Fe-C Compounds Research Articles

Two-dimensional FeC compound with square and triangle lattice structure – Molecular dynamics and DFT study

The stability, electronic structure and magnetic behaviors of the two-dimensional (2D) FeC compounds with square and triangle lattice structure are studied by the density functional theory (DFT) calculations. We find that both forms of 2D FeC, one with a flat square lattice structure denoted as t-FeC and another with a puckered triangle lattice structure denoted at tr-FeC, are stable after DFT optimization. However, calculations of the phonon dispersion and binding energy show that the puckered 2D FeC with a triangle lattice structure is the most stable form. We find that both forms of the 2D FeC compounds are metallic and ferromagnetic materials, however, the mean atomic magnetic moment of Fe in the puckered 2D tr-FeC is significantly smaller than that of the flat 2D t-FeC. We check various interatomic potentials for the Fe-C systems implemented in the LAMMPS package and find that only 2D t-FeC can be formed ‘naturally’ from the melt while it does not occur for the 2D tr-FeC. The crystallization temperature (TX≈2600K) and mean interatomic distance for the Fe-C pair of the flat 2D t-FeC have reasonable values. Various structural defects and mechanical behaviors of the flat 2D t-FeC compound are found and discussed.

Exploring the intrinsic ductile metastable Fe-C compounds: Complex chemical bonds, anisotropic elasticity and variable thermal expansion

As the vitally important phases in iron and steel, the Fe-C binary compounds are investigated by first-principles calculations combined with quasi-harmonic approximation (QHA). The formation enthalpy and Gibbs free energy are positive, but the phonon dispersion are stable, indicating θ-Fe3C, o-Fe7C3, h-Fe7C3, γ-Fe23C6, χ-Fe5C2, η-Fe6C, η-Fe2C and ε-Fe3C are metastable phases in this binary system for the first time. The electronic structure indicates that the complex bonding behavior of these carbides are the combinations of covalent, ionic and metallic bonds, which determines the anisotropic elastic properties and novel intrinsic ductility of these carbides. The mechanical modulus is nearly proportional to the carbon content. o-Fe7C3 has the largest B value as 379 GPa and The anisotropic bulk modulus, shear modulus and Young's modulus are discussed by the anisotropic index and three-dimensional surface contours. The shear modulus range from 40 GPa to 204.2 GPa and the Young's modulus vary from 113.7 GPa to 512.6 GPa. Intrinsic hardness is predicted by semiempirical models. The thermal electronic contributions to thermodynamic properties are studied using the Mermin statistics for the dynamic stable phases. The volumetric thermal expansion coefficients of these Fe-C compounds vary between 1.0 × 10−5 K−1 to 5.0 × 10−5 K−1 at 1200 K. θ-Fe3C, h-Fe7C3 and η-Fe2C have the closest thermal expansion coefficient to α-Fe. Considering the thermal electronic contribution, the calculated heat capacity of θ-Fe3C is in good agreement with experimental results. η-Fe2C has the largest CP value as 0.72 J g−1 K−1 and η-Fe6C has the smallest CP value as 0.56 J g−1 K−1 at 1200 K.

Stability, Magnetism and Hardness of Iron Carbides from First-Principles Calculations

The stability, electronic structure, magnetism and hardness of iron carbides in eight different space groups were investigated by the first-principles calculations. The electron-ion interactions were modelled using the Vanderbilt ultra-soft pseudopotentials (USPP) and norm-conserving pseudopotentials (NCPP). Generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formulation was used to describe the exchange-correlation energy. The calculated lattice parameters for USPP were in good agreement with experimental results. The formation enthalpy, cohesive energy and elastic constants revealed that with the exception of hex-Fe2C, all Fe-C compounds were both thermodynamically and mechanically stable. Meanwhile, the iron carbides’ bulk and shear moduli were also calculated and compared. Furthermore, magnetic properties were studied and it was found that the average magnetic moment of Fe23 C 6 (2.123 μ B) was the largest among these compounds, with a value close to that of pure α-Fe (2.24 μ B μ. As expected, hex-Fe2C is a paramagnetic metal based on the obtained densities of states and partial densities of states. Furthermore, the magnetic behaviour of iron carbides with polymorphic transformation was almost identical. Finally, several semi-empirical methods were used for estimating the hardness of the iron carbides.

