Evaluation of thickness and residual stress of shallow surface regions from diffraction profiles

Residual stresses resulting from surface treatment methods like shot peening generally show a more or less steep stress gradient versus depth. X-ray diffraction is the most used method for depth profiling in case of near-surface stress gradients due to the small penetration depth of X-rays resulting in a high depth resolution. When the stress gradient is very steep and the stressed surface layer is in the range of the penetration depth of the X-rays, asymmetric diffraction lines may occur and determination of the diffraction line position has to be done with great care. In case of the stressed surface layer being thinner than the penetration depth of the X-rays a (partial) splitting of the diffraction line may occur which may allow to separately calculate both the mean stresses of the layer and the base material and additionally to determine the thickness of the stressed surface layer. For a shot peened silicon nitride ceramic the evaluation of the mean stress in the thin surface layer, the stress in the underlying base material and the thickness of the stressed surface layer on basis of profile fitting methods is demonstrated with special emphasis on the influence of the method used for determining the diffraction line position.