Effect of preexisting surface cracks on the interfacial thermal fracture of thermal barrier coatings: an experimental study

The presence of a population of long surface pre-cracks with large crack density in thermal barrier coatings has been shown to increase the coating thermal strain tolerance, thus leading to the reduced crack driving force and less crack propagation at the TBC–bondcoat interface. Laser thermal shock tests were performed to experimentally study the effects of the pre-crack length and density. A high power CO2 laser was utilized to apply the concentrated high heat flux on the top surface of the 8 wt.% YSZ beam specimens with pre-crack length of 15, 50 and 100% of the coating thickness. The lengths of surface and interface cracks forming after the thermal shock were measured. The results showed that, depending on the coating thickness, the shortest and longest pre-cracks resulted in longer interface cracks. The results suggest that an optimum pre-crack length exists which would minimize the coating interface fracture.