Key Role of Transfer Layer in Load Dependence of Friction on Hydrogenated Diamond-Like Carbon Films in Humid Air and Vacuum.

Yunhai Liu,Bin Zhang,Pengfei Shi,Yong Peng,Junyan Zhang,Lei Chen,Linmao Qian,Ningning Zhou,Zhongyue Cao

doi:10.3390/ma12091550

Abstract

The friction of hydrogenated diamond-like carbon (H-DLC) films was evaluated under the controlled environments of humid air and vacuum by varying the applied load. In humid air, there is a threshold applied load below which no obvious friction drop occurs and above which the friction decreases to a relatively low level following the running-in process. By contrast, superlubricity can be realized at low applied loads but easily fails at high applied loads under vacuum conditions. Further analysis indicates that the graphitization of the sliding H-DLC surface has a negligible contribution to the sharp drop of friction during the running-in process under both humid air and vacuum conditions. The low friction in humid air and the superlow friction in vacuum are mainly attributed to the formation and stability of the transfer layer on the counterface, which depend on the load and surrounding environment. These results can help us understand the low-friction mechanism of H-DLC film and define optimized working conditions in practical applications, in which the transfer layer can be maintained for a long time under low applied load conditions in vacuum, whereas a high load can benefit the formation of the transfer layer in humid air.

Highlights

Nowadays, in order to reduce the huge space launch costs, recoverable carrier rockets, space shuttles and other recoverable spacecraft have been major areas of focus for industries and research groups from all over the world
It is essential to probe the effects of room air and vacuum, which respectively simulate the earth and space environments, on the friction behaviors of Diamond-like carbon (DLC) films
The results show that the friction behaviors of the hydrogenated diamond-like carbon (H-DLC) film strongly depend on the surrounding environment and the contact pressure

Summary

Introduction

In order to reduce the huge space launch costs, recoverable carrier rockets, space shuttles and other recoverable spacecraft have been major areas of focus for industries and research groups from all over the world. The reliability and service life of the key components of the recoverable spacecraft are greatly affected due to the need to repeatedly traverse the atmosphere and vacuum environments. It is necessary to develop an applicable coating to protect the key parts applied under these two conditions. The high sensitivity of the friction behaviors of DLC films to the working environment seriously restricts their practical applications, especially in the aerospace industries [6,7,8,9]. It is essential to probe the effects of room air and vacuum, which respectively simulate the earth and space environments, on the friction behaviors of DLC films

Methods

Results

Conclusion