| In the modern biomedical field,traumatic fractures and bone loss due to accidental traumatic accidents are increasingly common in clinical practice and the repair of bone structure and function with bone implants is a common tool in clinical medicine.The most widely used bone implants are metal implants and bioceramic implants.Titanium alloys are widely used due to their good mechanical properties and corrosion resistance,but their low wear resistance and biological inertness can affect the long-term stability of the implant.Among the bioceramic implants Hydroxyapatite(HA)is widely used in bone repair and bone tissue regeneration due to its good biocompatibility,bone-like properties and resistance to biological wear,but its low toughness and high brittleness make it difficult to be used in a large number of areas in implants.In this paper,HA gradient coatings with dense tissue structure,wear resistance and good bioactivity were prepared on the surface of Ti6Al4 V alloy using a laser melting process.In addition,since the original morphology of the laser clad coating surface was not conducive to the reduction of the biological wear rate and the adhesion and proliferation of osteoblasts,this paper uses a nanosecond laser to prepare a bone-like microtexture on the surface of the HA coating based on its superior bioactivity,so that the bone implant material has both a bone-like material and a bone-like surface microtexture.The main research of this paper is as follows:(1)Optimisation of the process of preparing HA gradient coatings by laser claddingThe pre-set powder ratios,pre-set layer thicknesses and laser parameters were regulated according to the thermodynamic properties of the materials,and the effects of such parameters on the physical phase and crystal structure characteristics of the laser melted layers were investigated.The strengthening mechanism of the melt solidification and the growth mechanism of ceramic grains in the laser melting HA gradient coating were investigated,and their effects on various properties such as hardness gradient,corrosion resistance,wear resistance and cytotoxicity of the gradient coating were further investigated to optimise the laser melting process.It was shown that when the laser cladding parameters were 50% lap rate,1 cm/s scanning speed and 700 W laser power,the surface of the coating was flat and the interface was dense,and it was tightly bonded to the substrate without cytotoxicity and with excellent biological wear properties.(2)Study on the ablation mechanism and removal law of HA coating by nanosecond laserA two-dimensional model of nanosecond laser ablation of HA gradient coatings on bone-like materials was established to study the heat transfer and flow field changes on the material surface during laser ablation,and the evolution of the ablation pits was analysed.The simulation results showed that the temperature field of the melt pool was mainly influenced by heat transfer,and the depth of the ablation pits increased significantly with the increase of the number of ablations,under the combined effect of vapourisation and liquid phase migration.As the melt pool gradually solidified,the flow rate within the melt pool under the action of the Marangoni flow gradually decreased,and the recoil pressure determined the shape and ablation area inside the ablation pits by influencing the flow rate of the liquid phase.HA materials have a lower thermal conductivity than metallic materials,and the ablation pits show a shallow and wide shape formation.The experimental validation using the same parameters showed that the experimental results were more similar to the simulation results,which verified the feasibility of this model.The effect of the laser parameters on the ablation groove morphology was investigated by orthogonal and response surface methods.It was found that the thermal accumulation in the ablation trough of HA coatings increases with increasing laser frequency and number of ablations,resulting in an increasing width and depth of the ablation trough.When the number of ablations was high and the laser energy density was high,a multiphoton absorption effect occurred,resulting in the material breaking away from the substrate in the form of an explosion.(3)Extraction and preparation of bone-mimicking microtextures on HA coating surfacesBased on the microstructural characteristics of the bone surface,bone-mimicking microtextures with length-diameter ratios ranging from 1.5:1 to 4.0:1 and depths of up to10.9-23.4 μm were designed and fabricated to achieve controlled processing of bone-mimicking microtextures with depth gradients,and the cellular activity of microtextures of different sizes was investigated to optimise the microtexture geometry.It was found that microtextures with bone-mimicking characteristics can mimic native bone well and control the interaction between the cells and the material surface.The osteoblasts showed good proliferation rates and adhesion strength when the microtexture had a length to diameter ratio of 2.5:1 and a maximum depth of 15.7 μm.The geometry and depth gradient of the osteomimetic microtexture create an important environmental induction signal for osteoblast adhesion and proliferation,with the cell morphology guided by it showing different adhesion patterns from the front to the end of the microtexture.(4)Study of the biological wear and cell adhesion properties of the bone-mimicking microtexture arrayParallel,staggered,end-to-end microtexture arrays were machined using the optimised bone like microtexture structure as a unit.The effect of the bone-mimicking microtexture arrays on the bio-friction properties was investigated by means of "walking state" joint loading friction wear tests on the surface of the microtexture samples.In addition,the effect of different arrangements of microtexture arrays on the adhesion and proliferation status of osteoblasts was investigated by in vitro culture of osteoblasts.It was shown that the osteomimetic microtexture arrays improved the bio-wear performance of the implants and were effective in reducing the wear rate on the wear substrate,increasing the service life of the bone implants.In vitro adhesion tests of osteoblasts showed that parallel and staggered microtexture arrays have better cell adhesion and proliferation properties.The smaller spacing of parallel and staggered bone mimicking microtexture arrays showed both better biological wear properties and better cell adhesion and proliferation properties.In summary,this paper addresses the urgent need to improve the performance of bone implants in the medical device field by preparing bone-like microtextured HA-coated implants with both bone-like material and bone-like surface characteristics using laser melting and nanosecond laser microweaving processes.The surface properties,physicochemical properties,wear properties and cellular adhesion properties of the materials were investigated to demonstrate the excellent biological wear properties and cellular activity of the bone-like microtextured HA coatings.These studies provide the experimental and theoretical basis for laser-controlled preparation of bone-like implants. |
