Effect of water content on the nanomechanical properties and microtribological behaviour of human tooth enamel

Human tooth enamel exhibits excellent wear-resistance in mouth. It consists of 92–96% inorganic substances, 1–2% organic materials, and 3–4% water by weight. The contribution of water content to its wear mechanism has not been fully understood yet. In this paper, nano-indentation and scratch tests were performed on human tooth enamel after different drying time to study the effect of water content on the nanomechanical properties and microtribological behaviour. The recovery of enamel's tribological properties in water was also investigated by immersing the dried enamel specimens in deionized water. Results showed that the water content within enamel had a significant influence on its nanomechanical properties and microtribological performance. The nanoindentation hardness of enamel increased with the increase of drying time, but its elastic modulus decreased. The friction coefficient and wear loss of enamel also increased. Moreover, an increasing fluctuation occurred to the friction coefficient of enamel as the drying time increased. Microscopic examinations showed that a few micro-voids appeared on the worn surface of dried enamel, and the voids became more and more obvious with the increase of drying time. Additionally, both the friction coefficient and wear loss of the dried enamel specimen were found to significantly decrease after being immersed in water. These results indicated that the water content within human tooth enamel could reduce the value and fluctuation of friction force on the surface and protect the surface from wear damage. The anti-wear properties of enamel decreased with the decrease of its water content. The results would be helpful to extend an understanding of the relationship between the enamel chemical composition and its tribological properties and then develop new dental materials.