Carbide Overlayer Research Articles

Accelerated design of chromium carbide overlays via design of experiment and machine learning

Chromium carbide overlays with MC-type primary carbides show a significantly lower cracking susceptibility during welding. However, optimizing the composition and improving the wear-resistant properties is difficult by traditional one-variable-at-a-time methods. Here we propose a method using design of experiments and machine learning to accelerate this process. A closed-loop design process was developed by using existing data to predict future experiments and suggest new ones. The influence of Nb, V, and Ti on the overlay hardness was established in an active database. XRD analysis shows that Nb would promote the formation of austenite in the matrix while Ti would foster the formation of martensite. The efficacy of this method is demonstrated by predicting the simultaneous effects of the carbide formers on overlay hardness, which highlights the possibilities of using machine learning in the alloy design of chromium carbide overlays.

Step-confined thin film growth via near-surface atom migration

Understanding of thin film growth mechanism is crucial for tailoring film growth behaviors, which in turn determine physicochemical properties of the resulting films. Here, vapor-growth of tungsten carbide overlayers on W(110) surface is investigated by real time low energy electron microscopy. The surface growth is strongly confined by surface steps, which is in contrast with overlayer growth crossing steps in a so-called carpet-like growth mode for example in graphene growth on metal surfaces. Density functional theory calculations indicate that the step-confined growth is caused by the strong interaction of the forming carbide overlayer with the substrate blocking cross-step growth of the film. Furthermore, the tungsten carbide growth within each terrace is facilitated by the supply of carbon atoms from near-surface regions at high temperatures. These findings suggest the critical role of near-surface atom diffusion and step confinement effects in the thin film growth, which may be active in many film growth systems.

Important factors affecting the gouging abrasion resistance of materials

Gouging abrasion occurs in many processes in comminution where large and hard abrasives such as rocks/ores are handled with high contact or impact stresses. In the past decade or so, the National Research Council Canada has systematically investigated the gouging abrasion performance of a wide range of materials per the ASTM G81 standard testing method using a jaw crusher. Materials evaluated include carbon and low alloy steels, high alloy tool steels, austenitic manganese steel, white cast irons, austempered ductile irons, chromium carbide and tungsten carbide overlays. In this paper, the main test results are summarized and the key properties/characteristics that are conducive to enhancing materials’ gouging abrasion resistance are discussed.

Growth mechanism of primary needles during the solidification of chromium carbide overlays

A mechanism for modelling the growth of faceted primary M7C3 needles during solidification of chromium carbide overlays is presented for the first time. In contrast with existing models that consider diffusion-controlled growth over the length of the needle, this work disaggregates the flow of solute at the tip (diffusion-controlled) and the faceted sides (interface-controlled). The proposed solution predicts an initial transient followed by a steady state longitudinal growth. The outward growth of the needle prism faces is linear and slow, consistent with interface-dominated kinetics. Abrupt decreases in longitudinal growth rate was observed when the tip of the carbides approached other carbides, consistent a soft-impingement effect when the diffusion field of two carbides a overlap. The model was compared to in-situ visualisation of the solidification of an alloy with primary chromium carbide needles, and was found to match closely to observations.

The Effect of pH on Slurry Erosion–Corrosion of Tungsten Carbide Overlays Alloyed with Ru

The aim of the study was to determine the effect of Ru additions to WC-Fe overlays when exposed to low pH slurry erosion conditions. These overlays were applied through Plasma Transferred Arc, and the original bulk Ru powder concentrations varied from 0.5 to 5 wt.%. A slurry jet impingement erosion–corrosion test rig was used to evaluate wear, and electrochemical measurements were performed to characterize the corrosion properties. The slurry mixtures contained silica sand and synthetic mine water. The pH was varied between 3 and 6.5 for the slurry erosion tests and lowered further for the corrosion characterization. Samples were examined optically and with a scanning electron microscope using energy-dispersive x-ray spectroscopy. X-ray diffraction analysis was used to determine the phases present. For the slurry erosion–corrosion results at the pH of 6.5, addition of Ru did not show a decrease in erosion–corrosion rates. However, when the pH was decreased to 3, by the addition of HCl, Ru improved the resistance. From the electrochemistry, it was also clear that Ru additions improved the corrosion resistance, but more than 1 wt.% Ru was required. At very low pH levels, the presence of Ru was not able to prevent corrosion.

