Influence of columnar grain microstructure on thermal shock resistance of laser re-melted ZrO₂-7wt.% Y₂O₃ coatings and their failure mechanism

In this study, influence of columnar grain morphology on thermal shock resistance of laser re-melted ZrO₂-7wt.% Y₂O₃ thermal barrier coatings (TBCs) has been researched. The influence of variation in columnar grain microstructures on the fatigue mechanism of laser re-melted coatings in thermal shock experiments has been investigated. The thermal shock experiments have confirmed the positive effect of small grain size on thermal fatigue life. This is thought to have resulted from an increase in the number of grain boundaries that restrict the grain motion during deformation. Further, "void effect" could be responsible for promoting peeling of the laser re-melted coatings when spallation is less than 10% because it can cause a relatively large tensile stress between the columnar grains and the underlying splats. However, it is found to have negligible effect on the shedding of LRCs shed when the spallation percentage is 20% or more because the remaining LRCs are relatively homogeneous having less or no voids. The presence of separate columns and large intergranular clearance may have allowed better compliance of stress through free-movement of the individual columnar grains. There are many more restrictions on stress relaxation. An additional moving resistance is found to be present at the undulate intergranular interfaces for the irregular gear-like columnar grains with small inter-crystalline space and large intergranular contact area. The one-piece structure with pseudo-columnar grains was found to improve the thermal cycle lifetime slightly. It is believed to be due to its relatively lower strain tolerance as well as lower fracture toughness as compared to that of the structure with separate columnar grains.