| Medical titanium alloy is the optimal choice for use in prosthetics and substitution of hard tissues such as bone. It is used in joint prostheses, dental implants and bone trauma products for its excellent mechanical strength and corrosion resistance. Titanium alloy implants need certain surface modifications prior to implantation in order to improve their corrosion resistance and bioactivity. Currently, both in China and abroad, preparation of titanium alloy implants is carried out after mechanical processing shaping, where the process chain is long, the equipment processing is complicated, and the efficiency is low while the cost is high. For these reasons, this study proposes a new technique for the preparation of titanium alloy implants, i.e., conducting surface modification while carrying out dry cutting and shaping of implants, where the heat produced during cutting is utilized for oxidative modification to improve corrosion resistance, and certain surface topography is obtained by machining to improve bioactivity. This technique plays a vital role in shortening the process chain, improving efficiency, reducing cost, protecting the environment, enhancing the preparation technology of titanium alloy implants, and ensuring the health of users of such implants. This is a prototypical, "green" machining technique for the preparation of titanium alloy implants with the additional benefit of sustainable development. The main research work is as follows:First, the formation mechanism of titanium alloy oxide films is studied and the formation condition of compact titanium dioxide films is determined according to oxidation thermodynamics. The movement of titanium ions and oxygen ions during the oxidation process of titanium alloy is analyzed to reveal the main controlling factors of the oxidation rate of titanium alloy on the basis of theories of physical chemistry. The study results show that the ideal temperature range for the formation of compact oxide films of titanium dioxide is 600-900℃, and the main controlling factor of the oxidation rate of titanium alloy is the diffusion rate of oxygen ions passing through the oxide film within this temperature range.Second, the main influencing factors of oxidation rate are explored based on the titanium alloy oxidation kinetics model established according to Fick’s first law and the Arrhenius formula, and the influence of each factor on oxidation rate is determined by analyzing the titanium alloy oxidation kinetics model with MATLAB. The results show that the main influencing factors of the oxidation rate of titanium alloy are oxidizing temperature, oxygen concentration and pressure, fineness of grains, and oxide film thickness. The oxidation rate of titanium alloy can be accelerated by improving the above factors with exception to the oxide film thickness, as increasing oxide film thickness will decrease the oxidation rate of titanium alloy.Then, a test platform of the cutting technique under oxygen-enriched atmosphere is set up. The cutting parameters are optimized with the finite element simulation software Deform 3D to control the cutting temperature within the optimum temperature range. Cutting tests are conducted under different cutting parameters and oxygen-enriched environments, and varying composition, thickness, surface appearance and surface grains of the titanium alloy’s finished surface oxide film. The influences of cutting temperature, oxygen concentration and fineness of grains on the oxidation rate are analyzed. The validity of the oxidation kinetics model is verified by comparing the test results and theoretical results. The results show that the cutting technique under oxygen-enriched atmosphere can significantly increase the thickness and compactness of the oxide film on the finished surface of titanium alloy. The influence rules of cutting temperature, oxygen concentration and fineness of grains on oxidation rate are found to be consistent between the test results and theoretical results, which validates the oxidation kinetics model.Lastly, the biocompatibility of machining titanium alloy surface under oxygen-enriched atmosphere is evaluated. The corrosion resistance of the finished surface is studied, the corrosion mechanism of titanium alloy in simulated body fluid is analyzed and the main factors that influence the corrosion resistance of titanium alloy are determined through simulated body fluid soaking tests and electrochemical corrosion tests. Cell behaviors such as adhesion, spreading, proliferation, differentiation and mineralization are observed, the bioactivity of the finished surface is studied, and regulation mechanism of the finished surface on cell behaviors is analyzed through in vitro cell culture. The results show that machining under oxygen-enriched atmosphere can significantly improve the corrosion resistance of titanium alloy, and facilitate protein absorption with micro/nanometer composite structures formed on the surface. This also has a good control effect on cell behaviors such as adhesion, spreading, and proliferation, thus prominently improving the biocompatibility of biomedical titanium alloy implants. |
PDF Full Text Request