect of uniform feed. Drive rolls must grip the wire/rod sufficiently to prevent slipping while not 

deforming the wire or crushing the rod. Drive rolls vary according to the wire properties. Rolls are usually 

made of metal or fiber-reinforced phenolic. Phenolic rolls are used for ceramic rods, whereas metal rolls 

are used for wires. Metal rolls can be knurled for hard wires or grooved for wires; many variations exist. 

Source: Process Control Equipment, revised by Daryl E. Crawmer, Thermal Spray Technologies Inc., as 




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3.10. Fluidized-Bed Powder Feeders 

Fluidized-bed powder feeders are commonly used in thermal spray processes. In these units, a “bed” of 

powder is continuously agitated by a fluidizing gas, in this case the carrier gas. The intent by design is to 

suspend a controlled volume of powder in a bed using a controlled volume of gas such that a sample of 

powder can be drawn off at a uniform rate. To accomplish this, the bed must be in continuous motion— 

that is, fluidized—and the volumes of gas and powder being removed must be continuously replaced. 

Vibrators and/or gravity feed are used to assist and maintain flow from the hopper into the bed. Powder 

feed rate in fluidized-bed feeders is affected by powder characteristics and particle size distribution. 

Source: Process Control Equipment, Revised by Daryl E. Crawmer, Thermal Spray Technologies Inc., as 




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3.11. Gravity-Based Powder Feeding 

Gravity- and vibration-based powder feeding system 

In a gravity-based powder feeding device, the powder falls into the thermal spray source. Some vibratory 

action, normally achieved by using compressed air to drive a ball bearing around a bearing race, may be 

supplied to agitate the powder into the gas stream and prevent blockages. The throttle control for the 

powder flow is a compressible rubber grommet that can be pinched to either allow or stop powder flowing 

into the carrier gas stream. This device relies on a full canister of powder so that the gravity head of 

material is approximately constant. There are no powder delivery tubes connecting the powder supply to 

the thermal spray source. Uneven and somewhat sporadic powder flow often occurs when the canister 

nears an empty condition. The powder flow rate is controlled by altering the inside diameter of a rubber 

grommet. This type of powder feeding device is simple and robust, but not particularly accurate or 

reproducible with respect to powder flow rate. 

Source: Feedstock Material Considerations, Christopher C. Berndt, as published in Handbook of Thermal 

Spray Technology, J.R. Davis (Ed.), ASM International, 2005, p 138. 



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3.12. Powder Feeding Systems: Rotating Wheel Devices 

Rotating wheel (a) and rotating disk (b) powder feeding systems 

A rotating wheel device delivers precise parcels of powder to the powder feed delivery tube. The wheel 

can be mounted either vertically or horizontally. When oriented in the vertical direction (in the geometry of 

a simple water wheel), the powder is dropped into the carrier gas stream. A gas tube is connected 

between both sides of the rotating wheel to equalized gas pressures on both sides of the wheel. 

Otherwise, the powder would be blown off the pockets that are machined into the outside rim of the 

wheel. The horizontally oriented wheel (or disk) has slots that pass over the gas delivery system. In this 

fashion, powder is taken out of the slots and enters the carrier gas stream. 

In both cases, the powder delivery rate is controlled by (a) the physical dimensions of the slots or pockets 

that hold the powder and (b) the rotational speed of the wheel. In either geometry, it is easy to overload 

the carrier gas with solids and either block the powder feed tube or cause the saltation effect by using too 

high a powder feed rate or an insufficient carrier gas flow rate. Of course, the dimensions of the powder 

feed tube and the powder injection port, as well as the pressure conditions into which the powder and 

carrier gas are injected, also influence the powder delivery. 

Source: Feedstock Material Considerations, Christopher C. Berndt, Stony Brook University (at the time of 

publish date), as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM 




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4. Surface Preparation 

4.1. Dry Abrasive Grit Blasting 

Mechanical bonding, showing a properly grit-blasted surface with a large number of reverse draft pits into 

which sprayed particles flow to achieve positively keyed mechanical interlocking 

Dry abrasive grit blasting is the most commonly used surface roughening technique. Dry abrasive 

particles are propelled toward the substrate at relatively high speeds. On impact with the substrate, the 

sharp angular particles act like chisels, cutting small irregularities into the surface. The amount of 

substrate (plastic) deformation is a function of the angularity, size, density, and hardness of the particles 

and the speed and angle at which the particles are directed toward the substrate. The action of the 

grit/shot causes irreversible plastic deformation of the surface material. The material beneath the 

deformed surface material remains elastic and tries to return the deformed surface material to its original 

(shorter) length. The resulting balance of forces places the outer layer of material in residual compressive 

stress, while the underlying material is in tension. To avoid reduction of fatigue life, titanium substrates 

are sometimes shot peened before dry abrasive grit blasting to reduce the underlying tensile stresses, 

effectively balancing the compressive/tensile forces. Irregularities on a perfectly grit-blasted surface will 

have a large number of reverse draft pits, providing a partially hidden space into which sprayed molten 

particles can flow. Coatings sprayed to this surface are said to be positively keyed, creating what is 

commonly referred to as a mechanical interlocking bond. 

Source: Coating Processing, Frank N. Longo, Longo Associates, as published in Handbook of Thermal 




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4.2. Alumina Blasting Abrasives 

Aluminium oxide (Al2O3) particles. 130x. (a) 240 mesh (b) 150 mesh 

Alumina (Al2O3) grit works very well in blasting cabinets. The specific gravity of alumina is just over half 

that of chilled iron, so that this type of abrasive is readily picked up by the suction feed and is effectively 

accelerated through the blasting nozzle. The rate of breakdown is not excessive, except when very high 

air pressures are used to blast hardened steel or other very hard substrate materials. Alumina, fused and 

pure, is extremely hard (>9 on the Mohs scale), and when properly crushed has sharp cutting edges. 

