Thermal oxidation and its effect on the wear of Mg alloy AZ31B

Magnesium alloy AZ31B is an important lightweight, high specific strength material for new generations of energy-effective vehicles. Presently, a critical challenge that the vehicle production is facing is how to prevent the material from surface oxidation at elevated temperatures. This paper aims to investigate the effect of thermal oxidation and chemical compositions of oxides on the wear of AZ31B. The tribology experiments were conducted by a contact pair of pin-on-disc under elevated temperatures. The disc and pin were made by AZ31B and the production tooling steel 86CrMoV7, respectively. The composition and morphology of the disc surface were examined by the scanning electron microscope (SEM). It was found that the initial surface oxide film naturally formed prior to testing at room temperature (RT) was thin, compact and protective. However, the film would lose its protection capability when the temperature rose because of the thickened film layer and the emergence of the oxide nodules. At a higher temperature, the oxide nodules grew more quickly. Besides, more oxide nodules emerged, and the film started to crack as the temperature increased, leading to a higher wear rate. The friction coefficient varied at different temperatures due to the change in the dominant wear mechanism of AZ31B. The minimum wear rate was found to occur at 200 degrees C and the maximum at 400 degrees C. It was also found that the surface hardness of AZ31B decreased from 75 HV2 at 25 degrees C to 50 HV2 at 200 degrees C, and then fluctuated around 50 HV2 at higher temperatures, which further accelerated the wear process.