In-line Process Control in the Active Screen Plasma Nitrocarburizing Using a Combined Approach Based on Infrared Laser Absorption Spectroscopy and Bias Power Management

摘要

The active screen plasma nitrocarburizing (ASPNC) technology is an extension of the ASPN method in that carbon-bearing gases such as CH_4 are admixed to the N_2-H_2 process gas. Besides process gas composition and working pressure, a bias power applied to the component surface is an important parameter in the nitrocarburizing process governing the competing uptake between nitrogen and carbon on the metallic surface. A variation of the bias power during the treatment provides the possibility of the dynamic control of both the nitriding and the carburizing potential without change of process gas composition. A two-stage process was implemented to improve the quality of the compound layer. The application of advanced in-situ and non-intrusive spectroscopic method of laser absorption spectroscopy in the mid infrared spectral range provides the valuable information about the plasma chemical reactions in the afterglow plasma, supplying the absolute concentrations of stable and transient molecular species monitored direct in process. The amount of hydrogen cyanide (HCN) measured in-situ by QCLAS diagnostics was found to be a reliable parameter to control the carburizing potential of the nitrocarburizing atmosphere.