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Salt Lake City: A playground for hardmetal research
technical trends
Salt Lake City: A playground for hardmetal research Academic research has often been suppressed due to industrial competition and secrecy. Students of the University’s department of metallurgical engineering in Salt Lake City invited Ken Brookes into their labs to show him what goes on in one of the world’s largest centres for academic hardmetal research…
I
n the three-quarters of a century since the first commercial hardmetals were introduced, academic research has progressed through a number of phases. At first it hardly existed, due to industrial secrecy, much of which has been preserved to the present day. In the second there was much reinvention of the wheel, since advanced technology developed by companies was kept firmly under wraps, often unpatented, and low-budget university departments could hope for no more than to catch up. In my own case, for example, hard-won and valuable know-how from the 1940s and 50s was still being ‘discovered’ or ‘invented’ elsewhere in the 1960s, 70s and even later. In the third phase, much more has been published, but certain technologies now require investments worth millions of dollars. Nevertheless, there are aspects where ingenuity can be as valuable as financial outlay, providing substantial opportunities for academic advancement. Sponsored collaboration involving industry, academia and governments is increasingly popular. Of the world’s major centres of academic hardmetal research, one of very few in the Americas is Utah University’s Department of Metallurgical Engineering in Salt Lake City. Led by Professor Zhigang Zak Fang, the students – both graduate and post-graduate – undertake significant investigations that form the hardmetal ‘backbone’ of important USbased technical conferences. At least
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five such contributions were included in MPIF’s recent Denver conference**. These two contributions refer to the Department’s innovative UPRC process, under development for ultrafine hardmetal compacts and the near-net-shape production of sintered tungsten components. Because of its potential for the large-scale production of kinetic penetrators, the latter is clearly of interest to the US Army, whose Army Research Laboratory is a key sponsor of this research.
Professor Zhigang Zak Fang’s research group at the University of Utah focuses on three topic areas, featuring sintered carbides, refractory metals and polycrystalline diamond (PCD): 1. Sintering and grain growth of ultrafine sintered tungsten carbide and metallic tungsten; 2. Functionally designed composites of WC/Co and PCD including WC/Co with graded cobalt compositions and microstructures;
Figure 1. Professor Fang demonstrates the rotary evaporator used to dry and granulate WC/Co powder after wet milling.
0026-0657/08 ©2008 Elsevier Ltd. All rights reserved.
3. Studies of the mechanical behaviour of WC/Co materials using Hertzian indentation techniques. ‘Nano’ WC research tackles the issue of rapid dynamic grain growth during sintering, which takes place during heating up and almost always causes the material to lose its ‘nanocrystalline’ characteristics (‘nanocrystalline’ defined as having sintered grain sizes greater than 100nm), even before the start of isothermal sintering. Current research is aimed at understanding the mechanisms and kinetics of this dynamic grain growth and developing techniques for its control. The ultimate goal is to manufacture WC/Co materials with true nano-sized grains (significantly less than 100nm) so that the ultimate potential of nanocrystalline WC/ Co can be explored and exploited. The investigation of functionally designed composites of WC/Co and PCD deals with composites of hard and superhard materials with custom-designed structures to improve the fracture resistance of the material without sacrificing wear resistance, or vice versa. Double cemented tungsten carbide is an example. Others have been looking at the fundamental principles governing liquid-phase migration during the sintering of WC/Co. By controlling initial gradients of grain size, cobalt and carbon content in green compacts, fully dense materials with
graded Co content can be manufactured by standard liquid-phase sintering procedures. In the application of Hertzian indentation techniques, results have shown that it is not only a useful tool for evaluating brittleness index – an opposite indicator of fracture toughness of materials – but also for studying the failure processes of WC/Co from quasi-plastic deformation to the formation of micro- and macrocracks. The Department’s diverse research projects are funded by the US Department of Energy, US Army Laboratory and several industrial companies. On a recent visit to Salt Lake City, Professor Fang gave me a quick tour of the advanced equipment installed in his department, much of it employed in research described in this and previous editions of Metal Powder Report. Firstly I was shown a customdesigned and manufactured planetary milling machine, capable of generating a centrifugal acceleration up to 100G, for extremely high energy powder milling. Compared to commercial planetary milling machines, it offers higher energy, higher efficiency and more flexibility by controlling critical speed through independently adjusted motor speeds for arm revolution and rotation of the milling canister. This machine is used to produce of nano-sized metal, ceramic and tungsten carbide powders.
Figure 2. Jane Guo presses green compacts on a 50-tonne hydraulic press.
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In Figure 1, Zak Fang demonstrates a rotary evaporator, used for drying and granulating hardmetal grade powders after wet milling. The diminutive Jane Guo, a PhD candidate, is in Figure 2 pressing green compacts on a simple 50tonne hydraulic press. A Fractometer II is used to measure the fracture toughness KIc of sintered hardmetals by the shortrod method. Operated by Qingzhong Lu, a scientist from China, this unit has been modified for PC control and operation. Data collection and processing is also accomplished by the software, eliminating the usual tedious manual operation. High temperature sintering is carried out in two R D Webb ‘Red Devil’ research vacuum sintering furnaces, with graphite heating elements and chamber. The furnaces operate at temperatures up to 2000°C and require no water cooling. The thermocouple can be adjusted on a closed furnace, and an opened furnace to expose the high-temperature zone. Finally I was shown the ultrahigh pressure rapid hot consolidation (UPRC) machine, designed and built inhouse by Professor Fang, his students and the University technologist, Robert Byrnes. This equipment was employed in the investigations described below. Samples are placed in the high-pressure cell (Figure 3) and can be heated at up to 300K per minute. At these high
Figure 3. Close-up view of upper punch and high-pressure die of the UPRC system.
