WC Crystals Research Articles

Formation of WC/Ni hard alloy coating by laser cladding of W/C/Ni pure element powder blend

WC/Ni coating was formed by laser cladding of a W/C/Ni powder blend. The formed WC crystals have rectangular or quadrangle cross-section shapes with size of 2–30 μm. Step, twist and cross growth morphologies of WC formation were observed. The coating contains WC, CW 3, WNi, FeW 3C, Fe 6W 6C, W 3O, W, C, and (Fe,Ni) phases.

The contiguity of carbide crystals of different shapes in cemented carbides

Equilibrium and geometry models for the contiguity of carbide crystals in cemented carbides are discussed. The equilibrium approach is applicable to rounded carbide crystals with a sufficient amount of the binder phase. Increase of the angularity of carbide crystals and/or decrease of the binder content lead to the geometrical constituent of the contiguity to be taken into account. In both cases, the energy advantage of the carbide–carbide contact over replaced carbide–binder interfaces determines the existence of the contact. An effect of the shape equiaxiality on the contiguity of WC crystals is observed in WC–Ni cemented carbides with small additions of TiC and explained by a model combining geometry and energy advantage of the contacts. The basal-to-basal, prismatic-to-prismatic, and prismatic-to-basal WC/WC contacts and their contribution to the decrease of the total contiguity are considered and evaluated separately in regard to the decrease of the shape equiaxiality of WC crystals.

Accommodation of the Lattice Mismatch at the VCx-WC Interface

A VC doped WC-Co alloy is investigated using high resolution transmission electron microscopy. The VC grain growth inhibitor induces the presence of a thin layer on the surfaces of the WC grains in contact with Co and precipitates in the corners of Co pockets. These (VW)Cx compounds adopt an epitaxial orientation relationship with regards to the (0001) base facets of the WC crystals. Due to the small difference in lattice parameters, misfit dislocations are expected in the interfaces. Unlike the thin layers where no defects are observed, two kinds of dislocations are pointed out for larger precipitates. 1/6〈112〉VC interfacial dislocations are sometimes present while more often 1/2〈1¯10〉VC dislocations lying above the interface in the (VW)Cx phase are visible.

Geometric and Crystallographic Characterization of WC Surfaces and Grain Boundaries in WC-Co Composites

Electron backscattered diffraction has been used to determine the orientation of WC crystals in a WC-Co composite and atomic force microscopy has been used to measure the shapes of planar sections of the same crystals. A stereological analysis has been used to determine that {101¯0} prism facets and the {0001} basal planes are the WC surfaces that are most frequently in contact with Co. Further, the WC habit is an approximately equiaxed trigonal prism bound by three prism facets and two basal facets. An analysis of ∼15,600 grain boundaries shows that certain interfaces occur with a frequency that is much higher than would be expected in a random distribution and that the grain boundary habit planes also have {101¯0} and {0001} orientations. Eleven percent of all the observed WC-WC interfaces are 90° twist boundaries about [101¯0]. Two types of boundaries with a 30° rotation about [0001], a twist and an asymmetric tilt, comprise 3% of the population.

Role of Vanadium Carbide Additive during Sintering of WC–Co: Mechanism of Grain Growth Inhibition

In a WC–Co specimen, the shape of WC crystals was a triangular prism with truncated corners. When VC was added to inhibit grain growth, the crystal shape changed to a triangular prism without truncation. This shape change was related to the variation of edge energy, which has a significant influence on the coarsening process of WC grains.

On the formation of very large WC crystals during sintering of ultrafine WC–Co alloys

During liquid phase sintering of very fine-grained WC powders (average particle size in the range 100–200 nm) very large WC crystals form with sizes of > 20 μm . Such grains typically exhibit the shape of plates. On tracing back the formation of these crystals during sintering it was observed that already on heating to 1150 °C small platelets were present in the still porous sintering body. At 1250 °C, i.e. before eutectic liquid formed, they had grown to a plate length of up to 7 μm with aspect ratios higher than 1:10. This early rapid WC crystal growth was observed preferentially in the outer areas of the sintering body where the sample is in closer contact with the sintering atmosphere. SEM imaging and TEM studies reveal the presence of twin boundaries in the plate-like WC crystals, rendering the possibility of a defect-assisted growth of the WC. Intermediate formation of η-carbides (M 12C, M 6C) and their subsequent transformation might be an explanation for this early occurrence of extraordinarily large grains.

