Porous HA and nanocomposite nc-TiO2/HA coatings to improve the electrochemical corrosion resistance of the Co-28Cr-5Mo alloy

Abstract

The development of porous hydroxyapatite (HA) and composite TiO2/HA coatings on metallic biomaterials has received a great deal of attention during recent years. This is due to their superior properties in biomedical applications, including the better biointegration and mechanical stability of implants in the bone. Their poor adhesion strength to metallic substrates often makes them unsuitable for use in medicine, therefore the present work has focused on the electrophoretic deposition of the porous HA and nanocrystalline nc-TiO2/HA coatings which adhere well to the metallic substrate, as well as investigation of their influence on the Co-28Cr-5Mo alloy’s electrochemical corrosion resistance in Ringer’s solution. The mixture of nanometric and submicron size HA and nanometric TiO2 powders were used to electrophoretically deposit porous coatings on a cobalt alloy. The microstructure of the coating and substrate has been examined by scanning- and transmission electron microscopy and X-ray diffractometry. Both types of coatings, 6 μm thick, were highly porous with an open pore diameter up to 1 μm. The HA coatings were composed of nanocrystalline nc-HA particles uniformly distributed within submicrocrystalline sm-HA particles. In the nc-TiO2/HA coating, three types of particles occurred, namely nc-HA, nc-TiO2 and sm-HA. The HA coating was free of microcracks and voids, while “dried mud” microcracks occurred in the nanocomposite nc-TiO2/HA coating due to a mismatch in the coefficient of thermal expansion for both coating components. The transformation of anatase-to-rutile was observed after sintering of the nc-TiO2/HA coatings. The adhesion of the coatings to the underlying substrate material was investigated by the micro-scratch technique. The results show that EPD is convenient for deposition of well-adhered porous HA and nc-TiO2/HA coatings on a cobalt alloy. However, due to a homogeneous morphology free of microcracks, the HA coating exhibited much better adhesion to the substrate material than that of the nc-TiO2/HA coating. The application of both types of coating improved the corrosion resistance of the cobalt alloy in Ringer’s solution. The corrosion resistance of the HA coated alloy free of microcracks was higher than that of the nc-TiO2/HA coated one. The results of this work confirm EPD as a convenient method to develop porous coatings on cobalt alloys with potential medical application.