- Email: [email protected]
WC composites
Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Wear performance of spark plasma sintered Co/WC and cBN/Co/WC composites Bilge Yaman a,⁎, Hasan Mandal b a b
Eskisehir Osmangazi University, Department of Metallurgical & Materials Engineering, Meselik Campus, TR-26480 Eskisehir, Turkey Sabanci University, Faculty of Engineering and Natural Sciences, TR-34956 Tuzla, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 25 July 2013 Accepted 10 October 2013 Keywords: Tungsten carbide composites Cubic boron nitride Spark plasma sintering technique Wear resistance
a b s t r a c t The present study investigates the efficiency of spark plasma sintering (SPS) technique on the tribological behavior of tungsten carbide composites, and aims to develop the wear resistance of these composites by addition of cubic boron nitride (cBN). Wear tests of spark plasma sintered 6(wt.%)Co/WC, 25(vol%)cBN/6Co/ WC and conventionally fabricated 6(wt.%)Co/WC as a reference sample were performed by ball-on-disk contact with dry and rotational sliding at room temperature in order to determine the friction coefficient and wear rate. Wear mechanisms were explained by using SEM observations and the wear rates were computed by a surface profilometer. In all cases, the major wear mechanisms were observed gradually in the form of microcraking, material removal by grain pull out, and generation and spalling of a tribochemical layer. Based on the experimental results, the addition of cBN considerably enhanced the wear resistance of tungsten carbides. In addition, these results revealed that SPS process has outstanding potential for the fabrication of tungsten carbides with high wear properties for tribological applications. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction For energy saving, environmental, economic and safety aspects, controlling and minimizing friction and wear are crucial issues in tribological applications and hence, there is a rising industrial demand for wear resistant and high performance materials to be applied under heavy tribological conditions [1–3]. Tungsten carbide has been often used in industry in cutting tools, drilling tools, dies etc., owing to its excellent physical and mechanical properties [4–6]. Because of the industrial importance, the wear behavior and mechanisms of these materials have been widely investigated so far [7–10]. On the other hand, only very few references [11,12] can be found in the field of tribology related to tungsten carbides fabricated by spark plasma sintering technique. It is well-known that the tribological, mechanical and microstructural properties of composites vary strongly depending on the processing used in the manufacturing of the product [13,14]. SPS process plays a key role in the formation of the microstructure and on the performance of these materials with controlled grain growth and phase transformation by enhancing the diffusion rates and sinterability in the presence of the pulsed direct electric current [15,16]. SPS combines simultaneously the application of pressure and a high-intensity, low-voltage pulsed DC electric current, flowing directly through the powder that enables very fast heating rates. This high heating rate provides relatively lower ⁎ Corresponding author. Tel.: +90 222 2393750x3696; fax: +90 222 2213918. E-mail addresses: [email protected] (B. Yaman), [email protected] (H. Mandal). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.10.010
sintering temperatures and shorter processing times compared to conventional sintering methods which leads to significant improvements in mechanical properties attained with very fine and homogeneous microstructure, and also prevents undesirable phase transformations such as in the case of cBN transformation to hexagonal BN (hBN) at high temperatures [17–20]. Amongst the few studies related to the wear of spark plasma sintered tungsten carbides, Picas et al. [11] studied the wear resistance of Co/Cr/WC and Co/WC coatings deposited by high velocity oxy-fuel (HVOF) and laser engineered net shaping (LENS) processes in comparison to 12 wt.%Co/WC bulk material produced by SPS. Wear tests were carried out with a pin-on-disk tribometer at a constant linear speed of 0.10 m/s with an applied load of 30 N. They reported that the SPSed sample exhibited a very low wear rate (3.5 m3(Nm)−1 × 10−18) as well as the HVOF sample (3.6 m3(Nm)−1 × 10−18) and both samples had better wear resistance than the LENS coating (8.9 m3(Nm)−1 × 10−18). Staia et al. [12] explored the sliding wear performance of binderless WC specimens fabricated at 1600– 1700 °C by using Plasma Pressure Compaction (P2C). Friction coefficients of 0.26–0.31 against an alumina ball and very low wear constant values, k, of 10−8 mm3/Nm were obtained for binderless WCs. It has been shown that wear behavior was not considerably influenced by the sintering temperature, nor by the initial particle size of the WC powder in the related study [12]. On the other hand, Staia et al. [12] reported that additional research should be performed in order to optimize the processing parameters and evaluate the microstructural and mechanical properties. Many researches on the tribological behavior of tungsten carbides fabricated by traditional methods have been focused on reducing the
10
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
cobalt content to obtain incremental increases in hardness, however resulting in a reduction in both fracture toughness and ductility of carbides. As a solution, it has been tried to maintain a smaller grain size, which is not easy to obtain it after sintering with the conventional processes in most cases [12]. In this study, the reason for preferring the SPS technique for the fabrication of WC composites was to enable a fine microstructure with a corresponding increase in both hardness and toughness without the need to decrease cobalt (Co) content. Moreover, it is one of the most appropriate technique for consolidation of cBN added composites since it allows suppressing of the hBN transformation during sintering. Due to the advantages offered by SPS process and the limited literature available on the subject of wear performance in spark plasma sintered (SPSed) WC composites, the objective of this study is to evaluate if the technique will offer significant improvement to these composites with respect to the microstructure, mechanical and wear properties. Besides it was aimed to improve wear performance by the addition of cBN to the investigated composites. The microstructure, mechanical properties and wear behavior correlation are discussed in detail. 2. Experimental procedures In the present study, prior to the powder processing, in the compositional design step, 25(vol%) cBN (equals to 7.25% of cBN by weight) additive was chosen since this amount in the composite may be high enough to achieve a sufficient increment in mechanical properties to determine the differences in comparison to the WC matrix, but low enough to avoid obtaining polycrystalline cubic boron nitride (PcBN), which forms when high amounts (mostly N 40% by volume) of cBN phase are added. More importantly, this low amount of cBN was preferred as additive since high cBN contents reduce the sinterability of tungsten carbides owing to strong covalent bonding of cBN, low diffusion coefficient of B and N and restriction of the sintering temperatures where cBN is thermodynamically stable [21]. Cobalt, as a second addition to tungsten carbide, was selected at a relatively low amount since the presence of a cobalt-rich liquid phase enhances the tendency of cBN to transform into hBN during sintering [22,23]. Hence, as a starting composition, 6(wt.%)Co/WC (Boehlerit GmbH&CO KG, D50:0.8 μm) and 7.25(wt.%) (25(vol%))cBN (Zibo ShineSo Chemical Material Co. Ltd, D50:5 μm) were mixed by wet ball-milling for 8 h and dried in a rotary evaporator. cBN added composites were designated as 25cBN/6Co/WC according to the volume ratio of cBN in this study. In addition, a reference sample (Boehlerit cutting tool, HB10F Grade, ISO-application area K05–K15) was selected in the same composition (6(wt.%)Co/WC) with the SPSed sample in order to compare and determine the wear performance of tungsten carbide composites obtained by spark plasma sintering technique. Sintering experiments were carried out at 1200°C, for 5min, under a pressure of 50 MPa for 6Co/WC samples, and 1300 °C, 75 MPa pressure was used for 7.5 min in the sintering of 25cBN/6Co/WC composites to achieve high densification. The details of the sintering processes can be found in the previous work [24]. All of the sintered composite's densities were determined according to Archimedes method. The theoretical density for the composite materials was calculated from the rule of mixture according to the following formula; ρth ¼