Synthesis of low density titanium-magnesium-silicon alloys: F.H.Froes et al. (University of Idaho, USA.)

The macrocrystalline thermit process provides a highly effective method for the production of tungsten carbide crystals directly from ore concentrates. The unique properties of macrocrystalline WC are being exploited in many special applications including hardfacing products, diamond matrix oil tools, hardrock mining and construction bits. WC crystals grown to 500 microns in an iron menstruum, with a stoichiometric 6.13% carbon, yield a stable carbide ideal for infiltration and wear applications. Charles Terry and James Morris of Kennametal, Inc, describe the process and the resulting properties and applications.

Macrocrystalline thermit process revealed

The macrocrystalline thermit process provides a highly effective method for the production of tungsten carbide crystals directly from ore concentrates. The unique properties of macrocrystalline WC are being exploited in many special applications including hardfacing products, diamond matrix oil tools, hardrock mining and construction bits. WC crystals grown to 500 microns in an iron menstruum, with a stoichiometric 6.13% carbon, yield a stable carbide ideal for infiltration and wear applications. Charles Terry and James Morris of Kennametal, Inc, describe the process and the resulting properties and applications.

Grooving wear of single-crystal tungsten carbide

The anisotropic nature of tungsten carbide (WC) single crystals has been evaluated in single-tip scratch testing and in multiple-tip abrasion. The single-tip grooves were made with a Vickers diamond indenter and the abrasion tests were performed with diamond and silica grits. All tests were performed on both the prism and basal planes of the WC crystals. A polycrystalline binderless carbide (Bl) was also evaluated. Optical surface profilometry was used to estimate the amounts of displaced, removed and ridge-formatted material in the scratch tests and the wear volumes in the abrasion tests. The scratches and wear scars were studied with scanning electron-, atomic force- and light optical microscopy (SEM, AFM, LOM). In situ studies of the scratch process were also performed. Wear debris were analysed with transmission electron microscopy (TEM). The results show that there are differences in both the amount of wear and the wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms change from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). It is also shown that WC tends to wear by a formation of angular rod-shaped wear debris with the slip planes as the preferred surface planes.

Epitaxial growth of WO3−x needles on (10 1̄0) and (01 1̄0) WC surfaces produced by controlled oxidation with CO2

The controlled oxidation with CO2 of both high surface area (high Sg) and low surface area WC to form WO3–x needles, has been observed in situ, inside a controlled environment transmission electron microscope (CETEM), and also ex situ, in a standard flow reactor; the tungsten oxide needles, which consist mainly of distorted γ-WO2.72, propagate in an epitaxial fashion from the (10 0) and (01 0) surfaces of the parent WC crystals; possible mechanisms for the formation of the needles are proposed.

The Effect of Elastic Relaxation on the Indentation Size Effect in Tungsten Carbide

where a is the apex half-angle of the pyramidal indenter, P is the indenting load in kgf and d is the indentation diagonal in mm. The diagonal, d, is measured by means of an optical microscope after unloading the indenter, while the apex half-angle of the indentation, a, is assumed to be constant at all applied loads and equal to the apex half-angle of the indenter, i.e., half of the angle between opposite triangular faces of the indenter (standard a ˆ 688). It is well established that in most materials the microhardness measured by means of indentations decreases with increasing indenting load down to a constant value, which coincides with the macrohardness of the material [1]. This phenomenon is called indentation size effect (ISE) and has been observed in many materials, including tungsten carbide (WC) single crystals [2]. This work aimed at assessing the role of elastic relaxation in the observed ISE. For this purpose, a (0 0 0 1) face of a WC crystal was indented at 25, 50, 100, 200 and 500 3 10y3 kg and, after unloading the indenter, the depth and diagonal lengths of the indentations were measured by atomic force microscopy (AFM). The results were compared with the depth and diagonal of the indentations before unloading, which were calculated from the geometry of the indenter. The indenter was a standard Vickers indenter, i.e., a square pyramid of apex half-angle equal to 688. For the calculation of the size of the indentation before unloading, it was assumed that the intrinsic hardness of the WC (0 0 0 1) surfaces was 2000 kgf mmy2, i.e., the macrohardness measured at loads of 1 kg or higher [3]. Fig. 1 shows the percentage change in the depth and the diagonal length of the indentations before and after unloading each of the ®ve indenting loads, as well as the percentage change in Vickers microhardness, relative to the known macrohardness. The percentage change decreases in all cases with increasing indenting load. The microhardness tends to the macrohardness value, and the percentage change in diagonal length tends to zero, but the change in depth tends to approximately 30%. The half-angle â between two opposite edges of the indentation, which, before unloading, is related to the apex angle a by the equation tan ⠈  2 p tan a (2)

