Preparation Of A Novel Ultrafine Grained Cobalt-chromium-silver Alloy For Dental Application

Cobalt-based alloys have been widely used as dental and orthopedic implant materials due to their high strength,high hardness,good wear resistance and superior corrosion resistance,etc.However,the release of nickel ions in cobaltbased alloys may cause toxic effects such as allergic reactions and carcinogenesis in human body;on the other hand,the wear resistance and strength can also be enhanced.Moreover,the emergence of drug-resistant super bacteria has accelerated the development of antibacterial alloys,but limited research efforts have beed devoted to the cobalt-based alloys.Nanocrystalline/ultrafine-grained metals/alloys have demonstrated unusual physical and mechanical properties compared to their coarse-grained counterparts and may be promising candidates for biomedical applications.However,there still have been rare explorations on nanocrystalline/ultrafine-grained cobalt-based alloys for medical implants.In this context,using a combination of high energy ball milling(HEBM)and spark plasma sintering(SPS),and ultrafine-grained Co-30 at% Cr-5at% Ag twophase immiscible alloy was fabricated by controlling the ball milling times and sintering parameters.X-ray diffraction,scanning electron microscopy and transmission electron microscopy were used to characterize the phase and microstructure.The mechanical properties including Vickers hardness,yield strength,and compressive strength were first measured.Then,the pin-on-disk dry sliding wear performance,and electrochemical corrosion behavior in artificial salvia solution were systematically investigated.Finally,the antibacterial activity and in vitro cytocompatibility of the alloy were evaluated.The following major conclusions can be drawn:(1)The elemental powders of Co and Cr with the nominal atomic ratio of 65:30 can be forced into the formation of a single-phase hexagonal-close-packed(hcp)?-phase supersaturated solid solution by HEBM.The Co-Cr solid solution with addition of 5 at% was further ball-milled for 6 h.The ball-milled ternary powder was spark plasma sintered into bulk.(2)The relative density of the alloy measured by the Archimedes method is over 97%.The microstructure of the alloy consists of a ?-Co matrix with uniformly dispersed nanoscale ? precipitate and Ag particles.The average grain size of the matrix is 167 nm,and the average size of the ? precipitate and Ag particles are 23 nm and 72 nm,respectively.The alloy shows yield strength of up to 2.09 GPa and the compressive strength of 2.29 GPa.A wear factor of 10-5 mm3/(N·m)was measured upon sliding against alumina,suggesting that the alloy has excellent wear resistance.(3)The Co-Cr-Ag alloy has a passivation zone in the artificial saliva solution(ASS)at 37 °C and corrosion current densities of 0.188 ?A/cm2,indicating that the alloy has good corrosion resistance.The antibacterial rates of Co-30Cr-5Ag alloy against E.coli and S.aureus reaches 90.5% and 72.6%,respectively,compared with Co-30 at% Cr alloy,indicating that the Co-30Cr-5Ag alloy has demonstrated high antibacterial activity.Furthermore,the fabricated Co-30Cr-5Ag alloy also shows good cytocompatibility.In summary,the present ultrafine-grained Co-30 at% Cr-5 at% Ag alloy with superior mechanical properties,excellent wear-and corrosion-resistance,high antibacterial activity and good cytocompatibility provides a promising candidate for medical implants.