Carbide Grains Research Articles

Effects of CaCO3 and ZrO2 on microstructure and properties of silicon-bonded SiC porous ceramics

The present study aims to determine the effects of calcium carbonate (CaCO3) and zirconium dioxide (ZrO2) on the microstructure and properties of silicon-bonded silicon carbide (SBSC) porous ceramics. The SBSC porous ceramics were prepared by adding CaCO3 and ZrO2 as composite additives in an argon atmosphere. Then, the impact of CaCO3 and ZrO2 on the properties and microstructure of SBSC porous ceramics was investigated. The results revealed that a proper amount of CaCO3 and ZrO2 can be effective for improving porous structures and increasing bending strength. Calcium zirconium silicate (Ca3ZrSi2O9) was identified to have a well-grown neck connection phase, which is the result of the reaction of CaO, ZrO2 and impurity SiO2. Simultaneously, the disappearance of the impurities of silicon dioxide (SiO2) facilitated the combination of fused silicon (Si) and silicon carbide (SiC) grains, thereby creating pores in the original location of the Si and optimizing the micropore structure. In conclusion, SBSC materials with 6 wt% ZrO2 and 4 wt% CaCO3 sintered at 1450 °C have the best apparent porosity (38.86%), bending strength (68.7 MPa), mean pore size (7.45 μm), and gas permeability (98.5 m3/m2·h1·kPa1).

Slow Neutron-Capture Process: Low-Mass Asymptotic Giant Branch Stars and Presolar Silicon Carbide Grains

Presolar grains are microscopic dust grains that formed in the stellar winds or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar silicon carbide (SiC) grains, including types mainstream (MS), Y, and Z, came from low-mass C-rich asymptotic giant branch (AGB) stars, which is supported by the ubiquitous presence of SiC dust observed in the circumstellar envelope of AGB stars and the signatures of slow neutron-capture process preserved in these grains. Here, we review the status of isotope studies of presolar AGB SiC grains with an emphasis on heavy element isotopes and highlight the importance of presolar grain studies for nuclear astrophysics. We discuss the sensitives of different types of nuclei to varying AGB stellar parameters and how their abundances in presolar AGB SiC grains can be used to provide independent, detailed constraints on stellar parameters, including 13C formation, stellar temperature, and nuclear reaction rates.

Relaxation time shift of Cobalt related internal friction peak in WC-Co cemented carbide

Cemented carbides are widely used in the cutting tools industry for their mechanical properties. They are composite materials made of hard tungsten carbide grains jointed together by a ductile cobalt binder. Due to the extreme conditions of use of the tools, understanding the evolution of the microstructure of the two phases is essential. This evolution can be followed by Mechanical Spectroscopy, either as a function of the temperature, or of the frequency. A general overview of the spectrum reveals three mechanical loss peaks. The second peak presents a transition in its Arrhenius plot at 1140 K, when measured in isothermal frequency scans. The temperature of this transition corresponds with the Curie temperature of the cobalt in WC-Co. However, temperature scans do not reveal any shift in the Arrhenius plot. This result shows how the presence of a phase transition leads to the measurement of apparent activation energies when a relaxation peak is measured in out-of-equilibrium conditions.

Effect of forging ratio on the microstructure, mechanical properties and abrasive wear behavior of a new C–Cr–Mo–V martensitic steel

C–Cr–Mo–V martensitic steel possesses high hardness, toughness and wear resistance, which benefits from the coordination of tempered martensite and alloy carbide in its microstructure. The size, content and distribution of alloy carbide have a great influence on the properties of steel. In this paper, the carbide characteristics and matrix microstructure of experimental steel were adjusted by controlling forging ratio. The results show that with the increase of forging ratio, the primary carbides distribute more homogeneously in the matrix and the amount of secondary carbides increases gradually, which enhances the pinning effect on grain boundary and refines the prior austenite grain. The hardness and impact toughness increase simultaneously with the increase of forging ratio, which benefits from the refinement of carbide and prior austenite grain. The increase of forging ratio not only promotes the refinement and dissolution of primary carbides, but also contributes the precipitation of secondary carbides. Therefore, the influence of forging ratio on the wear resistance is more complex. When the forging ratio reachs 6 or12, the wear resistance of experimental steel is enhanced which benefits from the higher content and finer size of primary carbides under small forging ratio, and the refinement of martensite matrix, uniform distribution of primary carbides and precipitation of secondary carbides under large forging ratio.

