Densification and FIB, SEM, TEM Microstructures of WC Composites with Fe or Co Matrices

Available online at www.sciencedirect.com

ScienceDirect Procedia Materials Science 8 (2015) 406 – 413

International Congress of Science and Technology of Metallurgy and Materials, SAM CONAMET 2013

Densification and FIB, SEM, TEM Microstructures of WC Composites with Fe or Co Matrices Esteban A. Alvareza,b,*, Carlos González Olivera,c, Flavio Solderad, José L. Garcíae a

Centro Atómico Bariloche, Av. E. Bustillo 9500, Bariloche 8400, Argentina. b ANPCYT, Av. Córdoba 831, Bs. As. 1054 , Argentina. CONICET, Av. Rivadavia 1917, Bs. As. 1033 , Argentina. d Functional Materials, Saarland University, Saarbrücken, Alemania. e Sandvik Tooling Sverige, Estocolmo, Suecia. *e-mail (author): [email protected] c

Abstract The present work refers to results obtained in the synthesis of tungsten carbide embedded in a metal matrix such as Co or Fe, through different routes and from various precursors. The two principal methods were, an aqueous, and the other one, a sol-gel using alkoxides. Heat treatments were carried out in an alumina oven with SiC heating elements, and covered different heating schedules under several atmospheres (gas flows and vacuum). Resulting phases after the different heat treatments were studied by X ray diffraction (showing the formation of desired phases, WC and the metal matrices Fe or Co) and final microstructure waqs analyzed through techniques suchs as SEM (Scanning electron microscopy), FIB (Focused ion beam) and TEM (Transmission electron microscopy). It was observed that sol-gel internally precipitated WC grains (within a matrix, rich in Fe, and having excess C) had average dimensions of 2 microns. Finally, microhardness (Vickers) values in the range between 937 and 1242 HV were measured for the various kinds of composites. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the scientific committee of SAM - CONAMET 2013. Selection and peer-review under responsibility of the scientific committee of SAM - CONAMET 2013 Keywords: Densification, WC, Metal matrix composite materials

1.Introduction Tungsten carbide (WC) embedded in a metal matrix is a metal matrix composite (MMC), broadly used in the fabrication of cutting tools, coatings and parts subjected to wear, González Oliver, Alvarez et al. (2012), because of their high hardness, toughness and wear resistance.

2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the scientific committee of SAM - CONAMET 2013 doi:10.1016/j.mspro.2015.04.091

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This work exhibits various results concerning to samples of WC in a metal matrix such as iron (Fe) or cobalt (Co). Composites having an iron matrix were produced by a sol-gel route, using alkoxide percursors. The other composites with cobalt matrices, were obtained by two different routes, an aqueous and a solid one (both of them from different precursors). A significant aspect of the aqueous route is that the WC composite is obtained after a heat treatment carried up to 800°C, while the other methods require higher temperatures, above 1250°C. Then, a comparison between Co and Fe samples is made in terms of grain size, grain distribution and Vickers hardness (due to the importance of this property, hardness was measured for these composites and are compared with those used in cutting industry). Besides, other results include micrographs obtained by diverse techniques such as SEM (Scanning electron microscopy), TEM (Transmission electron microscopy) and sections done with FIB (Focused ion beam). 2.Experimental Various analyses were made over samples obtained by different synthesis routes: x Sol-gel route: Using alcoxides, tungsten (W), iron (Fe) and carbon (C), a powder mix is produced and then subjected to a specific heat treatment , sintered and the in-situ formation of WC in the metal matrix, González Oliver et al. (2012). x Aqueous route: Through tungstic acid, a Co precursor, a carbon source and water as solvent, another mixture is made and subjected to heat treatment to produce the WC composite. x Solid route: From WC powder (4NPO, Starck, with a specific area of 4 m2/g) and Co powder, the WC composite is produced by similar heat treatments. The initial 4NPO powder has submicron grain size. The next table lists the samples under study and the final temperature and time of the heat treatments under vacuum (except for E1(800), which was only under 10% H2 -Ar flow) Table 1. Samples under study. Sample I-4NPO II-4NPO III-4NPO M1b M1b2 E1(800)

Route 4NPO 4NPO 4NPO Sol-gel Sol-gel Acuosa

Composition 20% P/P Co 20% P/P Co 20% P/P Co 20% P/P Fe 20% P/P Fe 20% P/P Co

Heat treatment 1364ºC, 2hs 1364ºC, 4hs 1364ºC, 6hs 1250ºC, 2 hs 1093ºC, 2hs 800ºC, 2hs

