Influence of electrical discharge machining on the reciprocating sliding friction and wear response of WC–Co cemented carbides

Influence of electrical discharge machining on the reciprocating sliding friction and wear response of WC–Co cemented carbides

Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & H...

1MB Sizes 0 Downloads 29 Views

Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Influence of electrical discharge machining on the reciprocating sliding friction and wear response of WC–Co cemented carbides K. Bonny a,*, P. De Baets a, W. Ost a, S. Huang b, J. Vleugels b, W. Liu c, B. Lauwers c a

Ghent University (UGent), Department of Mechanical Construction and Production, IR04, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium Catholic University Leuven (K.U. Leuven), Department of Metallurgy and Materials Engineering, MTM, Kasteelpark Arenberg 44 (Bus 2450), B-3001 Leuven, Belgium c Catholic University Leuven (K.U. Leuven), Department of Mechanical Engineering, PMA, Celestijnenlaan 300 B (Bus 2420), B-3001 Leuven, Belgium b

a r t i c l e

i n f o

Article history: Received 27 April 2008 Accepted 5 September 2008

Keywords: WC–Co cemented carbide Wire-EDM Dry reciprocative sliding wear Pin-on-flat

a b s t r a c t The dry friction and wear behavior of a number of WC–Co cemented carbides, exhibiting 6 up to 12 wt.% cobalt as binder phase and average carbide grain sizes ranging from 0.3 up to 2.2 lm, combined with various surface finishing variants of wire electrical discharge machining (EDM), was evaluated by performing linearly reciprocating pin-on-flat sliding experiments against WC–Co cemented carbide using normal contact loads of 15 N up to 35 N and a sliding velocity of 0.3 m/s. Post-mortem obtained wear volumes and volumetric wear rates were correlated to real-time recorded penetration depth curves versus sliding distance. Consecutive execution of gradually finer EDM regimes onto the WC–Co alloys was found to considerably enhance the wear performance. Scanning electron microscopy on the worn surfaces revealed the occurrence of several wear mechanisms, i.e., grain polishing, abrasion, grain cracking and surface binder removal, mainly depending on the original surface finish EDM regime. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Based on economic reasons but especially today on the basis of ecological considerations as well, there is a rising need for an adequate limitation of wear and corrosion damage of machines and construction tools with attention to the efficient application of scarce materials and resources such as energy. In this way there is an obvious industrial demand for advanced materials to be applied under heavy tribological circumstances and preferably without lubrication as for instance for tools (chisels, cutting tools, metal forming dies, punches, etc.), various machine parts and in the fields of aerospace and automobile. Engineering ceramics are very attractive for these purposes because of their ultra high hardness and wear resistance as well as their excellent high-temperature resistance. However, a significant drawback of these materials is their relatively high coefficient of friction in dry contact conditions, involving heat development and energy loss. Moreover, their high hardness renders them intrinsically difficult to be shaped and finished by conventional methods. Today tungsten carbide (WC) based cemented carbides are extensively used in engineering industries because they not only combine excellent mechanical properties with an outstanding wear performance, but they are also suitable to be machined by electrical discharge machining (EDM) due to the binder phase which renders them sufficiently * Corresponding author. Tel.: +32 485523004; fax: +32 92643295. E-mail addresses: [email protected], [email protected], [email protected] (K. Bonny). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.002

electrically conductive [1]. More specifically, EDM has successfully proven to be feasible for manufacturing WC–Co cemented carbides [2–5]. One of the key advantages of EDM is the possibility of creating intricate shapes in a highly automated way, irrespective of strength or hardness of the base material [6]. However, difficulties also arise with respect to the control of surface finish [7–9], the corrosion of these materials during machining [10] and the influence the machining parameters may have on final properties such as strength [11,12], fracture toughness and fatigue behavior [13–15], hardness [16] and tribological characteristics [17–21]. EDM surface damage is given by residual stresses and cracks produced within the thermally affected zone (recast layer and adjacent regions) beneath the shaped surface [11,22,23]. The aim of this paper was to elucidate the influence of different EDM cutting regimes on the wear behavior of a number of WC–Co cemented carbide grades sliding reciprocally against WC–Co cemented carbide pins. Correlations between friction coefficient, wear volume and wear rate on the one hand and surface conditions associated with sequential EDM of the investigated WC–Co alloys on the other hand were investigated.

2. Experimental The chemical, physical, mechanical and microstructural properties of the distinctive WC–Co cemented carbides, i.e., grades WC10Co, WC12Co(V), WC12Co(Cr), WC10Co(Cr/V) and WC6Co(Cr/ V), are listed in Table 1. The HV10 Vickers hardness was measured

351

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359 Table 1 Chemical, physical, mechanical and microstructural properties of WC–Co cemented carbides WC–Co grade

WC10Co

WC12Co(V)

WC12Co(Cr)

WC10Co(Cr/V)

WC6Co(Cr/V)

Co binder content (wt.%) WC grain growth inhibitor Density (g/cm3) Thermal conductivitya (W m1 K1) Vickers hardness HV10 (kg/mm2) Fracture toughness KIC(30kg) (MPa m1/2) E-modulus [GPa] Mean grain size, dav (lm) WC grain size, d50 (lm) WC grain size, d90 (lm) WC grain size, d95 (lm)

