Effect of reflectivity and porosity analysis of plasma sprayed lanthanum zirconate/yttria stabilized zirconia based thermal barrier coating: design of experimental approach

Lanthanum zirconate (LZ) is a promising material for thermal barrier coatings (TBCs) due to its low thermal conductivity, high phase stability, reflectivity, and lower annealing activity compared to yttria-stabilized zirconia (YSZ). However, LZ's insulating performance deteriorates at elevated temperatures due to increased radiative heat transfer. In this study, plasma-sprayed LZ and YSZ in equal weight proportions (50:50) onto Inconel 718 substrates. Reflectivity was measured using UV-VIS spectrometers, and porosity was determined through optical microscopy with image analysis. Empirical models were developed to correlate atmospheric plasma spraying (APS) parameters (power, spray distance, and powder feed rate) with porosity and reflectivity using central composite design (CCD) and response surface methodology (RSM), validated by ANOVA. The models achieved R2 values of 0.9972 for porosity and 0.9903 for reflectivity, indicating strong agreement with experimental data. Power was the most influential parameter, followed by spray distance and feed rate. Annealing at 1200 degrees C for 4 h resulted in a maximum reflectance of 89% and porosity of 26% (experimental condition 7), while condition 17 yielded the lowest reflectance of 45% with 4% porosity. These results highlight the critical role of microstructural control, as higher porosity enhances reflectivity through increased light scattering. The findings suggest that optimizing the porosity within a specific range improves both thermal insulation and reflectivity, which is essential for high-temperature applications in industries such as aerospace and power generation.