Research On The Microstructure,properties And Biocompatibility Of Selective Laser Melting Processed Ni@Al₂O₃/CoCrMo Composites

The aging trend of the Chinese population is becoming more and more obvious,and the number of orthopaedic patients is gradually increasing.At present,the traditional hip implants are mainly cast and cut from CoCrMo alloy.However,with the improvement of people’s living standards,higher requirements are put forward for the service life,biocompatibility,and personalized customization of CoCrMo alloy implants.By incorporating Al₂O₃into the CoCrMo alloy,one may significantly enhance its wear resistance and biocompatibility due to the advantageous properties of Al₂O₃,such as high hardness and good wear resistance.Selective Laser Melting（SLM）technology enables the rapid manufacture of small,personalized metal implants.In this paper,Ni@Al₂O₃/CoCrMo composite powder preparation for SLM forming,bionic bone structure design,SLM forming Ni@Al₂O₃/CoCrMo composite microstructure properties and biocompatibility were studied.The main research contents are as follows:（1）Ni coated Al₂O₃（Ni@Al₂O₃）composite powder with core-shell structure was prepared by electroless plating process with Ni coating on the surface of Al₂O₃powder,while the Ni plating layer with high density was evenly distributed.Then the Ni@Al₂O₃/CoCrMo composite powder for SLM forming was obtained by ball mixing Ni@Al₂O₃powder and CoCrMo alloy powder.（2）The Ni@Al₂O₃/CoCrMo composite was formed by SLM technology.It was found that the hardness and elongation of the composite increased firstly and then decreased with the increase of laser power or scanning speed.When the laser power was160 W and the scanning speed was 900 mm/s,the surface of SLM formed composite demonstrated no defects,with the density of 8.88 g/cm³.Besides,the molten pool was arranged in a regular order,with a high-quality fish-scale morphology.Moerover,the tensile strength of the Ni@Al₂O₃/CoCrMo composite reached 1170.5 MPa,and the elongation reached 16.8%.（3）A comprehensive study was conducted to investigate the impact of Ni plating on the characteristics of composites generated by SLM technology.With the addition of Ni element,the wear rate of Ni@Al₂O₃/CoCrMo composite decreased from 25.5×10^-6mm³·N^-1·m^-1to 9.6×10^-6mm³·N^-1·m^-1,and the elongation increased from 10.5%to18.6%.It was discovered that the mechanism behind the mechanical characteristics of Ni-layer reinforced composites is as follows:The introduction of Ni facilitated the separation of Co,Cr,and Mo elements,the creation of carbides,and theγ-εmartensitic transformation at grain boundaries.Additionally,the addition of Ni enhanced the binding energy between the reinforced phase and the CoCrMo phase.Besides,the lattice distortion of CoCrMo solid solution increased the slip resistance of dislocation,improved the friction resistance of Ni@Al₂O₃/CoCrMo composites.（4）The effects of particle size of Al₂O₃powder and Ni@Al₂O₃powder content on the properties of SLM formed Ni@Al₂O₃/CoCrMo composites were studied.When the particle size of Al₂O₃increased from 2μm to 30μm,the yield strength of the composite material increased from 706.9 MPa to 794.2 MPa,and the elongation increased significantly from 5.7%to 18.6%.The presence of theε（HCP）phase and the occurrence of specific grain boundary CSL（Σ≤29）were shown to be directly proportional to the particle size,resulting in a significant reduction in dislocation slip and an enhancement in the structural integrity of the composite materials.Moreover,at a concentration of 1 wt%Ni@Al₂O₃powder,the presence of(Al_0.9Cr_0.1)₂O₃and Cr₇C₃resulted in the creation of a continuous friction layer through a pinning effect.As a result,the CCM1N composite exhibited the highest level of wear resistance.（5）Three interface models,Ni（111）/CoCrMo（100）,Ni（111）/CoCrMo（110）and Ni（111）/CoCrMo（111）,were established.Thermodynamic properties of interface structure,such as interface binding work,interface energy,binding energy,state density and differential charge density,were calculated.The interfacial bonding strength of Ni and Co Cr was obtained.The results show that the Co-Ni and Cr-Ni at the Ni（111）/CoCrMo（111）interface tended to form metal bonds.The interface binding work(W_ad)was-3.79J/m²,the interface separation work(W_sep)was-3.73 J/m²,and the interface formation energy(E_for)was-0.64 J/m².Compared to Al₂O₃/CoCrMo composite,Ni@Al₂O₃/CoCrMo composite exhibited superior charge density,bonding strength,and interface electron cloud overlap.（6）Three periodic minimal surface（TPMS）porous structure composites with uniform pore distribution and porosity of 60%（G60）and 75%（G75）and gradient pore distribution of 50-80%（F70）were designed respectively.The three structures all experienced elastic,yield,fluctuation and densification stages during the compression deformation process,the elastic modulus ranged from 0.28 GPa to 12.51 GPa,and the yield strength varied from 78.26 MPa to 771.95 MPa,which was higher than that of natural bones.With the decrease of porosity,the elastic modulus and yield strength of G75,F70 and G60 structures increased gradually,and the F70 structure was closer to the mature skeleton.In addition,the strength and elastic modulus of the materials with gradient porosity distribution were significantly higher than those of the non-gradient structural materials with the same porosity.（7）The biocompatibility of Ni@Al₂O₃/CoCrMo composite implants with different porosity and pore distribution was studied.The immerse experiment show that the content of Co and Cr ions of F70 with gradient structure were 0.64 and 0.16 ppm respectively,which was lower than that of G60 and G75 with uniform structure.The optical density（OD）of F70 implant demonstrated the highest value of 2.75.The results demonstrated that the cell population was dispersed evenly throughout the area with a high degree of local curvature,and that the F70 implant had the lowest level of toxicity and the best level of cell compatibility.The F70 implant showed a bone growth area of up to 90%in animal studies.In addition,the implant demonstrated good structural stability due to its tight interweaving with the interior and exterior bone tissue.These findings suggest that the implant discussed in this study possessed exceptional bone integration capabilities.Consequently,this research provided valuable insights for the development and implementation of bionic bone structures in orthopedic implants.