There are two grades: C (coarse) and F (fine). A good all-purpose grit for general use in a blasting 

cabinet would be to start with a 50/50 mixture of C and F, with periodic small additions of the C grade to 

maintain a balanced grit range of the mix. If the -50 mesh fraction is occasionally removed by screening, it 

can be set aside and used where a fine, light grit blast is needed. 

Source: From Coating Processing, Frank N. Longo, Longo Associates, as published in Handbook of 

Thermal Spray Technology, J.R. Davis (Ed.), ASM International, 2005, p 114. 



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4.3. Machining and Macroroughening 

Comparison of thermal spray coatings deposited on macroroughened and smooth surfaces. (a) Sprayed 

metal over grooves; shrinkage constrained by grooves (b) Sprayed metal on smooth surface; effect of 

shear stress on bond due to shrinkage 

Macroroughening is usually accomplished by machining grooves or threads into the surface to be 

sprayed. Typically, rough machined surfaces are also grit blasted prior to spraying. Surfaces roughened 

to this magnitude are often used for thick coatings to restrict shrinkage stresses and to disrupt the 

lamellar pattern of particle deposition in order to break up the shear stresses parallel to the substrate 

surface. Groove threads should be considered for coatings used on machine-element repairs for 

thicknesses greater than 1.3 mm (0.050 in.) on the radius, for high shrinkage coatings, and for coatings 

that may be subject to high shear stresses (i.e., loads parallel to the interface). 

Source: From Coating Processing, Frank N. Longo, Longo Associates, adapted from H.S. Ingham and 

A.P. Shepard, Eds. Metallizing Handbook, 7th ed., Metallizing Engineering Co. Inc., (Metco), p-A 10, 

1959, as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, 

2005, p 115. 



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5. Coating Processing and Operation 

5.1. Part Configuration and Coating Location Considerations 

Thermal spray stream positions. (a) Good (b) Acceptable (c) Minimum acceptable (d) 

Formation of porosity in sprayed coatings when the spray angle is reduced to approximately 

45 degrees. Particles impacting at angles of less than 90 degrees create a shadowing effect 

that results in increased coating porosity. 

After identifying an area to be coated, determine whether the part is symmetrical about the coating 

location, which would allow fixturing for rotation and/or translation about an axis perpendicular to the 

spray torch. The stream of spray particles should impact the target surface as close to normal (90 

degrees) as possible. The minimum acceptable impact angle is 45 degrees to the target area. A 45 

degree spray angle impact should be used only as a last resort, bearing in mind that some coating 

property deviation from the optimum can be expected. Both bond strength and adhesion will be 

compromised. Porosity increases dramatically as the angle of the spray torch moves from normal to 45 

degrees. 

Source: Coating Operations, Frank N. Longo, Longo Associates, as published in Handbook of Thermal 




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5.2. Contact and Shadow Masking 

Coatings resulting from contact masks (a) versus shadow masks (b) 

Masking protects component areas next to the thermal spray target zone from impact by overspray 

particles. Methods include metal “shadow” masks, high temperature tapes, and paint-on masking 

compounds. 

Metal shadow masks are placed approximately 2-3 times the total coating thickness away from the part to 

be sprayed and in front of the spray torch. The unwanted spray collects on the mask and is prevented 

from reaching the substrate. 

Tape, or contact, masks are applied by hand wrapping areas that do not require a coating. After spraying, 

the tape is easily removed. Many different tapes are commercially available. The best tapes have 

sufficient toughness to resist grit blasting and sufficient heat resistance to withstand the hot gas and 

particle impact during spraying. 

With a shadow mask, the coating ends in a narrow feathered band rather than a sharply defined edge as 

with a contact mask. The sharp edge could lead to chipping if the coating is thick, or it could serve as a 

stress raiser that leads to debonding of the coating. 

Source: F.N. Longo, Coating Processing, Handbook of Thermal Spray Technology, J.R. Davis (Ed.), 

ASM International, 2005, p 117. 



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5.3. Thermal Spray Pattern 

Spray pattern illustrating particle deposition and the effect of size and debris on thickness and porosity in 

cross section. 

All thermal spray processes involve molten or semi molten droplets or particles travelling at some velocity 

in a gas stream and impacting onto a substrate to form a coating. A spray pattern cross section in planar 

view, parallel to the substrate, is circular or oval in shape. The fastest and densest deposits will build up 

at the center of the jet, where most particles are entrained and where the highest degree of melting 

occurs. Moving radially out from the center (where fewer particles are entrained and where they tend to 

be coarser and perhaps semi molten), combined with impact at angles of less than 90 degrees, results in 

increased porosity. At the jet periphery, fine particles oxidize readily due to entrained air from the 

surrounding atmosphere and deposit as debris. Small particles oxidize rapidly, sometimes completely, to 

form the major source of oxide inclusions in sprayed coatings. 


Spray Technology, J.R. Davis (Ed.), ASM International, p 121, 2005. 



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5.4. Relationship between Particle Melting and Coating Structure 

The effect of the degree of particle melting at or just before impact on shape and coating structure. 

(a) Particle is heat softened or beginning to resolidify. At impact, it does not flow out and begins to 

lift at the edges. (b) Properly melted particle impacts and flows to form a well-bonded classical 

lamellar splat shape. (c) Superheated particle impacts and splashes, creating debris, satellites, and 

dusting. 

In addition to the effects of peripheral particles stemming from the spray pattern, consideration must be 

given to center-stream particles and how variations in the degree of melting affect coating structure. 