The Influence of the Technological Process on the Surface Quality and Tribological Properties of Powder Details

<p class="R-AbstractKeywords"><span lang="EN-US">Nowadays the materials made by methods of powder metallurgy are widely used for producing details of the heavy-duty friction pair. Due to high reliability and effective combination of mechanical and tribological properties such materials are usually used in transport industry, for example for gear assemblies of different kinds of automobiles, brake system of high speed trains, aircrafts etc. There are many variations of powder materials, but for friction pairs the iron based antifriction materials, for example Fe-C-Cu, Fe-P, Fe-Cu-Sn, Fe-Cu-Sn-Pb-MoS<sub>2</sub>, Fe-C-Cu-Ni-Mo and others are successfully used. In recent years, the requirements for antifriction materials are raised highly because of more intensive freight traffic and speed of vehicles. On the other hand, the demand of more inexpensive materials and effective using of natural resources is the issue of the day.</span></p><p class="R-AbstractKeywords"><span lang="EN-US">The aim of this paper is to offer the new low-cost metal powder material based on Fe-C compound with content of Ni and Mo up to 0.3%. The influence of the parameters of technological process (pressing and sintering) on physical properties (porosity, density), mechanical properties (hardness) as well as on 2D roughness and 3D texture parameters was studied and is presented in this paper as well. The “Taylor Hobson Ltd” 3D measurement system has been used in metrological study. The analysis of the influence of the technological process on the surface roughness parameters is given. The tribological properties (friction coefficient) were evaluated using “ball-on-disk” testing without lubrication. The analysis of achieved results is prove the appropriateness of using of new low-alloy metal powder material on the iron base for producing details of friction pair. One of the possible applications - bushes for braking systems of railway rolling stock. Using offered material allows reducing the product cost in complex with acceptable wear resistance and durability.</span></p>

Microstructure, mechanical properties and tribological behavior of tribofilm generated from natural serpentine mineral powders as lubricant additive

Surface-modified serpentine powders with an average size of 1.0μm were dispersed into mineral base oil to improve the lubricating properties of oil, as well as to generate a thin tribofilm on the worn surface. SEM, TEM, nano-indentation and Stribeck testing were performed to study the morphology, microstructure, micromechanical properties and tribological behavior of the tribofilm, respectively. Results show that a nanocrystalline tribofilm, with a thickness of 500–600nm, is formed on the worn surface under the lubrication of oil with 1.5wt% serpentine. The film is mainly composed of Fe3O4, FeSi, SiO2, AlFe and Fe-C compound (Fe3C). A phenomenological model of the tribofilm generated by serpentine was developed based on the experimental results. The excellent mechanical properties, reinforced phase of embedded particles and porous structure of the tribofilm contribute to the reduction of friction and wear, especially in the case of boundary and mixed lubrication.

The Microstructures and Catalysis of Metallic Films Covering Diamond Single Crystals Synthesized from Fe-Ni-C System

The metallic films covering diamond play a very important role in the course of diamond nucleation and growth from Fe-Ni-C system at high-temperature and high-pressure (HTHP). The microstructures and surface morphologies of the metallic films are investigated by field-emission scanning electron microscope (FESEM) in this paper. It is shown that there are large numbers of small particles and a series of serrations on the inner surface of the films. It is calculated that the metallic film is mainly composed of (Fe, Ni) 3C, (Fe, Ni)2 3C6, but no diamond structure. The results by energy dispersive spectrometer (EDS) analysis are in good agreement with these by transmission electron microscopy (TEM). It can be concluded that the carbon atom groups coming from the decomposition of Fe-C compound rich in carbon in the film are deposited on the diamond surface directly and diamonds grow in the model of sandwich.

Characterization of carbon and iron nanostructures synthesized by the DC arc discharge method: influence of the location in the reactor and of the pressure

X-ray diffraction, Mossbauer spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques were used to characterize the Fe-C phases in the soots synthesized by the DC arc discharge method. Various equilibrium and non-equilibrium Fe-C compounds were identified, with fractions depending on both the location in the reactor and the helium gas pressure. The soots obtained are composed of the same five phases (C-graphite, α-Fe, γ-Fe, Fe3C and Fe5C2) whatever the helium gas pressure and wherever they are situated in the reactor. However, the location in the reactor has a considerable influence on the size of the particles in the nanostructure. The Fe-C compounds in the Pyrex vessel samples (CL) seem only to be present in the form of nanoparticles embedded in an amorphous gangue, while the water-cooled copper cylinder samples (RS) contain, in addition to these nanoparticles, large composite crystalline particles.

Temperature and pressure effects on the iron carbides formed by carbon ion implantation

Metallic iron foils were implanted at near room temperature, with 3.8 × 10 17 carbon ions cm −2 and subjected to thermal treatments up to 450°C. By means of conversion electron Mössbauer spectroscopy (CEMS), the thermal evolution of the iron carbide series, ϵ-Fe 2C, χ-Fe 5C 2, and θ-Fe 3C was followed. In the initial stages, the predominance of the ε precipitates was observed, changing later to χ and θ carbides. The implanted carbon atoms combine in the form of Fe-C compounds, and start diffusing out of the CEMS region at a temperature around 450°C. Application of a pressure of 3 GPa enhances the annealing process by reducing the temperature necessary for the onset of χ and θ carbides. This may be interpreted as being caused mainly by a pressure increase in the solubility of C in Fe.