| Because magnesium and its alloys can be used as biodegradable material for implants, the corresponding research has very important significance in the biomedical field. However, magnesium alloy has poor wear and corrosion resistance and brush-plating on its surface can overcome the shortcoming. In this paper, the microstructure and mechanical behavior of nanocomposite coating reinforced with HAP (Hydroxyapatite) and CNTs (Carbon nanotubes) on the surface of AZ91D Mg alloy have been studied. The Ni-Co was used as the metal matrix, and nano-HAP and CNTs were chosen as the hybrid reinforcements. The composite coating is bright and compact, and it has strong interface binding strength and excellent biocompatibility. So the coating is a new type of potential biomaterial in bone tissue engineering.According to the characteristics of magnesium alloys, its optimal brush plating process was determined and composite coatings with different content and ratio of nano-sized HA particles and CNTs were prepared. The surface and cross section morphology, microstructure and tensile performance of the coatings were analyzed by the means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tensile testing. The results show the hybrid nano-reinforcements is well distributed in the coating, the tensile properties of particle and fiber reinforced composite coatings are better than that of pure nickel and cobalt coating. The tensile properties of the coating are increased with the increase of particle content, while the tensile properties and interface bonding strengths are decreased with the increase of fiber content. In the experimental range, the coating has excellent comprehensive properties such as higher strength and hardness when the concentration of HAP is 5g/L and the concentration of CNTs is 0.1 g/L.Comparing with thermal spraying and vapor deposition etc., there are small temperature changes in the brush plating process. However, there often existed large thermal stress due to bigger thermal expansion coefficient changes. Using ABAQUS finite element software, the thermal residual stresses of the coating during the cooling process are calculated and the simulation of indentation is also conducted to describe the influence of the thermal residual stresses. The results show that the maximum thermal residual stress is increased with the increase of temperature, which has been verified by theoretical calculation and FEM. In the simulation of indentation, for the same impression depth, the higher the initial temperature is, the smaller the pressure is, and so the lower the hardness of the coating is. In order to compare with the above results, the thermal stress analysis and indentation simulation of rough film-substrate systems are also carried out.The results show that the binding strength is greater and the coating hardness is relatively higher for rough film-substrate systems.Cracking process and its stress distribution of vertical crack under thermal residual stresses have been simulated.Using periodic boundary conditions, the finite element model based on graphical vectorization of SEM and TEM images by software coretrace is built, and the tensile properties of coating are simulated. Effects of volume fraction, hybrid ratio and size distribution on the coating are calculated. The results show that when more CNTs are added and the box dimension is bigger, the more uniform the distribution of the particles and fibers are, thus the better the tensile properties of coatings are. The simulation results are consistent with that of the experiment. The mechanical property improvements of the hybrid nanocomposites can be attributed to CTE mismatch between the matrix and the reinforcements, Orowan strengthening, grain refinement, and the load bearing effects.
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