M. Koike, B. W. Johnston, T. Okabe, Baylor College of Dentistry, Dallas, TX

Summary: Titanium and titanium alloys have been used in dental applications because of their excellent biocompatibility. The most common method of fabricating dental metallic prostheses is by investment casting. Even with investment material that is specifically made for titanium casting, a hardened surface layer formed. This surface layer, called the Ą-case, is formed by the solid state diffusion of oxygen and some metallic elements into the solidifying titanium at the mold surface. The depth of the Ą-case is dependent on the temperature of the solidifying titanium at the mold surface, which dynamically changes with time. Castings of Commercially pure Ti (CP Ti: ASTM grade 2) and Ti-6Al-4V (Ti64: ASTM grade 5) (10 mm x 5 mm) with different thickness (0.5, 0.75, 1.0, 1.5 and 2.0 mm) were made in a magnesia-based mold (Selevest CB, Selec Inc., Japan) using a centrifugal casting machine (Ticast Super R, Selec Inc.) and then bench cooled. The thickness of the Ą-case was determined from the microhardness profile near the cast surface where the decreasing microhardness with the depth coincided with that of the bulk, interior microhardness. The depth of the Ą-case (n=4) from the specimens of different thicknesses were statistically analyzed using one-way ANOVA (Ą=0.05). The cooling rate of the castings was computed as a function of the casting size using the procedure described by Frueh et al (1997). Once the cooling rate was defined, the diffusion of oxygen from the surface to the interior of the casting was computed using the analysis of Boettinger et al (2000). Using empirically established relations between microhardness and oxygen content, the computed oxygen profile was utilized to calculate the microhardness value as a function of depth below the surface of the Ą-case. The Ą-case thickness profile curves fit well with the measured Ą-case thickness for each metal. The Ą-case is thicker in the thicker specimens, which cooled at a slower rate since the oxygen can diffuse into the deep interior structure. The estimated Ą-case was thicker in CP Ti compared to Ti-6Al-4V in identical specimen sizes probably because the oxygen diffusivity is higher in CPTi than in Ti-6Al-4V [(DCPTi=1.4x10-2exp(-2.3x105/RT), vs. D64=5.1x10-3exp(-2.5x105/RT)]. Supported by NIH/NIDCR grant DE 11787.