Fracture mechanics tests for measuring the adhesion of magnetron-sputtered TiN coatings

Experiments for determining the adhesion strength between TiN coatings and high speed steel substrates have been performed by using the three-point bending test and a modified shear test. Sample preparation is easier for the modified shear test; however, the interpretation of results is more complicated. The three-point bending test yields fracture mechanics data such as the interface fracture energy G_c and the fracture toughness K_c. The most critical problem is to prepare a sample with a notch which initiates crack propagation at the film-substrate interface. Various sublayers with weak adhesion, such as carbon, oxide and metals, have been tested as a notch, as well as thin mechanical slits. The results obtained so far demonstrate that the adhesion strength depends on the substrate cleaning treatment and on the impurity gas content in the receiver. Typical values of the fracture toughness K_c for substrates that have not been sputter cleaned are in the range 1–3 MN m^-3/2. Advantages and disadvantages of the various methods used are analysed and critically discussed with respect to sample preparation, reproducibility and data evaluation.