Degradation mechanism of high temperature protective coatings

       After the protective coating is applied to the surface of the superalloy, especially the coating system of Al2O3 is formed, the oxidation resistance of the alloy can be greatly improved.



However, when the coating is exposed to an oxidizing environment for a long time, the performance of the coating will still degrade, so that the anti-oxidation effect will eventually be lost. Coating degradation is the result of the continuous depletion of oxidative-resistant elements in the coating. There are two ways for the loss of elements in the coating: one is the outward diffusion oxidation to form a surface oxide film; the other is the interdiffusion between the coating and the base alloy. When the concentration of oxide-forming elements in the coating decreases so much that the oxide cannot grow, the coating loses its protective effect.



Take aluminide coating as an example to illustrate the degradation mechanism of the coating. The oxidation of aluminide-coated superalloys is similar to that of superalloys with strong oxidation resistance, except that the content of aluminum on the surface is higher, and an Al2O3 film is formed when oxidized. Due to the oxidative consumption of aluminum, the aluminum content of the coating surface decreases. As the oxidation progresses, aluminum continues to be consumed, the coating surface is depleted of aluminum, β-NiAI is transformed into γ'-Ni3Al, and finally a γ-Ni solid solution is formed. When the mass fraction of aluminum is reduced to about 4%, the Al2O3 film cannot be formed, rapid oxidation will occur, and the coating will fail.



On the other hand, interdiffusion occurs first at the coating/substrate interface, aluminum diffuses into the alloy, and other elements in the alloy diffuse into the coating, and the amount of aluminum in the coating near the interface decreases. Over time, the aluminum-depleted zone in the coating continues to expand. If the coating is thin, the aluminum-depleted zone caused by interdiffusion reaches the surface of the coating in a relatively short time, causing the coating to fail. On the premise that the oxide film remains intact, the degradation of the coating is mainly controlled by the interdiffusion because the interdiffusion rate between the coating and the substrate is much faster than that in the oxide film.



Table 1-1 gives the oxidation life of aluminide coatings on several superalloys. The composition of the base alloy plays an important role in coating performance. The large difference in coating life on several alloys is due to the difference in the aluminum content of the substrate, which affects the speed of aluminum diffusion and removal of the coating; it is also due to the difference in the content of other elements in some substrates, which are oxidized. During the process, it diffuses to the surface of the coating, thereby increasing (chromium or a certain amount of hafnium and tantalum) or reducing (titanium, vanadium, tungsten, molybdenum) the oxidation resistance of the coating.