1. State Key Laboratory of Tribology,Institute of Manufacturing Engineering,Department of Mechanical Engineering,Tsinghua University,Beijing 100084, China;
2. Department of Mechanical Engineering,Carnegie Mellon University,Pittsburgh, PA 15213e-mails: zhenguon@andrew.cmu.edu; zhenguonie@gmail.com
3. State Key Laboratory of Tribology,Institute of Manufacturing Engineering,Department of Mechanical Engineering,Tsinghua University,Beijing 100084, Chinae-mail: gwang@tsinghua.edu.cn
4. State Key Laboratory of Tribology,Institute of Manufacturing Engineering,Department of Mechanical Engineering,Tsinghua University,Beijing 100084, Chinae-mail: lpwang@tsinghua.edu.cn
5. Department of Mechanical and Energy Engineering,Southern University of Science and Technology,Shenzhen 518055, Chinae-mail: rongym@sustc.edu.cn