Measuring Substrate Temperature Variation During Application of Plasma-Sprayed Zirconia Coatings
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JTTEE5 16:580–587 DOI: 10.1007/s11666-007-9050-7 1059-9630/$19.00 ASM International
Measuring Substrate Temperature Variation During Application of Plasma-Sprayed Zirconia Coatings H.R. Salimijazi, L. Pershin, T.W. Coyle, J. Mostaghimi, S. Chandra, Y.C. Lau, L. Rosenzweig, and E. Moran (Submitted February 14, 2007; in revised form April 19, 2007) Substrate temperature variation was measured during plasma spraying of ZrO2 7% Y2O3 powder using fast-response thermocouples embedded in the stainless steel surface. Coatings were deposited with both stationary and moving torches. The substrate was either kept at room temperature at the start of coating deposition or pre-heated to 270-300 °C. Peak temperature during spraying reached 450 °C for a surface initially at room temperature, and 680 °C for a surface preheated to 300 °C before coating deposition. Preheating the substrate reduced coating porosity by approximately 40%. The porosity at the center of the deposit was significantly lower than that at its periphery since particle temperature and velocity were lower at the edges of the plasma plume than along its axis. When a coating was applied with a moving torch the substrate temperature did not increase above 450 °C, at which temperature heat losses to the ambient equalled the heat supplied by the plasma plume and particles. Coating porosity decreased with distance from the substrate. As sequential layers of coating are applied surface temperature increases and roughness decreases. Both of these factors suppress break-up of particles landing on the substrate and thereby reduce coating porosity.
Keywords
porosity of coatings, preheating of substrate, roughness effects, substrate temperature during TS process
1. Introduction The spread and solidification histories of sprayed particles have a strong effect on the microstructure and physical properties of thermal-sprayed coatings (Ref 1, 2). Experimental studies have shown that in-flight particle characteristics such as temperature and velocity and substrate surface temperature influence the dynamics of particles impact on the substrate in a thermal spray (Ref 36). The morphology of molten particles landing and freezing on a flat surface changes from fragmented splats to disk-shape splats with increase of substrate temperature (Ref 5-11). Experiments have shown that when YSZ particles are sprayed on substrates at temperatures above 300 C, the majority of splats formed are disk-shaped (Ref 7, 9, 11). Figure 1 shows splats of zirconia plasma sprayed on stainless substrates, which were kept at either 300 or 600 C. At the lower temperature the splats fragmented and are irregular in shape, while at the higher temperature they are mostly disk-shaped. H.R. Salimijazi, L. Pershin, T.W. Coyle, J. Mostaghimi, and S. Chandra, Centre for Advanced Coating Technologies, University of Toronto, Toronto, ON, Canada; Y.C. Lau, L. Rosenzweig, and E. Moran, GE Global Research, Niskayuna, NY, USA. Contact e-mail: [email protected].
580—Volume 16(4) December 2007
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