1. School of Mechanotronics & Vehicle Engineering, Chongqing Jiaotong University, Chongqing, P. R. China;
2. State Key Laboratory of Traction Power, Tribology Research Institute, Southwest Jiaotong University, Chengdu, Sichuan, P. R. China;
3. School of Engineering, University of Liverpool, Brownlow Street, Liverpool, United Kingdom