Mechanical And Tribological Behaviours Of Several Mammal Keratin Materials

mechanical behavior of natural biological materials. Mammal keratin material is a natural biological composite with hierarchical structure from nanometer to centimeter scale. In order to develop the bionic material and bionic tribology system, the structure, mechanical properties and tribological behaviors of the four mammal keratin materials (bovine hoof, bovine horn, sheep hoof and pig hoof), relation between their structure and their mechanical properties, and micro-mechanism of mechanical and tribological behavior were examined in this work., which is projects supported by National Science Fund for Distinguished Young Scholars of China (Grant No. 50025516), by National Science Foundation of China (Grant No. 50275037) and by Special Research Fund for the Doctoral Program of High Education of China (Grant No. 20020183029). The results obtained in this work have provided a new clue and method for developing biomimetic materials and bionic tribology systems. FTIR spectroscope, FTR spectroscope and X-ray diffraction instrument were used for systematically analyze structure and chemical constituent of the four keratin materials. It was found that FTIR spectrogram and FTR spectrogram of the four keratin materials is similar, except for little difference in exceptional peak position. Stretching and deformation vibration on hydrocarbon and amide three band was embodied in FTIR spectrogram. Vibration mode and deformation character of various chemical bonds can be found from FTR spectrogram. α-helix and β-sheet structure can be found in the four keratin materials. There all existed two peaks in X-ray diffractogram of the four keratin materials. This showed that keratin material is a natural biological composite material intervenient between crystalline state and amorphous state. The mechanical behavior of hoof and horn material relate to the microelement content within these materials, especially such heavy metal element as zinc, copper, manganese, and others. The ICP-ACES instrument was used for determining metal elements and their contents and components. It was found that the content of zinc and copper was highest in bovine horn, pig hoof took second place, thirdly was bovine hoof, the content of zinc and copper was lowest in sheep hoof. The content of manganese was highest in pig hoof, bovine horn took second place, and the content of manganese was lowest in bovine hoof. Polarizing microscope was used for analyzing morphological structure of the cross section of the four keratin materials. It was demonstrated that the inside layer of bovine hoof consisted of longitudinal cutin lobule, the outside layer and middle layer were composed of tubules and intertubular material and outside layer was very thin where tubule shape was elongated. Size and shape of the tubule were dependent on the position through the hoof wall thickness. From the outside to the inside through the middle layer, the ratio of the major axis to the minor axis gradually decreased, the tubules gradually change from ellipse shape to round shape and the tubule density gradually reduced. There were not obvious limit between the outside layer and middle layer of sheep hoof and the tubule size of sheep hoof were smaller than those of bovine hoof. There was a smooth band and no obvious tubule structure in middle part of middle layer. There were rough alveolate holes in the inside layer. There is no tubule structure in the outside layer of the pig hoof. The cutin lobule can be clearly observed in the inside layer of pig hoof. There were the tubule structure across the whole thickness of the middle layer, size of the inside layer tubule is much larger than that of the tubule size of outside layer, and tubule shape were similar as ellipse shape; Other than hoof structure, there no appeared tubule in cross section of bovine horn, where same oriented fiber structure can be seen. Tensile mechanical properties of three keratin materials (bovine hoof, bovine horn and sheep hoof) were examined by WDW-10 electron universal testing machine controlled by computer, and effect of experimental conditions andmaterial factors on the mechanical properties was discussed. The results showed that tensile stress-strain curve of the keratin material was similar to those of continuous fiber-reinforced ceramics composite material, which can be described by exponential model and polynomial model whose correlation coefficient was high. Tensile mechanical behavior has close relationship with strain rate. The strain rate used during the test spanned four orders of magnitude from 0.0001s-1 to 0.1s-1. Elastic modulus of bovine hoof wet sample increased 35.4%, ultimate strength increased 45%, and failure strain increased 10.5% from the least strain rate to the highest strain rate. Elastic modulus, ultimate strength and failure strain increased with strain rate according to linear