Microstructure and oxidation behavior of Ti(C,N)-based cermets with in situ synthesized Ni3Al phase

Ni3Al phase was synthesized by in-situ reaction of Ni binder and AlN addition during liquid phase sintering. The microstructure and properties of the Ni3Al-containing Ti(C,N)-based cermets were investigated. The hardness of the cermet increased after addition of 2.5 wt%AlN due to the solid-solution strengthening effect of Ni3Al, and further addition of AlN yielded coarser core-rim grains and decrease of hardness. The transverse rupture strength declined rapidly after addition of 5 wt% AlN, which was attributed to the increase of porosity. After static oxidation at 900 degrees C, the mass gain of the cermet without AlN addition increased rapidly with oxidation time and followed a quasi-parabolic law. After addition of 2.5 wt%AlN, both mass gain and oxidation rate declined obviously, and the oxidation kinetics curve still presented a quasi-parabolic law. The mass gain increased slightly after addition of 5.0 wt% AlN, and larger addition of AlN up to 7.5 wt% resulted in the changing of oxidation kinetics to a linear law. At 900 degrees C, the Ti(C,N)-based hard phase and Ni binder phase in the cermet all suffered serious oxidation. The Ti(C,N)-based hard phase was mainly oxidized to TiO2, and Ni binder transformed to complex oxides such as Ni3TiO7 and NiTiO3. As for the cermet with 2.5 wt% AlN addition, the oxidation behavior showed much difference. Though Ni3Al-containing binder was oxidized to Ni3TiO7 and NiAl2O4, both the intermediate oxidation product of Ti(C,N) hard phase, i.e. Ti(O-0.12,C-0.53,N-0.32) and Ni3Al remained stable after dwelling for 4 h. The oxide layer on the cermet with 2.5 wt% AlN addition appeared to be dense and protective, which inhibited the diffusion of oxygen into the cermet and improved the oxidation resistance of the cermets.