Quantitative optical fluorescence microprobe measurements of stresses around indentations in Al2O3 and Al2O3/SiC nanocomposites: The influence of depth resolution and specimen translucency

Residual stresses around 1 kg Vickers indentations in Al2O3 and Al2O3/SiC nanocomposites were measured using high-resolution Cr3+ fluorescence microscopy. Experiments and modelling showed that the use of non-confocal microscopes can lead to significant underestimation of the surface stress in Al2O3 because of the sampling of subsurface regions where the stresses are lower. The nanocomposites were less sensitive to the depth resolution of the microscope because their strong absorption limited the depth from which fluorescent radiation was collected. The use of confocal microscope settings allowed accurate measurements to be made and the indentation stresses were found to be very similar in Al2O3 and the Al2O3/SiC nanocomposites. The stresses measured were significantly different from the predictions of the Yoffe model for indentation stresses. This was because of indentation cracking, which is not accounted for in the model. Cracking was also considered to be important in determining the plastic zone size in ceramics, which is much smaller relative to the indentation size than in metals.