10 None 14.33 105 1149 ± 10 >15.5 578 ± 6 2.2 1.8 4.2 6.0

12 VC 14.08 95 1286 ± 8 15.4 ± 0.5 563 ± 2 0.9 0.7 1.5 1.8

12 Cr3C2 14.01 95 1306 ± 5 15.5 ± 0.6 546 ± 2 0.9 0.8 1.7 2.1

10 Cr3C2/VC 14.23 85 1685 ± 38 9.7 ± 0.2 541 ± 4 0.3 0.3 0.6 0.7

6 Cr3C2/VC 14.62 90 1913 ± 13 8.8 ± 0.2 609 ± 4 0.6 0.5 1.0 1.2

a

Data specified by manufacturer.

with an indentation load of 10 kg (Model FV-700, Future-Tech Corp., Tokyo, Japan). The fracture toughness KIC(30kg) was obtained by the Vickers indentation technique, based on crack length measurements of the radial crack pattern produced by Vickers HV30 indentations. The KIC values were calculated according to the Shetty formula [24], which is given by Eq. (1):

pffiffiffiffiffiffi P  K IC ¼ 0:0889 HV 4l

ð1Þ

with HV, is the Vickers hardness, P, is the indentation load (N) and l, is the total crack length (m), which is defined as the radial crack length (c) minus half the indentation diagonal length (a). The Young’s modulus E was measured by the resonance frequency method. The resonance frequency was obtained on a Grindo-Sonic (J.W. Lemmens, Elektronika N.V. Leuven, Belgium), by means of the impulse excitation method (ASTM E 1876–99). The electrical resistivity was measured by a four terminal method on a Resistomat (TYP 2302 Burster, Gernsbach, Germany). The grain size distribution of the cemented carbide grades was acquired using computer image analysis software according to the linear intercept method. At least 1000 grains were measured for each grade. As can be derived from Table 1, the WC10Co grade exhibits the coarsest WC grain structure, with 50% of the grains being smal-

ler than 1.8 lm and 95% being smaller than 6.0 lm. The WC10Co(Cr/V) alloy has the finest microstructure, with 95% of the grains smaller than 0.7 lm. The WC–Co cemented carbide grades were surface finished by wire-EDM on a ROBOFIL 2030SI (Charmilles Technologies, Switzerland) in deionized water (dielectric conductivity 5 lS/cm), with a brass (CuZn37) wire electrode (diameter 0.25 mm, tensile strength 500 MPa). The wire-EDM process was performed using one rough cut (e.g. regime E3 in this paper) with high spark thermal energy to obtain a high material removal rate, and several consecutive finish cuts (e.g. E8, E21, E23 in this paper) with globally decreasing energy input and pulse duration, aiming to optimize the surface integrity. For more details about generator settings for the EDM regimes one is referred to [4,19]. The regarded EDM regimes of the hardmetal grades were characterized by surface profilometry (SomicronicÒ EMS Surfascan 3D, needle type ST305) and scanning electron microscopy (SEM, XL30-FEG, FEI, Philips). Cross-sectioned SEM views of WC–Co alloys after rough EDM cutting and after finer EDM surface variants are presented in Fig. 1. In all cases, a small heat affected zone (HAZ) containing recast material, droplets, craters and micro cracks was observed, in agreement with e.g. [4,5,7]. The thickness of this HAZ was determined to be approximately 30 lm after EDM regime E3 and decreases with consecutive finer

Fig. 1. Cross-sectioned SEM micrographs (SE mode) of (a) rough EDM’ed WC6Co(Cr/V) and (b,c) fine EDM’ed WC12Co(Cr).

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

EDM cuts to approximately 15 lm, 10 lm and 5 lm for regimes E8, E21 and E23 respectively. It should be emphasized that the regarded wire-EDM regimes cannot be considered as the baseline reference surface finish condition of the pristine WC–Co alloys, nor represent a condition matching the bulk WC–Co material. Future investigation will be focused on a tribological comparison between distinctive surface finishing operations, i.e., wire-EDM, grinding and polishing. The friction and wear behavior of wire-EDM’ed WC–Co grades against WC6Co(Cr/V) cemented carbide pins was evaluated using a Plint TE77 tribometer, in an air-conditioned atmosphere of 23 ± 1 °C and 60 ± 1% relative humidity, corresponding to the ASTM G133 linearly reciprocating sliding wear test standard [25]. The test grades were plate specimens (width 38 mm, length 58 mm

0.70

0.70

0.60

0.65

E3 E8 E21 E23

0.55 0.50

μ

0.60

E3 E8 E21 E23

0.55 0.50 0.45

µdyn

0.45 0.40

0.40

2

3

4

5 6 s [km]

7

8

9 10

0

0.75

0.70

0.70

µstat

0.75

0.65 0.60

0.50

2

3

μ

0.60

5 6 s [km]

7

8

9 10

7

8

9 10

E3 E8 E21 E23

0.55 0.50 0.45

µdyn

0.45

4

0.65

E3 E8 E21 E23

0.55

1

µstat

1

0.40

µdyn

0

μ

µstat

0.75

µstat

0.75

0.65

μ

and thickness 4 mm), whereas the pin was a hemisphere of which the average rounding radius and roughness parameters Ra and Rt were determined to be 4.08 mm, 0.35 lm and 2.68 lm respectively, analogously with wear experiments described in [26,27]. The applied normal force (FN) and the concomitant tangential friction force (FT) as well as the penetration depth (Ddp) of each pin-on-flat sliding pair were recorded continuously using a loadcell, a piezoelectric transducer and an inductive displacement transducer respectively. The FT/FN forces ratio is defined as the coefficient of friction (l), which can be differentiated in a static (lstat) and a dynamic component (ldyn). Contact loads were varied from 15 N up to 35 N. The stroke length of the oscillating motion was 15 mm. A sliding velocity of 0.3 m/s was applied. The test duration was associated with a sliding distance of 10 km, allowing