Investigation of erosion-corrosion behavior of (WTi)C based weld overlays

Erosion-corrosion is responsible for severe damage and high maintenance costs in many industrial operations involving slurry handling such as oil sands and mineral processing. While tungsten carbide reinforced Ni-based alloy overlays are frequently used for applications requiring extremely high wear resistance, they tend to suffer from extensive carbide degradation during the weld cladding process which has been shown to severely impair their erosion-corrosion resistance. Recently, new (WTi)C based overlays are being developed to alleviate some of the limitations associated with the nickel-based tungsten carbide overlays. In this study, three different overlays reinforced with (WTi)C and one tungsten carbide were assessed using a slurry pot erosion-corrosion testing apparatus where the total erosion-corrosion (E-C) rate as well as the separate components of synergistic effect was determined. Erosion-corrosion test was performed at 30°C, in an aqueous slurry containing 35wt% AFS 50–70 silica sand and 3.5wt% NaCl. A special SEM technique was employed to examine changes on the specimen surface at specific overlay locations before and after erosion-corrosion test in order to identify the effect of corrosion on erosion and vice-versa. It was found that tungsten is more prone to dissolution in the matrix during the welding process than titanium, which forms W-depletion/Ti-rich layer around the (WTi)C particle. Dissolution of tungsten and titanium promotes the formation of secondary carbides in the matrix. Erosion-corrosion performance of the overlays largely depends on increased fraction of retained primary (WTi)C phase.

Characterization of phases and defects in chromium carbide overlays deposited by SAW process

Owing to their enhanced wear resistance, chromium carbide overlay (CCO) plates deposited by submerged arc welding (SAW) process have been widely used in various branches of the mining industry. Defects in the deposited CCO directly determine the wear performance of this material. Several characterization techniques, including optical microscope (OM), optical emission spectroscopy (OES), and field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD), have been used to identify the most common defects and phase distribution in the SAW-deposited CCO. The main phases are consist of primary carbide, secondary Cr7C3 and Cr23C6 carbides, austenite, and a small amount of ferrite. Defects in CCO, such as cracks, porosity, and clusters of particles known in the industry as “rice crispies” are observed near the top surface of the CCO. The undissolved Cr7C3 particles with surrounding porosity seem to have provided the preferential nucleation sites for the initial micro-cracks. These initial cracks are observed to propagate along the interface between the austenite and Cr7C3 carbides.

Cracks formation and residual stress in chromium carbide overlays

A chromium carbide overlay is often welded on the surfaces that are subject to severe wear attack, of mining equipment in order to prolong its service time. The welding causes unequal and unsynchronized thermal expansion/shrinkage between the overlay and its substrate, leading to a significant level of thermal strain/stress in the overlay. When the stress level at any location becomes higher than material strength, at that location a crack will start to form and propagate, resulting in partial release of stress. The residual stress varying by location can lead to delamination and spalling of chunks of the overlay. This paper tries to give better understanding of cracks-formation and residual stress, covering the driving force for cracks formation and residual stress, the locations where cracks initiate, the directions towards to which the cracks propagate, the resulting patterns of the cracks and the residual stress at the interfaces between the overlay and its substrate. The approach to realize this is through mechanics modeling and simulation plus experimental observations. The practical significance of this work lies in its values to improvement of welding processes and the basis for evaluating overlay quality.

Slurry erosion resistance of boride-based overlays containing boride crystals oriented perpendicularly to the wearing surface

Components exposed to the flow of liquid solutions containing hard particles experience significant material loss. For defined slurry conditions, the extent of damage to the components depends upon their microstructure and the slurry particle impingement angles. This paper presents the research work carried on to develop a gas metal arc welding (GMAW) clad overlay that resists slurry erosion at both low and high particle angles. GMAW overlays containing hard primary Fe2B crystals in a supporting matrix enriched in molybdenum, carbon and silicon have been considered. Cored wires of specific compositions deposited with adapted welding parameters produce weld overlays presenting a peculiar microstructure. These iron boride-based overlays contain fine elongated boride crystals aligned mainly perpendicularly to the wearing surface. This peculiar microstructure is responsible for the outstanding slurry erosion resistance observed at both impinging angles of 30° and 90°. These iron boride-based overlays present a slurry erosion resistance at low and high impinging angles far beyond that observed with known materials including chromium carbide overlays.

