Surface engineering, powder, PM processes and recycling

Violet tungsten oxide’s in-situ reduction technology for the preparation of ultrafine grained WC-Co hardmetals, C. Wu, China National R&D Centre for Tungsten Technology, China

(I can vouch for the technology behind this development. Back in the 1950s, when very little was published about major developments in hardmetals, violet tungsten oxide was our first choice as raw material for the finest submicron WC – KJAB). 

Sintered WC/Co hardmetals with ultrafine grain sizes (≤0.5 μm), which possess optimised properties such as hardness and toughness, are the preferred materials for efficient tools. In spite of only 15-year production history of ultrafine grained hardmetals in China, that country now claims to be the largest producer of these materials, thanks to the invention (re-invention?) of the preparation technology of “violet tungsten oxide’s in-situ reduction.” In the paper the development is systematically introduced and reviewed, focusing on aspects of content, principles and advantages. The high quality hardmetals derived from these ultrafine tungsten carbide powders are characterised by single-phase WC hard particles, uniform grain size and excellent crystal structure. The production equipment supporting these technologies is also discussed. 

Study on brush wear and processing efficiency of brush mechanical polishing, W. Zeng, Zigong Cemented Carbide Co.,Ltd, China 

According to different installation methods, mechanical brush polishing equipment can be divided into two categories: horizontal wheel brush and vertical disc brush polishing machines. In the paper, brush motion is analysed, processing efficiency of the brush being higher on the entrance face than on the exit face. Factors affecting brush wear included brush media, effects of material hardness, (including elastic coefficient, diameter and length), working temperature and humidity on the wear rate of the brush. Based on the formula D=WL3/0.1473Ed4N, the larger the brush filament diameter, the higher the hardness and greater the wear rate. The longer the brush filament, the lower the hardness. Keeping factors constant apart from brush filament hardness; for the same materials, the harder the filament, the faster the wear. Wear rate increases as the temperature is raised, but reduces as humidity is increased. However, under atmospheric environment, humidity increases with rising temperature, thus effects caused by two changes can cancel out. The author points out that, by a comparing the processing efficiency of the two equipment types, the horizontal wheel brush was found best suited for the cutting-edge passivating treatment of carbide turning inserts, while the vertical disc brush was better for the surface polishing treatment of carbide inserts. 

The synthesis of borides of refractory and rare earth metals by reduction-distillation, A. Mukherjee, Bhabha Atomic Research Centre, India 

This paper reports research on the synthesis of borides of group IV, V and VI refractory metals and of the rare earths lanthanum, neodymium, europium and yttrium starting from their compound intermediates, by a novel reduction/distillation process. The process, carried out at temperatures in the range 1000-1500°C under dynamic argon flow, comprises reduction and in-situ separation of the reduction products by distillation. The thermodynamic basis of the process and experimental results on practical application to the synthesis of borides of selected metals are described in the paper. Both oxides and halides have been used as the starting materials for synthesis and boron and aluminium as reducing agents. 

Magnetic characteristics of nanocarbon of different modifications, G. Bogatyreva, ISM - V.Bakul Institute of Superhard Materials, National Academy of Sciences of Ukraine, Ukraine

During recent years many promising studies have used nanocarbon in medicine as a platform for targeted delivery of drugs in magnetic resonance tomography and magnetically controlled local hyperthermia, as well as in electronics to create sensor devices. In this context it was important to have data of magnetic properties of nanocarbon.

The work focused on the magnetic properties of nanocarbon materials: carbon nanotubes produced by methane pyrolysis and nanodiamonds produced by detonation of explosives ("dry" and "wet" methods of synthesis), with follow-up special chemical treatment. 

The studies were carried out by magnetometric analysis with oscillating magnetometer 7404 VSM (Lake Sore Cryotronics, Inc., USA) in magnetic fields up to 13 000 Oe. Device sensitivity was 10-7 emu. Measurement range of magnetic moment was 10-7-103 emu. It made possible the measurement of magnetic moments of samples weighing only a few milligrams. Weight of the samples was measured using electronic microbalance AB135-S/FACT with autocompensation (METTLER TOLEDO, Switzerland). Sensitivity of the balance was 10-5 grams. 

Results showed that coercitivity of nanocarbon samples varied from 26 to 160 Oe and saturation magnetic moment from 0.08 to 0.28 emu/g. Nanodiamonds and carbon nanotubes are ferromagnetic materials whose magnetic properties can be controlled by special chemical treatment. 

New analytical computer-based methods of diagnostics of characteristics of powders of superhard materials, G. Bogatyreva, ISM - V.Bakul Institute of Superhard Materials, National Academy of Sciences of Ukraine, Ukraine 

(Following is the original wording of the abstract, which proved too difficult for this writer to summarise. Those interested are advised to contact the author. – KJAB)

New analytical computer-based methods of diagnostics of grain number in 1 carat of powder of superhard material (SHM), number and average value of edge angles of cutter bits of the grains have been developed. 

