Reciprocative sliding wear of ZrO2–TiCN composites against WC-Co cemented carbide

Wear 265 (2008) 1767–1775

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Reciprocative sliding wear of ZrO2 –TiCN composites against WC-Co cemented carbide K. Bonny a,∗ , P. De Baets a , J. Vleugels b , A. Salehi b , B. Lauwers c , W. Liu c a b c

Ghent University (UGent), Department of Mechanical Construction & Production, IR04, Belgium Catholic University Leuven (K.U.Leuven), Department of Metallurgy & Materials Engineering, MTM, Belgium Catholic University Leuven (K.U.Leuven), Department of Mechanical Engineering, PMA, Belgium

a r t i c l e

i n f o

Article history: Received 5 September 2007 Received in revised form 7 April 2008 Accepted 29 April 2008 Available online 10 June 2008 Keywords: ZrO2 –TiCN composite Nanocrystalline Wire-EDM WC-Co cemented carbide Dry reciprocative sliding wear

a b s t r a c t ZrO2 -based composites with 40 vol.% TiC0.5 N0.5 addition were processed from both nanocrystalline and micrometer sized TiC0.5 N0.5 starting powders by hot pressing. Flat samples were manufactured and finished by wire electrical discharge machining (EDM) in order to investigate their sliding-wear behavior against a WC-Co cemented carbide. Reciprocative sliding experiments were performed under unlubricated conditions using a small-scale pin-on-plate tribometer. The worn surfaces of the investigated composites were analyzed by X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and surface topography. Post-mortem obtained wear volume loss was compared to online measured wear depth. Wear rates were correlated to sliding distance and material parameters and microstructural characteristics. The experimental results revealed a significant dependence of the wear resistance of the ZrO2 -based composites on loading conditions and the nature of the secondary phase. © 2008 Elsevier B.V. All rights reserved.

1. Introduction A recent trend in the development of ceramic materials is to refine the grain size to sub-micron (ultra-fine-grained) and nanometer (nanoceramics) ranges with the anticipation of improving their mechanical properties [1,2]. This approach has met with limited success in ultra-fine-grained ceramics and nanoceramics because larger extrinsic flaws (processing defects, pores, surface flaws, in-service damage) become dominant over grain-sizedcontrolled intrinsic flaws. In this context, sliding-wear resistance does benefit from grain refinement, as demonstrated before for Al2 O3 ceramics [3]. This is because cumulative material removal during the sliding wear of non-transforming polycrystalline ceramics occurs by two successive mechanisms, i.e., dislocation plasticity, followed by fracture, both of which are controlled by the grain size [3,4]. A number of studies have demonstrated that fracture toughness plays a significant role in the wear process of ceramics [5–7]. ZrO2 -based ceramics represent a class of new structural materials, which have been proven to be the toughest and strongest oxides yet

∗ Corresponding author. E-mail address: [email protected] (K. Bonny). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.020

produced [8]. The superior mechanical properties are associated with the stress-induced phase transformation from tetragonal to monoclinic ZrO2 [9]. The incorporation of hard secondary phases such as TiB2 , TiN, TiC, TiC0.5 N0.5 and WC into the zirconia matrix were found to increase the hardness while maintaining the high toughness due to transformation toughening and crack deflection [10–14]. An additional key feature of these reinforcements is their electrical conductivity, allowing to manufacture zirconia based composites on which electrical discharge machining (EDM) can be employed successfully [15–20]. This technology not only allows to machine intricate parts with high accuracy irrespective of hardness and strength, but also exhibits the possibility for mass production, thus avoiding the expensive mechanical shaping and surface finishing operations such as grinding and/or polishing. This paper focuses on yttria-stabilized zirconia based composites with 40 vol.% TiCN, which were completely laboratory-made based on distinctive TiC0.5 N0.5 powder sources. The tribological behavior of ZrO2 –TiCN samples, manufactured and surface finished by wire-EDM, against a WC-Co cemented carbide was investigated in dry reciprocating sliding experiments on a pinon-plate tribometer. Correlation between wear volume and wear rate on the one hand, and sliding distance, material parameters, microstructural properties and loading conditions on the other hand were elucidated.

