1. Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education and Department of Applied Physics, School of Science, Northwestern Polytechnical University, Xi'an 710072, People's Republic of China
2. School of Mechanical Engineering, P.O. Box 552, Northwestern Polytechnical University, Xi'an 710072, People's Republic of China
3. State Key Lab of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People's Republic of China
4. State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China