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Effect of sliding mating materials on vacuum tribological behaviors of sintered polycrystalline diamond
Int. Journal of Refractory Metals and Hard Materials 54 (2016) 116–126
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Effect of sliding mating materials on vacuum tribological behaviors of sintered polycrystalline diamond Xiaohua Sha a, Wen Yue a,b,⁎, Yihui Zhao a, Fang Lin a, Chengbiao Wang a,b a b
School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 12 May 2015 Received in revised form 16 July 2015 Accepted 19 July 2015 Available online 23 July 2015 Keywords: Polycrystalline diamond Vacuum Mating material Tribological behaviors
a b s t r a c t The comparative tribological behaviors of the sintered polycrystalline diamond (PCD) sliding against SiC, Si3N4, GCr15 and Al2O3 mating balls have been investigated by a ball-on-disk tribometer under vacuum conditions. Scanning electron microscope (SEM), energy dispersive X-ray spectrum (EDS) and Raman spectroscopy were performed to study the morphologies and chemical composition of the worn surfaces on the PCD and mating balls. The results showed that the coefficients of friction (COFs) of PCD/SiC and PCD/Si3N4 tribopairs were about ten times as high as those of PCD/GCr15 and PCD/Al2O3 tribopairs. The EDS and Raman spectroscopy results demonstrated that the higher friction and wear of PCD/SiC and PCD/Si3N4 tribopairs were attributed to serious adhesion caused by the formation of C–C and Si–C bonds at the contact interface. Some diamond grains were found in the wear scar of GCr15 mating ball, which indicated that the embed diamond grains spalling from the PCD surface resulted in abrasive wear by plowing mechanism. Sliding against Al2O3, the low ultimate surface roughness and chemical inertness maintained the super low COF and minimal wear of both the PCD disc and mating ball. These results proposed that the vacuum tribological behaviors of the PCD were significantly affected by mating materials. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The polycrystalline diamond (PCD), sintered under high temperature and high pressure, is composed of diamond micro-powders and a “binder phase” of Co [1,2]. It is one of the potential materials for space drilling bits and aerospace components, including thrust bearings, due to its extremely high hardness, toughness and thermal conductivity, and high wear resistance [3–7]. The much-needed comparative study for tribological mechanism on variant mating materials in vacuum can offer engineering reference for proper mate selection, and make it more effectively applicable in aerospace field [8,9]. The vacuum tribological behaviors of diamond materials vary when sliding against dissimilar mating materials [10–12]. Liu et al. [10] examined the friction and wear behaviors of diamond-like carbon films sliding against GCr15, bronze, ZrO2, Al2O3, SiC, WC, and Si3N4 mating balls under high vacuum conditions, and the contact radius and the contact pressure were introduced to explain the diverse friction behaviors. Yu et al. [11] studied the vacuum tribological behavior of chemical vapor deposition (CVD) diamond films sliding against Si3N4, and observed a
⁎ Corresponding author at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China. E-mail addresses: [email protected], [email protected] (W. Yue).
http://dx.doi.org/10.1016/j.ijrmhm.2015.07.025 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
mass of transferred layers on the worn track of diamond. Zeiler et al. [12] investigated the friction behaviors of CVD diamond sliding against different ceramic materials. And the results show that Al2O3 and Si3N4 exhibit excellent wear behaviors accompanied by very low friction, and SiC and ZrO2 show somewhat inferior wear and friction behaviors. The different tribological behaviors of diamond materials result from diverse tribological mechanisms. Adhesion was proposed explaining diamond friction in vacuum [13–16]. Grillo et al. [13] investigated the vacuum tribological properties of self-mated natural diamond, and suggested that adhesion plays a fundamental role in diamond tribological behavior in vacuum. Bowden and Tabor [14] proposed that vacuum friction resulted from atomic bonding between diamond surfaces. Under vacuum conditions, the surfaces are not passivated by adsorbed molecules and therefore the dangling σ bonds of the carbon atoms on diamond surfaces are free to join at the sliding interfaces. Such interactions can cause strong adhesion and hence high friction [15,16]. Besides, the effect of mate surface roughness on tribological behaviors of diamond materials has been investigated [17–19]. Schade et al. [17] announced that the self-mated fine-grained CVD diamond coatings with low surface roughness and good flatness were best for dry sliding, and they indeed revealed in some cases longer running-in times. Similar speculation on the role of roughness was made by Hayward et al. They concluded that running-in period depended on surface roughness, and that the friction would decrease only after the interface had been
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smoothed by abrasive wear [18,19]. Based on previous work, the diamond materials exhibit significantly various vacuum tribological behaviors by different tribological mechanisms when sliding against diverse mating balls. The study on friction and wear behaviors of the PCD sliding against Si3N4 ball indicated that the friction coefficients obtained under vacuum are much higher than those under ambient air, and Zhao et al. [2] attributed it to the serious adhesion between counter surfaces and weak bonding strength among diamond grains. Nevertheless, few researches have been performed on the influence of mating materials on the tribological behavior of PCD under vacuum conditions. Therefore, in the present paper, vacuum sliding wear tests were carried out using SiC, Si3N4, GCr15 and Al2O3 mating balls, to investigate the role of mating materials on the tribological behaviors of the PCD. The wear track and wear scar were examined using scanning electron microscope (SEM)/energy dispersive X-ray spectrum (EDS) and Raman spectroscopy to explore the vacuum tribological mechanisms of the PCD when sliding against different materials.
