The role of chemical composition on the oxidation performance of aluminide coatings

The durability of aluminide bond coatings is critical to the life of thermal barrier coatings (TBCs). With service, the coating composition continues to change due to oxidation and interdiffusion. In order to better understand the effect of coating composition on its oxidation resistance, model alloys are being studied with various levels of Al, Pt and Hf. In hypostoichiometric β-NiAl, the formation of faster-growing Ni-rich oxide was observed, and this problem intensified at lower Al contents. Platinum additions were found to decrease this problem. With two-phase Ni–35Al–6Pt compositions, a phase transformation upon heating to 1100 °C caused macroscopic specimen deformation when the specimen was repeatedly cycled. The degradation of oxidation resistance with lower Al contents indicates that the loss of Al due to back-diffusion into the superalloy substrate is the critical performance-limiting problem for aluminide coatings. One strategy for improved coating performance suggested by Gleeson uses higher Pt and Hf contents and lower Al contents.