µdyn

352

0.40 0

1

2

3

4

5 6 s [km]

7

8

9 10

0

1

2

3

4

5 6 s [km]

0.75

µstat

0.70 0.65

μ

0.60

E3 E8 E21 E23

0.55 0.50

µdyn

0.45 0.40 0

1

2

3

4

5 6 s [km]

7

8

9 10

Fig. 2. Static and dynamic friction coefficient curves for wire-EDM’ed (a) WC10Co, (b) WC12Co(V), (c) WC12Co(Cr), (d) WC10Co(Cr/V), (e) WC6Co(Cr/V) grades sliding against WC6Co(Cr/V) pins at 0.3 m/s under a 35 N contact load.

353

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

Fig. 3. Ra and Rt surface roughness as function of EDM regime and sliding distance (s) during wear testing at 0.3 m/s under 35 N for a WC12Co(V) cemented carbide.

Ra [µm] 1.5 1

E21

E8

0.5

0

b 0.75

0.7

µstat

0.7

0.65

0.65

0.5

0.45

0.45

0.4

µdyn

0.55

0.5

0.35 20

WC10Co WC12Co(V) WC12Co(Cr) WC10Co(Cr/V) WC6Co(Cr/V)

0.6 µ [-]

0.55

WC10Co WC12Co(V) WC12Co(Cr) WC10Co(Cr/V) WC6Co(Cr/V)

µdyn

µ [-]

0.6

E3

2 E3

2.5 0.75

E23

a

µstat

3.1.1. Coefficient of friction Friction coefficient curves as function of sliding distance (s) under a 35 N normal contact force for wire-EDM (regimes E3, E8, E21 and E23) WC–Co flat versus WC6Co(Cr/V) pin combinations are shown in Fig. 2. Each curve is an average of at least two wear experiments performed under identical conditions. The error bars indicating the extent of the variations are excluded to make the figures better readable. The static and dynamic friction coefficient of the investigated tribopairs was measured to be in the range of 0.60–0.74 and 0.40–0.48 respectively. The dynamic and static component of fric-

E21

3.1. Online friction and wear

E23

3. Results

tion show similar evolution as function of the sliding distance, however at a different level. In all cases, the coefficient of friction is noticed to increase abruptly during the first meters of sliding, due to the quickly growing contact surface area. The initial friction should be mainly caused by strong asperity interaction (fragmentation and deformation), particularly for the rough wire EDM surface finish variant. As the sliding proceeds, the asperities are worn down, the initial surface roughness diminishes and the interlocking friction component decreases. This surface smoothening is confirmed by Fig. 3, which compares Ra and Rt roughness for distinctive EDM surface finish variants of a WC12Co(V) grade, before and after wear testing with various sliding distances. Furthermore, it is obvious that the surface roughness immediately reduces from the initial sliding pin-on-flat contact onwards. For example, the surface roughness in the wear track of the WC12Co(V) cemented carbide with surface finish variant E3 after a 0.1 km sliding wear path is characterized by Ra- and Rt-values of 0.15 and 1.98 lm respectively. This surface roughness is already much lower compared to the surface roughness of the original surface finishing variant E3, for which the Ra- and Rt-values were determined to be 2.21 lm and 15.84 lm respectively. It is worth noting that similar trends in wear track surface roughness were measured for the other WC–Co alloys. The small fluctuations in the friction curves, especially during running-in wear, are caused by continuous breaking and regeneration of micro junctions as a result of asperity interaction and should be related to the constantly changing degree of interlocking between the contact surfaces. The instabilities in the friction curves during the initial stage turn out to be more pronounced for the rough cut EDM surfaces and should be related to the thermally induced recast layer, exhibiting modified surface properties compared to the bulk material. The traces of copper on the rough cut wire EDM surface did not seem to have a beneficial impact on the initial friction coefficient. This trend is in agreement with [18]. The relatively small variation patterns in the coefficient of friction for each WC–Co grade and each EDM finishing regime points out the low incidence of adhesive wear. The lowest friction levels are recorded for the WC10Co(Cr/V)-WC6Co(Cr/V) tribopairs. The mutual differences in friction level between the distinctive cemented carbides are small and can be explained in terms of tribological compatibility, depending on the binder content and grain size distribution. The effect of the distinctive EDM-regimes on the friction

E8

post-mortem wear volumes to be compared. Before each test, the specimens were cleaned ultrasonically with acetone and a new WC6Co(Cr/V) pin was used in order to ensure equal initial surface conditions. The wear tracks were quantified volumetrically using surface scanning topography (SomicronicÒ EMS Surfascan 3D, needle type ST305).

0.4 0.35

15

10 Rt [µm]

5

0

30

25 20 15 10 5 HAZ thickness [µm]

0

Fig. 4. Influence of Ra and Rt surface roughness and thickness of heat affected zone (HAZ) on the friction coefficient for WC–Co flat/ WC6Co(Cr/V) pin pairs after a 4 km sliding distance (v = 0.3 m/s, FN = 35 N).

