| Titanium and titanium alloys are considered to be widely used as medical implant materials due to their low density,low elastic modulus,high specific strength,excellent corrosion resistance and good biocompatibility,especially when used as acetabular cups and the femoral stems.It should be noting that friction/wear between implants and the surrounding tissues,and corrosion in the fluid of the body would occur,which subsequently lead to aseptic loosening of the implanted prostheses and shortening their life span.Therefore,to further improve the wear resistance and corrosion resistance of the Ti and its alloys,developing Ti matrix composite should be of significant importance in the field of artificial load-bearing prostheses.To this end,the purpose of this thesis was to develop titanium-based composites with improved mechanical properties,wear resistance and corrosion behavior,in which spark plasma sintering(SPS)was used to fabricate multi-layer graphene reinforced Ti(MLG/Ti)composites and graphene oxide reinforced Ti6A14V(GO/Ti6A14V)nanocomposites.The phase analysis and microstructural observations of the composite powders and sintered samples were conducted using XRD,Raman,TGA,SEM,and TEM,and their mechanical,tribological and corrosive behaviors were in-depth investigated.Importantly,the strengthening mechanism,wear resistance mechanism,and anti-corrosion mechanism of GO/Ti6A14V composites were elaborated.Phase constituents of the MLG/Ti composites were α phase and Ti oxide,and the latter mainly came from oxidation of titanium during the milling.Micromechanical evaluation illustrated that the hardness,elastic modulus and indentation yield strength of the MLG/Ti composites were improved.Nevertheless,once the addition of MLG was higher(1.5wt.%in this research),the mechanical properties of the designed nanocomposite would be deteriorated.Meanwhile,drastic collision during the milling would also lead to structural damage of the added MLG,giving rise to the decreased strengthening effect.Hence,as an important derivative of graphene,graphene oxide(GO)with the reactive groups populating on its edge(carbonyl and carboxyl groups)and planes(hydroxyl and epoxide groups),which has been demonstrated to possess desirable dispersion behavior in aqueous media,was employed as the strengthening nanofillers in the follow-up study of this research.Moreover,ultrasonication,electrostatic self-assembly process and SPS were employed to fabricate the GO/Ti6A14V composites.Thermal stability of as-received GO was evaluated using thermal gravimetric analyzer,indicating that GO experienced thermal reduced reaction in the SPS process,leading to partial evolution of reduced graphene oxide(rGO),which homogeneously distributed in the Ti6A14V matrix.More importantly,as compared with the Widmans(?)tten microstructure,the addition of GO resulted in the formation of a typical basket-weave structure.Furthermore,high-resolution transmission electron microscopic observation of GO-Ti6A14V interface revealed that local in-situ formed TiC nanolayer and diffusion layer occurred,which is expected to improve the bonding strength of the interface,subsequently improving load transfer efficiency though the interface.Hardness,elastic modulus,yield strength,tensile strength of the sintered GO/Ti6A14V composite significantly increase with the GO content while elongation decrease with the GO content in this research,as compared to those of the Ti6Al4V sample.0.27GO/Ti6Al4V composite displayed a better balance of strength and ductility than that of composite with relatively higher GO addition(0.54wt.%).Moreover,strengthening mechanisms such as Orowan strengthening,enhanced dislocation density and load transfer mechanism along with the addition of GO fillers were investigated,and load transfer mechanism was considered to contribute greatly to the improved composites strength.Results of the dry wear experiments of the sintered samples showed that coefficient of friction(CoF)and wear rate of the composites significantly decreased with increasing of the GO content,strongly implying that GO/Ti6Al4V exhibited improved tribological resistance with low CoF.Careful observations of the worn surface,wear debris and subsurface of the wear track revealed that,despite abrasive wear and delamination occurred in the Ti6A14V samples reinforced with GO and without GO,the addition of GO possessed significant strengthening effect and made the composites much stronger.Such characteristics are expected to effectively resist the microcutting and plastic deformation of subsurface beneath the wear track,avoiding the damage of the wear surface associated with abrasive and delamination to some extent.As for the corrosive behavior in the simulated body fluid(SBF),the sintered GO/Ti6A14V composites exhibited an enhanced corrosion resistance.In particular,corrosion rate of the 0.54GO/Ti6A14V composite(～7.10×10-4 g·m-2·h-1)was only about 1/7 of that of Ti6A14V sample(～48.3×10-4 g·m-2·h-1),and it dispayed little tendency to pitting corrosion.Along with analysis of electrochemical impedance spectroscopy(EIS),the main mechanisms for the improved corrosion resistance of the GO/Ti6A14V composite were as follows:(1)Homogeneous dispersion of rGO in the Ti6A14V matrix are expected to form numerous galvanic couples with the matrix,in which rGO with excellent electric conductivity acts as cathode and Ti6A14V matrix acts as anode.These galvanic couples further accelerated the dissolution of Ti6A14V shortly after it was soaked in SBF,and then the critical value of[TiCl6]2-complex in SBF would easily reach,which in turn promoted rapid formation of stable TiO2 passive film to prevent the Ti6A14V matrix from being eroded.(2)GO with such characteristics as hydrophobic nature,chemically inert,and large specific surface area could act as the barrier to penetration of these aggressive ions such as Cl-,and OH-,and therefore avoiding localized attack and/or pitting corrosion to the great extent. |
PDF Full Text Request