BENDING TESTS TO ESTIMATE THE THROUGH-THICKNESS STRENGTH AND INTERFACIAL SHEAR-STRENGTH IN COATED SYSTEMS

As an alternative to scratch testing, four-point bending of coated coupons has been used to investigate the fracture and interfacial behaviour of steel samples with thin hard TiN coatings. The bending jig is small enough to fit into the chamber of a scanning electron microscope. Simple bending of strips has produced crack arrays running right across the sample width but which can only be seen in the scanning electron microscope with the sample bent. The spatial distribution of these cracks was determined for various degrees of bending. This was found not to obey a Poission distribution, so interference of cracks was inferred. With the aid of Monte Carlo computer simulation techniques, the degree of this interference was determined allowing us to estimate the shear strength of the interface between the coating and the substrate. The analytical model used is similar to that for stress transfer in fibre composites such that the presence of the crack lowers the tensile stress over a critical distance. The results are presented and the implications for the use of coated systems discussed.