A comparative study of the microstructure and oxidation behavior in supersonic and conventional air plasma sprayed thermal barrier coatings

Abstract

This paper reports the results from a comparative study on the microstructure and oxidation behavior in yttria-stabilized zirconia (YSZ) top-coat produced using supersonic and conventional air plasma spraying process. Microstructure analysis showed that the air plasma sprayed (APS) YSZ possessed porosity (~12%) two times higher than that of the supersonic air plasma sprayed (SAPS) YSZ. Higher porosity seems to provide more channels to the oxidative and corrosive gasses to percolate the ceramic layer in APS YSZ top-coat. At 400°C, the measured total ionic conductivity of APS YSZ was 1.5 times higher than that of SAPS YSZ. This indicated that the migration of oxygen ions was restricted in SAPS YSZ top-coat. The analysis showed that the SAPS YSZ with lower ionic conductivity and lower porosity offered a better oxygen barrier to produce a finer thermally grown oxide (TGO) during isothermal oxidation. This results in a lower weight gain during cyclic oxidation than APS YSZ samples at 1000°C. The effect of microstructures on the ionic conductivity of top-coat were analyzed experimentally as well as by applying molecular dynamic (MD) simulations. The results confirmed that the size of single pore or crack volume in SAPS YSZ was much smaller than that of APS YSZ. The improvement in ionic conductivity has strong relationship between the total porosity and volume of the single pore. The oxygen ions are found to trap in the regions around the pores, which led to the lower tracer diffusion coefficient of YSZ ceramic coating.