Particles in the center of the spray pattern may include one or all of the following states on impact: 

• Fully molten, or at just above the materials melting temperature 

• Superheated, well above the melting temperature and perhaps close to the vaporization point 

• Semimolten, with the outside liquid but the core still solid 

• Molten then solidified while in-flight before impact 

Fully melted particles at or just above the material melting point arrive at the substrate, impact, flow, and 

flatten. Particle material spreads and cools rapidly as heat is conducted into the substrate. The classic 

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lamellar splat morphology is thus achieved. Lamellar particle thickness depends on particle velocity, 

substrate surface tension, starting particle size, and substrate temperature. Superheated particles may 

splatter on impact, sending out radial splashes of fine droplets—generally spherical—that end up in the 

coating as satellites or debris. Debris formed from splatter differs from debris produced in the jet in that 

splatter lodges in the coating, building up at the first ridge that stops its radial travel. Air blasting does not 

remove splatter debris. Superheating is a condition that generally should be avoided. Particles that 

resolidify in flight after having been melted may not deposit; it they do, they may appear as unmelted 

particles but with clearly discernable oxide layers on the particle surfaces. 


Spray Technology, J.R. Davis (Ed.), ASM International, p 123, 2005. 



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5.5. Air-Cooling Devices to Minimize Debris 

Positioning of an air-cooling device to blow debris off the surface before it 

rotates into the center spray region and is incorporated into the coating. 

To spray large target areas, an ancillary air-cooling device can be attached to the spray torch, such that a 

stream of compressed air blows peripheral debris from the substrate surface in advance of the traversing 

spray stream. Debris is blown off the surface before it sticks and passes under “good” spray material to 

become permanently incorporated into the coating. When spraying flat surfaces, air coolers should be 

positioned on both sides of the spray stream to help remove debris. Air coolers are usually adjusted to 

precede and follow the spray torch along the transverse direction. While air is used to blow loose debris 

off the surface, it also helps cool the surface, as the name implies. 

Source: Gravity- and vibration-based powder feeding system. Coating Operations, Frank N. Longo, 

Longo Associates, as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM 




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5.6. Influence of Temperature Control on Substrate/Coating 

Effect of coating shrinkage on interfacial shear stresses. The sprayed metal cools, creating a 

tensile stress in the coating and a compressive stress in the underlying substrate material. These 

stresses may deform the substrate or weaken the bond between the sprayed coating and the 

substrate, leading to debonding of the coating. the bottom illustration shows deformation of a thinsection 

substrate and debonding of the coating due to stresses generated by cooling of the 

sprayed metal. 

When substrate preheating has been completed and spraying has begun, it is necessary to control the 

temperature of both the sprayed coating and the substrate to avoid substrate degradation, oxidation, and 

shrinkage/expansion. 

Substrate Degradation. For those substrate materials susceptible to physical changes at high 

temperatures, irreparable damage can be inflicted when temperature is not controlled. Physical properties 

(e.g., hardness) established by heat treating are most likely to change. 

Oxidation. Coating layers deposited and allowed to reach high temperatures during spraying will oxidize 

and appear to darken. Surface oxides so formed leave a plane of weakness in the coating and could lead 

to delamination under applied stresses. The critical oxidizing temperature for each material varies: 

nevertheless, keeping surface temperatures below 150 to 205 °C (300 to 400 °F) is a good general rule 

when spraying under air conditions. 

Shrinkage/Expansion. Sprayed coatings shrink as they cool, exerting considerable shear stress on the 

substrate and leading to warpage and possible separation. Substrate materials having a high coefficient 

of thermal expansion that are allowed to expand because of high temperatures during the spray process 

will experience major debonding at the substrate/coating interface. 

Source: H.S. Ingham and A.P. Shepard, Ed., Metallizing Handbook, 7th ed., Metallizing Engineering Co. 

Inc., (Metco), 1959, p A-65; as published in Coating Operations, Frank N. Longo, Longo Associates, 

Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 125, 2005. 



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6. Coating Structures, Properties and Materials 

6.1. Microstructures of thermal spray coatings 

Thermal spray coatings consist of many layers of thin, overlapping, essentially lamellar particles, 

frequently called splats. Cross sections of detonation gun deposited alumina and titania are shown in the 

photomicrograph. Generally, the higher-particle-velocity coating processes produce the most dense and 

better bonded coatings, both cohesively (splat-to-splat) and adhesively (coating-to-substrate). 

Metallographically estimated porosities for detonation gun coatings and some HVOF coatings are less 

than 2%, whereas most plasma sprayed coating porosities are in the range of 5 to 15%. The porosities of 

flame sprayed coatings may exceed 15%. 

Microstructure of detonation gun deposited alumina and titania. As-polished 

Most of the thermal spray processes lead to very rapid quenching of the particles on impact. Quench 

rates have been estimated to be 104 to 106 °C/s for ceramics and 106 to 108 °C/s for metallics. As a 

result, the materials deposited may be in thermodynamically metastable states, and the grains within the 

splats may be submicron-size or even amorphous. 

The metastable phases present may not have the expected characteristics, particularly corrosion 

characteristics, of the material, and this factor should be kept in mind in the selection of coating 

compositions. 

Source: R.C. Tucker, Jr., Thermal Spray Coatings, ASM Handbook, Vol 5, Surface Engineering, ASM 

International, 1994, p 507 



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6.2. Structure of Thermal Spray Coatings 

Schematic showing the buildup of a thermal spray coating. Molten particles spread out 

and deform (splatter) as they strike the target, at first locking onto irregularities on the 

substrate, then interlocking with each other. Voids can occur of the growing deposit traps 

air. Particles overheated in the flame become oxidized. Unmelted particles may simply be 

embedded in the accumulating deposit. Courtesy of the Materials Engineering Institute, 

ASM International. 

The deposited structures of thermal spray coatings differ from those of the same material in the wrought 

form because of the incremental nature of the coating buildup and because the coating composition is 

often affected by reaction with the process gases and the surrounding atmosphere while the materials are 

in the molten state. 

For example, where air or oxygen is the process gas, oxides of the material applied may be formed and 

become part of the coating. The as-applied structures of all thermal spray coatings are similar in their 

lamellar nature; the variations in structure depend on the particular process, the processing parameters 

and techniques, and the material applied. 

The diagram illustrates the microstructure that results. As shown in this figure, the molten particles spread 

out and deform (splatter) as they impact the substrate, at first locking onto irregularities on the roughened 

surface, then interlocking with each other. Voids can occur if the growing deposit traps air. Particles 

overheated in the flame become oxidized. Unmelted particles may simply be embedded in the 

accumulating deposit. 

