Carbide Layer Growth Research Articles

Cathodic Wear by Delamination of the Al4C3 Layer During Aluminium Electrolysis

In aluminium reduction cells, an electrochemical reaction occurs between the molten electrolyte film below the aluminium pad and the carbon cathode blocks, leading to the formation of an Al4C3 layer on the cathode blocks. The properties and role of this Al4C3 layer are therefore important for the aluminium production industry, as they could help increase the life expectancy of electrolysis cells and impact the resistive voltage losses. The purpose of this study is to gain a better understanding of the formation, growth and mechanical stability of the aluminium carbide layer formed on top of the cathode block. A reliable scenario describing both the mechanical and electrochemical behaviours of the Al4C3 layer is proposed. For different industrial graphitized cathode grades, a series of experiments were carried out in a bench-scale Hall-Heroult electrolysis cell and the Al4C3 layer formed on top of the cathode was characterized. Thereafter, the CALPHAD method was combined with density functional theory simulations to estimate the electrical and physical properties of Al4C3 together with the phase equilibria occurring at the interface between the carbide layer and the aluminium pad and the cathode blocks respectively. From these calculations, a scenario for carbide layer growth and mechanical stability was established.

Thermal stability and growth kinetics of the interfacial TiC layer in the Ti alloy/carbon steel system

The thermal stability and growth kinetics of the titanium carbide interfacial layer, formed in the course of the diffusion bonding of low-alloy carbon steel (0.3wt.% C) and Ti alloy, were investigated. Thermal stability of the titanium carbide interfacial layer was evaluated based on the thermodynamic analysis of the Fe–Ti–C ternary system. Thermodynamic analysis of the Fe–Ti–C system confirmed that the titanium carbide layer is stable in contact with the steel part of the diffusion couple. An experiment with inert markers at the interface confirms that growth kinetics of the TiC layer is governed by carbon diffusion from steel to titanium alloy through the titanium carbide phase. In the 800–950°C temperature range, carbon diffusion in austenite was found to be a rate-determining step of the titanium carbide layer growth during the initial stage of the interaction (<40min). For advanced stages, the thickness of the layer depends on two simultaneously occurring processes, namely flow of carbon atoms through the titanium carbide layer and that from the titanium carbide layer into the titanium alloy. The estimated values of the carbon diffusion coefficient and the activation energy of the process reflect the grain boundary mechanism of carbon diffusion through the interfacial layer.

Reactive Chemical Vapour Deposition of Titanium Carbide from H2-TiCl4 Gas Mixture on Pyrocarbon: A Comprehensive Study

Abstract In Reactive Chemical Vapour Deposition (RCVD), the absence of one element of the deposited carbide in the initial gas phase involves the consumption/conversion of the solid substrate. In this way, the growth of a continuous carbide layer on the substrate requires solid-phase diffusion of the reagent. In this work, a parametric study of the RCVD of titanium carbide from pyrocarbon (PyC) and an H2-TiCl4 mixture has been carried out. Conversion ratio, PyC consumption and carbide layer growth kinetics have been determined at 1000C. The influence of the H2/TiCl4 dilution ratio has been also investigated. The apparent inter-diffusion coefficient of the carbon through the TiC deposited layer and the direct apparent reaction rate were determined from a comparison between simulations based on a Deal-Grove-type model and the experimental results. The study has been completed with FTIR spectrometry analyses of the gases.

Microstructure of alumina-matrix composites reinforced with nanometric titanium and titanium carbide dispersions

-titanium powders which were thoroughly mixed under high energy ball-milling and through the second step it was induced the formation of titanium carbide during different times at 500 °C by the cementation packing process. SEM and EDS analysis of the microstructures obtained in both sintered and cemented bodies were performed in order to know the effect of the activated carbon used as cementing agent on the titanium for each studied composite. It was observed that a titanium carbide layer growth from the surface into the bulk and reaches different depth as the titanium content in the composites increases. On the other hand, the use of ductile titanium notably enhanced density level and fracture toughness of the composites.

