Thermal Stability of Plasma-Sprayed Thick Thermal Barrier Coatings Using Triplex ProTM-200 Torch

Segmentation-crack structured yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS) using a Triplex Pro™-200 gun. In this work, free-standing coating specimens (~700 μm) were isothermally heat-treated in air from 1200 to 1600 °C for 24 h and at 1550 °C for 20 to 100 h, respectively. The thermal aging behaviors such as microstructures, phase compositions, grain growth and mechanical properties were characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and a Vickers hardness test. The results indicated that the as-sprayed coatings mainly consisted of metastable tetragonal (t′-YSZ) phase, but the t′-YSZ gradually partitioned into equilibrium tetragonal (t-YSZ) and cubic (c-YSZ) phases due to yttrium diffusion during thermal exposure, and with an improvement in temperature, the c-YSZ may retain or transform into another yttrium-rich tetragonal (t″-YSZ) phase. The transformation of t-YSZ to monoclinic phase (m-YSZ) has occurred after 1550 °C/40 h heat treatment, and the content of the m-YSZ phase increased with the prolongation of the thermal exposure time. The variations of Vickers hardness have a correlation with pores healing and grain growth, which might be attributed to the coating sintering and m-YSZ phase formation. Furthermore, the growth pattern of the grains was investigated in detail. In service, cracks and pores proceeded along the grain boundaries, especially surrounding the small grains. It is conducive to the engineering application of TBCs fabricated with the Triplex Pro™-200 gun.