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ARTICLE IN PRESS+ModelECS-9042; No. of Pages 6

Available online at www.sciencedirect.com

Journal of the European Ceramic Society xxx (2013) xxx–xxx

Nanoindentation of WC–Co hardmetals

Annamaria Duszová a, Radoslav Halgas a,b, Marek Bl’anda a,b, Pavol Hvizdos a,Frantisek Lofaj a, Ján Dusza a,c,∗, Jerzy Morgiel d

a Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04353 Kosice, Slovak Republicb Faculty of Material Science and Technology of STU, Paulínska 16, 917 24 Trnava, Slovak Republic

c Óbuda University, Donát Bánki Faculty of Mechanical and Safety Engineering, Népszínház Street 8, 1428 Budapest, Hungaryd Institute of Metallurgy and Materials Science of Polish Academy of Sciences, Reymonta 25, 30 059 Krakow, Poland

bstract

C–Co cemented carbide has been investigated using instrumented indentation with maximum applied loads from 0.1 to 10 mN. The hardnessnd indentation modulus of individual phases and the influence of crystallographic orientation of WC on the hardness and indentation modulusave been studied. The hardness of the Co binder was approximately 10 GPa and that of WC grains up to 50 GPa with relatively large scatter underhe indentation load of 1 mN. Investigation of the role of crystallographic orientation of WC grains on hardness at 10 mN load revealed average

alues of HITbasal = 40.4 GPa (EITbasal = 674 GPa) and HITprismatic = 32.8 GPa (EItprismatic = 542 GPa), respectively. The scatter in the measured valuest low indentation loads is caused by the effects of surface and sub-surface characteristics (residual stress, damaged region) and at higher loads bymix-phase” volume below the indenter.

2013 Elsevier Ltd. All rights reserved.

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eywords: WC crystals; Nanoindentation; Hardness; Load-size effect; Orienta

. Introduction

Cemented carbides are widely used as cutting, forming andachining tools in different areas of industry because of their

igh hardness and strength, good fracture toughness and excel-ent wear resistance.1 This is due to their complex compositetructure of interpenetrating networks of a hard, brittle car-ide phase, usually WC, and a tough metallic binder, usuallyo, with dissolved tungsten and carbon. Structurally, dilute Colloys including Co–W–C can exist in either of two allotropicorms, hcp or fcc. The ratio of these two forms is determined byrocessing treatment and composition. Both tungsten and car-on stabilize the fcc phase and in most hardmetals, the binder isresent largely in fcc form.2

The physical properties of tungsten carbide are generally

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nown very well; WC is a non-oxide ceramic where hexago-al closely packed layers of W atoms are separated by closelyacked layers of C filling one-half of the interstices, giving

∗ Corresponding author.E-mail address: jdusza@imr.sske.sk (J. Dusza).

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955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2012.12.018

ffect

ise to a six-fold trigonal prismatic coordination for the atomictructures.

The lattice shape is hexagonal, with lattice parameters = 0.2906 nm and c = 0.2837 nm.2 The WC grains generatehree types of facets: two types of prismatic facets and theasal (0 0 0 1) facet which delimit the flat triangular prismFig. 1).3

The individual grains of WC within the WC–Co aressentially single crystals, each with orientation-dependentechanical properties. Understanding this orientation depend-

nce is important in optimizing microstructures for enhancedombinations of hardness, toughness and wear resistance,.g., in the case of composites with the preferred orientatedrains.

A number of studies have been devoted to characterization ofhe effect of the microstructure of WC–Co system on its hard-ess and the effect of the crystallographic orientation of WCingle crystals on their hardness.4−8 French and Thomas5 usednoop indentations to indent single crystals at basal and pris-atic planes and found that the Knoop hardness could vary by

entation of WC–Co hardmetals. J Eur Ceram Soc (2013),

factor of 2 for indentations of the prismatic planes. Knoopardness values were higher in the basal plane with valuesrom 2300 to 2500. On the prismatic planes (1 0 1 0) the hard-ess varied from 1000 to 2400 as the direction of the indent

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The nanoindentation tests have been performed usingBerkovich diamond indenter with a tip radius less than 20 nm

ig. 1. Crystal lattice of WC with C atoms at (1/3, 2/3, 1/2) sites, while the (2/3

eclined from the [0 0 0 1] direction. Takahashi and Freise6 usedickers microhardness measurements at an applied load of 1 kgfnd reported values of 2100 for the basal plane and 1080 for the1 0 1 0) prismatic plane. Pons7 used Vickers microhardness at aoad of 0.1 kgf and reported hardness of 1950 and 1360 for theasal and prismatic planes, respectively.

