Victor Ioan STANCIU*, Véronique. VITRY*, Fabienne DELAUNOIS*

Transcription

1 A READY TO USE CR 3C 2 DOPED COBALT POWDER AS BINDER IN CEMENTED CARBIDES Abstract Victor Ioan STANCIU*, Véronique. VITRY*, Fabienne DELAUNOIS* Metallurgy Department, University of Mons, Belgium, victorioan.stanciu@umons.ac.be Mechanical alloying is a method that allows producing mixtures of compounds in solid state. This paper investigates the possibility to produce by mechanical alloying ready-to-use cobalt powder doped with chromium carbide to be used as binder in cemented carbides. The role of inhibitors in cemented carbides is to prevent the growth of tungsten carbide particles during the sintering process. A review of previous work leads to the conclusion that chromium carbide is a good candidate for an inhibitor: it ensures the best ratio between hardness and toughness for cemented carbides. Various amounts (1%, 5%, 10% and 20% in weight) of chromium carbide (Cr3C2) are mixed and milled together with cobalt in a Fritch planetary ball mil. They are compared to a batch of cobalt only that was milled in the same conditions, as reference. The milling conditions were as follow: vials rotation :600 rpm; Milling sequence: 5 minutes of milling followed by 5 minutes of cooling; 10 mm WC-Co milling balls; WC-Co milling bowls ; maximal milling time 10h. Samples were taken every 2 hours. They were analyzed to study the evolution of particles size and the dispersion of chromium carbide into the cobalt matrix, using the following methods: SEM, laser granulometry and X-ray diffraction. A reduction of particle size was observed for longer milling times and the best particles size distribution was obtained for the materials containing 5% wt and 10% wt of chromium carbide. Keywords: cobalt, inhibitors, cemented carbides, mechanical alloying, properties 1. INTRODUCTION Mechanical Alloying (MA) is solid state treatment for powders that involves repeated welding, fracturing and re-welding of the powder particles in a high energy ball mill. This method is increasingly used for the manufacture of raw powders for cemented carbides [1-2]. Cobalt has been the most frequently used binder in cemented carbides industries from the beginning [3]. Its presence brings strength and toughness to the tungsten carbide particles constituting the part [4]. During the sintering process liquid cobalt dissolves some of the tungsten carbide particles that will then re-precipitate on the surface of other particles upon cobalt solidification, thus contributing to their increase in size [5]. The WC grains have a faceted shape and have thus a strong tendency to abnormal grain growth (AGG), where a few large grains grow much larger than the average. This is really detrimental for the mechanical properties, notably because abnormal grains act as starting points for cracks [6, 7]. In conventional sintering conditions, grain size cannot be maintained within acceptable limits, because high temperature sintering is needed to attain the high density required by operating conditions of carbides parts [8]. In sintered systems with multiple solid phases, inhibition of grain growth is usually observed, with a decrease of grain growth in the case of mixing two insoluble solid phases [9, 10]. Increasing the amount of inhibitor added to the alloy brings a finer structure by ensuring a better densification [11]. A typical inhibitor acts by decreasing surface transport phenomena during the leaching process, and reprecipitation. For example the addition of cubic carbides such as VC, Cr3C2 and NbC to WC-Co mixtures inhibits grain growth of the tungsten carbide [12, 13]. The work of Huang et al. [14] did show that VC provides greater hardness to the final product then Cr3C2, who allows a better tensile strength (toughness).

