Tribological manufacturing of ZDDP tribofilms functionalised by graphene nanoplatelets

Abstract 3D Tribo-Nanoprinting (3D TNP), which uses a highly controllable tribological contact to deposit tribofilms, has been proposed as a manufacturing method for nanoscale structures. Inspired by this, we show for the first time, as a proof of concept, the ability to electrically functionalise tribofilms for potential use in the manufacture of structures with nanoscale thickness. Zinc dialkyldithiophosphate (ZDDP) tribofilms have been generated to include varying concentrations of graphene nanoplatelets (GNPs) resulting in them becoming electrically conductive when tested using conductive atomic force microscopy. In its highest GNP concentration, approximately 55% of the surface of the tribofilm was able to sustain current up to a threshold of 245 pA. The higher graphene content led to a suppression in film formation and decreased substrate coverage. Transmission electron microscopy revealed a dual-layered tribofilm with a carbon-rich layer above a pure layer of ZDDP tribofilm. Within the carbon-rich layer, the GNPs formed into scrolls which created an internal network through which current could flow, being limited by the insulating pure ZDDP layer at the film-substrate interface, and the presence of surface graphene sheets. A modified lateral force microscopy procedure supported the presence of surface graphene sheets. Despite limited deposition precision in terms of homogeneity and distribution of the tribofilms, this work provides a step towards the use of 3D TNP for the manufacture of electronic structures on the nanoscale by proving that tribofilms can be functionalised by the addition of particle additives.