Source: J.R. Davis, Hardfacing, Weld Cladding, and Dissimilar Metal Joining, ASM Handbook, Vol 6, 

Welding, Brazing, and Soldering, ASM International, 1993, p. 803. 



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6.3. Sources of Coating Porosity 

Porosity created by shadowing resulting from off-axis spraying 

One of several sources of porosity is shadowing due to the angle of impingement of the spray stream. 

Shadowing generally is associated with coatings sprayed at angles below 45 degrees from the optimal 

“normal” angle of incidence. Coating porosity decreases (i.e., density increases) as the angle of spray 

approaches 90 degrees (normal to the surface being coated). More advanced coating systems are 

sprayed using tighter tolerances on fixture alignment. Plasma coatings may be sprayed at ±15 degrees, 

whereas low-end coatings may be sprayed at ±30 degrees. In shadowing, surface protrusions build up 

and then shadow interstices of voids adjacent to and behind the protrusions. These protrusions produce 

even more shadowing, particularly as the angle of incidence is lower relative to the surface; that is, less 

than 90 degrees. 

Source: D.E. Crawmer, Coating Structures, Properties, and Materials, Handbook of Thermal Spray 

Technology, J.R. Davis (Ed.), ASM International, 2005, p 50. 



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6.4. Influence of Porosity on Coating Properties 

Typical thermal spray coating defects 

Porosity is an important coating feature that strongly influences coating properties. As with oxide 

inclusions, porosity can be a desirable characteristic. This discussion takes the general position that 

porosity is undesirable. Porosity creates poor coating cohesion and allows for higher wear and corrosion 

rates. Porosity is generally associated with a higher number of unmelted or resolidified particles that 

become trapped in the coating. Poor splat or particle cohesion leads to premature coating cracking, 

delamination, or spalling. Open porosity can interconnect to the coating interface, enabling corroding or 

oxidizing elements to attack the base material. Porosity can thus "short-circuit" the inherent corrosion 

resistance of a coating. For hardface or wear-resistant coatings, porosity lowers coating hardness and 

contributes to poor surface finishes, thus decreasing wear resistance. Porosity in wear coatings can also 

lead to the generation of coating fragments that break away and become abrasive cutting agents, 

increasing coating wear rates. 

Source: Coating Structures, Properties, and Materials, Revised by Daryl E. Crawmer, Thermal Spray 

Technologies Inc., as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM 




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6.5. Coat Bonding: Mechanical Interlocking 

Interlocking of coating particles 

Bonding of the coating to the substrate and cohesion between consecutive splats is affected, in rough 

order, by: 

• Residual stresses within the coating 

• Melting and localized alloying at the contact surfaces between particles and between the 

substrate and adjoining particles 

• Diffusion of elemental species across splat boundaries 

• Atomic-level attractive forces (van der Waals forces) 

• Mechanical interlocking 

Mechanical interlocking has been historically viewed as the main mechanism of thermal spray coating 

adhesion. Mechanical interlocking can play a part in coating adhesion and cohesion when the surfaces 

being coated have features that allow molten material to flow into and fill negative relief or where the part 

has negative relief, as with shafts. In this case, the bond between impacting particles and the surface is 

established largely through the impact of particles that flow and solidify around the substrate surface 

asperities. Substrate asperities with negative relief can be formed prior to coating by grit blasting and 

other mechanical surface preparation techniques, or by process-induced irregularities on the actual 

coating surface. 

Source: Coating Structures, Properties, and Materials, Revised by Daryl E. Crawmer, Thermal Spray 

Technologies Inc., as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM 




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7. Post Coating Operation 

7.1. Sealing 

Common sealants used with thermal spray coatings 

Organic material Characteristic 

Paints Water and solvent soluble 

Waxes Low-temperature melt 

Phenolics Air dry or heat cure 

Epoxy phenolics Air dry or heat cure 

Epoxy resins One-part catalyst 

Polyesters Air or heat cure or one-part catalyst 

Silicones Heat cure 

Polyurethanes Air dry or one-part catalyst 

Linseed oil Air dry 

Polyimides Heat cure 

Coal tars Air dry 

Anaerobics Cure in absence of air 


Thermal Spray coatings, being somewhat porous by nature, often are sealed with organic materials that 

penetrate and fill the pores. The sealant is then cured, which effectively creates a barrier to penetration of 

unwanted materials. Benefits of sealing include: 

• Preventing corrosive species (liquids and gases) from penetrating the coating and attacking the 

substrate 

• Preventing grinding debris from lodging in the coating 

• Enhancing interparticle cohesion 

• Providing special surface properties such as non-stick and release characteristics 

The choice of sealer dictates the curing temperature and time. The sealer should have a low viscosity to 

penetrate the coating to a satisfactory depth. Also, it is important to know the chemical resistance and 

service temperature of the sealant after curing. Sealants can be applied by brushing, spraying, and 

dipping. 



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7.2. Recommended Metallographic Practice: Sectioning 

Proper sectioning of thermal spray coatings, (a) Sectioning of dense, nonfriable coatings, (b) Sectioning 

of porous, friable coatings. 

