Preparation of Al₂O₃ coating on Nb fiber and the effect on interfacial microstructure of Nb_f/TiAl composite

Continuous Nb fiber-reinforced TiAl-matrix composites are a potential structural material to satisfy the service requirements in space industry. However, the premature cracking failure is resulted from the brittle reaction products formed at the interface between Nb fiber and TiAl matrix, which accelerates the structural failure of Nb_f/TiAl composite. Fortunately, Al₂O₃ coating could be introduced on the surface of Nb fiber to tackle the problem. In this study, Al₂O₃ coating was prepared on the surface of Nb fiber by the cathodic plasma electrolytic deposition (CPED) technology, and then the Al₂O₃-coated Nb fibers were used to reinforce TiAl alloy by using vacuum hot-pressing sintering. Furthermore, the influence of the Al₂O₃ coating on the interfacial microstructure of Nb_f/TiAl composite after hot-pressing sintering and thermal exposure was studied. The results show that deposition voltage plays a key role in the microstructure of Al₂O₃ coating on the Nb fiber. As the voltage increases from 200 V to 400 V, the thickness, density and Al₂O₃ content of the coating increase. Meanwhile, the bonding between Nb fiber and Al₂O₃ coating is better when the deposition voltage exceeds 350 V, and the components of Al₂O₃-coated Nb fibers prepared from fiber to coating are as follows: Nb/Nb₂O₅/Nb₂O₅ + Al₂O₃/Al₂O₃. The Al₂O₃ coating can restrain the formation of the interfacial brittle phases including σ-Nb₂Al and α₂-Ti₃Al during hot-pressing sintering process of Nb_f/TiAl composite. In addition, the introduction of Al₂O₃ coating can reduce the defects in interface, as well as avoid the formation of the crisscrossed and deleterious oxides, which improves interfacial oxidation resistance of the Nb_f/TiAl composite.