354

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

coefficient is considerable. The highest friction coefficients are encountered with the rough EDM cut specimens. Moreover, finer executed EDM is noticed to result in a decreased friction coefficient. The differences in friction level between wire-EDM regimes are noticed to diminish with increasing sliding distance. The dependency of the coefficient of friction on the original surface roughness of the wire-EDM’ed WC–Co alloys after sliding for 4 km using a 35 N contact load and a 0.3 m/s sliding speed is compared in Fig. 4a. Within the range of measured Ra and Rt values, both static and dynamic friction coefficient are noticed to decrease with enhanced surface finish refinement. A similar trend is noticed

a

when the coefficient of friction for the cemented carbides after a 4 km sliding distance is plotted against thickness of the wireEDM induced heat affected zone, obtained from cross-sectioned SEM views of wire-EDM’ed cemented carbides, Fig. 4b. 3.1.2. Penetration depth Penetration depth curves (Ddp) resulting from the vertical displacement of pin towards counter plate, recorded during dry reciprocative sliding experiments for WC–Co flat versus WC6Co(Cr/V) pin combinations, are plotted as function of wear path and distinctive EDM surface finish variants in Fig. 5. Each curve is an average

b

v=0.3 m/s; FN=35 N; WC10Co E3

2

v=0.3 m/s; FN=35 N; WC12Co(V) E3

2

E8

E8 10

10

E21

Δ dp [µm]

Δdp [µm]

8 6 4

E23 2

1

4

0.01

0.1 s [km]

1

10

0.01

d

v=0.3 m/s; FN=35 N; WC12Co(Cr) E3

2

4

Δdp [µm]

E8 E21

2

E23

1

1

10

E3 E8

10

8 6

0.1 s [km]

v=0.3 m/s; FN=35 N; WC10Co(Cr/V) 2

10

Δdp [µm]

2

6

4

8 6

E21

4

E23 2

1

8

8

6

6

4

4

0.01

0.1 s [km]

1

10

v=0.3 m/s; FN=35 N; WC6Co(Cr/V) 2

0.01

0.1 s [km]

1

10

f

E3 E8

10

Δdp [µm]

E23

8

6

e

4

1

8

c

E21

8 6

8 6 4

E21 E23

2

1

8 6 4

0.01

0.1 s [km]

1

10

Fig. 5. Penetration depth (a–e) and penetration rate (f) as function of sliding distance and EDM regime for WC–Co flat/ WC6Co(Cr/V) pairs sliding at 0.3 m/s under a 35 N contact load.

355

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

of at least two wear experiments performed under identical conditions. Error bars indicating the extent of the variations were excluded in order to improve the readability of the figures. Simultaneously, penetration rate curves (kd) were derived, e.g. for a WC6Co(Cr/V) grade in Fig. 5f. The kd number is defined as the ratio of penetration depth (Ddp) and the product of the applied load (FN) and sliding distance (s), Eq. (2):

kd ðsÞ ¼

Ddp ðsÞ F N :s

h m i N:m

ð2Þ

In all cases, the combined pin/plate penetration depth appears to remain below 35 lm after a 10 km sliding distance. Furthermore, the lowest penetration depth and concomitant wear rate are encountered with the WC10Co(Cr/V) grade, whereas the largest wear damage occurs with the WC10Co grade. This trend is commensurate with [21] and also agrees with the respectively lower and higher friction level for these grades, and should be related to the mutual differences between the distinctive grades regarding their WC grain size, i.e., hardness, and their cobalt binder content. The penetration depth is noticed to increase abruptly during the first sliding cycles, owing to the quickly growing contact surface area, in correspondence with a maximum penetration rate. After the initial stage, which is mainly determined by strong asperity interaction (fragmentation and deformation) and wearing down asperities, penetration depth is found to increase at a continuously reducing speed. The concomitant penetration rate appears to decrease in a linear logarithmic way with sliding distance after the ‘running-in’ wear. The effect of the distinctive EDM-regimes on the wear performance is quite pronounced. The rough cut EDM conditions yield the largest penetration depth and wear rate. However, the wear damage is noticed to decrease with finer-executed EDM. The differences in wear damage between the distinctive surface finish variants can be correlated to the surface roughness of the concomitant finish operation and the occurrence of a wire EDM induced heat affected zone. Indeed, as indicated by Fig. 3, the initial wear process is mainly caused by strong asperity interaction and interlocking, wearing down the asperities and smoothening the contact interface, and thus, causing larger wear damage at higher rate for rougher surface finish variants. Furthermore, the wear process for each WC–Co grade appears to reach an equilibrium regime after a certain sliding distance. This equilibrium state is characterized by a constant slope in penetration depth as function of sliding distance, indicated by a striped line in Fig. 5a–e. In fact, the slope of the penetration depth for each individual WC–Co alloy with E8, E21 and E23 surface finish is demonstrated to be similar, and thus, presumably represents the wear behavior of the pristine WC–Co cemented carbide. After rough cut EDM (regime E3), the WC–Co alloy is covered by an approximately 20–25 lm thick recast layer, which is obviously less wear resistant compared to the bulk material. This recast material is cut off by consecutive finer EDM cuts, i.e., E8, E21 and E23, each leaving a gradually thinner recast layer, i.e., ca. 10 lm, 5 lm and less than 5 lm respectively, on the cemented carbide surface. Therefore, both penetration depth and wear rate exhibit the largest values for rough cut WC–Co cemented carbides and decrease when the cemented carbide is surface finished up to a finer EDM cutting regime. It is worth noting that the penetration depth for the WC–Co versus WC6Co(Cr/V) combinations, after a 10 km sliding path using a 35 N contact load, ranges within or slightly exceeds the recast layer thickness for the rough EDM cutting regime, and thus, in most cases, all traces of recast material should be removed. With decreasing recast thickness (regime E8), or amount of recast (regimes E21 and E23), however, the recast material was worn off