X
vi ρi
ð1Þ
where ρth is the theoretical density of the composite, vi; the volume fraction of phase i, and ρi its theoretical density (for designation; 6CoWC:14.85 g/cm3(TD), cBN:3.45 g/cm3(TD)). The percentage of the theoretical density was determined by the ratio of measured density to the theoretical density. Crystalline phases were characterized by X-ray diffraction (XRD, Rigaku Rint 200, Tokyo, Japan) using Cu-Kα radiation. The cross sections of the sintered and the reference sample were ground, polished and platinum (Pt) coated for microstructural observation by scanning electron microscopy (SEM) (Zeiss Supra 50 VP) in secondary electrons (SE) mode. The hardness (HV10) and fracture toughness (KIC) measurements were carried out by the Vickers indentation technique, by applying a 10 kg load for 10 s. Fracture toughness was evaluated by the following formula; h i 1=2 ‐3=2 =Φ KIC ¼ 0:15 k HV a ðc=aÞ
ð2Þ
where a is equal to the half length of indentation diagonal in μm, c is the mean of all crack lengths, Φ is a constant (~3), and k is a correction factor. Thermal diffusivity measurements of sintered samples with 13 mm diameters were measured from room temperature (RT) to 600 °C by using the laser-flash method (LFA 457-Netzsch) under a flowing nitrogen gas atmosphere at a flow rate of 100 mbar/s. The thermal diffusivity (a) was calculated using the formula below; a¼
h i 2 2 W x xd = π xt x
ð3Þ
where Wx is a dynamic correction factor to account for the heat losses, d is the thickness of the sample, and tx is the time to half of the maximum of the temperature peak. Wear tests were conducted against a 6Co/WC ball of 3 mm diameter (Table 1) in the ball-on-disk configuration with a microtribometer (CSM Instruments) with 0.15 mm contact radius of rotational (600 min−1) unlubricated sliding under a 15 N constant test load at a linear velocity of 0.92 cm/s at a frequency of 8 Hz for 100.000 cycles and prolonged to a 100 m wear distance. All tests were performed at room temperature (23–25 °C) in air with a relative humidity of 40 ± 5%. The contact schematic diagram used in the wear tests is shown in Fig. 1. Prior to the wear tests, all samples were ground and polished in order to achieve average surface roughness, Ra, in the range of 0.01–0.02 μm in order to minimize the effect of asperities on wear. The coefficient of friction (COF) of investigated samples were obtained online via a computerbased data system by the variation in tangential force recorded during wear tests. After tests, the worn areas of each sample were examined as 2D profiles by a stylus surface profilometer (Mitutoyo SJ-401) and the wear volumes were calculated by integrating these areas over diameter. Specific wear rates were computed by dividing the wear volume by the applied load and sliding distance. Worn surfaces were characterized by using SEM (in SE mode) (Zeiss EVO 50 EP) with an energy-dispersive X-ray spectroscopy (EDS) attachment in order to explain wear mechanisms. 3. Results and discussion Sintered 6Co/WC (1200 °C, 5 min, 50 MPa) and 25%cBN reinforced composites (1300 °C, 7.5 min, 75 MPa) achieved densities of 99.86% and 99.52% of theoretical, respectively. According to XRD investigations (Fig. 2) no compositional changes or no peaks of hBN for cBN/WC/6Co
Table 1 Properties of ball used in wear tests. Balla
Hardness HV10 (kg/mm2)
Compression strength (N/mm2)
Bending str. (N/mm2)
Elasticity constant (Gpa)
Thermal cond. (W/mK)
Heat capacity (J/kg K)
WC–6Co
1400
5300
1600
610
50
201
a
Data from supplier (CSM Instruments).
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
11
Fig. 1. Schematic illustration of the wear test setup used in the present study.
composites, which may form during sintering, were detected. It should be mentioned that the low intensity of cBN and Co peaks resulted from the strong mass-absorption coefficient of the WC phase [25]. SEM observations of the SPSed 6Co/WC (Fig. 3(a)) demonstrate a homogeneous distribution of Co (dark areas) around WC grains with a finer microstructure in comparison to the reference sample in the same magnification (Fig. 3(b)). For the cBN added composites, as seen from Fig. 3(c), cBN particles (dark areas) were uniformly distributed and embedded well with the matrix (Fig. 3(d)). The sintered samples were observed to be dense, apart from a small amount of porosity which can be attributed to the grain pull-out of hard particles in the polished surface that occurred while cutting of cross-section of samples during metallographic preparation. The thermal stability of cBN under the applied sintering conditions in this study was also confirmed by SEM observations. The presence of hBN as platelets precipitated around cBN particles was not observed on the fracture surface of the composites (Fig. 3(d)). According to the few works that have been done on the conversion of cBN to hBN at normal pressure, Eremets et al. [26], showed that cBN “graphitizes”, which means it transforms into hBN (low-hardness form), at ~1227 °C at ambient pressure and Fukunanga [27] demonstrated that the phase transformation from cBN to hBN was very slow at lower temperatures at normal pressure. For example, the ratio of the XRD peak intensity of cBN:hBN was 0.3 at after 48 h at 1250 °C and 18 5 h at 1300 °C, the sintering temperature used in this study. The short sintering time of 7.5 min should have helped prevent
the hBN formation in the composite. In addition, although tungsten carbide with Co binder is generally consolidated above ~1350 °C (the eutectic temperature of the WC–Co–C system) with conventional processes, the high density achieved by the SPSed composite at 1200 °C and 1300 °C may indicate that the pulsed direct current promotes material transfer and diffusion while inhibiting the phase transformation due to the shortened process time. Amongst the tungsten carbides of the same compositions (6Co/WC), similar mechanical properties in the SPSed one (Table 2) as compared to the reference sample (according to data from supplier; 1850 HV30 (kg/mm2) in hardness and 8.9 (MNm−3/2) in fracture toughness) were achieved, even though the SPSed 6Co/WC was fabricated in the sintering temperature of 1200°C which is lower than the sintering temperatures of conventionally fabricated tungsten carbides (1350–1550 °C) and in very short time (5 min). Even the mechanical properties of tungsten carbides depend on several parameters, such as; sintering conditions, Co content and grain size, the results of SPSed 6Co/WC are in agreement with the findings of other researches for similar compositions. Wang et al. [21], reported ~16.5 GPa the hardness value (under a load of 49 N), and ~6.5 MPa.m1/2 the fracture toughness for the 9.7(wt.%)Co/WC samples sintered at 1200 °C, under 50 MPa for 10 min by pulse electric current sintering with average grain size of 1.07 μm. In the same study, it is also investigated the effects of cBN addition to this composites and they obtained 17 GPa hardness and 11 MPa·m1/2 fracture toughness values for 20(vol%)cBN added composites with
Fig. 2. XRD patterns of sintered 6Co/WC and 25cBN/6Co/WC composite (the positions of hBN peaks are indicated by (*)) [24].
12
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
Fig. 3. SE micrographs of (a) SPSed 6Co/WC, (b) 6Co/WC (ref. sample), (c) polished and (d) fractured surface of 25cBN/6Co/WC.