Gee et al.8 were possibly the first who used instrumentedanoindentation for determination of the mechanical propertiesf constituent phases of WC–Co on a local scale. They foundifficulty in the mapping of individual phases owing to uncer-ainties in stage positioning and reliability of software during the

easurement, but found the technique promising to yield valu-ble information about the in situ properties of different phases.onache et al.9 used nanoindentation in a depth-controlled

egime with very shallow nanoindentations (30 nm depth) toeasure hardness and Young’s modulus of the constituents of

he WC–Co composite. They reported a hardness and indenta-ion modulus for the WC prismatic planes (1 0 1 0) being withinhe range of 40–55 GPa and 700–900 GPa respectively. Thesealues decrease to a hardness in the range of 25–30 GPa and aodulus in the range from 450 to 550 GPa for the basal plane

0 0 0 1).Recently, Cuadrado et al.10 used a Berkovich diamond inden-

er with loads up to 0.25 N to measure the hardness of individualrystals of WC in a WC–Co system. Electron backscatter diffrac-ion (EBSD) techniques were used to obtain individual crystalrientations and the hardness values were measured for basallanes (0 0 0 1) and prismatic (1 0 1 0) and (1 1 2 0) planes of WCrystals. Hardness values for 20 GPa (basal plane) and 17 GPaprismatic plane) were obtained.

More recently, Roebuck et al.11 applied depth-sensing micro-ardness mapping to measure the variation of microhardnessith an applied load and orientation of WC crystals of approxi-ately 50 �m in size, embedded in a copper alloy matrix. They

ound that the most significant effect on microhardness of WCas a deviation angle between the plane of measurement andither the basal or prismatic planes. The grains with a plane closeo the basal plane (0 0 0 1) were found to be considerably harder

Please cite this article in press as: Duszová A, et al. Nanoindhttp://dx.doi.org/10.1016/j.jeurceramsoc.2012.12.018

approx. 55 GPa at 0.4 N load) in comparison to prismatic planesapprox. 25 GPa at 0.4 N). Obviously, these results are contraryo the results of Bonache et al.9 which can be explained by the

1/2) sites are empty (a), shape of WC grains in the WC–Co system (b).3

ifferences in the size of the investigated WC grains and/or inpplied indentation loads, respectively.

The aim of the present contribution is to evaluate the hardnessf individual phases of WC–Co systems and the hardness andndentation modulus of WC crystals as a function of orientationsing instrumented nanoindentation in a load range from 0.1 mNo 10 mN.

. Experimental procedure

The experimental material was supplied by Pramet SumperkCzech Republic). The microstructure has been evaluated usingtandard metallographic procedures (cutting, grinding, pol-shing, etching) and scanning electron microscopy (SEM)bservation (Fig. 2). The microstructure parameters of thenvestigated WC–Co composite are: volume fraction of binder,Co = 10.7; mean grain size of WC, DWC = 1.7 �m, meanree path in binder, LCo = 0.3 �m and contiguity, CWC = 0.37.rior to nanoindentation testing, the samples were alsoround and polished, with a final step using 0.5 �m diamond

entation of WC–Co hardmetals. J Eur Ceram Soc (2013),

Fig. 2. Microstructure of the WC–Co cemented carbide investigated.

