IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 18 (2007) 185609 (9pp)
doi:10.1088/0957-4484/18/18/185609
Microstructural evolution during solid-state sintering of ball-milled nanocomposite WC–10 mass% Co powders 1 ˜ E Men´endez1 , J Sort2,3 , A Concustell1 , S Surinach , J Nogu´es2 and 1 M D Bar´o 1
Departament de F´ısica, Universitat Aut`onoma de Barcelona, 08193 Bellaterra, Spain Instituci´o Catalana de Recerca i Estudis Avanc¸ats (ICREA) and Departament de F´ısica, Universitat Aut`onoma de Barcelona, 08193 Bellaterra, Spain
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Received 18 December 2006, in final form 15 March 2007 Published 11 April 2007 Online at stacks.iop.org/Nano/18/185609 Abstract The microstructural evolution during solid-state sintering of nanocrystalline WC–10 mass% Co powders, prepared by different ball-milling processes to give rise to diverse mechanical activation states, is investigated. During sintering, two ternary carbides, Co6 W6 C and Co3 W3 C, form at the binder phase which surrounds WC particle aggregates. The overall microstructure of the sintered material (including the formation of these carbides) is found to be very sensitive to the initial state of the powders, even more than to the sintering temperature and/or sintering time. In addition, the nanocrystalline nature of the composites is preserved during the solid-state sintering, in part assisted by the formation of these complex carbides and decarburization. This nanostructure yields moderate microhardness and fracture toughness values, in spite of the remaining levels of porosity.
Even though sintering of microstructured WC–Co composites is usually achieved via a liquid-phase sintering process [5], solid-state sintering procedures, which are carried out below the lowest melting point of the system [6–9], have received much attention since nanocrystalline powders are used as a starting source with the aim of producing bulk nanostructured materials. Furthermore, it is known that submicron WC– Co composites can exhibit superior properties (e.g., hardness or fracture toughness) than conventional (microstructured) cemented carbides [3, 10–13]. It should be mentioned that if the crystallite size is reduced in the initial powders, the sintering activity increases due to the large amount of interfaces created, which make the system more reactive owing to the increase of interfacial free energy. This brings about a pronounced densification and a rapid grain growth, which can take place even during early stages of sintering [13, 14]. Hence, retaining nanometric grain sizes during sintering is rather challenging, and remains an issue of technological interest.
1. Introduction Cemented carbides, such as WC–Co, are commonly used to manufacture cutting and mining tools, wear-erosion resistant parts or pressing dies, since they combine a high hardness with considerable fracture toughness, owing to the Co-based phase that forms around WC, which provides a ductile bonding matrix for these ceramic particles (note that this Co-based matrix will be referred to as the binder phase, as it is commonly termed in the field of cemented carbides). WC– Co composites also exhibit high-temperature strength, good corrosion resistance and an important chemical stability even at high temperatures [1–3]. Several varieties of sintering routes have been proposed to synthesize these composites from powders into bulk materials [4]. Nevertheless, a major classification, depending on whether or not any liquid phase is present during sintering, can be made between liquid-phase and solid-state sintering. 3 Author to whom any correspondence should be addressed.
