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 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. Source: Coating Operations, Frank N. Longo, Longo Associates, as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 121, 2005. Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 48 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 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 49 Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ Back to TOC Back to Overview
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Thermal Spray Tips - Swinburne University of Technology

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