more quickly, and thus, the earlier described ‘bulk’ equilibrium wear regime has been reached within the 10 km sliding distance. 3.2. Post-mortem wear characteristics This research was mainly focused on the investigation of wear characteristics in the equilibrium regime. In order to enable wear volumes to be compared, the sliding wear experiments were performed up to a total sliding distance of 10 km. The obtained 3D wear track surface topographies allow the corresponding wear track volumes (Vwear) to be quantified. Based on these results, a post-mortem volumetric wear rate (kV) can be derived according to Eq. (3):

kV ¼

V wear F N :s

  mm3 N:m

ð3Þ

The post-mortem obtained wear track dimensions, originating from the sliding wear tests described in Figs. 2 and 5, together with the corresponding volumetric wear rates (kV) are summarized in Table 2 for the distinctive cemented carbides and surface finish variants. Under identical conditions of 10 km sliding wear distance, 0.3 m/s sliding speed and 35 N contact load, the volumetric wear rates are noticed to vary between 6.1  108 and 4.5  107 mm3 N1 m1. Consistent with the online recorded results shown in Fig. 5, the smallest depth and width of the wear scars are measured for the samples of WC10Co(Cr/V) cemented carbides with the E23 surface finish variant, whereas the highest values are found with the rough cut EDM WC10Co grade specimens. Comparing the post-mortem, i.e., obtained by surface profilometry (see Table 2), with the online recorded, i.e. from the pin displacement (see Fig. 5), penetration depth after 10 km of sliding distance reveals small deviations. This is mostly attributed to the wear of the pin, which was not taken into account during postmortem quantification. The impact of surface roughness, obtained by distinctive wireEDM regimes, on wear volume (Vwear) and volumetric wear rate (kV), is demonstrated in Fig. 6 for the regarded WC–Co cemented carbides after reciprocative sliding wear experiments using a 35 N contact load and a 10 km sliding distance. Within the range

Table 2 Wear scar dimensions, wear volume and volumetric wear rate for wire-EDM’ed cemented carbide flats sliding against WC6Co(Cr/V) (v = 0.3 m/s, FN = 35 N, s = 10 km) Grade

Surface finish

Width (mm)

Depth (lm)

Vwear (103 mm3)

kV (10 km) (106 mm3 N1 m1)

WC10Co

E3 E8 E21 E23

1.10 1.05 1.00 0.95

24.3 21.3 13.2 10.1

156.3 124.3 52.65 31.72

0.447 0.304 0.150 0.091

WC12Co(V)

E3 E8 E21 E23

1.10 1.00 0.95 0.90

20.1 14.1 8.8 7.2

128.5 81.11 37.71 23.63

0.367 0.232 0.108 0.067

WC12Co(Cr)

E3 E8 E21 E23

1.10 1.00 0.95 0.90

19.6 12.5 8.0 7.1

122.3 69.39 35.36 22.52

0.349 0.198 0.101 0.064

WC10Co(Cr/V)

E3 E8 E21 E23

1.05 0.95 0.85 0.80

16.2 10.6 7.1 6.5

96.80 44.80 23.33 21.40

0.276 0.128 0.066 0.061

WC6Co(Cr/V)

E3 E8 E21 E23

1.05 0.95 0.85 0.80

17.0 12.0 8.7 7.0

114.3 72.60 37.03 21.66

0.327 0.207 0.106 0.062

356

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

a

Ra [µm]

2

1

Ra [µm]

b

0

2

WC10Co WC12Co(V) WC12Co(Cr) WC10Co(Cr/V) WC6Co(Cr/V)

0

120 100 80 60 40

15

10

5

0.3 0.25 0.2 0.15 0.1

0

15

0

E21 E23

0.05

E23

E21

0

E8

E3

20

0.35

E8

140

0.4

E3

160

WC10Co WC12Co(V) WC12Co(Cr) WC10Co(Cr/V) WC6Co(Cr/V)

0.45

kV (10km) [10-6 mm 3.N-1.m -1]

180

Vwear (10km) [10-3 mm 3]

1

0.5

200

10

Rt [µm]

5

0

Rt [µm]

Fig. 6. Wear volume (a) and wear rate (b) as function of Ra and Rt surface roughness of WC–Co cemented carbides, slid for 10 km against WC6Co(Cr/V) at 0.3 m/s under 35 N.

