The decisive role played by graphene nanoplatelets on improving the tribological performance of Y₂O₃-Al₂O₃-SiO₂ glass coatings

Graphene nanoplatelets (GNPs) have proved to be effective fillers for enhancing the mechanical and tribological properties of bulk ceramics and glasses, also with the added benefit of developing electrical and thermal functionalities. Similarly, enhanced transport performance has recently been shown for glass-ceramics coatings of the Y₂O₃-Al₂O₃-SiO₂ (YAS) system containing a small amount of GNP fillers, intended for applications in the aerospace industry. In the present work, the wear and friction behaviour of GNP/YAS coatings –containing 0, 1.2 and 2.3 wt.% GNPs- on silicon carbide substrates is evaluated. The flame spraying process used for coating fabrication induces a structure of splats oriented parallel to the substrate with GNPs located at the inter-splat boundaries forming a connected network of platelets mainly oriented parallel to the surface as well. Unlubricated ball–on-plate reciprocating wear tests show that both the friction coefficient and the wear rate decreased by 35% and 65%, respectively, for 2.3 wt.% of GNPs. A wear mechanism for GNP/YAS coatings based on both the progressive exfoliation of the graphene sheets and the effect of the GNPs on preventing crack propagation within the coating is proposed.