Each tooth has two parts: the crown and the root. The dental crown is the entire visible aspect of a tooth in the mouth, while the root is the invisible aspect. The root of each tooth is firmly fixed in alveolar bone by a periodontal ligament, which is a compact connective tissue. Alveolar bone is softer than teeth. When hard particles are bitten with the teeth, the periodontal ligament can serve as a buffer to distribute occlusal loads evenly over alveolar bone to prevent teeth and alveolar bone from overloading. In addition, the surfaces of the tooth neck and alveolar bone are covered with gingiva in the oral cavity. Gingiva is a special oral mucosa and can dissipate a few occlusal stresses [1].

Human teeth possess a unique structure composed of enamel, the dentin-enamel junction (DEJ), dentin, and pulp; each zone is anisotropic. With a hardness of about 360 HV_50g, enamel is the hardest tissue in body, while the dentin is usually considered to be elastic and soft (its hardness is about 60 HV_50g) [2]. The hardness of the DEJ zone varies from 360 HV_50g to 60 HV_50g. Initially, the enamel is exposed to the occlusal surface and chemical environment within the mouth. With aging and various pathological factors, the enamel is gradually ground down by mastication.

The natural dental structure is really ingenious. Whether viewed from a macrostructural (teeth/bone/muscle tissue) or microstructural (enamel/dentin-enamel junction/dentin/pulp) aspects, the dental friction pair is a structural system composed of hard tissue in combination with soft tissue. Such a structure can help to absorb and dissipate occlusal stress and then prevent teeth from lesions caused by overload or impact wear during mastication.

As mentioned above, each tooth consists of outer enamel and inner dentin. The enamel and dentin have different chemical compositions and microstructures. The enamel, composed of 92–96 % inorganic substances, 1–2 % organic materials, and 3–4 % water by weight [3], is the hardest tissue in the human body and has an excellent wear resistance. The dentin is a hydrated biological composite composed of 70 % inorganic material, 18 % organic matrix, and 12 % water by weight [4]; thus, it has a lower hardness and lower wear resistance. In general, the enamel is about 2–3 mm thick and the dentin is 4–5 mm thick. From the viewpoint of surface engineering, the enamel layer acts as a hard surface coating. Such a natural coating not only has a much better wear resistance than the matrix, namely, the dentin, but also has an excellent bonding strength. Between the enamel and dentin is the dentin-enamel junction, a biological interface. It has been reported that the dentin-enamel junction is not a beeline, but a scallop shape, with its convexities directed toward the dentin and concavities directed toward the enamel [5–8]. The enamel rods are arranged in fasciculation and are perpendicular to the dentin-enamel junction. The hydroxyapatite crystals from the enamel and dentin are penetrative and overlap each other within the dentin-enamel junction. Such a constitution and structure could increase the contact area between the enamel and dentin, thereby endowing them with the best bonding strength.

As mentioned above, the enamel possesses a much better wear resistance than the dentin. Microexaminations reveal that the excellent antiwear properties of enamel are mainly attributed to its compact and orderly microstructure.

Human enamel, the most highly calcified tissue in the body, is one of those unique natural substances that still cannot be effectively replaced by artificial restorative materials. The enamel is composed of hierarchical structures. Based on its primary structure, the enamel can be regarded as a fiber-reinforced composite. As described in Chap. 1, keyhole-like rods (6–8 μm in diameter) are embedded in the interrod enamel, the matrix. The rods are over 95 % mineralized, while the interrod enamel is rich in protein and mostly a result of the incoherence of combining crystals of different orientations [9]. Hence, the Young’s modulus and the hardness are lower in the interrod enamel than those in the area of the rods. The rod sheath, a natural coupling agent, locates where the enamel rods meet the interrod enamel; the rod sheath consists of more protein than both the interrod enamel and the rods. Obviously, the primary structure of enamel is a compact alternate arrangement of mineral and organic phases.

Considering the physiological function of enamel, the enamel rod is its functional unit. The rods align in parallel and run approximately perpendicular from the DEJ toward the tooth surface. Each rod consists of tightly packed hexagonal carbonated hydroxyapatite particles, which are covered by a nanometer-thin layer of enamelin and orient along the rod axis [9, 10]. The hydroxyapatite particles are the fundamental hierarchical structure level, which then assemble into nanofibrils and fibers level by level. The fibers have a mean width of 68.3 nm and a mean thickness of 26.3 nm [9]. Finally, the fibers assemble into an enamel rod through a unique arrangement,

An important mechanical function of the composite nature of enamel is related to its antiwear performance. As described above, the interrod enamel is a weaker phase that can be easily worn out. However, the protein-rich interrod enamel could act as a stress buffer for the brittle enamel rods. When occlusal stress is applied on the surface of enamel during chewing, it is carried mostly by the high-stiffness rods, producing little plastic deformation and then low wear loss, while the soft interrod enamel may produce a large deformation and then partially dissipate the stress, preventing the rods from cracking. An additional contribution of enamel rods to the tribological behavior of enamel is attributable to their orientation. Mass [11] pointed out that the variation in the crystallite orientation of prismatic enamels may contribute to optimal dental function through the property of differential wear in functionally distinct regions of teeth. As described in the Chap. 3, the orientation of the enamel rods plays an important role in the wear resistance of enamel, which is related to the alignment of fiber-like apatite crystals and the composite nature of enamel rods.