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, Si3N4 and SiC. Understanding of the differences and similarities of Si3N4 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 Si3N4 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 Si3N4 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 Si3N4 than for SiC. Finally, we concluded that this novel understanding of the differences and similarities of the tribochemical reaction mechanisms of Si3N4 and SiC is likely to contribute to the design of sliding materials with high performance and high wear resistance.