80 60 40

E3

0

E8

20

30

25

20

15

10

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05

5

0

HAZ thickness [µm]

0 30

25

20

15

E23

100

0.45

E21

120

WC10Co WC12Co(V) WC12Co(Cr) WC10Co(Cr/V) WC6Co(Cr/V)

E8

Vwear (10km) [10-3 mm 3]

140

0.5

E3

160

b kV (10km) [10-6 mm 3.N-1.m -1]

WC10Co WC12Co(V) WC12Co(Cr) WC10Co(Cr/V) WC6Co(Cr/V)

180

E21 E23

a

10

5

0

HAZ thickness [µm]

Fig. 7. Wear volume (a) and wear rate (b) as function of heat affected zone thickness of WC–Co cemented carbides, slid for 10 km against WC6Co(Cr/V) at 0.3 m/s under 35 N.

of Ra and Rt, both the wear volume and wear rate are noticed to decrease with enhanced surface finish refinement. The rough cut EDM cemented carbides exhibit the highest wear level, which improves drastically by the execution of gradually finer EDM finishing cuts. Not only surface roughness, but also the thickness of the wireEDM induced heat affected zone influences the wear performance of the WC–Co cemented carbides. This is illustrated in Fig. 7, in which wear volume and volumetric wear rate are plotted against the HAZ thickness, which was obtained from cross-sectioned SEM views of wire-EDM’ed cemented carbides. 3.3. Wear surface analysis Wear mechanisms occurring at the surface of the envisaged WC–Co alloys were identified by analyzing the wear scars using scanning electron microscopy. The results of SEM investigation

Fig. 8. SEM surface view of wire EDM (regime E3) WC10Co(Cr/V) after sliding for 10 km against WC6Co(Cr/V) at 0.3 m/s under 35 N.

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

357

Fig. 9. Cross-sectioned wear tracks of WC10Co with EDM finishing regimes E23 (a), E21 (b) and E8 (c and d), after 10 km sliding against WC6Co(Cr/V) at 0.3 m/s under 35 N.

Fig. 10. Top view wear surfaces of WC10Co(Cr/V) grade with EDM surface finish E21 (a) and E3 (b) after 10 km sliding against WC6Co(Cr/V) at 0.3 m/s under 35 N.

on the central part of wear scars originating from dry reciprocating sliding wear tests using a 0.3 m/s sliding speed and a 35 N contact load for WC–Co cemented carbides with distinctive wire-EDM surface finishing variants are presented in Figs. 8–12. Fig. 8 displays a top view of rough cut (EDM regime E3) WC10Co(Cr/V) cemented carbide after sliding for 10 km against WC6Co(Cr/V). The surface within the wear track appears smoother compared to the original wire-EDM surface. The wear track can be clearly distinguished from the original wire-EDM’ed area. This observation points out that the surface of the WC–Co cemented carbide has been polished as a result of the sliding contact with the WC-6 wt.%Co pin. This polishing effect was observed on the wear surfaces of other WC–Co alloys as well and confirms the results given in Fig. 3. Additionally, abrasive grooves can be found in the wear surface. Cross-sectioned wear tracks of WC10Co cemented carbide with different EDM surface finishing regimes after sliding for 10 km against WC6Co(Cr/V) pins at 0.3 m/s under 35 N are shown in Fig. 9, revealing the occurrence of surface cobalt binder removal, WC grain cracking and WC grain pull out. These wear mechanisms are found to occur to another extent and each with modified relative importance for the distinctive EDM surface finish variants. The

recast layer is noticed to be completely removed on both fine and rough cut EDM surfaces. However, small remainders of the wireEDM induced heat affected zone, such as cobalt binder modifica-

Fig. 11. Cross-sectioned wear track of WC12Co(V) with EDM regime E3 surface finish, after 10 km sliding against WC6Co(Cr/V) at 0.3 m/s under 35 N.

358

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

Fig. 12. Top view SEM micrographs of rough cut (EDM regime E3) WC12Co(Cr) after 10 km sliding against WC6Co(Cr/V) at 0.3 m/s under 35 N.

tion, are still visible on rough cut EDM surfaces, Fig. 9c and d, despite the imposed 10 km sliding distance and a 35 N normal contact force. Similar observations are made in Fig. 10, which displays surface views in the central part of the wear tracks on WC10Co(Cr/V) grades, surface finished by rough (regime E3) and fine (regime E21) EDM, after sliding 10 km against WC6Co(Cr/V) cemented carbide at 0.3 m/s under 35 N. Secondary electron (SE) SEM images reveal the presence of surface cracks, Fig. 10a. These surface cracks are more likely to occur at a wire-EDM’ed surface compared to the bulk material, owing to thermally induced pre-existing cracks, Fig. 1, serving as notches for the surface and/or subsurface material during reciprocal sliding. Surface cracks were also identified on the wear surface of rough cut (EDM regime E3) WC10Co(Cr/V) cemented carbide, together with islandized, small remainders of recast material, Fig. 10b. A cross-sectioned view on the central part of the wear track on rough cut (wire-EDM regime E3) WC12Co(V) cemented carbide is presented in Fig. 11, confirming the occurrence of several coexisting wear mechanisms. Small amounts of recast material are still present on the wear surface of rough cut (EDM regime E3) samples. Top view wear tracks on rough cut (EDM regime E3) WC12Co(Cr) cemented carbides, after sliding 10 km against WC6Co(Cr/V) at 0.3 m/s under 35 N, are presented in Fig. 12. Both secondary electron (SE) and backscattered electron (BSE) SEM images reveal that the recast material has not been completely removed by the imposed wear test conditions. Furthermore, micro cracks are noticed on the wear surface. These micro cracks should be attributed to frictional stresses building up during the reciprocating sliding pin-on-flat contact. 4. Discussion The general wear process of WC–Co cemented carbides is evidenced to occur by the accumulation of damage, fracture and removal of carbide grains [28–30]. Based on the obtained wear data and SEM analysis on the wear surfaces of the WC–Co cemented carbides envisaged in this research, the main wear mechanisms for the WC–Co alloys could be identified as polishing, (micro) abrasion, surface binder removal, grain cracking and grain pull out, Figs. 8–12. These wear mechanisms are found to occur to varying extent and each with modified relative importance for the distinctive EDM surface finish variants, owing to the different thermal impact of each EDM regime. The significance of each distinctive wear mechanism as function of the original surface finishing operation will be studied thoroughly in a near future investigation. From the recorded tribological responses of the WC–Co cemented carbides with distinctive EDM surface finishing conditions, it can be inferred that the presence of recast layer primary deter-

