AN XPS AND SEMS STUDY OF SILICA SOL-GEL METAL-SUBSTRATE INTERACTION

Some applications of ceramic materials are difficult because of the markedly differing lattice bonding characteristics of ceramic and metals. This 'affinity barrier' can be overcome by the modifying of structure and composition in the interphase region. In our work the silica sublayer was deposited by the sol-gel method on metal implants prior to electrophoretical deposition of hydroxyapatite coatings to improve the oxidation resistance of the metal surface and the adhesion of apatite to metal during the annealing. The aim of this study is to determine changes in chemical state and structure of the coatings caused by thermal treatment for the better understanding of a relation between their composition and properties. Plates of titanium, WT 1-0, Fe-Cr-Ni stainless steel and of low-carbon steel were covered with silica layer by the sol-gel method. The coating on low-carbon steel was produced by the deposition of silica sol-gel directly on the metal or, the silica-titania opacified porcelain enamel was obtained by electrophoresis. In the case of direct deposition, below 400-degrees-C, an amorphous layer was formed while at 800-900-degrees-C for only 2-4 min the dense, fine crystalline structure was obtained including small amount of silicates and iron scales. The silica-titania enamel was electrophoretically applied on low-carbon steel pre-coated with nickel. During annealing nickel is alloyed with iron and decreases oxygen transport towards iron decreasing the rate of FeO formation. The silica from the enamel reacts with FeO producing a small amount of silicates. Titania partially precipitates at the metal boundary in the form of the needle-like crystals of FeTiO3, growing from the steel surface into the enamel. When stainless steel was annealed at 700-degrees-C in air, the surface oxide was enriched in chromium and manganese while the silica surface was enriched in iron and manganese.