Study On Preparation Of Rare Earth Doped Gradient Bioceramic Coating By Composite Process On Titanium Alloy Surface

Titanium alloys,with superior mechanical properties and excellent corrosion resistance,are widely used in bone tissue engineering.However,as bioinert materials,they are not suitable for inducing the formation of the interface between biomaterials and bone tissue,because their limited osteoconductivity delays the healing time.As well known,hydroxyapatite(HA)is considered to be an attractive bone substitute materials because of its special biocompatibility and osteoconductivity Thus,the bioactivity and biocompatibility of the titanium implants have been improved by introduction of a bioceramic coating,which expands the application fields of titanium alloys.In the present work,5wt% CaB₆/HA bioceramic coatings with different La₂O₃contents(0-0.6 wt%)have been fabricated by a laser cladding technique on Ti-6Al-4V.The La2O3-doped coatings show a significantly higher bone-like apatite precipitation after immersion in Simulated body fluid(SBF)compared with La₂O₃-free coating.In vitro experiment also shows that 5wt%CaB₆/HA bioceramic coating with0.2-0.4 wt%La₂O₃are more suitable for the attachment and proliferation of MG63 cells,exhibiting superior bioactivity and biocompatibility.A two-layer composite coating of(HA+5wt% CaB₆)+0.4wt%La₂O₃/HA+50wt%Ti(transition layer)was prepared on the surface of Ti-6Al-4V substrate by plasma spraying technology.Post heat treatment and laser remelting were used to crystallize the sprayed coating,and then to explore and understand the properties of the composite coating.The surface morphology,microstructure,phase composition,corrosion resistance,friction and wear performance and biological behavior of the coatings were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),and Fourier transform infrared(FTIR)spectroscopy.The results showed that the as-sprayed coating was mainly composed of HA(partially amorphous and few crystalline),calcium phosphate(TCP)and few peaks of Ca O.Heat treatment and laser remelting were used to crystallize the coating.The XRD results showed that the types and positions of the diffraction peaks of the coating did not change after heat treatment.However,different laser energy densities will cause crystals to produce different preferred orientation coatings after laser remelting,which is completely different from the results produced by the heat treatment process.After heat treatment(at 650?for 2 h)and laser remelting(rapid solidification),the amorphous and decomposed phases can be transformed into crystalline HA,where the selection of process parameters depends on the surface quality of the coating and the XRD results.The surface of heat treated coating contains a lower area fraction of porosities and a significant reduction in surface roughness as compared to as-sprayed coating.After laser remelting,the original layered structures of plasma sprayed coating have completely disappeared,but the formation of cracks caused by melting is a serious side effect of laser surface treatment.There is a lower corrosion current density and an improved corrosion resistance after heat treatment(as compared to plasma sprayed coating)which is,however,increased after laser remelting.The friction and wear test shows that the heat-treated coating has a lower coefficient of friction in the SBF solution environment,and the wear resistance of the coating has been greatly improved.Bioactivity tests shows that the dissolution of the amorphous phase of the plasma sprayed coating during the SBF immersion is beneficial to the nucleation and growth of apatite on the coating surface.However,the coating after the crystallization treatment has higher stability than the sprayed coating.The cultivation of MG63 cells on these coatings shows that surface chemistry and topography of the heat-treated coating might be favorable to cell attachment after being cultured for 4d.The MG63 cells spread well,and its filiform pseudopodia adhere closely to the coating surface.All three coatings exhibit improved bioactivity and biocompatibility compared to titanium alloy substrates.