Research On Micro Mechanical And Frictional Behaviors Of Human Fingernails

The fingernails are generally used for a variety of functions including protecting, foraging, and gripping. The achievement of these functions depends on the structure and physical properties of fingernails. However, the nails are subjected to large numbers of different extents of deformations and traumas during the processes. Water plays an important role in the mechanical properties of nails. Therefore, the investigation of the micro mechanical and frictional properties of fingernails can not only enrich the knowledge of bio-tribology, but also provide suggestions for the care and maintenance of fingernail as well as design of bionic composites.With a scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), the microstructure and component of fingernail was characterized. The micro mechanical and frictional behaviors of fingernails were investigated by an optical microscope (OM), a microhardness indenter and a nano-indentation/scratch tester (NHT/NST). Finally, the effects of microstructure and water on the performance of the fingernails were discussed based on the comparison with the chicken claw keratin. The main conclusions of the research can be summarized as following:1. Water in the fingernails plays an extremely important role in its mechanical properties. With the water in hydrated fingernails evaporating in air, the microhardness of fingernails shows a quick increase in the first 100 min and levels off after 200 min. Finally, the microhardness of fingernails in the fully dehydrated state is 2-3 times larger than that in the hydrated state. However, the hardness of fingernail presents obvious anisotropy on the different sections. In the dehydrated state, the microhardness of surface and cross section samples are 156 MPa and 205 MPa.2. Owing to the special orientation of filament in the intermediate layer of fingernails, the friction on the cross section of fingernail shows an anisotropic behavior. Either on the dehydrated or hydrated fingernails, the friction coefficient along the parallel direction of filament is lower than that along the vertical direction of filament. The water in fingernails reduces the fluctuation of friction force on surface and protects the surface of fingernails from scratch damage. The nail enamel and varnish could protect the nail from scratch damage to some extent.3. Water is very important for the protection of nails. Owing to the swelling of fingernail matrix and recovery of fibers, the indents and scratches on the fingernail samples could be recovered by dipped in water within 10 min. Compared to normal force, the fingernail is more easily damaged by shear force. The recovery of the deformations in water reveals obvious anisotropy on the different sections of fingernails. The recovery speed of the indents and scratches on the surface of fingernails is the fastest, and the slowest recovery happens for these deformations on the longitudinal section.4. The outer layer of chicken claw is sheet structure, and the inner layer is amorphous. From outside to inside, the content of sulfur gradually decreases. As a result, the nano-hardness and elastic modulus also decrease. The indentations on the claws could be fully recovered after the samples were dipped in water for 30 min, which is much slower than that of nails. The reason may be attributed to the different structure and composition of nails and claws. Such behavior is the result of adaptability to nature.