Preparation And Tribological Properties Of Anti-wear And Friction-reducing Plasma Electrolytic Oxidation Coatuings On Al Alloys

Aluminium alloys are widely used in various machinery and transportation system, especially in aerospace and automobile products, due to their advantages of high strength to weight ratio, proper corrosion resistance and lightweight. However, their shortcomings, such as low hardness, poor tribological properties, have seriously restricted their extensive applications. Employing plasma electrolytic oxidation (PEO) technique can deposit hard ceramic alumina coatings on Al alloys, which greatly improves their tribological properties. In this work, the preparation and tribological properties of anti-wear and friction-reducing PEO coatings on Al alloys were studied.1. Firstly, the controllable preparation of anti-wear PEO coatings was realized by adjusting the applied cathodic voltages. The PEO coatings with same coating thickness were prepared on Al alloy by adjusting the processing time under different cathodic voltages. The scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and tribometer were employed to investigate the microstructure, element content, phase composition and tribological properties of the coatings, respectively. It was found that the coatings exhibited more compact structure and improved wear resistance with the applied cathodic voltage increasing.2. Secondly, the graphite-containing friction-reducing PEO composite coatings were prepared on Al alloy in graphite-dispersed electrolyte, using periodically constant voltage. The scanning electron microscope (SEM), X-ray diffraction (XRD) and tribometer were applied to investigate the microstructure, composition and tribological properties of the coatings, respectively. The results showed that the graphite-containing PEO composite coatings could be prepared in graphite-dispersed electrolyte, the friction coefficient of the yielded coatings decreased from about0.6to about0.4.3Thirdly, the tribological properties of the PEO coatings were further optimized by introducing ultrasonic wave in the graphite-dispersed electrolyte during the PEO processes. The scanning electron microscope (SEM), X-ray diffraction (XRD) and tribometer were employed to investigate the microstructure, composition and tribological properties of the coatings, respectively. It was found that the introduction of ultrasonic wave improved the microcirculation of the electrolyte and caused the graphite particles dispersed more evenly. This resulted in more compact coating structure and caused the graphite particles embedded in the coatings more evenly. Therefore, the friction coefficient of the coatings further decreased to about0.3and the coatings exhibited improved anti-wear performance.