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 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 Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 34 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 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. Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 35 Back to TOC Back to Overview
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Thermal Spray Tips - Swinburne University of Technology

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