Dynamic nitriding zone development during plasma nitriding for the prediction of mechanical surface properties (DynP²)

• Applicants: Professor Dr.-Ing. Horst Biermann; Professor Dr.-Ing. Thomas Lampke
• Subject Area: Coating and Surface Technology ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 547149748

The present project application "Dynamic nitriding zone development during plasma nitriding for the prediction of mechanical surface properties" entails the development and validation of a nitriding model for predicting the nitriding zone formation and property profile of steels after plasma nitriding. The focus is on the diffusion-dependent nitrogen distribution and the mutual influence of carbon present in steel. By using diffusion-reaction and cross-diffusion approaches, existing models for the simulation of plasma nitriding processes are expanded to calculate not only nitrogen concentration but also carbon concentration and microstructure development within the compound layer. The required diffusion coefficients and solubility limits of the occurring iron nitride phases gamma' and epsilon, as well as the matrix, are determined through derivation and numerical approximation of inverse problems for carbon and nitrogen. The goal is to describe the diffusion coefficients and solubility limits as a function of temperature and alloy system. All input and validation data needed for modelling are experimentally determined using a full factorial test matrix with variations in process parameters such as temperature, time, and atmospheric composition. The resulting nitriding zones are structurally, chemically, and mechanically characterized. In-situ measurements by laser absorption spectroscopy are used to quantify the NH₃ concentration within the plasma-activated gas. The compound layer structures resulting from tests on pure iron samples are correlated with NH₃ concentration, analogous to the interaction between nitriding potential and iron nitride phases in gas nitriding processes. The concentration of NH₃ resulting from specific process parameters (power, pulse-pause/duty cycle, plasma power density) in a plasma nitriding process, considering further process parameters (pressure, H₂ content in the process), forms the basis for developing a robust process parameter. In addition to the specific variation of carbon content, the alloying element chromium is also investigated to achieve a transfer from pure iron and unalloyed steels to classical (nitriding) steels. The main goal of this project is to predict the complex dependencies between process parameters and resulting layer properties in plasma nitriding processes through a significantly reduced experimental scope, using the developed nitriding model. This removes the need to evaluate large parameter windows for steels with different chemical compositions.

• DFG Programme: Research Grants