Influence of stainless steel substrate preheating on surface topography and on millimeter- and micrometer-sized splat formation

Abstract

Many properties (thermal, electrical, mechanical…) of thermal sprayed coatings are strongly linked to the real contacts between the “piled-up” splats. The quality of this contact depends on droplet parameters at impact (size, temperature, velocity,…) and substrate parameters (temperature, topography…). Two different techniques have been developed in order to study the plasma sprayed particle behavior at impact. The first one allows direct studying under direct current (dc) plasma spray conditions, while the latter one, based on the millimeter-sized free-falling drop, enables the visualization of flattening phenomena, but at larger (about three orders of magnitude) time and size scales. These two techniques applied to zirconia and nickel droplets or drops bring complementary approaches and results. With millimeter-sized nickel drops impacting on stainless steel substrates, the flattening time and cooling rate of the lamellae are improved when the substrate surface is modified at the nanoscale, corresponding to a positive skewness S k parameter, by preheating it over the transition temperature. Resulting splats are disk shaped. Static wettability experiments show that the presence of nanopeaks increases the contact angle of the liquid on the substrates thus reducing the thermal contact resistance at the interface. With sprayed ZrO 2 particles the same phenomena (better wettability, and increased cooling rate) are observed on stainless steel substrate with S k > 0. It has also been shown that, when adsorbates and condensates are not eliminated from the surface, by preheating for zirconia droplets either on zirconia or stainless steel substrate (even with a positive skewness for the latter), the thermal contact resistance is increased and a fingered splat morphology is obtained.