Effect of reactive gas composition on the microstructure, growth mechanism and friction coefficient of TiC overlayers

Titanium carbide overlayers were synthesized by gas phase carburization of titanium at 1150°C with varying reactive gas compositions and durations. These layers were investigated by X-ray diffraction, tribometry, Raman spectroscopy and scanning electron microscopy. Titanium carbide layers exhibited very low coefficient of friction due to the presence of amorphous carbon on the surface. The growth mechanism and the surface microstructure of the layers showed a critical dependence on methane content in the reactive gas mixture.

Determination of carbide structures on an Mo(110) surface by scanning tunnelingmicroscopy and low energy electron diffraction

Surface properties of transition metals can be modified considerably byforming carbide overlayers. In this study we fabricated molybdenumcarbide overlayers by carburizing the Mo(110) surface with ethylene(C2H4) and successfully determined their real space structures by using low energy electrondiffraction (LEED) and scanning tunneling microscopy (STM). With various carburizationrecipes, three carbon-terminated structures , (12 × 4)-2C,and (4 × 4) ofcoverages 1/30,1/24, and 1/16 monolayer (ML) were formed on the surface, respectively. Since the carbide overlayers wereformed at high temperatures, inward diffusion of carbon atoms played a key role instructure formation.

Oxide and Carbide Formation at Titanium/Organic Monolayer Interfaces

X-ray photoelectron spectra (XPS) are reported from a series of buried titanium/organic monolayer interfaces accessed through sample delamination in ultrahigh vacuum (UHV). Conventional characterization of such buried interfaces requires ion-mill depth profiling, an energetic process that frequently destroys bonding information by chemically reducing the milled material. In contrast, we show that delaminating the samples at the metal/organic interface in vacuum yields sharp, nonreduced spectra that allow quantitative analysis of the buried interface chemistry. Using this UHV delamination XPS, we examine titanium vapor deposited onto a C18 cadmium stearate Langmuir-Blodgett monolayer supported on Au, SiO2, or PtO2 substrates. Titanium is widely used as an adhesion layer in organic thick film metallization as well as a top metal contact for molecular monolayer junctions, where it has been assumed to form a few-atoms-thick Ti carbide overlayer. We establish here that under many conditions the titanium instead forms a few-nanometers-thick Ti oxide overlayer. Both TiO2 and reduced TiOx species exist, with the relative proportion depending on oxygen availability. Oxygen is gettered during deposition from the ambient, from the organic film, and remarkably, from the substrate itself, producing substrate-dependent amounts of Ti oxide and Ti carbide "damage". On Au substrates, up to 20% of the molecular-monolayer carbon formed titanium carbide, SiO2 substrates approximately 15%, and PtO2 substrates <5%. Titanium oxide formation is also strongly dependent on the deposition rate and chamber pressure.

Angle-resolved XPS investigations of the interaction between O 2 and Mo 2C/Mo(1 0 0)

The interaction of oxygen with a carbidized Mo(1 0 0) surface was investigated at different temperatures (300 K–1000 K) with-angle resolved X-ray photoelectron spectroscopy. A carbide overlayer with a homogeneous Mo 2C stoichiometry (down to the information depth of XPS) was produced by the high-temperature decomposition of C 2H 4 on Mo(1 0 0). O 2 adsorbs dissociatively on Mo 2C/Mo(1 0 0) at room temperature. Oxidation of the carbide at 800 K results in the partial removal of carbon and leads to sub-surface O migration, accompanied by the appearance of highly oxidized Mo states. Raising the O 2 adsorption temperature to 900 K decreased the carbon content further, without affecting the amount and the distribution of adsorbed O. Performing the oxidation at 1000 K led to an even more effective removal of carbon, but the oxygen content of the surface region was also reduced.

