Reciprocating sliding friction and wear behavior of electrical discharge machined zirconia-based composites against WC–Co cemented carbide

Reciprocating sliding friction and wear behavior of electrical discharge machined zirconia-based composites against WC–Co cemented carbide

Int. Journal of Refractory Metals & Hard Materials 27 (2009) 449–457 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & H...

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Int. Journal of Refractory Metals & Hard Materials 27 (2009) 449–457

Contents lists available at ScienceDirect

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

Reciprocating sliding friction and wear behavior of electrical discharge machined zirconia-based composites against WC–Co cemented carbide K. Bonny a,*, P. De Baets a, J. Vleugels b, O. Van der Biest b, A. Salehi b, W. Liu c, B. Lauwers c a b c

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

a r t i c l e

i n f o

Article history: Received 27 April 2008 Accepted 2 July 2008

Keywords: ZrO2 composite WC–Co cemented carbide Wire-EDM Dry reciprocating sliding wear

a b s t r a c t The tribological characteristics of hot pressed zirconia-based composites containing 40 vol.% of WC, TiC0.5N0.5 or TiN and surface finished by electrical discharge machining (EDM) were evaluated by performing linearly reciprocating pin-on-flat sliding experiments against WC–Co cemented carbide under unlubricated conditions. The wear tests were executed on a Plint TE77 tribometer using normal contact loads of 15 N up to 35 N and a sliding velocity of 0.3 m/s. The ZrO2-40 vol.% WC grade displayed an undoubtedly better wear resistance compared to the ZrO2-40 vol.% TiCN and ZrO2-40 vol.% TiN composites. The morphology of the worn surfaces and the wear debris was investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), revealing several wear mechanisms such as polishing and abrasion, mainly depending on the imposed contact load and the material composition. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Engineering ceramics display exceptional properties of hardness, chemical stability, refractoriness and resistance against erosion/friction, which make them extremely adequate for tribological applications where high wear resistance is required [1]. In recent times, yttria stabilized tetragonal zirconia (Y-TZP) ceramics have been recognized as one of the strongest and toughest single phase oxides owing to the stress-induced phase transformation from tetragonal to monoclinic zirconia [2,3]. Additionally, the capacity to preserve these properties in a wide range of temperatures together with the excellent chemical inertness and low specific density has brought zirconia ceramics to the front end material to meet a large number of industrial applications [4] in the fields of manufacturing, cutting and tools [5,6], punches [7] and biomedical applications [8–13]. A growing trend in the development of advanced ceramics is the introduction of a secondary phase into the microstructure combined with grain size refinement towards nanometer scale with the aim of improving mechanical properties [14–16] and wear resistance [17]. The beneficial effect of grain refinement on the sliding wear resistance in ceramics can be explained by the fact that cumulative material removal during the sliding wear of non-transforming polycrystalline ceramics * Corresponding author. Tel.: +32 485523004; fax: +32 92643295. E-mail addresses: [email protected], [email protected] (K. Bonny). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.07.004

occurs by two successive mechanisms – dislocation plasticity, followed by fracture – which are both controlled by the grain size [18,19]. In sliding wear, an initial period of mild wear can be followed by a transition to severe wear promoted by the formation of surface fractures [20–22]. The incorporation of hard metallic phases such as WC, TiC0.5N0.5 or TiN into a ZrO2 matrix not only improves the hardness but also reduces the electrical resistivity of the resulting composites [23–26], making them suitable to be machined by electrical discharge machining (EDM) [27–31] and therefore avoiding the expensive grinding operation for final shaping and surface finishing of these otherwise hard to machine materials. This paper focuses on self-developed ZrO2-based composites with 40 vol.% WC, TiC0.5N0.5 or TiN addition and grain sizes in the sub-micrometer and nanometer range. Flat samples of ZrO2-40 vol.% WC, ZrO240 vol.% TiCN and ZrO2-40 vol.% TiN composites were produced by hot pressing and were surface finished by wire-EDM in deionized water. Their tribological behavior against WC–Co cemented carbide was investigated in dry reciprocating sliding experiments on a pin-on-plate testing rig using distinctive normal contact loads. Post-mortem obtained wear volume and volumetric wear rate were correlated to real-time recorded penetration depth as well as to mechanical properties, microstructure and contact load. The worn surfaces and the wear debris of the zirconia composites were analyzed thoroughly by scanning electron microscopy and Xray diffraction analysis in order to identify the occurring wear mechanisms.