The first method is based on advanced extrapolation-affine 3D simulation of spatial shape of grain with application of mathematical apparatus of affine space transformation. Special computational algorithms of engineering of extrapolation-affine 3D simulation of spatial shape of grain ensure getting final result with miscalculation not exceeding 7%. 

Analytic definition of number (n) and average value of edge angles of cutter bits (F) of grains of SHM powder is carried out by solution of combined transcendental equations. Initial data for solution of this problem are shape factor (f) and roughness (R) of grains of abrasive powder. Shape factor describes degree of roundness of grain projection, it expresses by formula: ratio of square of actual perimeter of grain projection to area of grain projection multiplied by 4pi. 

Roughness expresses through formula: ratio of actual perimeter of grain projection to convex perimeter. Both characteristics are diagnosed by modern devices of automated diagnostics of morphometric characteristics of abrasive powders. Experimental check of suggested method has been shown close coincidence of calculation and experimental data. 

Developed new methods are provided with special software support; it simplifies and intensifies process of generating of high valid data of values of diagnosed characteristics. 

Influence of process parameters on microstructure and density of an ultrafine WC-6Co hardmetal produced by spark plasma sintering (SPS), M. Zivcec, IWM RWTH Aachen, Germany 

In spark plasma sintering (SPS) process parameters like pressure, heating rate, sintering temperature and holding time must be set to optimise material properties. The achievement of dense hard metals with homogeneous distribution of the Co phase and low WC grain growth during sintering is the target for an optimal combination of parameters. 

WC/6Co samples were consolidated by SPS using various sintering values. A systematic study of these parameters and their influence on material properties is reported. 

Starting with a combination that gave 99.8 % of theoretical density, the maximum sintering temperature was reduced stepwise. In the following studies the heating rate and holding time were varied independently for each holding temperature. The influence of each factor was estimated by measuring the change in density and analysing microstructural images. Additionally, the influence of pressure was determined at one sintering temperature. The significance of the process parameters to material densification is discussed. 

Use of mechanically induced self-sustaining reactions in the design of cermets based on complex transition metal carbonitrides, E. Chicardi, Instituto de Ciencia de Materiales de Sevilla (US-CSIC), Spain

Cermets based on titanium carbonitride are sometimes used as cutting tools instead of WC-base cemented carbides due to their better mechanical behaviour and chemical stability at high temperature. The ceramic components are usually obtained by carbothermal reduction from metal transition oxides. After liquid phase sintering, ceramic grains show the characteristic core-rim microstructure. 

It is reported that the use of complex transition metal carbonitrides as raw materials improves control of the ceramic phase composition and microstructure, and of the final properties. It would also be possible to obtain cermets without the core-rim microstructure, avoiding the interface between core and rim that generates residual stresses and crack propagation. 

The paper proposes the mechanochemical MSR (mechanically induced self-sustaining reaction) method to synthesise directly and simply complex transition metal carbonitrides starting from elemental powder blends using N2 as reactive gas. This raw ceramic material had been applied to develop cermets (TixTa1-xCyN1-y/Co) with and without the core-rim microstructure. By varying the nature and composition of starting elements, the microstructure and properties were varied. 

Thermal compression treatment of WC-Co and TiC-VC-NbC-WC-Ni-Cr hard alloys, H. Kramar, Ternopil Ivan Pul'uj National Technical University, Ukraine

Hard alloys on the basis of tungsten and titanium carbides are still the most effective such materials. Their properties are claimed to be improved by using new technological approaches - thermal compression treatment (?CT) in particular. The aim of the work was to research ?CT influence on microstructure and mechanical properties of WC/10Co and TiC/5VC/5NbC/5WC/18(Ni/Cr) hard alloys. The alloys sintered by standard technology were subject to TCT at 1370º?, argon pressure 3.0 ?Pa for 75 minutes. Decrease in porosity, increase of medium size carbide grains by 30% and appearance of separate recrystallized grains of 12-15μm sizes were noticed in ultrafine WC/Co alloy after TCT. Transverse rupture strength increased by 20% and fracture toughness decreased by 6.5% compared with untreated alloys. Considerable increase of carbide grains in TiC based alloys was not available. Unlike the WC based alloys, fracture toughness increased by 11.5%, explained by the change of size of the carbide rim and redistribution of carbides and binder metals. 

Influence of milling conditions in processing of submicron to near-nano WC/Co powders, B. Caspers, H.C. Starck GmbH, Germany 

This presentation follows the influence of different milling media on particle size distribution after milling. Pressing behaviour, shrinkage and coercive force of the sintered hardmetals are also analysed. WC powders from near-nanosize of ~100nm up to submicron size (0.8µm FSSS) were investigated in an alloy consisting of 90wt% WC (including 1 wt% Cr3C2) and 10wt% Co. 

The results showed that pressing behaviour strongly depended on the grain size of the WC powder and on the milling media. While for submicron WC powders the influence of the milling media was limited for near-nanosized powders, the difference in shrinkage for different milling media was significant. As a consequence. the right choice of milling media is gaining importance when processing near-nanosized WC powders. 