Thermal spray coating specimens should be sectioned perpendicular to their axis using a precision 

diamond saw with either a metal-bonded diamond wafering blade or an ultrathin aluminium oxide 

abrasive blade. Generally, an abrasive cut off blade selected to cut the substrate effectively will be the 

best blade for the combination of thermal spray coating and substrate. The specimen should be rigidly 

clamped in the vise and positioned so that the cutting blade enters the coated side and exits the substrate 

side, thus substantially reducing fracturing of the coating. Friable, porous, or brittle coatings to be 

sectioned may be vacuum impregnated with epoxy mounting compound prior to sectioning to protect the 

specimen. Typical sectioning parameters are: 

• Speed: 2,500 to 3,500 rpm 

• Applied load: 300 to 500 gf 

• Lubricant: water soluble solution 

Source: ASM Handbook: Metallography and Microstructures, Vol.9, 2004, p 1040 



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8. Coating Characterization and Testing 

8.1. Measuring Deposition Efficiency 

Variation of cold spray deposition efficiency of titanium coatings with powder feed rate 

Deposition efficiency (DE) is defined as an idealized measure of the percentage of particles introduced 

into a spray jet that actually deposits onto a flat substrate without overspray considerations. Deposition 

efficiency can be measured according to the following procedure: 

• Determine the mass (X) of a clean, grit blasted 4 in. × 6 in. × 0.125 in. (100 × 150 × 3 mm) flat 

steel ate 

• Place a known mass of powder into the powder feeder 

• After stabilizing the spray parameters and powder feed, spray material onto the steel plate for a 

known period of time (e.g., 60 s) at a predetermined feed rate (e.g., 20 g/min) ensuring that the 

spray pattern remains on the plate at all times. (The plate may be cooled as necessary using 

air/gas jets directed at the rear face of the plate.) 

• Measure the mass (Y) of the plate plus the coating. 

• Determine the gain in mass (Z) due to the coating deposited (Z = Y - X). 

• Divide the mass deposited in one minute (Z) by the powder feed rate and multiply by 100 to give 

DE in %. 

• Repeat the measurement a minimum of three times and take the average. 



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Related terms: target efficiency, sticking efficiency. 

Notes: 

1) A scale capable of measuring to an accuracy of 0.1 g should be used to measure the changes in 

mass. 

2) Deposition efficiency is only useful insofar as it provides a measure for optimizing spray 

parameters. 

3) When substrate geometry, size, and overspray are taken into consideration, the ideal DE 

decreases substantially, yielding a true deposition efficiency, properly termed target efficiency 

(TE). 

Reference: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, 2004, p 79 

Source: Dr. Richard Knight, FASM, Auxiliary Professor and CPPM Director, Drexel University, 

Philadelphia, Pa. 



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8.2. Tensile Adhesion Testing of Thermal Spray Coatings 

The coating/fixture assembly used for tensile adhesion testing of thermal spray coatings. Dimensions are 

in inches (1 in. = 25.4 mm) 



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Adhesion is a property of major concern for thermal spray coatings because it is necessary for the coating 

to adhere to the substrate throughout the design life of the coating system. The standard coating/fixture 

assembly used for tensile adhesion testing of thermal spray coatings is adapted from ASTM C 633-01, 

“Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings.” The coating is 

applied to a 25 mm (1 in.) diameter cylindrical bar that is threaded for gripping at its end. A somewhat 

viscous adhesive bonding agent is then applied to the surface of the coating, and the coating/bonding 

agent is applied downward in a suitable alignment fixture onto another bar. The entire assembly is placed 

into an oven for curing at moderate temperatures (120 to 175°C, or 250 to 350°F). Following the cure 

cycle, the assembly is pulled apart to determine its strength. 

Source: Walter Riggs, Testing of Coatings, Handbook of Thermal Spray Technology, J.R. Davis (Ed.), 




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8.3. Bond Pull Testing of Thermal Spray Coatings 

A bond pull test measures the force required to separate a thermal spray coating from its substrate 

Adhesion (bond-strength) testing of thermal spray coatings is performed in accordance with ASTM C 633- 

01 “Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings.” The test 

measures the force required to separate a thermal spray coating from its substrate. Depending on its 

bond strength, the coating can fail at any of several locations including the coating-substrate interface, 

within the coating, and at the interface between the coating and the bonding agent. For some coatings 

(such as HVOF WCCo), the adhesive strength of the coating is typically greater than the strength of the 

bonding agent. The two most common bonding agents are FM-1000 and EC-2214. FM-1000 comes in 

the form of a wafer, while EC-2214 is a liquid. Both bonding agents are applied between the coating and 

a mating metallic slug. After curing, the coating is subjected to increasing tensile load until failure. While 

FM-1000 can be used for any coating, EC-2214 should only be used for dense coatings. Because it is a 

liquid, it is possible for EC-2214 to penetrate a porous coating and bond to the substrate. As a result, 

artificially high bond strength values could be measured. 

Source: Doug Puerta, KHA Metallurgical, Portland, Oregon 



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8.4. Rolling Contact Fatigue Failure Modes in Thermal Spray Coatings 

Rolling-contact fatigue failure modes in Thermal Spray cermet (WC-Co) and ceramic (Al2O3) coatings 

have been studied. Investigations relating to the RCF failure modes of TS coatings have classified the 

fatigue failure modes on the basis of surface and subsurface observations in pre- and post-RCF 

conditions. 

Rolling-contact fatigue failure of TS coatings are generally categorized in four main modes: 

• Abrasion 

• Delamination 

• Bulk failure 

• Spalling (M1–M4), as indicated in the diagram 

Rolling-contact fatigue failure modes of thermal spray cermet and ceramic coatings. 

Source: Rolling Contact Fatigue, R. Ahmed, Failure Analysis and Prevention, Vol 11, ASM Handbook, 

ASM International, 2002, p 941-956. 



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8.5. Residual Stress Determination in Thermal Spray Coatings 

Hole-drilling method for measuring residual stresses. (a) Typical three-element strain-gage rosette (b) Inplane 

strain components caused by release of residual stresses through the introduction of a hole 

The hole-drilling method for measuring residual stresses in thermal spray coatings involves drilling a 

shallow hole in the test specimen to a depth approximately equal to the hole diameter. Typical hole 

diameters range from 0.8 to 5.0 mm (0.03 to 0.20 in.). The creation of the hole redistributes the stresses 

in the material surrounding the hole. A specially designed three-element strain-gage rosette measures the 

associated partial strain relief. The in-plane residual stresses that originally existed at the hole location 

can then be calculated from the measured strain reliefs using the method described in ASTM E 837, 

“Measurements of Residual Stresses by Hole-Drilling Strain-Gage Method.” The ASTM standard also 

gives details of practical drilling procedures. 