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2. Experimental procedure 2.1. ZrO2-based composites The zirconia-based composites were obtained by hot pressing (W 100/150-2200-50 LAX, FCT, Rauenstein, Germany) ZrO2 powder mixtures with 40 vol.% of WC, TiC0.5N0.5 or TiN in vacuum (0.1 Pa) for 1 hour at 1450 °C under a mechanical load of 28 MPa. A combination of starting powders were used, i.e., (i) a pure monoclinic ZrO2 powder (Tosoh grade TZ-0, Japan, crystal size 27 nm) mixed with 3 mol% yttria (Y2O3) co-precipitated ZrO2 powder (Daiichi grade HSY-3U, Japan, crystal size 30 nm and Tosoh grade TZ-3Y, Japan, crystal size 27 nm), (ii) WC (MBN grade J550 Mechanomade nanocrystalline 20 nm powder agglomerates < 10 lm) or TiC0.5N0.5 (H.C. Starck grade B, deagglomerated TiC0.5N0.5 particles with a crystal size of 3–5 lm) or TiN (Kennametal Jetmilled, USA, crystal size 1.03 lm) powder with the function of zirconia matrix reinforcement and (iii) alumina (Al2O3) powder (Baikowski grade SM8, France, crystal size 0.6 lm) acting as ZrO2 grain growth inhibitor as well as sintering aid and densifier. The concentration of secondary phase was fixed at 40 vol.%, whereas the content of Y2O3 matrix stabilizer and Al2O3 additive was fixed at 2 mol% and 0.8 wt.%, respectively for ZrO2-40 vol.% WC and 1.75 mol% and 0.75 wt.%, respectively for both ZrO2-40 vol.% TiCN and ZrO2-40 vol.% TiN composites, because these amounts were found to result in the best combination of mechanical properties and were also considered as a good compromise with respect to the electrical conductivity and the EDM machinability, as indicated by previous investigations [23–26], to which one is also referred for more information on the processing and characterization of the ZrO2-based composites. Typical scanning electron micrographs (SEM, XL30-FEG, FEI, Philips, The Netherlands) revealing the microstructure of the obtained ZrO2-based composites are shown in Fig. 1. The secondary phases are identified as the bright phase in the ZrO2–WC composite, Fig. 1a, and the darker phase in ZrO2–TiCN an ZrO2–TiN, Figs. 1b

and c, whereas the ZrO2 matrix appears as the grey phase in Fig. 1a and the white phase in Figs. 1b and c. The physical, mechanical and microstructural properties of the ZrO2-based composites are listed in Table 1. It is worth noting that all reported values are the average of at least five measurements. The elastic modulus, E, was obtained by the resonance frequency method (ASTM C 1259-94) on a Grindo-sonic (J.W. Lemmens, Elektronika N.V. Leuven, Belgium). The Vickers hardness, HV10, was measured on a Zwick hardness tester with an indentation load of 10 kg. The fracture toughness, KIC,10 kg, was calculated using the formula of Anstis [32], based on crack length measurements of the radial crack pattern produced by Vickers HV10 indentations. The electrical resistivity was obtained by the 4-point contact method using a Resistomat Mikroohmmeter (Type 2302, Gernsbach, Germany). Grain size analysis of the secondary phases in the ZrO2based composites was executed using Imagine-Pro Plus software. It is obvious that the incorporation of the secondary phases into the zirconia matrix involves a substantial enhancement of the Emodulus, hardness and strength properties compared to the pure ZrO2 material. The best results were obtained with the ZrO2–WC composite, for which the HV10 hardness increases from about 1200 kg/mm2 for pure zirconia up to approximately 1700 kg/ mm2 for the composite material, while the corresponding flexural strength was noticed to increase from about 1250 MPa up to nearly 2000 MPa. However, a decrease in toughness of the composites, particularly for the ZrO2–TiN grade, has been measured as well. The grain size distribution of the TiN and TiCN phase in the respective ZrO2–TiCN and ZrO2–TiN composites is comparable, but significantly coarser compared to the WC phase in the ZrO2–WC grade, with 50% of the grains being smaller than 0.11 lm and 90% being smaller than 0.54 lm. The electrical resistivity of the pure zirconia was so large (> 50 kX) that it could not be measured. The addition of 40 vol.% secondary phase clearly reduced the electrical resistivity of the ZrO2-based composites down to a level where electrodischarge machining (EDM) is possible [27].