117
Cobalt catalyst
Diamond grains
2. Experimental details Fig. 2. SEM morphology of the PCD surface.
2.1. Materials The flat samples used in this work are commercial grade polycrystalline diamond compacts (PDCs), which contain coarse grain (half-content diameter, D50 = 25 μm) diamond, and WC-Co (16 wt.% Co) substrate. The diamond grains are typically placed adjacent to WC-Co substrate that provides a source for catalyst metal. The PDC disc was polished to make the surface roughness reach a value of 3–4 nm. The cross-sectional image and optical photo of the PDC are shown in Fig. 1. It can be seen that the dimensions of the disc are 45.0 mm in diameter, 2.9 mm in thickness, and 540 μm in thickness of the PCD layer and 2.37 mm in thickness of WC-Co substrate. Fig. 2 presents the surface morphology of the PCD. It is identified that the binder phase corresponded to the bright regions distribute along the grain boundaries of diamond corresponded to the dark ones. In this study, SiC, Si3N4, GCr15 and Al2O3 balls were used as mating samples. The physical properties of the PCD and mating balls are listed in Table 1 [2,9,20]. 2.2. Tribological tests The tribological tests were performed in vacuum (~7.0 × 10−4 Pa) by a ball-on-disk space tribometer (MSTS-1) [21]. The PDCs were used as the disks. The ball specimens were SiC, Si3N4, GCr15 and Al2O3 balls
540 µm
2370 µm
PCD layer
WC/Co substrate
with diameter of Φ 9.525 mm. During the tests, the mating ball was fixed while the PDC disc was rotating with a turning radius of 10 mm, and the duration of tribological test was 30 min. The friction forces were measured by the dynameter and then converted into friction coefficients throughout the tests. Prior to the tribological test, both the PDC discs and mating balls were rinsed with hexane, and then ultrasonically cleaned in fresh hexane, finally cleaned by ultrasonic with acetone for 30 min. The investigation aims to study the effect of mating materials on vacuum tribological behaviors of the PCD. Generally, the tribological behaviors of the PCD should be affected by the applied load and revolution speed. Therefore, the impact of applied load (30 N and 45 N) on the vacuum tribological behaviors of the PCD was investigated under a constant revolution speed of 100 r/min. Moreover, the applied load was constant at 30 N to evaluate the effect of revolution speed (100 r/min and 150 r/min) on the tribological behaviors of the PCD in vacuum. The selected parameters of the applied load and revolution speed were based on previous tribotests under diverse applied loads and revolution speeds.
2.3. Surface analysis The surface analysis of the PCD discs and mating balls tested under 30 N and 100 r/min was carried out to study the vacuum tribological mechanisms. Because the tribological behaviors of the PCD sliding against different mating materials under various applied loads and revolution speeds present a similar trend. The three-dimensional wear profiles across wear tracks of the PCD were measured by NanoMap-D three-dimensional white light interferometer. The wear scar morphologies of mating balls were observed using the Olympus BX51M optical microscopy and VK-X100K confocal microscope. The surface roughness of the mating balls before and after tribological tests were measured by VK-X100K confocal microscope. The CS3400 scanning electron Table 1 Physical properties and surface roughness of test specimens.
Fig. 1. The cross-sectional image and optical photo (the inset) of the polycrystalline diamond compact.
