Formation and oxidation behavior of TiO2 modified Al2O3-Nb2O5/NbAl3 composite coating prepared by two-step methods

To improve the high-temperature oxidation resistance, Al2O3-Nb2O5/NbAl3 composite coating with or without TiO2 particles was prepared on Nb-based alloy through a two-step method of halide activated pack cementation (HAPC) and micro arc oxidation (MAO). NbAl3 layer was firstly formed on the substrate by HAPC treatment. Then, TiO2 was introduced into the Al2O3-Nb2O5 coating on NbAl3 by hydrolysis reaction of K2TiF6 during MAO process. In this paper, the effect of TiO2 on the morphology and formation of the MAO coating was studied. Furthermore, the oxidation resistances of the TiO2-Al2O3-Nb2O5/NbAl3 and Al2O3-Nb2O5/NbAl3 composite coated Nb alloy were investigated. The results showed that the addition of K2TiF6 could promote the growth of MAO coating and increase the density. After oxidation at 1200 °C for 100 h, a dense protective oxides scale was found on the TiO2-Al2O3-Nb2O5/NbAl3 coating sample, and the weight gain was only 4.75 mg/cm2, which was significantly lower than the coated sample without TiO2. The mechanism of oxidation resistance enhancement of coating with TiO2 should be ascribed to the formation of a dense TiO2 + Al2O3 scale, which could effectually prevent the diffusion of O and suppress the formation of Nb2O5.