Splat formation and cooling of plasma-sprayed zirconia

The aim of this paper is to show, through investigations of splat formation, the influence of particle parameters at impact and of the substrate temperature on thermomechanical properties of plasma-sprayed zirconia coatings. During plasma spraying, the flattening of particles on the substrate was studied using either a linescan test or a system integrating two fast two-colour pyrometers. The size and shape factor distribution of the collected lamellae on polished stainless steel substrates as well as their cooling rate can thus be determined. A one-dimensional splat cooling model is used to predict the splat-substrate thermal contact resistance for each spraying condition. Fine (22-44 mu m) and coarse (45-90 mu m) cuts of fused and crushed yttria-stabilized zirconia (YSZ) were sprayed with an r.f. plasma torch and a d.c. plasma torch to obtain a range of impact velocities (50-200 ms(-1)). The polished (R-a similar to 0.05 mu m for steel and R-a similar to 0.2 mu m for plasma-sprayed YSZ) or grit blasted or as-sprayed (R-a > 0.5 mu m) substrates were kept either below 100 degrees C or above 300 degrees C. The main results obtained are the following. Almost perfect lenticular shaped splats with thermal contact resistance R-th < 10(-7) m(2) KW-1 are obtained for smooth, non-oxidized hot substrates (T > 150 degrees C). Cooling rates of 22-45 mu m particles propelled at 200 ms(-1) in a d.c. torch are approximately 10(9) and 109 Ks(-1) on 304 stainless steel and YSZ substrates respectively. The splat behaviour is similar on rough hot non-oxidized substrates. Thicker splats with lower degrees of flattening have lower cooling rates, owing to the surface asperities. The thermal contact with the substrate is also excellent. As soon as the substrate is cold (T < 150 degrees C) or hot but oxidized (oxide layer thickness greater than 40 nm), the splats are very distorted with fingers of splashed material at their periphery, and their contact with the substrate is rather poor (R-th > 10(-6) m(2) KW-1). Adhesion-cohesion measurements are in good agreement with results obtained on the splats. The coating adhesion increases with preheated substrates (300-500 degrees C). When the particles are fully molten, adhesion is highest on hot substrates and for high particle impact velocities. The larger particles (45-90 mu m) are not completely melted and are less adherent to the substrate, However, for both particle size distributions, when preheating the substrate, especially up to 500 degrees C, the preheating time has to be limited to 90-120 s in order to limit the thickness of the resulting oxide layer. When the preheating time increases over 120 s, the adhesion of the coating decreases. (C) 1997 Elsevier Science S.A.