Materials
Hardness (GPa)
Young's modulus (GPa)
Surface roughness (nm)
Thermal conductivity (W/(m·K))
Poisson's ratio
PCD SiC Al2O3 Si3N4 GCr15
40–50 25–27 15–17 14–16 6–8
810 440 370 300 210
3–4 30–40 300–350 20–30 20–30
700.0 33.0–33.4 25.2–32.9 16.2–29.5 45.2–51.3
0.07 0.17 0.27 0.25 0.30
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Fig. 3. The COFs of the PCD sliding against dissimilar mating balls under different loads and revolution speeds in vacuum. (a) 30 N, 100 r/min; (b) 30 N, 100 r/min and (c) 30 N, 100 r/min.
microscope (SEM) was used to observe the surface structures and topographies of the PCD discs and counter balls. The Oxford EDX-450 energy dispersive X-ray spectrum (EDS) was used to investigate the element chemical compositions of the worn PCD surfaces. The compositions and structures of the materials on the worn surfaces of mating balls and PCD discs were studied using the LabRAM HR Evolution Raman spectrometer with 514.5 nm wavelength of Ar+ laser. 3. Results 3.1. Friction and wear behaviors The research focuses on the effect of mating materials on vacuum tribological behaviors of the PCD. Besides, the impacts of the applied load and revolution speed on vacuum tribological behaviors of the PCD are also investigated. Fig. 3 shows the coefficients of friction (COFs) of the PCD discs sliding against SiC, Si3N4, GCr15 and Al2O3 mating balls under different loads and revolution speeds in vacuum. The impacts of mating materials on vacuum tribological behaviors of the PCD are presented in Fig. 3(a). It reveals that the average COFs of PCD/SiC and PCD/Si3N4 tribopairs are around 1.15 and 0.85, respectively, while those of PCD/GCr15 and PCD/Al2O3 tribopairs are about 0.08 and 0.05, respectively. It is clear that the COF curves of PCD/SiC and PCD/Si3N4 tribopairs fluctuant great, whereas the PCD/GCr15 presents steady curve. As for the PCD/Al2O3 tribopair, it presents an exceptional variation of COF during sliding operation. The initial COF of PCD/Al2O3 tribopair is about 0.04, increasing with sliding operation, peaking after about 5 min, and then decreasing slightly to a steady stage of around 0.05. The COF curve implies that the PCD/Al2O3 tribopair undergoes a long running-in period for 20 min. It is revealed that the COFs of the PCD depend on mating materials. The PCD presents higher COFs when
sliding against SiC and Si3N4, and exhibits super low COF in the case of Al2O3. The effects of the applied load on vacuum tribological behaviors of the PCD are shown in Fig. 3(a) and Fig. 3(b). It presents that the COFs appear to rise as the load increases. Similar results were found for the CVD diamond and natural diamond studied by Grillo and Field [16]. Moreover, the COFs of the PCD sliding against dissimilar mating balls follow the same trends under loads of 30 N and 45 N. The results imply that the applied load probably plays no role on the vacuum tribological behaviors of the PCD sliding against different mating materials. The roles of the revolution speed on the vacuum tribological behaviors of the PCD are shown in Fig. 3(a) and (c). The COFs of the PCD/SiC, PCD/Si3N4 and PCD/Al2O3 tribopairs decrease with the increasing speed of revolution, except for the PCD/GCr15 tribopair. Totally, the tendencies of COFs for different tribopairs do not occur, which means that the revolution speed may be independent of the tribological behaviors of the PCD sliding against dissimilar mating materials in vacuum. The three-dimensional wear profiles across wear tracks of the PCD discs sliding against different mating balls were carried out, and the results are presented in Fig. 4. When sliding against SiC (Fig. 4(a)) and Si3N4 (Fig. 4(b)), irregularly distributed spalling pits and conical uplifts were found on the worn surfaces of the PCD. A similar observation reported for PCD/Si3N4 tribopair under vacuum conditions attributed the high friction property obtained in vacuum to serious adhesion across contact interfaces [2]. In contrast, the worn regions on the PCD sliding against Al2O3 (Fig. 4(d)) present mild wear. Fig. 4(c) indicates that spalling pits and plowing grooves present on the surface of the PCD against GCr15 ball, which implies the occurrence of abrasive wear. The optical morphologies of worn surfaces on different mating balls after sliding operation under vacuum conditions are illustrated in Fig. 5, and the arrows indicate the sliding directions. The following features are
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(a) SiC
(b) Si3N4
(c) GCr15
(d) Al2O3
Fig. 4. Three-dimensional surface topographies of the wear tracks formed on the PCD discs.
