TiO2 film formation by electron cyclotron resonance plasma oxidation

Introduction: Definite difference of thermal expansion between Ti and TiO2 relates with the difficulty to prepare stable TiO2 film on Ti. Electron cyclotron resonance (ECR) plasma is high-active plasma and is suitable for generating highly quality crystalline films at low temperatures. This study aimed to estimate the characteristics of TiO2 film prepared on Ti substrate by ECR plasma oxidation.

Methods: Conditions of ECR plasma oxidation are as follows. Microwave power was set at 900 W. Oxygen gas pressure was 3.3 × 10-3~9.3 × 10-1 Pa and the oxidation time was 5~60min. The substrates (10 × 10 × 1 mm) of Ti (CP-Ti, JIS Grade 2) were used. An infrared lamp was used for controlling the substrate temperature in the range from room temperature (RT) to 600°C. Calcification experiment was conducted by immersing the TiO2 films on Ti substrates in phosphate-buffered saline at RT. The solution used was slightly supersaturated with respect to dicalcium phosphate dehydrate (DCPD) and supersaturated with both octacalcium phosphate (OCP) and hydroxyapatite, which promotes potentially the nucleation and calcification of these calcium phosphate crystals. After the calcium phosphate was deposited on the substrate, the substrate was examined by x-ray diffraction and scanning electron microscope to identify the crystalline phase of the deposits.

Results: The crystalline phase of the calcified deposits was identified to be OCP and was accompanied by DCPD as a minor constituent, which is consistent with the prediction by the degree of the supersaturation. However, the formation of HA was inhibited, suggesting that the surface oxidation by ECR modifies the surface characteristics to enhance OCP precipitation. OCP on the TiO2 was converted into HA 1day after simulated body fluid immersion

Conclusion: The surface conditions, obtained by various ECR plasma oxidation, control the induction of calcium phosphate nucleation and the growth.