CoB Phases Research Articles

Composition dependent characteristic transition temperatures of Co-B melts

The temperature-dependent liquid structures of Co-B melts were studied by in-situ measuring the magnetization. A magnetization anomaly in term of the non-Curie-Weiss temperature dependence of magnetization was observed in the overheated state, demonstrating a temperature-induced liquid-liquid structure transition. This anomalous behavior was found to be a universal formula for the Co-B binary alloy system. The transition point (T0) and two paramagnetic Curie temperatures (θp(LI), θp(LII)) corresponding to two distinct kinds of liquids (i.e., high-magnetization liquid (HML) I and low-magnetization liquid (LML) II) were measured. With the increased concentration of Co, T0, θp(LI) and θp(LII) shift to higher temperatures and the Curie constants for the HML and LML decrease. Based on the location of T0, a guideline is drawn above the liquidus in the Co-B phase diagram, which could provide a significant guidance to the practical melt treatment.

Improved nanostructured diamond adhesion on cemented tungsten carbide with boride interlayers

The application of diamond coatings for strengthening cemented tungsten carbide has been previously attempted, but suffers from delamination. Plasma enhanced chemical vapor deposition boriding improves the strength of cemented carbides by forming WCoB, W2CoB2, and/or CoB phases using controllable diborane stoichiometry; this research explores these borides as interlayers for nanostructured diamond coatings. Diamond deposition occurred between 600°C and 1100°C at 100°C increments for 30min to 4h. Raman spectroscopy showed enhanced diamond growth compared to untreated WC-Co, however predominantly WCoB and CoB phase interlayers suffered from diamond film delamination. Examination by scanning electron microscopy and energy dispersive X-ray spectroscopy showed interfacial cobalt clusters on these interlayers. Predominantly W2CoB2 phase interlayers showed improvement with a reduction in reactive cobalt, and improved adhesion of nanostructured diamond coatings. Diamond on W2CoB2 was well adhered with deposition temperature dependent nanoindentation hardness ranging from 10 to 60GPa and elastic modulus of 400 to 750GPa. Scratch testing revealed cohesive and adhesive failure of the diamond coatings at 5N±2N and 8N±2N respectively and epoxy pull testing resulted in a surface adhesion tensile strength of 8.2MPa±0.1MPa. The W2CoB2 phase is shown to be desirable as an interlayer for improved nanostructured diamond adhesion on cemented tungsten carbide. Prime novelty statementThe scope of diamond deposition parameters used and the comparison of interlayer effect for CVD diamond coatings on ternary borides (WCoB, W2CoB2) with a cemented tungsten carbide substrate has not been previously examined to this level.

The role of boron segregation in enhanced thermoelectric power factor of CoSi1−xBx alloys

We report on the influence of boron segregation on the thermoelectric properties of CoSi. Contrary to previous suggestions, and in stark contrast to aluminum substitution, boron does not enter the lattice on the Si site, but rather segregates to the grain boundaries in these alloys. Through a combination of x-ray diffraction, scanning electron microscope, and scanning Auger techniques, we present clear evidence of the formation of a CoB phase at the grain boundaries. Consistent with the failure of B to substitute for Si, we observe no changes in the electron concentration or the Seebeck coefficient under boron substitution. The electrical resistivity, on the other hand, displays a non-monotonic behavior with increasing boron concentration, first decreasing for small amounts of boron, before increasing at higher levels of substitution. We attribute this behavior to a combination of an initial healing effect of boron on microcracks, followed by the eventual increase in electron scattering by the secondary CoB phase at higher concentrations.

