Preparation And Oxidation Properties Of The Si-modified Aluminized Coating On Ni-based Superalloy K438

As is well-known that the hot end components of aero-engines always working in the high temperature environment. It cannot improve the oxidation resistance if we improved the technology of cooling and the preparation technology of alloy only. Now many countries exploited the protective coating to improve the oxidation resistance. Al-Si coating has the high research significance because it has the simple preparation technology and the high oxidation resistance.The thesis is focusing on the preparation and the high temperature oxidation behaviors of the substrate, aluminized coating and Si-modified aluminized coating. The Si-modified aluminized (Al-Si) coating has been prepared by two steps that the Si has been acquired by slurry method and the Al by the pack cementation aluminizing. The microstructure evolution and degradation mechanism of the coatings were also discussed after the isothermal oxidation behavior at 1100℃ by the tools of scanning electron microscopy (SEM) with energy dispersive X-ray spectrum (EDS) and X-ray diffraction (XRD).The SEM results demonstrate that the aluminized coating and Al-Si coating are dense and adhere tightly to the substrate. The microstructures of the aluminized coating and Al-Si coating are characterized in two distinguished areas:the outer layer and the inter diffusion zone. The(3-NiAl phase and the Ni2Al3 phase can be find in the outer layer of the aluminized coating. While the Al-Si coating consists of β-NiAl phase, Ni2Al3 phase and Cr3Si phase in the outer layer.The high temperature isothermal oxidation properties at 1100℃ and the microstructure of oxide scale of the substrate, aluminized coating and Al-Si coating have been comparatively investigated and compared. In the preliminary stage of oxidation, there is a large amount of mixed oxides include Cr2O3 and TiO2 on the surface of the substrate. And then the Cr2O3 disappeared and the NiAl2O4 and NiCr2O4 spinel have been found. There are many holes in the surface and they have been oxidized seriously. The oxide scale on the surface of the aluminized coating is simplex α-Al2O3 at the initial stage, while prolonging the oxidation time it changes into less protective mixed structure composed of NiAl2O4 spinel anda-Al2O3 with massive internal oxidation and the reductive coating thickness.The surface of the Al-Si coating is covered with simplexa-Al2O3 at the initial stage, too. And with the oxidation time carried on, there is a Cr(W) zones growing in the inter diffusion zone which can delay the process of Al depletion by hindering the diffusion of Al from the coating to the surface. It give rise to narrowing of the outer layer and broadening of the inter diffusion zone. A mixed structure composed of less NiAl2O4 spinel and α-Al2O3 appeared at the later stage of the oxidation and the Al-Si coating degenerates primarily by consuming Al. The coating still possesses 9 wt.% Al which could benefit the oxidation resistance. The most of Si in the Al-Si coating can be found in the inter diffusion zone and it can dissolve many elements like Cr, Ti, Mo which are diffused from the substrate. It hinders the speed of inter oxidation effectively and benefits the oxidation resistance.