Influence of plasma spray conditions on the structure of Al₂O₃ coatings

The influence of plasma spray conditions on the structure of Al₂O₃ coatings was quantiatively estimated with three structural parameters: mean thickness of flattened particles, vertical crack density in the flattened particles and mean bonding rate at the interfaces between flattened particles. All the particles have experienced sufficient flattening to disks of mean lamellar thickness of 1.5 μm to 2 μm, which depends on the velocity, temperature and size of particles. It is quantiatively illustrated that there exists limited bonding at the interfaces between flattened particles. The maximum mean bonding rate is about 32%. The mean bonding rate is quickly saturated to the maximum when plasma power was increased to 24.5 kW and abrupt drop in mean bonding rate had been observed with the change of spray distance from 100 mm to 150 mm. The coating sprayed with powders of small mean grain size shows a less bonding rate. The mean bonding rate at the interfaces between flattened particles is likely much influenced by the temperature of particles. It is confirmly shown that vertical cracks occur in all Al₂O₃ coatings with vertical crack density over 1 per 10 μm. The fact that the higher mean bonding rate the higher vertical crack density suggests that the vertical cracks occur during rapid cooling of flattened particles after solidification.