Influence of Holding at High Temperatures and Heat Cycles on Young's Modulus and Hardness of Thermal Barrier Coatings Measured by Nano-Indenter

Abstract

Changes of the Young's modulus and hardness of thermal barrier coatings (TBCs) isothermally held at high temperatures or subjected to heat cycles were experimentally studied using a nano-indenter. 8mass% yittria-stabilized zirconia (YSZ) was plasma-sprayed on a Ni-based superalloy with a CoNiCrAlY bond coat layer. The specimen was isothermally held or heatcycled at temperatures of 1323 K, 1373 K, and 1473 K, and the Young's modulus and hardness were then measured using the nano-indenter. Both the Young's modulus and hardness results for the isothermally held specimens showed almost constant values at a maximum indentation load of 100 mN, and monotonically increased with holding time at a maximum indentation load of 1000 mN before reaching constant values. When the specimens were heatcycled at maximum temperatures of 1323 K, 1373 K, and 1473 K, both the Young's modulus and hardness, measured at a maximum indentation load of 1000 mN, decreased with increasing heating time after reaching maximum temperature. The heating times corresponding to the maximum indentation values decreased as the heating temperatures increased. Cross-sectional observation showed that sintering of the intersplat followed by cracking occurred in the top coat. Equations to predict the changes of the Young's modulus and hardness were semi-theoretically derived and agreed with the experimental result.