(b) Si3N4
(a) SiC
Sliding direction
Sliding direction
~ 3.6 mm
~ 3.4 mm
(d) Al2O3
(c) GCr15 Sliding direction
Sliding direction
~ 680 µm
~ 610 µm
Fig. 5. The wear scars of mating balls after sliding operation under vacuum conditions.
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observed when comparing the distinct tested mating balls. The wear scars of SiC (Fig. 5(a)) and Si3N4 (Fig. 5(b)) are about an order of magnitude larger in diameter than those of GCr15 (Fig. 5(c)) and Al2O3 (Fig. 5(d)). Comparing Si3N4 with SiC ball, it can be found that the wear scar of Si3N4 ball is slightly bigger, which may result from the different hardness of mating balls [22]. It is clearly showed in Fig. 5(c) that the wear scar with overlap issue and plastic deformation presents some obvious deep grooves for GCr15 mating ball. Moreover, black deposits are observed in the grooves, which may lead to the grooves by plowing mechanism. As for the Al2O3 (Fig. 5(d)) mating ball, the wear scar is relatively smaller in diameter and some debris are piled up at the leading edge. The three-dimensional views of the wear scars on mating balls were performed by the confocal microscope. It presents much larger wear scars in diameter for SiC and Si3N4 mating balls as shown in Fig. 6. It reveals in Fig. 6(c) that the worn surface of GCr15 mating ball presents evident grooves, which might be result from the abrasive wear. The Al2O3 ball behaviors the smallest wear scar. The results are consistent with the optical results shown in Fig. 5.
3.2. SEM and EDS analysis The SEM images of worn surfaces on the PCD discs sliding against different mating balls are shown in Fig. 7. The inset images reveal that the wear track of the PCD against SiC (Fig. 7(a1)) is similar to that of the PCD against Si3N4 (Fig. 7(b1)), while the wear track of the PCD against GCr15 (Fig. 7(c1)) is much alike with that of the PCD against Al2O3 (Fig. 7(d1)). Sliding against GCr15 and Al2O3, the PCD discs present much slighter and narrower wear tracks than the others, which is corresponding to the results revealed in the wear scars of mating balls. After sliding against SiC and Si3N4, a large number of white speckles are deposited on the wear tracks of the PCD, and the one against Si3N4 presents much wider wear track. For further investigation, the enlarged
views of worn surfaces and the corresponded EDS mapping analysis were performed. It is identified that the silicon, which corresponded to the white areas in the SEM images, distributed on the wear tracks of the PCD discs sliding against SiC and Si3N4. The results interpret that the SiC and Si3N4 coming from mating balls significantly adhered to the PCD surfaces during sliding operation in vacuum. By contrast, few adhered materials were observed for the ones against GCr15 and Al2O3. The SEM morphologies of the wear scars on mating balls are shown in Fig. 8, and the arrows indicate the sliding directions. It is observed that the worn surfaces of SiC (Fig. 8(a)) and Si3N4 (Fig. 8(b)) exhibit the severe adhesive wear, which is corresponding to spalling pits and conical uplifts. The GCr15 counterface (Fig. 8(c)) reveals the abrasive wear, while mild abrasive wear occurs on the worn surface of Al2O3 (Fig. 8(d)).
3.3. Raman spectrum analysis Raman spectroscopy is an effective way to characterize the composition and structure of materials with molecular (and bond) specificity [23–25]. The worn surfaces of different mating balls were analyzed by Raman spectroscopy aiming to ascertain whether adhesive materials can be found on the mating balls. The results are exhibited in Fig. 9. The Raman spectrum obtained from the wear scar of SiC (Fig. 9(a)), Si3N4 (Fig. 9(b)) and Al2O3 (Fig. 9(d)) presents characteristic peaks proving the existence of SiC [26], Si3N4 [27] and Al2O3 [25]. These confirm that no other materials are observed on the wear scars of SiC, Si3N4 and Al2O3 balls. In order to find out the composition of embed materials in the deep grooves on the wear scar of GCr15 mating ball (Fig. 5(c)), the detecting positions of Raman spectrum analysis were selected at the black scratch. The result shown in Fig. 9(c) reveals that an apparently diamond Raman single (~ 1332 cm− 1) occurs
(a) SiC
(b) Si3N4
(c) GCr15
(d) Al2O3
Fig. 6. Confocal micrographs showing three-dimensional views of wear scars on (a) SiC, (b) Si3N4, (c) GCr15 and (d) Al2O3 mating balls.
