| Titanium alloys are widely used in load-bearing bone prostheses(such as hip joint and knee joint)because of their high specific strength and good biocompatibility.However,titanium alloy is a bioinert material with poorwear resistance.After implantation into human body,the fixation interface of artificial joint is prone to aseptic loosening,and even lead to early implantation failure of prosthetics.Therefore,it is an effective method to prepare bioactive coating on the fixed surface of titanium alloy joint.Hydroxyapatite(Ca10(PO4)6(OH)2,HA)has a similar chemical composition and crystal structure to calcium phosphate in human bone tissue,and has excellent biocompatibility and bioactivity.HA is the preferred material for bioactive coating.HA coatings prepared by plasma spray on the surface of artificial joints have been widely used in clinic.However,the intrinsic brittleness(fracture toughness is much lower than human dense bone)of HA severely restricts its clinical application as a surface for load-bearing bone implants.Therefore,how to greatly improve the fracture toughness of HA coating without damaging its excellent bioactivity has been the focus of academic research.Based on the excellent mechanical properties and biocompatibility of graphene(GNS)and the lamellar structure characteristics of the plasma sprayed coating,this paper puts forward to the design idea of biomimetic bioactive coating on the fixed surface of artificial joints based on the multi-scale structure of "splats interface and a splat internal synergistic reinforcement and toughness".The microstructural,mechanical,tribological,biocompatibility and bioactivity in vitro were in-depth investigation to elucidate the strengthening and toughening mechanism and reveal the in vitro biological behavior of plasma sprayed GNS/HA composite coatings.(1)It is found that the main phases of plasma sprayed GNS/HA composite coatings are crystalline HA,amorphous HA,tricalcite phosphate(TCP)and CaO,and the two-dimensional structure characteristics of GNS can be retained effectively and evenly distributed in the composite coatings after plasma spray.(2)The results of mechanical properties show that GNS(2.0 wt.%)can effectively improve the strength and fracture toughness of the composite coating.It is found that the strengthening mechanism of composite coating is mainly the interfacial strengthening between splats and the stress transfer between GNS-HA interface,while the main toughening mechanism is crack deflection,crack branching,GNS pull out and GNS bridging,etc.(3)The coefficient of friction(CoF)and wear rate of plasma sprayed HA coating decreased with increasing of GNS content in the sliding friction process under simulated body fluid(SBF)lubrication condition,showing excellent wear resistance and friction reduction performance.The farther study found that GNS enhanced the strength and toughness of HA composite coating simultaneously,which greatly inhibited the crack initiation,propagation and brittle fracture of composite coating under contact stress condition.GNS transfer film on the worn surface,which can effectively protect the worn surface and exert its self-lubricating effect.The hydrophobicity,good chemical inertness and large surface area of GNS in the composite coating can significantly hinder the corrosive Cl-1 ions in SBF into the coating,and effectively reduce the corrosion induced damage of the coating during the sliding friction process.(4)Results of in vitro biocompatibility showed that the addition of GNS was beneficial to the adhesion,proliferation and differentiation of osteoblasts on the surface of the composite coating,which made the GNS/HA composite coatings have better biocompatibility.After 7 days of immersion in simulated body fluids,the morphology of mineralization products on the surface of the three coatings showed no significant difference,but the amount of mineralization products(spherical uplift)on the surface of composite coatings increased with increasing of GNS content.(5)Thermal treatment can effectively improve the mechanical properties of plasma sprayed coatings,but the improvement of indentation yield strength and fracture toughness of plasma sprayed composite coatings was much lower than that of HA coating.Thermal treatment further improves the sliding friction and wear properties of the coating under SBF conditions.Thermal treatment HA and GNS/HA coatings can further promote the adhesion,proliferation and differentiation of osteoblasts on the surface to a certain extent,but reduce the osteogenic mineralization performance of the coatings in SBF.The above results indicate that the addition of GNS and thermal treatment make the GNS/HA composite coatings have good mechanical properties,wear and friction reduction properties,and excellent biological properties.Therefore,GNS/HA composite coatings on titanium alloy surface are potential load-bearing joint prosthesis materials. |
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