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Figure 4. Schematic diagram of apparatus for ultrahigh pressure rapid hot consolidation process.
temperatures, pressure up to 1 GPa can be applied. The system has been used to consolidate ultrafine metal, ceramic and WC/Co powders.
High pressure carbides Presented by Xu Wang of Utah University, the subject of the first paper in this final excerpt from PowderMet2007 is well described by its title – Nanocrystalline cemented tungsten
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Figure 5. Morphology of as-milled WC/10Co powder.
carbide sintered by an ultra-high-pressure rapid hot consolidation process. The biggest problem with so-called nanocrystalline tungsten carbide powders is not their initial manufacture, but the preservation of ultrafine grain size after sintering. A single substantially oversize particle in the sintered compact can act like a stress-raising cavity in the microstructure. The effect on fracture toughness is especially deleterious. Unfortunately however, the usual
manufacturing process tends to generate significant quantities of extra-fine particles, which dissolve preferentially in the binder during sintering – even during solid-state heating above 1000°C – and precipitate out on the coarsest particles during cooling from sintering temperature. The most common method of slowing or preventing grain growth is to incorporate specific grain-growth inhibitors, such as vanadium or chromium carbides, in the grade powder. The basis of an alternative idea was that all phases of the process were time-dependent. By operating at ultrahigh pressures, the sintering cycle could be sped up so that all consolidation took place below the eutectic (liquids or liquidphase) temperature, or that grain growth had insufficient time to occur, even above the liquidus temperature. Cemented tungsten carbide is typically sintered in vacuo at approximately 1400°C. Techniques previously developed to control or diminish rapid grain growth included the addition of grain-growth inhibitors, rapid heating and pressureassisted sintering techniques such as hot pressing, HIPing and spark plasma sintering (SPS). Using these methods, the finest average grain size achievable was quoted in the range 100-300nm – distribution within and outside that range was not given. Nonetheless, rapid heating with ultrahigh pressure offered one of the most promising approaches for limiting grain growth while achieving full densification. Rapid heating avoided the temperature range for rapid grain coarsening resulting from the grain/particle coalescence controlled by surface diffusion, while ultrahigh pressure aided the densification process at relatively low temperature.
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Figure 6. Fracture surface of WC/10Co sintered by (a) 78 per cent power output (b) 81 per cent power output, under 1GPa pressure.
The new process, termed ultrahigh pressure rapid hot consolidation (UPRC), combining rapid heating (>300K/min) with pressures up to 1.0 GPa, produced dense nanocrystalline WC/Co with no grain-growth inhibitors, and minimal grain growth. Mechanical properties including Vickers hardness and Palmqvist surface fracture toughness were evaluated. The WC/10Co powder used in this investigation was manufactured by Nanodyne Inc. Grain size was thought to be about 50nm, according to the XRD line-broadening technique. The powder was milled in an attritor in heptane with two per cent wax for four hours, then dried and compacted at 200 MPa into cylinders. These compacts were dewaxed at 250°C and pre-sintered at 800°C for one hour in a vacuum furnace. The presintered samples were introduced into the UPRC system in Figure 4. Ultrahigh pressure was defined as pressures greater than 400MPa, typically the upper limit of commercial HIP (hot isostatic pressing). During the UPRC process, samples were heated by resistance heating, isostatic pressure being applied through a fluid medium in a hardmetal die. High resolution scanning electron microscopy (Philips XL30 ESEM) was used to examine microstructural changes in the samples, which had been polished and etched using a Murakami solution or broken for observation of the fracture surfaces. Grain sizes were measured from the fracture surface with Image Tool software, which converts the measured crosssectional area of a feature to the diameter of an equal area circle, termed the equivalent circle diameter (ECD). Densities and HV30 hardnesses were measured and Palmqvist fracture surface toughness calculated according to the
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conventional equation where KIc is the Palmqvist surface fracture toughness, H the hardness, P the applied load, and C the total length of cracks produced by the Vickers indentation. Morphology of the as-milled WC/10Co powder is shown in figure 5. The powder was largely agglomerated. Particle size was in the range 100-500nm but average grain size of the (unagglomerated) powder was said to be approximately 50nm. The relative density of WC/10Co samples as a function of percentage total powder output during UPRC. Powder level could be directly correlated with temperature. With applied pressure of 1.0 GPa, the sintering cycle was completed in 20 minutes. Because of the extremely high pressure and the UPRC assembly design, temperature measurement during UPRC was very difficult. The temperature was therefore estimated from the output power level. For 78 per cent powder output, temperature was estimated to be 1200°C. With further increases in output power, the temperature increased. When density increased with increase in sintering temperature, densification was complete at 78 per cent output power. However, it appeared that when a certain (though not accurately known) temperature was exceeded, density decreased slightly. Microstructures of WC/10Co samples sintered by UPRC at different output powers are shown in Figure 6. Average WC grain size was 95nm when sintered at 78 per cent power output (figure 6a) but, when power output was increased to 81 per cent, average grain size was 423nm (figure 6b). The rapid increase of grain size in the 78-81 per cent power output range implied a threshold temperature
range, above which grain growth took place much more rapidly. Hardness increased with increase of density. The highest hardness value from this study of 10Co samples was consolidated at 78 per cent output power, approximately 1200°C. The hardness of 1845HV was much larger than those of similar samples sintered by conventional liquid-phase sintering. Using measurements from an indentation on the sample sintered at 78 per cent output power, Palmqvist surface toughness was calculated as 9.8 MPa mm1/2. Though a number of important questions were left unanswered, the author concluded that UPRC was a promising technique for consolidation of nanosized WC/Co powders and production of bulk nanocrystalline WC/Co materials. The animated discussion following this presentation centred on the ill-defined but clearly critical temperature range in which massive grain growth took place (figure 6). Among the theories put forward was that it represented a solid-state transition, the transition between solid-state and liquid-phase sintering, or a complex but depressed eutectic temperature involving dopant elements (though the paper claimed none were present) as well as tungsten carbide and cobalt. Cemented carbides are by no means the only materials to benefit from techniques based on ultrafine powders. Because of their extraordinary properties, ‘nanocrystalline’ (that word again) materials create interest throughout the materials science community. Traditionally, processes that strengthened metals by impeding the movement of dislocations simultaneously decreased the ductility of the metal. However, reduction of grain size to less than 100 nm had been
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Figure 7. Comparison of common isostatic processes with UPRC.