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(a1) SiC
(a2) SiC
121
Si-K
Wear track
(b1) Si3N4
(b2) Si3N4
Si-K
Wear track
(c1) GCr15
(c2) GCr15
Fe-K
Wear track
(d1) Al2O3
(d2) Al2O3
Al-K
Wear track
Fig. 7. SEM images (a1, b1, c1 and d1) of worn surfaces of the PCD sliding against different mating balls and corresponded EDS mapping images (a2, b2, c2 and d2).
on detecting positions, implying that some diamond grains filled in the wear scar of GCr15 counterface [19]. It confirms that the abrasive wear at the interface of the PCD/GCr15 tribopair is caused by the plowing action of embed diamond grains spalled from the PCD disc. As this reason, the increased applied load results in more embedded diamond grains in the wear scar of GCr15 mating ball, leading to more serious abrasive wear (Fig. 3(a) and (b)).
4. Discussion The PCD performs quite different tribological behaviors when sliding against SiC, Si3N4, GCr15 and Al2O3 in vacuum. The COFs of PCD/SiC and PCD/Si3N4 tribopairs are about ten times as high as those of PCD/GCr15 and PCD/Al2O3 tribopairs, and the former tribopairs worn much more seriously. Corresponding to these, the four tribopairs present dissimilar tribological mechanisms.
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(a) SiC Sliding direction
(b) Si3N4
Sliding direction
Adhesive wear Adhesive wear
(c) GCr15
(d) Al2O3 Sliding direction
Abrasive wear
Sliding direction
Mild abrasive wear
Fig. 8. SEM micrographs of the wear scars on (a) SiC, (b) Si3N4, (c) GCr15 and (d) Al2O3 mating balls after sliding test.
4.1. Tribological mechanisms of the PCD/SiC and PCD/Si3N4 tribopairs The PCD/SiC and PCD/Si3N4 tribopairs present high and great fluctuated COFs. Moreover, as shown in Figs. 4 and 5, both the discs and mating balls wear seriously after sliding operation in vacuum. The SEM micrographs of the wear scars on SiC and Si 3 N 4 presented in Fig. 7(a) and (b) reveal obvious adhesive wear. The EDS mapping images (Fig. 8(a) and (b)) of worn surfaces on the PCD discs also confirm that adhesion occurs during sliding operation with SiC and Si 3N 4 mating balls. Sliding under vacuum conditions, two clean surfaces of the tribopair contacted directly due to the desorption of hydrogen and other adsorbate, creating dangling bonds on the surface of the PCD [12,28]. In this case, the formation of chemical bonds between the PCD and mating balls seems to be extremely ready to occur, which would lead to adhesion. In order to find out the structure of adhesive materials observed on the wear track of the PCD against Si3N4 ball, Raman spectra analysis was performed, and the result is presented in Fig. 10. The Raman spectrum obtained on the detecting positions exhibits the only one peak at around 733 cm− 1, which illustrates the formation of Si–C bond during sliding operation for the PCD/Si3N4 tribopair [26]. Presenting analogous adhesive characteristics, it may also occur the formation of Si–C and C–C bonds at the contact interface of PCD/SiC tribopair during sliding operation. A model is proposed to further illustrate the adhesion wear mechanism, as shown in Fig. 11. Such a serious adhesion caused by the formation of Si–C and C–C bonds at the contact interface of PCD/SiC and PCD/Si 3N 4 tribopairs is the main reason why the friction is higher and it presents much serious wear under vacuum conditions. The result that the PCD against SiC ball wears a litter slightly than that against Si3N4 may result from the higher hardness of SiC. Previous study has reported that the hardness of mating materials has an enormous effect on the wear behaviors of diamond materials [23,29,30]. Liu et al. [30] investigated the tribological properties of DLC films against both hard and soft mating materials and found that the harder the sliders, the lower their wear rates. It indicates that the observed phenomenon showed in Fig. 5 that the SiC mating ball wears a litter slighter than Si3N4 is attributed to its comparatively higher hardness.
