Fractal character and fracture toughness of plasma sprayed yttria-stabilized zirconia coatings

Yttria-stabilized zirconia (YSZ)-based thermal barrier coatings (TBCs) in micron size were deposited by supersonic atmospheric plasma spraying (SAPS) and conventional atmospheric plasma spraying (APS). During the spraying, a commercially available online diagnostic system (SprayWatch-2i) was used to monitor the velocity and surface temperature of the in-flight particles for each operating condition. The microstructures of the as-sprayed coatings in two spraying conditions were characterized by the scanning electron microscopy and the image analysis technique. The fracture toughness and elastic modulus were measured by the indentation method. The power law of area-perimeter which originated from the fractal theory was employed to quantitatively characterize the irregular morphology of the pores in two spraying conditions. The relationship of fractal dimension and fracture toughness was also investigated. The results show that, the fractal dimension can characterize the irregular morphology of pores. The coatings deposited by SAPS and APS show the good fractal character. The fractal dimension of the coatings deposited by SAPS is 1.12 times than that of the coatings deposited by APS, showing the more complex pore structure and the higher fracture toughness.