Fracture and deformation of brittle materials using in-situ indentation testing

This study develops methods of evaluating deformation and fracture behavior of thin films and bulk materials. Micro/nano indentation techniques are used to test materials in conditions which may alter their mechanical performance. These conditions include corrosive environments and applied stresses.; An anodic titanium oxide was selected as a representative metal oxide/metal system. Oxide films were electrochemically grown at different scanning rates. The in-situ mechanical testing of the titanium oxide/titanium composite was carried out by nanoindentation coupled with scanning probe microscopy in 0.1 M sulfuric acid. The variation in mechanical properties was revealed to be dependent on the film growth rate, with a higher film strengthening effect at faster film growth rate. TEM structural characterization showed that the films evolved from an amorphous structure, to a mixture of crystalline islands in an amorphous matrix, to a completely crystalline structure with the decrease in the film growth rate. Correlations between electrochemical polarization, structural characteristics and the mechanical behavior of these anodic films are discussed in relationship to electrostrictive stresses that may lead to the breakdown of passive films.; A model has been developed to quantitatively predict the mechanical response prior to oxide fracture for the case of a hard film (titanium oxide) on a soft substrate (titanium). During loading contact, the hard film undergoes elastic deflection that includes both bending and membrane-stretching effects, while the substrate is elasto-plastically deformed. The plastic deformation of the substrate is successfully solved by introducing a Hertzian-like solution. The model works well for surface films thicker than 20 nm. Additionally, the maximum radial tensile stress in anodically grown titanium oxide, which is responsible for film cracking at the critical load, is calculated to be approximately 15GPa.; In addition, the effect of an applied bi-axial stress field on the fracture behavior on silicon was studied via Vickers indentation. The principle of linear superposition of stress intensity factors for individual stress fields was applied for model development. The theoretical applied stress—crack size function fits the experimental data well. The deviation of theoretical function from experimental data due to scatter of fracture toughness and geometric constraints is discussed.