Kinetics of tungsten carbidization under non-isothermal conditions

Kinetic laws of high-temperature interaction between tungsten and methane are studied under non-isothermal conditions in the temperature interval 1273–2873 K. Computer Assisted Electrothermography was applied during which thin metallic wires 100 μm in diameter were directly heated up by electric current in the carbon-containing atmosphere. Experimental data on weight gain, carbide layer growth, and microstructure of phases are obtained in the linear heating regime at heating rates from 10 to 1500 K/s. We established that the interaction between tungsten and methane in a wide interval of pressure and heating rates may proceed with simultaneous or consecutive formation of W 2C and WC carbide phases. Experimental data on weight gain and carbide layer growth were processed using calculation schemes deduced within the framework of a reaction diffusion model with the first type boundary conditions under non-isothermal conditions. Kinetic and diffusion constants are determined for tungsten carbidization with formation of the W 2C phase.

Carbon diffusion in cementite (Fe 3C) and Hägg carbide (Fe 5C 2)

Pure iron samples were carburised at 500 ∘C in CO–H 2–H 2O–H 2S gas mixtures to study the growth kinetics of metastable iron carbides. The carbon activity was varied in the range 7 ≤ a C ≤ 4580 . Small amounts of H 2S were added to the atmosphere to prevent graphite deposition and metal dusting from occurring. The gas composition was optimised to obtain parabolic carbide layer growth over a sufficiently long time period. At activities a C < 150 parabolic growth of cementite (Fe 3C) was observed. At higher activities a C > 150 Hägg carbide (Fe 5C 2) formed on top of the cementite. From the dependence of the growth rate on carbon activity in the gas phase the self-diffusion coefficients D C ∗ of carbon at 500 ∘C were determined for both iron carbides Fe 3C and Fe 5C 2.

Inter-diffusion of carbon into niobium coatings deposited on graphite

The inter-diffusion of carbon (originating from a graphite substrate) into a niobium coating and the fabrication of its carbides by heat treatment in the temperature range of 1073–1773 K was studied. The thickness of the Nb 2C and NbC phases formed after heat treatment as well as the inter-diffusion coefficients for the formation of the carbide layers were also studied. It was found that the carbide layer growth displayed parabolic behavior patterns inherent in the growth rate constants (K) of Nb 2C and NbC layers. By assuming that the inter-diffusion coefficients are independent of concentration, it was possible to determine the inter-diffusion coefficients of carbon D c into Nb 2C and NbC layers as a function of temperature.

Annealing of niobium coatings deposited on graphite

Niobium coatings 8–12 μm thick were deposited by magnetron sputtering on ATJ graphite substrate. The kinetic growth of carbide layers into niobium coatings and the properties of the Nb 2C and NbC phases obtained after annealing in the temperature range of 1073–1773 K were studied. It was found that the carbide layer growth displayed parabolic behavior patterns establishing the growth rate constants ( K) of Nb 2C and NbC layers, as follows: K Nb 2 C = 3.7 × 1 0 - 8 exp ( - 190 kJ RT ) m 2 s ; K NbC = 4.5 × 1 0 - 9 exp ( - 164 kJ RT ) m 2 s .

Modelling of the Growth of Carbide Layers in Tantalum

Modelling of the Growth of Carbide Layers in Tantalum

SURFACE IMPROVEMENT OF DUCTILE CAST-IRON

Diffusion impregnation with chromium is widley used as a surface treatment for steel and gray cast-iron, but very limited in ductile cast-iron. Two types of ductile cast-iron, ferretic and 60 % pearlitic, are surface chromised under different conditions using powder pack method. The maximum attaind chromised layer thickness of 22 um with surface micro-hardness of 1950 HV. is achived after treatment at 1000.0 for 30 hours. The rate controlling of the carbide layer growth process during chromising is being the diffusion of the chroium element and the direction of layer growth is determined to be inward chromium diffusion. The evaluation of the newly formed layers by corrosion, oxidation and wear resistance are carried out and discussed in the light of the obtained results.

