1. Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, Sichuan Province, People's Republic of China
2. School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, United Kingdom
3. Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom