In-situ determination of bridging stresses in Al2O3/Al2O3-platelet composites by fluorescence spectroscopy

Bridging stresses arising from interlocking and frictional effects in the crack wake have been quantitatively evaluated in an Al2O3/Al2O3-platelet ceramic, using in-situ microprobe fluorescence spectroscopy. Crack opening displacement (COD) profile has also been quantitatively measured in the scanning electron microscope (SEM), in order to substantiate the reliability of the piezo-spectroscopic measurements of microscopic bridging stresses. Mapping the crack wake (at critical condition for crack propagation) with a laser probe of 2 mu m spatial resolution led to determine a discrete map of closure stresses over a crack extension of about 800 mu m. Relatively high bridging stress values approximate to 350 MPa were revealed due to platelet interlocking in a near-tip bridging zone less than 100 mu m, whereas frictional sites of lower stress magnitude less than 100 MPa were monitored in the crack profile farther away from the crack tip. The availability of microscopic fracture parameters like as the bridging stress distribution and the near-tip COD profile enables to quantitatively explain the rising R-curve behavior of the Al2O3/Al2O3-platelet material. Bridging stress distribution, COD profile and R-curve data are discussed in comparison with those collected in previous studies on equiaxed Al2O3 and roughened Si3N4 The present study supports the notion that crack bridging is by far the most important toughening mechanism in non-transforming ceramics. (C) 1999 Elsevier Science Limited. All rights reserved.