Study On The Wear Resistance Of Plasma Sprayed Al₂O₃, ZrO₂ And Al₂O₃-ZrO₂ Composite Coatings

NiCoCrAlY, NiCoCrAlY/ZrO₂, NiCoCrAlY/Al₂O₃ and NiCoCrAlY/ (ZrO₂-Al₂O₃) coatings were prepared by air plasma spraying (ASP) process on the surface of 20g steel. The microstructure, the surface morphology and wear resistance of the coatings were studied by X-ray diffraction (XRD), electron probe microanalysis (EPMA), scan electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanisms of wear resistance of these coatings were discussed.The results show that plasma sprayed NiCoCrAlY coatings consist of Ni-based solid solution phase, Ni₃Al, NiAl and some amorphous phases. Ni₃Al would benefit the high temperature properties of the coatings. The amorphous phases are located on the grain boundaries, which can improve the strength of the coatings. Microcrystalline structure and column crystal structure exist in the coatings, which could improve the mechanical properties and the resistance of thermal shocking. The coatings show typical lamellar structure. There are a small amount of microcracks and pores exist in the coatings. The element distributions in the coatings are uniform, except Al, which could be oxidated in the sprayed process and then formed the interlayer oxides. The bond between NiCoCrAlY coating and substrate is well. The micro-hardness of the coating is not high, which is only about 378HV_0.1. Though the hardness of substrate has been much increased by the coatings, but the wear resistance is still poor. The wear mechanism of coatings is delamination wear incorporated with slight and plough wear.Plasma sprayed ZrO₂ coatings consist of t'-ZrO₂,t-ZrO₂ and c-ZrO₂ phases. t'-ZrO₂ has good phase stability at high temperature and could offer good high temperature resistance. At the surface of the coating there were many unmelted or semi-fused particles, the microcracks and loose structure are much more than these exit in the NiCoCrAlY coatings. Interlaminar cracks were found in the coatings, and the bond between ZrO₂ coatings and NiCoCrAlY coatings is not very good. The average hardness of ZrO₂ is 772HV_0.1, which is much higher than that of NiCoCrAlY coatings. The wear resistance is not good enough, because of the existence of loose structure and interlayer crack in the coatings. The wear mechanism is microcracking wear incorporated with slight plough wear.Plasma sprayed Al₂O₃ coatings consist ofβ-Al₂O₃,γ-Al₂O₃ andβ-Al₂O₃ phases. Among them,α-Al₂O₃ is stable phase;γ-Al₂O₃ andβ-Al₂O₃ are metastable phase. The formability of Al₂O₃ particles is good, so the coatings show typical lamellar structure. Some unmelted or semi-fused particles and loose structure were still found in the coatings. The coatings and the interfaces were well integrated. The hardness of Al₂O₃ coatings was the highest, arrived 864HV_0.1, which increase the wear resistance greatly. The predominant wear mechanism of the coatings is microcracking wear.The phase composition of plasma sprayed ZrO₂-Al₂O₃ coatings is complicated, which consist ofα-Al₂O₃,γ-Al₂O₃, t'-ZrO₂, t-ZrO₂, C-ZrO₂ and a little m-ZrO₂ phases. A little m-ZrO₂ phase could be beneficial to the coatings. Microcrystalline structure can be found in the coatings, which would improve the mechanical properties of the coatings. The composite coatings could combine the nice formability and high hardness of Al₂O₃ with the perfect toughness of ZrO₂ to advance the performance of the coatings. Although the surface of the coatings is still unsmooth, but the loose and holes are much less than the ZrO₂ coatings. In coatings the adherence between the ZrO₂ and Al₂O₃ lamellar are good, and the bonding between the coatings and the interfaces are well too. The average hardness of ZrO₂-Al₂O₃ coatings is 783HV_0.1. The wear mechanism of ZrO₂-Al₂O₃ coatings is microcracking wear.