Source: From Testing of Coatings, Walter Riggs, Tubal-Cain Co. Inc., and Ken Couch, Protech Lab 

Corp., as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, 

2005, p 271. 



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8.6. Thermal Fatigue Testing of TBCs 

Schematic illustration of oxide surface temperature cycle used for thermal 

fatigue testing of ZrO3 coated test buttons 1.27 mm (0.050) thick, simulating a 

first-stage gas turbine outer air-seal application. 

Laboratory testing has been used, particularly in the coating development stage, to thermally cycle a 

thermal barrier coating (TBC) specimen, followed by post-test microscopic examination of spallation-type 

cracks. The thermal cycle for evaluation of thermal shock resistance uses rapid heating and cooling rates, 

using direct impingement flames or heating jets on the oxide face of the specimen, with little hold time at 

the maximum temperature. This test principally challenges the oxide layer, because the bond coat 

remains at relatively low temperatures due to the insulating nature of the zirconia layer and the short time 

at high temperatures. 

Source: Walter Riggs, Testing of Coatings, Handbook of Thermal Spray Technology, J.R. Davis (Ed.), 




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9. Applications 

9.1. Coatings for Corrosive Environment: Conventional Coal-Fired Boilers 

(a) (b) 

Economizer tube bends being coated with high-chromium steel alloy using the twin-wire arc process (a). 

The tubes after coating are shown in (b). Courtesy of ASB Industries Inc. 

Utility boilers suffer from high wear and corrosion caused by coal slagging. In recent years, there has 

been substantial research showing extended life for such boilers when corrosion/wear zones are coated 

by plasma spray, twin-wire arc, and HVOF with Inconel systems or other proprietary alloys. Most are now 

coated with wire-arc applications of high-chromium alloys or chromium-nickel alloys that effectively 

protect them from high-sulfur corrosion for periods of up to five years. It is estimated that 4,650 m 2 

(50,000 ft 2 ) of new surfaces are coated each year. 

Source: Selected Applications, as published in Handbook of Thermal Spray Technology, J.R. Davis 

(Ed.), ASM International, p 208, 2005. 



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9.2. Elevated-Temperature Corrosion Applications 

Thermal spray coatings for elevated-temperature service 

Service temperature, °C (°F) Coating metal or alloy Coating thickness 



µm mils 

Up to 550 (1020) Aluminium 175 7 

Up to 550 (1020) in the presence of 

sulphurous gases 

Ni-43Cr-2Ti 375 15 

550 – 900 (1020 – 1650) Aluminium or aluminium-iron 175 7 

900 – 1000 (1650 – 1830) Nickel-chromium or MCrAlY 375 15 

900 – 1000 (1650 – 1830) in the 

presence of sulphurous gases 

Nickel-chromium (a) 

Aluminium (a) 

(a) Coating system consists of an aluminium layer deposited on top of a nickel-chromium layer 

Common applications involving high-temperature corrosion include coating exhaust stacks, chimneys, 

flues, rotary kilns and dryers, catalytic crackers, and furnace parts. These generally involve the use of an 

aluminum or nickel-chromium alloy coating. One specific high-temperature corrosion application that has 

met with great success in recent years is the coating of boilers in paper mills, power plants, and chemical 

plants. Water-wall tubes in these applications suffer severe corrosion because of the high sulfur content 

of the burning fuel, the high operating pressures, and abrasion. A 375 μm (15 mil) coating of Ni-43Cr-2Ti 

alloy offers extremely good protection against this very severe corrosion at temperatures to 550 °C (1020 

°F). 

Source: M.L. Berndt and C.C. Berndt, Thermal Spray Coatings, Corrosion: Fundamentals, Testing, and 

Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 803-813; as published in Introduction to 

Applications for Thermal Spray Processing, Handbook of Thermal Spray Technology, J.R. Davis (Ed.), 


375 

100 

15 

4 

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9.3. Oxidation Protection 

Thermal spray coatings for elevated-temperature service 

Service temperature, °C (°F) Coating metal or alloy Coating thickness 



µm mils 

Up to 550 (1020) Aluminium 175 7 

Up to 550 (1020) in the presence of 

sulphurous gases 

Ni-43Cr-2Ti 375 15 

550 – 900 (1020 – 1650) Aluminium or aluminium-iron 175 7 

900 – 1000 (1650 – 1830) Nickel-chromium or MCrAlY 375 15 

900 – 1000 (1650 – 1830) in the 

presence of sulphurous gases 

Nickel-chromium (a) 

Aluminium (a) 

(a) Coating system consists of an aluminium layer deposited on top of a nickel-chromium layer 

Thermal spray coatings are extensively used by industry to protect steel components and structures from 

heat oxidation at surface temperatures to 1095 °C (2000 °F). By ensuring long-term protection, thermal 

spray coatings show real economic advantages during the service lives of such items. Coatings are 

particularly effective in protecting low-alloy and carbon steels. The table lists thermal spray coatings for 

high-temperature applications. 

Aluminum has been widely used to protect such steels in a number of applications involving high surface 

temperatures up to 550°C (1020 °F). Nickel-chromium alloys and some of the MCrAlY alloys have also 

provided protection in severe environments (up to 1000 °C, or 1830 °F). 