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2. Experimental procedure 2.1. ZrO2 –TiCN composites The ZrO2 -based composites were obtained by hot pressing a mixture of two zirconia powders, i.e., pure monoclinic ZrO2 (Tosoh grade TZ-0, Tokyo, Japan, crystal size 27 nm) and 3 mol.% Y2 O3 coprecipitated ZrO2 (Tosoh grade TZ-3Y, Tokyo, Japan, crystal size 27 nm), with 40 vol.% TiC0.5 N0.5 powder and 0.75 wt.% alumina (Baikowski grade SM8, Annecy, France, crystal size 0.6 ␮m) additive acting as ZrO2 grain growth inhibitor and sintering aid. Two different TiC0.5 N0.5 powder sources were used, i.e., HTNMC grade 50/50 (particles <100 nm) and H.C.Starck grade B (micrometer sized deagglomerated TiC0.5 N0.5 with a crystal size of 3–5 ␮m). The two grades are referred to as “Fine” and “Coarse” throughout the text. The yttria stabilizer content of the ZrO2 matrix was fixed at 1.75 mol.% for the Coarse grade and at 2.75 mol.% for the Fine grade, as these amounts were found to result in the best combination of mechanical properties. More information on the processing and characterization of the ZrO2 –TiCN composites is given elsewhere [10,11]. Representative scanning electron micrographs of the ZrO2 –TiCN composites obtained using coarse and fine TiC0.5 N0.5 powder sources are shown in Fig. 1. The bright phase is ZrO2 , whereas the grey particles represent TiC0.5 N0.5 . The small black particles are Al2 O3 grains which were added to the composite mixture to enhance densification. The small white particles are WC originating from WC-Co milling balls. The TiC0.5 N0.5 phase in the coarse ZrO2 –TiCN grade exhibits an angular morphology, whereas the TiC0.5 N0.5 particles in the fine ZrO2 –TiCN grade are agglomerated and display a necklace-like structure in the composite. ZrO2 phase transformation in the obtained composites did not occur due to the Y2 O3 stabilizer. Grain size measurements of the secondary phase in ZrO2 –TiCN composites by means of Imagine-Pro Plus software, as listed in Table 1, revealed that the grain size distribution of the TiC0.5 N0.5 phase in the coarse ZrO2 –TiCN grade is significantly coarser than that for the fine ZrO2 –TiCN composite. The mechanical and physical properties of the ZrO2 –TiCN composites are listed in Table 1. The use of TiC0.5 N0.5 nanocrystalline powder appears to result in a significantly increased composite hardness as well as a drastically decreased toughness. The fine grade ZrO2 –TiCN composite has a higher Young’s modulus, but a much lower electrical conductivity, compared to the coarse ZrO2 –TiCN composite. Close investigation of the crack lengths revealed that the toughness of the coarse ZrO2 –TiCN composite depends on the crack propagation direction. The crack length along the hot-pressing direction was systematically shorter than that along the perpendicular direction. The fracture toughness was calculated to be 8.5 ± 0.3 MPa m1/2 and 5.9 ± 0.4 MPa m1/2 along and perpendicular to the hot-pressing direction, respectively. The TiC0.5 N0.5 particles were observed to align along the direction perpendicular to the hot-pressing direction. In the fine ZrO2 –TiCN composites, however, the ZrO2 grains were found to fracture in a transgranular way, in correlation with the transformability of the ZrO2 matrix and the yttria stabilization [10,11].

Fig. 1. Microstructure of the coarse (a) and fine (b) ZrO2 –TiCN composites with 40 vol.% TiC0.5 N0.5 addition.

