Effect Of Internal Organic Matter And Water On The Micro-friction And Wear Behavior Of Human Tooth Enamel

As an impotant mastication organ, good tribological properties are the guarantee of normal function og huamn teeth. Human teeth possess a unique structure composing of enamel, dentin and pulp. Enamel, the hardest tissue in human body, has excellent anti-wear properties. It comprises96-97%inorganic substances,1-2%organic materials, and2-3%water by weight. It should be noted that human tooth enamel is one of those unique natural substances which still cannot be substituted effectively by artificial restorative materials. Hence, understanding of the effects of chemical composition and microstructure of enamelon its friction and wear behavior would help deepen the scientific understanding of tribological behavior of natural biological friction pair, provide important theoretical basis and key technical support for bionic tribological design, and then develop bionic dental restorative materials.In this paper, the effect of internal organic matters and water on the micro-friction and wear behavior of human tooth enamel has been studied using a nano-indentation/scratch tester. The main objective was to exploring the contribution of organic matters and water to the tribological properties of human tooth enamel. Main conclusions were drawn as follows:1. After the extraction of organic matter by sodium hypochlorite, the nanoindentation hardness of human tooth enamel did not change significantly. During the process of extraction, the elastic modulus of enamel rapidly decreased initially, and then decreased slowly. Finally, the elastic modulus kept stable. With the extraction time increasing, the friction coefficient of enamel decreased a little, but the scratch depth and the wear loss on the surface of enamel increased. Further microscopic examinations showed that the size of hydroxyapatite particles within the enamel increased after the sodium hypochlorite extraction.2. Water content within human tooth enamel had a significant influence on the micro-friction and wear behavior of enamel. With the drying time increasing, the elastic modulus of enamel decreased, but its hardness increased. Furthermore, as the drying time increased, the friction coefficient of enamel surface increased accompanied with obvious fluctuation, and the scratch depth on the surface of increased, resulting in more wear. Also found was that some voids appeared on the surface of enamel after the drying process, and the voids became more and more obvious with the increase of drying time. Obviously, the water within enamel could provide a lubricating effect and then reduce the friction and wear of the enamel. In addition, the friction and wear properties of the dried enamel were found to recover to a certain degree by being immersed in water. It could be inferred that when the dried enamel was immersed in water, the water attached to enamel surface could combine with the hydroxyapatite crystals within the surface layer of enamel, forming hydroxyapatite hydrate. As a result, the microstructure and tribological properties of the dried enamel was restored in some degree.