Process Mechanics And Mechanical Properties Of Surface Biomimetic Structures And Thin Film Materials

Laser surface micromachining and surface coating are often used to improve the biological activity and mechanical properties of medical metal alloys such as titanium and magnesium. First, surface temperature and stress fields during nanosecond laser micromachining of titanium alloy were calculated using finite element method. In the finite element simulation, laser heat source was presumed to be gaussian distribution, the influences of the laser power density, pulse number on the temperature and stress fields of the ablation crater were studied. When the pulse width, wavelength and frequency are constant, the optimal laser pulse intensity is 11.5×105W/cm2 and the pulse number is double pulses. In addition, the influences of the crater size and fractal dimension of laser micromachining surfaces on stresses at different directions and wear volume under different wear conditions were also studied. The optimal dimension and space for round pits are 25μm,50μm respectively, and the fractal dimension is 1.64. The studied result will have important theoretical significance in practical surface micromachining.Secondly, according to the theoretical investigation and experimental results at early stages, processing mechanical properties and indentation mechanical properties of the (CNTs+HA)/NiCo nanocomposite coating prepared by brush plating on the surface of magnesium alloy were simulated by FEM. The influences of the plating temperature, film and intermediate layer (HA) thickness, substrate surface roughness on the thermal residual stress and indentation mechanical properties of film-based systems (including the film-substrate system with pre-crack) were studied. Therefore, the optimized brush plating process was brought forward. For the film-substrate system with pre-crack, when the plating temperature is 40～50℃, coating thickness is 4μm, HA transition layer thickness is 3.5μm, the thermal residual stress of film-substrate system is 0.675GPa, the modulus and hardness of the coating are 17GPa and 210GPa separately, the interfacial bonding strength is 2.4GPa. Besides, when the interface bonding strength is greater than the critical stress of 2.4GPa, the crack will propagate along the interface of the coating and substrate. The present calculation results are good for design and optimization of the plating coating.Finally, based on TEM images of actual microstructures for the biocompatible nanocomposite coatings, the crack propagation path in the coating was simulated by using ANSYS element birth/death techniques. Besides, the crack propagation path and stress intensity factor at crack tip in the coating were compared under the condition of different hybrid ratios. It is concluded that the crack propagates along the particle boundaries, the stress intensity factor and tensile mechanical properties increase with increasing hybrid ratio for CNTs and HA in the nanocomposite coating. On the other hand, the biocompatibility of the coating has been verified by the biocompatibility experiment.