Figure 8. Small-scale UPRC sample: a) dark areas are oxide inclusions highlighted by back scatter SEM; b) at x20,000 ~80nm grains can be seen.
shown not only to strengthen material by retarding dislocation movement but also to allow deformation of the metal by mechanisms not possible with coarsegrained materials, including grain boundary rotation and sliding. Thus so-called ‘nano’ materials had been shown not only to have significantly increased strength but ductility near to or exceeding that of coarse-grained material. The attainment of such ultrafine grain structures in refractory metals seemed to be beyond the scope of lab-scale experiments. With tungsten, standard production methods involved severe plastic deformation (SPD) and/or high-pressure
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torsion (HPT), giving improved strength without losing ductility. However, these processes were very limited in shapes and sizes, either an irregular billet from SPD or a disc from HPT. Material processed with SPD or HPT was machined to obtain a usable form, understandable because the object of either process was to yield a high-strength material which was therefore difficult to machine. A more powder metallurgical approach for bulk materials was thus very attractive, with its ability to yield near-net geometries. The PM method aimed at consolidating to full density whilst maintaining grain size. However, due to rapid
grain growth at liquid-phase temperatures, traditional sintering proved incapable of producing the targeted superfine material, regardless of starting grain size. In the paper under consideration, James D. Paramore of Utah University’s Department of Metallurgical Engineering offered a new approach, the Production of nanocrystalline tungsten using Ultrahigh Pressure Rapid Hot Consolidation (UPRC). The UPRC process has two unique abilities allowing it to produce ultrafine refractory materials such as tungsten. The first of these is rapid heat-up and cooldown times. By eliminating the long ramp times associated with traditional sintering, it reduced substantially the amount of time the material was at a temperature that encouraged excessive grain growth. Additionally, the rapid heating technique provided means for future studies into the effect of heating rate on grain growth. The second ability was the ultrahighpressure sintering environment. It was expected that, at very high pressures, both the sintering temperature and the time required for full densification could be significantly reduced. As a result, excessive grain growth might be eliminated. Figure 7 is a comparison of UPRC with other common isostatic sintering processes. UPRC was much more energy-efficient and rapid than HIPing or hot pressing, as well as a commercially viable highpressure alternative to high-temperature, high-pressure (HTHP) diamond presses. The author explained that the UPRC process worked by pressurising a cell composed of the sample to be sintered, a liquid medium to produce an isostatic environment, and a resistance heating element. The experimental press was capable of creating a sintering environment at about 1 GPa and over 1500°C. Maximum temperature had been obtained with heatup and cool-down times shorter than five minutes. Two powders were used to study the ability of UPRC both to consolidate tungsten and to do so in a manner resulting in nanoscale microstructures of 45 and 15nm grain size. All powders were supplied by Kennametal. The 15nm powder was milled from 500nm powder in a high-energy, dual-drive planetary mill at the University of Utah. This powder was milled in heptane for four hours and dried at 80°C. Grain size of the powders was
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Figure 9. Fracture surface of sample pressed in UPRC with 25mm diameter cell: average grain size appears to be < 100nm
estimated by X-ray diffraction (XRD) and the Stokes and Wilson formula for grainsize determination from peak broadening. Two die designs were tested for the UPRC process, a smaller version with an internal cell volume roughly 9.5mm in diameter and 12.7mm in height, and a larger with cell volume roughly 25mm in diameter and 76mm in height. Green parts for the small-scale UPRC were prepared by compacting the powder at 200MPa into cylinders, which were placed in niobium cans to prevent impregnation by the liquid medium. Green parts for the large-scale UPRC were prepared by compacting the powder into cylinders at 200MPa and coating with ceramic slurry to prevent impregnation by the liquid. With the setup described, sintering temperature was estimated by correlating temperature with powder density in the heating element. It was assumed, and apparently confirmed by results, that steady state would be reached within the cell in a very short time. As a check, the cell was filled successively with metal powders having a variety of melting temperatures. A series of tests were conducted at various powder densities and the powders examined afterwards to determine which melting temperatures had been exceeded. A relationship was thus formulated between steady-state temperature and powder density in the heating element. After consolidation, density measurements were made of each sample and their microstructures examined by highresolution scanning electron microscopy (HRSEM) and X-ray diffraction. Figure 8 shows two micrographs from a sample pressed in the UPRC with the
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were not directly proportional, shown 9.5mm diameter cell. At x20,000 magnifiby comparing the sample of figure 10 cation in figure 8(b), grain outlines could with that of figure 8. The sample in be seen. From this inspection it was deterfigure 8 had a specific gravity of 17.0, mined that average grain size was approxiwhereas that of the sample in figure 10 mately 75-100nm. Using X-ray diffraction was 15.6, only 81 per cent of theoretical and the Stokes Wilson formula, grain size (19.35). However, the sample in figure 10 was estimated to be about 80nm. Figure had grains an order of magnitude larger 9 is a micrograph of a fracture surface than those shown in figure 8. A study of a sample pressed using UPRC with a of measured densities and grain sizes of 25mm diameter cell. As can be seen from all samples pressed with UPRC showed this micrograph, the grains are also on the ‘nanoscale’ (<100nm).These results showed no trend between the two parameters. It was deduced that full densification of the UPRC process was capable of producsamples consolidated with UPRC was not ing ultrafine refractory material. Thus dependent on grain growth beyond the the process could prove a valuable tool in nanoscale. further studies of the relationship between Within the reported investigation heating rate, sintering temperature and sintering pressure during the consolidation period, no UPRC sample had a specific gravity greater than 17.6. However, of ultrafine powders into bulk ultrafine materials. Moreover, industrial promise was demonstrated by the production of ultrafine tungsten in a heated pressure cell similar in volume to commercial HTHP diamond presses. In addition, the UPRC process involved considerably reduced capital and operating costs when compared with commercial HTHP presses. Densification and Figure 10. UPRC sample with 15.6 g/cc measured density. However, the micrograph seemed to indicate full consolidation. grain size of samples
Figure 11. Comparison of samples pressed in UPRC at varying powder densities, an increase in powder density by 33 per cent resulted in an increase of average grain size from 200nm to 5µm.