Both ZrO2 –TiCN composites are suitable for EDM as their electrical resistivity is in the order of 10−6 or 10−5  m [20]. They were wire-EDM’ed on a ROBOFIL 2000 (Charmilles Technologies, Switzerland) in demineralized water (dielectric conductivity 5 ␮S/cm), using a CuZn37 wire electrode with a diameter of 0.25 mm and a tensile strength of 500 MPa. Initially, rough cutting was performed, with the aim to obtain a higher material removal rate. In order to improve the surface quality, consecutive EDM finishing cuts were applied with gradually lower energy input and a shorter energy pulse duration. The wireEDM parameters for the applied cutting regimes are discussed in detail elsewhere [17]. Surface/sub-surface integrity was analyzed by SEM (Philips XL30-FEG, FEI, The Netherlands). In this paper, the finest EDM regime was selected for wear experiments in order to compare the sliding-wear response of the ZrO2 –TiCN composites. The coarse grade is noticed to exhibit a slightly higher Ra and Rt surface roughness compared to the fine grade, Table 2.

Table 1 Physical properties of ZrO2 –TiCN composites TiC0.5 N0.5 phase*

E (GPa)

HV10 (kg/mm2 )

KIC (MPa m1/2 )

Density (g/cm3 )

Coarse (3–5 ␮m) Fine (<100 nm)

284 ± 2 307 ± 2

1422 ± 10 1629 ± 8

7.0 ± 0.2 3.9 ± 0.1

5.76 5.59

*

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

Resistivity ( m) 17 × 10−6 3.0 × 10−6

TiC0.5 N0.5 D50 size (␮m)

TiC0.5 N0.5 D90 size (␮m)

0.23 0.12

0.84 0.33

K. Bonny et al. / Wear 265 (2008) 1767–1775 Table 2 Ra and Rt surface roughness for the EDM finish cut ZrO2 –TiCN composites

Ra (␮m) Rt (␮m)

Fine

Coarse

0.48 4.23

0.70 6.37

2.2. Wear testing The sliding-wear behavior of the wire-EDM ZrO2 –TiCN composites was evaluated using a Plint TE77 tribometer, in which a WC-Co cemented carbide pin (CERATIZIT grade MG12 cemented carbide with 6 wt.% Co) was reciprocally slid against a ZrO2 –TiCN counter plate, in an air-conditioned atmosphere of 23 ± 1 ◦ C and a relative humidity of 60 ± 1%, in conformity with ASTM G133. The pin material displays a compressive strength of 7.2 GPa, a Vickers hardness HV10 of 1913 kg/mm2 , a fracture toughness of 9.3 MPa m1/2 and a stiffness of 609 GPa. The average rounding radius and roughness parameters Ra and Rt were determined to be 4.08 mm, 0.35 ␮m and 2.68 ␮m, respectively, by means of surface scanning equipment [17]. Normal contact forces were varied from 15 N up to 35 N, corresponding to initial Hertzian contact pressures ranging between 1.32 and 1.81 GPa for both ZrO2 –TiCN composites (assuming elastic deformation). The stroke length of the oscillating motion was 15 mm. A sliding frequency of 10 Hz, i.e., a sliding velocity of 0.3 m/s, was applied. The test duration was associated with a travelling distance of 10 km. Before each test, the specimens were rinsed ultrasonically in distilled water with a detergent solution (2% Tickopur R33, 50 ◦ C, 15 min.) and immersed in acetone and in cold distilled water. After each sliding test, the worn surfaces were cleared by blowing pressurized air on the ZrO2 –TiCN surface before post-mortem observations. Two methods were utilized in order to quantify wear loss. Firstly, online measurements of penetration depth curves as a result of the vertical displacement from pin into the counter sample are obtained by an inductive displacement transducer. The second procedure comprises the assessment of the wear scar dimensions, i.e., depth, length, width and volume, using topographical scanning equipment (Somicronic® EMS Surfascan 3D, type SM3, needle type ST305). 3. Results and discussions 3.1. Wear characteristics Typical and representative wear data were obtained during real-time monitoring of combined wear depth for finish EDM cut ZrO2 –TiCN flat/WC-Co pin combinations as function of the sliding distance. For each sliding-wear test, a new WC-Co pin was used in order to ensure identical initial surface conditions. Each experiment was performed at least two times under identical testing parameters, with a standard deviation of less than 10% between different samples of the same material. The error bars indicating the extent