finer average grain size (0.94 μm) of WC by using the same sintering conditions. Rosinski et al. [22] studied the hardness of 30(vol%)cBN/ 6(wt.%)Co/WC sintered by pulse plasma sintering method at 1200 °C, under 100 MPa for 5 min and achieved 2330HK1 (Knoop hardness, 1 kg load) hardness value. Martinez et al. [25] reported 2100 HV5 (kg/mm2) and 7.3 MPa·m1/2 for hot isostatic pressed 5(wt.%)Co/WC (1100 °C, 150 MPa, 60 min); 2150HV5(kg/mm2) and 14.7 MPa·m1/2 mechanical properties for HIPed 30(vol%)cBN/ 5(wt.%)Co/WC composites (1100 °C, 200 MPa, 60 min). These results also agree well with the values determined in this study for cBN added composites (Table 2). Both hardness and toughness values were increased by incorporation of cBN. While high fracture toughness was obtained by the crack-deflection effect of cBN particles as determined according to the indentation tests, the increment in hardness can be attributed to the inhibition of WC grain growth due to the low sintering temperature, as can be clearly seen from fine, round WC grains in Fig. 3(d). Moreover, the cBN particles may also inhibit grain growth by decreasing the grain boundary migration of WC grains [21]. The thermal conductivity is one of the important parameters that influence the temperature build-up at tribocontacts in unlubricated dry sliding wear [28], depending on frictional forces. Since the higher thermal conductivity of cBN (~360–400 W/(m·K) at room temperature [29] is approximately three times more than WC(6Co) (~95 W/(m·K)), adding cBN particles was expected to improve the thermal properties of
tungsten carbides. The measured thermal diffusivities of the sintered samples are given in Fig. 4. The samples incorporating cBN had higher thermal diffusivity, which may help to reduce frictional heating, failure and wear rates of composites induced by thermal stresses at the contact area during sliding. The variation of friction coefficients of the samples against a tungsten carbide ball is illustrated in Fig. 5. The average COF of SPSed 6Co/WC (0.254) exhibited a similar value as the reference sample (0.258). The steady-state COF of 0.27 for the reference sample was revealed within 80–100 m. In order to characterize the steady-state friction, the wear tests were prolonged to 200 m. A slight increase in COF to 0.27 was observed for the SPSed 6Co/WC sample (Fig. 5(a)), which is close to the reference sample. According to these obtained similar COF values for the samples in the same composition, it has not been observed a significant influence of SPS fabrication technique on friction values under the same wear conditions. A higher steady state
Table 2 Mechanical properties of tungsten carbide composites, used as flat materials in wear tests. Sample
HV10 (GPa)
KIC (MPa·m1/2)
6Co/WC (SPS) 25cBN/6Co/WC (SPS)
18.59 ± 0.2 21.23 ± 0.4
6.68 ± 0.5 10.97 ± 0.5
Fig. 4. Thermal diffusivities of SPS sintered tungsten carbide composites [24].
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
13
Fig. 5. The evolution of frictional behavior of all investigated tungsten carbide composites against time (s), sliding distance (m) and lap: (a) SPSed 6Co/WC, (b) SPSed 25cBN/6Co/WC, (c) reference sample.
COF of 0.36 was recorded in cBN added tungsten carbide (Fig. 5(b)), as expected since it possessed the highest hardness amongst all the samples. The increased fluctuation within the 70 m was may be a result of pull-out of hard cBN particles from the interface between the flat and ball contact. The calculated worn volumes and specific wear rates are shown in Table 3. The 2D topography traces (Fig. 6) indicate that very shallow depth occurred in both SPSed composites. Based on the results, wear can be classified as mild wear regime under these selected test
conditions, according to the wear volumes which ranged at 10−6 mm3 (Table 3). The SPSed 6Co/WC sample exhibited a wear rate of 3.37 × 10−9 mm3/Nm, which means approximately 45% improvement
Table 3 Specific wear rates for investigated tungsten carbide composites. Sample 6Co/WC 6Co/cBN/WC/ 6Co/WC (ref)
Wear volume (mm3) −06
5.05 × 10 3.48 × 10−06 9.12 × 10−06
Wear rate (mm3/N·m) 3.37 × 10−09 2.32 × 10−09 6.08 × 10−09
Fig. 6. 2-D surface profiles measured using surface profilometer of worn surfaces on investigated materials.
14
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
Fig. 7. SEM (SE) micrographs of the worn surfaces of (a) SPSed 6Co/WC and (b) 6Co/WC (ref. sample), in insets, part of the wear region is presented with sliding direction indicated by double-pointed arrows, (c) and (d) EDS spectra of unworn surface and tribolayer of SPSed 6Co/WC, respectively.
was achieved in comparison to wear rate of the reference tungsten carbide fabricated by conventional sintering process. The sample with cBN content showed the lowest wear loss amongst all the investigated tungsten carbides. In order to understand and compare the wear mechanisms of tungsten carbides of the same composition fabricated with different methods, the topographical features of their worn surfaces were analyzed by SEM and presented in Fig. 7(a–b). In the case of SPSed 6Co/WC, as can be seen from the inset micrograph, the worn surface was smooth and covered continuously by a very thin tribolayer all over the surface as well as spalling around the edges of the wear scar. In contrast, the formation of the tribolayer was discontinuous in the worn zone of the reference sample. The tribolayers and also unworn surfaces were characterized by energy dispersive spectroscopy (EDS) and representative analyses are presented in Fig. 7(c–d). According to these analyses, higher oxygen content was found in the tribolayer zone of SPSed 6Co/WC than in the unworn surface and it was also richer in oxygen compared to the tribolayer of reference sample. This indicates that the tungsten and cobalt were oxidized as sliding proceeded depending on frictional heat generation, especially depending on contact flash temperature generated on asperities, and presumably the reaction products should consist of various types of oxides, such as WO3 and CoO2. Formation of this surface enrichment of oxidation may explain the reduction in wear rate of the SPSed sample due to its lubrication effect and it may play a role as a protector of the wear surface. Various reports [30] have shown that oxide ceramics, such as tungsten oxides, retain lubricious properties to protect such surfaces from damage in tribological environments. The smooth tribolayer surface in the SPSed sample might have diminished the mechanical stresses by reducing the number of asperity contacts. The reason that
the layer was regarded as stable during test can be attributed to the amount of grain removal, which can be clearly seen in the enlarged views of each worn zone (Fig. 7(a–b)). The amount of the carbide grain pull out for the reference sample fabricated by conventional method was significantly higher than for the SPSed 6Co/WC sample (Fig. 7(b)), which may resulted in weak formation and fragmentation of the tribolayer. As shown some representative grain pull-outs illustrated by circles in Fig. 7(a–b), less number of grain pull-out in SPSed 6Co/WC may have caused the permanency of this tribochemical layer during test which may help to reduce the adhesion wear of this sample. On the other hand, the repetition of the formation, fragmentation and the removal of this tribochemical layer at the contact area during all sliding should increase the adhesive wear of reference sample. This situation may also explain the frictional behavior of the reference sample as shown in Fig. 5(c), where friction might have been decreased by the generation of the tribochemical layer corresponding to lower frictional forces, fluctuated owing to the discontinuous formation and increased by removal of this layer within 20 m–80 m, whereas frictional behavior of SPSed tungsten carbide tended to be stable during all sliding distance with the more undeformable tribochemical coverage surface. Furthermore, even though they are the same composition; higher wear rate and deep grooves were observed in the conventional tungsten carbide, which indicates that more abrasive wear was generated. According to the SEM investigations, in both samples, material removal in the case of abrasive wear originated from the development of intergranular microcracks, followed by fragmentation and/or removal of hard carbide grains which were caused by the detachment of the Co binder phase at the grain boundaries of tungsten carbide. Deep abrasive grooves in the worn region of the reference sample may be caused by these removed and trapped hard carbide particles that act as abrasives,
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
such as third-body particles at the interface which can also be presume to increase the wear rate. On the other hand, the contribution of the SPS method in exhibiting higher wear resistance of samples and/or limiting the grain pull-out can be attributed to the benefit of enhanced sintering by pressure and short sintering time which leads to limitation of grain growth. The fine grain size of the SPSed sample played an important role in obtaining low wear rates. While weak grain boundaries in the reference sample facilitated crack propagation, in the SPSed sample, the fine microstructure reduced the probability of intergranular cracking due to possessing higher grain boundary strength. Owing to Joule heating and the applied pressure in SPS technique facilitates the formation of several necks between particles during sintering, which gradually develop and increase in a very short sintering time [17,19], the strength of bonding between WC grains might therefore be
15
enhanced. As observed in Fig. 7(a), WC grains remained well-bonded with the matrix in SPSed tungsten carbide and thus, the grain pull-out is less than the reference sample. As a result, the application of SPS technique makes it possible to enhance wear resistance of carbides with no need to remove or reduce the amount of the binder phase. The worn surface of 25cBN/6Co/WC sample is given in Fig. 8(a). The abrasion scratches were mild which can be seen in the enlarged view (Fig. 8(b)). According to microstructural analyses, although the wear mechanisms like abrasion and the formation of tribochemical layer were similar to the tungsten carbides with no addition, no large cavities or damages were observed in the worn area, and the detachment of cBN particles from the matrix was of minor importance. The amount of grain pull-outs for both cBN and WC particles were the least in this composite amongst all the other samples. As reasons mentioned before with regard to the SPSed 6Co/WC sample, very strong bonding between cBN particles and tungsten carbide matrix caused a minimum wear rate by reducing the fragmentation and removal of material. Furthermore, the positive effect of cBN addition on improving the wear resistance can be explained by assuming that the compressive stresses occurred during sliding contact were carried by cBN particles which has higher hardness than carbides and thus, the tungsten carbide matrix was protected. In addition, as can be observed from micrograph (Fig. 8(a)), a thicker and more adhesive tribochemical layer was obtained possibly by the oxidation of BN apart from tungsten and binder oxide (Fig. 8(c)), which may have served as a protection from surface damage. Moreover, the increased wear resistance by the addition of cBN phase can be attributed to increasing both hardness and fracture toughness which aids in resisting crack propagation through crack deflection. 4. Conclusions Tribological performance of tungsten carbide composites fabricated by spark plasma sintering technique has been characterized in reciprocating sliding dry contact by using ball-on-disk geometry. Friction coefficient of SPS sintered 6Co/WC was 0.25 as low as reference 6Co/WC and the 25cBN/6Co/WC's was determined as 0.36. The wear rates varied between (2.32 and 3.37) × 10−09 for SPSed composites. Wear data indicated that spark plasma sintered tungsten carbide composites showed better wear resistance as compared to conventional fabricated tungsten carbide. Besides, the addition of cBN phase significantly increased wear resistance by enhancing the mechanical properties. According to this promising result and owing to SPS being one of the most proper techniques for fabrication of cBN composites without phase transformation, these SPSed cBN/Co/WC composites have potential for tribological applications especially under these applied static loads. Besides, the wear behavior of WC composites containing cBN with different amounts can be investigated as a future research to explore the relationship between wear and properties in detail since there has not been much study in relevant literature. Dominating wear mechanisms can be summarized as microabrasion, microfragments, binder and carbide grains pull-out and formation of thin tribolayer containing oxides under selected operating conditions. The experimental results were attributed to features of spark plasma sintering technique which enabled control of microstructural characteristics such as a fine grain size, leading to improved mechanical properties. Due to high tool life becoming an important factor in tool design, it is believed that spark plasma sintering technique would play an important role for the material design process in tribological applications. References
Fig. 8. SEM micrographs (a–b) and EDS spectra (c) of the worn surface of the SPSed 25cBN/WC/6Co. Wear width in (a) and tracks in (b) are presented by double-pointed arrows.