Surface and Subsurface Oxidation of Mo2C/Mo(100): Low-Energy Ion-Scattering, Auger Electron, Angle-Resolved X-Ray Photoelectron, and Mass Spectroscopy Studies

The interaction of oxygen with a carburized Mo(100) surface was investigated at different temperatures (300-1000 K). The different information depths of low-energy ion-scattering (LEIS) spectroscopy, with topmost layer sensitivity, Auger electron spectroscopy (AES), and angle-resolved X-ray photoelectron spectroscopy (ARXPS) allowed us to discriminate between reactions on the topmost layer and subsurface transformations. According to ARXPS measurements, a carbide overlayer was prepared by the high-temperature decomposition of C(2)H(4) on Mo(100), and the carbon distribution proved to be homogeneous with a Mo(2)C stoichiometry down to the information depth of XPS. O(2) adsorbs dissociatively on the carbide layer at room temperature. One part of the chemisorbed oxygen is bound to both C and Mo sites, indicated by LEIS. Another fraction of oxygen atoms probably resides in the hollow sites not occupied by C. The removal of C from the outermost layer by O(2), in the form of CO, detected by mass spectroscopy (MS), was observed at 500-600 K. The carbon-depleted first layer is able to adsorb more oxygen compared to the Mo(2)C/Mo(100) surface. Applying higher doses of O(2) at 800 K results in the inward diffusion of O and the partial oxidation of Mo atoms. This process, however, is not accompanied by the removal of C from subsurface sites. The depletion of C from the bulk starts only at 900 K (as shown by MS, AES, and XPS), very probably by the diffusion of C to the surface followed by its reaction with oxygen. At T(ads) = 1000 K, the carbon content of the sample, down to the information depth of XPS, decreased further, accompanied by the attenuation of the C concentration gradient and a substantially decreased amount of oxygen.

The Band Offsets of Isomeric Boron Carbide Overlayers

ABSTRACTSemiconducting boron carbide overlayers, formed from the decomposition of orthocarborane and metacarborane have been studied by angle resolved photoemission. The incurrence of surface photovoltage and the photovoltaic process, from the photoemission experiment, reveal band offsets in the orthocarborane multilayer configurations that are invereted relative to single layer configurations. Defect induced gap states which trap charge at the heterostructure interface is used as one explanation of these results. The role of defects is also used to help illuminate why opposite semiconducting type materials are formed from the decomposition of isomer carborane molecules.

Substrate-Dependent Reaction Pathways of Ethylene on Clean and Carbide-Modified W(110) and W(111)

The decomposition of ethylene was utilized as a probe reaction to determine the chemical properties of carbide-modified W(110) and W(111) using high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) measurements. Ethylene adsorbed via a di-σ configuration on both clean and carbide-modified W(110). Most of the di-σ bonded ethylene converted to ethylidyne species by 200 K. In contrast, ethylene decomposed to form vinylidene and acetylide intermediates without forming ethylidyne at 90 K and 200 K on clean and carbide-modified W(111). Thermal desorption results showed clean W(111) to be twice as active as W(110) toward the decomposition of ethylene. The presence of the carbide overlayer reduced the activity of the W(111) surface by ∼50%, whereas carbide-modification only reduced the activity of the W(110) surface by ∼30%. This study showed that the reaction pathways of ethylene on carbide-modified surfaces were strongly dependent on the structure of the metal substrate.

Decomposition pathways of NO on carbide and oxycarbide-modified W( [formula omitted]) surfaces

The decomposition of NO has been studied on clean W(1 1 1), carbide-modified W(1 1 1), and oxycarbide-modified W(1 1 1) surfaces. The decomposition pathways are investigated using a combination of temperature programmed desorption, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and soft X-ray photoelectron spectroscopy. All these surfaces exhibit high activity toward the decomposition of NO, and the only N-containing products are N 2 and N 2O. Furthermore, all three surfaces preferentially produce N 2 over N 2O from the decomposition of NO. Oxygen atoms, produced from the decomposition of NO, react with carbide surfaces to produce gas-phase CO at high temperatures. In addition, our results demonstrate that cycles of alternate NO/hydrocarbon treatments can regenerate the carbide overlayer on W(1 1 1), and the regenerated C/W(1 1 1) surface remains active in the decomposition of NO.

