Andreas Klemenz

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)

Behaviour of materials is governed by physical phenomena that occur at an extreme range of length and time scales. Computational modelling requires multiscale approaches. Simulation techniques operating on the atomic scale serve as a foundation for such approaches, providing necessary parameters for upper-scale models. The physical models employed for atomic simulations can vary from electronic structure calculations to empirical force fields. However, construction, manipulation and analysis of atomic systems are independent of the given physical model but dependent on the specific application. matscipy implements such tools for applications in materials science, including fracture, plasticity, tribology and electrochemistry.

Graphite represents a promising material for solid lubrication of highly loaded tribological contacts under extreme environmental conditions. At low loads, graphite’s lubricity depends on humidity. The adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies. To explore the humidity dependence and validate the adsorption model for high loads, a commercial graphite solid lubricant is studied using microtribometry. Even at 1 GPa contact pressure, a high and low friction regime is observed - depending on humidity. Transmission electron microscopy reveals transformation of the polycrystalline graphite lubricant into turbostratic carbon after high and even after low load (50 MPa) sliding. Quantum molecular dynamics simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon, while low friction originates in molecular water films on surfaces. In this work, a generalized adsorption model including turbostratic carbon formation is suggested.

Thermal conductivities of single- and double-wall carbon nanotubes in contact with foreign atoms on their surfaces are investigated by means of molecular dynamics simulations. Small amounts of atoms on the surfaces of single-wall nanotubes drastically reduce their thermal conductivity, while the conductivities of the inner walls of double-wall nanotubes which are only weakly coupled to the outer walls are retained even for large amounts of atoms on the surfaces. Based on the simulation results, an estimation for the conditions under which an enhancement of thermal conductivities can be expected is presented.