Thick titanium coatings were prepared by warm spraying (WS) and cold spraying (CS) process to investigate the oxidation and microstructure of the coating layers. Prior to the coating formations, the temperature and velocity of in-flight titanium powder particle were numerically calculated. Significant oxidation occurred in WS process using higher gas temperature conditions with low nitrogen flow rate, which is mixed to the flame jet of an HVOF spray gun in order to control the temperature of the propellant gas. Oxidation, however, decreased strikingly as the nitrogen flow rate increased. In CS process using nitrogen or helium as a propellant gas, little oxidation was observed. Although most of the cross-sections of the coating layers prepared by conventional mechanical polishing looked dense, coating cross sections prepared by an ion-milling method revealed the actual microstructures containing small pores and unbounded interfaces between deposited particles. Even when scanning electron microscopy or x-ray diffraction method did not detect oxides in the coating layers by WS using high nitrogen flow rate or CS using helium, the inert gas fusion method revealed minor increase of oxygen content below 0.3 wt%.

In this study, we investigated microstructures of thermal sprayed coatings and single deposited splats using two types of ion beam milling: one is argon ion beam for the cross-sectioning of thermal sprayed coatings in a cross section polisher, the other is gallium focused ion beam for the cross-sectioning and TEM sample preparation of single deposited splats. The cross section of WC-Co coatings fabricated by the polisher showed that it created a mirrored surface with minimizing artifacts such as pull-outs of ceramic particles or smearing of pores during conventional metallographic preparations. A thin and locally re-thinned membrane of single warm-sprayed nickel splat was feasible to observe the internal interface of particle/substrate in high resolution electron images. The substrate was heavily deformed by the impact of nickel particle with high kinetic and thermal energies. The particle and the substrate were intimately bonded without any voids or gaps.