Evolution of Eutectic Carbide during M7C3/M23C6 in situ Transformation in Martensitic Stainless Steel

The M7C3/M23C6 in situ transformation in high‐carbon martensitic stainless steel is investigated. Eutectic Cr‐rich M7C3 carbide is generated during solidification; it is metastable and can almost completely transform into other phases when the temperature is <1000 °C. The eutectic M7C3 carbide transforms into an M7C3–M23C6 core–shell structure after heat treatment at 1000 °C for 1 h and then completely transforms into M23C6 carbides after heat treatment at 1000 °C for 7 h. The eutectic M7C3 carbide with the same orientation in the as‐cast state transforms into dozens of M23C6 carbide grains with varying orientations during in situ transformation, and the transformed M23C6 carbides have no constant orientation relationship with the adjacent matrix and eutectic M7C3 carbide core. The formation of the M23C6 shell is the result of not only the in situ transformation but also the coarsening of M23C6 carbides. The diffusion of carbon atoms in the M23C6 shell is the restrictive factor of M7C3/M23C6 in situ transformation and maintains transformation. The diffusion of chromium atoms in the matrix is the restrictive factor in the coarsening of the M23C6 shell, which continues coarsening and results in a significant increase in the final area fraction of carbides.

Carbides of transition metals: Properties, application and production. Review. Part 1. Titanium and vanadium carbides

The properties, application, and methods for producing titanium and vanadium carbides are considered. These carbides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Titanium and vanadium carbides exhibit significant chemical resistance in aggressive environments. For these reasons, they have found application in modern technology. These carbides are used as surfacing materials for the application of wear-resistant coatings to steel products. It is possible to use them as catalysts in organic synthesis. Titanium carbide is used in tungsten-free hard alloys, carbide steels. Due to its high hardness, it is used as an abrasive and as a component of ceramic cutting tools. Vanadium carbide serves as an inhibitor of the growth of tungsten carbide grains in hard alloys. The properties of refractory compounds depend on the content of impurities and dispersion (particle size). To solve a specific problem associated with the use of refractory compounds, it is important to choose the right method for their preparation and to determine the permissible content of impurities in the initial components. This leads to existence of different methods for the synthesis of carbides. The main methods for their preparation are: synthesis from simple substances (metals and carbon), metallothermal and carbothermal reduction. Plasma-chemical synthesis (vapor-gas phase deposition) is also used to obtain carbide nanopowders. A characteristic is given to each of these methods. Information on the possible mechanism of the processes of carbothermal synthesis is presented.

Studies on the phase formation of cobalt contacted with zinc vapour

In cemented carbides cobalt serves as a binding agent between tungsten carbide grains. The zinc process exhibits an important technique to recycle these materials. The decomposition takes place at temperatures of 900–1000 °C and the role of gaseous zinc in this process is poorly investigated. A specific experimental set-up was used to ensure that only gaseous zinc reacts with solid cobalt. By varying the temperatures, times and Zn:Co ratios, it was possible to ensure the formation of intermetallic phases. According to the binary CoZn phase diagram, phases of different composition are formed, depending on temperature and pressure. It was found that not all of the indicated phases occur simultaneously, but several do. With the support of the findings from the layer evolution between two solid as well as solid and liquid substances, it is explained which layers may form in the CoZn system. The multiple phase formation depends on diffusitivity and other factors such as the different melting points, the atomic radii and the occurrence of cracks. Of these, the occurrence of cracks across or between two layers represents the most likely reason.