[1] Holmberg K, Ronkainen H, Laukkanen A, Wallin K. Friction and wear of coated surfaces — scales, modelling and simulation of tribomechanisms. Surf Coat Technol 2007;202:1034–49. [2] Van Acker K, Vanhoyweghen D, Persoons R, Vangrunderbeek J. Influence of tungsten carbide particle size and distribution on the wear resistance of laser clad WC/Ni coatings. Wear 2005;258:194–202.
16
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
[3] Bonny K, De Baets P, Ost W, Vleugels J, Huang S, Lauwers B, et al. Influence of electrical discharge machining on the reciprocating sliding wear response of WC–Co cemented carbides. Wear 2009;266:84–95. [4] Liu K, Li XP, Rahman M, Liu XD. CBN tool wear in ductile cutting of tungsten carbide. Wear 2003;255:1344–51. [5] Hegeman JBJW, De Hosson JThM, De With G. Grinding of WC–Co hard metals. Wear 2001;248:187–96. [6] Andrén HO. Microstructure development during sintering and heat-treatment of cemented carbides and cermets. Mater Chem Phys 2001;67:209–13. [7] Beste U, Jacobson S. A new view of the deterioration and wear of WC/Co cemented carbide rock drill buttons. Wear 2008;264:1129–41. [8] Saito H, Iwabuchi A, Shimizu T. Effects of Co content and WC grain size on wear of WC cemented carbide. Wear 2006;261:126–32. [9] Sheikh-Ahmad JY, Bailey JA. The wear characteristics of some cemented tungsten carbides in machining particleboard. Wear 1999;225–229:256–66. [10] Gant AJ, Gee MG. Abrasion of tungsten carbide hard metals using hard counterfaces. Int J Refract Met Hard Mater 2006;24:189–98. [11] Picas JA, Xiong Y, Punset M, Ajdelsztajn L, Forn A, Schoenung JM. Microstructure and wear resistance of WC–Co by three consolidation processing techniques. Int J Refract Met Hard Mater 2009;27:344–9. [12] Staia MH, Torres IJ, Castillo C, Sudarshan TS, Lesage J, Chicot D. Tribological study of WC produced by plasma pressure compaction. Int J Refract Met Hard Mater 2006;24:183–8. [13] Chaim R. Densification mechanisms in spark plasma sintering of nanocrystalline ceramics. Mater Sci Eng A 2007;443:25–32. [14] Reis P, Davim JP, Xu X, Ferreira JMF. Tribological behaviour of colloidally processed sialon ceramics sliding against steel under dry conditions. Tribol Lett 2005;18(3):295–301. [15] Yucheng W, Zhengyi F. Study of temperature field in spark plasma sintering. Mater Sci Eng 2002;B90:34–7. [16] Belmonte M, González-Julián J, Miranzo P, Osendi MI. Spark plasma sintering: a powerful tool to develop new silicon nitride-based materials. J Eur Ceram Soc 2010;30:2937–46.
[17] Orru R, Licheri R, Locci AM, Cincotti A, Cao G. Consolidation/synthesis of materials by electric current activated/assisted sintering. Mater Sci Eng 2009;R 63:127–287. [18] Kim HC, Jeong IK, Shon IJ, Ko IY, Doh JM. Fabrication of WC–8wt.%Co hard materials by two rapid sintering processes. Int J Refract Met Hard Mater 2007;25:336–40. [19] Anselmi-Tamburini U, Garay JE, Munir ZA. Fast low-temperature consolidation of bulk nanometric ceramic materials. Scr Mater 2006;54:823–8. [20] Hungria T, Galy J, Castro A. Spark plasma sintering as a useful technique to the nanostructuration of piezo-ferroelectric materials. Adv Eng Mater 2009;11(8):615–31. [21] Wang B, Matsumaru K, Yang J, Fu Z, Ishizaki K. The effect of cBN additions on densification, microstructure and properties of WC–Co composites by pulse electric current sintering. J Am Ceram Soc 2012;95(8):2499–503. [22] Rosinski M, Michalski A. WC/Co/cBN composites produced by pulse plasma sintering method. J Mater Sci 2012;47:7064–71. [23] Upadhyaya GS. Cemented carbides production, properties and testing. William Andrew Publishing/Noyes; 1998 [420 pp.]. [24] Yaman B, Mandal H. Spark plasma sintering of Co–WC cubic boron nitride composites. Mater Lett 2009;63:1041–3. [25] Martinez V, Echeberria J. Hot isostatic pressing of cubic boron nitride–tungsten carbide/cobalt (cBN–WC/Co) composites: effect of cBN particle size and some processing parameters on their microstructure and properties. J Am Ceram Soc 2007;90:415–24. [26] Eremets MI, Takemura K, Yusa H, Golberg D, Blank VD, Sato Y, et al. Disordered state in first order phase transitions: hexagonal-to-cubic and cubic-to-hexagonal transitions in boron nitride. Phys Rev B Condens Matter Mater Phys 1998;57:5655–60. [27] Fukunanga O. The equilibrium phase boundary between hexagonal and cubic boron nitride. Diam Relat Mater 2000;9:7–12. [28] Basu B, Mukhopadhyay A, Mishra A, Sarkar J. Does thermal conductivity play a role in sliding wear of metals in cryogenic environment? J Tribol 2010;132(4):041604-1–5. [29] Wang ZG, Rahman M, Wong YS. Tool wear characteristics of binderless CBN tools used in high-speed milling of titanium alloys. Wear 2005;258:752–8. [30] Greenwood OD, Moulzolf SC, Blau PJ, Lad RJ. The influence of microstructure on tribological properties of WO3 thin films. Wear 1999;232:84–90.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Wear performance of spark plasma sintered Co/WC and cBN/Co/WC composites Bilge Yaman a,⁎, Hasan Mandal b a b
Eskisehir Osmangazi University, Department of Metallurgical & Materials Engineering, Meselik Campus, TR-26480 Eskisehir, Turkey Sabanci University, Faculty of Engineering and Natural Sciences, TR-34956 Tuzla, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 25 July 2013 Accepted 10 October 2013 Keywords: Tungsten carbide composites Cubic boron nitride Spark plasma sintering technique Wear resistance
a b s t r a c t The present study investigates the efficiency of spark plasma sintering (SPS) technique on the tribological behavior of tungsten carbide composites, and aims to develop the wear resistance of these composites by addition of cubic boron nitride (cBN). Wear tests of spark plasma sintered 6(wt.%)Co/WC, 25(vol%)cBN/6Co/ WC and conventionally fabricated 6(wt.%)Co/WC as a reference sample were performed by ball-on-disk contact with dry and rotational sliding at room temperature in order to determine the friction coefficient and wear rate. Wear mechanisms were explained by using SEM observations and the wear rates were computed by a surface profilometer. In all cases, the major wear mechanisms were observed gradually in the form of microcraking, material removal by grain pull out, and generation and spalling of a tribochemical layer. Based on the experimental results, the addition of cBN considerably enhanced the wear resistance of tungsten carbides. In addition, these results revealed that SPS process has outstanding potential for the fabrication of tungsten carbides with high wear properties for tribological applications. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction For energy saving, environmental, economic and safety aspects, controlling and minimizing friction and wear are crucial issues in tribological applications and hence, there is a rising industrial demand for wear resistant and high performance materials to be applied under heavy tribological conditions [1–3]. Tungsten carbide has been often used in industry in cutting tools, drilling tools, dies etc., owing to its excellent physical and mechanical properties [4–6]. Because of the industrial importance, the wear behavior and mechanisms of these materials have been widely investigated so far [7–10]. On the other hand, only very few references [11,12] can be found in the field of tribology related to tungsten carbides fabricated by spark plasma sintering technique. It is well-known that the tribological, mechanical and microstructural properties of composites vary strongly depending on the processing used in the manufacturing of the product [13,14]. SPS process plays a key role in the formation of the microstructure and on the performance of these materials with controlled grain growth and phase transformation by enhancing the diffusion rates and sinterability in the presence of the pulsed direct electric current [15,16]. SPS combines simultaneously the application of pressure and a high-intensity, low-voltage pulsed DC electric current, flowing directly through the powder that enables very fast heating rates. This high heating rate provides relatively lower ⁎ Corresponding author. Tel.: +90 222 2393750x3696; fax: +90 222 2213918. E-mail addresses: [email protected] (B. Yaman), [email protected] (H. Mandal). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.10.010
