Stress analyses and in-situ fracture observation of wear protective multilayer coatings in contact loading

A model was developed to describe stress distributions in hard multilayer coatings under applied contact load, simulating coating application for wear protection. Elastic deformations were considered and an algorithm was suggested to calculate normal and shear stress across the coating thickness for various numbers of layers and layer thickness. The model was applied to analyze contact stress in Ti–TiN multilayer coatings produced by vacuum arc deposition. Calculated locations of maximum shear stress agreed well with locations of actual coating failures, which were studied in-situ using a scanning electron microscope (SEM). A three-point bending test stage was designed for SEM observations of deformation and crack development under a cylindrical indenter. Results indicated that the highest shear stress occurs under the substrate/coating interface and that this region may plastically deform causing coating adhesive and cohesive failure. When using a multilayer coating design with 10 pairs, the peak stress was moved into the coating volume, reducing normal and shear stresses in the substrate by as much as 40 and 22%, respectively. A good correlation between computational and experimental studies verified the model applicability for optimizing the design of hard multilayer coatings in tribological contacts.