Atomistic insights into diamond-rock contacts through molecular modelling

Abstract

Understanding friction and wear properties at diamond-rock interfaces is crucial to increase the energy efficiency of not only drilling operations but several other processes. A significant proportion of the current research focuses on experimental work and continuum-level computation studies, which limits the fundamental understanding of the physicochemical mechanisms at these interfaces. Molecular modelling has been previously used to study tribological interfaces, and here, in particular, it can shed light on atomic-scale behaviour taking place at diamond-rock contacts to aid the development of new engineered surfaces and the design of new lubricants that can meet current and future demands.

In this thesis, rock drilling tribology is studied using nonequilibrium molecular dynamics (NEMD) and with a reactive forcefield (ReaxFF). Firstly, macroscale tribometer experiments reveal that a diamond-limestone (mostly calcite) gives significantly higher friction coefficients compared to granite (mostly quartz) in various humidity levels. NEMD simulations uncover the physicochemical mechanisms that lead to higher kinetic friction in the diamond-calcite system mainly due to increased interfacial bonding mostly formed through chemisorbed water molecules trapped between the tip and the substrate. For both rock types, interfacial bond formation increases exponentially with pressure, indicative of a stress-augmented thermally activated (SATA) process. The friction force is shown to be linearly dependent on the number of interfacial bonds during steady-state sliding. The agreement between the friction behaviour observed in the NEMD simulations and tribometer experiments suggests that interfacial bonding also controls diamond–rock friction at the macroscale. We anticipate that the improved fundamental understanding provided by this study will assist in the development of bit materials and coatings to minimize friction by reducing diamond–rock interfacial bonding.

Secondly, NEMD simulations of diamond-hexadecane-quartz systems show higher kinetic friction than diamond-water-quartz systems, contrary to the general understanding of lubrication at tribological contacts. This observation was corroborated by diamond-granite (mostly quartz) macroscale tribometer experiments. As previously, direct interfacial bonding between the asperity and substrate forms more readily in the diamond-hexadecane-quartz system due to little to no passivation of the quartz, which occurs with water. Again the friction force is shown to be linearly dependent on the number of interfacial bonds during steady-state sliding, and interfacial bond formation increases exponentially with pressure, suggesting that this process can be explained with pre-existing SATA models. When water is added to the diamond-hexadecane-quartz system, the kinetic friction decreases. We expect that the development of water-based lubricants will not only be more environmentally friendly but also improve the friction behaviour of diamond-rock interactions through mitigation of interfacial bonds.

Thirdly, we show that diamond wear on α-quartz involves atom-by-atom attrition of carbon atoms, which is initiated by the formation of C-O interfacial bonds, followed by C-C cleavage, and either diffusion into the substrate or further oxidation to form CO2 molecules. The presence of interfacial water molecules, which dissociate to form surface hydroxyl groups, passivate the sliding surfaces and markedly reduce both interfacial bonding and wear. At low loads, the initial wear rate of the diamond tip increases exponentially with temperature and normal stress, consistent with stress-augmented thermally activated wear models. At higher loads, the initial wear rate becomes less sensitive to the normal stress, eventually plateauing towards a constant value. The initial wear rate versus normal stress data can be accurately described over the entire load range using the multibond wear model. Over longer sliding distances, wear also occurs through cluster detachment via tail fracture. During this phase of the simulations, wear becomes approximately proportional to the sliding distance and normal load, which is consistent with the empirical Archard model. The normalised wear rates from the simulations are within the experimentally-measured range for single-crystal diamond on quartz glass surfaces.

Finally, in the same aforementioned system, quartz undergoes a phase transformation to a stishovite-like phase. This phase is formed through a three-step process: firstly, rupture of Si-O bonds, then a steady-state period, and finally the reformation of these bonds. Bond reformation only takes place when no water is present, and at high loads (>35 nN) and pressures (>4.5 GPa), the net number of Si-O bonds had increased at the end of the simulation when compared with the start. This phenomenon also becomes more apparent with increasing temperatures. Again, this phase transformation presents itself as a SATA process, with mechanochemical activation of Si-O bonds through sliding of the diamond asperity since this phase does not present itself with an absent asperity. We determine that the rate of bond rupture and activation volume (V) both decrease when water is present, and subsequently the activation energy (Ea) increases significantly. Mitigation of this phase transformation from α-quartz to an stishovite-like region should be an effective strategy to improve the tribological performance of drill bits and other diamond tools during SiO2-rich rocks.