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Figure 12. Graph showing the dependency of grain size on powder density (sintering temperature) during UPRC.
photomicrographs of samples consolidated by UPRC seemed to show much higher relative densities. According to the author, the illustration was indicative of a sample with >98 per cent theoretical density. The disparity between theoretical and measured density of samples was thought to be caused by a large volume fraction of oxide inclusions. Colouration of the micrographs (Figures 8 and 10) seemed to indicate a large volume fraction of a second, possible oxide, phase. Using energy dispersive Xray analysis (EDAX), it was determined that the darker phase contained a large amount of oxygen. X-ray spectrograms of UPRC samples contained WO2 peak patterns, the diffraction pattern showing a large volume fraction of WO2. This explained the low specific gravity of 15.6. The authors believed that while the oxide inclusions caused the density of the material to be lower than its theoretical value, this problem could be circumvented. The starting powder used in this study contained a large amount of oxide contamination resulting from insufficient care during milling and subsequent excessive exposure to the atmosphere. This not only accounted for the presence of oxide inclusions in the consolidated samples, but also explained the variation in density of two samples with similar microstructures. The metal powder would be unequally contaminated due to diffusion of oxygen through the powder. When milled to ultrafine powder, it became exceedingly vulnerable to oxygen contamination.
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Powders milled in the university laboratory to about 10nm were pyrophoric. It was intended that future tests with UPRC would take more note of oxygen contamination of the starting powder to prevent oxide inclusions in the sintered material. This problem is perhaps typical of what can happen when a university department seeks to develop a commercial process without sufficient reference to existing industrial practice. It is commonplace in the tungsten and hardmetals industries, not only that tungsten powder oxidises readily – and more rapidly as specific surface area increases with finer grain sizes – but that reduction of tungsten oxides is a simple, standard and wellunderstood part of the regular production process. Thus hydrogen reduction at elevated temperature of the oxidised tungsten powder should (in my opinion) have preceded the UPRC phase of the process, and the UPRC itself should have involved a protective or reducing environment to avoid reoxidation. However, oxidised surfaces on the particles in a pressed compact could be expected to have an inhibiting effect on possible grain growth, which might well disappear if the grains were to have clean, reduced surfaces (KJAB). Figure 11 illustrates the sensitivity of the grain size to an increase in powder density (sintering temperature). UPRC-W08 had a grain size of roughly 200-300nm. When the powder density was increased by 33 per cent, the grain size grew far beyond the nanoscale or ultrafine region
to approximately 5µm (figure 12, UPRCW-06). Figure12 shows grain size results from three samples sintered using UPRC with three different powder densities. As would be expected, an increase in powder density (sintering temperature) is followed by an increase in product grain size. There is, however – though not mentioned by the presenter – a clearly visible substructure within the large grains of the same order as the original grain size. It would be interesting to know the extent – if any – by which properties were governed by the substructure dimensions in addition to the recrystallised grain size. According to the authors, the UPRC process itself did not facilitate oxygen contamination during sintering. The reason advanced was that the pressure and heating cell was thoroughly packed before entering the press to ensure minimal presence of atmospheric oxygen and was sealed from the atmosphere during sintering. This, however, ignores the fact that the spaces between the packed particles would be filled with air, containing around 80 per cent oxygen, most of which would be likely to react with the tungsten during processing. Probably only a marginal factor in this investigation, it might gain in importance if and when the preexisting oxidation were reduced. In spite of the oxidation problem, the presenter considered that the UPRC process had been proven as a feasible means for the production of ultrafine sintered tungsten compacts. The process was therefore currently being employed for more comprehensive studies on the relationship between sintering conditions – time, temperature and pressure – and into the densification and grain-growth kinetics of ultra-fine tungsten.
