• PDF / 1,710,557 Bytes
• 10 Pages / 595.22 x 842 pts (A4) Page_size
• 69 Downloads / 198 Views

DOWNLOAD

REPORT

---

ISSN 2223-7690 CN 10-1237/TH

RESEARCH ARTICLE

Running-in behavior of a H‒DLC/Al2O3 pair at the nanoscale Pengfei SHI1, Junhui SUN1, Yunhai LIU1, Bin ZHANG2, Junyan ZHANG2, Lei CHEN1,*, Linmao QIAN1,* 1

Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China

2

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

Received: 28 December 2019 / Revised: 29 May 2020 / Accepted: 07 July 2020

© The author(s) 2020. Abstract: Diamond-like carbon (DLC) film has been developed as an extremely effective lubricant to reduce energy dissipation; however, most films should undergo running-in to achieve a super-low friction state. In this study, the running-in behaviors of an H–DLC/Al2O3 pair were investigated through a controllable single-asperity contact study using an atomic force microscope. This study presents direct evidence that illustrates the role of transfer layer formation and oxide layer removal in the friction reduction during running-in. After 200 sliding cycles, a thin transfer layer was formed on the Al2O3 tip. Compared with a clean tip, this modified tip showed a significantly lower adhesion force and friction force on the original H–DLC film, which confirmed the contribution of the transfer layer formation in the friction reduction during running-in. It was also found that the friction coefficient of the H–DLC/Al2O3 pair decreased linearly as the oxygen concentration of the H–DLC substrate surface decreased. This phenomenon can be explained by a change in the contact surface from an oxygen termination with strong hydrogen bond interactions to a hydrogen termination with weak van der Waals interactions. These results provide new insights that quantitatively reveal the running-in mechanism at the nanoscale, which may help with the design optimization of DLC films for different environmental applications. Keywords: hydrogenated diamond-like carbon (H–DLC) film; running-in; nanoscale; oxide film; transfer layer

1

Introduction

As an unavoidable phenomenon for most as-fabricated mechanical parts during the initial operation, runningin alters the performance of the moving components and plays a crucial role at all size scales, from macroscale engines down to dynamic nano-devices [1–8]. Diamond-like carbon (DLC) film is one of the most promising materials for reducing energy consumption and CO2 emission since it is capable of achieving super-low friction for sliding parts [9–12]. Except for a few specially treated DLC films, such as those with a pre-existing highly graphitized surface [8, 13], most kinds of DLC films must initially

undergo a transition to lower the friction level. The high friction and severe wear during this running-in process may greatly hinder the development of additional applications of DLC films [14–16]. Therefore, it is important to achieve a scientific understanding of the contact and friction mechanisms that occur during the running-in of a D