Influences on Quantitative Nitriding Layer Thickness Measurements using Model-Based Photothermal Radiometry

Abstract In nitriding furnaces, the nitriding result is currently only controlled indirectly via the nitriding potential based on gas sensors. Detrimental properties such as soft spots, insufficient compound layer thickness or strongly porous zones, which might result from reduced surface reactivity, are thus only detected post-process. Therefore, in-process measurements of the layer formation promise a real benefit for energy efficiency and process quality enhancement. Photothermal radiometry is a promising contactless method for layer inspection that so far showed qualitative correlations of the photothermal phase signal with material parameters and layer thicknesses. In this article, thickness and thermal conductivity of the compound layer are quantitatively determined by using a physical signal model for a least-squares approximation of in-process measured photothermal phase signals. In addition, the influence of roughness and surface curvature is investigated, with the model-based photothermal layer thickness measurement showing robustness to different surface conditions and allowing quantification of the layer thickness with uncertainties < 1 μm even during in-process measurement inside an industrial nitriding furnace.