Effects of Environmental Gas and Trace Water on the Friction of DLC Sliding with Metals.

Yoshihiro Kurahashi,Joichi Sugimura,Masaya Terayama,Hiroyoshi Tanaka

doi:10.3390/mi8070217

Abstract

This paper describes an experimental study on the friction of a-C:H diamond-like carbon (DLC) and ta-C DLC coatings in gas with different concentration of trace water. Pin-on-disk sliding experiments were conducted with DLC coated disks and aluminum pins in hydrogen, nitrogen, and argon. Trace oxygen was eliminated to less than 0.1 ppm, while water in the gas was controlled between 0 and 160 ppm. Fourier transform infrared spectroscopy (FT-IR) and laser Raman spectroscopy were used to analyze the transfer films on the metal surfaces. It was found that trace water slightly increased friction in hydrogen gas, whereas trace water caused a significant decrease in the friction coefficient in nitrogen and argon, particularly with a-C:H DLC. The low friction in hydrogen was brought about by the formation of transfer films with structured amorphous carbon, but no differences in the structure and contents of the films were found in the tests with and without trace water. In nitrogen and argon, the low friction with a-C:H DLC was achieved by the gradual formation of transfer films containing structured amorphous carbon, and FT-IR spectra showed that the films contained CH, OH, C–O–C, and C–OH bonds.

Highlights

Diamond-like carbon (DLC) is one of the most successful materials for tribological coatings
The objective of the present study is to investigate how a small amount of water of ppm level in the environmental gas affects friction and wear of two types of DLC
Sliding experiments with a-C:H DLC and ta-C DLC and aluminum were conducted in hydrogen, nitrogen, and argon with trace water and no trace oxygen, and the following conclusions were drawn: (1) The effect of trace water on friction of two types of DLCs in hydrogen was different from that in nitrogen and argon

Summary

Introduction

Diamond-like carbon (DLC) is one of the most successful materials for tribological coatings. DLCs were introduced for their low friction and high wear resistance, only in particular environments and under limited sliding conditions. After decades of studies and developments, DLCs come in a wide variety of types with different manufacturing processes and compositions, and can be used in severe practical conditions from dry contacts to oil lubricated contacts [1,2,3]. DLCs are important candidates in the context of reducing energy consumption and CO2 emissions through low friction and wear loss. The low and super-low friction exhibited by DLC strongly depends on their atomic structure, the formation of carbon transfer layers, the presence of hydrogen and the nature of the environments in which sliding takes place. Many of the earlier studies on undoped DLC revealed that hydrogen content in DLC gave rise to diverse frictional behaviors in different gas environments

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Conclusion