Tribochemical insights into the carbon film formation induced by metallic species derived from layered double hydroxide nanoadditives

Layered double hydroxides (LDHs) have demonstrated excellent tribological performance in various studies. However, there remains a fundamental knowledge gap concerning the chemical composition of tribofilms across the cross-section. In this study, we seek to elucidate this question by tribologically testing various binary LDHs. It was found that five out of eight LDHs, denoted as CoAl-, CoFe-, MgAl-, NiFe-, and ZnAl-LDHs could induce hierarchical protective tribofilms due to the formation of rigid oxide layers and in situ carbon-based films. The five tribofilms helped to reduce wear loss and friction by approximately 90% and 20%, respectively. Remarkably, the Co-based tribofilms showed the best antiwear performance because of the thick carbon tribofilm formation and the strong bonding at the metallic/tribo-oxide interfaces. The formation mechanisms of the carbon tribofilm and its characteristic nature were evaluated and revealed by scanning transmission electron microscopy and electron energy loss spectroscopy. Meanwhile, the intriguing nano-scale mechanical properties, i.e., the hardness and reduced elastic modulus of different tribofilms, were well-demonstrated by nanoindentation and nanoscratch experiments. The correlation between chemical structures and mechanical properties of the tribofilms derived from LDH lubricant additives was first conducted and discussed, unveiling the tribochemical and lubrication mechanisms between the LDHs and sliding surfaces.