CCD based emissivity measurements for surface characterization in heat treatment processes

Plasma nitriding is an industrial surface hardening process which involves diffusion of nitrogen atoms into the metal surface to enhance material properties. The aim of this work is the development of a low cost device (based on a standard near infrared enhanced CCD camera) for the inline-characterization of such surface modifications during thermo-chemical heat treatment for improved process control. Due to relatively low process temperatures of about 400°C - 600°C the emitted light intensity of the heated metal parts is very weak. The imaging device therefore primarily utilizes the CCD-silicon's spectral sensitivity for wavelengths around 1μm. As the emissivity of metals generally increases strongly at shorter wavelengths, the considered spectral range has certain advantages with regard to sensitivity for emissivity changes. Therefore, the measurement system is based on the principles of ratio pyrometry (dual-bandmethod), in which two images of the same scene, each taken at slightly different spectral bands, are used to determine the spectral light intensities. This results in an improved relative sensitivity for spectral changes (i.e. deviations from the gray-body hypothesis) which makes it possible to correlate the measured spectral intensities with surface modifications during the plasma nitriding process.