Influence of impact angle on the resistance of a DLC/steel coating system to cyclic micro-impact fatigue

To experimentally simulate the combination of compression and shear forces that coated systems can be subjected to in highly loaded contacts, inclined micro-impact tests have been performed on a multilayered DLC/steel coating system. The coating behaviour was strongly dependent on both applied load and impact angle. At 100 mN, the coating remained resistant to lateral chipping at all angles. Under inclined cyclic impact at higher load there was a marked reduction in the coating resistance to failure by through-thickness cracking and lateral chipping in comparison to tests under perpendicular impact. At 200 mN, chipping occurred in 7 of 10 tests at 70°, compared with only 2 of 10 tests at 80°. At 300 mN, the median number of impacts to failure decreased from 89 cycles at 90° to 48 cycles at 80° and 14 cycles at 70°. At 400 mN, failure occurred rapidly under all loading orientations, so the influence of angle became less pronounced. Analytical simulation of the induced stresses under inclined impact showed enhanced substrate and coating plasticity together with very high tensile stresses at the surface at the rear of the impact combined with very high compressive stresses at the opposite side. The reduced fatigue resistance under inclined impact results from the combined effects of enhanced substrate deformation, increased coating strain and asymmetric tensile stresses combined with higher compressive stresses that accelerate crack initiation and propagation. Depth-sensing inclined micro-impact testing, where surface degradation is continuously monitored throughout the test through changes in test probe position, provides a sensitive method to assess coating durability under combined normal and shear loading that is more representative of many practical tribological contacts.