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Table 4 Post-mortem obtained wear track dimensions as function of contact load for EDM finished ZrO2 –TiCN composites slid against WC-Co (s = 10 km) TiC0.5 N0.5 phase

Contact load [N]

Width [mm]

Coarse

15 25 35

1.15 2.05 2.15

12.8 37.3 60.6

Fine

15 25 35

1.4 2.3 2.6

15.1 110.4 272.4

Depth [␮m]

Volume [mm3 ] 0.092 0.51 0.98 0.20 1.61 5.29

of the variations are excluded in order to make the figures better readable. 3.1.1. Online wear Vertical displacement curves measured during reciprocative pin-on-flat wear experiments for the fine-EDM’ed ZrO2 –TiCN composite/WC-Co combinations as function of the sliding distance and applied contact loads are plotted in Fig. 2. The online measured penetration depth of the examined tribopairs turns out to be lower for the coarse grade ZrO2 –TiCN samples. Both axes are presented in a logarithmic scale as wear depth and sliding distance vary over several orders of magnitude and, moreover, this allows to investigate the initial wear process in more detail. For both ZrO2 –TiCN composites, wear loss is noticed to increase abruptly during the first meters of sliding and then gradually ascends further with growing pin-on-plate contact surface. Nearby a sliding distance of 1 km, wear depth appears to reach an almost constant level, and subsequently increases again at a nearly constant rate. At this point, the sliding-wear process is found in a steady-state situation. From the curves presented in Fig. 2, it can be estimated that the penetration depth, beyond a running-in stage, varies in a linear logarithmic correlation with sliding distance, with a deviation occurring near a wear path of 1 km. This discontinuity corresponds well to the phenomena established in the frictional characteristics of these ZrO2 –TiCN composites against WC-Co pins [17]. In agreement with the frictional behavior, the generated wear damage is found to increase with increasing contact load for both ZrO2 –TiCN composites, as illustrated in Table 3. 3.1.2. Post-mortem wear In order to examine the reciprocating sliding-wear behavior, research was concentrated on the steady-state wear regime. Wear tests were carried out under the condition of a constant total sliding distance of 10 km, ensuring that the wear regimes lay at a steady-state situation. From the 3D wear track topographies for the ZrO2 –TiCN composites, measured on wear surfaces generated during the sliding experiments described in Fig. 2, wear scar dimensions are derived, Table 4. Under identical conditions of wear path, sliding speed and contact load, the largest wear track depth, width and volume are encountered with the fine ZrO2 –TiCN composites, in full agreement with the results of Fig. 2 and Table 3.

Table 3 Instantaneous vertical displacement (d) as a function of sliding distance and contact load for the fine-EDM’ed ZrO2 –TiCN flat/WC-Co pin combinations d (15 m) [␮m]

d (1 km) [␮m]

d (4 km) [␮m]

d (10 km) [␮m]

TiC0.5 N0.5 phase

Imposed normal force [N]

Coarse

15 25 35

1.2 2.1 3.0

6.9 19.8 36.8

9.8 27.4 45.4

14.3 39.3 63.1

Fine

15 25 35

1.4 6.3 11.0

4.3 55.8 161.5

8.6 77.9 197.8

16.5 113.2 274.8

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Fig. 2. Penetration depth versus sliding distance for EDM finished coarse (a) and fine (b) ZrO2 –TiCN flat/WC-Co pin pairs sliding at 0.3 m/s under various contact loads.

