Microstructure, mechanical properties, and adhesion in IN625 air plasma sprayed coatings

The effects of feed stock particle size on the microstructure and the mechanical properties of IN625 air plasma sprayed coatings were investigated. It was found that the porosity increases with the particles size while the oxide content rises with decreasing particle size. Because of the lower velocity of large particles, these produce weakly bonded splats resulting in coatings with low elastic modulus, low residual stresses, small oxide content, and low hardness. The porosity is high and it is largely comprised of surface connected pores. Small particles produce splats that are bonded through oxide layers. Due to higher particle speed the porosity and the fraction of connected pores are low. The small porosity produces a relatively large value of Young's modulus and high residual stresses. The hardness increases as the porosity decreases and the oxide content increases. The coating adhesion exhibits a maximum at intermediate particle sizes. This is caused by the limiting effect of the oxide content for small particles and the limiting effect of low particle velocities for large particles.