Fig. 1. Backscattered electron images revealing the microstructure of (a) ZrO2-40 vol.% WC (bright phase is WC), (b) ZrO2-40 vol.% TiCN (dark phase is TiCN) and (c) ZrO240 vol.% TiN (dark phase is TiN).

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K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 449–457 Table 1 Physical, mechanical and microstructural properties of laboratory-made ZrO2 and ZrO2-based composites

E-modulus (GPa) Hardness HV10 (kg/mm2) Toughness KIC,10kg (MPa m1/2) 3-point Bending Strength [MPa] Density (g/cm3) Resistivity (X m) dav average grain sizeb (lm) d50 grain sizeb (lm) d90 grain sizeb (lm) a b

Pure ZrO2

ZrO2–WC (20–40 nm)a

ZrO2–TiCN (1.6 lm)a

ZrO2–TiN (0.8–1.2 lm)a

203 ± 1 1195 ± 3 10.1 ± 0.1 1257 ± 114 6.06 / – – –

328 ± 2 1691 ± 8 8.5 ± 0.4 1964 ± 88 9.80 4.3  106 0.25 0.11 0.54

284 ± 2 1422 ± 10 7.0 ± 0.2 1521 ± 61 5.76 17.0  10-6 0.37 0.22 0.84

274 ± 1 1370 ± 7 5.6 ± 0.1 1674 ± 314 5.81 4.6  106 0.39 0.25 0.86

The number indicates the crystal size of the secondary phase starting powders. Grain size of the secondary phase.

The hot pressed ZrO2-based samples were manufactured on a ROBOFIL 2000 (Charmilles Technologies, Switzerland) in de-ionized water (dielectric conductivity of 11 lS/cm), using a CuZn37 wire electrode with a diameter of 0.25 mm and a tensile strength of 500 MPa. One rough cutting together with several consecutive finishing cuts was applied with gradually decreased energy input and shorter energy pulses. Detailed information on the WireEDM settings and the EDM parameters for the distinctive cutting regimes are discussed elsewhere [31]. In this paper, the finest EDM regime was selected for wear testing. A surface view together with a cross-sectioned view of the wire-EDM’ed surface of ZrO2– TiCN are presented in Figs. 2a and b, respectively. A small amount of recast material, containing droplets, craters and microcracks, induced by sparking energy, is visible at the surface, Fig. 2a. The cross-section view confirms the presence of a thin, resolidified recast layer, Fig. 2b, but does not reveal any signs of subsurface cracks or a clearly defined heat affected zone, and thus, beneath the top layer, the microstructure of the composite material has been maintained. It is worth noting that the thermal impact of the EDM surface finishing operation on the friction and wear characteristics of zirconia-based composites will be the subject of a near future investigation. The Ra and Rt surface roughness of the ZrO2-based composites surface finished by wire-EDM are compared in Table 2. Under equal set-up and identical EDM parameters, the ZrO2–TiN specimens are discerned to display the smoothest surface finish. 2.2. Wear testing Tribological behavior of wire-EDM’ed zirconia composites was evaluated using a high frequency tribometer (Plint TE77), in which WC–Co cemented carbide pins were slid reciprocally against zirconia composite counter plates, in accordance with the ASTM G133 wear test principle. The pin material (CERATIZIT grade MG12 with 6 wt.% Co) displays a compressive strength of 7.2 GPa, a Vickers

Table 2 Ra and Rt surface roughness for the ZrO2-based composites after EDM surface finish Roughness (lm)

ZrO2–WC

ZrO2–TiCN

ZrO2–TiN

Ra Rt

0.87 7.39

0.70 6.37

0.65 5.16

hardness HV10 of 1913 kg/mm2, a fracture toughness of 9.3 MPa m1/2 and an E-modulus of 609 GPa. The surface topographies of the pins were obtained using surface scanning equipment (SomicronicÒ EMS Surfascan 3D, type SM3, needle type ST305), yielding an average rounding radius and Ra and Rt surface roughness of 4.08 mm, 0.35 lm and 2.68 lm respectively. The contact load was varied from 15 N up to 35 N, with a stroke length of the oscillating motion of 15 mm. A sliding velocity of 0.3 m/s was applied. The test duration was associated with a sliding distance of 10 km. For each sliding experiment, a new WC–Co pin was used in order to ensure similar initial surface conditions. The generated wear tracks were analyzed by scanning electron microscopy and quantified volumetrically by surface scanning topography.

