Comparative Study On The Tribological Properties Of Human Tooth Enamel And Three Typical Materials

Human teeth, the important masticatory organ in body, play an extremely significant role in human health. Dental friction and wear is an inevitable lifetime process due to normal oral function. However, most teeth can satisfyingly serve for tens years or even almost centenary, which is mostly attributed to the excellent tribological property of human tooth enamel. It is well known that human tooth enamel has been the hardest tissue in the body after evolution of million years, and has excellent resistance against wear. Knowledge of the wear mechanism of human tooth enamel could provide valuable insights into the scientific understanding about natural biomaterials, the bionics-tribology design, the study of toothwear, and the developments of improved dental materials and oral treatments.The composition of human tooth enamel comprises 96-97% inorganic substances,1-2% organic materials, and 2-3% water by weight. The inorganic substances are mostly hydroxyapatite. The effect of enamel microstructure on its wear mechanism has not been fully understood. Given that human tooth enamel and artificial hydroxyapatite have totally different microstructures, nanomechanical and microtribological properties of human tooth enamel have been studied by comparing it with that of artificial hydroxyapatite in this paper, using nanoindentation and nanoscratch techniques. Moreover, the wear behaviors of human tooth enamel, copper, and ceramic have been investigated and compared using a reciprocating apparatus both in dry condition and in distilled water. Main conclusions are drawn as follows:1.The mechanical property of human tooth enamel is different from that of the artificial hydroxyapatite. Both the hardness and elastic modulus of the artificial hydroxyapatite were higher than those of the enamel under the same normal loading level. With the load increasing, the values of hardness and elastic modulus decrease for the enamel and artificial hydroxyapatite, but the decreasing amplitude is a little bigger for the artificial hydroxyapatite than for the enamel.2.The friction coefficient of either enamel or artificial hydroxyapatite increases with normal load, but the coefficient of artificial hydroxyapatite significantly fluctuates especially under high loads. Compared with artificial hydroxyapatite, the wear volume of enamel is a little higher at low loads but becomes much lower at high loads. The surface lesion of enamel is dominated by plastic deformation at low loads. With the load increasing, sand-shaped wear particle packing happens at the edge of scratch, and no flaky exfoliation traces are observed. Hydroxyapatite crystal grain size is remarkably smaller on the worn enamel surface than on the unworn enamel surface. However, no significant difference appears in the hydroxyapatite crystal grain size between the worn and unworn surfaces of artificial hydroxyapatite, and obvious flaky exfoliation is observed on the surface of scratch at high loads. In conclusion, enamel wear at the nano-level differs significantly from that of artificial hydroxyapatite. and no brittle delamination mechanism occurrs, even at high loads. Enamel microstructure plays an extremely significant role in its microtribological behaviour.3. Compared with ceramic material, the reciprocating sliding wear behavior of human tooth enamel is more similar to that of copper material. For the enamel and copper, both small delamination pits and ploughing traces appear on the worn surfaces and the wear volume is bigger in dry condition than in distilled water. The worn surface of ceramic is characterized mainly by significant brittle delamination. And its wear volume is bigger in distilled water than in dry condition.