Core-Shell-Structured Tungsten Composite Powders. A. Bicherl*, A. Bock*, B. Zeiler*, W.D. Schubert**

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1 Bicherl, Bock, Zeiler et al. 17th Plansee Seminar 2009, Vol. 3 GT 27/1 ore-shell-structured Tungsten omposite Powders A. Bicherl*, A. Bock*, B. Zeiler*,.D. Schubert** *olfram Bergbau- und Hütten GmbH NfG KG, St. Martin i.s., Austria **Vienna University of Technology, Austria Abstract Unique particle design of tungsten metal and tungsten carbide composite powders in form of a core-shell structure is described. Such core-shell structures show an excellent distribution of the individual components. Focus of the presentation is on the systems -Mo and -(o, Ni, Fe). The specific characteristics of these powders are adjustable by selection of core-/shell-materials and process parameters. VT-mechanisms enable the preparation of core-shell composites by a simple process route. Keywords omposite powder, tungsten-molybdenum, tungsten carbide-cobalt, core-shell structure, VTmechanism Introduction omposite materials, as described in literature, consist of two ore more components which have different chemical composition and are separated from one another in the material by distinct boundaries. Furthermore, they are heterogeneous on a microscopic scale but homogenous on a macroscopic scale [1]. Thus composite materials are characterised by a uniform distribution of elements relative to each other. For tungsten composite powders several technological approaches have been reported: I) Preparation of -o composites by direct reduction or -carburisation of tungsten and cobalt oxide mixtures [2-4]. II) Precipitation of cobalt onto the tungsten carbide surface by chemical vapour deposition, direct reduction of cobalt ions from solution onto and deposition of cobalt precursors from solution onto surface followed by subsequent reduction (reviewed in [5]). III) Manufacturing of tungsten copper composite powders by direct reduction of tungsten copper composite oxides where the tungsten phase substantially encapsulates the copper phase [6]. IV) Tough coated hard powders where a covalent hard material is coated by tungsten carbide and surrounded by a cobalt layer via complex PVD- and VD-processes in a rotary reactor [7].

2 GT 27/2 17th Plansee Seminar 2009, Vol. 3 Bicherl, Bock, Zeiler et al. Most of the processes mentioned above are complex in processing and require a tricky process control. The resulting composite powders vary from intimate mixtures with partly adhesion of the one component to the other to composite powders with well defined particle structures, as, for example, the encapsulating of one component by the other or a coating. The present paper presents and characterises tungsten composite powders with a novel particle design by means of tungsten-ferrous group metals (o, Ni) and tungsten-molybdenum composite powders manufactured via a simple process route. Strategy and procedure The manufacturing of the composite powders is based on the observation that during reduction of solid metal oxides, which form volatile intermediate reaction products (oxide, oxide hydrate), foreign powder particles are overgrown by means of chemical vapour transport. The following schematic flow sheet visualises the simple process route for preparation of the core-shell structured (SS) composite powders (fig. 1) [8, 9]. ore () and shell (S) components are homogeneously blended by appropriate processes: dry milling or mixing, wet milling or mixing, precipitation or spray-drying. The blend is subsequently heat treated and the metal oxides are reduced by hydrogen [8, 9]. In case that both starting materials (S and ) are oxides, the reduction proceeds in two steps: at first, the reduction of the nobler element takes place, which then forms the core structure (), and in the second step, the less noble metal is reduced forming the shell structure (S). Shell forming component (S) ore component () for example: O x, MoO x Blending for example: O x, MoO x, Mo o-oxide, o Ni-oxide, Ni H 2-Reduction Nucleation of metal S from the volatile compound on metal nucleus S Metal composite powder Fig. 1: Schematic flow sheet for processing of core-shell-structured tungsten composite powders The formation of the SS-particles implies that the shell component is a metal whose reduction sequence proceeds via a chemical vapour transport reaction process. Furthermore, a distinct chemical or structural affinity between core and shell components is essential. In this study the shell component (S) is mainly a tungsten oxide (O 3, O 2 ), whereas cobalt and molybdenum metal as well as their oxides are selected as core components (). Fig. 2 shows tungsten metal containing composite powders with typical core-shell structure. During the reduction process core-shell structures are induced by epitaxial growth (for example Mo or Mo ) and/or by

