3. Results and discussion
3.1. Characteristics of the oxide layer formed on Ti–15Mo alloy after thermal oxidation
3.1.1. Surface morphology of the oxide layer and its evolution as a function of treatment temperature and time
The morphological features of the oxide layer developed on Ti–15Mo alloy after TO at 500 and 650◦C for 8 h is shown in Figs. 2(a)and (b), respectively. When treated at 500◦C for 8 h, the surface of Ti–15Mo alloy is covered with thin oxide grains (Fig. 2(a)). An increase in treatment temperature from 500 to 650◦C led to an increase in the rate of growth of the oxide film as well as the sizeof the oxide grains that makes the oxide film relatively rougher(Fig. 2(b)) than those prepared at 500◦C. Nevertheless, some defects in the form of fine pores and large craters are also observed(Fig. 2(b)). The occurrence of these defects could be attributed to the buildup of stress in the oxide layer following its higher rate of growth at 650◦C. An increase in the size of the oxide grains is also observed when the treatment time is increased from 8 to 24 h when treated at 500◦C while an increased surface coverage and formation of many oxide nodules are observed at 650◦C under similar conditions. Since the growth of the oxide is less pronounced at 500◦C, further characterization studies are limited to those treated at 650◦C or higher.