It is worth noting that the length of the wear scars corresponds with the 15 mm stroke length. Comparing the post-mortem, i.e., extracted from surface scanning topography, with the online measured, i.e., obtained by real-time vertical displacement recording, wear depth indicates small deviations. This is partly attributed to the fact that the pin wear loss contribution, included within the online registration, was not taken into account during postmortem quantification. However, the correspondence between the online and post-mortem obtained results appears quite acceptable, and thus, it may be concluded that the online wear monitoring technique produces a reliable sliding-wear prediction for the investigated tribocouples. In reciprocating sliding experiments, the specific wear rate (k) is defined as the wear loss per unit of applied normal contact force and sliding distance, and thus, a “volumetric wear rate” (kV ) can be derived from post-mortem measured wear scar volumes Vwear , Eq. (1):

kV =

Vwear FN s



mm3 Nm

 (1)

From the wear track dimensions presented in Table 4, the corresponding kV (10 km) wear rates for the coarse and fine ZrO2 –TiCN composites are derived, Table 5. In the full range of imposed normal contact forces, the lowest wear rate after a 10 km sliding distance is encountered for the coarse ZrO2 –TiCN composite. These findings confirm the previously observed impact of the secondary TiC0.5 N0.5 -phase on the wear process. This finding is in agreement with previous investigations [21–24], where, under dry sliding conditions, material properties such as hardness and fracture toughness, as well as microstructural features such as grain size and grain shape, were demonstrated to exhibit a considerable influence on wear resistance.

Table 5 Volumetric wear rates for the fine-EDM’ed ZrO2 –TiCN composite flat/WC-Co pin combinations as function of contact load kV (10 km) [10−6 mm3 N−1 m−1 ]

Contact load [N] 15 N

Coarse Fine

0.62 1.32

25 N

35 N

2.05 6.46

2.79 15.1

3.2. Effect of contact load In agreement with previous investigation on zirconia based ceramics [23] and more general research on ceramics [24], the wear behavior is strongly influenced by contact load. This is confirmed by Fig. 3, which compares the post-mortem obtained wear volume and volumetric wear rates resulting from a sliding-wear path of 10 km for the coarse and fine ZrO2 –TiCN composite as function of the imposed normal contact force. The wear volume is plotted on a logarithmic scale in order to display all the values, which vary over more than two orders of magnitude. Both wear volume loss and volumetric wear rate are noticed to increase with increasing contact load. The highest wear level was found for the fine ZrO2 –TiCN composite, whereas the best wear resistance was encountered with the coarse ZrO2 –TiCN grade. For example, after a sliding distance of 10 km under high-load testing, the wear rate for the coarse ZrO2 –TiCN composite was more than five times lower compared to the fine ZrO2 –TiCN composite. The distinction in wear loss between the ZrO2 –TiCN composites established for a 15 N contact load is noticed to increase with higher contact loads, particularly for contact loads higher than 25 N. The pronounced differences in wear levels between the ZrO2 –TiCN composites could point to a load- and material-dependent wear transition [4,25], which would occur at lower contact load for the fine composite of ZrO2 –TiCN. As indicated by Hamilton and Goodman [26], large tensile stresses are formed at the rear of a sliding contact. Under certain conditions, the tensile stress at the tribological contact surface could result in either crack propagation or brittle fracture, leading to a larger coefficient of friction and severe wear damage [27]. 3.3. Wear surface analysis The worn surfaces were investigated by scanning electron microscopy. Generally, the wear process in ceramic materials is mainly caused by plastic deformation controlled and microcrack controlled mechanisms [24,27,28]. In this paper, SEM analysis of the ZrO2 –TiCN wear track surfaces identified the occurring wear mechanisms as polishing, microabrasion, microcracking and wear debris film formation, Figs. 5 and 6. Within the range of applied test conditions, the optical appearance of the wear track was smooth, indicating that the original surface roughness of the wire-EDM’ed ZrO2 –TiCN composites was reduced as a result of the sliding contact with the WC-Co

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Fig. 3. Post-mortem wear volume (a) and volumetric wear rate (b) as a function of contact load for fine-EDM’ed ZrO2 –TiCN composite flat/WC-Co pin sliding pairs (v = 0.3 m/s, s = 10 km).

