Rockwell indentation testing as a method of establishing the interfacial fracture toughness of thermal barrier coatings is investigated. To this end, indentation tests have been systematically performed on coatings with yttria-stabilized zirconia top coat deposited by electron beam physical vapor deposition. Specimens in “as-coated” condition and after heat treatment in air have been studied. Unexpectedly, indentation of the heat-treated samples resulted in smaller delaminations than the as-coated samples, suggesting an increase in fracture toughness for coatings subjected to elevated temperatures. Careful image analyses of the cross-section of the indented area show that the ceramic top coat undergoes a complex damage evolution during indentation that is altered by thermal treatment. The consequences of this are discussed and we note that care must be taken when evaluating fracture parameters for multilayered structures based on indentation testing.