STRUCTURAL BEHAVIOR AND MICROSTRUCTURAL HARD METAL SINTERED AT 1350 C FROM THE POWDER OF NANOMETER WC WITH 10 wt% Co

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1 STRUCTURAL BEHAVIOR AND MICROSTRUCTURAL HARD METAL SINTERED AT 1350 C FROM THE POWDER OF NANOMETER WC WITH 10 wt% Co 1 A. C. Batista, 2 H. C. P. de Oliveira, 1 G. J. Perpétuo e 1 R. R. V. Leocádio 1 Rede Temática de Engenharia de Materiais REDEMAT Praça Tiradentes n 20, centro, Ouro Preto / Minas Gerais, , Brasil adrianocorrea77@gmail.com 2 Instituto Superior Técnico IST / Departamento de Materiais / Av. Rovisco Pais, 1, Campus Alameda, Lisboa, , Portugal. ABSTRACT The hard metal (WC-10%Co), processed via powder metallurgy, using powder of nanometer WC, were characterized from the point of view to the microstructural and structural techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDS) for mapping and punctual. To analyze the behavior of hard metal after the sintering process performed in resistive furnace at 1350 C for 1 hour under vacuum of 10-2 mbar, the analysis identified the formation of WC grains, the pore distribution and behavior and evolution of the phases WC and Co, generating phases (Co 3 W 3 C and Co 6 W 6 C). Keywords: Hard Metal, Sintering, Structural and Microstructural Characterization. INTRODUCTION The processing of Hard Metal - WC-10%Co is accomplished through a conventional route and industrial production, where the process is the preparation of tungsten powder and cobalt, followed by carbonization of tungsten and finally the mixture of powder of nanometric WC with Co powder to form the composite. Obtaining hard metal occurs through liquid phase sintering of WC Co with a temperature of approximately 1400 o C, in which the Co diffuses through the structure reaching the desired uniformity and density (1,2,3). Cobalt is the metal most frequently used as a binder for the production of hard metal due to the easy wettability of WC by the Co liquid at sintering temperature. In general, the sintering temperature of a material decreases with decreasing particle size. Up to 85% densification of WC-Co was obtained by performing sintering below 1280 C when the grain sizes were <0.3 m. But when the grain size was 0.7 m, up to 70% of the densification is carried out at the same temperature, which suggests that the onset of sintering is a function of grain size (4). 2692

2 Interest in hard metal with grain size nano comes with the understanding that there is some general increase in hardness with a decrease in grain size of WC (5). The hard metal based on WC-Co has limitations in the choice of C content, the thermodynamic point of view (6). With high content of C, the graphite becomes stable and its compounds are present in the final microstructure of the sintered. The compounds and Co 3 W 3 C Co 6 W 6 C known as phases are formed with low C. The phases and graphite are not desirable because they promote a reduction in mechanical properties of hard metal (7). The hard metal since the early 20th century has been widely used in many manufacturing processes because of its properties, which combine high hardness and wear resistance, and high fracture toughness. Its main applications include cutting tools, drill bits for drilling for oil and gas, matrix compression, puncture, components for high energy milling, among others (8). As mentioned above, the classical composition of the cemented carbide is WC-Co. The main microstructural characteristics are directly related to the WC grain size, the interfacial bonding between the phases WC, Co and the binder phase quantity of Co. What along the past two decades, has generated substantial research efforts directed at synthesis and sintering of nanometric powders of WC, with the objective of producing materials that have grain in their structure with nanometric dimensions (9,10). The influence of grain size on sintering and densification of hard metal provide a more homogeneous microstructure, changing and improving the mechanical properties considerably. The exploitation of the improvements on the properties of this material is intended to increase the lifetime and robustness of the tools of WC. The use of nanostructured materials, and should have guaranteed participation in all important industries. The industry of cutting tools is already investing and seeks to benefit from improved properties generated with decreasing grain size of WC. With the decrease of grain size, are expected to significantly improve the performance of these tools. The research in this area tends to move forward and continue to improve the processing capability of the powders and achieve acceptable production economy (5). During this study monitored the evolution of microstructure hard metal. After the sintering step carried out in the resistive furnace at 1350 C for 1 hour under vacuum of 10-2 mbar. The techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy X-ray mapping (EDS) showed the formation of grains, the pore distribution and behavior of the phases of WC and Co and phase transitions, occurring up to the level of sintering at 1350 C (11,12,13). MATERIALS AND METHODS The starting powders used in this study was the powder of nanometer WC supplied by Sumitomo - WC05NR/Lot. N o A-05119, with an average particle size of 125 nm and the Co powder supplied by Umicore, with average particle size of 3.6 µ. The mixture of powders of WC and Co was carried out in a ball mill of high energy - SPEX 8000 to dry for 40 minutes. The relation between the amount of mass 2693

