First-Principles Molecular Dynamics Study of Silicon-Based Ceramics: Different Tribochemical Reaction Mechanisms during the Running-in Period of Silicon Nitride and Silicon Carbide

Because silicon-based ceramics show superlow friction in aqueous environments, these materials have attracted much attention for the development of water lubrication systems. The superlow friction is thought to be derived from a tribolayer that is formed through complicated tribochemical reactions during the running-in period. Atomic-scale insights into the tribochemical reactions during the running-in period are crucial to the development of sliding materials with superlow friction and high wear resistance. This study was focused on the running-in period of two silicon-based ceramics, Si 3 N 4 and SiC. Understanding of the differences and similarities of Si 3 N 4 and SiC during the running-in period is expected to identify principles for the design of water lubrication systems with superlow friction and high wear resistance. We performed self-mated sliding simulations of Si 3 N 4 and SiC using first-principles molecular dynamics. We discovered that a lower tribochemical reaction energy barrier and stable highly coordinated Si-atom intermediates favored the tribochemical reactions of Si 3 N 4 versus SiC. This proposed nanoscale mechanism is in good agreement with previously reported experimental results in which the running-in period has been shorter for Si 3 N 4 than for SiC. Finally, we concluded that this novel understanding of the differences and similarities of the tribochemical reaction mechanisms of Si 3 N 4 and SiC is likely to contribute to the design of sliding materials with high performance and high wear resistance.