The representative curves of the evolution of friction coefficients of NCF with sliding time under an applied load of 10 N at a sliding speed of 0.188 m/s and different temperatures are shown in Fig. 1. The NCF can be seen to have high friction coefficients at room temperature, 200℃and 400℃; these are attributable to CaF2 acting as a high-
temperature solid lubricant only at temperatures 500℃. As the temperature increases to 600 ℃, the friction coefficient decreases to the value of about 0.35. The transformation in CaF2 from a brittle to a ductile state plays a dominant role in this decrease, as does the presence of little CaCrO4and CaMoO4(Fig. 2) on the worn track. However, above 800 ℃, NCF shows an excellent lubri cating property. A low and stable frictional curve at the steady-state period is obtained, with a value of about 0.2 at 800℃. With increasing temperature up to 1000℃, the friction coefficient remains constant at about 0.2 and is closely related to the formation of CaCrO4 and CaMoO4, both of which act as high-temperature solid lubricants on the worn surfaces. This result can be attributed to the complex reaction (including the high temperature and tribo-chemical reaction), as shown in Fig. 2. The high-temperature lubricious behavior of some chromates and
molybdates has been defined and found to be associated with low shear strength and high ductility at elevated temperatures.
Fig.1 The evolution of friction coefficients with sliding time and variation of wear rates of NCF at different temperatures
Fig. 2 X-ray photoelectron spectroscopy (XRD) patterns of the sintered sample (a) and worn surfaces of the NCF composite after tests at 600 ℃ (b), 800 ℃ (c), and 1000℃ (d)
The variation in wear rates of NCF at the end of the tests is shown in Fig. 1. The wear rate can be seen to be moderate at room temperature, with a value of about 7 ×10-5mm3N-1m-1, with a tendency to increase with increases in temperature between 400 and 600 ℃, reaching a peak at about 7×10-4mm3N-1m-1within this temperature range. However, at 800℃, the wear rate clearly decreases sharply to a very low value of about 8.77× 10-6mm3N-1m-1and with further temperature increases to 1000℃, the wear rate only increases slightly to 1.24 ×10-5mm3N-1m-1.
The worn surfaces of NCF after tests at different temperatures are shown in Fig. 3. At room temperature, the presence of some flaky wear debris on the worn surface suggests that the wear mechanism is mainly delamination. With increases in temperature to 400 ℃, some fine grooves and flaky wear debris appear on the worn track, indicating that the wear mechanism is both delamination and microploughing. At 600℃, coarse grooves and a severely deformed surface appear, revealing that the wear mechanism is dominated by microploughing and surface deformation. However, at 800 ℃, a smooth film is present on the worn surface, thereby implying the formation of the glaze film. As the temperature increases to 1000℃, the smooth film incompletely covers the worn surface compared to coverage at 800℃. The XPS results shown in Fig. 3 confirm the Ca peak at the high binding energy as the Ca present in CaF2 at low temperatures, whereas the Ca peak shifts to the low binding energy with increasing temperature. Above 800 ℃, it is clear that the XPS peaks of CaCrO4 and CaMoO4 are becoming stronger, indicating that the glaze film mainly consists of oxides, such as CaCrO4 and CaMoO4, which is consistent with XRD results in Fig. 2.
The wear resistance of material decreases with decreasing hardness, while the hardness of material decreases with increasing temperature. Thus, the change in worn surface morphologies from the relatively smooth surface to the severely deformed surface and the increase in wear rates with increasing test temperature from room temperature to 600℃, as shown in Figs. 1 and 3, correspond to the trend of wear rate reported elsewhere.However, at high temperatures (800 ℃), the presence of the glaze film plays an important part in the tribological properties of the material (Figs. 2, 3). The protective oxide glaze film formed in situ separates the sliding surfaces of the sample and the Si3N4 ceramic ball, providing effective wear protection and significantly decreasing the friction coefficient.
Fig.3 Worn surfaces and XPS results of worn surfaces of NCF after tests at different temperatures
In the case of the high-temperature self-lubricating NiAl–Cr–Mo–CaF2composite, NiAl acts as a high-temperature oxidation-resistant and high-strength matrix, CaF2 performs as a solid lubricant, and Cr and Mo work as reinforcement. In addition, the presence of CaCrO4 and CaMoO4 as high-temperature solid lubricants on the worn surface due to the complex reaction between CaF2 and the active elements (Cr and Mo) also markedly improves the sliding characteristics at higher temperatures. Thus, it should be noted that their formation on sliding surfaces at high temperatures is beneficial to both friction and wear and this fact taken into account when designing high-temperature self-lubricating composite.
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