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 using the three-point bend test and a modified shear test. Sample preparation is easier for the modified shear test; however, the interpretation of the results is more complicated. The three-point bend test yields fracture mechanics data such as the interface fracture energy Gc and the fracture toughness Kc. The most critical problem is to prepare a sample with a notch which initiates crack propagation at the coating/substrate interface. Various sublayers with weak adhesion have been tested as a notch, such as carbon, oxides, and metals, 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 contamination of the sputter gas with oxygen. Typical values of the fracture toughness Kc for non-sputter-cleaned substrates are in the range of 1-3 MN m 3/2. The advantages and disadvantages of the various methods used are analysed and critically discussed with respect to sample preparation, reproducibility, and data evaluation.