relationships in log-log coordinates. Material factors have important effects on tensile mechanical behavior. The toughness and strength of the three kinds of keratin samples decreased, but plasticity increased with their increasing of moisture content. Mechanical properties of three kinds of keratin samples were different under the same testing conditions. Elastic modulus, ultimate strength and failure strain of the sheep sample were minimal, those of bovine horn samples were maximal, and those of bovine hoof were intervenient between sheep hoof and bovine horn. The mechanical properties of materials from different sampling positions are different for the bovine horn. Both the elastic modulus and ultimate strength were largest at the top position of the horn, and elastic modulus and ultimate strength were least at the lowest position of the horn. Scanning electron microscope analysis on tensile fracture showed that fracture surface of the dry sample was smooth and flat, which attributed to brittle fracture having cleavage features. The fracture surface of the wet sample was very rough and had fiber-pulling off phenomena, which attributed to obvious tough fracture properties. Nanoindenter and micro-hardness tester were used for investigating into nano to micro mechanical properties of the four keratin materials. It was found that elastic modulus, nano-hardness and micro-hardness of the dry sample were all greater than that of the wet sample. Elastic modulus of vertically loaded samplewas obviously greater than that of laterally loaded sample, however there was no obvious difference on nano-hardness and micro-hardness between vertically and laterally loaded sample. The elastic modulus and nano-hardness was decreasing from the outside layer to the inside layer across the all hoof wall. And the decreasing amplitude of wet sample was greater than that of dry sample. The elastic modulus and nano-hardness of bovine hoof surface sample from the different positions had a little difference and had increasing trend from the proximal capitutum locations to the distal contact surface locations following growth direction of hoof wall. Comparative analysis of four keratin materials as for nano-hardness and micro-hardness showed that the elastic modulus, the nano-hardness and the micro-hardness of bovine horn sample were higher than other hoof sample, pig hoof sample take second place, elastic modulus, nano-hardness and micro-hardness of sheep hoof and bovine hoof sample had not obvious difference, those of sheep hoof sample in some degree was higher. It may be due to the difference of hardness of the four keratin samples has relation with chemical composition of materials and metal element content. Dry sliding friction and wear tests were run on a universal micro-tribometer for the examining the effects of the testing conditions and material factors on friction and wear properties of four keratin materials. The tribological behaviors between animal and plant cuticle on frictional behavior was discussed by the use of plant seed cuticle (maize seed epidermis) as comparative analysis. The result showed that friction coefficient of dry bovine hoof sample decreased but volume wear loss increased with the normal load at steady state. Friction coefficient of maize seed epidermis cuticle decreased with load following the linear relationships. Friction coefficient of dry bovine hoof sample had a decreasing trend with increasing sliding velocity at steady state, but volume wear loss increased and basically following a linear relationship. Orthogonal polynomial regression analysis has showed that the effect of the sliding velocity on volume wear loss was larger than that of the normal load for bovine hoof sample within the test range. Sliding velocity had no obvious influence on friction coefficient of maize seedepidermis cuticle. Sliding friction coefficient of both keratin materials and maize seed epidermis cuticle increased with the increasing moisture content. Volume wear loss of bovine hoof sample has not appeared mono-increasing or mono-decreasing trend with increasing moisture content. At the three moisture states, volume wear loss of fresh sample was lowest, volume wear loss of dry sample was highest, and volume wear loss of wet sample was intervenient between that of the dry and the fresh sample. Volume wear loss of both bovine horn and pig hoof were equivalent and smaller, moreover volume wear loss of bovine hoof and sheep hoof was correspondingly larger among the four keratin materials. Friction mechanism of keratin materials was complicated. Friction force consisted of adhesion component, plough component, plastic deformation component and hysteresis component. There existed two main wear mechanisms: the adhesive wear and fatigue cleavage wear. There also existed abrasive wear mechanism for bovine hoof materials during sliding wear. Wear of pig hoof material was mainly due to fatigue cleavage wear.