1. Professor, Institute of Tribology, Hefei Univ. of Technology, Hefei 230009, People’s Republic of China.
2. Lecturer, Faculty of Georesources and Materials Technology, RWTH Aachen, Brunsstraße 10, Hamburg D-21073, Germany (corresponding author). E-mail: andreas.momber@t-online.de
3. Researcher, Institute of Tribology, Hefei Univ. of Technology, Hefei 230009, People’s Republic of China.