** “Advances in Powder Metallurgy & Particulate Materials – 2007”, the Proceedings of the 2007 International Conference on Powder Metallurgy & Particulate Materials (PowderMet2007) in Denver, USA, is published on CD-ROM in searchable Adobe Acrobat format by the Metal Powder Industries Federation, 105 College Road East, Princeton, NJ 08540-6692, USA. Phone: +1 609452-7700, email: [email protected], web: www.mpif.org.
metal-powder.net
Salt Lake City: A playground for hardmetal research Academic research has often been suppressed due to industrial competition and secrecy. Students of the University’s department of metallurgical engineering in Salt Lake City invited Ken Brookes into their labs to show him what goes on in one of the world’s largest centres for academic hardmetal research…
I
n the three-quarters of a century since the first commercial hardmetals were introduced, academic research has progressed through a number of phases. At first it hardly existed, due to industrial secrecy, much of which has been preserved to the present day. In the second there was much reinvention of the wheel, since advanced technology developed by companies was kept firmly under wraps, often unpatented, and low-budget university departments could hope for no more than to catch up. In my own case, for example, hard-won and valuable know-how from the 1940s and 50s was still being ‘discovered’ or ‘invented’ elsewhere in the 1960s, 70s and even later. In the third phase, much more has been published, but certain technologies now require investments worth millions of dollars. Nevertheless, there are aspects where ingenuity can be as valuable as financial outlay, providing substantial opportunities for academic advancement. Sponsored collaboration involving industry, academia and governments is increasingly popular. Of the world’s major centres of academic hardmetal research, one of very few in the Americas is Utah University’s Department of Metallurgical Engineering in Salt Lake City. Led by Professor Zhigang Zak Fang, the students – both graduate and post-graduate – undertake significant investigations that form the hardmetal ‘backbone’ of important USbased technical conferences. At least
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five such contributions were included in MPIF’s recent Denver conference**. These two contributions refer to the Department’s innovative UPRC process, under development for ultrafine hardmetal compacts and the near-net-shape production of sintered tungsten components. Because of its potential for the large-scale production of kinetic penetrators, the latter is clearly of interest to the US Army, whose Army Research Laboratory is a key sponsor of this research.
Professor Zhigang Zak Fang’s research group at the University of Utah focuses on three topic areas, featuring sintered carbides, refractory metals and polycrystalline diamond (PCD): 1. Sintering and grain growth of ultrafine sintered tungsten carbide and metallic tungsten; 2. Functionally designed composites of WC/Co and PCD including WC/Co with graded cobalt compositions and microstructures;
Figure 1. Professor Fang demonstrates the rotary evaporator used to dry and granulate WC/Co powder after wet milling.
0026-0657/08 ©2008 Elsevier Ltd. All rights reserved.
3. Studies of the mechanical behaviour of WC/Co materials using Hertzian indentation techniques. ‘Nano’ WC research tackles the issue of rapid dynamic grain growth during sintering, which takes place during heating up and almost always causes the material to lose its ‘nanocrystalline’ characteristics (‘nanocrystalline’ defined as having sintered grain sizes greater than 100nm), even before the start of isothermal sintering. Current research is aimed at understanding the mechanisms and kinetics of this dynamic grain growth and developing techniques for its control. The ultimate goal is to manufacture WC/Co materials with true nano-sized grains (significantly less than 100nm) so that the ultimate potential of nanocrystalline WC/ Co can be explored and exploited. The investigation of functionally designed composites of WC/Co and PCD deals with composites of hard and superhard materials with custom-designed structures to improve the fracture resistance of the material without sacrificing wear resistance, or vice versa. Double cemented tungsten carbide is an example. Others have been looking at the fundamental principles governing liquid-phase migration during the sintering of WC/Co. By controlling initial gradients of grain size, cobalt and carbon content in green compacts, fully dense materials with
graded Co content can be manufactured by standard liquid-phase sintering procedures. In the application of Hertzian indentation techniques, results have shown that it is not only a useful tool for evaluating brittleness index – an opposite indicator of fracture toughness of materials – but also for studying the failure processes of WC/Co from quasi-plastic deformation to the formation of micro- and macrocracks. The Department’s diverse research projects are funded by the US Department of Energy, US Army Laboratory and several industrial companies. On a recent visit to Salt Lake City, Professor Fang gave me a quick tour of the advanced equipment installed in his department, much of it employed in research described in this and previous editions of Metal Powder Report. Firstly I was shown a customdesigned and manufactured planetary milling machine, capable of generating a centrifugal acceleration up to 100G, for extremely high energy powder milling. Compared to commercial planetary milling machines, it offers higher energy, higher efficiency and more flexibility by controlling critical speed through independently adjusted motor speeds for arm revolution and rotation of the milling canister. This machine is used to produce of nano-sized metal, ceramic and tungsten carbide powders.
Figure 2. Jane Guo presses green compacts on a 50-tonne hydraulic press.
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In Figure 1, Zak Fang demonstrates a rotary evaporator, used for drying and granulating hardmetal grade powders after wet milling. The diminutive Jane Guo, a PhD candidate, is in Figure 2 pressing green compacts on a simple 50tonne hydraulic press. A Fractometer II is used to measure the fracture toughness KIc of sintered hardmetals by the shortrod method. Operated by Qingzhong Lu, a scientist from China, this unit has been modified for PC control and operation. Data collection and processing is also accomplished by the software, eliminating the usual tedious manual operation. High temperature sintering is carried out in two R D Webb ‘Red Devil’ research vacuum sintering furnaces, with graphite heating elements and chamber. The furnaces operate at temperatures up to 2000°C and require no water cooling. The thermocouple can be adjusted on a closed furnace, and an opened furnace to expose the high-temperature zone. Finally I was shown the ultrahigh pressure rapid hot consolidation (UPRC) machine, designed and built inhouse by Professor Fang, his students and the University technologist, Robert Byrnes. This equipment was employed in the investigations described below. Samples are placed in the high-pressure cell (Figure 3) and can be heated at up to 300K per minute. At these high
Figure 3. Close-up view of upper punch and high-pressure die of the UPRC system.
