Tribology behaviour of graphene-modified nanostructured Al2O3/3 % TiO2 coatings under boundary and mixed lubrication conditions

Abstract

This study is focused on the preparation of a self-lubricating coating via thermal spraying. This coating was designed to control friction and lubrication in both boundary lubrication (BL) and mixed lubrication (ML) regimes. We aimed to compare the tribological behaviour of nanostructured Al2O3/3 % TiO2 and modified nanostructured Al2O3/3 % TiO2 incorporating 3 % GNPs coatings deposited by Oxygen-Fuel (OF). The critical conditions for the transition from BL to ML were experimentally studied using a fundamental tribological concept known as the Stribeck curve. A lubricating film consisting of detached GNPs, Al2O3 nanoparticle debris, and a conventional lubricant effectively covers the surface material and improves the mechanochemical interactions between the surface material and lubricating oil, thereby enhancing the tribological performance. The introduction of GNPs is found to play an effective role in the lubrication regime, leading to an increase in microhardness, a decrease in the friction coefficient and wear volume of the Al2O3/3 % TiO2 coating, and the steel counterpart disc. It noticeably improved the critical load-carrying capacity at different speeds during the transition from BL to ML regimes. The tribo-layer formed on the worn surfaces controls the tribological properties of the Al2O3/3 % TiO2 + 3 % GNPs coatings.