3 of powder and ball into the reservoir was 3:1, to fill about 25% of the reservoir, promoting adequate homogenization. The container mill was used Carbide not posing a risk of contamination. The samples were compacted by uniaxial pressing in a cylindrical steel matrix with compaction pressure of 300 MPa. The sintering step was performed in a tube resistive furnace, under vacuum BP Engineering mark of 10-2 mbar at a temperature of 1350 C for 1 hour. Through XRD phases present in the composite WC-10%Co sintered were identified with the equipment from Shimadzu, model XRD Using Cu K radiation, voltage 35 KV, current 40 ma, 2 varying from 5 to 120 with step of 0.03 and time of 3 seconds per point. The diffractograms were compared with files deposited in the database of crystallographic research portal for the Coordination of Improvement of Higher Education Personnel (CAPES) Brazil. With SEM were observed the microstructural aspects of powder samples, details of the morphology and grain size. Was utilized the SEM of Shimadzu, model SSX-550. With the help of an energy dispersive spectrometer (EDS) coupled with SEM, microanalysis was carried point, obtaining semiquantitative information about the presence of phases and solid solutions samples in areas micrometers referring to incidence of the electron beam. The distribution of chemical elements present in the sample and the value of semiquantitative chemical composition of the sample was obtained by mapping X-ray characteristic of the metallic constituents. RESULTS AND DISCUSSIONS The micrographs of the powders of WC and Co, obtained by SEM using secondary electrons (SE) increases of 10000x, are shown in Fig. 1. Were verified different morphologies and particle sizes, and the powders of WC nanoparticles having smaller and more agglomerated than the post of Co. A B Figure 1 Micrographs of powders (A) WC and (B) Co respectively, an increase of 10000x. 2694

4 intensity (cps) 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais The chemical composition analysis via EDS point of WC and Co powders has not detected the presence of impurities in significant quantities, proving the quality of the powders, as Tab. 1. Table 1 Semiquantitative analysis of the powders of WC and Co. Powder WC Powder Co Element Atom % Element Atom % C O O Co W In this context, the analysis using SEM / EDS showed the variation of morphology and chemical composition of the powders, being relatively low and incapable of form structures which could be identified in the diffractograms. The identification of phases present in the sample sintered at 1350 C offered some degree of difficulty. For the process of identification of the phases were compared the positions 2, interplanes distances and intensities. With the help of crystallographic database of CAPES, it was possible identify the phases WC (hp), Co (hp), Co 3 W 3 C (fcc) and Co 6 W 6 C (fcc). The diffractogram of the sample (figure 2) shows some degree of complexity with the background, with many peaks with intensities below 80 cps. This work was considered with peaks up to 60 cps, which has allowed for detailed analysis and systematic diffractogram WC (hc) Co (hc) Co 3 W 3 C (fcc) Co 6 W 6 C (fcc) Theta ( ) Figure 2 Diffractogram of sample sintered at 1350 C. 2695

