Stress mapping in engineering ceramics

In many wide bandgap materials, the wavelength of the luminescence signal is useful to determine stress and strain fields. Luminescence in these materials results from energy transitions involving defects—point defect clusters or impurities—that place an energy level in the forbidden energy gap of the crystal's electronic structure. In many materials, the precise energy of this transition varies as a result of changes in the electronic structure of the crystal under applied stress. Spectrum imaging with high spectral and spatial resolution enables the stress field to be determined. The most noted application example of stress mapping is associated with alumina (using the R-line doublet associated with Cr3+); however, other examples including dielectric materials such as SiO2 in semiconductor devices.

Stress mapping around a Vickers indent (white, dashed box) in an engineering ceramic. Non-linear least-squares fitting was used to map the position and separation of the R-line doublet of Cr3+ in alumina. The typical R-line doublet is shown in (a) and the map of peak energy of R1 enables hydrostatic stress to be determined (b) while the shear stress can be determined from the separation of the two peaks in the doublet (c).