January 2008 MPR
27
Figure 4. Schematic diagram of apparatus for ultrahigh pressure rapid hot consolidation process.
temperatures, pressure up to 1 GPa can be applied. The system has been used to consolidate ultrafine metal, ceramic and WC/Co powders.
High pressure carbides Presented by Xu Wang of Utah University, the subject of the first paper in this final excerpt from PowderMet2007 is well described by its title – Nanocrystalline cemented tungsten
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Figure 5. Morphology of as-milled WC/10Co powder.
carbide sintered by an ultra-high-pressure rapid hot consolidation process. The biggest problem with so-called nanocrystalline tungsten carbide powders is not their initial manufacture, but the preservation of ultrafine grain size after sintering. A single substantially oversize particle in the sintered compact can act like a stress-raising cavity in the microstructure. The effect on fracture toughness is especially deleterious. Unfortunately however, the usual
manufacturing process tends to generate significant quantities of extra-fine particles, which dissolve preferentially in the binder during sintering – even during solid-state heating above 1000°C – and precipitate out on the coarsest particles during cooling from sintering temperature. The most common method of slowing or preventing grain growth is to incorporate specific grain-growth inhibitors, such as vanadium or chromium carbides, in the grade powder. The basis of an alternative idea was that all phases of the process were time-dependent. By operating at ultrahigh pressures, the sintering cycle could be sped up so that all consolidation took place below the eutectic (liquids or liquidphase) temperature, or that grain growth had insufficient time to occur, even above the liquidus temperature. Cemented tungsten carbide is typically sintered in vacuo at approximately 1400°C. Techniques previously developed to control or diminish rapid grain growth included the addition of grain-growth inhibitors, rapid heating and pressureassisted sintering techniques such as hot pressing, HIPing and spark plasma sintering (SPS). Using these methods, the finest average grain size achievable was quoted in the range 100-300nm – distribution within and outside that range was not given. Nonetheless, rapid heating with ultrahigh pressure offered one of the most promising approaches for limiting grain growth while achieving full densification. Rapid heating avoided the temperature range for rapid grain coarsening resulting from the grain/particle coalescence controlled by surface diffusion, while ultrahigh pressure aided the densification process at relatively low temperature.
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Figure 6. Fracture surface of WC/10Co sintered by (a) 78 per cent power output (b) 81 per cent power output, under 1GPa pressure.
The new process, termed ultrahigh pressure rapid hot consolidation (UPRC), combining rapid heating (>300K/min) with pressures up to 1.0 GPa, produced dense nanocrystalline WC/Co with no grain-growth inhibitors, and minimal grain growth. Mechanical properties including Vickers hardness and Palmqvist surface fracture toughness were evaluated. The WC/10Co powder used in this investigation was manufactured by Nanodyne Inc. Grain size was thought to be about 50nm, according to the XRD line-broadening technique. The powder was milled in an attritor in heptane with two per cent wax for four hours, then dried and compacted at 200 MPa into cylinders. These compacts were dewaxed at 250°C and pre-sintered at 800°C for one hour in a vacuum furnace. The presintered samples were introduced into the UPRC system in Figure 4. Ultrahigh pressure was defined as pressures greater than 400MPa, typically the upper limit of commercial HIP (hot isostatic pressing). During the UPRC process, samples were heated by resistance heating, isostatic pressure being applied through a fluid medium in a hardmetal die. High resolution scanning electron microscopy (Philips XL30 ESEM) was used to examine microstructural changes in the samples, which had been polished and etched using a Murakami solution or broken for observation of the fracture surfaces. Grain sizes were measured from the fracture surface with Image Tool software, which converts the measured crosssectional area of a feature to the diameter of an equal area circle, termed the equivalent circle diameter (ECD). Densities and HV30 hardnesses were measured and Palmqvist fracture surface toughness calculated according to the
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conventional equation where KIc is the Palmqvist surface fracture toughness, H the hardness, P the applied load, and C the total length of cracks produced by the Vickers indentation. Morphology of the as-milled WC/10Co powder is shown in figure 5. The powder was largely agglomerated. Particle size was in the range 100-500nm but average grain size of the (unagglomerated) powder was said to be approximately 50nm. The relative density of WC/10Co samples as a function of percentage total powder output during UPRC. Powder level could be directly correlated with temperature. With applied pressure of 1.0 GPa, the sintering cycle was completed in 20 minutes. Because of the extremely high pressure and the UPRC assembly design, temperature measurement during UPRC was very difficult. The temperature was therefore estimated from the output power level. For 78 per cent powder output, temperature was estimated to be 1200°C. With further increases in output power, the temperature increased. When density increased with increase in sintering temperature, densification was complete at 78 per cent output power. However, it appeared that when a certain (though not accurately known) temperature was exceeded, density decreased slightly. Microstructures of WC/10Co samples sintered by UPRC at different output powers are shown in Figure 6. Average WC grain size was 95nm when sintered at 78 per cent power output (figure 6a) but, when power output was increased to 81 per cent, average grain size was 423nm (figure 6b). The rapid increase of grain size in the 78-81 per cent power output range implied a threshold temperature
range, above which grain growth took place much more rapidly. Hardness increased with increase of density. The highest hardness value from this study of 10Co samples was consolidated at 78 per cent output power, approximately 1200°C. The hardness of 1845HV was much larger than those of similar samples sintered by conventional liquid-phase sintering. Using measurements from an indentation on the sample sintered at 78 per cent output power, Palmqvist surface toughness was calculated as 9.8 MPa mm1/2. Though a number of important questions were left unanswered, the author concluded that UPRC was a promising technique for consolidation of nanosized WC/Co powders and production of bulk nanocrystalline WC/Co materials. The animated discussion following this presentation centred on the ill-defined but clearly critical temperature range in which massive grain growth took place (figure 6). Among the theories put forward was that it represented a solid-state transition, the transition between solid-state and liquid-phase sintering, or a complex but depressed eutectic temperature involving dopant elements (though the paper claimed none were present) as well as tungsten carbide and cobalt. Cemented carbides are by no means the only materials to benefit from techniques based on ultrafine powders. Because of their extraordinary properties, ‘nanocrystalline’ (that word again) materials create interest throughout the materials science community. Traditionally, processes that strengthened metals by impeding the movement of dislocations simultaneously decreased the ductility of the metal. However, reduction of grain size to less than 100 nm had been
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Figure 7. Comparison of common isostatic processes with UPRC.