Reaction pathways of cis- and trans-2-butene on Mo(110) and C/Mo(110):: selective activation of α and β C–H bonds

The reaction pathways of cis- and trans-2-butene on clean Mo(110) and carbide-modified Mo(110) surfaces have been studied using a combination of temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The vibrational data reveal that the decomposition pathways of cis- and trans-2-butene are different on clean Mo(110). In the case of cis-2-butene, the olefinic α(C–H) bonds are cleaved at 80 K to produce surface hydrogen and 2-butyne. In contrast, the initial decomposition of trans-2-butene also involves β(C–H) bond scission, such that at least one of the methyl groups of the molecule is converted to a CH 2 group at temperatures below 150 K. On carbide-modified Mo(110), the initial decomposition step for both cis- and trans-2-butene involves α(C–H) bond cleavage to form 2-butyne. This difference in reactivity of Mo(110) and carbide-modified Mo(110) towards the 2-butenes provides experimental evidence that the formation of the carbide overlayer enhances the selective activation of α(C–H) bonds.

Controlling surface reactivities of transition metals by carbide formation

The surface reactivities of transition metals, including the Groups 4–6 early transition metals as well as the late transition metals of the 3d series, can often be modified by the formation of a carbide overlayer. More importantly, the reactivities of carbide-modified surfaces frequently demonstrate strong similarities to those of the Pt-group (Pt, Pd, Ir, Rh, Ru and Os) metals. In this paper, we will summarize our recent surface science investigations of the electronic, structural, and catalytic properties of well-characterized model carbide systems. Using several characteristic surface probing reactions, we will provide experimental evidence for the similar reactivities of the carbide-modified surfaces and the Pt-group metals. We will also discuss the underlying structural and electronic properties that are controlling the reactivities of the carbide-modified surfaces, such as the binding sites and chemical nature of the carbon atoms, the ionicity of the metal–carbon bonds, and the band structure of the transition metal carbides. We will also show examples of experimental attempts to correlate findings from the single crystal model carbide systems to amorphous powder carbide catalysts.

Modifying Surface Reactivities by a Carbide Overlayer: A Vibrational Study of the Reaction Mechanisms of Cyclohexene and 1,3-Cyclohexadiene on Mo(110) and (4×4)-C/Mo(110) Surfaces

The dehydrogenation and thermal decomposition mechanisms of cyclohexene and 1,3-cyclohexadiene on clean Mo(110) and carbide-modified (4×4)-C/Mo(110) surfaces have been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On the clean Mo(110) surface, partial dehydrogenation of a fraction of the cyclohexene molecules occurs at temperatures as low as 80 K. When the surface is heated to 150 K, the HREEL spectra obtained are characteristic of a C6H9 intermediate, as seen by a comparison with HREEL spectra reported for C6H9 on Pt(111).1,2 At higher temperatures, competing C−C and C−H bond cleavage reactions lead to the formation of surface carbon and the evolution of hydrogen. In contrast, on the carbide-modified surface, the primary reaction pathway for cyclohexene is selective dehydrogenation to form benzene and hydrogen. In the case of 1,3-cyclohexadiene, the HREEL results suggest that dehydrogenation to form benzene occurs at 80 K on the clean Mo(110) surface, based on a comparison with the HREEL spectrum for benzene directly dosed onto Mo(110) at 80 K. However, upon heating, most of the benzene decomposes to form surface carbon and hydrogen, as shown by TPD studies. On the carbide-modified surface, the primary reaction pathway for 1,3-cyclohexadiene is selective dehydrogenation to form benzene, which desorbs at 313 K. Furthermore, the HREEL results also indicate that a competing reaction pathway occurs to form a surface intermediate which most likely has an tilted aromatic c-C6 ring, such as a surface phenyl species.