Anisotropic atomic packing model for abnormal grain growth mechanism of WC-25wt.%Co alloy

During liquid phase sintering, cemented carbide particles grow into either faceted or non-faceted grain shapes depending on ally system. In case of WC-Co alloy, prism-shape faceted grains with (0001) planes and {l_brace}1{bar 1}00{r_brace} planes on each face are observed, and furthermore an abnormal grain growth has been reported to occur. When abnormal grain growth occurs in WC crystals, dimension ratio, R, of the length of the side of the triangular prism face to the height of the prism is higher than 4 whereas that for normal grains is approximately 2. Abnormal grain growth in this alloy is accelerated by the fineness of starting powders and by high sintering temperature. To account for the mechanism of the abnormal grain growth, there are two proposed models which drew much research attention: nucleation and subsequent carburization and transformation of {eta} (W{sub 3}Co{sub 3}C) phase into WC, and coalescence of coarse WC grains through dissolution and re-precipitation. Park et al. proposed a two-dimensional nucleation theory to explain the abnormal grain growth of faceted grains. There are questions, however, on the role of {eta} phase on abnormal grain growth. The mechanism of coalescence of spherical grains as proposed by Kingery is also unsuitable for facetedmore » grains. So far theories on abnormal grain growth do not provide a satisfactory explanation on the change of R value during the growth process. In the present work a new mechanism of nucleation and growth of faceted WC grains is proposed on the ground of anisotropic packing sequence of each atom.« less

Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites

The microstructure and mechanical properties of nanograin-sized WC-Co composites were investigated and compared with those of conventional cermets. The dislocation density in the nanometer-sized WC crystals is lower than in the conventional ones, and no inclusions are observed in them. Nanostructured composites have higher tungsten content in the binder phase and a higher FCC HCP ratio of the cobalt. An amorphous phase is observed in the binder phase of the nanostructured samples. Hardness and surface toughness were investigated by performing Palmqvist indentations at loads from 0.025 to 40 Kg. The hardness increases with decreasing binder mean free path of dislocation in the binder phase. The high hardness of nanostructured cemented carbides results not only from the ultrafine microstructure, but also from alloy strengthening of the binder phase itself. The variations of hardness with load suggest that the finer grade conventional carbides have higher microfracture strength, and the nanostructured WC-Co composites are superior to the conventional ones in this respect. Bulkfracture toughness is related to cracks developing through the phases of the material. Palmqvist indentation toughness measurements show that the toughness decreases with increasing hardness in conventional composites, whereas the increase of hardness in nano-structured composites does not further reduce their bulk fracture toughness. This implies that different dominant toughening mechanisms exist in the conventional and nanostructured composites.

The shape of WC crystals in cemented carbides

A study has been made of the shape development of WC crystals in cemented carbides during liquid phase sintering. The relative influence of shape relaxation and carbide crystal growth processes on the shape is discussed. An effect of titanium additions on the shape of WC crystals in WC–Ni cemented carbides was observed. A shape parameter referred to as shape equiaxiality has been defined and calculated theoretically for various triangular prisms. The observed change of shape equiaxiality with titanium concentration in the binder phase of WC–Ni cemented carbides is discussed in terms of titanium segregation at the interphase boundary.

Effects of particle size and compaction of nano-structured tungsten carbide-cobalt: P. Seegopaul, L. Wu. (Nanodyne Inc., USA.)

