The lubricating properties of graphene at nano- and micro-scale as additive for oils and/or as self-solid lubricant in coatings and composites have extensively been demonstrated. In the case of ceramic/graphene composites, graphene fillers can reduce the friction coefficient of monolithic materials up to ~50% and increase the wear resistance up to 100%, although the tribological response of graphene-based composites seems to strongly depend on the testing parameters and composite characteristics.
In the present work, cubic yttria-stabilized zirconia (8YSZ) ceramic composites containing different amounts of graphene nanoplatelets (GNPs, 7, 11, 14 and 21 vol.%) were tested under dry linear reciprocating ball-on-plate configuration, highlighting the role played by GNPs in the tribological properties. 8YSZ ceramics were selected as they are commonly employed, among others, in new micro-energy conversion devices where the internal friction between their components can reduce the efficiency.
The addition of graphene-based fillers to an 8YSZ matrix extraordinarily increased its tribological response under dry testing conditions, showing a much better friction and wear performance than other ceramic/graphene tribosystems. This response was clearly associated to the development of a well adhered and continuous thick carbonaceous tribolfilm on the worn surface of composites that acted as self-lubricant and wear protecting layer and, more importantly, to the formation in the composites of hundreds of graphene-based twisted rolls on top of the tribofilms that produced a bearing-like effect, considerably contributing to lower the friction and wear. This study definitively clarifies the role played by graphene fillers in the tribological behaviour of ceramic composites.