375 

100 

15 

4 

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9.4. Thermal Barrier Coatings 

Effect of yttria content on the performance of zirconia-base thermal barrier coatings 

Thermal barrier coatings (TBCs) consist of a low thermal conductivity ceramic layer deposited over a 

MCrAlY bond coat. Such coatings have been the subject of extensive study since the early 1960s, and 

the evolution of TBCs has been steady since then. The ceramic of choice for TBCs is zirconia (ZrO2), but 

pure zirconia exhibits a phase change as the temperature approaches 425 °C (800 °F), resulting in 

substantial volume change, which can subsequently generate internal stresses and lead to premature 

coating failure. Oxide stabilizers (yttria, calcia, magnesia, and ceria, for example) are added to the 

zirconia to prevent this mode of failure. Yttria (Y2O3) is the most widely used stabilizer for TBCs; 

commonly known as yttria-stabilized zirconia (YSZ). Partially stabilized material improves TBS 

performance. The amount of stabilizer also is important. Initially, fully stabilized zirconia containing 12 to 

20% Y2O3 was used, but it has been demonstrated that partially stabilized material improves TBC 

performance. It was reasoned that allowing a controlled amount of the ceramic to undergo a phase 

change created microcracks that relieved high temperature stresses and promoted longer life. 

The state-of-the-art thermal spray TBC today is a 6 to 8 wt% Y2O3-ZrO2 ceramic deposited on a thickness 

of 0.25 to 1 mm (0.010 to 0.040 in.), with 10 to 15% porosity. A bond coat of NiCrAlY or CoNiCrAlY is 

used at a thickness of 0.125 mm (0.005 in.). 

Source: R.A. Miller, Thermal Barrier Coatings for Aircraft Engines-History and Directions, Thermal Barrier 

Coatings Workshop, March 1995 (Cleveland, OH), NASA Center for Aerospace Information, 1995, p 17- 

34, as published in Selected Applications, Handbook of Thermal Spray Technology, J.R. Davis (Ed.), 

ASM International, p 178, 2005. 



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9.5. Functionally Gradient Materials 

Functionally gradient materials (FGMs) are a rapidly growing application area with significant promise for 

the future production of (a) improved materials and devices for applications subject to large thermal 

gradients, (b) lower-cost clad materials for combinations of corrosion and strength or wear resistance, and 

(c) perhaps improved electronic material structures for batteries, fuel cells, and thermoelectric energy 

conversion devices. 

Thermal spray forming of such gradient structures has been proposed because of the unique ability this 

approach has of being able to deposit thin, individual layers of a wide range of materials--metals, 

intermetallics, and ceramics--thereby enabling layered or continuously graded structures to be produced. 

Schematic of a thermally sprayed FGM for burner nozzle applications. 

The most immediate application for FGMs is in thermally protective claddings, where large thermal 

stresses could be minimized and component lifetimes improved by "tailoring" the coefficients of thermal 

expansion, thermal conductivity, and oxidation resistance. These FGMs could, and indeed are, finding 

use in turbine components, rocket nozzles, chemical reactor tubes, incinerator burner nozzles, or other 

critical furnace components. The diagram illustrates an example of a thermally sprayed FGM proposed for 

the protection of copper using a layered FGM ceramic structure. Successful fabrication of this would have 

application as improved burner nozzles, molds, and furnace walls. 

Source: R. Knight and R.W. Smith, ASM Handbook, Vol 7, Powder Metal Technologies and Applications, 




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9.6. Wear and Abrasion Resistance of Thermal Spray Coatings 

Abrasive wear data for selected thermal spray coatings 

Material Type Wear rate, mm 3 /1000 rev 

Carballoy 883 Sintered 1.2 

WC-Co Detonation gun 0.8 

WC-Co Plasma spray 16.0 

WC-Co Super D-Gun 0.7 

WC-Co High-velocity oxyfuel 0.9 

ASTM G65 dry sand/rubber wheel test, 50/70 mesh Ottawa silica, 200 rpm, 30 

lb load, 3000-revolution test duration 

Erosive wear data for selected thermal spray coatings 

Material Type Wear rate, µm/g 

Carballoy 883 Sintered 0.04 

WC-Co Detonation gun 1.3 

WC-Co Plasma spray 4.6 

AISI 1018 steel Wrought 21 

Silica-based erosion test; particle size, 15 µm; particle velocity, 129 m/s; particle 

flow, 5.5 g/min, ASTM Recommended Practice G 75 

One of the most important uses of thermal spray coatings is for wear resistance. They are used to resist 

virtually all forms of wear, including abrasive, erosive, and adhesive, in virtually every type of industry. 

The materials used range from soft metals to hard metal alloys to carbide-based cermets to oxides. 

Generally, the wear resistance of the coatings increases with their density and cohesive strength, so the 

higher-velocity coatings such as HVOF and particularly detonation gun coatings provide the greatest wear 

resistance for a given composition. 

A variety of laboratory tests have been developed to rank thermal spray coatings and compare them with 

other materials. Examples of abrasive and erosive wear data are shown in Tables 10 and 11. It should be 

kept in mind that laboratory tests can seldom duplicate service conditions. Therefore these tests should 

only be used to help select candidate coatings for evaluation in service. Only rarely, with good baseline 

data, can any precise prediction of wear life in service be made from laboratory data. 

Source: R.C. Tucker, Jr., Thermal Spray Coatings, ASM Handbook, Vol 5, Surface Engineering, ASM 

International, 1994, p 508 



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9.7. Metallography of NiCrAl/Bentonite Abradable Coatings 

Typical microstructure for Ni-4Cr-4Al/Bentonite 

Abradable coatings (such as Ni-4Cr-4Al/Bentonite) entail a family of coatings used throughout jet 

engines, primarily as sacrificial coatings into which moving components wear. The coatings generally 

consist of a metallic phase and a nonmetallic phase, and contain relatively high porosity levels (to 40%). 

Typical locations for application of an abradable coating include the fan, and low- and high-pressure 

compressor sections. 

The composite nature of abradable coatings presents unique challenges from a metallography 

standpoint. Due to thickness and porosity considerations, vacuum impregnation with a low-viscosity cold 

mount epoxy is the recommended mounting method. To facilitate impregnation with fast-cure epoxies 

(which are typically more viscous than slow-cure epoxies), the resin can be heated to approximately 

150°F (65°C) prior to mixing with the hardener. Holding the epoxy at elevated temperature for 15 to 20 

minutes should result in a significant improvement in the viscosity of the epoxy. 

