Fluorescence studies of polycrystalline Al₂O₃ composite constituents: piezo-spectroscopic calibration and applications

Abstract

Optical fluorescence microscopy (OFM) was used to quantify the effect of applied stress or strain upon the position of the R fluorescence line of αAl₂O₃ composite constituents (fibers and matrices) prior to composite processing. Polycrystalline Nextel^TM Nextel 720 fibers were tested under tension and compression by means of a cantilever beam technique, whereas the polycrystalline matrix was tested in compression. The position of the R fluorescence line was correlated to applied strain and stress in order to provide the piezo-spectroscopic calibration curve and the corresponding coefficients for both sensors, which form the basis for interpretation of frequency shifts from full, all-alumina, composites. The piezo-spectroscopic coefficients of the polycrystalline matrix were found to be 2.57 cm^-1 GPa^-1 and 2.52 cm^-1 GPa^-1 for the R1 and R2 lines respectively, whereas the coefficients for the polycrystalline αAl₂O₃ Nextel 720 fibers were found to be 3.07 cm^-1 GPa^-1 and 2.91 cm^-1 GPa^-1 for the R1 and R2 lines, respectively. The effects of collection probe size, as well as penetration depth, are discussed. The established piezo-spectroscopic behavior is used inversely to quantify the residual stresses in the as-received fibers due to the presence of sizing, as well as in the thermally grown alumina layer of an industrial thermal barrier coating.