Maxwellian-averaged cross section of 181Ta (n,γ) reaction and its astrophysical implications

The neutron-capture and Maxwellian-averaged cross sections of 181Ta(n,γ) reaction have been computed making use of the TALYS code. Two main inputs of the TALYS calculation, nuclear level density (NLD) and radiative strength function (RSF), are calculated within a recent microscopic model that takes into account the exact thermal pairing plus independent-particle model and collective excitations within the phonon-damping model. A good agreement between the calculated and experimental NLD and RSF data of 182Ta was found, confirming the goodness of the microscopic nuclear model used. As the result, the deduced neutron-capture and Maxwellian-averaged cross sections of 181Ta(n,γ) reaction also agree reasonably well with the experimental and recommended data across the measured energy range. The impact of the predicted cross sections on the tantalum production in the slow neutron-capture process (s-process) is then studied by employing a recent version of FRUITY stellar evolutionary models in which the effect of magnetic fields is newly implemented. By calculating the isotopic composition ratios within the FUNS code, we obtain a slight depletion of tantalum as compared to the recommended data. The variation between the calculated and reference tungsten isotopic ratios in presolar stardust silicon carbide (SiC) grains is found to be negligible, as expected. However, the capability of our theoretical model to reproduce experimental nuclear data will allow us to investigate in detail the s-process branching points at 181W and 185W, for which experimental data from direct measurements are still unavailable.

Effect of heat treatment on corrosion resistance of supersonic atmospheric plasma-sprayed Cr3C2–NiCr coating

Supersonic atmospheric plasma spraying (SAPS) is a newly developed technique in preparing high-performance Cr3C2–NiCr coatings with a high deposition efficiency. In this work, the effect of post-heat treatment on carbide precipitation and corrosion property of the SAPS Cr3C2–NiCr coatings was investigated. The results suggested that heat treatment induced the precipitation of secondary carbide grains, and the size of these carbides increased with the increasing heat treatment temperature. In addition, the self-corrosion current density of the coatings thermally treated at 500∘C was seven times smaller than that of the coating without heat treatment. The corrosion morphology showed that significant corrosion cracks were present on the as-sprayed coating. In contrast, the heat-treated coatings demonstrated small corrosion holes due to the formation of small corrosion galvanic cells between the precipitation of carbides and the substrate.

CVD diamond-SiC composite films: Structure and electrical properties

Thin films of microcrystalline diamond-silicon carbide composite have been deposited by microwave plasma chemical vapor deposition (CVD) in methane-silane-hydrogen gas mixtures on Si substrates with an insulating polycrystalline diamond buffer layer. The structure and phase composition of the films were characterized by scanning electron microscopy, Raman and photoluminescence spectroscopy. The silicon carbide grains are identified to be cubic polytype 3C-SiC with only a minor admixture of hexagonal 6H-SiC. The measured electrical resistivity of the films is found to decrease with the temperature from ~10−1 Ω·cm at 300 K to ~10−2 Ω·cm at 460 K, with activation energy Ea = 0.20 eV. The conductivity is assumed to be realized via pathways in the SiC network immersed in isolating diamond matrix. The obtained results show that such diamond/SiC composites may be considered as an alternative to electrically conductive nitrogenated ultrananocrystalline diamond films for a number of practical applications.

Impact of cobalt content and grain growth inhibitors in laser-based powder bed fusion of WC-Co

Processing of tungsten carbide‑cobalt (WC-Co) by laser-based powder bed fusion (PBF-LB) can result in characteristic microstructure defects such as cracks, pores, undesired phases and tungsten carbide (WC) grain growth, due to the heterogeneous energy input and the high thermal gradients. Besides the processing conditions, the material properties are affected by the initial powder characteristics. In this paper, the impact of powder composition on microstructure, phase formation and mechanical properties in PBF-LB of WC-Co is studied. Powders with different cobalt contents from 12 wt.-% to 25 wt.-% are tested under variation of the laser parameters. Furthermore, the impact of vanadium carbide (VC) and chromium (Cr) additives is investigated. Both are known as grain growth inhibitors for conventional sintering processes. The experiments are conducted at a pre-heating temperature of around 800 °C to prevent crack formation in the samples. Increasing laser energy input reduces porosity but leads to severe embrittlement for low cobalt content and to abnormal WC grain growth for high cobalt content. It is found that interparticular porosity at low laser energy is more severe for low cobalt content due to poor wetting of the liquid phase. Maximum bending strength of σB > 1200 MPa and Vickers hardness of approx. 1000 HV3 can be measured for samples generated from WC-Co 83/17 powder with medium laser energy input. The addition of V and Cr leads to increased formation of additional phases such as Co3W3C, Co3V and Cr23C6 and to increased lateral and multi-laminar growth of the WC grains. In contrast to conventional sintering, a grain growth inhibiting effect of V and Cr in the laser molten microstructure is not achieved.

