Thermal Spray Tips - Swinburne University of Technology

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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 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. Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 26 Back to TOC Back to Overview
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 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 Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 27 Back to TOC Back to Overview
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