In-depth analysis of residual stress in an alumina coating on silicon nitride substrate using confocal Raman piezo-spectroscopy

Confocal Raman piezo-spectroscopy was applied to the non-destructive evaluation of residual stresses as they develop in a chemical-vapor-deposited Al2O3 coating on a Si3N4 ceramic substrate. According to a selected confocal configuration of the optical probe, with its focal plane set to in-depth scan the sample, the residual stress could be measured at various depths along the thickness of both coating and substrate. The residual stresses stored in the Al2O3 coating layer were measured using both the Cr3+ fluorescence band, located at 14,400 cm−1 (R1), and the Al2O3 Raman band at 417 cm−1. When the R1 fluorescence band was used, no variation could be resolved for the residual stress along the coating depth direction; in contrast, a clear in-depth stress distribution was observed when the 417 cm−1 Raman band was used. The minimum stress magnitude was located at the coating external surface and the maximum at the coating/substrate interface. Given the high transparency of the Al2O3 coating, the residual stress field stored within the Si3N4 substrate could also be measured as a function of depth (according to the piezo-spectroscopic shift of the 206 cm−1 Raman band of Si3N4). A deconvolution procedure of confocal spectra was proposed, which is based on the knowledge of the probe response functions of both coating and substrate materials. Laser probe deconvolution enabled us to retrieve the actual in-depth stress distribution from the stress distribution experimentally observed by defocusing experiments.