Research On Process And Performance Of Silicon-modified Aluminide Composite Coatings By CVD Methods

Simple aluminide coatings, which can inhance the high temperature corrosion resistance of the aeroengine turbine blades, are usually deposited on internal cooling channels in turbine blades and vanes by chemical vapor deposition(CVD) methods. However, the simple aluminide coatings have poor hot corrosion resistance, and the interdiffusion of elements between coating and substrate will occur under long-term service at elevated temperatures, resulting in reduction of aluminum in coatings. Thus, a continuous aluminum oxide film may not be formed on the coating surface. The high temperature corrosion resistance of alumide coatings can be improved by adding modifying elements, e.g. Si, thereby the life of the turbine blades in aeroengine is increased.In this topic, the simple aluminide coatings prepared in the pre-experiments were optimized with combined treatments of CVD depositing and diffusing. After the treatment, the diffusion of the elements in the coatings were relatively adequate, the microstructure tended to being homogeneous, coating thickness was increased,and the ratio of the thickness of the diffusion zone to the coating thickness also varied. According to the change of coating composition and microstructure, this topic demonstrated the formation of aluminide coatings on nickel-base superalloys under the condition of this CVD aluminizing experiment.This topic systematically studied the preparation technology of silicon-modified aluminide coatings by CVD methods. The results showed that the single-step co-deposition of Al-Si had poor controllability, the coating composition exibited an alternate distribution; two-step deposition(first CVD aluminizing and then CVD siliconizing) might cause the replacement of aluminide by silicide; two-step deposition(first CVD siliconizing and then CVD aluminizing) could easily prepare silicon-modified aluminide composite coatings with a good metallurgical bonding zone between a Al-rich outer layer and a Si-rich inner layer. Then, silicon-modified aluminide coatings prepared by the selected third process suffered from combined treatments of CVD depositing and diffusing, it had the similar effect on composition and microstructure with the simple aluminide coatings undergoing the same treatments, but a Al-rich intermediate layer was present between the outer layer and inner layer.High temperature oxidation test was also carried out at a temperature of 1100 ℃with a oxidation of 100 hours in this topic. The results showed that silicon-modified aluminide coatings exhibited a better high temperature oxidation resistance than simple aluminide coatings. During high temperature oxidation process, the drain of Al in the simple aluminide coatings occurred, in contrast, the inward aluminium diffusion was hindered by "obstacle" rich in Si. Both the two types of aluminide coatings experienced successively initial oxidation stage with a rapid mass gain and then a stable oxidation stage with a sluggish mass gain.