State-of-the-art ab-initio simulation unveils ways to reduce friction and wear

Famed for their outstanding mechanical properties, biocompatibility, and chemical stability, diamond and diamond-like carbon (DLC) have found widespread use in industries including electronics, optics, and medical devices. DLC in particular has been employed as a coating to reduce friction and protect from wear between contacting surfaces, and diamond is indispensable in cutting applications. However, open questions remain around its tribological properties at smaller scales.

Motivated by the need to establish “a clear understanding of basic chemical events responsible for diamond-based materials’ frictional and wear performances”, a group of Italian researchers undertook a simulation-based study into the interaction between diamond and one other material – silica. Silica was chosen because diamond makes direct contact with it in many applications – either as an amorphous native oxide formed on silicon substrates, or perhaps counterintuitively, when the much softer silica is used to polish ultrahard diamond films.

In the paper published in Carbon [DOI: 10.1016/j.carbon.2022.11.074], the authors write that “regardless of [prior] efforts, the comprehension of silica-driven diamond wear remains at a preliminary stage.” Their main aim was to identify the critical chemical processes that occur when silica and diamond interact tribologically (i.e., via friction). To do this, they analysed the effect of surface facets and chemical groups commonly produced at the interface between the two materials.

The computational tools used included static Density Functional Theory (DFT) and modified, large-scale ab-initio Molecular Dynamics (AIMD) simulations. They successfully modelled two slabs of amorphous silica, with surfaces that have either slightly higher or slightly lower silanol density than average (as measured experimentally). The resulting disordered structure, they say, produces “models which are more realistic representations of silica surface compared to models present in literature based on un-reconstructed crystalline surface.” For the diamond surface, they chose to model C(111) – both reconstructed and unreconstructed, C(110), and C(001). They also modelled C(111) passivated by water and oxygen, and at different levels of hydrogen passivation i.e. 0%, 50% and 100% for C(111), 25% and 50% for C(110), 50% for C(001).

These twelve interfaces formed the basis of their simulation, which they describe as “realistic and accurate in silico experiments”. Through these experiments, they found that dangling bonds on diamond surfaces are primary sources of friction. The authors suggest that they catalyze the formation of interfacial chemical bonds between the silica and diamond surfaces, which drives up the resistive force. As a result, “reducing the dangling bond density by hydrogenation” could be an “efficient way to reduce the adhesion and friction of silicon oxide on diamond.”

Among the diamond surfaces the team investigated, silica-driven wear of diamond was observed on just one facet, C(110). This confirms previous experimental and computational findings, but the authors say that longer simulation times would be needed to “observe wear events on the other surface orientations, where the energy cost for C detachment is higher.” In terms of passivation, the simulations show that the degree of passivation matters more to the lubrication efficiency than the chemical species; the higher the surface coverage, the lower the interfacial adhesion and friction. In general, hydrogen passivation of the diamond was found to be the most effective approach to reducing friction and wear – it worked for all of the diamond surface orientations considered in the study. The authors also observed oxygen atom transfer from silica to diamond C(110), which forms stable C=O double-bonds. They propose that over a longer time period, this C=O group “may further oxidize, leading to carbon dioxide formation, a standard product of diamond CMP [chemical–mechanical polishing].”

They conclude, “We believe that the insights presented in this work can help the engineering of better low-friction and wear diamond–silica interfaces that are relevant for many important technological applications.”


Michele Cutini, Gaia Forghieri, Mauro Ferrario and Maria Clelia Righi. “Adhesion, friction and tribochemical reactions at the diamond–silica interface,” Carbon 203 (2023) 601–610. DOI: 10.1016/j.carbon.2022.11.074