Mechanical evaluation of WC-Co materials with varying microstructures

Tungsten carbide‑cobalt materials are useful in a variety of extreme applications due to a desirable blend of properties, yet the technology has not significantly changed since their initial development in the 1920s. The mechanical properties of this class of materials is highly dependent on two variables, the size of the tungsten carbide grains, and the amount of binder phase present in the final body. In this study, the amount of binder phase is isolated across three commercial materials from the same manufacturer with three different grain sizes to investigate the effect on mechanical properties. The mechanical properties investigated are indentation hardness, flexure and tensile strength, as well as fracture toughness. In general, an increase in hardness and tensile strength with decreasing grain size was observed, while the fracture toughness showed the opposite trend with toughness increasing with increasing grain size. The flexure strength results did not show a correlation to grain size. Fractographic analysis identified the dominant strength-limiting flaw for each sample, which largely were in the form of porosity. Other flaws types, such as inclusions from the milling process, clusters of large grains, and machining cracks from the surface finishing process, were also identified. Weibull analysis was performed and deemed appropriate for analysis of these materials, but strength-size scaling was not conducted due to the variability in the strength-limiting feature between the different specimen geometries.

Influence of nano-Ni on the microstructure of multicarbide-based alloys

Titanium carbide alloys are used for manufacturing of cutting tools, dies, wear-resistant parts. Significant advantages of such materials are high hardness, wear resistance, resistance to build up edge, but they possess low strength and crack resistance. Since the mechanical properties are structurally sensitive, it is important to establish size of the starting powders effect on the alloy’s microstructure formation and the morphology of carbide grains. In this paper TiC-5NbC-5WC-x (NiCr) alloys have been investigated, where x is 10, 18, 24% (wt.) at the ratio Ni:Cr = 3:1. Fine and nano nickel were used as starting powders. The alloys were obtained by powder metallurgy technology, which included homogenization of carbide powders and binder metals, pressing, sintering at the temperature 1350 ̊C during 40 minutes. The microstructure of the alloys was examined by scanning electron microscopy using ZEISS EVO 40 XVP microscope and energy dispersive X-ray spectrometry. In all alloys, regardless of the chemical composition and size of the starting nickel powders, the core/rim structure of carbide grains specifying titanium carbide-based alloys, however, of different size and shape, were found. Comparison of alloys, containing 13.5% (wt.) of fine or nano nickel, revealed that the diffusion of carbide metals in the binder is more intense in alloys with nano nickel, which could result in its maximum alloying and strengthening.

Isotopic Compositions of Ruthenium Predicted from the NuGrid Project

Abstract The isotopic compositions of ruthenium (Ru) are measured from presolar silicon carbide (SiC) grains. In a popular scenario, the presolar SiC grains formed in the outskirt of an asymptotic giant branch (AGB) star, left the star as a stellar wind, and joined the presolar molecular cloud from which the solar system formed. The Ru isotopes formed inside the star, moved to the stellar surface during the AGB phase, and were locked into the SiC grains. Following this scenario, we analyze the Nucleosynthesis Grid (NuGrid) data, which provide the abundances of the Ru isotopes in the stellar wind for a set of stars in a wide range of initial masses and metallicities. We apply the C > O (carbon abundance larger than the oxygen abundance) condition, which is commonly adopted for the condition of the SiC formation in the stellar wind. The NuGrid data confirm that SiC grains do not form in the winds of massive stars. The isotopic compositions of Ru in the winds of low-mass stars can explain the measurements. We find that lower-mass stars (1.65 M ☉ and 2 M ☉) with low metallicity (Z = 0.0001) can explain most of the measured isotopic compositions of Ru. We confirm that the abundance of 99 Ru inside the presolar grain includes the contribution from the in situ decay of 99 Tc. We also verify our conclusion by comparing the isotopic compositions of Ru integrated over all the pulses with those calculated at individual pulses.

