AN ELECTRON-MICROSCOPE AND X-RAY INVESTIGATION OF THE MILLING OF TUNGSTEN CARBIDE/COBALT MIXTURES

Abstract

It is well known that some interaction takes place between the two components when mixtures of cobalt and tungsten carbide are milled. To gain a further insight into this phenomenon, the milling process has been studied by means of the electron microscope and by the BET and X-ray methods.

During the milling of cobalt powder the number of stacking faults and the amount of the hexagonal phase both increase. On milling 80:20 tungsten carbide/cobalt mixtures increasing agglomeration of cobalt and fine tungsten carbide particles was found with rise in milling intensity. The larger tungsten carbide particles appeared to have a smooth surface. However, if the cobalt was dissolved in hydrochloric acid, the true surface of the tungsten carbide particles was revealed. This became rougher with increasing milling intensity. From this it can be concluded that, during milling, cobalt settles between the surface irregularities.

The variation in distribution of the cobalt that results from different milling conditions leads to a difference in sintering behaviour. A dilatometric study has been made of this aspect.

The original particle size of the cobalt used for hard-metal mixtures does not affect the properties of the sintered product, if the mixtures are milled very intensively.     

Determination of optimum parameters for the detonation deposition of coatings

Conclusions An empirical formula is given expressing the relationship between the strength of adhesion of detonation-deposited coatings to substrates and the heat capacity of the particles of a powdered material being deposited at the instant of coating formation. An expression has been derived for the variation of a dimensionless temperature with powder particle size. A procedure has been developed for determining the optimum composition of an explosive gas mixture for the detonation deposition of various materials.Translated from Poroshkovaya Metallurgiya, No. 4(232), pp. 61–65, April, 1982.     

Structure of the surface layer of a hard-metal cold-upsetting tool

Conclusions In the structure of the WC and Co phases making up VK20K hard metal the presence was discovered, in the as-sintered condition, of dislocations and twins, which may be attributed to the generation, during the postsintering cooling of the alloy, of large thermal stresses as a result of the difference in the coefficients of thermal expansion of its tungsten carbide grains and metallic cobalt interlayers. Operation increased the number of dislocations and twins and brought about fragmentation of twins in the Co and WC phases, which is evidence that their wear was preceded by appreciable plastic deformation. In the structure of VK20K alloy in the as-sintered condition and in its surface layer after wear the carbide W₂C was found on the surface of the carbide WC between the WC grains and Co-phase interlayers, and it is therefore reasonable to assume the existence under such conditions of an extremely thin transition layer of virtually pure tungsten.Translated from Poroshkovaya Metallurgiya, No. 12(216), pp. 83–88, December, 1980.     

Use of x-ray photoelectron spectroscopy for the investigation of the surfaces of hard-alloy parts machined with polycrystalline superhard materials

Conclusions The thin (<600 Å) surface layers forming on VK20 and VK20K hard alloys during machining with polycrystalline superhard tool materials and diamond grinding under recommended conditions consist chiefly of tungsten and cobalt oxides, cobalt containing dissolved tungsten and carbon, and pure tungsten (or the compound W₂C). The formation of such layers must be regarded as a beneficial phenomenon, because the presence on the surface of a tool material of a thin layer of oxides of the elements of which the material is composed helps reduce tool wear.Translated from Poroshkovaya Metallurgiya, No. 11(191), pp. 73–79, November, 1978.     

Porosity measurements on detonation-deposited coatings

Conclusions Typical values of the open and total porosities of good-quality detonation-deposited alumina and VK alloy coatings are 4–6%. To obtain reliable values of porosity for detonation-deposited coatings by the hydrostatic weighing method, the coatings must be vacuum-treated and then impregnated with a liquid for not less than 1 h. The open porosity of an alumina coating does not differ greatly from its total porosity, but in a VK alloy coating the proportion of closed pores may be as high as 40%.Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 47–50, April, 1980.     

