Preparation and Tribological Behavior of the MoS2/MAO Wear-resistant and Anti-friction Composite Coating
The work aims to fabricate MoS2/MAO wear-resistant and anti-friction composite coatings on Al alloy surface by two-step method, and investigate the friction and wear behavior characteristics. The first step was to prepare a MAO coating on the surface of 7075 Al alloy by micro-arc oxidation (MAO) technique. The second step was to construct the MoS2/MAO wear-resistant and anti-friction composite coating on MAO coating by in-situ hydrothermal synthesis method. The micro-structure and composition of the coatings were characterized by Scanning Electron Microscope (SEM), X-ray Diffraction Spectrometer (XRD) and Raman spectrum. The wear resistance was measured with friction tester, and the wear scars were analyzed with 3D surface profiler.The characterization results showed that the MAO coating was mainly composed of Al2O3, accompanied by a small amount of SiO2. The surface was a typical porous structure with a large number of micropores and a large surface roughness. However, after the formation of MoS2/MAO composite coating, the number of micropores on the surface of the coating significantly decreased. MoS2 uniformly covered the concave areas of the ceramic coating in the form of particles and filled them into the micropores of the MAO coating, making the surface flat and smooth, and significantly improving the compactness. The friction test results showed that there were many volcanic micropores on the surface of the MAO coating, which were uneven and rough, resulting in a large fluctuation of the friction coefficient during the friction process. It took a long time to run in order to achieve a stable friction coefficient of 0.6. When the MoS2/MAO composite coating was formed, the friction coefficient was significantly reduced, with an average friction coefficient of around 0.25, and the friction curve became more stable. At the same speed, different loads had a great effect on the friction coefficient, which varied from 0.14 to 0.4. The friction coefficient was the lowest under 4 N load, and the average friction coefficient was 0.18. The wear rate decreased with the increase of load, but the difference was not large, ranging from 1×10-5 to 4×10-5 mm3/(N·m). Under the same load, the rotational speed had little effect on the friction coefficient, which varied between 0.15 and 0.3. The wear rates ranged from 1.6×10-5 to 2.2×10-5 mm3/(N·m).The MoS2/MAO wear-resistant and anti-friction composite coating can significantly improve the friction and wear properties of the substrate. Under low load conditions, MAO coating mainly plays a load-bearing and anti-wear role, while MoS2 particles play a good role in lubrication and friction reduction. The protruding microporous structure on the surface of MAO acts as a buffer for MoS2 in the depression, reducing the pressure it bears. MoS2 particles can effectively cover the surface and provide continuous lubrication, reducing the friction coefficient and maintaining a smooth friction curve. As the load further increases, the micropores on the surface are seriously worn or crushed because they cannot bear the pressure. More hard particles of Al2O3 and SiO2 are mixed into MoS2 particles, and the friction coefficient increases. With the increase of sliding distance, the hard particles in the wear scar gradually increase, and the MoS2 particles in the groove are more squeezed out of the wear scar by the friction pair. The lubrication effect decreases, the friction coefficient continues to increase, and the fluctuation of the friction curve also increases. In general, on the surface of aluminum alloy, the MoS2/MAO wear-resistant and anti-friction composite coating can play a good role in anti-friction and anti-wear compared with single MAO.
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