Fig. 1 shows the friction coefficient of pure Ni coating and Ni–TiO2 nanocomposite coatings containing different TiO2 wt.%, under the identical test conditions. The friction coefficients of all coatings are approximately same at the first 10 m. Thereby, the pure Ni coating friction coefficient increases dramatically to 1 and then maintains at a constant level. However the friction coefficients of Ni–TiO2 nano-composite coatings exhibited little change and keep stable during the test. The hard nano-scale reinforcements embedded in nanocomposite coatings reduce direct contact between metal matrix and abrasivesurface.
Fig. 1. Friction coefficient curves of (a) pure Ni coating, (b) Ni–6.5 wt.% TiO2 and(c) Ni–8.3 wt.% TiO2.
On other hand, the nanoparticles, separated from matrix due to abrasion, act as solid lubricants between two wear surfaces. Fig. 2 shows the wear loss of pure Ni coating and Ni–TiO2 nanocomposite coatings with different TiO2 contents. It is obvious that wear resistance of nanocomposite coatings are more than pure Ni coating. This can be attributed to the strengthening effect and friction coefficient decreasing which was explained previously. The hardness and friction coefficient are two parameters which affect the wear resistance.
Fig. 2. Variation of wear loss with wt.% of codeposited TiO2.
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