Effect of Magnetic Abrasive Finishing on the Chemical and Phase Composition of TiC Coatings on VK8 Alloy

Using x-ray diffraction analysis and electron-probe microanalysis, we have studied TiC coatings obtained by chemical vapor deposition, where the base and subsequently the coating have undergone magnetic abrasive finishing. We have found that treating the base before the coating is deposited prevents an η phase (Co₆W₆C) from forming in the surface layer of coated platelets. Magnetic abrasive finishing (MAF) has been found to cause an α → β transition in the cobalt phase.     

Reaction of Tungsten with Liquid Co ― Sn Alloys

The solubility of tungsten in Co Sn melts and the growth kinetics of a W₆Co₇ phase layer at the tungsten melt interface were studied at 1200°C. The liquid alloys composition in the three-phase equilibrium W W₆Co₇ melt was established as (at. fraction) 0.51 Co, 0.49 Sn, 2.3·10^–3 W. The solubility of tungsten in the investigated range of melt compositions is well represented by the equation lgx _W = –0.964-3.420x _Sn, where x _W and x _Sn are atomic fractions of the elements in the melt. The calculated thermodynamic properties can be used for the analysis of other systems which include cobalt and tungsten.     

Structural state,phase composition,and thermal fatigue resistance of detonation-deposited alumina coatings

Conclusions By suitable choice of gas ratio in the detonating mixture and degree of barrel filling, which determine the kinetic energy and degree of heating of the particles, it is possible to obtain good-quality alumina coatings exhibiting fairly high thermal fatigue resistance. For the application of satisfactory coatings it is necessary to employ powders of predominant particle size 20–40 m annealed at 1400–1700°C. Evidence has been obtained in support of literature reports concerning the beneficial effect of incomplete filling of the barrel of the detonation apparatus, which results in a higher proportion of -Al₂O₃ (up to 30%) in coatings. This is a key factor in the wear and thermal fatigue resistance of the coatings. The weakest structural elements in detonation-deposited oxide coatings under conditions of thermal cycling at 800°C are their interlayer boundaries, whose rupture leads to lamination and crumbling of the coatings.Translated from Poroshkovaya Metallurgiya, No. 5(269), pp. 32–38, May, 1985.     

A STUDY OF THE FACTORS CONTROLLING GRAIN SIZE IN SINTERED HARD-METAL

Abstract

Previous experimental work concerning the grain growth observed during the sintering of tungsten carbide–cobalt alloys is reviewed. Particle-sizing methods suitable for the examination of hard-metal powders are described, and techniques for the evaluation of the carbide grain size in the sintered compacts are discussed.

By using a Model A Coulter Counter to examine the size distribution of the carbide grains (obtained from the milled hard-metal powders by dissolution of the cobalt with hydrochloric acid), and by counting techniques on electron photomicrographs of carbon replicas of the sintered compacts, it has been established that the increase in grain size during sintering is quantitatively related to the carbon content of the material after pre-heating. The results presented indicate that the cobalt content exerts little influence on the average grain size of the sintered structures A cobalt content >10% by weight is shown to exert a strong damping effect on the rate of comminution during milling.

The linear relationships between the specific surface area of the carbide grains in milled powders (obtained using a Perkin–Elmer Sorptometer) and the specific surface of the carbide phase in sintered compacts are given. The influence of sintering temperature and time on average grain size and contiguity in a commercial alloy is shown. Some preliminary work indicates that the morphologies of the initial carbide powders may be important factors with respect to the grain size of sintered hard-metal.     

Effect of cobalt addition on the liquid-phase sintering of W-Cu prepared by the fluidized bed reduction method

A new process, fluidized bed reduction (FBR) method, was applied for fabrication of uniform W-Cu sintered material. Liquid-phase sintering was carried out to obtain fully densified W-Cu composite, and the effect of cobalt addition on the sintering behavior was investigated. It was found that fully densified material could not be obtained even after sintering at 1200 °C for 4 hours in the case of 75W-25Cu, while more than 96 pct density could be obtained as soon as the sintering temperature reached 1200 °C when 0.5 wt pct cobalt was added prior to the sintering. It has been found that the wetting angle of the liquid copper is reduced significantly by the addition of cobalt, and the formation reaction of Co₇W₆ intermetallic compound at the surface of the tungsten powder is mainly responsible for the enhancement of the densification process.     

