Diamond-like carbon coating under oleic acid lubrication: Evidence for graphene oxide formation in superlow friction

Maria Isabel De Barros Bouchet,Shandan Bai,José Avila,Nobuki Ozawa,Kentaro Yoshida,Momoji Kubo,Jean Michel Martin,Makoto Kano,Maria C Asensio,Takeshi Tsuruda,Yuji Higuchi

doi:10.1038/srep46394

Abstract

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Based on a combined high-resolution photoemission and soft X-ray absorption study, we report that superlubricity can be realized for engineering applications in bearing steel coated with ultra-smooth tetrahedral amorphous carbon (ta-C) under oleic acid lubrication. The results show that tribochemical reactions promoted by the oil lubrication generate strong structural changes in the carbon hybridization of the ta-C hydrogen-free carbon, with initially high sp3 content. Interestingly, the macroscopic superlow friction regime of moving mechanical assemblies coated with ta-C can be attributed to a few partially oxidized graphene-like sheets, with a thickness of not more than 1 nm, formed at the surface inside the wear scar. The sp2 planar carbon and oxygen-derived species are the hallmark of these mesoscopic surface structures created on top of colliding asperities as a result of the tribochemical reactions induced by the oleic acid lubrication. Atomistic simulations elucidate the tribo-formation of such graphene-like structures, providing the link between the overall atomistic mechanism and the macroscopic experimental observations of green superlubricity in the investigated ta-C/oleic acid tribological systems.

Highlights

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues
Through a complementary study of high–resolution synchrotron radiation photoemission (HRPES) together with soft X-ray absorption near edge structure (XANES), we demonstrate that superlubricity can be realized for engineering applications when tetrahedral amorphous carbon (ta-C) coatings are lubricated by fatty oleic acid owing to the presence of a partially oxidized ultrathin graphene-like film
The values in the curves are the average value of these three measurements. These results are shaped like a Stribeck curve, and they show the different regimes that are typically encountered in such a lubrication test, namely, elasto-hydrodynamic lubrication (EHL) above 100 mm/s, boundary lubrication (BL) under 10 mm/s and mixed lubrication (ML) between these two speeds

Summary

Introduction

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Through a complementary study of high–resolution synchrotron radiation photoemission (HRPES) together with soft X-ray absorption near edge structure (XANES), we demonstrate that superlubricity can be realized for engineering applications when ta-C coatings are lubricated by fatty oleic acid owing to the presence of a partially oxidized ultrathin graphene-like film. This protective film is produced by the tribochemical reactions that take place at the surface under friction. This is the first reported case of in situ formation of graphene oxide species directly induced by friction

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