Cobalt Matrix Research Articles

Synthesis and structural analysis of electrodeposited Pb-Co films for oxygen evolution applications

Pb-Co films have been studied due to the reduced oxygen overpotential at which they operate. However, the synthesis of these films is not trivial because Pb and Co are known to be immiscible metals. Thus, with the aim of understanding the process behind the formation of Pb-Co films, full structural characterization of such films is done. Moreover, to ease the electrodeposition of the films, ascorbic acid is used as a complexing agent. Various concentrations of ascorbic acid were studied. The sample grown in the presence of 90 mM showed the most compact morphology, free of voids and cracks. Further, the analysis of polarization curves revealed that this coating is the best suited as an electrocatalyst for the oxygen evolution reaction, allowing operation at a lower overpotential compared to that of conventional Pb-Ca-Sn anodes. The structure of the samples was studied employing X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). It was encountered that the obtained coatings were composed of nanocrystallites of lead immersed in a pseudo-amorphous matrix of lead and cobalt. By differential scanning calorimetry (DSC) analysis, the amorphous nature of the matrix was demonstrated. This nanocrystalline nature favors the electrocatalytic activity of the coating as every point in the surface of the electrode (anode) acts as a catalyst.

The Effects of Heat Treatment on Microstructure and Mechanical Properties of Tribaloy 400 Coatings Deposited by Laser Cladding

The effects of heat treatment on the microstructure, microhardness, nanohardness and wear resistance of Tribaloy 400 coatings prepared by laser cladding on 304 stainless steel were studied in this paper. The results show that the microstructures of Tribaloy 400 laser cladding layer are cobalt matrix dendrites and eutectic structure. After the heat treatment at 1250 °C for 1 h, the eutectic structure disappears and the strip-like Laves phase is formed on the Co matrix. Meanwhile, the hardness is obviously improved. After the heat treatment at 1250 °C for 1 h and then 900 °C for 4 h, a small amount of e-Co appears, the strip-like Laves phases become coarser and blunter, and some of the strip structures combine together to form blocky structures, and the hardness is decreased. The wear resistance of the samples at 600 °C is better than that at RT since the hardness of the Laves phase increases with the rise in temperature.

The influence of carbides on the microstructure, grain growth, and oxidation resistance of nanostructured carbides-strengthened cobalt-based multi-track laser-cladding layers

Cladding layers were prepared on carbon steel using nanostructured carbide-strengthened cobalt (Co)-based alloy (using nanostructured tungsten carbide as hard phases) as a deposited material by multi-track laser cladding. In addition to Co-based alloy, the deposited materials contained nanostructured chromium carbide (Cr3C2), vanadium carbide (VC), or Cr3C2 and VC. Compared with the substrate, all cladding layers had an improved oxidation resistance with an increased time (0–50 h) at 800 °C. Cr2C3 had a higher oxidation-resistance performance than VC and the mutual reaction of Cr2C3 and VC. The constitution and phases of the cladding layers indicated hexagonal-close-packed-α and body-centered-cubic-β cobalt and hard carbides, such as CoCx, V8C7, and Cr3C2, and some decarburization as Co3W3C and WC1-x. Elemental inter-diffusion occurred between the different phases as indicated by transmission electron microscopy and electron backscattered diffraction. Mixed carbide has a larger misorientation than the cobalt matrix, and electron backscattered diffraction has considerable potential to evaluate plastic deformation. The average grain size of the hard carbides was much larger than that of the original deposited material. The carbides grew rapidly through interfacial diffusion, boundary migration, and coalescence, which followed the Kirkendall effect and Ostwald-coarsening mechanism. This research helps to understanding the microstructure, oxidation resistance, grain growth, and interfacial behavior in multi-track laser-cladding layers.

Evaluation of a worn out WC–Co–TiAlN cutting tool used in industry

In this work, a cutting tool with premature wear was characterized by Rockwell hardness test, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). In addition, electrochemical impedance spectroscopy (EIS) measurements were carried out in order to investigate the characteristic properties of the wear coating (TiAlN) of tool (WC–Co substrate). The substrate hardness was 90 HRA, which is in accordance with the observed microstructure of a cobalt matrix with tungsten carbides. SEM-EDS analysis confirmed the presence of a TiAlN coating and of typical features of an adhesive wear mechanism. EIS results revealed typical behavior of a porous WC–C substrate. Hence, premature wear of cutting tool could be explained in terms of an inadequate performance of the defects of coating deposited on the WC-Co substrate, as suggested by porosity revealed in the SEM images and the EIS measurements.

