Tribological Behavior of SPS-Prepared Al-Matrix–ZrO2-Nanofiber Composites with Graphene Nanoplatelets Solid-Lubricating Surface Films

In this study, the tribological compatibility of ZrO2-nanofiber-strengthened Al-matrix composites with graphene nanoplatelets (GNPs)-derived surface film acting as a solid lubricant was investigated. The substrate materials prepared by Spark Plasma Sintering (SPS) included the pure aluminum monolith (reference material) and two Al–ZrO2 nanocomposites with either 1 or 3 wt.% of ZrO2 nanofibers. The GNPs-derived solid lubricant films were dry mechanically burnished into the metallographically polished surfaces. The durability of these burnished films was evaluated by performing tribological friction experiments using a ball-on-disk method. Thus, a friction load capacity of GNP-derived tribofilms on the substrate materials and its effect on the coefficient of friction (COF) were evaluated. The results showed that the films burnished on the surfaces of Al–ZrO2 nanofiber composites were more resistant to much higher loads than films burnished on monolithic aluminum. The obtained findings indicated that ZrO2 nanofiber protrusions likely stabilize a GNP-derived carbon tribolayer on the polished composite surfaces. As a result, the reinforcement of aluminum with ceramic nanofibers led also to a significant reduction in COF. The highest improvement of tribological performance was observed for the Al–ZrO2 nanofiber composite with 1 wt.% ZrO2 nanofibers. The increase of ZrO2 nanofibers up to 3 wt.% was no more efficient due to nanofiber clustering leading to lower stability of the carbon friction film. Our objective was to isolate the role of the aluminum substrate, specifically, ZrO2 nanofiber protrusions in the formation and retention of a GNP-derived carbon tribofilm under room-temperature, ambient-air dry sliding.