Hard alloys production during vacuum sintering of nanometric tungsten carbide-cobalt composition, Y. Blagoveshchenskiy, IMET RAS, Russia 

The process of vacuum sintering of tungsten carbide/cobalt composites with or without grain-growth inhibitors was investigated. Average grain size of pure tungsten carbide was 40-60nm. The influence of process parameters such as temperature and time on the structure and porosity of hardmetal was studied. The accurate regulation of carbon content in tungsten carbide made it possible to avoid eta-phase double carbide as well as abnormal grain growth, even in solid-phase sintering. The quantity of submicron pores formed during sintering of such powders was sharply decreased by holding in vacuo at appropriate temperature. The hard alloys had 25-30% higher Vickers hardness and fracture strength. 

Compaction process of pure tungsten carbide by spark plasma sintering, V. Chuvildeev, NIFTI UNN, Russia

This work investigated the sintering of nanometric tungsten carbide by spark plasma sintering, targeting high mechanical properties. 

Spark plasma sintering (SPS) was carried out in Syntex Inc Model 625 equipment. Tungsten carbide nanopowders with average grain size 50-60nm were sintered in the temperature range 1400-1950°C without time hold. Heating rate was 25 to 2400K/min. 

It was shown that near-theoretical density of pure tungsten carbide could be achieved at 1800°C sintering temperature and 500K/min heating rate. The average grain size of samples obtained at 1400°-1800°C was 90 to 150nm. The samples obtained at high heating rate had high hardness and a better combination of such properties as hardness and viscosity. The hardness of WC was HV 3180 and fracture strength was ?1?=5,2 with high density, sintered at 1800°C, 2400K/min. 

Short-term recycling technique for tungsten carbides scraps, X. Liu, Beijing University of Technology, China 

A short-term recycling technique for tungsten carbide scraps is proposed, combining the oxidation and reduction-carbonisation procedures. The effect of the oxidisation process on phase constitution and chemical composition in the reaction products of metal oxides was studied. Parameters for the reduction-carbonisation process were optimised for the synthesis of recycled WC/Co composite powder. The recycled WC/Co powder was used to press and sinter components, and the microstructures and properties of the resultant WC/Co were characterised. The results showed that WC/Co from recycled powders had “pure phase constitution,” homogeneous microstructure and good mechanical properties. In addition to the high quality of the recycled WC-Co powder products, the recycling technique had advantages of low energy cost, environmental friendliness and high potential for industrialisation. 

Hydrogen reduction of tungsten oxides: alkali additions and their interaction on the metal nucleation process, T. Zimmerl, Wolfram Bergbau und Hütten AG, Austria

It was known that the presence of alkali compounds influenced W grain growth during the reduction of tungsten oxide by hydrogen. Literature existed but the mechanism and mode of interaction remained unclear. To improve understanding of the reduction process, lithium, sodium and potassium compounds were added to a highly pure tungsten oxide prior to reduction, and interrupted reduction experiments were carried out at 750°C. The resulting powders were investigated by metallographic and chemical means, X-ray diffraction, SEM and EDX analysis. 

The experiments showed that intermediately formed tungsten bronzes played an important role in the early stages of reduction. In addition it was demonstrated that the kinetics of the reduction sequence differed significantly between doped and undoped tungsten oxide powders, as demonstrated by on-line measurement of reaction water in the hydrogen process gas. 

The results should aid understanding of how the presence of alkali compounds affects the reduction of tungsten oxides. 

Incorporation of titanium, tantalum, and vanadium into the hexagonal WC lattice, A. Pasquazzi, Vienna University of Technology, Austria

Based on the establishment’s previous work on the formation of hexagonal (W,Cr)C mixed crystal carbide, the formation of (W,Me)C (Me=Ti, Ta, V, V+Cr) solid solutions was studied in the temperature range 1450-1950°C. A two-step carburisation procedure was used for their preparation: In a first step, (W,Me)2C was formed, which in a second carburisation step was transformed into the (W,Me)C carbide. 

Experimental and theoretical study of the grain growth process of wc-co nano composites, A. Kumar, Sri Guru Granth Sahib World University, India

This poster describes both experimental and theoretical aspects of grain growth in different tungsten carbide/cobalt (WC/Co) nanocomposites sintered by liquid-phase sintering (LPS). The composites were prepared using tungsten carbide nanoparticles synthesised by solvo-thermal process and commercially available cobalt nanoparticles as binder metal for these particles. Sintering parameters; temperature, time and binder composition were varied to study the grain-growth process. In these composites no grain-growth inhibitors were used. Sintered composites were analysed using scanning electron microscope (SEM) and x-ray diffraction (XRD). Microstructural examination showed that the grain growth occurred through coalescence. XRD analysis showed that no η phase or other unwanted phases were present in any of the sintered composites. A theoretical model was developed for analysing the microstructure of such composites.