On the deformation mechanisms of cemented carbide in rock drilling – Fundamental studies involving sliding contact against a rock crystal tip

Cemented carbide is a composite material, most commonly consisting of tungsten carbide grains in a metallic matrix of cobalt. The combination of a hard ceramic phase in a ductile metallic matrix combines high hardness and ability to withstand plastic deformation with toughness to avoid cracking and fracturing. Since these properties are very important in rock drilling, cemented carbides are frequently used in such applications.In earlier work, it was found that granite in sliding contact with considerably harder cemented carbides not only results in plastic deformation of the cemented carbide composite, but also in plastic deformation of some of the individual WC grains. The latter observation is remarkable, since even the two hardest granite constituents (quartz and feldspar) are significantly softer than the WC grains. This tendency to plastic deformation of the WC grains was found to increase with increasing WC grain size.The present investigation aims to increase the understanding of plastic deformation of cemented carbides in general, and the individual WC grains in particular, in a situation representative for the rock drilling application. The emphasis is put on explaining the seemingly paradoxical fact that a nominally softer counter material is able to plastically deform a harder constituent in a composite material. The experimental work is based on a scratch test set-up, where a rock crystal tip slides against a fine polished cemented carbide surface under well-controlled contact conditions. The deformation and wear mechanisms of the cemented carbide are evaluated on the sub-micrometer scale; using high resolution FEG-SEM, EDS, EBSD, BIB and FIB cross-sectioning. The size of the Co-pockets, together with the shape and size of WC grains, turned out to be decisive factors in determining the degree of carbide deformation. The results are discussed with respect to their industrial importance, including rock drilling.

Microstructure, Wear Behavior and Corrosion Resistance of WC-FeCrAl and WC-WB-Co Coatings

The paper is focused on investigating the quality of two grades of thermally sprayed coatings deposited by high-velocity oxygen fuel (HVOF) technology. One grade contains WC hard particles in an environmentally progressive Ni- and Co-free FeCrAl matrix, while the second coating contains WC and WB hard particles in a cobalt matrix. The aim of the experimental work was to determine the effect of thermal cyclic loading on the coatings’ resistance to adhesive, abrasive and erosive wear. Abrasive wear was evaluated using abrasive cloth of two grit sizes, and erosive wear was evaluated by a dry-pot wear test in a pin mill at two sample angles. Adhesion wear resistance of the coatings was determined by a sliding wear test under dry friction conditions and in a 1 mol water solution of NaCl. Corrosion resistance of the coatings was evaluated using potentiodynamic polarization tests. Metallographic cross-sections were used for measurement of the microhardness and thickness and for line energy-dispersive X-ray (EDX) analysis. The tests proved the excellent resistance of both coatings against adhesive, abrasive, and erosive wear, as well as the ability of the WC-WB-Co coating to withstand alternating temperatures of up to 600 °C. The “green carbide” coating (WC-FeCrAl) can be recommended as an environmentally friendly replacement for Ni- and Co-containing coatings, but its operating temperature is strictly limited to 500 °C in air.

Sintering of tungsten carbide cermets with an iron-based ternary alloy binder: Processing and thermodynamic considerations

Cemented tungsten carbide materials are traditionally bonded using a cobalt matrix, resulting in a material that possesses a unique combination of exceptional properties. However, cobalt has been recently classified as “reasonably anticipated to be a human carcinogen,” and thus, removal of this hazardous phase is desired for many applications. This research focuses on the production of tungsten carbide bodies cemented with a non-hazardous iron-based alloy binder phase. This binder has roots in the previous transition metal binders and additives for tungsten carbide and is based on the principles of high-entropy alloys; however, it relies on a novel means of carbide formation and dispersion from prior investigations. Sintering studies were conducted through uniaxial hot pressing, field-assisted sintering techniques, and pressureless sintering techniques. While some of the results of hot pressing and field-assisted sintering technique were positive, the resultant mesostructure exhibited an undesired, graded, heterogeneous form. Pressureless sintering, on the other hand, was capable of producing a homogeneous microstructure with high theoretical density along with promising hardness and indentation toughness values.

