Study On Microstructure And Properties Of Titanium Alloys For Dentistry

Titanium and its alloys are promising candidate materials for dental applications due to their excellent mechanical properties, biocompatibility and corrosion resistance. Ti-6Al-4V and Ti-6Al-7Nb alloys have been used ago. But elements Al and V exhibit high cytotoxicity. Recently, many researchers have concentrated on developing new type of titanium alloys without toxic elements. Asβphase stabilizer in titanium alloys, Mo and Nb have shown good biocompatibility. Adding Mo and Nb to titanium alloy can increase its strength and ductility, while can decrease its elastic modulus. In present paper, Ti-Mo, Ti-Nb and Ti-Mo-Nb alloys were prepared using LZ5 type centrifugal casting technique. The microstructures, mechanical properties, wear resistance and corrosion resistance of Ti-Mo, Ti-Nb and Ti-Mo-Nb alloys were investigated. The compatibility and casting characteristic of selectedβtype Ti-Mo-Nb alloy that has the excellent properties have been studied. The dental crawn has been made by investment casting technique.The investigations have shown that with the increase of Mo content, Ti-Mo alloys revealed refined microstructure, reduced compression strength, but increased plasticity. For Ti-Nb alloys, the increase of Nb content modified the microstructure of Ti-Nb alloys significantly and decreased their compression elastic modulus, in which Ti-20Nb alloy showed the largest compression strength and Ti-5Nb alloy showed the best plasticity. With same content in Ti, Mo addition results in higher Vickers hardness, compression strength and elastic modulus than Nb does. All the three kinds of Ti-Mo-Nb alloys show equiaxed crystalline microstructure. The increasing Nb contents of Ti-Mo-Nb alloys result in the decrease of density, Vickers hardness, compression strength and compression elastic modulus, and the increase of bending strength and plasticity. Comparing to Ti-10Mo alloy, a lower Vickers hardness and compression elastic modulus and higher compression strength, bending strength and plasticity were obtained for Ti-10Mo-10Nb alloy.Wear tests showed that the increasing Mo content leads to a reduction of wear depth of Ti-Mo alloy and increase of wear resistance, with a smallest steady friction coefficient for Ti-10Mo alloy. For Ti-Nb alloys, Ti-10Nb alloy shows a smallest steady friction coefficient, Ti-5Nb alloy shows the smallest wear depth and best wear resistance and Ti-15Nb alloy shows the largest wear depth and worst wear resistance. For Ti-Mo-Nb alloys, the increasing Nb content reduced the wear depth and increased wear resistance. The applied load did not influence evidently the wear resistance of Ti-10Mo-10Nb alloys. Hank's solution was found to act as lubricant during wear tests for Ti-10Mo-10Nb alloys. After surface oxidation treatment, TiO₂ and Nb2O5 formed on the surface of Ti-10Mo-10Nb alloys, leading to an increase of wear resistance. The prolonged oxidation resulted in the formation of larger volume fraction of TiO₂ and surface became loosen. The wear mechanism for all the tested alloys is a combination of adherence wearing and debris wearing.Corrosion tests revealed Ti-10Mo alloy showed higher self-corrosion potential for all Ti-Mo and Ti-Nb alloys in Hank's solution and pH2.5 lactic acid solution. Ti-5Mo alloy showed higher self-corrosion potential for all Ti-Mo and Ti-Nb alloys in 0.9%NaCl solution. For all Ti-Mo and Ti-Nb alloys, Ti-5Nb alloy was easy to passivate with a wider passivation range in Hank's solution, Ti-20Mo alloy was easy to passivate with a wide passivation range in 0.9%NaCl solution and Ti-10Nb alloy was easy to passivate with a wide passivation range in pH2.5 lactic acid solution. For all Ti-Mo and Ti-Nb alloys, the passivation film of Ti-15Mo alloy was stabler in Hank's solution, 0.9%NaCl solution and pH2.5 lactic acid solution.In Hank's solution and pH2.5 lactic acid solution, the self-corrosion of Ti-10Mo-10Nb alloy and pure titanium was nearly. In Hank's solution, 0.9%NaCl solution and pH2.5 lactic acid solution, Ti-10Mo-10Nb alloy was easier to progress from activation state to passivation state with a wider passivation range, comparing to other two Ti-Mo-Nb alloys and pure titanium. However, no evident difference was found for the dissolution rate of the passivation film for all Ti-Mo-Nb alloys. Considering the stability and easiness of formation of the passivation film, Ti-10Mo-10Nb alloy showed a better corrosion resistance than pure titanium does. Ti-10Mo-10Nb alloy showed a highest self-corrosion potential in pH2 lactic acid solution, an easy passivation and wide passivation range in pH2.5 lactic acid solution, and a stable passivation film in pH4 lactic acid solution. The passivation film consists of mainly TiO2 and small amount of Nb2O5 and MoO3. Pit corrosion had taken place when Cl- ion contained solution was applied to Ti-10Mo-10Nb alloy.Biocompatibility tests revealed a hemolysis coefficient of 0.883% for Ti-10Mo-10Nb alloy, which was much smaller than 5% and wouldn't cause acute hemolysis. No evident inflammation and histological change such as epidermic growth were observed in stimulation tests of mucous membranes of mouth, indicating that Ti-10Mo-10Nb alloy isn't irritant and trauma to the mucous membranes of mouth. 4 weeks after implantation of Ti-10Mo-10Nb alloy into white rat, the fabulous capsule was found to be loose without a clear boundary with tissues, while small amount of inflammation cells were observed such as neutrophil cells, lymphocytes, monocytes and plasma cells, no multinuclear girnt cells and foreign body giant cells were found. 8 weeks after the implantation, fabulous capsule wall turned thinner, the amount of inflammation cells reduced with remaining monocytes and lymphocytes. The histological reaction is regarded as light for this alloy. Biocompatibility tests revealed that Ti-10Mo-10Nb alloy has the excellent biocompatibility.Casting characteristic tests revealed that an excellent casting performance of Ti-10Mo-10Nb alloy in the specified dental titanium casting machine. A 98% of casting value can be reached at room temperature. Crowns have been cast successfully using this alloy. In the reaction layer of Ti-10Mo-10Nb alloy with investment materials, Al showed a diffusion thickness of about 20μm, the distribution of Si and Zr was not uniform with a diffusion thickness of about 15μm, and segregation was observed for Ti and Mo. The all test results indicated that Ti-10Mo-10Nb alloy has the excellent properties and is suitable to be as dentisty mateial.