Damage analysis for thermally cycled (Ti,Al)N coatings—estimation of strength and interface fracture toughness

The thermal shock behaviour of (Ti,Al)N-coated WC/Co-hard metals was investigated using the laser thermal shock method. By short surface irradiation with a high power Nd-YAG laser, well defined thermal shock cycles were realised in the coating. Scanning electron microscopy and a focused ion beam based cross-section technique were used to characterize the damage in the surface. There are several damage stages depending on maximum surface temperature and number of thermal cycles. A systematic presentation of the observed damage stages is given in a damage map. The experimental results are explained by residual stress turning from compression to tension due to high temperature creep in the coating. The coating strength as well as the interface fracture toughness for the (Ti,Al)N coatings on the hard metal substrate can be estimated from the damage map in a straightforward way, without the knowledge of creep parameters.