WEB Phase transformation and damage behaviour in zirconia investigated by acoustic emission measurements
The stress-assisted martensitic phase transformation of partially MgO-stabilized zirconia (Mg-PSZ) from the cubic/tetragonal phase into the monoclinic phase was investigated under compressive loading by in situ acoustic emission measurements. The AE measurements were corroborated by scanning electron microscopy and X-ray diffraction. The continuously recorded AE data streams during compression tests up to 2 GPa were analysed by Fast Fourier Transformation and an adaptive sequential k-means clustering algorithm. This allows to study the kinetics both of the stress-assisted phase transformation as well as the damage process of Mg-PSZ. Two different AE sources were identified. The first type contains high amplitude transient signals characterized by a very short rise time. These AE signals are characterized by high energies and a narrow band of median frequency around 300 kHz. Signals of this cluster are caused by initiation and propagation of brittle cracks. A second type is characterized by lower amplitudes compared to the previous one. But these are still transient signals, however with an increased rise time. These signals have lower energies compared to previous ones. They occur also in a narrow band of median frequency, but at higher frequencies of about 400 kHz. These signals are caused by the martensitic tetragonal/monoclinic phase transformation. The results of the AE data analysis were supported by additional XRD measurements for interrupted tests at different stress levels, which clearly revealed an increase in the volume fraction of monoclinic phase above 1.2 GPa. Both the AE results and the results of XRD measurements are supported by literature data claiming on a transformation stress in compression of above 800 MPa for the tetragonal/monoclinic phase transformation.