1. Institute of Intelligent Flexible Mechatronics Jiangsu University Zhenjiang 212013 P. R. China
2. State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics Chinese Academy of Sciences Lanzhou 730000 P. R. China
3. Department of Mechanical Engineering Material Science & Engineering Program University of Colorado Boulder Boulder CO 80309 USA
4. School of Mechanical Engineering Yangzhou University Yangzhou 225009 P. R. China