5 For all the diffractogram for overlapping peaks, and manifestations of different phases with peak values at 2 very close. Peaks with scores below 80 cps show a certain characteristic of enlargement phases are not well formed, resulting in small differences in position 2. These differences are part of the identity of this material for this temperature range, where complex structures originating from the WC and Co phases are formed, and the phases Co 3 W 3 C Co 6 W 6 C. Fig. 3 shows the image of the microstructure with increasing 7000x obtained by SEM with secondary electrons (SE). It can be observed the morphology of phases with the identification of areas where analysis was performed by EDS point. Tab. 2 to 6 show the results of semiquantitative chemical analysis point. Which confirm the presence of gray areas with high concentrations of W indicating the chemical composition of WC grains and dark gray regions in the presence of Co almost always accompanied by a proportionate amount of W (atom%) indicating the possible formation of phases. Figure 3 Microstructure of sample sintered at 1350 C, with the identification of areas where we made the point by EDS microanalysis. It is noteworthy that with the formation temperature of the eutectic Co forms a "fine layer", percolating grains of WC. What is noticed by the values of Tab. 2 to 6 of the chemical analysis carried point with EDS, and frequent mutual presence of the elements W and Co with proportionate quantities and variations, indicating the possible formation of phases and the presence of grains of WC. 2696

6 Table 2 Semiquantitative chemical analysis of the sample sintered at 1350 C, microanalysis referring to mode point using the ZAF correction method for point 1 in Fig. 4. Point 1 Element Intensity Wt % Atom % K-value Z A F C 4,077 12,701 64,464 0, , , ,00000 Co 1,629 9,378 9,700 0, , , ,90418 W 2,201 77,921 25,836 0, , , ,00481 The point 1 has a dark gray color typical of Co rich regions and possibly phases. This point reveals a thin layer of binder percolating grain WC, possibly where the electron beam struck the structure of a grain of WC just below this fine layer with its volume of interaction. Table 3 Semiquantitative chemical analysis of the sample sintered at 1350 C, microanalysis referring to mode point using the ZAF correction method, to point 2 in Fig. 4. Point 2 Element Intensity Wt % Atom % K-value Z A F C 3,628 11,148 60,315 0, , , ,00000 Co 2,050 11,052 12,186 0, , , ,90655 W 2,310 77,800 27,499 0, , , ,00585 The point 2 shows a typical gray region of WC grains, with the greatest amount of C followed by W, in atom%, and with the help of the micrograph, morphologically can indicate the formation of WC grain. The amount of Co indicated in the table to this point can be justified by the fine layer covering the WC grain, leaving her lightly darkened shade of gray. Table 4 Semiquantitative chemical analysis of the sample sintered at 1350 C, microanalysis referring to mode point using the ZAF correction method, to point 3 in Fig. 4. Point 3 Element Intensity Wt % Atom % K-value Z A F C 3,458 10,802 57,014 0, , , ,00000 Co 2,933 16,730 17,997 0, , , ,91498 W 2,046 72,468 24,989 0, , , ,

7 With help of Fig. 4, we can see that the region of section 3 is more homogeneous than the area of point 1. At point 1 it is verified and contrasts different possible grain boundaries in this region. While the point 3, also has a unique tone, but does not show possible grain boundary compared to point 1. We have to point 1 may represent the start of the interaction between the elements, triggering the formation of phases. By observing this contrast regions with more homogeneous, the possibility of increased interaction between the elements may have occurred, with the diffusion between the elements of W and Co indicating the possible formation of additional phases. Table 5 Semiquantitative chemical analysis of the sample sintered at 1350 C, microanalysis referring to mode point using the ZAF correction method, to point 4 of Fig. 4. Point 4 Element Intensity Wt % Atom % K-valoe Z A F C 3,096 10,808 56,552 0, , , ,00000 Co 2,766 17,886 19,074 0, , , ,91665 W 30,712 71,306 24,374 0, , , ,00000 The 4 point, the region with more homogeneous shade of gray, shows similar values of quantities in atomic percentage of W and Co, indicating the possibility of formation of phases. Table 6 Chemical analysis of semiquantitative sample sintered at 1350 C, microanalysis referring to mode point using the ZAF correction method, to the point 5 in Fig. 4. Point 5 Element Intensity Wt % Atom % K-value Z A F C 2,711 8,136 47,880 0, , , ,91976 Co 3,802 20,616 24,727 0, , , ,91976 W 2,098 71,248 27,393 0, , , ,01144 The 5 point another example that shows a dark gray region with proportionate amounts of Co and W, possibly indicating the formation of phases. The cemented carbides are sintered by liquid phase, and after being hit the eutectic temperature, cobalt liquid will flow to wet and form a film around the solid particles of WC (14). 2698

