Understanding the formation of plasma-sprayed Ni20Cr splats through interface observation

In the formation of plasma-sprayed metallic splats, substrate preheating plays a critical role in determining the splat morphology as well as the splat-substrate and the splat-splat interfacial interactions. In the present work, the splat formation mechanism of regular-shaped Ni20Cr splats on stainless steel substrates preheated at 423 K,573 K and 673 K was studied. Focused ion beam (FIB) microscopy and transmission electron microscope (TEM) were used to characterize the splat solidification microstructure, pore formation, splat delamination, and the elemental diffusion along the splat-substrate interface. The substrate surface roughness was consistent up to 573 K preheating temperature, while the great degree of surface oxide formation increased the roughness at 673 K. For disk splats, good interfacial contact with a long distance of elemental diffusion was observed near the center. With increased substrate preheating temperatures, the degree of curling-up of the splat edge decreased. At high substrate preheating temperatures (673 K) where splats slowly solidified, Cr segregated to the top surface forming a thick layer of NiCr2O4 spinel During FIB milling, a dense layer consisting of Fe and Cr was formed at the delaminated splat edge due to substrate material redeposition.