mines the friction and wear characteristics. In fact, once the WC– Co pin has penetrated the recast layer, the wear behavior reaches a wear regime characteristic for the bulk material, as demonstrated by Fig. 5. The wear resistance of the recast material is substantially lower than that of the base material, as can be deduced from the much higher (initial) wear rate for the rough cut EDM surfaces (regime E3) compared to the finer EDM regimes. Indeed, wire EDM was found to induce a heat affected zone and a recast layer at the surface, in which resolidified molten material, voids, residual tensile surface stresses and (micro) cracks are generated [22]. These phenomena were already demonstrated to lead to a considerable deterioration of the surface quality [11]. The deteriorated wear resistance of rough cut wire-EDM specimens compared to their equivalent finer surface finish variants can be explained in terms of surface roughness, Fig. 6, and thermally induced surface damage originating from the EDM process, Fig. 7. Indeed, a higher surface roughness results in a higher abrasion level, owing to an increased ploughing component [31–33]. Furthermore, the wire-EDM induced heat affected zone was found to contain surface cracks, penetrating into the base material, Fig. 1a, and/or thermal cracks of WC grains, Fig. 1b, both after rough and finer EDM cutting. In addition, depletion of Co binder and concomitant loosening of surface WC grains due to the EDM process was discovered, Fig. 1c. These phenomena deteriorate the strength properties at the surface. The pre-existing transgranular and/or intergranular cracks after EDM serve as notches for the (sub)surface material during the reciprocating sliding wear tests. Growth and propagation of these cracks increases the probability of grain fracture (e.g. Figs. 9b, 10, 11d and 12). The inferior wear resistance of the heat affected material can be explained in terms of surface stress state. Actually, after cooling down the WC–Co based cemented carbide from the liquid phase sintering, the WC phase is in compression, whereas the cobalt binder phase is in tension, due to the higher thermal expansion coefficient of Co compared to WC. However, the material removal process of wire-EDM generates residual surface stresses which are tensile in nature [11]. As a result of the imposed reciprocating sliding friction, the tangential stresses are building up with the pre-existing residual tensile stresses in the wire-EDM surface. 5. Conclusions Dry reciprocative sliding experiments on WC–Co/WC–Co cemented carbide combinations revealed that wire-EDM affects wear performance considerably. This could be attributed to the thermally induced recast layer and the concomitant surface quality, i.e., binder depletion, residual tensile surface stresses, thermal grain cracking and surface roughness, each separately influencing the observed wear mechanisms, i.e., grain polishing, abrasion, sur-

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 350–359

face binder removal, grain cracking and grain pull out. The surface roughness and the thickness of the heat affected zone, exhibiting substantially lower wear resistance compared to the bulk material, mainly determine the (running-in) wear behavior. However, the recast material on rough cut wire-EDM cemented carbides was not always completely removed under the applied sliding wear conditions. Consecutive execution of gradually finer EDM finishing regimes increases the wear resistance significantly. The volumetric wear rate after a 10 km sliding distance using a 35 N contact load and a 0.3 m/s sliding speed varied in the range of 6.1  108 and 4.5  107 mm3 N1 m1. Acknowledgements This work was co-financed with a research fellowship of the Flemish Institute for the promotion of Innovation by Science and Technology in industry (IWT) under project contract number GBOU-IWT-010071-SPARK. The authors gratefully recognize all the support, scientific contributions and stimulating collaboration from the partners from the University of Ghent (UGent) and the Catholic University of Leuven (K.U.Leuven). Special acknowledgement goes to CERATIZIT for supplying the hardmetal grades and pins. References [1] Kozak J, Rajurkar KP, Chandarana N. Machining of low electrical conductive materials by wire electrical discharge machining (WEDM). J Mater Process Tech 2004;149(1–3):266–71. [2] Gadalla AM, Tsai W. Machining of WC–Co composites. Mater Manuf Process 1989;4:411–23. [3] Gadalla AM, Tsai W. Electrical discharge machining of tungsten carbide–cobalt composites. J Am Ceram Soc 1989;72:1396–401. [4] Lauwers B, Liu W, Eeraerts W. Influence of the composition of WC-based cermets on the manufacturability by Wire-EDM. Trans NAMRI/SME 2004;32:407–14. [5] Mahdavinejad RA, Mahdavinejad A. ED machining of WC–Co. J Mater Process Tech 2005;162–163(Spec Iss):637–43. [6] Lauwers B, Kruth JP, Liu W, Eeraerts W, Schacht B, Bleys P. Investigation of material removal mechanisms in EDM of composite ceramic materials. J Mater Process Tech 2004;149(1–3):347–52. [7] Puertas I, Luis CJ, Alvarez L. Analysis of the influence of EDM parameters on surface quality, MRR and EW of WC–Co. J Mater Process Tech 2004;153– 154(1–3):1026–32. [8] Khan AA, Mohd Ali MBt, Mohd Shaffiar NBt. Relationship of surface roughness with current and voltage during wire EDM. J Appl Sci 2006;6(10):2317–20. [9] Abdullah A, Shabgard MR, Ivanov A, Shervanyi-Tabar MT. Effect of ultrasonicassisted EDM on the surface integrity of cemented tungsten carbide (WC–Co). Int J Adv Manuf Tech. doi: 10.1007/s00170-008-1476-7. [10] Obara H, Satou H, Hatano M. Fundamental study on corrosion of cemented carbide during wire EDM. J Mater Process Tech 2004;149(1–3):370–5. [11] Jiang D, Anné G, Vleugels J, Vanmeensel K, Eeraerts W, Lauwers B, et al. Residual stress in hardmetals caused by grinding and EDM machining and its

