2. Experimental
2.1. Feedstock preparation
Commercial ZnO powder (≥99.0 wt%; D50 < 9 m) and aluminum sol with solid concentration 25 wt% were used as raw materials. The two raw materials were milled together in the weight ratio of 1:1 for 2 h by planetary milling using zirconia ball and deionized water as the medium, followed by which the finely dispersed mixtures were granulated by using spray-drying technology to produce composite feedstock particles. After oven-dried at 100 ◦C for 4 h, the resultant composite particles were passed through a 100–300 mesh sieve to acquire an appropriate particle size range for plasma spraying.