2 Of all the cubic carbides used as growth inhibitors, chromium carbide appears to give the best compromise between hardness and toughness [14]. The effect of inhibitors increases up to a concentration of 1% wt (for cemented carbide with 10% cobalt) after this amount, the improvement effect is limited [14, 15]. The aim of this study is to produce a cobalt powder doped uniformly with chromium carbide for the fabrication of cemented carbides. In this context a very good distribution of inhibitor in the binder matrix is desirable. To achieve this, we chose to grind the two components in a planetary mill, in order to produce a ready-to-use binder - inhibitor mixture. 2. EXPERIMENTAL Grinding is carried out in a Fritsch Pulverisette 7 premium line planetary mill, using tungsten carbide bowls and balls to avoid powder contamination. The following materials were used: cobalt with a purity of 99% and a lower grain size 90μm. Chromium carbide with a size of 3-5μm (up to 10 µm after SEM analyze), is provided by Inframat Advanced Materials. The following mixes were studied: cobalt doped with 1%, 5%, 10% and 20%wt chromium carbide. A batch of cobalt without chromium carbide was milled as reference. The milling was carried out at 600 rpm, the highest speed of our planetary ball mill. It was performed for 10 hours, in steps of 2 hours, with the following cycles: 5 minutes of grinding + 5 minutes of cooling, with reversal of the bowls rotation after each break. The balls to powder ratio was 4/1, with 10 mm balls (100 times bigger than the biggest particle). Samples were taken for analysis after every 2 hours milling step. Powders were analyzed using a JEOL scanning electron microscope, equipped with EDX and WDX analysis, which allowed to obtain an overview of powder and to check the aggregation (agglomeration), particle size and the spread of chromium carbide in cobalt matrix. Particle size analysis was carried out by humid laser diffraction tests (Malven Mastersizer 3000E) to determine the particle size distribution of the powders. X-ray diffraction tests were carried out to identify the crystalline species and measure the grain size. The degree of agglomeration of the particles was suited by pressing two grams of powder into a mold and embedding the compact in epoxy resin. The samples were then polished, etched with NITAL and Murakami reagents and studied by optical and scanning electron microscope (SEM) to observe the location of the chromium carbide particles in the cobalt matrix. 3. DISCUSSIONS Agglomeration of the powder was observed during solid state grinding, as well as adhesion to the walls of the grinding chamber. Joining of crushed material to the balls was minor and was observed only in the earlier milling hours. This is not surprising because, in order to avoid contamination of the powder, no lubricant is used to prevent particles agglomeration. Results of the study of the degree agglomeration of the powders, are shown on figures 1 to 4 (optical microscopy) and 5 to 8 (scanning electron microscope (SEM)). They show that increasing the proportion of chromium carbide favors powder agglomeration. The lamellar structure obtained by mechanical alloying is easily observable on figures 1 to 4. The large particles have a composite structure obtained by joining of several small particles, with chromium carbide particles at their interfaces. The chromium carbide has a relatively uniform distribution, with an observable diminution of particle size greatly, with many particles encrusted in the cobalt.

3 Fig. 1. Microstructure of Co+1% Cr3C2, after 10 Fig. 2. Microstructure of Co+5% Cr3C2, after 10 Fig. 3. Microstructure of Co+10% Cr3C2 after 10 Fig. 4. Microstructure of Co+20% Cr3C2 after 10 The reduction of the size of chromium particles can clearly be seen from figures 5 to 8: they do not exceed 2-3 µm, which is much smaller than their original size (up to 10µm). Most of them are embedded in the cobalt matrix. Chromium carbide particles are found at the former interfaces between cobalt particles, welded as a result of mechanical alloying processes. A thin layer of cobalt was detected by EDX analysis on the surface of free chromium carbide particles, as shown on figure 9. An important number of free carbide particles were observed in the mixture with the highest chromium carbide content. Laser particle size analysis shows a significant decrease in particle size distribution with increasing chromium carbide content, as shown on figure 10. The best results are obtained for the 5%wt Cr3C2, and 10%wt Cr3C2, which is in agreement with the literature [15], which recommends an inhibitor concentration of 5 wt% of the cobalt content of the cemented carbide. In figure 11 are presented graphically the mean particle diameter for 10%, 50% and 90% volume of the powder, for each composition tested.

4 Fig. 5. Microstructure of Co+1% Cr3C2 after 10 hours of milling. NITAL etching, SEM BSE 5000X Fig. 6. Microstructure of Co+5% Cr3C2 after 10 hours of milling. NITAL etching, SEM BSE 5000X Fig. 7. Microstructure of Co+10% Cr3C2 after 10 hours of milling NITAL etching, SEM BSE 5000X Fig. 8. Microstructure of Co+20% Cr3C2, after 10 hours of milling. NITAL etching, SEM BSE 5000X Fig.9. Cr3C2 particles covered with cobalt in Co+5% Cr3C2 after 10 hours of milling. SEM BSE 4500X

5 Fig. 10. Particle size distribution of the Cr3C2+Co powder mixtures, after 10 hours of milling time. Fig.11. Particles size mean after 10hours of milling for tested mixtures. Following the XRD analysis, in the composition of the mixtures, we can find cobalt, chromium carbide (Cr3C2) and some oxidation of cobalt powder (Co3O4) was observed. The intensity of the peaks corresponding to chromium carbide is not very high, because they are covered with a thin film of cobalt, which is observed in SEM analysis (figure 9).

