2.2. Coating preparation
The coatings were prepared by the low power plasma spraying system with Ar and N2 as plasma gases at a flow rate of 20 and 2 standard liters per minute (slpm), respectively. More details about the spraying parameters were shown in Table 1. The sprayed coatings with the thickness around 3 mm were produced on graphite substrates. After APS deposition, they were mechanically scratched from the substrates and thermally annealed at 300, 500, 900 ◦C in air and at 900 ◦C vacuum atmosphere. The heating rate and annealing time were fixed as 10 ◦C/min and 5 h, respectively. The composition of raw materials and annealing conditions are shown in Table 2.