Structure And Properties Of The Thermal Oxidation Of Commercially Pure Titanium Surface Hardened Layer

In an effort to improve the wear-resistance of titanium alloys for orthopaedic implants and understand the bonding behavior between dental titanium alloys and porcelain, thermal oxidation of pure titanium was systematically investigated. The microstructure, microhardness property and growth kinetics of the achieved oxide film and oxygen diffusion layer on the titanium surface were analyzed in the present research.After annealing and pretreatment, the thermal oxidation was carried out on pure titanium at 600-900for 0.5-24h in an air atmosphere. Specimens for surface analyses and cross-sectional analyses were prepared, respectively.The surface microstructure of the treated titanium was analyzed by X-ray diffraction method. The surface oxide films on all the specimens were identified to be rutile TiO2. When treated at low temperature and in short time, the oxide film bonds well with the metal substrate. While with increasing oxidation temperature and time, the adherence becomes poorer. The sublayer beneath the oxide film is an oxygen diffusion layer consisting of a -phase Ti-0 solid solution. With increasing oxidationtemperature and time, the oxygen content and thickness of oxygen diffusion layer increase.The microstructure of the diffusion layer was observed using a LEICA MPS60 optical microscope and a HITACHI S-4700 field emission scanning electron microscope (FESEM). The diffusion layer is comprised of an inner and an outer oxygen diffusion layers which are separated by a line that runs across the grain boundaries. With increasing oxidation temperature and time, the thicknesses of inner and outer diffusion layers are both increase, and the inner diffusion layer is always a little thicker than the outer diffusion layer.The surface and the cross-sectional hardnesses were measured using an HV-1000 Vickers microhardness tester. With increasing oxidation temperature and time, thesurface hardness increases. With increasing indenter load, i.e. increasing the distance from the surface, the hardness decreases. Comparing with the cross-sectional microhardness analysis, the surface microhardness analysis can detect more thinner hardening layer, so is a more sensitive method to identify the presence of surface hardening layers.The growth kinetics of oxide film and oxygen diffusion layer were characterized by measuring their thicknesses. The growth of the oxide film agrees with parabolic-linear law, and the oxygen diffusion layer agrees with parabolic law with a linear trend at the end of the growth curve. When oxidized in relatively short time, the oxygen diffusion layer thickens faster than the oxide film does. While with increasing oxidation time, the oxide film grows faster than the oxygen diffusion layer. Oxidation temperature has a more obvious effect on the thickening of oxygen diffusion layer than time dose.