3. Results and discussion 3.1. Online friction and wear monitoring During each wear experiment the imposed normal contact force (FN) and the concomitant tangential friction force (FT) of pin-on-flat sliding pairs were recorded continuously using a load-cell and a piezoelectric transducer respectively. The coefficient of friction (l) is calculated from the FT/FN forces ratio. Furthermore, the measured friction force can be differentiated in a static Eq. (1) and a dynamic Eq. (2) component, from which likewise a static and dynamic coefficient of friction are derived:

Fig. 2. SEM micrographs of ZrO2-40 vol.% TiCN composite after wire-EDM surface finishing; (a) surface view and (b) cross-sectioned view.

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F T;stat ¼

F T;dyn

jF T;min j þ jF T;max j 2

ð1Þ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z 1 T ¼ ðF T ðtÞÞ2 dt T 0

ð2Þ

Simultaneously, with the real-time registration of the friction coefficient, vertical displacement (Ddp) curves resulting from the combined wear of WC–Co pins penetrating the zirconia-based flat counter samples were recorded using an inductive displacement transducer. Typical and representative tribological data versus sliding distance, obtained during real-time monitoring of friction coefficient and penetration depth for a ZrO2–TiN flat/WC–Co pin combination, are shown in Fig. 3. Each curve is an average of at least two wear experiments performed under identical conditions. The deviation between different samples of the same material was determined to be less than 5% and less than 10% for the friction coefficient and the penetration depth respectively. The error bars indicating the extent of the variations are excluded to improve readability. The dynamic friction coefficient is noticed to exhibit lower values compared to the static coefficient of friction. Moreover, both static and dynamic component of the friction coefficient appear to vary similarly as function of the sliding distance, that is to say, they both increase abruptly during the first meters of sliding, subsequently decline and increase again towards a ‘steady state’ situation, in which the fluctuations in the friction curves are considerably smaller. The fluctuations in the friction curves observed throughout the entire wear experiment are due to a continuous breaking and regeneration of micro junctions at the contact surface. The temporary drop down followed by a steep increase in friction coefficient near a sliding distance of 1 km has not been fully elucidated and could be interpreted as edge phenomenon, but could also be related to the changes in the sliding contact surface as a result of the removal of wire-EDM induced recast material. Indeed, the decrease/increase behavior in the friction coefficient appears to correspond to the slope change in the penetration depth curve, i.e., when a penetration depth of approximately 10 lm has been reached. This penetration depth of 10 lm is related to the total removal of both the surface roughness

0.9

(Rt  5 lm) of the original wire-EDM surface and the layer in which the influence of EDM was observed (thickness  5 lm). The penetration depth is noticed to increase rapidly during the initial reciprocating sliding contact owing to the quickly growing contact surface area. After a wear path of about 1 km, the wear process reaches a ‘steady state’ regime in which the penetration depth increases at a lower rate due to a more gradually growing pin-onflat contact area. Furthermore, the ‘steady state’ penetration depth appears to vary almost linearly with sliding distance. The current wear experiment was found to result in a penetration depth of approximately 33 lm after a 10 km wear path Fig. 3. Some numerical data are provided in Table 3, in which friction coefficient and penetration depth at various sliding distance and contact load are compared for the zirconia-based composites. Within the full range of sliding distance and imposed contact loads, the friction coefficient of the ZrO2–WC, ZrO2–TiCN and ZrO2–TiN grades against WC–Co cemented carbide pins was measured to be in the range of 0.48–0.80, 0.53–0.82 and 0.51–0.83, respectively for the static component and in the range of 0.36–0.53, 0.38–0.55 and 0.37–0.59, respectively for the dynamic component. For each zirconia-based composite, the coefficient of friction is noticed to increase with higher contact loads, in agreement with [31,33,34]. Furthermore, the effect of the secondary phase on the resulting coefficient of friction is quite significant. This is in full agreement with literature data on friction and wear behavior of aluminabased and zirconia-based ceramic composites [34,35]. The lowest friction level was recorded for the ZrO2–WC/WC–Co sliding combination, whereas the ZrO2–TiN/WC–Co tribopairs yield the highest coefficient of friction. The significant impact of the secondary phase and the contact load on the coefficient of friction is also found in the online recorded penetration depth. For each zirconia-based composite, higher penetration depth is discerned when higher contact loads are imposed, in agreement with [33–38]. For the ZrO2–TiCN grade for instance, the penetration depth after a 10 km wear path and contact loads of 15 N, 25 N and 35 N were measured to be 14.3 lm, 39.3 lm and 63.1 lm, respectively. In the full sliding distance and contact load range, the lowest wear depth is encountered for the ZrO2–WC composite, whereas wear testing of the ZrO2–TiN grade yields the largest penetration depth. The relative ranking of the wear depth of the three zirconia-based composites correlates well to their corresponding relative hardness and fracture toughness, as will be discussed in Section 3.4.