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A. Duszová et al. / Journal of the Euro

n the Nano Hardness Tester (CSM-Instruments SA) andn Nano Indenter G200 (Agilent Technologies) fitted with aanovisionTM module.The CMC (Continuous Multi Cycle) method under the

oads of 0.25, 0.5, and 1.0 mN has been applied to measurehe load-size effect using arrays of 20 × 20 indents. Singleoading–unloading cycles in a load controlled regime and depthontrolled regime (1 mN and 30 nm) were applied for nanohard-ess study of the individual phases with small horizontal andertical dimensions and for the study of the orientation effect ofC crystals on their hardness. Single indentations with higher

oads up to 10 mN were used to minimize the influence of the sur-ace effects when the influence of crystallographic orientation of

C planes on hardness was studied. The indentation hardness,IT, and indentation modulus, EIT, have been automatically

alculated using the Oliver–Pharr method.12

Additional SEM and atomic force microscopy (AFM) obser-ations were used to visualize the topographical details ofndividual indents. The indents in individual phases have beenoupled with the corresponding load–displacement diagramso recognize the effect of the microstructure and crystal-ographic orientation of WC crystals on hardness. EBSDnalyses have been used for the identification of the crys-allographic orientation of WC crystals. TEM investigationas used for the study of the microstructure of the subsur-

ace area of the investigated samples. The FIB technique waspplied for the preparation of thin foils below the polishedurface.

. Results and discussion

In Fig. 3 three different load–displacement curves are illus-rated during the CMC tests under the maximum load of 1 mN.hese curves correspond to the tests with a maximum penetra-

ion depth of around 35 nm, then of around 40 nm and finally

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hose with the depth of approximately 70 nm. The residualepths of these curves are approximately 20 nm, 28 nm and5 nm, respectively. The coupling of the indentation curves with

ig. 3. Characteristic load–displacement curves of indents in individual phasesf WC–Co.

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ig. 4. AFM image of the indent in binder phase after indentation with a 10 mNoad.

he individual indents using SEM and AFM revealed that thendents with the highest penetration depth and the lowest hard-ess of approximately 10 GPa belong to indents in the Co phase.he curves with smaller displacements and hardness valuesbove ∼20 GPa can be attributed to WC grains with differ-nt orientations. The indents with hardness values between 10nd 20 GPa were usually located not exactly only in one indi-idual phase: e.g., even if they were originally only in a Coinder, the indenter may hit a WC grain located under the sur-ace or horizontally when the indent is close to a WC grain.his effect was even more visible at higher indentation loads,ig. 4.

In Fig. 5 the hardness values obtained at three indentationoads from 0.25 mN to 1 mN are summarized. A decrease inardness with load increase was found. This indentation loadize effect (ISE) is obvious in the WC grains, however, it isegligible in the binder phase.

entation of WC–Co hardmetals. J Eur Ceram Soc (2013),

Single loading–unloading cycles at the load-controlledegime and depth-controlled regime (1 mN and 30 nm) werepplied for study of the orientation effect of WC crystals on their

ig. 5. Load-size effect during the CMC indentation of different phases ofC–Co system at loads of 0.25, 0.5 and 1.0 mN.

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Fig. 6. AFM images of indent in planes of WC close to basal (a) and pris

ardness. In some cases it was easy to recognize the crystallo-raphic orientation vs hardness relationship, but in the majorityf indents it was not unambiguous. In Fig. 6, indents createdsing a 1 mN load in the planes close to the basal and pris-atic plane are illustrated using AFM technique. According

o the corresponding load–displacement curves these exhibitimilar hardness values, therefore the effect of orientation wasimited. One of the reasons for this behavior could be com-ressive residual stresses in WC–Co systems after grinding.hey were reported to be in the range level of 1.0–1.5 GPa, andecrease after polishing and heat treatment to values between50 and 200 MPa.2,13 High residual stresses can overshadowhe influence of crystallographic orientation of WC crystals onardness under a low indentation load/depth. Similarly, the dis-ocation structure in WC grains, as the result of ceramographicreparation of the surface, can influence hardness results at low

Please cite this article in press as: Duszová A, et al. Nanoindhttp://dx.doi.org/10.1016/j.jeurceramsoc.2012.12.018

oads. Indeed, such dislocation networks have been identified byEM in WC grains in the subsurface area in our samples, afterolishing and prior to indentations (Fig. 7).

ig. 7. Dislocations in WC grains in the sub-surface area of a polished WC–Courface.

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(b) plane after indentation with a 1 mN load with very similar hardness.

