Calibration of the Raman effect in α–Al₂O₃ ceramic for residual stress measurements

The pressure shifts of the 418 and 379 cm⁻¹ Raman lines of polycrystalline α-Al₂O₃ ceramic were measured at room temperature to 3190 MPa. A diamond anvil high pressure cell was used in conjunction with a microRaman spectrometer system. Pressures were measured by the ruby fluorescence method. Distilled water was used as the pressure transmitting medium. The pressure dependence of the wave number for the hydrostatic range (to 1585 MPa) was found to be 0.00220 ± 0.00007 cm⁻¹ MPa⁻¹ for the 418 cm⁻¹ Raman line and 0.0011 ± 0.0001 cm⁻¹ MPa⁻¹ for the 379 cm⁻¹ line. Both shifts are based on a linear least squares fit to the data within a 95% confidence interval. Measurements that included the nonhydrostatic regime (to 3190 MPa) gave similar shifts, but with almost twice the uncertainty. Absolute residual stresses in alumina ceramics are in the 200 to 600 MPa range with higher values, 1790 MPa, reported for single-crystal alumina. For the 418 cm⁻¹ Raman line, these values correspond to shifts of 0.4, 1.4, and 3.9 cm⁻¹, respectively. The sensitivity of the microRaman technique is sufficient to detect and measure residual stresses of this magnitude.