Design And Impact Resistance Of HA/ZrO₂ Gradient Coatings

Due to its favorable biocompatibility and mechanical properties,plasma sprayed HA coatings on the titanium alloy substrate has been accepted as one of the most promising material for artificial joint implant.However,HA coating has a problem of poor interfacial bonding strength and easy falling off from the titanium alloy substrate when HA coating is subjected to impact loading,which reduces the service life of artificial joint implant.Therefore,improving the properties of HA coating materials to increase their bonding strength and impact resistance is of great significance for the study of biological implant materials.Using finite element software to simulate the change of the Mises stress on the coating surface with different structures when coating is subjected to impact loading.On this basis,pure HA coating,HA/ZrO₂ coating and HA/50wt%ZrO₂+50wt%HA/ZrO₂ gradient coating?named H1,H2 and H3?were deposited on titanium alloy substrate by plasma spraying.The microstructure,phase composition,tensile bonding strength and interfacial properties of HA gradient coating was characterized by means of SEM,XRD and tensile test.The biological activity of the coating was studied by immersion test in simulated body fluid.The impact resistance performance of the three coatings was studied by the falling ball impact test,the impact failure mode and failure mechanism of the gradient coating are analyzed.The main work and achievement are as follows:?1?It is shown from the results of numerical simulation that when the coating thickness is the same,the gradient structure can reduce the Mises stress value after the impact load on the coating surface,and the more coating layers,the smaller the Mises stress value.Gradient coating can also effectively slow down the stress abrupt change between HA coating and titanium alloy substrate.Provided that the number of layers and the composition of the coating remain unchanged,the Mises stress also increases with the increase of coating thickness.According to the result of simulation,the factors such as practical application are considered synthetically,The three layer HA gradient coating with a total thickness of 0.3mm is selected as the best coating design scheme.The components from the titanium alloy substrate to the top HA layer are 100wt%ZrO₂,50wt%ZrO₂+50wt%HA and 100wt%HA.?2?The surface of HA coating prepared by plasma spraying is more uniform and the HA particles are spread well,but some holes and unmelted particles still existed.The sections of all samples showed significant lamellar structure with some holes and pits.What's more,the boundary between the HA layer and the intermediate transition layer on the surface of H3coating was not particularly obvious,and the microstructure between HA layer and the intermediate transition layer was closely.ZrO₂ intermediate layer combined with titanium matrix in the form of interleaving junction,which showed good mechanical bonding.?3?From XRD pattern of H1,H2 and H3,all coatings existed a small amount of?-TCP and?-TCP.The main phase of H1 coating was HA,and the diffraction peak intensity of ZrO₂which existed as cubic phase form in H2 and H3 coatings was higher than HA.With the increase of ZrO₂ gradient layer,the porosities of coatings showed decreasing trend,however,the bonding strength of coatings showed increasing trend.The porosities of H1,H2 and H3coatings were 13.7%,10.6%and 9.3%.The bond strength of H1,H2 and H3 coatings were13.92MPa,20.46MPa and 24.37MPa.The morphology of tensile surface of coatings consisted of smooth area and stepped area.With the increasing of ZrO₂ gradient layer,the growing proportion of smooth area in tensile surface area had been achieved.?4?SBF solution immersion test proved that after a certain time of immersed,there was a white flocculant surface similar to apatite on the surface of the gradient coating.The white flocculant contained a lot of elements such as calcium,phosphorus and so on.This indicated that apatite began to deposit on the coating surface,which indicated that the addition of ZrO₂had no effect on the bioactivity of HA coating.?5?Three kinds of coatings with different structures were tested by falling ball impact test machine and the impact times were recorded.The experimental results show that the impact resistance of three-layer HA gradient coating is the strongest,while that of pure HA coating is the worst,which is consistent with the results of computer simulation.The thickness of gradient coating in a certain thickness range,under the same impact conditions,the coating is more serious of damage with the increase of coating thickness,which is consistent with the simulation results.By observing the microstructure of the coating after impact damage,we can see that the coating with transition layer has less shedding,which is mainly due to the delamination flake caused by the internal stress of the coating under the impact load.The coating without transition layer falls off more seriously,and the failure is mainly due to the shear stress and brittle fracture of the coating under impact load,which indicates that the coating with transition layer has stronger impact resistance than the coating without transition layer.Under the impact load,the gradient coatings exhibit longitudinal impact coating breakaway and transverse crack propagation.The numerical simulation results of HA gradient coating under impact load are basically consistent with the impact test,which can provide theoretical guidance for the application of artificial joint implants material.