Online inspection of thermo-chemical heat treatment processes with CCD camera system

Plasma nitriding belongs to the group of the thermo chemical surface heat treatments. During this process nitrogen is dissociated into the surface of the material increasing hardness, wear resistance, endurance strength and/or corrosion resistance. This paper presents a new inspection system based on a CCD camera system for monitoring such heat treatment processes (PACVD, plasma assisted chemical vapour deposition). Treatment temperatures commonly used are within the range of 350°C to 600°C. A near infrared enhanced CCD camera system equipped with specifically chosen spectral filters is used to measure spectral emittances during the surface modification. In particular the spectral operating range of 950nm to 1150nm of the silicon CCD camera is utilized. The measurement system is based on the principles of ratio pyrometry (dual-band method) known from non-contact temperature measurements, in which two images of the same scene, each taken at slightly different spectral bands, are used to determine the spectral light characteristics. This results in an improved relative sensitivity for spectral changes (i.e. deviations from the gray-body hypothesis) during the surface modification.