[12]

[13] [14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23] [24] [25] [26]

[27]

[28] [29] [30] [31] [32]

[33]

359

influence on flexural strength. In: Proc 16th Int Plansee Seminar. Powder Metallurgical High Performance Material, vol. 2, 2005. p. 1075–85. Lin Y-C, Hwang L-R, Cheng C-H, Su P-L. Effects of electrical discharge energy on machining performance and bending strength of cemented tungsten carbides. J Mater Process Tech 2008;206(1–3):491–9. Casas B, Torres Y, Llanes L. Fracture and fatigue behavior of electrical-discharge machined cemented carbides. Int J Refract Met H 2006;24(1–2):162–7. Llanes L, Casas B, Idanez E, Marsal M, Anglada M. Surface integrity effects on the fracture resistance of electrical-discharge-machined WC–Co cemented carbides. J Am Ceram Soc 2004;87(9):1687–93. Tsutsui T, Tamura T. Effect of the electro-discharge machined surface on the mechanical properties – On the fatigue strength of cemented carbide. B Jpn Soc Prec Eng 1987;21(1):70–1. Qu J, Riester L, Laura Shih AJ, Scattergood RO, Lara-Curzio E, Watkins TR. Nanoindentation characterization of surface layers of electrical discharge machined WC–Co. Mat Sci Eng A 2003;344(1–2):125–31. Ishikawa K, Iwabuchi A, Shimizu T. Influence of EDM on the wear characteristics of WC–Co cemented carbide. J Jpn Soc Tribol 2003;48(11):928–35. Llanes L, Idanez E, Martinez E, Casas B, Esteve J. Influence of electrical discharge machining on the sliding contact response of cemented carbides. Int J Refract Met H 2001;19(1):35–40. Bonny K, De Baets P, Lauwers B, Vleugels J, Van Der Biest O. Influence of Electro-discharge machining, microstructural and mechanical properties on wear behavior of hardmetals. In: Proc 16th Int Plansee Seminar, Powder Metallurgical High Performance Materials, vol. 2, 2005. p. 863–77. Bonny K, De Baets P, Vleugels J, Van Der Biest O, Lauwers B. Influence of surface finishing and binder phase on friction and wear of WC based hardmetals. Mater Sci Forum 2007;561–565(3):2403–6. Bonny K, De Baets P, Lauwers B, Vleugels J, Van Der Biest O. Reciprocating friction and wear behavior of WC–Co based cemented carbides manufactured by electro-discharge machining. Mater Sci Forum 2007;561–565(3):2025–8. Yakou T, Hasegawa T. Relations between condition and electrically discharge machining and depth of surface crack in cemented carbides. Trans Jpn Soc Mec 1995;61:1192–7. Stössel-Sittig C, Schnyder B, Kotz R. XPS and AES surface characterization of WC–Co after electro-discharge machining. Surf Interf Anal 2004;36(8):777–9. Shetty DK, Wright IG, Mincer PN, Clauer AH. Indentation fracture of WC–Co cermets. J Mater Sci 1985;20(5):1873–82. ASTM G 133-05, Standard Test Method for Linearly Reciprocating Ball-on-Flat SlidingWear. Annual Book of ASTM Standards, vol. 03.02, 2005. Bonny K, De Baets P, Vleugels J, Salehi A, Lauwers B, Liu W, et al. Influence of secondary electro-conductive phases on the electrical discharge machinability and frictional behavior of ZrO2-based ceramic composites. J Mater Process Tech 2008;208(1–3):423–30. Bonny K, De Baets P, Vleugels J, Salehi A, Lauwers B, Liu W, et al. Sliding wear of electrically conductive ZrO2-WC composites against WC–Co cemented carbide. Tribol Lett 2008;30(3):191–8. Gee MG, Gant A, Roebuck B. Wear mechanisms in abrasion and erosion of WC/ Co and related hardmetals. Wear 2007;263(Spec Iss 1–6):137–48. Gee MG, Gant A, Byrne LP, Roebuck B. Abrasion and reciprocating wear of hardmetals and ceramics. NPL Report CMMT(A) 1999. p. 166. Pirso J, Letunovit S, Viljus M. Friction and wear behaviour of cemented carbides. Wear 2004;257(3–4):257–65. Sayles RS. Basic principles of rough surface contact analysis using numerical methods. Tribol Int 1996;29(8):639–50. Lubrecht AA, Ioannides E. A fast solution of the dry contact problem and associated surface stress field using multilevel techniques. ASME J Tribol 1991;113:128–33. Bailey DM, Sayles RS. Effect of roughness and sliding friction on contact stress. ASME J Tribol 1991;113:729–38.