Source: Doug Puerta, KHA Metallurgical, Portland, Oregon 



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9.8. Abradable Seals 

Suggested temperature range for turbine compressor abradables. Those shown to 

be compatible with titanium blading also will function against steel and superalloy 

blades. However, the reverse is not true 

Thermally sprayed abradable seal coatings are employed in turbomachinery and can be machined in situ 

by the rotating components, such as blades, resulting in very close tolerances, reduced blade wear and 

overall improved engine efficiency. The abradable materials are deposited to a sufficient hardness to 

withstand gas and particle erosion, yet maintain abradability and functionality for the designed life span. A 

variety of coating materials are used; most notable for compressor cold sections are polymer filled 

aluminum alloys (325°C max. use temperature) or graphite or hexagonal boron nitride soft phases, 

incorporated within an aluminum alloy, for use up to 450°C. These are generally applied using APS or 

combustion spray systems. Hot compressor section abradables include combustion sprayed NiCrAlbentonite 

(650°C max. temperature) or plasma-sprayed composites consisting of MCrAlYs, polyester, and 

hexagonal boron nitride (650-750°C max.). Figure shows temperature ranges for different compressor 

abradables. 



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For maximum turbine service temperatures up to 1200°C, seals can be manufactured from porous yttriazirconia 

based ceramics. The use of alternative zirconia stabilizers such as Dy2O3 can lead to drastic 

improvements in thermal shock resistance as is seen in the case of the Dy2O3 stabilized coatings. 

Depending on the specific application conditions (temperature, incursion speed/blade rotation/material), 

tipped blades may or may not be required. A typical high temperature tip material is cubic boron nitride; in 

some cases, silicon carbide tips are also used. Benefits of DySZ ceramic abradables are the ability to cut 

under certain conditions with untipped blades and excellent thermal shock properties. 

Source: R.K. Schmid, F. Ghasripoor, M. Dorfman, and X. Wei, An Overview of Compressor Abradables, 

ITSC 2000, First International Thermal Spray Conference, May 2000, Montreal, Quebec, Canada, ASM 

International, p 1087-1093; as published in Selected Applications, Handbook of Thermal Spray 

Technology, J.R. Davis (Ed.), ASM International, p 178, 2005. 



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9.9. Processing Electronic Devices: Beryllia 

A BeO meanderline substrate produced in near-net shape by inert chamber plasma spraying. Crosssectional 

dimensions are 1.016 x 1.016 mm (0.040 x 0.040 in.). the device was formed by milling the 

meanderline pattern in high-quality graphite. Inert chamber plasma spraying with a helium atmosphere 

was used to fill the milled pattern, the ceramic flashing being ground away. The net shape was completed 

by firing the structure at >800 °C (>1470 °F), which removed the graphite, leaving the freestanding 

ceramic. 

Spraying dense, pure beryllia (BeO) requires higher substrate temperatures and very hot plasma. 

Occupational Safety and Health Administration (OSHA) requirements for handling this toxic, diseasecausing 

material are stringent and usually economically prohibitive. Excellent thermal and dielectric 

properties have been developed in plasma sprayed products, and structures have been produced for 

microwave and nuclear fusion applications. 

Source: From Selected Applications, as published in Handbook of Thermal Spray Technology, J.R. Davis 

(Ed.), ASM International, 2005, p 199. 



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9.10. Aluminum Coatings and Zinc Coatings 

Corrosion rate of zinc and aluminium in NaCl aqueous solution as a function of pH 

The TSA and TSZ coatings are usually sprayed onto large structures via flame and electric arc-based 

processes, and with the coating material supplied in wire form to the spray gun. There are standards for 

the composition and diameter of these wires: ISO 14919 and AWS C2.25/C2.25M. Among these, four 

classes of materials—zinc, aluminum, Zn-15Al, and Al-5Mg—are specified for the general purpose of 

corrosion protection in ISO 2063. 

Alloys with other compositions, as well as composite coatings, can be used for corrosion protection, but 

applications are limited. One way to fabricate composite coatings is to simultaneously feed wires of 

different materials, such as zinc and aluminum, to the arc spraying gun. The molten droplets of the two 

metals mix on the substrate and form a composite coating called a pseudoalloy. 

Source: S. Kuroda and A. Sturgeon, Thermal Spray Coatings for Corrosion Protection in Atmospheric 

and Aqueous Environments, ASM Handbook, Vol 12B, Corrosion: Materials, ASM International, 2005, p 

423. 



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9.11. Thermal Spray Polymer Coatings 

Selection of polymers applied as coatings via thermal spray processes 

Polymer Maximum temperature resistance 



°C °F 

Ethylene methacrylic acid copolymer (EMAA) 40 – 60 105 – 140 

Polyethylene (PE) 40 – 80 105 – 175 

Polypropylene (PP) 70 160 

Nylon 6, 6 (PA) 65 150 

Polyphenylene sulphide (PPS) 110 230 

Polyethylene-tetrafluoroethylene copolymer (PE-TFE) 160 320 

Polyetheretherketone (PEEK) 125 255 

Liquid-crystal polymers (LCPs) 250 480 

Phenolic epoxy 130 265 

Polyimide (PI) 300 570 

Polymer spraying is a one-coat process that serves as both the primer and the sealer, with no additional 

cure times. Polymer powders are specified by their chemistry, morphology, molecular weight distribution 

or melt-flow index, and particle size distribution. Spray parameters must be selected to accommodate 

each particular polymer formulation. The heat source of the thermal spray torch can be a plasma, 

combustion flame, or combustion exhaust, as in the HVOF process. The technique is chosen on the basis 

of the polymer-melting characteristics. A simple combustion torch is well-suited for use with low meltingpoint 

polymers having a large processing window, such as polyethylene. High melting-point polymers 

may require plasma deposition for maximum quality to melt the polymer without causing oxidation. 

Therefore, processing parameters must be selected for each polymer chemistry and deposition 

technique. 

Source: Selected Applications, Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM 


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Thermal Spray Tips - Swinburne University of Technology

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