A thermal sprayed coating is produced by the deposition of numerous consequent layers formed by flattening and solidifi- cation of molten or partially molten particles impinging at high velocity on the substrate. Fig. 1(b)–(d) are cross-sectional SEM morphologies of the sprayed ZnO/Al2O3 composite coatings and the coatings annealed in air and vacuum atmospheres. The bright areas correspond to ZnO phase while the gray areas are related to Al2O3 and ZnAl2O4 phases. It can be observed from Fig. 1(b) that A1coating shows a dense and homogenous structure. In addition, a small amount of pores can be found in A1 coating, as indicated by arrows, and these pores are found to appear preferentially around the unmelted or partially melted particles trapped in the coating. The porosity of the composite coating increased evidently after annealing at 900 ◦C for 5 h in air and vacuum, respectively, as shown in Fig. 1(c) and (d). The coating porosity with different annealing treatment condition were measured and exhibited in Table 2 according to Archimedes principle. The porosity of the coatings annealed in air increased gradually with higher annealing temperature. Moreover, the porosity of the coating annealed in vacuum is lower than that annealed in air, which is in agreement with the SEM results between Fig. 1(c) and (d). It is noteworthy that proper porosities would increase thermal shock and cycling resistances, which is advantageous to its application in the high-temperature environment [26].