3 Bicherl, Bock, Zeiler et al. 17th Plansee Seminar 2009, Vol. 3 GT 27/3 a chemical reaction, such as formation of solid solutions or intermetallic phases (for example o forming o 7 6 or o 3 ). Interaction between core and shell material: o, Fe or Ni react with tungsten to form intermetallic phases ideal System: crystal lattice of core and shell component = bcc similar lattice parameters: a Mo = Ǻ a = Ǻ Fig. 2: Example for metal composite powder structure (powders embedded in copper, etched) Mechanism of shell formation The mechanism of shell formation is based on a tungsten or molybdenum transport via a volatile tungsten oxide hydrate (O 2 (OH) 2 ) [2] or a volatile molybdenum oxide (MoO 3-X ), followed by reduction of the volatile species at the surface of the core component (fig. 3). Thus homogeneous coating of the core component during VT crystal growth of tungsten takes place in the reduction step: tungsten oxide tungsten metal. The particle surface of the core component acts as nucleation aid for tungsten deposition and enables epitaxial growth on the core component. O x Ox, = shell component = core component O x O 2(OH 2) O x Fig. 3: Scheme of mechanism for metal composites If core and shell components are isostructural (example (d) in fig. 4), the shell component continues the lattice structure of the core component (3-dimensional). In the majority of cases, there exists only a two dimensional structural relationship or a high chemical affinity between core and shell component. This leads to directional growth of the shell being visible by the formation of grain boundaries within the shell structure (example (a) in fig. 4). Tungsten growth and the structure of the shell will depend also on the agglomeration and the morphology of the core particles, respectively, as being illustrated in fig. 4 (b) and (c).

4 GT 27/4 17th Plansee Seminar 2009, Vol. 3 Bicherl, Bock, Zeiler et al. (a) (b) (c) (d) Fig. 4: reconstruction of the core particle morphology and formation of the shell structure Results and Discussion The composite particle structure and size is primarily determined by the size of the nucleus (= core particle size), the reduction conditions (humidity, powder layer height), and the mass relation of both components. Reduction conditions and core particle size For optimal chemical vapour transport a minimum humidity is essential, which corresponds to a certain combination of temperature, powder layer height and hydrogen flow. During reduction of the shell-forming oxide the tungsten/molybdenum nucleation and growth are possible at much higher average humidity than encountered for the pure tungsten/molybdenum oxide, thus indicating that the core component acts as nucleation aid (see SEM picture on the left side for Mo and ). The nucleation of tungsten or molybdenum metal takes place on the surface of the core particle. The size and morphology of the core particle defines the resulting particle size and morphology of the metal composite particle (fig. 4 and 5). The core particle size distribution, any core particle agglomeration or core particle intergrowth remain the same during reduction. The better the phase distribution of the staring materials, the more homogeneous is the shell formation around the core particles. Fig. 6 illustrates the maintenance of the particle distribution of the core material during the reduction process for tungsten and molybdenum composite powders. Between tungsten and molybdenum a three dimensional structure relationship exists which is the prerequisite for a homogeneous growth of one element on the other. The polished and etched microsections of Mo- or -Mo in fig. 6 show an optimal overgrowth of the particles and an excellent distribution of -Mo.

5 Bicherl, Bock, Zeiler et al. 17th Plansee Seminar 2009, Vol. 3 GT 27/5 10 wt% o 90 wt% o ~0,5µm 30 wt% o 70 wt% o ~9,5µm 20 wt% Ni 80 wt% Ni ~3µm metal composite starting material / core structure Fig. 5: Influence of the core particle size on the composite particle properties starting material metal composite powder Mo Mo- (Mo=grey, =white) Mo (core) (shell) 10w% Mo 90w% -Mo (Mo=grey, =white) (core) Mo (shell) 50w% 50w% Mo Fig. 6: Metal composites: SEM images of tungsten overgrown by molybdenum and vice versa, to the right copper embedded powders (etched).