35

3.2. Wear volume 30

0.8

Δd

25

μ

20

μ stat

0.6

15

Δdp (µm)

0.7

0.5 10

μ dyn 0.4

5

All pin-on-flat sliding wear experiments were ceased after a sliding distance of 10 km. This approach ensured that the wear process had reached a ‘steady state’ condition and allowed the post-mortem assessed wear volumes to be compared. From surface scanning profilometry measurements on the wear tracks of the zirconia-based composites, the corresponding wear track dimensions, i.e., depth and width, as well as wear track surface topographies and wear volumes (Vwear) were extracted. The concomitant volumetric wear rate (kV) was determined by calculating the quotient of the wear volume on the one hand and the imposed normal contact force multiplied by the sliding distance, e.g. 10 km, on the other hand Eq. (3):

Kv ¼ 0.3

0 0

2

4

6

8

10

Sliding distance (s) [km] Fig. 3. Static and dynamic friction coefficient (lstat, ldyn) and penetration depth (Dd) for a ZrO2-40 vol.% TiN flat/WC–Co pin pair sliding at 0.3 m/s, under a 15 N contact force.

  V wear mm3 FN  s N m

ð3Þ

The results of the post mortem wear quantification of the zirconiabased composites after a sliding wear path of 10 km and various contact forces are compared in Table 4. The wear volumes are noticed to vary over more than three orders of magnitude. Furthermore, the highest wear levels are found for the ZrO2–TiN composite, whereas

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Table 3 Static and dynamic friction coefficient (lstat, ldyn) and penetration depth (Ddp) as function of sliding distance (s) and contact load (FN) for ZrO2-based composites slid at 0.3 m/s against WC–Co pins s (km)

FN (N)

ZrO2–WC 15

ZrO2–TiCN 25

35

15

ZrO2–TiN 25

35

15

25

35

0.015 1 4 10

lstat

0.59 0.48 0.55 0.56

0.72 0.70 0.68 0.69

0.80 0.77 0.77 0.75

0.53 0.57 0.58 0.59

0.66 0.70 0.71 0.71

0.77 0.82 0.79 0.78

0.51 0.61 0.65 0.66

0.65 0.69 0.72 0.70

0.79 0.83 0.81 0.82

0.015 1 4 10

ldyn

0.42 0.36 0.38 0.39

0.50 0.47 0.45 0.46

0.53 0.51 0.50 0.51

0.38 0.39 0.40 0.40

0.46 0.48 0.48 0.48

0.54 0.55 0.55 0.54

0.37 0.42 0.47 0.48

0.46 0.48 0.49 0.52

0.58 0.59 0.59 0.58

0.015 1 4 10

Ddp

0.8 3.9 4.8 6.9

2.6 14.0 22.2 32.1

6.4 22.2 40.3 53.6

1.2 6.9 9.8 14.3

2.1 19.8 27.4 39.3

3.0 36.8 45.4 63.1

1.4 15.1 21.5 33.0

5.9 74.3 142.6 248.2

9.9 99.6 200.5 362.7

Table 4 Wear track dimensions, wear volume (Vwear) and volumetric wear rate (kV) as function of imposed contact load (FN) for ZrO2-based composites slid against WC–Co pins (v = 0.3 m/s, s = 10 km) Depth (lm)

Vwear (10 km) (103 mm3)

kV (10 km) (106 mm3 N1 m1)

Grade

FN (N)

Width (mm)

