2, North Carolina State University, Raleigh, North Carolina, United States
The addition of nano-particles to liquid lubricants often leads to a reduction in both friction and wear rates. While the lubricating properties of nano-particles are well documented, the detailed physical mechanisms remain to be fully explored. Results from prior experiments (Z. Liu, et al., RSC Advances 5, 78933 – 78940 (2015)) suggest that nano-diamond charge, as measured by the Zeta potential, can have a large influence on tribological performance. Using molecular dynamics simulations, we have characterized the interaction between charged nano-diamond particles and a gold surface in water. Two atomic models of octahedral nano-diamonds in an aqueous solution were created that mimic the negatively and positively charged experimental nano-diamonds, one with chemisorbed carboxyl groups (COO-) and Na+ counter ions, and one with chemisorbed amino groups (NH3+) and Cl- counter ions. To explore the influence of particle–surface electro-static interactions, the simulations were carried out with and without induced electrostatic forces between the nano-diamonds and the gold substrate. For both types of nano-diamond, the electro-static interactions enhance surface adhesion and increase the applied force needed to slide the nano-diamonds along the gold surface. However, the magnitude of these effects and their adhesion mechanisms were found to depend on the nano-diamond surface groups. The simulations predict that the positively charged nano-diamonds are both more strongly adhered to the gold (by a factor of almost three), and require a larger force for sliding (by a factor just greater than two). These results are consistent with the interpretation of the prior experimental studies from Liu et al. The relatively large size of surface carboxyl groups allows Na+ counter ions to reside in a thin water layer between the nano-diamond and gold substrate, which partially screens the nano-diamond/gold electrostatic interactions. In contrast, the positively charged nano-diamonds sit closer to the gold surface without the interface water layer and associated electro-static screening from the counter ions. This difference in electrostatic screening and in the thin water layer results in the different sliding and adhesion behavior.