Temperature dependence of single-asperity diamond-diamond friction elucidated using AFM and MD simulations

Complementary experimental (atomic force microscopy) and theoretical (molecular dynamics) techniques were used to investigate friction between diamond-diamond junctions as a function of temperature. The simulation and experimental conditions were designed to correspond as closely as possible. In the atomic force microscopy (AFM) experiments, two microcrystalline-diamond (μCD) AFM tips of differing contact radii were used to examine the friction of diamond (111) and (001) single crystals from 24 to 225 K in an ultrahigh vacuum. At all temperatures, the experimentally determined dependence of friction on load was consistent with the occurrence of single-asperity interfacial friction, where friction is proportional to contact area. In addition, the behavior of the contact was fit well by the Derjaguin-Muller- Toporov continuum model. Friction measurements within a given series were highly repeatable; however, as is typical with AFM measurements, there was some variation in measurements taken from different regions of the sample and with different tips. Interfacial shear strength, or the intrinsic resistance to sliding, decreased slightly with increasing temperature for both surfaces. To shed additional insight into the AFM results, MD simulations were performed with the diamond single crystals of the same orientation. The calculations also show that the average friction force decreased slightly as the temperature increased for both diamond surfaces and for all sliding directions. Both AFM and MD results agree with the numerical analysis of friction as a function of temperature published by Sang et al. (Sang, Y.; Dube, M.; Grant, M. Phys. Rev. Lett. 2001, 87, 174301).