INFLUENCE OF SUBSTRATE SURFACE OXIDATION STAGE ON THE MORPHOLOGY AND FLATTENING OF SPLATS

When spraying on a smooth surface (Ra < 0,05 µm), two types of splats can be observed: disk shaped ones and extensively fingered ones - the transition between both depending on a substrate critical temperature (Tc). Even when Ts > Tc the splat shape depends strongly on the substrate surface oxidation stage; the aim of this paper being to study this effect. For that two substrates have been chosen: low carbon 1040 steel and low alloy steel with 1.6 wt % Cr. Alumina and 316L austenitic stainless steel particles have been sprayed with a d.c. plasma torch (PTF4 type) with a 7 mm internal diameter anode nozzle. For these substrates and particles, the transition temperature was found to be Tc ~300°C. Extensively fingered splats were obtained after preheating substrates to temperatures higher than 380°C. These substrates have been, thus, plasma preheated at 400°C and 450°C in air or nitrogen atmosphere.
Splats with bubbles were created on oxidized 1040 substrates with a morphology that suggests an expulsion of entrapped gases from the underneath porous oxide layer. The entrapped gases could also explain the splashing of splats on substrates preheated to temperatures higher than Tc. A higher degree of splashing was observed in 316L steel splats compared to A1₂O₃ ones keeping the same substrate conditions. Average splat diameter (D) decreases with an increase in formed oxide layer thickness. At Ts = Tc, flattening degree for steel splats were of order 1.2 higher than those of Al₂O₃. Splashing can be suppressed by preheating the substrates with an oxygen free gas flowing around the substrates that results in limiting the oxidation and substrates can be maintained at higher temperatures.