The microstructure and mechanical properties of nanograin-sized WC-Co composites were investigated and compared with those of conventional cermets. The dislocation density in the nanometer-sized WC crystals is lower than in the conventional ones, and no inclusions are observed in them. Nanostructured composites have higher tungsten content in the binder phase and a higher FCCHCP ratio of the cobalt. An amorphous phase is observed in the binder phase of the nanostructured samples. Hardness and surface toughness were investigated by performing Palmqvist indentations at loads from 0.025 to 40 Kg. The hardness increases with decreasing binder mean free path of dislocation in the binder phase. The high hardness of nanostructured cemented carbides results not only from the ultrafine microstructure, but also from alloy strengthening of the binder phase itself. The variations of hardness with load suggest that the finer grade conventional carbides have higher microfracture strength, and the nanostructured WC-Co composites are superior to the conventional ones in this respect. Bulkfracture toughness is related to cracks developing through the phases of the material. Palmqvist indentation toughness measurements show that the toughness decreases with increasing hardness in conventional composites, whereas the increase of hardness in nano-structured composites does not further reduce their bulk fracture toughness. This implies that different dominant toughening mechanisms exist in the conventional and nanostructured composites.

Microstructural Characterization of Cobalt-Tungsten Coated Graphite Fibers.

The present research concerns an electrodeposited cobalt-tungsten alloy coating on graphite fibers. Annealed and unannealed coated fibers were analyzed by scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The mole fraction of tungsten in the as-deposited cobalt-tungsten coating was found to be (7.10 ± 0.82) %, and the crystalline lattice was determined to be hexagonal close-packed. Note: The uncertainties reported here are expanded uncertainties (i.e., 2 standard deviation estimates). After annealing at 1100 °C for 1.5 h, the lattice was found to be a cobalt-tungsten-carbon (Co-W-C) alloy with a face centered cubic lattice. The mole fraction of tungsten in the annealed coating was shown to be (5.30 ± 0.66) %. XRD analysis revealed that the annealed coating contained regions having three slightly different lattice parameters. The lattice parameters in the three regions of the Co-W-C alloy coating corresponded to a mole fraction of carbon of 0.66 %, 0.40 %, and 0.19 % (± 0.18 %). These compositions are discussed with respect to the Co-W-C phase diagram. Various tungsten carbide species were identified in the coating and fiber, and a network of overlapping WC crystals was found at the coating-fiber interface. This network appeared to serve as a protective barrier to extend the lifetime of the fibers at elevated temperatures.

The anisotropic work-hardening of WC crystals

It has been found that indented ( 10 10 ) surfaces of WC crystals exhibit piled-up material next to the indentations while (0001) surfaces exhibit sunk-in material. Since in some metals sunk-in material around indenters indicates a higher work-hardening capacity than piled-up material, slip line and etch pit patterns around indentations were analysed, in order to deduce the dislocation reactions occurring in each case. It was found that 1 6 [ 12 10] sessile dislocations can be produced only when indenting (0001) surfaces, which is consistent with a higher work-hardening capacity of (0001) surfaces.

Subsurface modification of WC single crystals by nitrogen implantation

Specific crystallographic planes of WC single crystals have been studied by means of SEM and TEM, both before and after nitrogen implantation. It is found that nitrogen implantation causes large increases both in susceptibility to fracture and in dislocation density. The results suggest that an increase in the hardness of the subsurface of WC crystals is induced by nitrogen implantation.

Some stereological properties of hexagonal prisms

A method is described by which the ratio of base to height in crystals of trigonal symmetry may be determined from metallographic sections of the crystals in a matrix. The method is applied to a WC-25 wt.% Co sample, liquid-phase sintered at 1400°C for 100 hr, during which the WC crystals form triangular prisms. It is shown that the prisms have a base to height ratio of 2.0 ± 0.1 and are slightly truncated along the edges parallel to the prism axis.

Studies on Direct Carburization of WC form the Mixture lf WO3 and Carbon

The reason why WC powder of good quality cannot be obtained by the process of heating the WO3 and C mixture in H2 was examined by simple experiments. The authors obtained the following conclusions so that the 2 stage-carburization was adequate to solve the problem.1) The reduction by H2 occurs preferentially to the reduction by C when the mixture of WO3 and C is heated in H2.2) The H2O formed by the reaction of WO3+H2 reacted with carbon at the reacting zone. The quantity of the H2O depends upon both the way of charging and heating conditions, affecting the carbon content of WC.3) WC crystals of abnormal shape were found when the coarse WO3 powder was used. The reason was discussed.4) The WC powder of good quality was obtained when the H2O was either completely removed or not formed and when fine WO3 powder was used.