ZrO2–WC

15 25 35

1.05 1.35 1.75

4.8 35.2 58.1

8.8 378 699

0.058 1.51 2.0

ZrO2–TiCN

15 25 35

1.15 2.0 2.6

12.8 37.3 274

92.3 602 987

0.61 2.41 2.82

ZrO2–TiN

15 25 35

1.55 2.7 3.1

28.6 284 302

320 6.9  103 9.8  103

the best wear resistance was encountered with the ZrO2–WC grade. In agreement with previous findings for ceramics [21,35] and zirconia ceramics [33,34,39], the wear resistance of the ZrO2-based composites regarded in this paper is noticed to be strongly influenced by the applied contact load, i.e., wear volume increases with higher normal contact force. At the lowest contact load, the ZrO2–WC material, for instance, exhibits a wear volume of 8  103 mm3 for a 10 km wear path length. At 25 N, a much higher wear volume of 0.38 mm3 is measured. After the high-load experiments, the wear volume increased to 0.7 mm3. For the low-load case, the ZrO2–WC sample displays a much better wear resistance than the other composites. With rising contact load however, the difference in wear volume between the ZrO2–WC and ZrO2–TiCN grades tends to decrease. This behavior could indicate a transition from a low to a higher wear regime for the ZrO2–WC composite [1,20,22]. In addition, the lowload wear volume of ZrO2–TiN occurs in the same range compared to the high-load wear level of the other composites, whereas its high-load levels are an order of magnitude higher. Hence, a similar transition from an intermediate to a high wear regime can be presumed for the ZrO2–TiN material. Comparing the wear depth in Table 4 with the real-time penetration depth in Table 3 after a 10 km wear path reveals small deviations. This is mostly attributed to the fact that the contribution of the pin wear, included throughout the online registration, was not taken into account during post-mortem quantification. However, the online and the post-mortem wear determination match pretty well, and thus, the online wear monitoring technique is found acceptable for reliable sliding wear estimation for the investigated tribocouples. 3.3. Wear surface analysis Visual inspection of the wear surface after sliding wear testing revealed the presence of wear debris, mainly located at the

2.13 27.6 28.0

extremities of the wear track, but occasionally also inside, along and adjacent to the wear track of the zirconia-based composites. Furthermore, a smooth appearance of the wear track surface was discerned, indicating that the originally wire-EDM’ed surface of the zirconia-based composites had been polished as a result of the sliding contact with the WC–Co pin. The polishing effect is thought to occur by ploughing away the surface asperities and roughness peaks of the original wire-EDM’ed surface and explains the previously observed friction coefficient behavior in Fig. 3. Indeed, the initial coefficient of friction is high due to intense asperity interaction when the sliding starts. After the initial stage, however, the pin-on-flat interface becomes smoother and the coefficient of friction fixes on a more or less constant level. The phenomenon of polishing is illustrated by Fig. 4, in which the Ra and Rt surface roughness of the zirconia-based composites are compared before and after wear experiments with a 15 N contact load, a 0.3 m/s sliding speed and various wear paths. It is clear that the roughness measurements in the wear track of the zirconia-based composite yield lower Ra- and Rt-values compared to the original roughness after wire-EDM surface finishing. Furthermore, it can be seen that the roughness diminishes strongly during the first sliding meters, in agreement with [1], and thus, the smoothening occurs particularly during the ‘running-in’ stage of the wear process. It is worth noting that the original Ra and Rt surface roughness after wire-EDM for the three zirconia-based composites was not identical, as indicated by Table 2. This discrepancy in surface finish should not be neglected when the differences in wear volume between the three zirconia-based composites are considered. The influence of the wire-EDM induced surface modification (recast layer, residual surface stresses) on the resulting friction and wear characteristics will be the subject of a near future investigation. The worn surface morphologies of the zirconia-based composites were investigated by SEM analysis, Fig. 5, from which following wear mechanisms could be identified: microcracking,

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Fig. 4. Ra and Rt surface roughness before and after wear testing of zirconia-based composites against WC–Co pins for a 15 N contact force, a 0.3 m/s sliding speed and various sliding distances.

spalling, delamination, abrasion and wear debris layer formation. After sliding experiments on fine EDM’ed ZrO2–WC against WC– Co for 10 km at 15 N, the microstructure in the central wear track corresponds to that of the pristine material, Fig. 5a. With a contact load of 35 N, however, abrasive grooves are created all along the wear track, which is covered with a thin layer of wear debris, Fig. 5b. Microcracks perpendicular to the sliding direction are observed in the wear debris layer all along the wear track, and additionally, the debris layer tends to delaminate, Fig. 5c. In delaminated areas

