Fracture in single-layer zirconia (YSZ)–bond coat alloy (NiCoCrAlY) composite coatings under thermal shock

Abstract

Yttria-stabilized zirconia (YSZ)–NiCoCrAlY bond coat (BC) alloy composites constitute the individual layers of functionally graded thermal barrier coatings (TBCs). Characterizing the fracture response of these composites under thermo-mechanical loads is vital for the design of durable graded TBCs. Accordingly, thermal fracture in plasma sprayed single-layer YSZ–BC alloy composite coatings, subjected to a single high heat flux laser heating-ambient cooling cycle is investigated. Coatings with three different YSZ volume fractions (100%, 80% and 40% YSZ), all having similar thermal resistance were studied. It was found that under similar thermal loads, longer surface cracks as well as horizontal cracks near the coating–BC layer interface resulted in coatings with higher BC alloy (lower YSZ) content. These findings are discussed by considering the effective time-dependent (viscoplastic) behavior and fracture toughness of these composites predicted from micromechanics models. Potential implications of these results on the design of graded TBC systems are briefly discussed.