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

Abstract

Confocal Raman piezo-spectroscopy was applied to the non-destructive evaluation of residual stresses as they develop in a chemical-vapor-deposited Al 2O 3 coating on a Si 3N 4 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 Al 2O 3 coating layer were measured using both the Cr 3+ fluorescence band, located at 14,400 cm −1 (R 1), and the Al 2O 3 Raman band at 417 cm −1. When the R 1 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 Al 2O 3 coating, the residual stress field stored within the Si 3N 4 substrate could also be measured as a function of depth (according to the piezo-spectroscopic shift of the 206 cm −1 Raman band of Si 3N 4). 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.