SPLAT FORMATION AND COOLING OF PLASMA-SPRAYED ZIRCONIA

The aim of this paper is to show, through investigations of splat form ation, the influence of particle parameters at impact and of the subst rate temperature on thermomechanical properties of plasma-sprayed zirc onia coatings. During plasma spraying, the flattening of particles on the substrate was studied using either a linescan test or a system int egrating two fast two-colour pyrometers. The size and shape factor dis tribution of the collected lamellae on polished stainless steel substr ates as well as their cooling rate can thus be determined. A one-dimen sional splat cooling model is used to predict the splat-substrate ther mal contact resistance for each spraying condition. Fine (22-44 mu m) and coarse (45-90 mu m) cuts of fused and crushed yttria-stabilized zi rconia (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 poli shed (R-a similar to 0.05 mu m for steel and R-a similar to 0.2 mu m f or 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 lenticul ar shaped splats with thermal contact resistance R-th < 10(-7) m(2) KW -1 are obtained for smooth, non-oxidized hot substrates (T > 150 degre es C). Cooling rates of 22-45 mu m particles propelled at 200 ms(-1) i n 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 degre es of flattening have lower cooling rates, owing to the surface asperi ties. The thermal contact with the substrate is also excellent. As soo n as the substrate is cold (T < 150 degrees C) or hot but oxidized (ox ide layer thickness greater than 40 nm), the splats are very distorted with fingers of splashed material at their periphery, and their conta ct with the substrate is rather poor (R-th > 10(-6) m(2) KW-1). Adhesi on-cohesion measurements are in good agreement with results obtained o n 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. Th e larger particles (45-90 mu m) are not completely melted and are less adherent to the substrate, However, for both particle size distributi ons, when preheating the substrate, especially up to 500 degrees C, th e 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 incre ases over 120 s, the adhesion of the coating decreases. (C) 1997 Elsev ier Science S.A.