Figure 8. Small-scale UPRC sample: a) dark areas are oxide inclusions highlighted by back scatter SEM; b) at x20,000 ~80nm grains can be seen.
shown not only to strengthen material by retarding dislocation movement but also to allow deformation of the metal by mechanisms not possible with coarsegrained materials, including grain boundary rotation and sliding. Thus so-called ‘nano’ materials had been shown not only to have significantly increased strength but ductility near to or exceeding that of coarse-grained material. The attainment of such ultrafine grain structures in refractory metals seemed to be beyond the scope of lab-scale experiments. With tungsten, standard production methods involved severe plastic deformation (SPD) and/or high-pressure
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torsion (HPT), giving improved strength without losing ductility. However, these processes were very limited in shapes and sizes, either an irregular billet from SPD or a disc from HPT. Material processed with SPD or HPT was machined to obtain a usable form, understandable because the object of either process was to yield a high-strength material which was therefore difficult to machine. A more powder metallurgical approach for bulk materials was thus very attractive, with its ability to yield near-net geometries. The PM method aimed at consolidating to full density whilst maintaining grain size. However, due to rapid
grain growth at liquid-phase temperatures, traditional sintering proved incapable of producing the targeted superfine material, regardless of starting grain size. In the paper under consideration, James D. Paramore of Utah University’s Department of Metallurgical Engineering offered a new approach, the Production of nanocrystalline tungsten using Ultrahigh Pressure Rapid Hot Consolidation (UPRC). The UPRC process has two unique abilities allowing it to produce ultrafine refractory materials such as tungsten. The first of these is rapid heat-up and cooldown times. By eliminating the long ramp times associated with traditional sintering, it reduced substantially the amount of time the material was at a temperature that encouraged excessive grain growth. Additionally, the rapid heating technique provided means for future studies into the effect of heating rate on grain growth. The second ability was the ultrahighpressure sintering environment. It was expected that, at very high pressures, both the sintering temperature and the time required for full densification could be significantly reduced. As a result, excessive grain growth might be eliminated. Figure 7 is a comparison of UPRC with other common isostatic sintering processes. UPRC was much more energy-efficient and rapid than HIPing or hot pressing, as well as a commercially viable highpressure alternative to high-temperature, high-pressure (HTHP) diamond presses. The author explained that the UPRC process worked by pressurising a cell composed of the sample to be sintered, a liquid medium to produce an isostatic environment, and a resistance heating element. The experimental press was capable of creating a sintering environment at about 1 GPa and over 1500°C. Maximum temperature had been obtained with heatup and cool-down times shorter than five minutes. Two powders were used to study the ability of UPRC both to consolidate tungsten and to do so in a manner resulting in nanoscale microstructures of 45 and 15nm grain size. All powders were supplied by Kennametal. The 15nm powder was milled from 500nm powder in a high-energy, dual-drive planetary mill at the University of Utah. This powder was milled in heptane for four hours and dried at 80°C. Grain size of the powders was
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Figure 9. Fracture surface of sample pressed in UPRC with 25mm diameter cell: average grain size appears to be < 100nm
estimated by X-ray diffraction (XRD) and the Stokes and Wilson formula for grainsize determination from peak broadening. Two die designs were tested for the UPRC process, a smaller version with an internal cell volume roughly 9.5mm in diameter and 12.7mm in height, and a larger with cell volume roughly 25mm in diameter and 76mm in height. Green parts for the small-scale UPRC were prepared by compacting the powder at 200MPa into cylinders, which were placed in niobium cans to prevent impregnation by the liquid medium. Green parts for the large-scale UPRC were prepared by compacting the powder into cylinders at 200MPa and coating with ceramic slurry to prevent impregnation by the liquid. With the setup described, sintering temperature was estimated by correlating temperature with powder density in the heating element. It was assumed, and apparently confirmed by results, that steady state would be reached within the cell in a very short time. As a check, the cell was filled successively with metal powders having a variety of melting temperatures. A series of tests were conducted at various powder densities and the powders examined afterwards to determine which melting temperatures had been exceeded. A relationship was thus formulated between steady-state temperature and powder density in the heating element. After consolidation, density measurements were made of each sample and their microstructures examined by highresolution scanning electron microscopy (HRSEM) and X-ray diffraction. Figure 8 shows two micrographs from a sample pressed in the UPRC with the
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were not directly proportional, shown 9.5mm diameter cell. At x20,000 magnifiby comparing the sample of figure 10 cation in figure 8(b), grain outlines could with that of figure 8. The sample in be seen. From this inspection it was deterfigure 8 had a specific gravity of 17.0, mined that average grain size was approxiwhereas that of the sample in figure 10 mately 75-100nm. Using X-ray diffraction was 15.6, only 81 per cent of theoretical and the Stokes Wilson formula, grain size (19.35). However, the sample in figure 10 was estimated to be about 80nm. Figure had grains an order of magnitude larger 9 is a micrograph of a fracture surface than those shown in figure 8. A study of a sample pressed using UPRC with a of measured densities and grain sizes of 25mm diameter cell. As can be seen from all samples pressed with UPRC showed this micrograph, the grains are also on the ‘nanoscale’ (<100nm).These results showed no trend between the two parameters. It was deduced that full densification of the UPRC process was capable of producsamples consolidated with UPRC was not ing ultrafine refractory material. Thus dependent on grain growth beyond the the process could prove a valuable tool in nanoscale. further studies of the relationship between Within the reported investigation heating rate, sintering temperature and sintering pressure during the consolidation period, no UPRC sample had a specific gravity greater than 17.6. However, of ultrafine powders into bulk ultrafine materials. Moreover, industrial promise was demonstrated by the production of ultrafine tungsten in a heated pressure cell similar in volume to commercial HTHP diamond presses. In addition, the UPRC process involved considerably reduced capital and operating costs when compared with commercial HTHP presses. Densification and Figure 10. UPRC sample with 15.6 g/cc measured density. However, the micrograph seemed to indicate full consolidation. grain size of samples
Figure 11. Comparison of samples pressed in UPRC at varying powder densities, an increase in powder density by 33 per cent resulted in an increase of average grain size from 200nm to 5µm.
