Influence of AlN addition on the microstructure, mechanical properties and oxidation characteristics of NbC-Ni cermets

Abstract

Herein, NbC–Ni cermets with various AlN contents (0–9.0 wt%) are prepared using powder metallurgy, and their microstructure, mechanical properties, and oxidation characteristics are investigated. The addition of AlN dissolves the binder and forms Ni3Al, which can inhibit the dissolution–reprecipitation process and refine the average grain size of NbC, thereby resulting in enhanced mechanical properties of the cermets. Cermets with 1.0 wt% AlN exhibit the highest hardness (HV101,470 kg/mm2) and TRS (1365 MPa). With an increase in AlN content, excess Al exists as a metallic phase in the cermets. This destroys the continuity of the cermet structure and decreases its hardness and transverse rupture strength. For oxidation test at 700 °C, NbC in the cermets is oxidized primarily to Nb2O5, and the Ni binder is transformed into binary oxides, such as (NiNb2O6)0.6667. The oxidation weight gain of the NbC cermets decreases after the addition of AlN, which can be attributed to the formation of Ni3Al, thus slowing the oxidation of the binder. Further addition of AlN up to 3.0 wt% deteriorated the oxidation resistance because cracks were generated at the boundary of the Nb2O5/Al2O3 interface, which accelerated the oxidation process.