Contact damage and fracture of ceramic layer structures

Contact damage and fracture of ceramic layer structures are studied in this thesis. Hertzian indentation is used to generate contact damage in hard coatings on soft substrates in a wide range of material combinations: ceramic/ceramic, ceramic/metal, and ceramic/filled-polymer. Specimen preparation methods for each system and experimental procedures for contact damage and fracture analysis are described. Test variables studied include elastic-plastic mismatch (controlled by changing material combinations) and coating thickness. The evolution of mechanical damage within the coating and substrate layers is studied. Upon loading, "quasi-plasticity" occurs in the ceramic substrates, plasticity in the metal substrates, and visco-plasticity in the filled-polymer substrates. During loading, fracture occurs in the coatings in ceramic/ceramic and ceramic/filled-polymer systems: cone cracks initiate at the top surface and propagate downward, while inverted cones initiate at the coating/substrate interface and propagate upward. In ceramic/metal systems quasi-plasticity occurs in the coating. Finite element analyses of the stress fields in contact loading reveal direct correlations between the damage patterns and appropriate stress components: tensile stresses in the case of fracture, and shear stresses in the case of plasticity or quasi-plasticity.