Mesoscale levels of strain and fracture of coated materials

According to investigation on plastic deformation and failure within the framework of physical mesome-chanics, surface layers of a loaded solid are an individual structural level of strain origin and development. If surface layers are treated or a protective (strengthening) coating is deposited, the pattern of strain origin and development is substantially changed. The restriction of dislocation plasticity results in increase of local stress concentrator level, while the character of strain development is governed by pattern of their occurrence and relaxation. Development of the considered processes depends on thickness and structure of a coating, relationship between mechanical properties of the coating and substrate, thickness ratio of the interfaced materials, presence of transient layers etc. Also, the basic factor is composition loading scheme, since the latter defines strain localization degree under applying external loading to a composition. In the present work investigations of plastic deformation origin and evolution in materials with protective coatings during their stretching, compression, 3-point static and cyclic bending were carried out using high-resolution television-optical technique TOMSC. It is shown, that incompatibility of strain development in a coating and substrate under tension results in occurrence of local bending-rotation-torsion zones in the vicinity of the interface. The size of such zones can vary from a scale of entire specimen cross-section down to areas comparable with thickness of the coating. During investigations carried out under cyclic bending the compositions, in which mechanical characteristics of the substrate significantly and insignificantly differed from those of the coating were studied. Complex pattern of localized plastic deformation evolution under friction and wear is related to extremely localized pattern of loading applying. On the basis of the results obtained some recommendations on coating and interface formation are formulated with the purpose of increasing their performance.