sintering temperatures and shorter processing times compared to conventional sintering methods which leads to significant improvements in mechanical properties attained with very fine and homogeneous microstructure, and also prevents undesirable phase transformations such as in the case of cBN transformation to hexagonal BN (hBN) at high temperatures [17–20]. Amongst the few studies related to the wear of spark plasma sintered tungsten carbides, Picas et al. [11] studied the wear resistance of Co/Cr/WC and Co/WC coatings deposited by high velocity oxy-fuel (HVOF) and laser engineered net shaping (LENS) processes in comparison to 12 wt.%Co/WC bulk material produced by SPS. Wear tests were carried out with a pin-on-disk tribometer at a constant linear speed of 0.10 m/s with an applied load of 30 N. They reported that the SPSed sample exhibited a very low wear rate (3.5 m3(Nm)−1 × 10−18) as well as the HVOF sample (3.6 m3(Nm)−1 × 10−18) and both samples had better wear resistance than the LENS coating (8.9 m3(Nm)−1 × 10−18). Staia et al. [12] explored the sliding wear performance of binderless WC specimens fabricated at 1600– 1700 °C by using Plasma Pressure Compaction (P2C). Friction coefficients of 0.26–0.31 against an alumina ball and very low wear constant values, k, of 10−8 mm3/Nm were obtained for binderless WCs. It has been shown that wear behavior was not considerably influenced by the sintering temperature, nor by the initial particle size of the WC powder in the related study [12]. On the other hand, Staia et al. [12] reported that additional research should be performed in order to optimize the processing parameters and evaluate the microstructural and mechanical properties. Many researches on the tribological behavior of tungsten carbides fabricated by traditional methods have been focused on reducing the
10
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
cobalt content to obtain incremental increases in hardness, however resulting in a reduction in both fracture toughness and ductility of carbides. As a solution, it has been tried to maintain a smaller grain size, which is not easy to obtain it after sintering with the conventional processes in most cases [12]. In this study, the reason for preferring the SPS technique for the fabrication of WC composites was to enable a fine microstructure with a corresponding increase in both hardness and toughness without the need to decrease cobalt (Co) content. Moreover, it is one of the most appropriate technique for consolidation of cBN added composites since it allows suppressing of the hBN transformation during sintering. Due to the advantages offered by SPS process and the limited literature available on the subject of wear performance in spark plasma sintered (SPSed) WC composites, the objective of this study is to evaluate if the technique will offer significant improvement to these composites with respect to the microstructure, mechanical and wear properties. Besides it was aimed to improve wear performance by the addition of cBN to the investigated composites. The microstructure, mechanical properties and wear behavior correlation are discussed in detail. 2. Experimental procedures In the present study, prior to the powder processing, in the compositional design step, 25(vol%) cBN (equals to 7.25% of cBN by weight) additive was chosen since this amount in the composite may be high enough to achieve a sufficient increment in mechanical properties to determine the differences in comparison to the WC matrix, but low enough to avoid obtaining polycrystalline cubic boron nitride (PcBN), which forms when high amounts (mostly N 40% by volume) of cBN phase are added. More importantly, this low amount of cBN was preferred as additive since high cBN contents reduce the sinterability of tungsten carbides owing to strong covalent bonding of cBN, low diffusion coefficient of B and N and restriction of the sintering temperatures where cBN is thermodynamically stable [21]. Cobalt, as a second addition to tungsten carbide, was selected at a relatively low amount since the presence of a cobalt-rich liquid phase enhances the tendency of cBN to transform into hBN during sintering [22,23]. Hence, as a starting composition, 6(wt.%)Co/WC (Boehlerit GmbH&CO KG, D50:0.8 μm) and 7.25(wt.%) (25(vol%))cBN (Zibo ShineSo Chemical Material Co. Ltd, D50:5 μm) were mixed by wet ball-milling for 8 h and dried in a rotary evaporator. cBN added composites were designated as 25cBN/6Co/WC according to the volume ratio of cBN in this study. In addition, a reference sample (Boehlerit cutting tool, HB10F Grade, ISO-application area K05–K15) was selected in the same composition (6(wt.%)Co/WC) with the SPSed sample in order to compare and determine the wear performance of tungsten carbide composites obtained by spark plasma sintering technique. Sintering experiments were carried out at 1200°C, for 5min, under a pressure of 50 MPa for 6Co/WC samples, and 1300 °C, 75 MPa pressure was used for 7.5 min in the sintering of 25cBN/6Co/WC composites to achieve high densification. The details of the sintering processes can be found in the previous work [24]. All of the sintered composite's densities were determined according to Archimedes method. The theoretical density for the composite materials was calculated from the rule of mixture according to the following formula; ρth ¼
X
vi ρi
ð1Þ
where ρth is the theoretical density of the composite, vi; the volume fraction of phase i, and ρi its theoretical density (for designation; 6CoWC:14.85 g/cm3(TD), cBN:3.45 g/cm3(TD)). The percentage of the theoretical density was determined by the ratio of measured density to the theoretical density. Crystalline phases were characterized by X-ray diffraction (XRD, Rigaku Rint 200, Tokyo, Japan) using Cu-Kα radiation. The cross sections of the sintered and the reference sample were ground, polished and platinum (Pt) coated for microstructural observation by scanning electron microscopy (SEM) (Zeiss Supra 50 VP) in secondary electrons (SE) mode. The hardness (HV10) and fracture toughness (KIC) measurements were carried out by the Vickers indentation technique, by applying a 10 kg load for 10 s. Fracture toughness was evaluated by the following formula; h i 1=2 ‐3=2 =Φ KIC ¼ 0:15 k HV a ðc=aÞ
ð2Þ
where a is equal to the half length of indentation diagonal in μm, c is the mean of all crack lengths, Φ is a constant (~3), and k is a correction factor. Thermal diffusivity measurements of sintered samples with 13 mm diameters were measured from room temperature (RT) to 600 °C by using the laser-flash method (LFA 457-Netzsch) under a flowing nitrogen gas atmosphere at a flow rate of 100 mbar/s. The thermal diffusivity (a) was calculated using the formula below; a¼
h i 2 2 W x xd = π xt x
ð3Þ