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Figure 12. Graph showing the dependency of grain size on powder density (sintering temperature) during UPRC.
photomicrographs of samples consolidated by UPRC seemed to show much higher relative densities. According to the author, the illustration was indicative of a sample with >98 per cent theoretical density. The disparity between theoretical and measured density of samples was thought to be caused by a large volume fraction of oxide inclusions. Colouration of the micrographs (Figures 8 and 10) seemed to indicate a large volume fraction of a second, possible oxide, phase. Using energy dispersive Xray analysis (EDAX), it was determined that the darker phase contained a large amount of oxygen. X-ray spectrograms of UPRC samples contained WO2 peak patterns, the diffraction pattern showing a large volume fraction of WO2. This explained the low specific gravity of 15.6. The authors believed that while the oxide inclusions caused the density of the material to be lower than its theoretical value, this problem could be circumvented. The starting powder used in this study contained a large amount of oxide contamination resulting from insufficient care during milling and subsequent excessive exposure to the atmosphere. This not only accounted for the presence of oxide inclusions in the consolidated samples, but also explained the variation in density of two samples with similar microstructures. The metal powder would be unequally contaminated due to diffusion of oxygen through the powder. When milled to ultrafine powder, it became exceedingly vulnerable to oxygen contamination.
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Powders milled in the university laboratory to about 10nm were pyrophoric. It was intended that future tests with UPRC would take more note of oxygen contamination of the starting powder to prevent oxide inclusions in the sintered material. This problem is perhaps typical of what can happen when a university department seeks to develop a commercial process without sufficient reference to existing industrial practice. It is commonplace in the tungsten and hardmetals industries, not only that tungsten powder oxidises readily – and more rapidly as specific surface area increases with finer grain sizes – but that reduction of tungsten oxides is a simple, standard and wellunderstood part of the regular production process. Thus hydrogen reduction at elevated temperature of the oxidised tungsten powder should (in my opinion) have preceded the UPRC phase of the process, and the UPRC itself should have involved a protective or reducing environment to avoid reoxidation. However, oxidised surfaces on the particles in a pressed compact could be expected to have an inhibiting effect on possible grain growth, which might well disappear if the grains were to have clean, reduced surfaces (KJAB). Figure 11 illustrates the sensitivity of the grain size to an increase in powder density (sintering temperature). UPRC-W08 had a grain size of roughly 200-300nm. When the powder density was increased by 33 per cent, the grain size grew far beyond the nanoscale or ultrafine region
to approximately 5µm (figure 12, UPRCW-06). Figure12 shows grain size results from three samples sintered using UPRC with three different powder densities. As would be expected, an increase in powder density (sintering temperature) is followed by an increase in product grain size. There is, however – though not mentioned by the presenter – a clearly visible substructure within the large grains of the same order as the original grain size. It would be interesting to know the extent – if any – by which properties were governed by the substructure dimensions in addition to the recrystallised grain size. According to the authors, the UPRC process itself did not facilitate oxygen contamination during sintering. The reason advanced was that the pressure and heating cell was thoroughly packed before entering the press to ensure minimal presence of atmospheric oxygen and was sealed from the atmosphere during sintering. This, however, ignores the fact that the spaces between the packed particles would be filled with air, containing around 80 per cent oxygen, most of which would be likely to react with the tungsten during processing. Probably only a marginal factor in this investigation, it might gain in importance if and when the preexisting oxidation were reduced. In spite of the oxidation problem, the presenter considered that the UPRC process had been proven as a feasible means for the production of ultrafine sintered tungsten compacts. The process was therefore currently being employed for more comprehensive studies on the relationship between sintering conditions – time, temperature and pressure – and into the densification and grain-growth kinetics of ultra-fine tungsten.
** “Advances in Powder Metallurgy & Particulate Materials – 2007”, the Proceedings of the 2007 International Conference on Powder Metallurgy & Particulate Materials (PowderMet2007) in Denver, USA, is published on CD-ROM in searchable Adobe Acrobat format by the Metal Powder Industries Federation, 105 College Road East, Princeton, NJ 08540-6692, USA. Phone: +1 609452-7700, email: [email protected], web: www.mpif.org.
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