Graded thermal barrier coatings. Cracking due to laser irradiation and determining of interface toughness

The damage resistance of (graded) thermal barrier coatings (TBCs) can be quantified by cyclic surface heating with laser beam. Damage was found to develop in several stages: vertical cracking -> delamination -> blistering -> spalling. This sequence can be understood as an effect of progressive shrinkage due to sintering during thermal cycling, which subsequently turn into delamination cracks. Taking into account TBC shrinkage, delamination crack propagation is modelled by means of FEM. An increase of interface fracture toughness due to grading and a decrease due to aging has been measured in a 4-point bending test modified by a stiffening layer. Correlation with the observed damage in cyclic heating is discussed. It is explained in which way grading is able reduce the damage.