Sintering involves two principal aspects: the densification of the compact mass and grain-growth. Regarding this last issue, while some grains increase their size, others must shrink and disappear. The driving force for sintering is the surface free energy excess that a material has when it is divided into many small grains. It can be assumed that the surface energy of the powder that forms the compact is the actual excess energy, Budworth (1970). To analyze the size of the grain as a function of time and temperature at which the samples are subjected, polishing of these ones are made with diamond impregnated polishing cloths and lapping films, both suitable for hard materials. The last stage consists of using diamond paste of 1 micron. Then, the specimen-images were obtained by SEM and the grain growth of WC was studied. Regarding the dilatometric test, a sol-gel sample was studied in a vertical dilatometer to analyse the densification of such material as a function of temperature. The dilatometric curve obtained represents (L(t, T)-Lo)/Lo as a function of temperature “T”, where “L” is the linear length (pellet height) and “Lo” is the initial thickness. In this case, the sample was subjected to a heat treatment with 10% H2 -Ar up to 1093°C (at a heating rate of 5 °C/min ). A high hardness is characteristic of the WC composites used in industry. Several specimens were tested for Vickers microhardness, to estimate these values. The first diagram in Fig. 1 shows the indenter acting over the sample surface, and the angles between the opposite faces. On the other hand, below that diagram, it is shown the pyramidal form of such indenter, Gordon England (2013).

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Fig. 1. Indenter

Several microhardness indentations were made on these samples, to do a statistic and estimate a mean value for each of them. 3.Results and Discussion 3.1 Sol-gel route In this section are analyzed the results corresponding to M1b, obtained by the sol-gel route through a heat treatment up to 1250°C. The diffractogram (Fig.2) shows the peaks associated with WC, Fe and C phases. This last one is associated to some excess of carbon level (it was added as solid C powder) relative to the stoichiometric value as required to precipitate WC. 800

1 - WC (025-1047) 2 - Fe (006-0696) 3 - C (025-0284)

1

Int. a.u.

600

400

3

2 1

1

1

200

0

30

40

50

60

70

2T(°) Fig. 2. Diffractogram of M1b sample.

Fig. 3 was taken by STEM, and various zones are detected. The sample under study consists of a cut (fraction of a micron in thickness) made by FIB. Region I indicates a WC grain; the larger ones have sizes of 2 microns. It can be observed that this grain is touching another WC grain and these are surrounded by the Fe matrix. Regarding region II, a large Fe grain can be seen. In third place, region III exhibits a metal interface and a clear division between two Fe grains.

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Fig. 3. STEM micrograph of M1b sample (FIB cut).

Going further in the analysis of this sample, Figs. 4 and 5 show different pictures with various grain boundaries of the metal matrix composite.

Fig. 4. TEM micrograph of M1b sample.

Fig. 5. STEM micrograph of M1b sample.

Two separated WC grains of different shapes can be observed in Fig. 4 (the light gray area corresponds to the metal matrix). The distance between the two parallel boundaries of these grains is about 350 nm. Besides, some defects which are possibly dislocations can be seen in the bigger WC grain. Three large dark WC grains embedded in a Fe matrix are shown in Fig. 5. An interface (grain boundary) between two of them can be observed, and another one between two Fe grains (gray and light-gray) is clearly defined. 3.2 Aqueous route In this case the E1 (800) sample is analyzed. The X-rays diffraction pattern (Fig.6) exhibits the peaks associated to WC, Co and C.

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1 - WC (072-0097) 2 - Co (015-0806) 3 - C (012-0212)

1 3000

1 Int. a.u.

2250

1

1500 750 0

3

2 30

40

1

2 50

2T(°)

60

Fig. 6. Diffractogram of E1(800) sample.

70 Fig. 7. STEM micrograph STEM of E1(800) sample.

Similarly to the sol-gel case, Fig.7 shows WC (II) grains distributed within the Co matrix (I). Again, the grains have a few microns in size. 3.3 Qualitative grain growth analysis Three samples, “I-4NPO”, “II-4NPO” and “III-4NPO”, produced by solid route, were subjected to heat treatments at 1364°C, for 2, 4 and 6 hours, respectively. The qualitative grain growth study was made from these micrographs (Figs.8-10).

Fig. 8. SEM micrograph of sample I-4NPO (2 hs).

Fig. 9. SEM micrograph of sample II-4NPO (4 hs).

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Fig. 10. SEM micrograph of sample III-4NPO (6 hs).

In the analysis of these figures, strong differences in grain sizes are observed between sample I-4NPO and the others II-4NPO and III-4NPO. These last two, have larger grains than the first one. Although no big differences are detected between II-4NPO and III-4NPO, by increasing the time (from 4 to 6 h) at final temperature, it is obtained larger grains grow at the expense of shrinkage and disappearance of others; like a kind of coarsening effect, Herber et al. (2006), in this case driven by the probable existence of a liquid phase, rich in Co, at these temperatures, Upadhyaya and Bhaumik (1988). 3.4 Dilatometry The dilatometric curve in Fig.11 shows the densification of a sol-gelM1b2 (pressed pellet) sample at a controlled heating rate of 5ºC min-1 up to 1093°C. M1b2

'l/lo

0,0

-0,1

-0,2

0

200

400

600

800 1000 1200

T [ °C ] Fig. 11. Densification under H2 -Ar flow up to 1093ºC.