pin. This phenomenon is exemplified for the coarse ZrO2 –TiCN grade in Fig. 4, in which normal roughness surface profiles before and after wear testing are compared. From the profile in Fig. 4(b), Ra - and Rt -values of 0.01 and 0.08 ␮m, respectively, were derived, which is much below the roughness level obtained for the original wire-EDM surface finishing conditions, Table 2. Furthermore, after each sliding-wear experiment, wear debris were found, especially at the outer extremities but occasionally

Fig. 4. Normal roughness profile for a ZrO2 –TiCN composite (a) before wear testing and (b) in the wear scar after sliding 10 km against WC-Co cemented carbide.

also inside, along and adjacent to the complete wear tracks. The wear debris that was located outside the wear tracks has been collected for detailed SEM analysis. Scanning electron micrographs of the wear debris derived from fine ZrO2 –TiCN surface finished by wire-EDM and slid against WC-Co pins at a 10 Hz oscillating frequency and 35 N contact load are presented in Fig. 7. No phases could be differentiated in the debris by means of backscattered electron images, indicating that the original ZrO2 and TiCN phases are integrated in the debris material, which contains Zr, Ti, Al and O. The examined wear debris particles generally displayed the original (nanometric) size. However, some of the generated wear debris particles remain for a certain time in the contact between the two surfaces [29], where they were observed to be transformed into rolls, loose particles or a compact surface film by the continuous sliding movement of the pin, and also are able to modify the contact stresses [30,31]. Previous research has demonstrated that high normal loads, and thus, high contact pressures can promote the mechanical bonding of wear debris [32]. In addition, the circulation of wear debris developing in the sliding interface appears to simultaneously exhibit a strong influence on both friction and wear rate response [32,33]. These trends were confirmed by comparing the sliding-wear responses of the ZrO2 –TiCN composites, i.e., the strong impact of the secondary phase and the contact load on the wear characteristics, as well as by a detailed examination of the wear surface morphologies. Secondary electron (SE) and atomic number (BSE) morphologies of the worn surface of the fine ZrO2 –TiCN composite after reciprocal sliding at 10 Hz against a WC-Co pin for 10 km under distinctive normal contact forces are presented in Fig. 5. For the wear surface caused by a 15 N contact load, Fig. 5(a, b), the microstructure in the wear scar corresponds to the microstructure of the base material. The grain size of the TiCN phase is not changed due to the sliding of the pin. Abrasive microgrooves parallel to the sliding direction of the pin are clearly visible, Fig. 5(a). From Figs. 4 and 5(a, b), it can be inferred that polishing and abrasion are the primary wear mechanisms, within a mild wear regime. As for the 25 N/0.3 m/s/10 km test conditions, the microstructure in the central part of the wear scar of the fine ZrO2 –TiCN composite still corresponds to that of the base material, Fig. 5(c). Similar to the observations in Fig. 5(a, b), surface damage occurs as abrasive grooves in a burnished wear surface. Furthermore, a small amount of wear debris appears on the wear track surface,

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Fig. 5. SE and BSE micrographs and EDX analysis in the central wear track of ZrO2 –TiCN composites finished by wire-EDM, after sliding 10 km at 0.3 m/s under (a, b) 15 N (c, d) 25 N (e, f) 35 N contact load.

Fig. 5(c). These wear debris occur in the form of some loose particles, existing of very fine TiC0.5 N0.5 and ZrO2 , and some adhering material in the form of band-like zones as well. This is evidenced by EDX analysis, which clearly shows the presence of the element tungsten (W), originating from the cemented carbide pin, on the ZrO2 –TiCN wear surface, Fig. 5(d). Hence, the phenomenon of adhesive wear should be considered as an additional wear mechanism for the fine ZrO2 –TiCN flat/WC-Co pin sliding combinations.