8 With the grinding and polishing the surface of the samples was the formation of fine layers which allow the interaction volume generated by the electron beam exciting transposed another structure just below the chosen point. May be a region with different composition of the expected when it evaluated the region of the beam by the shade of gray. Fig. 4 shows the image of the microstructure obtained by SEM in secondary electron with increasing 3000x. Analysis was performed by EDS mapping by X-ray characteristic of C, Co and W. Figure 4 Mapping by X-ray emission characteristic of the sample sintered at 1350 C. Figure 5 shows the characteristic X-ray emission for the selected area, noting the good dispersion of the elements in the sample, and for the elements W, Co and C, these are superimposed, but in some areas are higher amounts of W to Co. Even though there are regions with higher concentrations of W or Co, the interaction between these elements is intense, while C is noticed in all these regions. This interaction between the elements of the composite it help the process of formation of phases. Table 7 Chemical analysis of semiquantitative sample sintered at 1350 C, microanalysis on the mapping using the ZAF correction method to fig. 5. Compositional analysis Element Intensity Wt % Atom % K-value Z A F C 2,189 7,330 49,753 0, , , ,00000 Co 1,751 9,739 13,472 0, , , ,90395 W 2,409 82,931 36,775 0, , , ,

9 Table 7 shows the percentage by weight values that maintain the relation of the composite with 90% WC and 10 wt% Co indicating good mixing of the powders during the homogenization of the components of the composite. CONCLUSIONS The technique of X-ray diffraction was utilized for qualitative analysis phase, where the phases were identified WC hexagonal primitive, the primitive hexagonal phase Co and also the significant contribution to the diffractogram phases, and the Co 3 W 3 C Co 6 W 6 C both cubic face centered. Micrographs of starting powders of WC and Co showed different morphologies and particle sizes, and the powders of WC nano particles were smaller and more agglomerated than the post of Co. Analysis using SEM / EDS showed the variation of the chemical composition of the powders, being relatively low and possibly incapable of to form structures which could be identified in the diffractograms. With SEM performed on sintered samples was obtained micrographs, which revealed good sintering with small pores and circular in shape, grain morphology WC showing more united and not so rectangular regions with a predisposition to formation of phases rich regions in Co. Through EDS confirmed the distribution of elements of C, W and Co of form homogenous, but able to identify areas rich in W and Co, as well as grains of WC and Co lakes with the possible formation of phases. The study of the micrographs 3 and 4 associated with the XRD pattern of sintered sample, as they relate to the phases, is based on information from X-ray diffraction, which, although of a qualitative order. Indicate little contribution to the peaks of the Co phase when compared with representatives of the peaks in the XRD pattern phases, suggesting intense amount of these phases in the sample. REFERENCES [1] Fang, Z.; Maheshwari, P.; Wang, X.; Sohn, H. Y.; Griffo, A.; Riley, R. Int. J. Ref. Met. & H. Mater. (2005). [2] Schubert, W.D.; Neumeister, H.; Kinger, G.; Lux, B. Int. J. Ref. Met. & H. Mater (1998). [3] Milheiro, F.A. Dissertação de Mestrado. PPGECM/CCT/UENF.91p. (1996). [4] Schubert WD. (2000). In: 2000 International conference on tungsten hard metals and refractory alloys, Annapolis, MD, USA. 2700

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