6 GT 27/6 17th Plansee Seminar 2009, Vol. 3 Bicherl, Bock, Zeiler et al. Proportion of core and shell component Assuming ideal composite particles (complete overgrown structures, spherical particles) the size of the composite particles can be calculated based on following mathematic model [8, 9]: d composite particle = d core ((V shell / V core )+1) 1/3 (1) The higher the amount of core particles (at constant diameter), the smaller is the resulting composite particle size. Fig. 7 demonstrates the correlation to the measured values (FSSS), based on an average core particle size of ~0.60µm. Fig. 7 exemplifies the influence of the core particle size on the resulting composite particle size as well as the shell thickness omposite particle size (theoretical) 1.70 omposite particle size (measured) 1.50 d [µm] d 0.60µm o [w%] Fig. 7: Dependence of o--composite particle size on the proportion of core (cobalt) to shell (tungsten) components. Further processing (carburisation) The metallic SS-powders possess a stable particle structure at temperatures below the melting point of the lower melting component. Therefore carburisation of SS particles can be carried out by standard processing (mixing with carbon black and heat treatment in hydrogen atmosphere). Fig. 8 demonstrates composite carbide structures manufactured by standard carburisation process for o- and -Mo 1-x core-shell structures.

7 Bicherl, Bock, Zeiler et al. 17th Plansee Seminar 2009, Vol. 3 GT 27/7 o TEM image Mo 1-x - copper embedded powder (Murakami etched) Fig. 8: Illustration of carbide composite powders metal particle structure survives further processing Fig. 9 gives a comprehensive view of the powder manufacture from starting core particle to tungsten carbide composite powder. The close relation between starting material, metal composite and carbide composite is clearly demonstrated. starting material metal composite carbide composite o-metal (core) 10o-90 [w%] 9.5o-90.5 [w%] -metal (core) 50-50Mo [w%] 50-50Mo 1-x [w%] Fig. 9: From starting material to carbide composite powder; effect of particle size, size distribution and morphology of the core material.

8 GT 27/8 17th Plansee Seminar 2009, Vol. 3 Bicherl, Bock, Zeiler et al. onclusion omposite powders containing tungsten and molybdenum metal and carbide with a well-defined core-shell structure can be obtained by a simple process route. This route is based on the blending of two powder components and subsequent reaction during hydrogen reduction. The core-shell constitution of the particles is the result of the formation of volatile tungsten or molybdenum oxides, and a VT-reaction occurring during the reduction process. One constituent acts as a nucleation aid (core component) for the growth of the shell structure (shell component). The core component can be added prior to reduction, or, alternatively, can form in-situ during the early stages of reduction. The powder properties can be controlled by the core powder properties (in particular the particle size and morphology), the reduction conditions, and the proportion of the core to shell component. Several metal composite powders were obtained and described, such as -Mo, Mo-, o- and Ni- (ore-shell). The core-shell structure of the metal composite powders is maintained during carburisation and results in core-shell structured -Mo 1-x, Mo 1-x -, -o or -Ni composite powders. References 1. K.I. Portnoi, A.A. Zabolotskii, S.E. Salibekov and V.M. hubarov; Poroshkovaya Metallurgiya, No. 12 (180), pp (1977) 2. E. Lassner,.D. Schubert; Tungsten: Properties, hemistry, Technology of the Element, Alloys, and hemical ompounds ; Kluwer Academic/Plenum Publishers, New York (1999) 3. P. Seegopaul et al.; International Journal of Refractory Metals & Hard Materials, Vol. 15(1-3), p (1997) 4. Z. Yl et al.; hina Particuology, Vol. 3(5), p (2005) 5. h. Adorjan, A. Bock, S. Myllymäki,.D. Schubert, K. Kontturi ; /o-composite powders via hydrothermal reduction of o 3 O 4 -suspensions ; International Journal of Refractory Metals and Hard Materials, Volume 26(6), p (2008) 6. L.P. Dorfman and M.J. Schreithauer; European Patent EP (1996) 7. R.E. Toth et.al, United States Patent US (2000) 8. A. Bock, A. Bicherl, A. Schön, B. Zeiler,.D. Schubert; International Patent O2008/ A. Bock, A. Bicherl, A. Schön, B. Zeiler,.D. Schubert; International Patent O2008/031121