Magnetization studies of binary and ternary Co-rich phases of the Co–Si–B system

CoB, Co 2B, CoSi, Co 2Si and Co 5Si 2B phases can be formed during heat-treatment of amorphous Co–Si–B soft magnetic materials. Thus, it is important to determine their magnetic behavior as a function of applied field and temperature. In this study, polycrystalline single-phase samples of the above phases were produced via arc melting and heat-treatment under argon. The single-phase nature of the samples was confirmed via X-ray diffraction experiments. AC and DC magnetization measurements showed that Co 2Si and Co 5Si 2B phases are paramagnetic. Minor amounts of either Co 2Si or CoSi 2 in the CoSi-phase sample suggested a paramagnetic behavior of the CoSi-phase, however, it should be diamagnetic as shown in the literature. The diamagnetic behavior of the CoB phase was also confirmed. The paramagnetic behavior of Co 5Si 2B is for the first time reported. The magnetization results of the phase Co 2B have a ferromagnetic signature already verified on previous NMR studies. A detailed set of magnetization measurements of this phase showed a change of the easy magnetization axis starting at 70 K, with a temperature interval of about 13 K at a very small field of 1 mT. As the strength of the field is increased the temperature interval is enlarged. The strength of field at which the magnetization saturates increases almost linearly as the temperature is increased above 70 K. The room temperature total magnetostriction of the Co 2B phase was determined to be 8 ppm at a field of 1 T.

Effect of Cu Addition on Nanocrystallization Behavior in a Co-Based Soft Magnetic Metallic Glass

Microstructure and magnetic properties of a nanocrystalline soft magnetic material having composition Co64.5 Fe3.5 Si16.5 B13 Ni1.5 Cu1 has been studied. Amorphous ribbon could be produced by melt spinning unit. DSC analysis showed four distinct crystallization events. Heat treated samples were characterized using XRD and TEM techniques. Co2B, and CoB phases were found to crystallize before magnetic phase a−Co. Addition of copper was proved to have adverse effect on soft magnetic properties.

Chemical vapor deposition of metal borides: 6. The formation of neodymium boride thin film materials from polyhedral boron clusters and metal halides by chemical vapor deposition

AbstractThe chemical vapor deposition (CVD) of crystalline thin films of neodymium hexaboride (NdB6) was achieved using either nido‐pentaborane(9) or nido‐decaborane(14) with neodymium(III) chloride on different substrates. The highly crystalline NdB6 films were formed at relatively moderate temperatures (835 °C, ca. 1 µm/h) and were characterized by scanning electron microscopy, X‐ray emission spectroscopy, X‐ray diffraction and glow discharge mass spectrometry. The NdB6 polycrystalline films were found to be pure and uniform in composition in the bulk material. Depositions using CoCl2, NdCl3 and B5H9 as the CVD precursors resulted in the formation of a mixture of NdB6 and CoB phases, rather than the ternary phase. Copyright © 2008 John Wiley & Sons, Ltd.

Adherent diamond coatings on cemented carbide substrates with different cobalt contents

A new process has been employed for preparing diamond coatings on cemented carbide substrates. Before depositing the diamond coatings, a diffusion barrier interlayer was prepared by solid diffusing boron into the substrates and this interlayer has the function of suppressing catalytic effect of Co on non-diamond formations. It was confirmed that on cemented carbide substrates with Co contents ranging from 3 to 8%, high quality adherent diamond coatings could be deposited. The formation of the boronized interlayer changed the catalytic character of the substrate surface and the interlayer functioned as a diffusion barrier layer against outward Co diffusion. The latter beneficial effect was found to be accompanied by phase transformations in the interlayer, from one composed of mainly WC, CoW 2B 2 and CoB phases to that of CoWB, WC and Co 2B phases, during the high temperature diamond deposition processes.

Superhard boride layer deposition on a carbide-cobalt hard alloy

The interaction of a carbide-cobalt hard alloy (K 10, International Standards Organization) with powdery B 4C and the product Borozar-HM and the effect of this interaction on the structure and hardness of the diffusion layer obtained are investigated. The phase composition of the diffusion layers is shown to depend on the type of the powders and on the temperature and duration of the boronizing process. As a result of thermal treatment with Borozar-HM, the phase CoWB is formed in the diffusion layer whereas in the case of B 4C the boron-richer CoW 2B 2 and CoB phases prevail. The Co-W-B phase diagram, the concentration of dissolved tungsten in the binder and the difference in mass transfer of boron from the two boride powders are used to determine the phase composition of the layer. The homogeneous diffusion layer with a small number of defects, formed during the interaction of the alloy with Borozar-HM, was studied by electron microscope analysis. It is shown that the difference in phase composition of the diffusion layers obtained with the two boron compounds affects the layer hardness. The thermochemical treatment of the alloy with Borozar-HM leads to the appearance of superhard surface layers exceeding in hardness those obtained with B 4C. The maximum Vickers hardness measured IIV 01 = 23.4 GPa is attributed to the single-phase diffusion layer of CoWB obtained.