where Wx is a dynamic correction factor to account for the heat losses, d is the thickness of the sample, and tx is the time to half of the maximum of the temperature peak. Wear tests were conducted against a 6Co/WC ball of 3 mm diameter (Table 1) in the ball-on-disk configuration with a microtribometer (CSM Instruments) with 0.15 mm contact radius of rotational (600 min−1) unlubricated sliding under a 15 N constant test load at a linear velocity of 0.92 cm/s at a frequency of 8 Hz for 100.000 cycles and prolonged to a 100 m wear distance. All tests were performed at room temperature (23–25 °C) in air with a relative humidity of 40 ± 5%. The contact schematic diagram used in the wear tests is shown in Fig. 1. Prior to the wear tests, all samples were ground and polished in order to achieve average surface roughness, Ra, in the range of 0.01–0.02 μm in order to minimize the effect of asperities on wear. The coefficient of friction (COF) of investigated samples were obtained online via a computerbased data system by the variation in tangential force recorded during wear tests. After tests, the worn areas of each sample were examined as 2D profiles by a stylus surface profilometer (Mitutoyo SJ-401) and the wear volumes were calculated by integrating these areas over diameter. Specific wear rates were computed by dividing the wear volume by the applied load and sliding distance. Worn surfaces were characterized by using SEM (in SE mode) (Zeiss EVO 50 EP) with an energy-dispersive X-ray spectroscopy (EDS) attachment in order to explain wear mechanisms. 3. Results and discussion Sintered 6Co/WC (1200 °C, 5 min, 50 MPa) and 25%cBN reinforced composites (1300 °C, 7.5 min, 75 MPa) achieved densities of 99.86% and 99.52% of theoretical, respectively. According to XRD investigations (Fig. 2) no compositional changes or no peaks of hBN for cBN/WC/6Co
Table 1 Properties of ball used in wear tests. Balla
Hardness HV10 (kg/mm2)
Compression strength (N/mm2)
Bending str. (N/mm2)
Elasticity constant (Gpa)
Thermal cond. (W/mK)
Heat capacity (J/kg K)
WC–6Co
1400
5300
1600
610
50
201
a
Data from supplier (CSM Instruments).
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
11
Fig. 1. Schematic illustration of the wear test setup used in the present study.
composites, which may form during sintering, were detected. It should be mentioned that the low intensity of cBN and Co peaks resulted from the strong mass-absorption coefficient of the WC phase [25]. SEM observations of the SPSed 6Co/WC (Fig. 3(a)) demonstrate a homogeneous distribution of Co (dark areas) around WC grains with a finer microstructure in comparison to the reference sample in the same magnification (Fig. 3(b)). For the cBN added composites, as seen from Fig. 3(c), cBN particles (dark areas) were uniformly distributed and embedded well with the matrix (Fig. 3(d)). The sintered samples were observed to be dense, apart from a small amount of porosity which can be attributed to the grain pull-out of hard particles in the polished surface that occurred while cutting of cross-section of samples during metallographic preparation. The thermal stability of cBN under the applied sintering conditions in this study was also confirmed by SEM observations. The presence of hBN as platelets precipitated around cBN particles was not observed on the fracture surface of the composites (Fig. 3(d)). According to the few works that have been done on the conversion of cBN to hBN at normal pressure, Eremets et al. [26], showed that cBN “graphitizes”, which means it transforms into hBN (low-hardness form), at ~1227 °C at ambient pressure and Fukunanga [27] demonstrated that the phase transformation from cBN to hBN was very slow at lower temperatures at normal pressure. For example, the ratio of the XRD peak intensity of cBN:hBN was 0.3 at after 48 h at 1250 °C and 18 5 h at 1300 °C, the sintering temperature used in this study. The short sintering time of 7.5 min should have helped prevent
the hBN formation in the composite. In addition, although tungsten carbide with Co binder is generally consolidated above ~1350 °C (the eutectic temperature of the WC–Co–C system) with conventional processes, the high density achieved by the SPSed composite at 1200 °C and 1300 °C may indicate that the pulsed direct current promotes material transfer and diffusion while inhibiting the phase transformation due to the shortened process time. Amongst the tungsten carbides of the same compositions (6Co/WC), similar mechanical properties in the SPSed one (Table 2) as compared to the reference sample (according to data from supplier; 1850 HV30 (kg/mm2) in hardness and 8.9 (MNm−3/2) in fracture toughness) were achieved, even though the SPSed 6Co/WC was fabricated in the sintering temperature of 1200°C which is lower than the sintering temperatures of conventionally fabricated tungsten carbides (1350–1550 °C) and in very short time (5 min). Even the mechanical properties of tungsten carbides depend on several parameters, such as; sintering conditions, Co content and grain size, the results of SPSed 6Co/WC are in agreement with the findings of other researches for similar compositions. Wang et al. [21], reported ~16.5 GPa the hardness value (under a load of 49 N), and ~6.5 MPa.m1/2 the fracture toughness for the 9.7(wt.%)Co/WC samples sintered at 1200 °C, under 50 MPa for 10 min by pulse electric current sintering with average grain size of 1.07 μm. In the same study, it is also investigated the effects of cBN addition to this composites and they obtained 17 GPa hardness and 11 MPa·m1/2 fracture toughness values for 20(vol%)cBN added composites with
Fig. 2. XRD patterns of sintered 6Co/WC and 25cBN/6Co/WC composite (the positions of hBN peaks are indicated by (*)) [24].
12
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
Fig. 3. SE micrographs of (a) SPSed 6Co/WC, (b) 6Co/WC (ref. sample), (c) polished and (d) fractured surface of 25cBN/6Co/WC.
finer average grain size (0.94 μm) of WC by using the same sintering conditions. Rosinski et al. [22] studied the hardness of 30(vol%)cBN/ 6(wt.%)Co/WC sintered by pulse plasma sintering method at 1200 °C, under 100 MPa for 5 min and achieved 2330HK1 (Knoop hardness, 1 kg load) hardness value. Martinez et al. [25] reported 2100 HV5 (kg/mm2) and 7.3 MPa·m1/2 for hot isostatic pressed 5(wt.%)Co/WC (1100 °C, 150 MPa, 60 min); 2150HV5(kg/mm2) and 14.7 MPa·m1/2 mechanical properties for HIPed 30(vol%)cBN/ 5(wt.%)Co/WC composites (1100 °C, 200 MPa, 60 min). These results also agree well with the values determined in this study for cBN added composites (Table 2). Both hardness and toughness values were increased by incorporation of cBN. While high fracture toughness was obtained by the crack-deflection effect of cBN particles as determined according to the indentation tests, the increment in hardness can be attributed to the inhibition of WC grain growth due to the low sintering temperature, as can be clearly seen from fine, round WC grains in Fig. 3(d). Moreover, the cBN particles may also inhibit grain growth by decreasing the grain boundary migration of WC grains [21]. The thermal conductivity is one of the important parameters that influence the temperature build-up at tribocontacts in unlubricated dry sliding wear [28], depending on frictional forces. Since the higher thermal conductivity of cBN (~360–400 W/(m·K) at room temperature [29] is approximately three times more than WC(6Co) (~95 W/(m·K)), adding cBN particles was expected to improve the thermal properties of
tungsten carbides. The measured thermal diffusivities of the sintered samples are given in Fig. 4. The samples incorporating cBN had higher thermal diffusivity, which may help to reduce frictional heating, failure and wear rates of composites induced by thermal stresses at the contact area during sliding. The variation of friction coefficients of the samples against a tungsten carbide ball is illustrated in Fig. 5. The average COF of SPSed 6Co/WC (0.254) exhibited a similar value as the reference sample (0.258). The steady-state COF of 0.27 for the reference sample was revealed within 80–100 m. In order to characterize the steady-state friction, the wear tests were prolonged to 200 m. A slight increase in COF to 0.27 was observed for the SPSed 6Co/WC sample (Fig. 5(a)), which is close to the reference sample. According to these obtained similar COF values for the samples in the same composition, it has not been observed a significant influence of SPS fabrication technique on friction values under the same wear conditions. A higher steady state
Table 2 Mechanical properties of tungsten carbide composites, used as flat materials in wear tests. Sample
HV10 (GPa)
KIC (MPa·m1/2)
6Co/WC (SPS) 25cBN/6Co/WC (SPS)
18.59 ± 0.2 21.23 ± 0.4
6.68 ± 0.5 10.97 ± 0.5
Fig. 4. Thermal diffusivities of SPS sintered tungsten carbide composites [24].