When a 35 N contact load was applied during the sliding experiments, the wear track of the fine ZrO2 –TiCN composite was covered with a thin layer of wear debris in band-like zones, Fig. 5(e). Moreover, microcracks are observed in the wear debris layer, Fig. 5(f), which are induced by tangential stresses due to the reciprocal sliding movement of the cemented carbide pin. Atomic number (BSE) and secondary electron (SE) micrographs of the worn surfaces of the coarse ZrO2 –TiCN composite after reciprocal sliding at 10 Hz against a WC-Co pin for 10 km under

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Fig. 6. BSE and SE micrographs in the central wear track of coarse ZrO2 –TiCN composites finished by wire-EDM, after sliding 10 km at 0.3 m/s under contact load of (a) 15 N (b) 25 N (c, d) 35 N.

distinctive normal contact forces are compared in Fig. 6. Contrary to the wear surface morphologies for the fine ZrO2 –TiCN composite, all wear tracks displayed a microstructure corresponding to the microstructure of the base material, Fig. 6(a, b, c), and wear debris layer formation did not occur at all. However, debris layer was barely initiated for the 35 N contact load, Fig. 6(c, d). For the 25 N loading conditions a small amount of wear debris appeared as bright particles on the wear track surface, Fig. 6(b). From Figs. 5 and 6, it can be inferred that the increasing wear volume with rising contact load is associated with more pronounced occurrence of wear mechanisms: polishing, abrasion, adhering wear debris layer formation and microcracking. The observed microcracks do not seem to affect the substrate but only a shallow film of debris laying on the worn surface. They are only resulting from shear stresses caused by deformation of the transfer layer. Indeed, tensile stresses appear around Hertzian contacts but they produce deep macroscopic cracks just at the back of the track. Inside the track, shear stresses are predominant. A typical zirconia phase transformation induced by mechanical stresses was not evidenced. The 15 N contact loading conditions are linked to a mild wear regime for both investigated ZrO2 –TiCN composites. The counter faces are only polished and micro-abraded by asperities of the harder pin material. At 25 N, the formation of adhering

wear debris is initiated for the fine grade ZrO2 –TiCN composite, whereas the coarse ZrO2 –TiCN composite only displays a small amount of loose wear debris particles. The 35 N load fine grade ZrO2 –TiCN wear surface turns out to be covered by a debris film, whereas of a wear debris layer is barely initiated on the coarse ZrO2 –TiCN composites. These wear surface state observations are summarized as function of contact load in Table 6 and the differences between the coarse and fine ZrO2 –TiCN grade can be associated with the previous findings of wear volume and volumetric wear rate being higher for the fine ZrO2 –TiCN composite. Combined with the findings in Fig. 4, the change from clean wear surface to the formation of a continuous debris film presumes the occurrence of a load-dependent wear transition, which turns out to depend on the secondary TiC0.5 N0.5 phase as well.

Table 6 Correlation between wear surface state observations, starting powder TiC0.5 N0.5 size and load during wear testing of ZrO2 –TiCN composites against WC-Co pins (C = clean, D = debris layer) Load

Fine

Coarse

15 N 25 N 35 N

C D (start) D

C C D (start)

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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 cemented carbide pins.

References

Fig. 7. SE micrographs of the wear debris originating from the fine ZrO2 –TiCN composite, slid against WC-Co pins.

4. Conclusions Dry reciprocative sliding experiments on ZrO2 –TiCN composites with 40 vol.% TiC0.5 N0.5 phase, obtained from distinctive ZrO2 and TiC0.5 N0.5 powder sources and surface finished by wireEDM, against WC-Co pins under contact loads of 15, 25 and 35 N, revealed several mechanisms involved in their wear process: polishing, microabrasion and adhering wear debris film formation. Microstructure refinement of the secondary phase does not improve the wear performance of ZrO2 –TiCN at all. After a 10 km sliding distance under high-load testing, the volumetric wear rate for the Coarse ZrO2 –TiCN composite was 1.5 × 10−5 mm3 N−1 m−1 , or more than five times lower compared to the Fine ZrO2 –TiCN grade. Furthermore, wear rate was found to increase considerably with higher contact loads. These trends could be associated with the wear debris circulation inside the wear tracks and the concomitant proneness for amorphous wear debris film formation on top of the base material of ZrO2 –TiCN: finer microstructure as well as higher contact load were evidenced to promote this phenomenon. 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

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