The friction coefficients of 1.15 and 0.85 for PCD/SiC and PCD/Si3N4 tribopairs mean that the friction force hindering the relative motion of tribopairs is extremely high under vacuum conditions. The high friction force mainly comes from the serious adhesion action of sliding tribopair ball during sliding operation [31,32]. It implies that under vacuum conditions, the PCD applied in such as thrust bearings and drilling bits, sliding against SiC or Si3N4 is prone to be stuck and is thus hardly applicable due to the severe adhesion action during movement operation. 4.2. Tribological mechanism of the PCD/GCr15 tribopair Comparing the EDS results of SiC and Si3N4 shown in Fig. 7, the PCD/ GCr15 and PCD/Al2O3 tribopairs do not exhibit much adhesion, which result in a lower COF and slighter wear. Sliding against GCr15, the PCD presents low and stable COF, being accompanied by wild wear of both the PCD and mating ball. In addition, the SEM micrograph (Fig. 8(c)) of the wear scar on GCr15 mating ball presents an obvious abrasive wear. The result exhibited in Fig. 9(c) confirms that some diamond grains shedding from the PCD disc embed in the mating ball during sliding operation, which is responding to the appearance of spalling pits on the worn surface of the PCD shown in Fig. 4(c). Therefore, it is reasonable that the vacuum wear mechanism of PCD/GCr15 tribopair is mainly the secondary abrasive wear caused by the embed diamond grains in the counterface of GCr15 mating ball. A model is proposed to further illustrate the wear mechanism, as shown in Fig. 12. When sliding against GCr15 mating ball in vacuum, some of tinier diamond grains on the contact surface of the PCD are likely to shed off during sliding operation. Moreover, during subsequent sliding, some spalling diamond grains may embed in the GCr15 mating ball, simultaneously scratch the PCD surface in return. 4.3. Tribological mechanism of the PCD/Al2O3 tribopair Sliding against Al2O3 in vacuum, the COF of PCD presents an enormous variation shown in Fig. 3(a). It undergoes a long running-in period and gradually reaches a steady stage of around 0.05. Besides, both Al2O3 and the PCD wear mildly. These phenomena may be related to the changes of surface roughness for Al2O3 mating ball during sliding
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(a1)
123
(a2) Detecting positions
(b1)
(b2) Detecting positions
(c1)
(c2)
Detecting positions
(d2)
(d1)
Detecting positions
Fig. 9. Raman spectra of wear scars on (a) SiC, (b) Si3N4, (c) GCr15 and (d) Al2O3 balls.
operation. Al2O3 is one of the rough surfaced ceramics. In this study, the surface roughness of Al2O3 ball is approximately 10 times as high as that of other mating balls, as shown in Table. 1. Researchers [17,23,33] demonstrated that the rough surface not only gives a high COF during the
longer running-in period, but also governs the overall tribological behavior. To further investigate the effect of mating ball surface roughness on the vacuum tribological behavior of PCD/Al2O3 tribopair, a worn Al2O3
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(a)
(b) Detecting positions
Fig. 10. Raman spectrum of adhered material observed on the worn surface of the PCD sliding against Si3N4 ball: black adhesion on the wear track and Si–C vibration standard.
(a)
(b) Si-C
C-C
Si-C
Fig. 11. Vacuum tribological mechanisms of the (a) PCD/SiC and (b) PCD/Si3N4 tribopairs: adhesion caused by the formation of Si–C and C–C bonds. The gray, purple and blue atoms correspond to C, Si and N elements, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ball obtained after sliding against a PCD disc for 30 min was tested against a new PCD disc, and the COF curve is presented in Fig. 13. It reveals that the COF of the PCD sliding against the worn Al2O3 keeps a steady-state for a value of around 0.05, which is accordance with the stable ultimate COF of the new Al2O3 ball. Besides, almost no evidence of running-in period was observed. Moreover, the surface roughness of Al2O3 mating ball before and after tribological tests was measured by confocal micrographs, presenting in Table 2. The surface roughness of mating ball reduces much from 300–350 nm to 40–50 nm after 30 min sliding, and nearly kept at a low value during the further sliding.
It illustrates that the uneven protruding surface asperities of Al2O3 mating ball are gradually polished during the long initial running-in period, leading to a dropping friction value. In this way, the low ultimate surface roughness of Al2O3 maintains the final super low COF and minimal wear of the PCD/Al2O3 tribopair. Under vacuum conditions, two clean surfaces of the tribopair were contacted directly due to the breakdown of surface adsorption. The analogous elements are prone to form chemical bonds for strong affinity [34]. Therefore, SiC and Si3N4 are significantly likely to adhere to the PCD surfaces by the formation of Si–C and C–C bonds at the contact
Fig. 12. Vacuum wear mechanism of the PCD/GCr15 tribopair: secondary abrasive wear caused by embedded diamond grains spalling from the PCD.