where the top layer spalls off, the base material becomes visible, displaying clear evidences of a fracture process, which indicates that the adhesion strength of the debris layer to the substrate is quite high. Material removal might be due to the process of delamination, i.e. initiation and subsequent propagation of subsurface lateral cracks. It is not elucidated yet whether the microcracks penetrate into the pristine material or whether they are restricted to the debris layer. These microcracks are induced by tangential stresses building up during the oscillating sliding contact. However, this stress development could not be associated with an occurrence of tetragonal to monoclinic ZrO2 transformation. This is confirmed by XRD-analysis on the wear debris, Fig 5d, which did not reveal any traces of crystalline ZrO2, nor monoclinic, nor tetragonal, but rather presumed an amorphous nature of the debris layer containing Zr–O–Al–W–C phase. When contact loads of 35 N were imposed, a thin wear debris layer occurred on the wear surface. The continuous character of the wear debris layer at a 35 N normal contact force is in contrast with the 15 N wear experiments, after which the wear debris in and around the wear track was less numerous and mainly consisted of loose particles. These observations invigorate the presumption of load-dependent wear transitions for the ZrO2-based composites. SEM analysis on the wear tracks of zirconia-based composites after sliding 10 km at 0.3 m/s against WC–Co pins under a 35 N contact load is compared in Fig. 6. Abrasive grooves and wear debris in and around the wear scars were observed for all zirconiabased composites. With ZrO2–TiN and ZrO2–WC, the wear track was covered by a continuous wear debris layer with microcracks normal to the sliding direction and delaminated areas as well, as shown in Figs. 6a and b, respectively. The ZrO2–TiCN composite appears to be least prone to the formation of a wear debris layer, that is to say, the original microstructure is still visible, together with some abrasive grooves, but no microcracks are detected, Fig. 6c.

Fig. 5. SEM micrographs in the wear track on fine EDM’ed ZrO2-40vol.% WC after sliding 10 km at 0.3 m/s against WC–Co pins under contact force of (a) 15 N, (b, c) 35 N and (d) XRD analysis on the resulting wear debris.

K. Bonny et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 449–457

455

Fig. 6. SEM micrographs in the wear track of ZrO2-40vol.% TiN (a), 40vol.% WC (b) and 40vol.% TiCN (c,d) composites after sliding 10 km at 0.3 m/s under a 35 N contact load.

In some regions however, the formation of the wear debris layer is initiated, as shown in Fig. 6d.

3.4. Correlations with material properties The influence of material properties and loading parameters on wear behavior of ceramic materials is very complex [20–22,39]. This paper has already demonstrated that wear volume increases with the applied contact load. From the wear data presented in Tables 3 and 4 it can be inferred that the highest wear resistance is found for the ZrO2–WC composite. The ZrO2–WC composite also exhibits a finer microstructure compared to the ZrO2–TiCN and ZrO2–TiN grades, as indicated in Table 1. This finding could point out a beneficial effect of grain refinement on the wear resistance. The grain size effect on wear resistance is thought to be attributed to the smaller grain boundaries or defects in fine grain ceramics which require higher stress to propagate pre-existing defects. However, relative comparison between the microstructural characteristics and wear properties of ZrO2–TiCN and ZrO2–TiN composites in Table 1 on the one hand and Tables 3 and 4 on the other hand reveals large differences in wear resistance for similar grain size. Thus, the influence of other properties such as hardness and fracture toughness on the wear resistance should be considered at the same time. In this research, an attempt has been made to express the dependence of the wear resistance on the loading parameters as well as the material properties of the three zirconia-based composites. Jianxin et al. [35] suggested to apply the theory of Evans et al. [40] that the volumetric wear varies proportionally to P9/8  KC1/2  H5/8  (E/H)4/5, where P is the normal load, KC is the fracture toughness, H is the hardness, E is the Young’s modulus and the E/H ratio represents the plasticity index of the ceramic material. Yang et al. [39] concluded that wear rate of fine-grain zirconia was propor-

tional to G1/2  KIC1/2  HV1/8  (E/H)4/5, with G the average grain size, KIC the fracture toughness in mode I, HV the Vickers hardness and E/H the plasticity index. The plasticity index of the zirconiabased composites regarded in this investigation is comparable, i.e., 0.194, 0.199 and 0.2 for the ZrO2–WC, ZrO2–TiCN and ZrO2–TiN grade, respectively. Based on a curve fitting procedure of the experimental results, following preliminary equation was proposed in order to describe how the measured wear volume of zirconia-based composites is affected by material properties and contact load: 2 v ¼ 106  F 6N  dav  E3  H6 V;10  K IC;10

