In-Situ Determination of bridging stresses in Al 2O 3/Al 2O 3-platelet composites by fluorescence spectroscopy

Abstract

Bridging stresses arising from interlocking and frictional effects in the crack wake have been quantitatively evaluated in an Al 2O 3/Al 2O 3-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 μm spatial resolution led to determine a discrete map of closure stresses over a crack extension of about 800 μm. Relatively high bridging stress values ≈350 MPa were revealed due to platelet interlocking in a near-tip bridging zone <100 μm, whereas frictional sites of lower stress magnitude <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 Al 2O 3/Al 2O 3-platelet material. Bridging stress distribution, COD profile and R-curve data are discussed in comparison with those collected in previous studies on equiaxed Al 2O 3 and toughened Si 3N 4. The present study supports the notion that crack bridging is by far the most important toughening mechanism in non-transforming ceramics.