High performance Co–Cr3C2 composite coating by jet electrodeposition

ABSTRACTThis paper studied the Co–Cr3C2 composite coatings on the nickel-based super alloy GH4169 deposited by pulse reverse jet electrodeposition in conjunction with ultrasonic vibration. The GH4169 substrates were first annealed at 500°C for 20 min to eliminate the internal stress, and then the surface was activated chemically. The surface activation procedure and ultrasonic pulse interval time in the jet electrodeposition were investigated. The results showed that the Cr3C2 particle contents, microhardness, wear resistance of the coatings were significantly improved, while the surface roughness and the coefficient of friction were reduced, when the ultrasonic vibration is applied in the jet electrodeposition of the Co–Cr3C2 composite coatings. It is found that the intermittent ultrasonic vibration at 8 s interval time has the greatest effect on the coatings. The Cr3C2 particle content at this condition is at its maximum of 23.85%, with evenly particle distribution and good adhesion with the cobalt matrix. The hardness of the composite coating is 652 HV(0.5), and frictional coefficient is 0.15. It has also high thermal stability, and the microhardness can hold up to 800°C without degradation, showing a great potential for high-temperature applications.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Stellite 12 Fabricated by Laser Additive Manufacturing

Microstructure evolution and wear resistance of Stellite 12 produced by laser additive manufacturing were studied in untreated and heat-treated conditions. In untreated conditions, cobalt matrix dendrites and inter-dendritic eutectic carbides were the primary microstructure, blocky and lamellar eutectic carbides can be found, and M7C3 and M23C6 are the primary carbides. Heat treatment can modify the microstructure of this alloy. Primary carbides were dissolved into the cobalt matrix after solution treatment. Aging treatment resulted in the precipitation of carbides, the transformation of M7C3 into M23C6 carbides, and the increase in microhardness. Solution and aging treatment can also cause the precipitation of carbides, but the morphology and distribution of carbides were absolutely different. The wear resistance of untreated samples at room temperature (RT) and 600 °C is better than that at 200 and 400 °C due to the high hardness at RT and the formation of oxide film at 600 °C. The wear resistance of heat-treated samples was tested at 600 °C; it was dominated by the formation and flake-off of oxide film. The sample that underwent solution plus aging treatment has a superior wear resistance.

Microstructure Transformation during Isothermal Aging of Biomedical Co-Cr-Mo Alloy

Co-Cr-Mo alloys are currently applied as major materials for orthopedic implant because of their excellent wear resistance. The main strengthening mechanism of this alloy is the transformation of γ-phase Co-Cr-Mo alloys to the ε-phase. In this study, the evolution of the microstructure of a Co-28Cr-6Mo-0.08C-0.2N alloy has been investigated during isothermal aging. Solution treatment at 1275°C for 15 hours was carried out for as-cast alloy, followed by aging treatments at 700°C and 800°C for up to 15 hours. Microstructure evaluation, XRD analysis and micro-hardness test were carried out for both as-cast and aged alloys. From XRD analysis showed that the transformation of γ-phase to ε-phase occurred during the isothermal aging. The amount of ε-phase increased with increasing aging temperature and time, while the hardness of the alloy increased with increasing the amount of ε-phase. This led to the increasing of the hardness of alloy at higher aging temperature and time. In addition, the very fine precipitation in the cobalt matrix was observed in the aged-specimens. Aging at 800°C caused the progressive formation of very fine precipitates along intra-granular striation and the matrix. It is suggested that the precipitation was took place in the grain on the dislocations and the stacking faults.

Integrity of Co-Cr-C coated P92 steel for power plant pipework applications

The main limiting factor for P92 steel, in power plant at high temperature, is the increased oxidation damage on the inside surface which causes enhanced damage of components. Industry have attempted to address this problem by applying oxidation resistant coatings to the inside surface of the P92 pipework to prevent damage. Aluminide diffusion coatings have been a particular focus for research to date, however they have been found to have a number of detrimental effects on the creep properties and coating-substrate integrity. This paper introduces a Co-Cr-C type coating, composed of Cr3C2 particles electro-deposited within a cobalt matrix. On exposure to high temperature oxidation conditions the coating is shown to form a cobalt and chromium rich oxide which is slow growing, adherent and ideal for oxidation resistance. When applied to P92 substrate and exposed at service relevant temperatures the coated system retains its integrity and appears suitable for long term service. The coated P92 system is also shown to retain its integrity during high temperature creep testing and coating application does not have a negative effect on the mechanical properties of P92. Overall the Co-Cr-C coating has a number of superior properties compared to previously investigated coatings.

Synthesis and Multi Scale Tribological Behavior of WC-Co/Nanodiamond Nanocomposites

Nanodiamonds (ND) present a unique combination of desirable mechanical, functional, and chemical characteristics that are ideally suited for reinforcing and enhancing the wear resistance of carbide based materials. Tungsten carbide cobalt (WC-Co) matrix nanocomposites reinforced with varying amounts of ND (2 – 10 vol.%) were synthesized here by spark plasma sintering. The rapid thermal consolidation route enabled attainment of dense samples with a significant retention of the metastable diamond phase. NDs affected the microstructural evolution, chemistry, and mechanical properties of WC-Co. Macroscale reciprocating pin-on-disk tests were conducted to assess wear behavior under conditions relevant to service environments, e.g., high cycles and high contact pressure. Microscale tribological properties were assessed using microscratch tests in order to investigate the intrinsic effects of ND on the localized mechanical and tribological response of WC-Co-ND composites. The incorporation of 10 vol.% ND enhanced wear resistance at both the micro- and macroscale, by 28% and 35%, respectively.