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Stage
Fig. 13. The COF curve of the PCD sliding against a worn Al2O3 ball (obtained after sliding against a new Al2O3 ball for 30 min).
Table 2 The variation of surface roughness for wear scar on Al2O3 ball. Sliding time (min) Surface roughness (nm)
0 300–350
30 40–50
60 30–40
surfaces during vacuum sliding operation, leading to the high COFs [2]. On the contrary, The PCD/Al2O3 tribopair is hardly possible for chemical bonds forming due to the weak affinity between Al and C elements [34, 35]. The comparative experimental results shown in Fig. 14 reveal that the COFs of the PCD sliding against different mating balls in vacuum were about ten times as high as those in air. This may be attributed to the difference between the termination and desorption of dangling bonds on the PCD surface in air and vacuum, respectively. Overall, the low ultimate surface roughness and chemical inertness of Al2O3 maintain the final low and steady COF and minimal wear of the PCD/Al2O3 tribopair. 5. Conclusions In the present study to evaluate tribological behaviors of the PCD sliding against dissimilar mating materials in vacuum, it can be concluded as follows: (1) Under vacuum conditions, the PCD performs low stable COFs around 0.05 and 0.08 sliding against Al2O3 or GCr15, respectively, whereas those of the PCD sliding against Si3N4 or SiC are about 0.85 and 1.15, respectively. (2) The adhesion of SiC or Si3N4 caused by the formation of Si–C and C–C bonds at the contact interface, is observed on the surface of the PCD, which lead to their high average COFs. What's more, the slighter wear of SiC mating ball is attributed to its higher hardness than Si3N4.
(3) The low ultimate surface roughness and chemical inertness of Al2O3 mating ball result in the super low COF and minimal wear of the PCD/Al2O3 tribopair. (4) As for the PCD/GCr15 tribopair, the embed diamond grains spalling from the PCD surface resulted in abrasive wear by plowing mechanism.
Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51375466), International Science and Technology Cooperation Project of China (2011DFR50060) and Fundamental Research Funds for the Central Universities (2652015074, 2652015072) for the financial support. The authors are grateful to Prof. Haidou Wang and Dr. Guozheng Ma from National Key Lab for Remanufacturing, Academy of Armored Forces Engineering, for their help with the use of the vacuum tribometer. References [1] Y.X. Wu, H.X. Li, L. Ji, Y.P. Ye, J.M. Chen, H.D. Zhou, Vacuum tribological properties of a-C: H film in relation to internal stress and applied load, Tribol. Int. 71 (2014) 82–87. [2] Y.H. Zhao, W. Yue, F. Lin, C.B. Wang, Z.Y. Wu, Friction and wear behaviors of polycrystalline diamond under vacuum conditions, Int. J. Refract. Met. Hard Mater. 50 (2015) 43–52. [3] X. Yu, C.B. Wang, G.W. Jiang, H.S. Liu, M. Hua, Tribological mechanism and property of 9Cr18 friction pair at atmosphere and in vacuum, Vacuum 72 (2004) 461–466. [4] D. Miess, G. Rai, Fracture toughness and thermal resistance of polycrystalline diamond compacts, Mater. Sci. Eng. A 209 (1996) 270–276. [5] C.L. Liu, Z.L. Kou, D.W. He, Y. Chen, K.X. Wang, B. Hui, R. Zhang, Y.F. Wang, Effect of removing internal residual metallic phases on wear resistance of polycrystalline diamond compacts, Int. J. Refract. Met. Hard Mater. 31 (2012) 187–191. [6] A. Lammer, Mechanical properties of polycrystalline diamonds, Mater. Sci. 4 (1988) 948–956.
Fig. 14. The COFs of the PCD sliding against different mating balls under (a) ambient air and (b) vacuum.
