M. F. Bahbou, I. Choquet, P. Nylen, University West, Trollhättan, Sweden, Trollhättan, Sweden
Phenomena involved at impact of thermal sprayed particles on a substrate have a strong influence on the coating build-up and microstructure. It has been shown that the way sprayed particles flatten and solidify at impact is a key factor controlling the coating mechanical properties. It is also well established that preheating a substrate to a certain temperature leads to a shape transition of the splats from an exploded “star-like” to a disk shape resulting in increased coating adhesion strength. However, the physical phenomena behind this transition have not been demonstrated yet. It is believed that preheating desorbs the surface from contaminants thus improving the wetting and the contact at the interface and leading to a faster cooling of the splat that prevents it from splashing. In this study, the impact of nickel particles on a titanium substrate is investigated both numerically and experimentally. Concerning numerical simulations, two surface conditions are considered, clean and oxidised. The effect of surface oxidation is introduced in the model through parameters such as the surface topography, contact angle and thermal contact resistance. To characterise the important phenomena at impact, the average cooling rate for solidification and flattening speed for wetting are used. In parallel, experimental measurements of particle impacts are carried out for comparison with the simulation results. Sieved Ni5%Al powder in a narrow diameter range (-65 + 75 µm) was sprayed on two sets of polished titanium alloy surfaces. One set is a clean surface and the other one is a previously oxidised surface at 600oC for two hours. Resulting splats after impact were characterised by scanning electron microscopy. The effects of the substrate’s oxide layer on the splat final shape, also on the cooling rate and flattening speed during impact are discussed.
Summary: Phenomena involved at impact of thermal sprayed particles on a substrate have a strong influence on the coating build-up and microstructure. It has been shown that the way sprayed particles flatten and solidify at impact is a key factor controlling the coating mechanical properties. It is also well established that preheating a substrate to a certain temperature leads to a shape transition of the splats from an exploded “star-like” to a disk shape resulting in increased coating adhesion strength. However, the physical phenomena behind this transition have not been demonstrated yet. It is believed that preheating desorbs the surface from contaminants thus improving the wetting and the contact at the interface and leading to a faster cooling of the splat that prevents it from splashing. Experimental investigations showed that the splat cooling rate on oxiddised surfaces is higher than on clean ones.The aim of this study is to build a model that could predict this behaviour, the impact of nickel particles on a titanium substrate is investigated. The effect of surface oxidation is introduced in the model through parameters such as the surface topography, contact angle and thermal contact resistance. To characterise the important phenomena at impact, the average cooling rate for solidification and flattening speed for wetting are used.
