Advanced Materials Research Vols. 41-42

Abstract: Nano-scale Au/Cu multilayers were investigated by nano/microindentation. It was found that the hardness of the multilayers increases with decreasing individual layer thickness (λ), and shear band deformation can occur more easily in the multilayer with small λ. For comparison, the same experiments were also performed on Cr/Cu multilayers with the same layer structure. The results show that the Cr/Cu multilayer can be more effective in resisting shear band deformation than the Au/Cu multilayer. Finally, the λ dependence of shear band deformation and the difference between plastic deformation behaviors of the two multilayers were analyzed based on dislocation plasticity.

Abstract: Articular cartilage is a semitransparent elastic material that covers on the two articulating bones in synovial joints. It acts as a cushion between the bones that transfers loads from one to another while attenuating dynamic stresses and providing almost frictionless contact surfaces for normal use of synovial joints without pains. Osteoarthritis causes a chronic joint pain and it is mainly due to malfunction of articular cartilage. The mechanical function of articular cartilage is derived from its unique microstructure. Therefore, study of the relationship between the mechanical function and microstructure of articular cartilage comprehends the aetiology and pathology of osteoarthritis. Confocal microscopy permits studying the internal microstructure of buck biological tissues without tissue sectioning and dehydration. This provides a way to study the relationship between the mechanical function and microstructure of articular cartilage. Using a fibre optic laser scanning confocal microscope, this study examines the pathological status of articular cartilage in relation to the mechanical function and 3D collagen network of articular cartilage. The results show that the 3D collagen structure and the mechanical function are different between normal and arthritic cartilage. Loss of the integrity of the 3D collagen network is closely related to cartilage softening.

Abstract: In this work, deformation of monocrystalline silicon (Si) under nanoscratching was investigated using transmission electron microscopy (TEM). The results indicated that no fracture occurred during nanoscratching with loads ranging from 1 to 6 mN. The damaged regions induced by nanoscratching included an amorphous Si region and a damaged crystalline Si region. Detailed TEM analyses revealed that at the lowest load of 1 mN no dislocation was observed in the damaged crystalline region, and only stacking faults were observed at the boundary between the damaged crystalline Si and amorphous Si. Dislocations started to nucleate along (111) planes and penetrated into the bulk Si when the normal load was increased to 2 mN and above. Defects perpendicular to the scratched surface were initiated when the load was greater than 4 mN. The density of dislocations also increased rapidly with the increase of the applied load.

Abstract: The effects of cyclic loading on contact damage in curved bi-layer systems are investigated. Dome structures consisting of glass shells, filled with epoxy resin, simulate the essential structure of monolithic all-ceramic dental crowns on natural tooth dentine. Cyclic loading, with only a vertical component, was carried out with the Multi-Functional Chewing Simulator (Willytec. Munich, Germany). The specimens were tested by indentation with hard spheres of tungsten carbide, with the load applied axially at the apex of the dome. This project reports some new results on the effect of cyclic loading on curved bi-layer systems. In addition, the effect of aqueous environments is addressed. In both air and water tests, observations taken throughout the cyclic loading course indicated that the outer cone cracks inhibited the propagation of radial cracks. Results confirm that crack initiation occurred more rapidly in wet conditions of testing, emphasizing the influences of the moist environment of the oral cavity. Furthermore, the experiments took into account the evolution of inner cone cracks observed in wet cyclic loading tests. The new results are important since nearly all-dental crowns exhibit some curvature. The implications of the results on the failure of dental crowns are discussed.

Abstract: Two available strength data sets of single-walled and multi-walled carbon nanotubes are analysed, and the effects of sample sizes on their tensile strengths are investigated. A minimum information criterion is applied to determine the optimal strength distribution. The results show that, in contrast to a two-parameter Weibull distribution, lognormal distribution seems to be a more suitable choice. A simple extrapolation of classical Weibull statistics to nanoscales may result in overestimation on the tensile strength of carbon nanotubes.

Abstract: This paper summarizes some of the recent advances that have been made as a result of contact damage tests on bi-layer structures containing one brittle layer on a polymeric support base. The effects of indenter modulus (hard/soft indenters) and of the sate of loading (load location reference to the specimen axis of symmetry) on contact damage in bi-layer systems were investigated. Convex specimens having curvature of 12 mm inner coating diameter and 1mm thick were produced, and loaded along the axis of symmetry and off axis at 45o using flat indenters of six different moduli. The influence of indenter modulus on radial crack initiation and damage evolution was examined in respect to the load location, with particular attention paid to the relevance of such damage to lifetime-limiting failures of biomechanical layered systems. The results of this study illustrates that the fracture behaviour of brittle layered structures is not dominated by certain parameters. Critical loads for initiation of radial cracks are sensitive to indenter modulus (hardness) but not sensitive to load location. Load location plays an important role in crack propagation and subsequent damage patterns, especially at specimen margins.

Abstract: Hydroxyapatite (HA) powder was synthesized by a sol-gel method with Ca(OH)2 and H3PO4 as reactants. The HA granules were then coated with TiH2 powder using a mechanical mixing method. The HA-TiH2 material system produced HA-Ti composites after hot-pressing at 1050°C. The HA-Ti composites are mainly composed of HA and Ti, with small amounts of Ca2P2O7 and Ca3(PO4)2 phases. Fracture toughness and bending strength are 2.4 MPa·m1/2 and 54.3 MPa, respectively for the HA-20vol%Ti composite, higher than those of the pure HA ceramic. The improvement in properties is because of the unique 3D network structure of Ti, which is an ideal reinforcement structure for the weak and brittle HA. According to ISO/TR 7405-1984, hemolysis test was performed to evaluate the blood compatibility of the material. The results show that the hemolysis rate of the HA-20vol%Ti composite is 0.56%. Relative growth rates (RGR) of L-929 cells soaked after 6 days in the HA-20vol%Ti group, pure Ti group, black group and pure Pb group were 132%, 100%, 90% and 6% respectively, while the level of cytotoxicity was grade 0 in HA-Ti composite group. These results imply that the HA-20vol%Ti composite has good biocompatibility and bioactivity.