Behavior of nucleation and growth of carbide layers on alloyed carbide particles in substrates in salt bath carbide coating

In salt bath immersion carbide coating, the effect of alloyed carbides in substrates is significant from the scientific and industrial points of view. This is because carbide layers grow by chemical bonding between carbon atoms in substrates and carbide forming elements in the baths, and by continuous supply of carbon atoms from substrates. It is concluded, from the use of substrates with very coarse carbide particles, that carbide growth behavior is highly dependent on combinations of species of the coating carbides and carbide particles in substrates. This can be explained in terms of the free energy for carbide formation of both coating carbide elements and substrate carbide elements.

Chemically vapour-deposited Mo/Re double-layer coating on graphite at elevated temperatures

The diffusion process operating in a double-layer coating at elevated temperatures was investigated experimentally by performing isothermal annealing tests (at 900–1200 °C) for a chemically vapour-deposited molybdenum coating on graphite with an interlayer of rhenium. The Mo/Re double-layer coating with low impurity and good adhesion to the graphite substrate was fabricated by thermal decomposition of the metal carbonyls at fairly low temperatures (<700° C). Isothermal annealing tests showed a parabolic growth of Mo 2C within the molybdenum layer with annealing time and no rhenium carbide in the rhenium layer. The parabolic rate constant of the carbide layer growth was decreased by increasing the thickness of the rhenium layer, and this tendency was more significant at lower annealing temperatures. The carbide layer growth also depended on the microstructure of the rhenium layer; it was restricted as the columnar diameter of the rhenium layer increased.

Chemical vapour deposition of rhenium on graphite

Adhesive and dense rhenium coatings were fabricated on graphite substrates by thermal decomposition of Re 2(CO) 10. The rhenium coatings possessed columnar grains at deposition temperatures above 600 °C and fine fibrous grains below 500 °C. The degree of crystallinity increased as the deposition temperature increased. The 〈110〉 preferred orientation was favoured above 600 °C. Hardness and grain size of the coating strongly depended upon the deposition temperature. Annealing above 1000 °C caused recrystallization of the coatings. All the annealed coatings showed no thermal damage, such as ablation or microcracking, at 800–1100 °C. Lattice parameters and microhardness increased after the annealing due to carbon diffusion from the graphite substrate. However, no carbide layer growth was observed at the interface between the rhenium coating and graphite substrate.

ChemInform Abstract: Carbide Layer Growth Behavior in a Molybdenum Coating on Graphite at Elevated Temperatures

Abstract(800‐1000 °C).

Carbide layer growth behaviour in a molybdenum coating on graphite at elevated temperatures

Carbide layer growth behaviour in a chemically vapour deposited molybdenum coating on graphite was evaluated by performing isothermal annealing tests at elevated temperatures. By measuring the carbide layer growth in the coating, it was determined that the layer growth obeyed a parabolic law with annealing time. The diffusion coefficient of carbon in the carbide layer grown in the molybdenum coating was several times larger than that in the carbide layer grown in the bulk molybdenum sheet. The activation energy for the carbon diffusion in the coating was estimated to be smaller than that of in the bulk molybdenum sheet. It was also found that the diffusion coefficient increased and the activation energy decreased when the diameter of the columns in the coating decreased. These results suggest that grain boundary diffusion of carbon in the coating played an important role in the carbide layer growth behaviour in the coating.

Observations of EFG die material interactions with liquid silicon

Investigation into the short-term interaction of various graphite and silicon carbide surfaces with liquid silicon droplets have been carried out. Details of the carbide morphologies formed and contact angles found are described. In addition, an investigation into the interaction over longer periods of time of Poco DFP-2 graphite, after different purification treatments, has been carried out. Results of these investigations have suggested that the rate of silicon carbide layer growth on fine-grained graphite and infiltration of graphite by liquid silicon obey a parabolic law and are dependent on the crucible material and on the pre-treatment given to the graphite.

NbC中のCの自己拡散

Self-diffusion coefficients of carbon in a carbide of niobium has been measured in the temperature range 1800∼2200°C, using radioactive tracer techniques. The self-diffusion coefficients can be expressed by(This article is not displayable. Please see full text pdf.) Comparing the present results with the chemical diffusivities obtained by the carbide layer growth method, it is shown that the relations between self-diffusivities and chemical diffusivities of carbon in the carbides of transition metals can be explained qualitatively on the assumption that carbon diffuses in the carbide by a vacancy mechanism.