Direct measurement of crack shielding in ceramics by the application of Raman microprobe spectroscopy

Abstract

Raman microprobe spectroscopy was employed to measure stresses in ceria partially stabilized zirconia/alumina (Ce-TZP/Al2O3) composites toughened by transformation zone shielding. The near crack-tip stresses in in situ loaded compact specimens were measured by measuring stress-induced shifts in the frequency of a Raman peak corresponding to the tetragonal zirconia phase. The peak shift as a function of the applied stress was separately calibrated using a ball-on-ring flexure test. Both analytical function-fitting methods and numerical methods that determine centroids were evaluated for measuring the shift in the frequency with applied stress. A method based on locating the centroid of the intensity peak with a moving range of frequencies was selected because it gave the best correlation between the frequency shift and the applied stress. The stresses measured within an inner core of the transformation zone (∼30 μm), where the volume fraction of the transformed monoclinic phase was essentially constant, were used to estimate a local crack-tip stress intensity (Kl). The amount of total crack shielding (ΔKs) was then estimated from the applied stress intensity (Ka) and the estimated local crack-tip stress intensity. Comparisons with the predictions of two different theories of zone shielding indicated that transformation toughening models can account for 80 to 89% of the measured crack shielding, depending upon the value of the crack-tip stress intensity assumed in the models.