| 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. |