6 4. CONCLUSIONS Mechanical alloying is a technique that allows obtaining homogeneous mixtures of compounds that are immiscible in solid state. The results of this investigation confirms that the value of 5 to 10 wt. % of inhibitor, found in the literature, yields the best granulometry. The process of fabrication of cobalt binder doped with chromium carbide still requires optimization because there remains significant amounts of agglomerates. In further work, smaller balls and wet milling will be used to avoid agglomeration of the particles. ACKNOWLEDGEMENTS The authors wish to thank MATERIA NOVA and INISMA research centers for their help in carrying out part of the analytical methods. LITERATURE [1] SURYANARAYANA C., Mechanical alloying and milling, Progress in Materials Science, 46 (2001) [2] M.H. ENAYATI, G.R. ARYANPOUR, A. EBNONNASIR, Production of nanostructured WC Co powder by ball milling, International Journal of Refractory Metals and Hard Materials, Volume 27, Issue 1, January 2009, Pages [3] SPRIGGS, Geoffrey E., History of Fine Grained Hardmetal, Int. J. of Refractory Metals & Hard Materials, 13 (1995) [4] UPADHYAYA, G.S., Materials science of cemented carbides - an overview, Materials and Design, Volume 22, Pages [5] SEEGOPAUL, P., McCANDLISH, L. E., GAO L., Sintering of WC-Co nanocomposites, 14th Plansee Seminar 97, Plansee Proceedings, v. 4, Cemented Carbides and Hard Materials, 1997, p [6] SHATOV, A.V., FIRSTOV, S.A., SHATOVA, I.V., The shape of WC crystals in cemented carbides, Materials Science and Engineering, Volume 242, Issues 1 2, February 1998, Pages 7-14 [7] MANNESSON, Karin, BORGH, Ida, BORGENSTAM, Annika, ÅGREN, J., Abnormal grain growth in cemented carbides Experiments and simulations, International Journal of Refractory Metals and Hard Materials, Volume 29, Issue 4, July 2011, Pages [8] LEIDERMAN, M., BASTEIN, O., ROSEN, A., Sintering, microstructure and properties of submicron cemented carbides, 14th Plansee Seminar 97, Plansee Proceedings, v. 2, Cemented Carbides and Hard Materials, 1997, p [9] CARROLL, D. F., Processing and properties of ultrafine WC/Co hard materials, 14th Plansee Seminar 97, Plansee Proceedings, v. 2, Cemented Carbides and Hard Materials, 1997, p [10] PORAT, R., BERGER, S., ROSEN A., Synthesis and processing of nanocrystalline WC/Co powders, 14th Plansee Seminar 97, Plansee Proceedings, v. 2, Cemented Carbides and Hard Materials, 1997, p [11] SUN, Lan, JIA, Chengchang, CAO, Ruijun, CHENGUANG, Lin Effects of Cr3C2 additions on the densification, grain growth and properties of ultrafine WC-11Co composites by spark plasma sintering, International Journal of Refractory Metals and Hard Materials, Volume 26, Issue 4, July 2008, Pages [12] TANIUCHI, T., OKADA, K., TANASE, T. Sintering behavior of VC dopped micro-grained cemented carbide, 14th Plansee Seminar 97, Plansee Proceedings, v. 2, Cemented Carbides and Hard Materials, 1997, p [13] SUN, Lan, YANG, Tian'en, JIA, Chengchang, XIONG, Ji, VC, Cr3C2 doped ultrafine WC Co cemented carbides prepared by spark plasma sintering, International Journal of Refractory Metals and Hard Materials, Volume 29, Issue 2, March 2011, Pages [14] HUANG, S.G., LI, L., VANMEENSEL, K., Van DER BIEST, O., VLEUGELS, J., VC, Cr3C2 and NbC doped WC Co cemented carbides prepared by pulsed electric current sintering, International Journal of Refractory Metals and Hard Materials, Volume 25, 2006, Pages [15] LUYCKX, S., ALLI, M. Z., Comparation between V8C7 and Cr3C2 as grain refiners for WC-Co, Materials and Design, nr. 22/2001, p