Design of iron system for enhancement of nitridation rate: multi-scale computer simulation

Abstract

Nitridation has been the most promising heat treatment which is aimed at improving surface hardness. Moreover, it also leads to multi-functional properties such as heat resistance, wear and corrosion resistance owing to incorporation of chemically stable nitride compounds. Those are multi-functional nitriding products to be expected to utilize in various industries. To form a nitride compound, nitrogen should be diffused into pure bcc Fe. But nitrogen has extremely low solubility in iron (0.1~0.3 at%). Thus, nitridation has not happen well. Among various nitridation methods, I focused on gas nitridation method which is the typical method in the nitridation technology due to low cost and eco-friendly. However, that is time consuming process (process time: 40~100h). Until now, the most studies which improve the nitridation rate have been conducted by experimental approach. So various research groups has underwent trial and errors. In this thesis, alternative methods, I approached to understand the theoretical knowledge. For approaching the fundamental theory and detailed discussion, I attempted to use the multi-scale atomistic simulation such as Density Functional Theory (DFT), Molecular Dynamics (MD) and Kinetic Monte Carlo (KMC). In chapter 1, the overview of my research which includes objective of this study and recent trends as well as outline will be treated. And then, I will introduce the scheme of atomistic computational methods as the theoretical method. Those are divided by three kinds of methods (DFT, MD, KMC). In addition, those methods are also described in detail. As well as, I will be mainly dealing with estimation of nitridation rate and suggesting iron system to obtain activation energy of nitrogen behavior. Therefore, I present theory of CI-NEB method in DFT, MSD method in MD and probability of reaction event in KMC which is available to real-time scale simulation. In chapter 2, I focused on the full exact nitridation mechanism to understand fun...