Structure and Properties of Wear-Resistant Detonation Coatings Based on Titanium Carbonitride

An investigation was made of the composition, structure and wear rate of detonation coatings on steel 30KhGSNA deposited from composite powders based on TiC_0.5N_0.5 with refractory additions of SiC, AlN, and a Ni – Cr metallic binder. It was shown that at a load of 10 MPa coatings based on titanium carbonitride exhibit substantially less wear and a larger range of sliding velocities with a stable value of wear than coatings of the hard alloy VK15 (WC – 15% Co). The physico-chemical reactions in the process of tribo-oxidation were analyzed.     

WC-Co by Direct Reduction Carburisation Reactions

Abstract

Thermogravimetry, XRD and SEM have been used to study the direct reduction/carburisation of non-stoichiometric CoW0₄ with hydrogen/methane mixtures over the temperature range 900°C to 1100°C. The evolution of phases during reduction/carburisation has been related to differences in starting materials, temperature and gas phase composition. The thermochemistry of the reduction/carburisation process is detailed and actual results of processing are related to thermodynamic predictions. It is shown that the first stage of reaction is reduction of the tungstate to give the intermetallic compounds Co₃W and Co₇W₆ as well as tungsten metal. During this stage no carburisation occurs. The subsequent carburisation stage of the process yields a sequence of phases passing through carbides of the general formulae M₁₂C and M₆C before giving a mixture of WC and Co. In W-rich mixtures a transitory existence of W₂C is shown to exist.     

News & Views

Abstract

Although hardmetal has been manufactured, by the powder metallurgy route for well over 55 years, the characterization of the starting powders, basically tungsten carbide and cobalt, still commands the attention of powder metallurgists. Three fine cobalt powders and two 1·2 μm (FSSS) tungsten carbide powders, freely available, have been both chemically and physically characterized. Various techniques of measuring physical characteristics, such as FSSS, BET gas adsorption, pore volume and area, sedimentation, SEM powder, and SEM polished and etched sections of copper impregnated powders, have been studied. The techniques have been discussed in relation to their advantages and disadvantages. The measurement by FSSS and BET gas adsorption give fairly reproducible results, as does the sedimentation technique. However, by reference to the two SEM techniques it has been shown that the very important characteristic of particle shape and, more importantly for tungsten carbide powder, particle agglomeration (due to the manufacturing route) is not fully revealed. The two SEM techniques are indispensable for defining shape and agglomeration characteristics. It is suggested that by using the SEM copper impregnated powder method and a semi-automatic image analysing system the true crystal (grain) size distribution of tungsten carbide powders can be evaluated. The chemical purity of cobalt and tungsten carbide powders has significantly increased in recent years. Some preliminary results from milled tungsten carbide powders are discussed. PM/0393     

Physicochemical interaction and structure development during the formation of metal gas-transfer coatings on diamond (review). 1. Kinetics

Work devoted to studying the phase composition and thickness of chromium, titanium, molybdenum, vanadium, and tungsten coatings which form on diamond powder during vacuum annealing of this powder mixed with chromium powder or oxidized powders of Ti, Mo, V, and W is analyzed. Coatings consist of metal (Cr, Ti, Mo, V, W) and carbide (Cr₇C₃, Cr₃C₂, TiC, -Mo₂C, V₂C, VC, W₂C, WC) phases. Diffusion of carbon during coating growth with increased metallizing time and temperature causes carbidization of chromium, titanium, and vanadium (it causes a reduction in the content of metal phase and an increase in carbide phases is coatings), growth of higher carbides (Cr₃C₂, VC, WC) at the expense of lower carbides (Cr₇C₃, V₂C, W₂C), and filling of carbon vacancies in the lattices of TiC and VC. Saturation of coatings with carbides correlates with the temperature-time range in which a further increase in coating weight slows down.Translated from Poroshkovaya Metallurgiya, No. 7(355), pp. 34–40, July, 1992.     