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
13
Fig. 5. The evolution of frictional behavior of all investigated tungsten carbide composites against time (s), sliding distance (m) and lap: (a) SPSed 6Co/WC, (b) SPSed 25cBN/6Co/WC, (c) reference sample.
COF of 0.36 was recorded in cBN added tungsten carbide (Fig. 5(b)), as expected since it possessed the highest hardness amongst all the samples. The increased fluctuation within the 70 m was may be a result of pull-out of hard cBN particles from the interface between the flat and ball contact. The calculated worn volumes and specific wear rates are shown in Table 3. The 2D topography traces (Fig. 6) indicate that very shallow depth occurred in both SPSed composites. Based on the results, wear can be classified as mild wear regime under these selected test
conditions, according to the wear volumes which ranged at 10−6 mm3 (Table 3). The SPSed 6Co/WC sample exhibited a wear rate of 3.37 × 10−9 mm3/Nm, which means approximately 45% improvement
Table 3 Specific wear rates for investigated tungsten carbide composites. Sample 6Co/WC 6Co/cBN/WC/ 6Co/WC (ref)
Wear volume (mm3) −06
5.05 × 10 3.48 × 10−06 9.12 × 10−06
Wear rate (mm3/N·m) 3.37 × 10−09 2.32 × 10−09 6.08 × 10−09
Fig. 6. 2-D surface profiles measured using surface profilometer of worn surfaces on investigated materials.
14
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
Fig. 7. SEM (SE) micrographs of the worn surfaces of (a) SPSed 6Co/WC and (b) 6Co/WC (ref. sample), in insets, part of the wear region is presented with sliding direction indicated by double-pointed arrows, (c) and (d) EDS spectra of unworn surface and tribolayer of SPSed 6Co/WC, respectively.
was achieved in comparison to wear rate of the reference tungsten carbide fabricated by conventional sintering process. The sample with cBN content showed the lowest wear loss amongst all the investigated tungsten carbides. In order to understand and compare the wear mechanisms of tungsten carbides of the same composition fabricated with different methods, the topographical features of their worn surfaces were analyzed by SEM and presented in Fig. 7(a–b). In the case of SPSed 6Co/WC, as can be seen from the inset micrograph, the worn surface was smooth and covered continuously by a very thin tribolayer all over the surface as well as spalling around the edges of the wear scar. In contrast, the formation of the tribolayer was discontinuous in the worn zone of the reference sample. The tribolayers and also unworn surfaces were characterized by energy dispersive spectroscopy (EDS) and representative analyses are presented in Fig. 7(c–d). According to these analyses, higher oxygen content was found in the tribolayer zone of SPSed 6Co/WC than in the unworn surface and it was also richer in oxygen compared to the tribolayer of reference sample. This indicates that the tungsten and cobalt were oxidized as sliding proceeded depending on frictional heat generation, especially depending on contact flash temperature generated on asperities, and presumably the reaction products should consist of various types of oxides, such as WO3 and CoO2. Formation of this surface enrichment of oxidation may explain the reduction in wear rate of the SPSed sample due to its lubrication effect and it may play a role as a protector of the wear surface. Various reports [30] have shown that oxide ceramics, such as tungsten oxides, retain lubricious properties to protect such surfaces from damage in tribological environments. The smooth tribolayer surface in the SPSed sample might have diminished the mechanical stresses by reducing the number of asperity contacts. The reason that
the layer was regarded as stable during test can be attributed to the amount of grain removal, which can be clearly seen in the enlarged views of each worn zone (Fig. 7(a–b)). The amount of the carbide grain pull out for the reference sample fabricated by conventional method was significantly higher than for the SPSed 6Co/WC sample (Fig. 7(b)), which may resulted in weak formation and fragmentation of the tribolayer. As shown some representative grain pull-outs illustrated by circles in Fig. 7(a–b), less number of grain pull-out in SPSed 6Co/WC may have caused the permanency of this tribochemical layer during test which may help to reduce the adhesion wear of this sample. On the other hand, the repetition of the formation, fragmentation and the removal of this tribochemical layer at the contact area during all sliding should increase the adhesive wear of reference sample. This situation may also explain the frictional behavior of the reference sample as shown in Fig. 5(c), where friction might have been decreased by the generation of the tribochemical layer corresponding to lower frictional forces, fluctuated owing to the discontinuous formation and increased by removal of this layer within 20 m–80 m, whereas frictional behavior of SPSed tungsten carbide tended to be stable during all sliding distance with the more undeformable tribochemical coverage surface. Furthermore, even though they are the same composition; higher wear rate and deep grooves were observed in the conventional tungsten carbide, which indicates that more abrasive wear was generated. According to the SEM investigations, in both samples, material removal in the case of abrasive wear originated from the development of intergranular microcracks, followed by fragmentation and/or removal of hard carbide grains which were caused by the detachment of the Co binder phase at the grain boundaries of tungsten carbide. Deep abrasive grooves in the worn region of the reference sample may be caused by these removed and trapped hard carbide particles that act as abrasives,
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
such as third-body particles at the interface which can also be presume to increase the wear rate. On the other hand, the contribution of the SPS method in exhibiting higher wear resistance of samples and/or limiting the grain pull-out can be attributed to the benefit of enhanced sintering by pressure and short sintering time which leads to limitation of grain growth. The fine grain size of the SPSed sample played an important role in obtaining low wear rates. While weak grain boundaries in the reference sample facilitated crack propagation, in the SPSed sample, the fine microstructure reduced the probability of intergranular cracking due to possessing higher grain boundary strength. Owing to Joule heating and the applied pressure in SPS technique facilitates the formation of several necks between particles during sintering, which gradually develop and increase in a very short sintering time [17,19], the strength of bonding between WC grains might therefore be
15
enhanced. As observed in Fig. 7(a), WC grains remained well-bonded with the matrix in SPSed tungsten carbide and thus, the grain pull-out is less than the reference sample. As a result, the application of SPS technique makes it possible to enhance wear resistance of carbides with no need to remove or reduce the amount of the binder phase. The worn surface of 25cBN/6Co/WC sample is given in Fig. 8(a). The abrasion scratches were mild which can be seen in the enlarged view (Fig. 8(b)). According to microstructural analyses, although the wear mechanisms like abrasion and the formation of tribochemical layer were similar to the tungsten carbides with no addition, no large cavities or damages were observed in the worn area, and the detachment of cBN particles from the matrix was of minor importance. The amount of grain pull-outs for both cBN and WC particles were the least in this composite amongst all the other samples. As reasons mentioned before with regard to the SPSed 6Co/WC sample, very strong bonding between cBN particles and tungsten carbide matrix caused a minimum wear rate by reducing the fragmentation and removal of material. Furthermore, the positive effect of cBN addition on improving the wear resistance can be explained by assuming that the compressive stresses occurred during sliding contact were carried by cBN particles which has higher hardness than carbides and thus, the tungsten carbide matrix was protected. In addition, as can be observed from micrograph (Fig. 8(a)), a thicker and more adhesive tribochemical layer was obtained possibly by the oxidation of BN apart from tungsten and binder oxide (Fig. 8(c)), which may have served as a protection from surface damage. Moreover, the increased wear resistance by the addition of cBN phase can be attributed to increasing both hardness and fracture toughness which aids in resisting crack propagation through crack deflection. 4. Conclusions Tribological performance of tungsten carbide composites fabricated by spark plasma sintering technique has been characterized in reciprocating sliding dry contact by using ball-on-disk geometry. Friction coefficient of SPS sintered 6Co/WC was 0.25 as low as reference 6Co/WC and the 25cBN/6Co/WC's was determined as 0.36. The wear rates varied between (2.32 and 3.37) × 10−09 for SPSed composites. Wear data indicated that spark plasma sintered tungsten carbide composites showed better wear resistance as compared to conventional fabricated tungsten carbide. Besides, the addition of cBN phase significantly increased wear resistance by enhancing the mechanical properties. According to this promising result and owing to SPS being one of the most proper techniques for fabrication of cBN composites without phase transformation, these SPSed cBN/Co/WC composites have potential for tribological applications especially under these applied static loads. Besides, the wear behavior of WC composites containing cBN with different amounts can be investigated as a future research to explore the relationship between wear and properties in detail since there has not been much study in relevant literature. Dominating wear mechanisms can be summarized as microabrasion, microfragments, binder and carbide grains pull-out and formation of thin tribolayer containing oxides under selected operating conditions. The experimental results were attributed to features of spark plasma sintering technique which enabled control of microstructural characteristics such as a fine grain size, leading to improved mechanical properties. Due to high tool life becoming an important factor in tool design, it is believed that spark plasma sintering technique would play an important role for the material design process in tribological applications. References
Fig. 8. SEM micrographs (a–b) and EDS spectra (c) of the worn surface of the SPSed 25cBN/WC/6Co. Wear width in (a) and tracks in (b) are presented by double-pointed arrows.
