Influences of Annealing on Microstructures and Tribological Properties of Ni/Ti₂AlC Composites

The Ni/Ti₂AlC composites were prepared by vacuum hot-pressing sintering at 1 200 ℃/25 MPa for 1 h and annealing was conducted for composites. The effects of two different annealing processes at 1 200 ℃ and 1 350 ℃ for 16 h in Ar atmosphere respectively on the microstructures and the tribological properties at room temperature and 800 ℃ were investigated. The wear tests were carried out in the UMT-3 friction test machine under a load of 10 N with a sliding speed of 0.105 m/s coupled with Al₂O₃ ball of Φ6 mm. The results showed that the Ti₂AlC was decomposed fully and reacted with Ni during the hot-pressing sintering process and the sintered Ni/10%Ti₂AlC composite contained Ni-based solid solution, TiC_x, Ni₃Al and a small quantity of Al₂O₃, while the Ni/50%Ti₂AlC composite was mainly composed of Ni₂TiAl, TiC_x, Ti₃NiAl₂C and a small amount of Al₂O₃. After annealing at 1 200 ℃ and 1 350 ℃ for 16 h, the Ni₃Al phase in Ni/10%Ti₂AlC composite and the Ti₃NiAl₂C phase in Ni/50%Ti₂AlC composite disappeared. The annealing processes also gave rise to the growth of TiC_x particles and the optimization of microstructures and compositions of composites, meanwhile the compactness of composites was maintained. The Vickers hardness of sintered Ni/10%Ti₂AlC and Ni/50%Ti₂AlC composites reached 566.10HV and 1 065.88HV respectively, while that of annealed at 1 350 ℃ was reduced to 411.52HV and 786.17HV respectively, which was attributed to the disappearance of Ni₃Al and Ti₃NiAl₂C strengthening phases and the growth of TiC_x particles. After friction test at room temperature, with the rise of annealing temperature, the friction coefficients and wear rate of composites presented a decreasing trend and shortened running-in period. The wear rates of the composite sintered at 1 200 ℃ and annealed at 1 350 ℃ were 33.13×10⁻⁵ mm³/(N·m), 20.43×10⁻⁵ mm³/(N·m), 8.64×10⁻⁵ mm³/(N·m) for Ni/10%Ti₂AlC and 5.56×10⁻⁵ mm³/(N·m), 4.25×10⁻⁵ mm³/(N·m), 0.78×10⁻⁵ mm³/(N·m) for Ni/50%Ti₂AlC. No new phases were formed on the wear surface at room temperature, and the wear mechanism of sintered composites were abrasive wear and adhesive wear, but that of annealed composites transformed to adhesive wear and fatigue wear. The alleviation of abrasive wear and the decrease of wear rate of annealed composites was attributed to the improved bonding strength between TiC_x and metal matrix and the resulted stronger inhibiting effect of TiC_x extrusion out of matrix caused by annealing. Under wear test at 800 ℃, for Ni/10%Ti₂AlC, the friction coefficients and wear rates of annealed were lower than that of sintered, and the lowest friction coefficient of 0.2 and wear rate of 8.64 ×10⁻⁵ mm³/(N·m) appeared after annealing at 1 350 ℃ for 16 h. For Ni/50%Ti₂AlC, the wear rates increased slightly with the rise of annealing temperature, nevertheless, the wear resistance were excellent with the lowest wear rate of 0.31×10⁻⁵ mm³/(N·m). Adhesive wear and oxidation wear were confirmed as the main wear mechanism at 800 ℃, and the NiO, NiTiO₃, TiO₂, and Al₂O₃ phases were generated and the glaze layer consisting of these oxides and bimetallic oxides formed on the wear surface, which was responsible for the low friction coefficient and wear rate. In addition, Raman spectra showed higher characteristic intensities of TiO₂ and NiTiO₃ inside wear track than that outside wear track, and hinting that annealing processes can promote the formation of TiO₂ and NiTiO₃ as the high temperature lubricating phases on wear surface, which was beneficial for the tribological properties at high temperature.