Improving the wear resistance of hard-alloy rolling disks by pulsed-plasma treatment

Pulsed-plasma surface treatment of VK20 hard-alloy (WC + 20% Co) components changes the crystallite size by 10–20% and enriches the binding metal Co with complex carbides of type W₆Co₆C and W₃Co₃C and nonequilibrium carbides of composition W₂C, WC_1-x, and W₆C_2.54. The internal stress (compression) reaches 800 N/mm². Industrial tests show that pulsed-plasma treatment increases the wear resistance of the working surface of VK 20 hard-alloy (WC + 20% Co) rollers (rolling disks) by a factor of 2–4.     

Effect of interaction between the structural components of W-Co-Sn pseudo-alloys on the kinetics of their compaction during liquid-phase sintering

Compaction kinetics during liquid-phase sintering of W-Co-Sn powder composites containing 90 mass% refractory component and 10 mass% readily-melting component is studied. It is established that compaction kinetics depends markedly on cobalt content in the melt. Specimens with a cobalt content up to 3 mass% at 1200 °C (in the nonisothermal heating stage) undergo an increase in volume, and then they are compacted at a rate typical for liquid-phase sintering. The nonuniform nature of compaction is observed with an increase in cobalt in the test composites. Specimens with a cobalt mass fraction of more than 2% (cobalt content with three-phase equilibrium) experience considerable additional growth due to formation of the intermetallic compound W₆Co₇ whose decomposition temperature exceeds the liquid-phase sintering temperature.     

Interphase reactions in forming plasma coatings

Nickel-plated powders and coatings of titanium and chromium carbides and borides are studied. Components of the starting powders react between themselves and with oxygen from the atmosphere forming solid solutions, intermetallic compounds, and a shielding oxide film. During pulverization the surface of particles does not lose a plated layer and nickel metal is always available in the coating. In order to increase the life of coatings it is advisable to use powders with the minimum possible amount of oxygen which promotes decomposition of higher carbide. In order to reduce the amount of complex oxides and intermetallic compounds in the coating which make a brittle matrix it is recommended to use boride powders in a mechanical mixture with metal (alloy) powders.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 5–6, pp. 54–60, May–June, 1994.     

Synthesis of Co3W–Cu composite nanopowders by mechanical milling and hydrogen reduction process

Abstract

In this research, synthesis of Co₃W–Cu composite nanopowders based on Co₃W intermetallic compound by mechanical milling and hydrogen reduction process was investigated. Powder mixture of Co₃O₄, WC and CuO with Co₅₀W₄₀Cu₁₀ stoichiometry was first milled by high energy planetary ball mill and then reduced in a hydrogen reduction system. Crystallite structure of milled mixture and reduced powders was determined by X-ray diffraction. Particles size, morphology and cross-section of reduced samples were studied by SEM, TEM and SEM back scattered electron microscopy. Optimum condition of reduction under hydrogen atmosphere was found at 900°C. Particles have Cu coring structure surrounded with Co₃W intermetallic compound. Mean particle size was observed less than 50 nm with six fold hexagon morphology.     

Reaction of Tungsten with Melts in the Cu - Co System

We have studied the solubility at 1200°C of tungsten in Cu - Co melts and the growth kinetics of a W₆Co₇ layer at the tungsten — melt interface. We have established the composition of the melt in the three-phase equilibrium tungsten — W₆Co₇ — melt: 0.0195 Co, 4.8 · 10⁻⁵ W, the rest is Cu (in atomic fractions). In the studied composition range for the melt, the solubility of tungsten is described well by the expression: lgX_W = −7.117 + 25.7 · X_Co ^1/2 − 41.06 · X_Co, where X_W and X_Co are the atomic fractions of the corresponding elements in the melt. We have determined the correlation between the growth rate for a layer of tungsten-containing phase at the tungsten — melt interface and the thermodynamic characteristics of the melt. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 81–86, May–June, 2005.