Effect of substrate temperature on the microstructure and interface bonding formation of plasma sprayed Ni20Cr splat

Ni20Cr splats were sprayed onto polished substrates at different preheating temperatures in an Ar atmosphere by low pressure plasma spray to reveal the dominating factor driving the bonding formation of the splats. The splat morphology, microstructure and splat-substrate interface were characterized by SEM, EBSD and TEM. The interface for examination of typical splats was prepared by focus ion beam (FIB). Disk-like splats were obtained on AISI 304 stainless steel substrate preheated to temperatures of 100 °C (cooling down from 350 °C) and 350 °C, whereas few short projections in the periphery were observed on the splat deposited at 550 °C. For all splats, three typical distinct zones with different microstructure features were observed on the splat surface by SEM and EBSD, including central coarse grain zone, transitional medium grain zone and peripheral fine grain zone. The interface bonding formation is closely related to the microstructure distribution. TEM observations demonstrated that the bonded interface only formed in the central coarse grain zone which provides a facile approach to access the bonding of plasma sprayed splat. The present work also reveals that increasing the substrate temperature cannot significantly improve the interface bonding due to the existence of oxide layer. The mechanism dominating the formation of those microstructures was also discussed.