The thesis is focusing on the high temperature oxidation behaviors and corrosion behaviors of the AlSiY diffusion coating, the composite and gradient MCrAlY coatings prepared by arc ion plating. The microstructure evolution and degradation mechanism of the coatings were also discussed.
An AlSiY diffusion coating was developed by a combined approach of arc ion plating and subsequent diffusion treatment. Cyclic and isothermal oxidation behavior and hot corrosion behavior of the coating specimens were tested. The AlSiY diffusion coating consists of an outer layer and an interdiffusion zone. The outer layer is composing of β-NiAl matrix with dispersed precipitates of α-W and CoWSi. The β-NiAl matrix in the outer layer sustains the formation and repair of alumina scale. As the oxidation continued, the γ/γ′phase was formed due to the phase transformation of β to γ/γ′, which was caused by the consumption of Al. Finally it replaced β as the matrix. During the oxidation, the precipitates in the outer layer congregated and grew up without causing any deterioration to the coating. The slight internal oxidation and sulfidation occurred in the AlSiY diffusion coating, but the coating still has the enough Al reservoirs to offer protection to the substrate during the corrosion process.
A NiCoCrAlYSiB+AlSiY composite coating was prepared by arc ion plating. After annealing treatment, the composite coating consisted of β-(Ni,Co)Al with dispersed σ-NiCoCr and Cr3Si phases in outer layer, and Cr-rich phase plus minor β-(Ni,Co)Al phase in inner layer. During the oxidation, the spinels and NiO were formed on the normal NiCoCrAlYSiB coating due to the depletion of aluminum. While the composite coating showed better oxidation resistance due to its prosession of more Al reservoirs and the addition of Si. In Na2SO4+K2SO4, NiO and (Ni,Co)Al2O4 were formed on the surface of NiCoCrAlYSiB coating due to the depletion of aluminum and the dissolution of alumina scale. The composite coating contained sufficient β-(Ni,Co)Al reservoirs to supply formation of continuous α-Al2O3 scale. The presence of NaCl aggravated the corrosion extent of the two coatings. Compared with the NiCoCrAlYSiB coating, the composite coating postponed the formation of internal oxidation and sulfidation, which could be attributed to the gradient distribution of Al-enriched outer layer and Cr-enriched inner layer.
A gradient NiCoCrAlYSi coating was prepared by a combination of arc ion plating and subsequent diffusion treatment. The outer layer of the gradient coating consisted of β-NiAl phase and α-Cr phase, while the inner layer was composed of β-NiAl, γ/γ′and α-Cr phase. During the oxidation, it is evident for the vanishment of β phase and enlargement of interdiffusion zone for the NiCoCrAlYSi coating, while adequate β phase is still preserved despite increased γ/γ′phase in the gradient coating. An in-situ diffusion barrier of Cr(W)-rich σ phase was formed during the oxidation, and it slowed down the degradation by hindering the diffusion of Al to substrate in the gradient coating. The gradient coating provided better protection against corrosion attack than the normal NiCoCrAlYSi coating. The favorable corrosion resistance should be attributed to the gradient distribution and enrichment of Al.
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