To minimize the surface effects, a higher load of 10 mN wassed for clear measurement and statistical evaluation of thenfluence of the crystallographic orientation of WC grains onardness. A composite image, Fig. 8, showing indents createdsing a 10 mN load in the planes close to the basal and prismaticlanes together with their corresponding load–displacementurves. The measurements on approximately 25 indents, whichere considered sufficient for statistical purposes, revealed

he average hardness of the basal planes and prismatic planess 40.4 ± 1.6 GPa and 32.8 ± 2.0 GPa, respectively. The cor-esponding average values of the indentation modulus forasal and prismatic planes are 674 ± 14 GPa and 542 ± 34 GPa,espectively.

These results are in good agreement with the results ofuadrado et al.10 as regarding the influence of the crystallo-raphic orientation on the hardness of WC crystals. They usedigher applied loads in comparison to our experiment with anndentation depth between 600 nm and 800 nm, and reportedardness for basal and prismatic planes of the WC crystals5.6 GPa and 17.2 GPa, respectively. Taking into considerationhe load-size effect, these values are not only in agreement withur results as regarding the effect of the crystallographic orienta-ion of WC crystals on their hardness, but the absolute hardnessalues are similar as well. Our results show a similar influence ofhe orientation of WC crystals on indentation moduli as for hard-ess. This is different in comparison to the results of Cuadradot al.10 who found a slightly higher indentation modulus for pris-atic planes than for basal planes of WC crystals. Our results

re also in good agreement with the results of Roebuck et al.s regarding the tendency of the hardness vs crystallographicrientation of WC crystals.11 They used similarly higher inden-ation loads in the range from 100 mN to 500 mN and reportedardness of basal and prismatic planes of WC crystals with sizepproximately 50 �m at indentation load of 400 mN 55 GPa and5 GPa, respectively. Similarly as in our experiment at nano-evel, they found a load-size effect at a higher load/depth interval,

entation of WC–Co hardmetals. J Eur Ceram Soc (2013),

hich was most evident on basal planes of WC crystals whereardness values increased from 50 GPa to 120 GPa when inden-ation loads decreased from 500 mN to 100 mN. The very high

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ardness values in this case are probably the result of the non-tandard instrumented indentation used. Bonache et al.9 used aimilar indentation procedure as in the present investigation andeported values of hardness and an indentation modulus of WCrystals in a similar range as in our case, with hardness from5 GPa to 55 GPa and modulus from 400 GPa to 900 GPa. How-ver, they found higher hardness and indentation modulus forlanes close to prismatic planes in comparison to the hardnessf basal planes. Clarification of the effect of crystallographicrientation of WC crystals on hardness and indentation modu-us at very low load/depth conditions, where the preparation ofhe surface of samples is crucial, will be the subject of our futurenvestigation.

. Conclusions

Hardness of individual phases of the WC–Co system and thenfluence of crystallographic orientation of WC crystals on theirardness and indentation modulus was investigated.

Indentation loads below 1 mN are suitable for mapping thehardness of individual phases in WC–Co systems. An evi-dent load-size effect was found at the nano level during theindentation of WC grains at loads up to 1 mN;

significant influence of the crystallographic orienta-tion of WC crystals on the hardness and indentation

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modulus has been found at the 10 mN load withvalues of HITbasal = 40.4 GPa, EITbasal = 674 GPa andHITprismatic = 32.8 GPa, EItprismatic = 542 GPa, respectively.

n of WC on hardness at the indentation load of 10 mN.

scatter in the measured results at low indentation loads canbe attributed to residual stresses and dislocation networksdeveloped in the surface and subsurface zone during thegrinding/polishing and to “mixed-phase” effect in the volumebelow the indenter at higher loads.

cknowledgements

Work was supported by the project Slovak Grant Agencyor Science, grant no. 2/0122/12 by the NanoCEXmat I. ITMSo: 262200120019, NanoCEXmat II, ITMS no: 26220120035,eKSiM, ITMS no: 26220120056 APVV-0520-10, APVV-04206, LPP-0174-07 and CE SAS CLTP-MREC.

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nnamarya Duszová is a PhD student at the IMR SAS working in the area ofesearch “Indentation testing of hardmetals and advanced ceramics”. She is theuthor of 10 scientific papers which have been cited 52 times.