At low temperatures, approximately 100°C, a light expansion is detected and this can be associated to the PVB added as a binder in the pressing stage. Then, a densification up to 540°C is observed, and then two more at 550°C and 850°C, respectively. From 900°C, a strong densification takes place and is supposed that the WC is being formed at this point. With increasing temperature, the slope gets flat, where W might be produced because of the reduction of WC. Finally, the system shows another clear densification until it reaches 1093°C. The suggested interpretation of such behaviour is that up to 950ºC the formation of WC or subcarbides is obtained. Then, for

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T!950ºC the carbides are reduced to W metal, possibly doped with iron, as shown by the XRD analysis (not shown here). 3.5 Vickers Hardness Table 2 (a.u.: arbitrary units) indicates the partial results and the final mean value for Vickers hardness (HV) estimated for pellet of WC in a Co matrix, which was subjected for 6 hs up to 1364°C. Table 2. Vickers microhardness in sample III (WC in Co matrix). Test

Diagonal 1[u.a.]

Diagonal 2 [u.a.]

Diagonals average value [u.a.]

Vickers hardness [HV]

1

112

2

108

106

109

1249

115

111,5

3

1193

115

114

114,5

1132

4

109

107

108

1272

5

105

107

106

1320

6

108

107

107,5

1284

Final average number

1242 HV

The final values for all samples tested are given in the following table: Table 3. Final values for Vickers microhardness. Sample I III M1b2

Route/Heat treatment Solid 4NPO / 2hs up to 1364ºC Solid 4NPO / 6hs up to 1364ºC Sol-gel / 2hs up to 1364ºC

Vickers hardness [HV] 1017 1242 937

Four indentations are shown in Fig. 12. Besides, through the picture can be seen that the indentations size is approximately 30 microns (diagonals):

Fig. 12. Hardness tests on sample III (WC in Co matrix).

Finally, comparing these results with those taken as reference, the Vickers hardness for a WC/Co composite used as cutting tool is in the range 1550-1700 HV, J. Gurland (1969), Goodfellow (2013).

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4.Conclusions Several WC composites with metal matrix of Co or Fe samples were obtained by various synthesis routes implying partial reduction in H2 -Ar flow and vacuum. Micrographs allowed the analysis of their microstructure, and to see the grain distribution in the metal matrix, their shape and size. Besides, comparing the grain dimensions for the three synthesis routes, it is concluded that there are not strong differences and oscillate in a range of a few microns. Besides, through X-rays diffraction the phases of each one of the samples were determined. The effect of time exposure of the samples at the final temperature, could be seen. Going from two to four hours, produce an increase in WC grain size. Finally, Vickers hardness was estimated for three samples, and one of these values slightly differs from those of composites used as cutting tools in industry (the closest value corresponds to a 4NPO sample, with 1242 HV). Acknowledgements Acknowledged are Mr. Miguel Sanfilipo (Nuclear Materials, CAB), Mrs. Paula Troyon and Mrs. Carolina Ayala (U.A. TEMADI, CAB) for their important collaboration. Besides, this work was carried out through “PICT-20101759” (Scientific and Technological Promoting National Agency (a governmental entity dependent of the Ministry of Science, Technology and Productive Innovation), International cooperation project “NanoCom Network” and finally, the C372 project of the National University of Cuyo. References González Oliver, Carlos J.R., Alvarez Esteban Alejandro, et al., 2012. Densificación de compuestos Fe/Co - WC sinterizados bajo atmósfera de H2-Ar. SAM-CONAMET, p.2. González Oliver, J.RC, et al., 2012. Formation of nanocrystalline phases in sol-gel masses in the Fe- W-C(O) system and densification up to 1100ºC. Procedia Materials Science 1 (2012), p.95 – 103. Budworth, D.W., 1970. An introduction to ceramic science. Pergamon Press. Gordon England, 2013. Available: http://www.gordonengland.co.uk/hardness/vickers.htm Herber, R.P., Schubert, W.D., Lux, Benno, 2006. Hardmetals with rounded WC grains. Int. J. of Ref. Metals & Hard Mat., 24 (2006), p.360-364. Upadhyaya, G.S., Bhaumik, S.K., 1987. Sintering of submicron WC-10wt.%Co Hard Metals Containing Nickel and Iron. Materials Science and Engineering, A105/106 (1988), p.249-256. Gurland, J., 1969. Microstructural aspects of the strength and hardness of cemented tungsten carbide, p.3. Goodfellow, 2013. Available: http://www.goodfellow.com/A/Tungsten-Carbide-Cobalt-Ceramic.html