Gianpietro Moras (Fraunhofer IWM)
Tribology of diamond and silicon: atomic-scale insights from computers simulations
Tribological processes involving diamond and silicon are very common in technology. Chemical-mechanical polishing of diamond films and their ultralow friction in presence of water, wear of diamond-coated cutting tools, multi-wire sawing of silicon for photovoltaic applications, wear of MEMS and atomic force microscopy tips are just a few examples. In all these processes, friction and wear are strongly influenced by material transformations that are not fully understood as they occur at buried interfaces and are hardly accessible by in situ experiments.
In this seminar I will present the results of atomic-scale computer simulations that contribute to shed light on the tribological evolution of these two crystals. Interestingly, in spite of their common crystal structure and brittle behaviour, diamond and silicon can respond in completely different ways to tribological loads. First, I will show how both diamond and silicon can undergo shear-induced amorphization. The pressure-dependence of the amorphization rates is however opposite in the two materials. This is due to the opposite volume change upon shear-induced amorphization: carbon expands while silicon becomes denser. This can be related to the opposite slopes of the melting curves of diamond and diamond cubic silicon in their pressure-temperature phase diagram. Finally, I will present atomic-scale mechanisms for ultralow friction in water-lubricated diamond. It is known that water dissociative chemisorption can lead to surface passivation and low friction in both diamond and silicon. Our simulations show that water can also induce aromatic surface reconstructions that are responsible for ultralow friction in diamond and diamond-like carbon.