0957-4484/07/185609+09$30.00
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© 2007 IOP Publishing Ltd Printed in the UK
Nanotechnology 18 (2007) 185609
E Men´endez et al
All BM processes were performed in a planetary ball mill (Fritsch Pulverisette 7), using hardened stainless steel balls and vials, which where sealed under Ar atmosphere, a ballto-powder ratio of 6:1 and a disc angular frequency (equal to the vial frequency) of 500 rpm. Fe contamination was evaluated for the powdered mixture II using a flame atomic absorption spectrophotometry (FAAS) procedure, resulting in a maximum of 0.5 mass% of Fe with a relative standard deviation value of 4%. The as-milled powder mixtures were then pre-compacted, under Ar atmosphere, using a WC hot press apparatus under a uniaxial pressure of 650 MPa for 10 min at T = 600 ◦ C. Typical values of relative densities of the green compacts are around 55% [29, 30]. A significant reduction of the distances between particles is reached, leading to a shorter atomic diffusion length and, as a result, increasing the reactivity of the system due to the better connection of the particles. Subsequently, to obtain bulk materials, the compacts were sintered either at 1000 ◦ C or at 1250 ◦ C, for 20, 60, 100 and 140 min of isothermal holding, in a tubular furnace under vacuum (air pressure < 10−3 mbar). The heating and cooling rates were 5 ◦ C min−1 ; the employed cooling rate was sufficiently slow to promote the sintering process. Note that the maximum temperature used in this study, 1250 ◦ C, is below the eutectic temperature of the microstructured system [8]; thus the sintering process can be considered as solid-state sintering. The carbon content was evaluated by means of an elemental organic CHNS analysis. The morphology of the composites was examined using a JEOL JSM 6300 scanning electron microscope (SEM) equipped with an energy-dispersive x-ray (EDX) analysis system and a JEOL JEM 2011 HR high-resolution transmission electron microscope (TEM) operating at 200 kV. The TEM samples were mechanically ground using a dimple grinder apparatus in order to thin the specimens down to 50 μm in thickness. As a final step, an ion-milling process was performed using a precision ion polishing system. The sample composition at the nanometre scale was studied by electron energy-loss spectroscopy (EELS). Porosity was estimated from optical micrographs. The microstructural parameters (phase percentages, crystallite sizes—average coherently diffracting domain sizes—and microstrains—atomic level deformations) were evaluated by fitting the x-ray diffraction (XRD) patterns (recorded using Cu Kα radiation) using a full patternfitting procedure (Rietveld method) [31, 32]. The Vickers microhardness was determined using a load of 0.1 kg by means of an MHT-10 microhardness indenter. Finally, a Vickers hardness tester (FRANK type 532) was used, with 40 kg load, to estimate the fracture toughness of the produced composites.
Nanocrystalline powders are commonly prepared by mechanochemical processes [15, 16] or mechanical alloying [1, 17]. Diverse consolidation techniques, such as hot isostatic pressing [14, 18], spark plasma sintering [19], microwave sintering [14], powder injection moulding [20] or extrusion [5], have been reported. To restrain grain growth, sometimes, inhibitors (such as VC or Cr3 C2 ) are added in small percentages, either during the milling [11, 17, 21] or during the chemical carburization routes [15] used to prepare the cemented carbides. It is worth mentioning that although the influence of the sintering conditions on the properties of nanocrystalline WC– Co powders has been investigated to some extent [1, 3, 13, 14], the formation of new phases (e.g., Co6 W6 C or Co3 W3 C ternary carbides, both known as η-phases) and their role during sintering have not been systematically addressed. For conventional composites and according to thermodynamic reasons, the two-phase region (FCC-Co + WC) is limited to the carbon range 5.38–5.54 mass%. These limiting carbon contents are approximately the corresponding threshold values where the phase fields (M6 C + FCC-Co + WC) and (C + FCC-Co + WC) become stable. Thus, the M6 C carbide and pure carbon start to form when the carbon content is somewhere around 5.38 and 5.54 mass%, respectively [22]. Furthermore, these ternary carbides can also be synthesized by mechanical alloying of elemental W, C and Co powders [23] and are known to be rather brittle [24]. The study of ternary carbides is also of technological importance since, in general, carbides involving two or more metals have been reported to exhibit novel properties (e.g., magnetic [25] or electrical [26]) compared with individual monometal carbides. In this work, we perform a detailed investigation on the microstructural evolution during solid-state sintering of nanocrystalline WC–10 mass% Co powders (without grain growth inhibitors or extra addition of carbon) under reducing conditions, previously subjected to different ball-milling (BM) processes (i.e., leading to different mechanically activated states). Particular emphasis is given to the formation of ternary carbides, which is promoted, as other reactions, by a prior ball milling [27, 28]. The overall microstructure of the sintered material is very sensitive to the milling state of the powders before consolidation. Interestingly, the formation of ternary carbides as binder phase is found to assist the preservation of the nanocrystalline nature in the sintered composite materials, thus avoiding crystallite growth, which in conventional sintered cemented carbides results in the detriment of mechanical properties.
2. Experimental details 3. Results and discussion
Powder mixtures consisting of WC (ALDRICH, powder,
