A new method of determining strength and fracture toughness of thin hard coatings

Abstract A new method is presented for measuring strength and fracture toughness of thin films. For that purpose a bulk steel substrate is pre-cracked by means of fatigue loading for enabling partly or complete separation of the specimen prior to the coating process. During bend loading the film bridging the crack breaks first at the front face of the specimen; from this the film strength can be calculated. The successive crack growth on the sides faces caused by further loading allows the evaluation of the fracture toughness of the film. The method is successfully applied to PACVD hard coatings of the system Ti–Al–C–N on S6-5-2 steel substrate. For TiN hard coatings the film strength is determined to be 491 MPa. The fracture toughness amounts to 8.7 MPa √m for TiN, 7.9 MPa √m for TiCN and 3.8 MPa √m for TiAlN.