High hardness alumina coatings prepared by low power plasma spraying

Alumina coatings were prepared by low power plasma thermal spraying (2.5–4.0 kW). The powders were directly injected into the region between the cathode and the anode intake inside the plasma torch, and the carrier gases were transferred to the plasma flame. The results show that the hardness of Al2O3 coatings by low power plasma spray with an internally-fed powder system is higher than that of the coatings by APS with an externally-fed powder. The hardness of the coating increases with increasing plasma energy up to HV=1500 kg/mm2, when the plasma arc voltage and current are 80 V and 50 A, respectively, and the porosity is estimated lower than 1%. The coatings are mainly composed of γ-Al2O3 although the original powders were nearly all α-Al2O3. The amount of the γ-Al2O3 in the coating increases, and the α-Al2O3 decreases with increasing plasma power. The grain size of the coating is 0.1–1.2 μm. The optimum spraying conditions have been established by analyzing the relation of the coating properties with spray parameters.