Structure, Mechanical and Tribological Properties of HVOF Sprayed (WC-Co+Al) Composite Coating on Ductile Cast Iron

The paper presents the results of examinations of WC-Co coating sprayed on ductile cast iron by high velocity oxygen fuel spray process (HVOF) with powder containing Al particles in an amount of 10%. The impact of Al particles added to the tungsten carbide coating on the structure, mechanical and tribological properties in the system of (WC-C)/ductile cast iron was examined. The microstructure of the thermal sprayed WC-Co+Al coating was characterized by light, scanning electron (SEM) and transmission electron (TEM) microscopes as well as the analysis of chemical and phase composition in micro areas (EDS, XRD). It was found that by supersonic thermal spraying with WC-Co powders with the addition of Al particles, the coatings of low porosity, high hardness, a very good adhesion to the substrate, compact structure with molten Al particles and finely fragmented WC particles embedded in a cobalt matrix, reaching the nanocrystalline sizes were obtained. Moreover, the results were discussed in reference to examination of bending strength considering cracking and delamination in the system of (WC-Co+Al)/ductile cast iron as well as hardness and wear resistance of the coating. It was found that the addition of Al particles was significantly increase resistance to cracking and wear behaviour in the studied system

Investigation of pulse electrodeposited Ni–Co/SiO2 nanocomposite coating

Ni–Co/SiO2 nanocomposite coating was deposited on the steel substrate by pulse electrodeposition method. The coatings were characterised by X-ray diffraction, field emission scanning electron microscopy technique, energy-dispersive X-ray spectroscopy and X-ray map (dispersive elemental mapping) analysis. The influences of stirring rate, pH and SiO2 concentration parameters on the mechanical properties and corrosion resistance were studied. The zeta potential of SiO2 nanoparticles was decreased by mounting the pH and also adsorption of the surfactant SDS. Microhardness, friction coefficient, wear and anodic polarisation were determined through Buhler microhardness tester, pin-on-disc wear test and potentiostat galvanic device, respectively. By applying optimum electrodeposition condition (stirring rate 200 rev min−1, pH 4.6 and SiO2 concentration 20 gL−1) incorporation of SiO2 particles into the nickel–cobalt matrix increases which results in the preparation of coatings with high microhardness and corrosion resistance, low friction coefficient and wear mass loss.

STEM analysis of WC-Co coatings modified by nano-sized TiC and nano-sized WC addition

Thermally sprayed hardmetal WC-Co-based coatings are widely applied in order to provide good wear resistance to mechanical components working in demanding environment. The WC-Co coatings are obtained by various spraying methods such as High Velocity Oxygen Fuel (HVOF) or High Velocity Air Fuel (HVAF) techniques. Additional improvement of coatings' properties can be achieved by adding very fine carbides to the WC-Co powder feedstock. The usual approach is the agglomeration and sintering of submicron-sized carbides without or with micro-sized carbides (bimodal distribution). In this work, WC-Co powders were modified by blending them with 5wt% nano-sized TiC (<100nm) or 5wt% nano-sized WC (<100nm). The coatings were deposited by High Velocity Air Fuel (HVAF) on a steel substrate. As reference, pure WC-Co coating was also sprayed. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were employed for the microstructural characterization of the deposited coatings. The formation of solid solution of tungsten in cobalt with the composition Co0.9W0.1 by selected area electronic diffraction, was detected. W dissolution effect was observed to be lower in the case of nano-sized TiC-added coatings. Microstructural investigation by STEM indirectly indicates that nano-sized WC completely dissolved in the cobalt matrix, inducing solid solution of tungsten in cobalt formation in the 5wt% WC-added coating. The formation of new phases containing Ti, except TiC, was not found in the coating. Obtained results suggest that nano-sized TiC may increase the stability of metallic matrix microstructure in a WC-Co-based coating.

Determination of emissivity coefficient of heat-resistant super alloys and cemented carbide

Abstract This paper presents the analysis of emissivity engineering materials according to temperature. Experiment is concerned on difficult to machine materials, which may be turned with laser assisting. Cylindrical samples made of nickel-based alloys Inconel 625, Inconel 718, Waspaloy and tungsten-carbides based on cobalt matrix were analyzed. The samples’ temperature in contact method was compared to the temperature measured by non-contact pyrometers. Based on this relative, the value of the emissivity coefficient was adjusted to the right indication of pyrometers.

