Microstructure formation in plasma-sprayed functionally graded NiCoCrAlY/yttria-stabilized zirconia coatings

Functionally graded NiCoCrAlY/yttria-stabilized zirconia coatings were fabricated by plasma spraying using mechanically alloyed, plasma-spheroidized NiCoCrAlY/yttria-stabilized zirconia (YSZ) composite powders. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis revealed that oxidation of aluminum, chromium and yttrium in the NiCoCrAlY alloy occurred in the high-temperature plasma-spray stream during powder preparation and coating deposition. The oxidized products, predominantly aluminum oxide, subsequently mixed with zirconia at elevated temperatures in a wide composition range. These mixtures are observed as fine lamellae structures with different gray levels in the back-scattered electron (BSE) mode. Among the lamellae structures, the regions containing high aluminum oxide concentration appeared darker, compared with the regions with high zirconia concentration. It was observed that aluminum oxide formed a preferential combination with zirconia in the coating. Dendritic structures were frequently observed in the coating interlayers and were considered to have formed during the solidification process of the molten splats. The observations on polished cross-sections also showed that the resultant coatings were dense and had no clear interface between adjacent layers. It is believed that the graded distribution of the coating compositions helped to decrease the high level of residual thermal stresses experienced during thermal cycling, and hence contribute positively to the bond strength of the coating. These functionally graded coatings have demonstrated superior tensile adhesive bond strength to duplex coatings.