ELECTRICAL RESISTIVITY AND DYNAMIC TEMPERATURE X-RAY DIFFRACTION OF Co-B LIQUID QUENCHED AMORPHOUS ALLOYS

The crystallization behaviour of Liquid Quenched Amorphous alloys of Co-B has been investigated by Dynamic Temperature X-Ray Diffraction (DTXD) at a heating rate of 50 K per hour. The result of DTXD were consolidated by measuring electrical resistivity. The products of devitrification indicates that crystallization of the amorphous mass occurs in stages and the sequence of phases appearing during heating follows the same order as has been depicted in Co-B phase diagram obtained during solidification from liquid state.

Activities of boron in the binary Fe-B, Co-B, and Cu-B melts

Activities of boron in the binary Fe-B, Co-B, and Cu-B melts have been directly determined by the electromotive force (emf) measurement. Boron-saturated liquid Cu-B alloy was used as the reference electrode and a ternary 28 wt pct Al2O3-29 wt pct B2O3-43 wt pct CaO oxide melt was used as the electrolyte. Deviations of the boron activities from Raoult's law have been found largely negative for the Fe-B and Co-B systems but largely positive for the Cu-B system. Boron activities calculated from the literature data have not been in good agreement with the measured data. Activities of iron, cobalt, and copper have been calculated from the obtained boron activities by means of the Gibbs-Duhem equation. Some modifications to the liquidus curves on the Fe-B and Co-B phase diagrams have been presented.

Nucleation and growth of CoSi 2 on (100) and (111) Si

CoB, Co2B, CoSi, Co2Si and Co5Si2B phases can be formed during heat-treatment of amorphous Co–Si–B soft magnetic materials. Thus, it is important to determine their magnetic behavior as a function of applied field and temperature. In this study, polycrystalline single-phase samples of the above phases were produced via arc melting and heat-treatment under argon. The single-phase nature of the samples was confirmed via X-ray diffraction experiments. AC and DC magnetization measurements showed that Co2Si and Co5Si2B phases are paramagnetic. Minor amounts of either Co2Si or CoSi2 in the CoSi-phase sample suggested a paramagnetic behavior of the CoSi-phase, however, it should be diamagnetic as shown in the literature. The diamagnetic behavior of the CoB phase was also confirmed. The paramagnetic behavior of Co5Si2B is for the first time reported. The magnetization results of the phase Co2B have a ferromagnetic signature already verified on previous NMR studies. A detailed set of magnetization measurements of this phase showed a change of the easy magnetization axis starting at 70 K, with a temperature interval of about 13 K at a very small field of 1 mT. As the strength of the field is increased the temperature interval is enlarged. The strength of field at which the magnetization saturates increases almost linearly as the temperature is increased above 70 K. The room temperature total magnetostriction of the Co2B phase was determined to be 8 ppm at a field of 1 T.

ＴｉＢ２‐１％ＣｏＢおよびＴｉＢ２‐５％ＴａＢ２‐１％ＣｏＢ材のち密化機構

Sintered TiB2-1%CoB and TiB2-5%TaB2-1%CoB materials have high density and high trasverse rupture strength. In the present study, the densification mechanism of the above materials has been investigated. It has been found that even a small amount (1 wt% or less) of CoB added to TiB2 or to TiB2-5%TaB2 has a remarkable effect on densification, and that the mixture of TiB2-1%CoB shows remarkable shrinkage during sintering under the die pressure of 200 kg/cm2 even at 1400°C which is lower than the melting point of CoB. These results suggest that the densification of these materials is caused mainly by an activated sintering process under die pressure. In this process, atoms on the surface of TiB2 (or TaB2) grains dissolve into the CoB phase so that a carrier phase may be formed in the contact area between the TiB2 (or TaB2) grain and the CoB phase. Then, the surface atoms on any TiB2 (or TaB2) grain can diffuse rapidly to other grains through the carrier phase.