Thermal-sprayed MCrAlY coatings are widely used for land-based gas turbine applications. The cold spray may increase the coating density owing to the high-velocity particle impacts during spraying. Many researchers have considered critical velocity to be the most important factor of the deposition mechanism of cold-sprayed coatings. However, this dominant parameter of critical deposition condition has not been completely understood. In order to understand the mechanism, two approaches were used in this study. One is the transmission electron microscope (TEM) observation of the interface between the coating and the substrate, and the other is the cross-sectional observation of the deposited particle by using the focused ion beam (FIB) cutting technique. From the TEM observations, there are no evidences of melting at the interface, and it is found that the actual bonding occurred at the nascent surfaces. Generally, there is a native oxide on the surface of the particles and substrate. After the plastic deformation of the particles and substrate, the native oxide breaks down; subsequently, a nascent surface can be created and direct contact initiates deposition. From the results of these investigations, it is thought that the dominant factor for deposition is the plastic deformation of the particles and substrates.

The substrate surface roughening and cleaning are very important process for pre-treatment of a substrate for thermal spraying. There are some methods for the pre-treatment. Especially, the grit blasting process is the most general process. However, some grits embed into the blasted substrate, and the embedded grits remain on it. The girt embedment is affected by various conditions of the blasting process. It is important to make known the effect of the blasting time on the residual grit amount. A carbon steel substrate was blasted by white alumina grit. The blasting time was changed. The surface roughness of the blasted substrate increased monotonically with increasing blasting time. During very short time at the starting of blasting, the residual grit amount increased with increasing blasting time. Then the amount of the residual grit decreased with increasing blasting time. The penetration depth of the residual grit on the blasted substrate denoted the same tendency of the residual grit amount. There may be a critical time that the residual girt amount becomes the local minimum value.