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Nanodiamond particles (NDPs) have been considered as a potential lubricant additive to various tribological applications, such as water lubrication systems. In this study, the tribological properties of silicon carbide (SiC) lubricated by NDPs dispersed in water are investigated utilizing the ball-on-disk tribometer. It is found that the slight addition of NDP to water (i.e., 0.001 wt%) can distinctly accelerate the running-in process, which is necessary to achieve a friction coefficient (μ) as low as 0.01. This study also discusses two NDP functional terminations —hydroxyl and carboxyl. It is demonstrated that the use of carboxyl-terminated NDP over a wide range of concentration (0.001 wt%–1 wt%) yields a low friction force. In contrast, the ideal effective concentration of hydroxyl-terminated NDP is considerably limited because agglomeration in this material is more probable to occur than in the former. Meanwhile, when utilizing NDPs, the input friction energy (Pin, defined as the product of sliding speed and applied load) is found to have an essential function. Several sliding tests were implemented at various Pin values (50–1,500 mW) using carboxyl-terminated water-dispersed NDPs. It was observed that the μ and wear decreased with increasing Pin when 200 mW < Pin < 1,500 mW. However, when Pin < 200 mW, low friction with high wear occurs compared with the resulting friction and wear when pure water is used.

Effect of carbon nanohorns (CNHs) as additives on tribological properties of self-mated silicon carbide (SiC) in water was investigated. Friction coefficient could decrease with increasing concentration of CNH in water, and minimum friction coefficient of 0.04 was obtained with water containing 1.00 mass% of CNH in severe contact condition. It was found that CNHs form lubricious carbon films on the rubbed surfaces. Formation of carbon film was controlled actively by introduction of laser-surface texturing to substrate, resulting that standard deviation of friction coefficient decreases more than 50% as compared to that without texturing.

Low, stable friction, a higher load-carrying capacity, and lower leakage of sealed blood are required for mechanical seals in implantable ventricular assist devices. One solution is to apply surface texture to the mechanical seal surface, consisting of self-mated silicon carbide. However, the effect of surface texture on the frictional properties of mechanical seals under blood sealing conditions has not yet been studied. Therefore, this study aimed to clarify the effect of surface texture on fundamental frictional properties of mechanical seals and to propose design concepts for mechanical seals in ventricular assist devices. The results show that surface texture increases the critical load of mechanical seals, although it also causes periodic peaks in friction. Further, it was found that surface texture induces the formation of denatured protein aggregates on the sealing surface, also inducing periodic surface friction peaks. The frictional properties of the mechanical seals were stabilized by creating small, dispersed concave features with wet blast fabrication, followed by coating with diamond-like carbon. Lower and more stable frictional properties were thus achieved, while simultaneously ensuring a higher critical load.

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