Effect of tribochemistry on lubricity of DLC films in hydrogen

Abstract

This paper presents the results of a systematic study demonstrating how controlled or targeted tribochemistry can lead to super-low friction and wear in diamond-like carbon (DLC) films. Specifically, how hydrogen can lead to dramatic reductions in the friction and wear of hydrogen-free DLC films which otherwise experience high friction and severe wear losses. When sliding tests are performed with a hydrogen-free DLC film in an inert gas (like Ar), the friction coefficients were in the range of 0.4 to 0.7 and it wore out rather quickly. However, when the same tests were run under the 1333Pa partial pressure of hydrogen, the friction coefficients were reduced by factors of more than 10 and the wear damage became very difficult to quantify. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses provided strong chemical evidence that the sliding contact surfaces of these films underwent tribochemical reactions with hydrogen effectively passivating dangling bonds of surface carbon atoms, which is believed to be responsible for ultralow friction and wear. In an effort to rule out the possibility of contamination and confirm the pivotal role of hydrogen in lubricity of DLC films, we ran similar tests with the same hydrogen-free DLC in deuterium (an isotope of hydrogen) and achieved essentially the same level of reduction in friction. TOF-SIMS clearly confirmed the formation of a deuterium-rich tribofilm on rubbing surfaces thus confirming the critical role of tribochemistry in super-low friction and wear of DLC.