| With the development of the aerospace industry and the new energy industry,the requirement of protecting the environment against pollution,the service temperature of the gas turbine becomes higher and higher.The water vapor content increases in gas turbine due to the using of fuel with hydrogen containing,when the gas turbine works.In recent years,the preparation of protective coatings has become the effective way to promote the oxidation resistance behavior and the hot corrosion resistance behavior of the superalloy,and it has played an important role in the field of defense technology.The Si-modified diffusion aluminide coating is prepared on the surface of the nickel-based superalloy by hot dipping in this research.Then,the cyclic oxidation test is taken in dry air at 1050?.the chemical composition and micro-structure of the simples are analyzed through the SEM which equipped with EDX.And the phase of it is detected by X-ray diffractomete.The results are as follows:1.Compared with the nickel-based superalloy samples without coating,both the hot dipping pure Al and the nickel-based superalloy with Al-lOSi coating has better effect on the spallation of the oxide scale.Significantly,the hot dipping Al-lOSi coating shows better oxi'dation resistance behavior than pure Al coating at high temperature.2.In an argon atmosphere at 850? for 24 hours,the diffusion of the elements such as Al,Ni,Hf,W and Ta et al.leads to the transforming form the unstable aluminide coating to a stable aluminide coating.And then a continuous alloy layer,which containing Si and Al as the main element and gathering element Ta and Hf,is formed on the surface of the Al-lOSi coating.3.In the hot dipping Al-10Si coating,the SiCr3 phase forms at the grain boundary and gathers the highly segregated elements(W and Ta),which increases the diffusion of aluminum atoms at the grain boundaries.4.The effect of Si on the promotion of high temperature oxidation resistance behavior is to promote the formation of a continuous and dense a-Al2O3 layer.This layer is a barrier to prevent the diffusion of element O into the coating and the strong segregation elements Hf,W and Ta diffusing to the outermost layer ?-Al2O3 layer.In order to research the effect of water vapor on the oxidation behavior of the Si-modified diffusion aluminide coating.this coating is prepared on the surface of the nickel-based superalloy with element Ti by hot dipping,and then cyclic oxidation test is taken in different environments(high temperature dry air environment and high temperature wet air environment).the results and discussions are shown as following:1.The obvious spallation of the Al-Si diffusion coating with Ti occurs in the high temperature wet air environment,so the Al-Si coating in the high temperature wet air environment shows a lower kinetics curve than that in dry air environment.2.The oxide scale surface has a rough and porous structure in wet air environment,Whereas,the oxides scale growth in dry air is very compact and smooth with little spallation,when these Al-Si coatings are operated with cyclic oxidation test.3.The cross-section structure shows that the rough and porous oxide scale is formed and the Si-rich layer under the oxide scale is also destroyed,which lead to the serious inner oxidation.Then the oxide scale in the outmost coating become thicker and thicker.Finally,the spallation occurs.4.the element Al on the surface of the coating is consumed rapidly,which increase the diffusion speed of element Al inside the coating.the area and the number of bulk Si-rich zones inside the coating become little.This Si-rich zones can gather element Ti and Nb to prevent it diffusing to the surface of the coating.The inner oxidation will be serious,when the zones decompose in high temperature wet air environment.When the out-diffusion speed of Al is higher than filling speed of Si,some holes will form around the bulk Si-rich zone. |
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