Enhancing the CMAS resistance of YSZ thermal barrier coatings via hybrid suspension/solution precursor plasma spraying with gadolinium doping

Liquid feedstock plasma spraying is a promising method for producing thermal barrier coatings (TBCs) with a microporous structure, offering low thermal conductivity and good compliance to thermal stresses. However, the high porosity reduces their resistance to CMAS (calcium–magnesium–alumino–silicate) infiltration. This study explored a hybrid suspension/solution precursor plasma spraying (SPS/SPPS) technique to improve CMAS resistance and thermal cycling performance. A 25 wt% 8YSZ suspension in ethanol was doped by dissolving various concentrations (0.25 M, 0.50 M, and 0.75 M) of Gd(NO3)3·6H2O. Coatings were deposited on Hastelloy-X substrates with a NiCoCrAlY bond coat, both as single layers and as top layers over conventional porous or dense-vertically cracked (DVC) 8YSZ. Microstructural, phase, and micromechanical analyses, including pillar splitting tests, were performed. Gd-doping enhanced the CMAS resistance by forming a protective apatite phase, although its benefits plateaued beyond a certain concentration. This method offers a flexible, cost-effective route to advanced TBCs.