Generation and functionalisation of ZDDP tribofilms for additive manufacturing via 3D Tribo-Nanoprinting

The growth and modification of Zinc Dialkyldithiophosphate (ZDDP) tribofilms has been investigated with the aim of furthering the additive manufacturing technique, 3D Tribo-Nanoprinting (3D TNP). 3D TNP allows for the high precision deposition of tribofilms under a controlled tribological contact; in this work, that of either an Atomic Force Microscope (AFM) or a Mini Traction Machine (MTM). Two aspects of the tribofilm growth have been investigated.

Firstly, the growth of ZDDP tribofilms under an AFM contact was measured with respect to varying temperatures, contact pressures and ZDDP concentrations. It was found that temperature had a large impact on the formation of tribofilms, with room temperature conditions leading to a significant reduction in tribofilm growth and tribofilms forming in a banded structure; both pressure and temperature increased growth rate, but these were seemingly limited by the ZDDP concentration; and substrate features such as scratches acted as seeding locations for film growth.

Secondly, larger tribofilms, generated on a mini traction machine (MTM), were modified using three additives: Copper Nanoparticles (Cu NPs), Graphene Nanoplatelets (GNPs) and Molybdenum Dithiocarbamate (MoDTC) additive. The aim was to change the properties of the tribofilms, making them conductive. Here it was seen that all additives were able to induce currents to flow in high enough additive quantities. Copper proved the least effective, with only the highest concentrations providing any measurable current; graphene and MoDTC both performed well but differed in that graphene produced patches of current, whereas MoDTC made a uniformly conductive tribofilm. It was seen that the likely cause of the graphene patches was due to the limited openings for charge carriers to enter the bulk of the film, in the form of surface graphene sheets.

This work demonstrates that the controlled deposition of ZDDP tribofilms for manufacture is feasible, and that ZDDP tribofilms can be modified to produce conductive structures. These may serve as building blocks for more complex 3D printed devices.