Properties dependence of castable samples from low-cement silicon carbide castable on the firing temperature and the gas atmosphere

The influence of firing temperature of 1000, 1100, 1200, 1300, 1400, 1580 °C and atmosphere (air and reducing) on the main properties of castable samples from low-cement silicon carbide castable grade of “NKBS” with a combined additive has been investigated.
 It was shown that, in air with an increase in the firing temperature in the range 1000—1580 °C, the content of silicon carbide in the castable samples decreases. Up to temperatures of 1200—1300 °C, the content of silicon carbide in the castable samples practically does not change, and oxidation slows down due to the formation of a viscous melt of a glassy phase of an alumina silicate composition on the surface of silicon carbide grains. An increase in the firing temperature above 1300 °C leads to a more intense oxidation of silicon carbide grains to SiO2, which causes a change in the porosity, density and strength of the samples. As a result of the carried out studies, it was found that, the using temperature in the air of the above mentioned low-cement silicon carbide castable should not exceed 1300 °C.
 It was found that, after firing in a reducing atmosphere in the temperature range of 1000—1580 °C in the samples of silicon carbide castable, the content of silicon carbide remains almost the same, and the density and strength increase, which ensures the efficiency of its use for lining thermal units operating in a reducing atmosphere up to temperatures of 1580 °C.

SHS compaction of TiC cermets using mechanically activated mixtures

This paper focuses on obtaining cermet composite materials by SHS compaction. The study covers the effect of mechanical activation of metal components contained in reaction mixtures based on the Ti + C + Cr + Ni system when treated with grinding media in a ball mill. Two mechanical activation methods were used for Ti, Cr and Ni metal powders. In the first method, Cr and Ni powders were activated with grinding media separately from other reaction mixture components, and then mixed with titanium and carbon black powders. It is shown that the preliminary mechanical activation of inert components reduces the temperature and rate of combustion and increases the average size of carbide grains. In the second method, Ti + + Cr, Ti + Ni, and Ti + Cr + Ni powder mixtures were jointly processed in a ball mill, and then mixed with carbon black. This method provided mechanical activation of titanium particles with a minimum effect of grinding media on Cr and Ni powders. This led to an increase in the combustion rate and temperature, a decrease in the average size of carbide grains, and an increase in the composite structure homogeneity. A mechanism is proposed for the interaction of reagents (Ti + C) with the participation of activated Cr and Ni particles in combustion and structure formation zones, according to which the mechanical activation of inert components leads to their direct participation in the reaction interaction of titanium with carbon, which determines a decrease in the combustion rate and temperature and affects the fineness and structural homogeneity of compact composites. The results obtained were used to increase the structural homogeneity and fineness of the STIM-3B composite (Grade 3B synthetic hard tool material).

Evaluation of the classification of pre-solar silicon carbide grains using consensus clustering with resampling methods: An assessment of the confidence of grain assignments