ð4Þ

with FN the imposed normal contact force, dav the average grain size, HV10 the Vickers hardness, KIC,10 the fracture toughness for a HV,10 indentation load and E the Young’s modulus. The results of these calculations are presented in Fig. 7, in which the influence of material properties and applied contact load on wear volume and wear rate for the fine EDM’ed zirconia-based composites is correlated. Each linear fitting curve in the figure seems to apply only within one magnitude of wear volume. This invigorates the earlier made presumptions of the occurrence of wear transitions from mild to intermediate and severe wear regimes. For example, for the ‘intermediate’ wear regime, indicated by line (II) in Fig. 7, following equations are obtained, Eqs. (5) and (6):

V wear ¼ 0:3813 : K V ¼ 1:485 :

v0:2632 R2 ¼ 0:9877

v0:1849 R2 ¼ 0:957

ð5Þ ð6Þ

with Vwear and kV the post mortem wear volume and volumetric wear rate after a sliding distance of 10 km. The R-square value, R2, indicates that both equations match the experimental results pretty well. Substituting Eq. (4) into Eqs. (5) and (6) confirms that the wear volume and the volumetric wear rate can be described in terms of material properties and loading parameters. More particularly,

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a

b

v=0.3 m/s; s=10 km

v=0.3 m/s; s=10 km

10

kV (10km) [10 mm³.N .m ]

100 -1

(II) 0.1

ZrO2-TiN ZrO2-TiN ZrO2-TiCN ZrO2-TiCN ZrO2-WC ZrO2-WC

(I) 0.01 0.01

0.1

1

10

100

106×(FN)6×dav×(E)3×(Hv,10)-6×(KIC)-2

-1

1

10

-6

Vwear (10 km) [mm³]

(III)

1

(III)

(II) ZrO2-TiN ZrO 2-TiN ZrO2-TiCN ZrO 2-TiCN ZrO2-WC ZrO2-WC

(I)

0.1

0.01 0.01

0.1

1

10

100

106×(FN)6×dav×(E)3×(Hv,10)-6×(KIC)-2

Fig. 7. Correlation between material properties, contact load and (a) wear volume and (b) volumetric wear rate, for zirconia-based composites after 10 km reciprocative sliding at 0.3 m/s against WC–Co cemented carbide.

wear damage is found to decrease with improved hardness, fracture toughness and stiffness as well as reduced grain size and contact load. Furthermore, the fitting lines (I), (II) and (III) in Fig. 7 imply the occurrence of distinctive load-dependent wear transitions in the wear process of the zirconia-based composites. Indeed, the wear transitions are caused by external stresses which exceed the critical strength of the material. Hence, wear damage increases as the normal load or the contact pressure is increased. When the wear experiments were performed using a 15 N contact load, both ZrO2–WC and ZrO2–TiCN samples can be classified into a ‘mild’ wear regime, indicated by line (I) in Fig. 7, whereas the ZrO2–TiN specimens seem to wear according to a more severe regime, categorized by line (II) in Fig. 7. A contact load of 25 N apparently involves a wear transition towards an ‘intermediate’ regime for the ZrO2–WC and ZrO2–TiCN composites, pointed out by line (II) in Fig. 7, and towards a ‘severe’ wear regime for the ZrO2–TiN specimens, as indicated by line (III) in Fig. 7. When a 35 N contact load is applied, the ZrO2–WC and ZrO2–TiCN grades still wear according to regime (II), whereas the ZrO2–TiN grades remain classified in regime (III). From these findings it can be inferred that increased mechanical properties shift the wear transition tendency of the studied zirconia-based composites towards higher loads. Furthermore, a wear transition from regime (II) to (III) is expected to occur as well for the ZrO2–WC and ZrO2–TiCN composites when contact loads above 35 N would be applied, just like a wear behavior according to regime (I) is assumed for the ZrO2–TiN composites when contact loads below 15 N would be applied. This should be confirmed by equal sliding experiments with lower and higher normal contact forces, which will be reported in the near future. 4. Conclusions Comparative dry sliding pin-on-flat experiments on wireEDM’ed ZrO2–based composites in reciprocating contact with WC–Co cemented carbides under various contact loads revealed several mechanisms involved in their global wear process: polishing, abrasion, microcracking, spalling, delamination and the formation of an adhering wear debris layer. The wear volume and volumetric wear rate of the zirconia-based composites were found to be affected considerably by the nature of the secondary phase and the imposed contact loads, indicating presumably three distinctive wear regimes depending on the sliding material combination. Amongst the ZrO2-based composites with equal volumetric

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