Effect of Melting Behavior of Al2O3 Powder on the Structure and Dielectric Properties of Plasma-Sprayed Coatings

Depositing an Al2O3 insulating coating on the inner or outer ring of a bearing can effectively prevent shaft current from passing through it, thereby preventing electrical deterioration. Herein, the effect of spraying parameters on the temperature and velocity of in-flight Al2O3 powder particles, the morphology of the resultant splats, and the microhardness and dielectric characteristics of related coatings were thoroughly investigated. The results reveal that the densest Al2O3 coatings with a porosity of 3.43% can be achieved when the powders are thoroughly melted in the plasma flame without over-melting. Otherwise, porosities and microdefects increased in coatings deposited by semi-molten or over-molten powders. Moreover, due to the higher density, the dielectric strength of the sealed coating is greatly boosted. The number of defects (particularly longitudinal cracks) in the coating is the major reason for the decrease in dielectric strength. Furthermore, the increase in coating thickness significantly reduces the dielectric strength of the coating, which should be attributed to the increase in the number of defects generated by the thickness increase. The essence of dielectric breakdown of the Al2O3 coating is the combined mechanisms of corona discharge and thermal breakdown, and the corresponding dielectric breakdown model was constructed.