The effects of temperature and sliding speed on the tribological behavior of a SIC ceramic by sliding on the same material in deoxygenated water were investigated from room temperature to 300°C under the corresponding saturated vapor pressures. The friction coefficient and specific wear rates of both plates and disks increased at elevated temperatures at all sliding speeds, but decreased with increasing sliding speed at 120" and 300°C. Fine mirrorlike worn surfaces were observed without wear debris under all sliding conditions. The wear mechanism appears to consist of hydrothermal oxidation of SIC and dissolution of reaction products such as silica.
Biomedical titanium metals subjected to gas under precisely regulated oxygen partial pressures (P(O2)) from 10(-18) to 10(5) Pa at 973 K for 1 h were soaked in a simulated body fluid (SBF), whose ion concentrations were nearly equal to those of human blood plasma, at 36.5°C for up to 7 days. The effect of oxygen partial pressures on apatite formation was assessed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements. After heating, the weight of the oxide layer (mainly TiO(2)) formed on the titanium metal was found to increase with increased oxygen partial pressure. Nitrogen (N)-doped TiO(2) (Interstitial N) was formed under a P(O2) of 10(-14) Pa. At lower P(O2) (10(-18) Pa), only a titanium nitride layer (TiN and Ti(2)N) was formed. After soaking in SBF, apatite was detected on heat-treated titanium metal samples. The most apatite was formed, based on the growth rate calculated from the apatite coverage ratio, on the titanium metal heated under a P(O2) of 10(-14) Pa, followed by the sample heated under a P(O2) of 10 and 10(4) Pa (in N(2)). The titanium metal heated under a P(O2) of 10(5) Pa (in O(2)) experienced far less apatite formation than the former three titanium samples. Similarly, very little weight change was observed for the titanium metal heated under a P(O2) of 10(-18) Pa (in N(2)). During the experimental observation period (5 days, 36.5°C, SBF), the following relationship held: The growth rate of apatite decreased in the order P(O2) of 10(-14) Pa > P(O2) of 10 Pa ≥ P(O2) of 10(4) Pa > P(O2) of 10(5) Pa > > P(O2) of 10(-18) Pa. These results suggest that N-doped TiO(2) (Interstitial N) strongly induces apatite formation but samples coated only with titanium nitride do not. Thus, controlling the formation of N-doped TiO(2) is expected to improve the bioactivity of biomedical titanium metal.
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