ABSTRACTWe report the use of several cluster analysis techniques to evaluate the classification of pre-solar silicon carbide (SiC) grains. The stability of clusters and the confidence of individual cluster assignments of grains are assessed using consensus clustering with resampling methods. Our analysis shows that pre-solar SiC grains can be divided into seven groups that are found to be highly stable with most of the grains being assigned to the same cluster for at least 90 per cent of the time over multiple aggregated clustering. Among the seven groups, two groups are dominated by AB grains, three groups by MS grains, one group by Z grains, and one group by X grains. The further division of X grains into two groups is highly dependent on the chosen algorithm and is therefore uncertain. Z and Y grains are clustered jointly with MS grains, with one group dominated by Z grains, pointing to their common origins from low-mass asymptotic giant branch stars. The most stable N grain-containing clusters are dominated by 15N-rich AB grains. However, some methods assign N grains with X grains, but in less stable clusters. The suggested genetic relationship among 15N-rich AB, N, and X grains is in line with the recent proposal that all three types of pre-solar SiC grains came from core collapse supernovae. We discuss the results from different clustering techniques based on our assessment of the cluster stabilities and the extent to which the cluster assignments overlap across the different methods.

Contact fatigue of WC-6%wtCo cemented carbides: Influence of corrosion-induced changes on emergence and evolution of damage

The effect of corrosion-induced changes on the contact fatigue response and associated damage in a medium-grained WC-6%wtCo hardmetal grade is investigated. Cyclic contact loading tests are conducted by means of Hertzian indentation techniques. Corrosion damage is introduced in a controlled way by previous immersion of specimens in a stirred acidic medium. Results reveal that corrosion significantly affects the contact fatigue behavior of the material under consideration, in terms of decreasing the effective load bearing capability of the material as well as earlier emergence and evolution of specific damage features, as compared to those discerned under monotonic loading and in a pristine hardmetal respectively. As cycle number increases, damage evolution in the corroded condition is more gradual and diffuse than for the uncorroded one. In general, it is dominated by existence of ill-defined radial cracks together with pronounced carbide grain pull-out which finally evolve into cohesive-like spallation in regions close to the periphery of indentation imprints. These findings are rationalized on the basis of phase assemblage changes linked to corrosion, particularly in terms of lessening of toughening capability and easiness of WC grain pull-out intrinsic to the loose and porous binderless WC skeleton remnant at the surface level, after exposure to the acid medium. Although some removal of carbide grains and discrete cohesive chipping is also observed in the pristine sample for very high number of cycles, such different response, as compared to monotonic loading, is linked to oxidation phenomena of the metallic binder for this reference condition.

A new Cr25Ni35Nb alloy critical failure time prediction method based on coercive force magnetic signature

High-temperature damage in Cr25Ni35Nb alloys can be differentiated into four stages: the almost perfect stage, Carbide grain boundary coarsening stage, early creep stage and critical failure stage. In this paper, a novel approach based on a magnetic signature to monitor the alloy transitions between the four stages is proposed. Since the difference (ΔNi/Cr) between the content ratio of Ni to Cr in grains and the average value of the ratio of Ni to Cr of the alloy vary at different stages, these variations are highly correlated to the magnetic signature, coercive force (Hc). Thus, the curve of Hc vs ΔNi/Cr, which is “M” shaped over the whole life of the alloy, can be used to estimate the remaining life of the alloy tube using the easy measurement of coercive force. The first “U” inflection point indicates that the early creep stage has started and the second inverted “U” inflection point indicates that the critical failure stage is coming up. By identifying the inflection points, the alloy critical failure time can be predicted.

First principles analysis of impurities in silicon carbide grain boundaries

Silicon carbide is an important structural and electronic ceramic material that finds many uses in a wide variety of applications that require stability at extreme conditions. In this study, we provide a detailed investigation of the formation energies of point defects and the stability of a wide variety of dopants in bulk cubic silicon carbide (3C-SiC) and in 3C-SiC grain boundaries (GBs) using first principles methods and a detailed charge distribution analysis. We also determine the driving force (segregation energies) of these dopants towards GBs. Our results show that smaller, more electronegative elements such as oxygen and nitrogen occupy carbon substitutional sites whereas larger transition metals such as molybdenum and rare earth elements such as cesium, substitute for silicon sites in SiC. Such dopants tend to migrate to more open spaces as provided in the Σ9 GB. This suggests that Σ9 GB is more effective in pinning the defect centers as compared to Σ3 GB. These findings provide the chemical landscape for defect engineering through which doped 3C-SiC can be designed for targeted materials development purposes.