[1] Holmberg K, Ronkainen H, Laukkanen A, Wallin K. Friction and wear of coated surfaces — scales, modelling and simulation of tribomechanisms. Surf Coat Technol 2007;202:1034–49. [2] Van Acker K, Vanhoyweghen D, Persoons R, Vangrunderbeek J. Influence of tungsten carbide particle size and distribution on the wear resistance of laser clad WC/Ni coatings. Wear 2005;258:194–202.
16
B. Yaman, H. Mandal / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 9–16
[3] Bonny K, De Baets P, Ost W, Vleugels J, Huang S, Lauwers B, et al. Influence of electrical discharge machining on the reciprocating sliding wear response of WC–Co cemented carbides. Wear 2009;266:84–95. [4] Liu K, Li XP, Rahman M, Liu XD. CBN tool wear in ductile cutting of tungsten carbide. Wear 2003;255:1344–51. [5] Hegeman JBJW, De Hosson JThM, De With G. Grinding of WC–Co hard metals. Wear 2001;248:187–96. [6] Andrén HO. Microstructure development during sintering and heat-treatment of cemented carbides and cermets. Mater Chem Phys 2001;67:209–13. [7] Beste U, Jacobson S. A new view of the deterioration and wear of WC/Co cemented carbide rock drill buttons. Wear 2008;264:1129–41. [8] Saito H, Iwabuchi A, Shimizu T. Effects of Co content and WC grain size on wear of WC cemented carbide. Wear 2006;261:126–32. [9] Sheikh-Ahmad JY, Bailey JA. The wear characteristics of some cemented tungsten carbides in machining particleboard. Wear 1999;225–229:256–66. [10] Gant AJ, Gee MG. Abrasion of tungsten carbide hard metals using hard counterfaces. Int J Refract Met Hard Mater 2006;24:189–98. [11] Picas JA, Xiong Y, Punset M, Ajdelsztajn L, Forn A, Schoenung JM. Microstructure and wear resistance of WC–Co by three consolidation processing techniques. Int J Refract Met Hard Mater 2009;27:344–9. [12] Staia MH, Torres IJ, Castillo C, Sudarshan TS, Lesage J, Chicot D. Tribological study of WC produced by plasma pressure compaction. Int J Refract Met Hard Mater 2006;24:183–8. [13] Chaim R. Densification mechanisms in spark plasma sintering of nanocrystalline ceramics. Mater Sci Eng A 2007;443:25–32. [14] Reis P, Davim JP, Xu X, Ferreira JMF. Tribological behaviour of colloidally processed sialon ceramics sliding against steel under dry conditions. Tribol Lett 2005;18(3):295–301. [15] Yucheng W, Zhengyi F. Study of temperature field in spark plasma sintering. Mater Sci Eng 2002;B90:34–7. [16] Belmonte M, González-Julián J, Miranzo P, Osendi MI. Spark plasma sintering: a powerful tool to develop new silicon nitride-based materials. J Eur Ceram Soc 2010;30:2937–46.
[17] Orru R, Licheri R, Locci AM, Cincotti A, Cao G. Consolidation/synthesis of materials by electric current activated/assisted sintering. Mater Sci Eng 2009;R 63:127–287. [18] Kim HC, Jeong IK, Shon IJ, Ko IY, Doh JM. Fabrication of WC–8wt.%Co hard materials by two rapid sintering processes. Int J Refract Met Hard Mater 2007;25:336–40. [19] Anselmi-Tamburini U, Garay JE, Munir ZA. Fast low-temperature consolidation of bulk nanometric ceramic materials. Scr Mater 2006;54:823–8. [20] Hungria T, Galy J, Castro A. Spark plasma sintering as a useful technique to the nanostructuration of piezo-ferroelectric materials. Adv Eng Mater 2009;11(8):615–31. [21] Wang B, Matsumaru K, Yang J, Fu Z, Ishizaki K. The effect of cBN additions on densification, microstructure and properties of WC–Co composites by pulse electric current sintering. J Am Ceram Soc 2012;95(8):2499–503. [22] Rosinski M, Michalski A. WC/Co/cBN composites produced by pulse plasma sintering method. J Mater Sci 2012;47:7064–71. [23] Upadhyaya GS. Cemented carbides production, properties and testing. William Andrew Publishing/Noyes; 1998 [420 pp.]. [24] Yaman B, Mandal H. Spark plasma sintering of Co–WC cubic boron nitride composites. Mater Lett 2009;63:1041–3. [25] Martinez V, Echeberria J. Hot isostatic pressing of cubic boron nitride–tungsten carbide/cobalt (cBN–WC/Co) composites: effect of cBN particle size and some processing parameters on their microstructure and properties. J Am Ceram Soc 2007;90:415–24. [26] Eremets MI, Takemura K, Yusa H, Golberg D, Blank VD, Sato Y, et al. Disordered state in first order phase transitions: hexagonal-to-cubic and cubic-to-hexagonal transitions in boron nitride. Phys Rev B Condens Matter Mater Phys 1998;57:5655–60. [27] Fukunanga O. The equilibrium phase boundary between hexagonal and cubic boron nitride. Diam Relat Mater 2000;9:7–12. [28] Basu B, Mukhopadhyay A, Mishra A, Sarkar J. Does thermal conductivity play a role in sliding wear of metals in cryogenic environment? J Tribol 2010;132(4):041604-1–5. [29] Wang ZG, Rahman M, Wong YS. Tool wear characteristics of binderless CBN tools used in high-speed milling of titanium alloys. Wear 2005;258:752–8. [30] Greenwood OD, Moulzolf SC, Blau PJ, Lad RJ. The influence of microstructure on tribological properties of WO3 thin films. Wear 1999;232:84–90.