Abstract:
The Ni/Ti
2AlC 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 Al
2O
3 ball of
Φ6 mm. The results showed that the Ti
2AlC was decomposed fully and reacted with Ni during the hot-pressing sintering process and the sintered Ni/10%Ti
2AlC composite contained Ni-based solid solution, TiC
x, Ni
3Al and a small quantity of Al
2O
3, while the Ni/50%Ti
2AlC composite was mainly composed of Ni
2TiAl, TiC
x, Ti
3NiAl
2C and a small amount of Al
2O
3. After annealing at 1 200 ℃ and 1 350 ℃ for 16 h, the Ni
3Al phase in Ni/10%Ti
2AlC composite and the Ti
3NiAl
2C phase in Ni/50%Ti
2AlC 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%Ti
2AlC and Ni/50%Ti
2AlC 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 Ni
3Al and Ti
3NiAl
2C 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 mm
3/(N·m), 20.43×10
−5 mm
3/(N·m), 8.64×10
−5 mm
3/(N·m) for Ni/10%Ti
2AlC and 5.56×10
−5 mm
3/(N·m), 4.25×10
−5 mm
3/(N·m), 0.78×10
−5 mm
3/(N·m) for Ni/50%Ti
2AlC. 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%Ti
2AlC, 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 mm
3/(N·m) appeared after annealing at 1 350 ℃ for 16 h. For Ni/50%Ti
2AlC, 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 mm
3/(N·m). Adhesive wear and oxidation wear were confirmed as the main wear mechanism at 800 ℃, and the NiO, NiTiO
3, TiO
2, and Al
2O
3 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 TiO
2 and NiTiO
3 inside wear track than that outside wear track, and hinting that annealing processes can promote the formation of TiO
2 and NiTiO
3 as the high temperature lubricating phases on wear surface, which was beneficial for the tribological properties at high temperature.