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 behaviour at impact. The first one allows direct observation under direct current (dc) plasma spray conditions, while the latter one, based on the millimetre sized free falling drop, enables the visualization of flattening phenomena, but at larger scale. These two techniques bring complementary approaches and results. The latter show that flattening time and cooling rate of the lamellae (metallic and ceramic) are improved with the stainless steel substrate surface modification at the nanoscale when corresponding to a positive skewness parameter obtained by preheating it over the transition temperature. Experiments of wettability show that the presence of nanopeaks increases the contact angle of the liquid on the substrates and reduces thermal contact resistance at interface. It has also been shown that, when adsorbates and condensates are not eliminated from the surface, even with a positive skewness, the thermal contact resistance is increased.
