Study On Bioactive Composite Coating On The Surface Of Ti-6Al-4V Alloy By Laser Cladding

The Ti-6Al-4V alloy is widely applied as medical implant because of whosedensity is similar to human bone and its excellent mechanical properties. However, itslow wear resistance, corrosion resistance and bad bioactivity limit its further use in allorganisms. Hydroxyapatite(HA) has excellent biocompatibility and tissue bioactivity,but its mechanical properties is poor. Therefore, hydroxyapatite-coated Ti-6Al-4V alloy,which can take advantages of both the metal substrate and the ceramic coating, has beenaccepted as one of the most promising implanting material. In this paper, as to the needin the clinical application, the bioactive composite coating of HA and Ti-6Al-4V wasfabricated on the surface of Ti-6Al-4V alloy by squash presetting CO2laser cladding.The composition and thickness of the coating were studied and laser claddingparameters were optimized in single cladded experiment with circular and rectangularbeam. The weight percent of HA in pre-placed powders was chosen as40%,60%and80%. The thickness of pre-placed layer was0.3mm. The defocusing amount in claddingwith circular spot was30mm and optimal parameters were as follows: P=0.6kW,V=0.3m/min; the beam size in cladding with rectangular beam was D=2×4mm andoptimal parameters were as follows: P=1.0kW, V=0.3m/min. The result showed thatmultiple cladded composite coating with flat surface was obtained in the overlappingratio of20%.The microstructure and mechanical properties of laser cladded bioactive compositecoating were investigated. The result showed that microstructure in the top of thecoating were fine-equiaxed grains while in the middle and bottom of the coating weredendrites. The coating was metallurgically bonded to the Ti-6Al-4V alloy substrate bythe action of laser. The laser cladded composite coating contained such phases as-Ti,Ti3P, TiO and biocompatible ceramic phases as CaTiO3and β-Ca3(PO4)2. Due to grainrefinement and ceramic phases, the microhardness of the coating increased prominentlycompared with the substrate. The friction coefficient, wear weight loss and width ofwear track of composite coating were all lower than those of the substrate, whichindicated that the wear resistance of Ti-6Al-4V alloy was significantly improved.Thewear mechanism of coating was mainly adhesive wear and abrasive wear. Besides, the wettability of SBF on the surface of coating and in vitro bioactivity ofcomposite coating were evaluated. The contact angle of SBF against coating wassmaller than that against the substrate. After soaking in SBF for14days, the bulk andflocculent precipitates of apatite were observed on the sueface of composite coating.The precipitates composed of a grey composite layer and an apatite layer. The resultsindicated that the surface wettability and in vitro bioactivity on Ti-6Al-4V alloy wereimproved by laser cladded composite coating.