Tribological behavior of AZ31/ZrO2 surface nanocomposites developed by friction stir processing

The present study focused on the poor wear resistance of the AZ31 magnesium alloy and improving its tribological applications. Surface nanocomposites reinforced with nano-sized ZrO2 particles on the AZ31 magnesium alloy substrate were fabricated by friction stir processing (FSP) in two different pass number. The average grain size of the as-received AZ31 base metal reduced from 11 pm to 2 pm in the case of AZ31/ZrO2 surface nanocomposite developed by four passes of FSP. Secondary electron microscopy (SEM) images of the 4-pass FSPed nanocomposite showed that the ZrO2 nanoparticles distributed homogenously in the magnesium matrix. Microhardness measurements revealed a significant improvement by adding ZrO2 nanoparticles and also increasing in FSP pass number. The microhardness of the AZ31 base metal improved about 90% in the case of four passes FSPed AZ31/ZrO2 surface nanocomposite. The wear performance of the samples was evaluated using a reciprocal wear machine and AISI 52100 steel with the hardness of 63 HRC as the counterpart. Investigating the wear performance and friction coefficient of the samples revealed enhanced tribological behavior in FSPed samples. The wear rate of the AZ31 base metal reduced about 40% for the 4-pass FSPed AZ31/ZrO2 nanocomposite. The average friction coefficient of the 4-pass FSPed nanocomposites was about half of the base metal. The SEM observations of the worn surface, wear track cross-section, counterpart tip and wear debris indicated that the wear mechanism changed from severe abrasion and adhesion for AZ31 base metal to mild abrasion for the 4-pass FSPed AZ31/ZrO2 nanocomposite.