On growth of suspension plasma-sprayed coatings deposited by high-enthalpy plasma torch

Introduction of suspension plasma spraying (SPS) brought novel coating microstructures unachievable by other thermal spraying techniques. Both columnar and dense layers are of particular interest for various applications. Deeper understanding of the coating buildup mechanisms is essential for tailoring of the coatings microstructure and properties. This paper contributes to elucidation of the buildup mechanisms with focus on high enthalpy plasma torches, in particular the hybrid water/argon-stabilized plasma torch WSP-H 500. The main differences between the formation of finely structured dense coatings and columnar coatings were studied for alumina and yttria-stabilized zirconia (YSZ) by comprehensive evaluation of individual splat formation, gradual coating growth, and geometrical characteristics of the deposits. It was clearly demonstrated that the coating microstructure is generally controlled by morphology of individual deposited splats which is influenced namely by formulation of the suspension and mean trajectory of the fragmented suspension droplets in the plasma jet, both influencing size, flattening ratio and thermal history of deposited splats. Larger droplets having proper thermal treatment provided miniature lamellae similar to conventional plasma spraying, eventually leading to the development of low porosity (dense) structures. With decreasing splat size and increasing distance from the plasma jet axis, coatings tended to develop more porous columnar structures. After characteristic number of deposition cycles, lateral growth of the columnar structures became saturated, i.e. the equivalent diameter of the columnar structures became stabilized at about 75 μm, when observed from the free-surface. Material deposited from the plasma fringes was only loosely bonded and could be easily blown away from the surface which explains absence of interpass porosity in the deposited coatings.