Electroless deposition and investigation of composite coatings based on nickel or cobalt matrix, including boron nitrides as dispersoids, on polyethylene terephthalate substrate

Composite coatings, based on nickel or cobalt matrix with incorporated boron nitride particles (BN) as dispersoid, have been chemically deposited and investigated. The mass, and relative thicknesses, of the obtained composite coatings, as well as the number of co-deposited particles, depending on the type (alpha boron nitride – BN(h) or cubic boron nitride – BN(c)) and particle size of the dispersoid, as a function of dispersoid concentration in the working electrolyte and the concentration of the reducing agent in the plating electrolyte have been determined. The morphology, structure and chemical composition of the deposited coatings have been characterised by means of SEM and EDS analysis.

Cobalt-Palladium Alloys for Hydrogen Generation Via Water Electrolysis

The d-Block metals such as cobalt and nickel are widely applied in chemical industry as catalysts in reactions producing, among others, liquid or gaseous fuels. The best parameters are featured by precipitation catalysts modified with precious metals. Due to a very high price of platinum metals, it is important to search for solutions that could lower their quantity while maintaining similar catalytic properties.An innovative approach is the use of a magnetic field to modify the crystalline structure of coatings, which, especially for ferromagnetic elements, can improve their catalytic properties. Magnetic field applied parallel to the surface of the electrode generates convection of the electrolyte; it results in a laminar flow on the surface of the electrode which reduces the diffusion layer and increases the concentrations gradients. This results in change of the size of the grains and thus can also influence the texture and formation of various phases of the deposits. These various effects can be caused at the same time by the above mentioned convection but also by the magnetic properties, when the field is superimposed the growth in the direction of easier magnetization appears. Optimal composition, structure and morphology of cobalt matrix alloys with respect of hydrogen evolution performance will be sought through selection of proper composition of electrolyte, changes of electrolysis parameters and by application of external electromagnetic field with intensity up to 12 Tesla.

Electrochemical Codeposition of UHMWPE Biopolymer into Cobalt Matrix for Biomedical Applications

The aim of this work was to obtain hybrid Co/UHMWPE composite biocoatings reinforced by UHMWPE (ultra high molecular weight polyethylene) biopolymer microparticles in the cobalt matrix, by electro-codeposition technique, with possibilities to use them as biomaterials. UHMWPE was selected as surface modifier element, due to its high biocompatibility and low coefficient of friction being used in many biomedical applications. Cobalt is already used in biomedical implants as cobalt – chromium alloys. The obtained coatings were investigated in terms of surface morphology (scanning electron microscopy - SEM), chemical composition and inclusion percentage (energy dispersive X-ray spectroscopy - EDX), roughness and microtopography (atomic force microscopy - AFM), coating thickness and microhardness. The SEM morphologies of electrodeposited pure cobalt and Co/UHMWPE composite biocoatings, show differences due to UHMWPE biopolymer particles incorporation in the cobalt matrix. The inclusion of UHMWPE microparticles increases with increasing the UHMWPE concentration in the electrolyte as was demonstrated by EDX investigations. The addition of the UHMWPE biopolymer microparticles to the deposition bath led to an increase of the roughness of hybrid coatings comparatively with pure cobalt coating obtained under the same conditions. The coating thickness of the electroplated surfaces as were observed by cross sectional scanning electron micrographs confirm higher adhesion strength of the coatings on the stainless steel support.

The Structure and Bond Strength of Composite Carbide Coatings (WC-Co + Ni) Deposited on Ductile Cast Iron by Thermal Spraying

An investigation was conducted to determine the role of Ni particles in the WC-Co coating produced with the supersonic method on microstructure, mechanical, and wear properties in a system of type: WC-Co coating/ductile cast iron. The microstructure of the thermal-sprayed WC-Co + Ni coating was characterized by scanning electron and transmission electron microscopes as well as the analysis of chemical and phase composition in microareas (EDS, XRD). The microstructure of the WC-Co + Ni coating consisted of large, partially molten Ni particles and very fine grains of WC embedded in cobalt matrix, coming to the size of nanocrystalline. Moreover, the results were discussed in reference to examination of bending strength considering cracking and delamination in the system of (WC-Co + Ni)/ductile cast iron as well as hardness and wear resistance of the coating. It was found that the addition of Ni particles was significantly increase resistance to cracking and wear behavior in the studied system.