Transient Formation of Single Layer Diamond During Friction Force Microscopy of SiC-Supported Epitaxial Graphene

Carbon allotropes are crucial to advanced interfaces to control friction and wear because of their unique range of mechanical properties: from diamond's hardness to graphite's lubricity. Friction force microscopy (FFM) is reported for diamond tips sliding on SiC(0001)-supported epitaxial graphene. A sharp friction increase is observed at a threshold normal force, linked to an intermittent graphene rehybridization. Comparing the FFM response of a diamond tip to that of a previously studied silicon tip with a comparable radius reveals a similar abrupt friction increase, though at roughly half the threshold force. Atomistic simulations of SiC(0001)-supported graphene sliding against hydroxylated amorphous carbon (a-C) and silicon oxide show low shear stress at low pressures for both systems. The shear stress increases at higher pressures due to bond formation between graphene and the counterbody. For a-C, the transition threshold shifts to higher pressures, consistent with FFM results. In simulations with high normal pressures, epitaxial graphene undergoes a structural transformation into single-layer diamond, contributing to the abrupt increase in friction. The graphene structure recovers after lifting the a-C counterbody, demonstrating structural robustness under tribological stress. These findings provide insights into the stability of low-friction interfaces between epitaxial graphene and key materials for current micro-electro-mechanical systems (MEMS).