Theoretical investigations of surface interactions on chromium carbide and fe-substituted chromium carbide surfaces

Abstract

Chromium carbide is used as a protective coating on various industrial tools and components due to its hardness, toughness and chemical stability. The wear resistant properties of chromium carbide materials have been known but the corrosion resistance of chromium carbide as a coating agent is yet to be considered. Also, during surface coating, diffusion of Fe (base metal) into the chromium carbide surface takes place. Thus, considering these two phenomena, this project has been aimed towards the investigation of the surface oxidation phenomenon on the chromium carbide surface and Fe-substituted chromium carbide surface. Using molecular modelling approach based on Density Functional Theory (DFT), the electronic and structural properties of the bulk and surfaces of chromium carbide and Fe-substituted chromium carbide has been investigated.

It has been widely known that Cr₂₃C₆, Cr₇C₃ and Cr₃C₂ are the three stable crystallographic phases of chromium carbide materials and Cr₃C₂ is the most stable phase. Using electronic and structural properties obtained from computational calculations, the chromium carbide phases were revaluated and Cr₃C₂ was found as the most stable chromium carbide bulk phase which is in line to information available in the literature. Additionally, the stability and surface free energy values of different Cr₃C₂ surfaces, when determined, revealed Cr₃C₂ (1 1 1) as the most stable surface. Three different Fe-substituted chromium carbide bulk systems, Cr₂Fe1C₂, Cr₂Fe2C₂ and Cr₂Fe3C₂ were also studied using DFT calculations. Cr2Fe3C2 was found to be the most stable system. Further, various surfaces of Fe-substituted chromium carbide systems were also investigated. Based on stability and surface free energy values, Cr₂Fe3C₂ (1 1 1) was obtained as the most stable Fe-substituted chromium carbide surface.
The process of surface oxidation was then investigated using stepwise adsorption method on Cr₃C₂ (1 1 1) and Cr₂Fe3C₂ (1 1 1) surfaces. It was determined that O₂, followed by H₂O and CO₂ oxidants, is the adsorbent primarily responsible for degrading these surfaces at room temperature and normal pressure. The influence of temperature, oxygen partial pressure and chemical potential on the stable oxygen coverages on Cr₃C₂ (1 1 1) and Cr₂Fe3C₂ (1 1 1) surfaces was also obtained. Additionally, on the basis of phase diagrams, the stable oxygen coverage at different oxygen partial pressure and temperature values was easily identified. It was revealed that diffusion of Fe into the hard-facing surface, enhanced oxidation and reduced the strength of the surface coating.

Date of Award	Aug 2019
Original language	English
Supervisor	Vinuthaa Murthy (Supervisor) & Kannoorpatti Krishnan (Supervisor)