3. Results and discussion
3.1. Microstructure and crystalline phase of coatings
Fig. 1(a) shows the SEM images of the spray-granulated feedstock for APS deposition. Most feedstock exhibit a typical spherical morphology with narrow particle size distribution, which is beneficial for the flowing and close packing of particles during the deposition process. In order to verify the dispersion homogeneity of the composite powders after the spray drying process, some granules were immersed in epoxy resin, ground and polished for cross-sectional analysis as shown in the inset in Fig. 1(a). It is found that the ZnO phase (bright particles) are well dispersed in the amorphous Al2O3 (gray zones) matrix. Compared to the addition of Al2O3 powder, the use of aluminum sol as the Al2O3 resource could significantly improve the dispersion of ZnO absorbers thus the reproducibility of the method. Furthermore, the fully mixed composite suspension with a certain viscosity can be used for spray granulation process directly. Therefore,the experimental processes were simplified simultaneously.