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[7] C.W. Knuteson, T.N. Sexton, C.H. Cooley, Wear-in behaviour of polycrystalline diamond thrust bearings, Wear 271 (2011) 2106–2110. [8] T.N. Sexton, C.H. Cooley, Polycrystalline diamond thrust bearings for down-hole oil and gas drilling tools, Wear 267 (2009) 1041–1045. [9] M.W. Cook, Wear-resisting properties and application examples of PCD, Ind. Diamond Rev. 4 (1996) 107–111. [10] X.F. Liu, L.P. Wang, Z.B. Lu, Q.J. Xue, Vacuum tribological performance of DLC-based solid–liquid lubricating coatings: influence of sliding mating materials, Wear 292–293 (2012) 124–134. [11] X. Yu, J. Ai, L. Yang, C.B. Wang, Exploring tribological behaviour of diamond film by hot-filament chemical vapour deposition on tungsten carbide for lunar exploration, Vacuum 100 (2014) 41–45. [12] E. Zeiler, D. Klaffke, K. Hiltner, T. Grögler, S.M. Rosiwal, R.F. Singer, Tribological performance of mechanically lapped chemical vapor deposited diamond coatings, Surf. Coat. Technol. 116–119 (1999) 599–608. [13] S.E. Grillo, J.E. Field, The friction of natural and CVD diamond, Wear 254 (2003) 945–949. [14] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, 2001. [15] Z. Feng, Y. Tzeng, J.E. Field, Friction of diamond on diamond in ultra-high vacuum and low-pressure environments, J. Phys. D. Appl. Phys. 25 (1992) 1418–1424. [16] J. Andersson, R.A. Erck, A. Erdemir, Friction of diamond-like carbon films in different atmospheres, Wear 254 (2003) 1070–1075. [17] A. Schade, S.M. Rosiwal, R.F. Singer, Influence of surface topography of HF-CVD diamond films on self-mated planar sliding contacts in dry environments, Surf. Coat. Technol. 201 (2007) 6197–6205. [18] I.P. Hayward, I.L. Singer, L.E. Seitzman, Effect of roughness on the friction of diamond on CVD diamond coatings, Wear 157 (1992) 215–227. [19] I.P. Hayward, Friction and wear properties of diamonds and diamond coatings, Surf. Coat. Technol. 49 (1991) 554–559. [20] T. Hisakado, H. Tani, Effects of elevated temperatures and topographies of worn surfaces on friction and wear of ceramics in vacuum, Wear 224 (1999) 165–172. [21] G.Z. Ma, B.S. Xu, H.D. Wang, X.H. Wang, G.L. Li, S. Zhang, Research on the microstructure and space tribology properties of electric-brush plated Ni/MoS2-C composite coating, Surf. Coat. Technol. 221 (2013) 142–149. [22] K.P. Shaha, Y.T. Pei, D. Martinez-Martinez, J.Th.M. De Hosson, Influence of hardness and roughness on the tribological performance of TiC/a-C nanocomposite coatings, Surf. Coat. Technol. 205 (2010) 2624–2632.
[23] D.S. Knight, W.B. White, Characterization of diamond films by Raman spectroscopy, J. Mater. Res. 4 (1988) 385–393. [24] C.A. Charitidis, Nanomechanical and nanotribological properties of carbon-based thin films, Int. J. Refract. Met. Hard Mater. 28 (2010) 51–70. [25] A. Bharatish, H.N. Narasimha Murthy, G. Aditya, B. Anand, B.S. Satyanarayana, M. Krishna, Evaluation of thermal residual stresses in laser drilled alumina ceramics using Micro-Raman spectroscopy and COMSOL Multiphysics, Opt. Laser Technol. 70 (2015) 76–84. [26] M.I. Baraton, M.S. El-Shall, Synthesis and characterization of nanoscale metal oxides and carbides: II. Micro-Raman and FT-IR surface studies of a silicon carbide powder, Nanostruct. Mater. 6 (1995) 301–304. [27] W.B. Qin, W. Yue, C.B. Wang, Understanding integrated effects of humidity and interfacial transfer films formation on tribological behaviors of sintered polycrystalline diamond sliding against silicon nitride, RSC Adv. 5 (2015) 53484–53496. [28] Awadesh K. Mallik, S.A. Shivashankar, S.K. Biswas, High vacuum tribology of polycrystalline diamond coatings, Sadhana 34 (2009) 811–821. [29] V. Podgursky, T. Hantschel, A. Bogatov, E. Kimmari, M. Antonov, M. Viljus, V. Mikli, M. Tsigkourakos, W. Vandervorst, J.G. Buijnsters, A.T. Raadik, P. Kulu, Rippling on wear scar surfaces of nanocrystalline diamond films after reciprocating sliding against ceramic balls, Tribol. Lett. 55 (2014) 493–501. [30] H.W. Liu, Akihiro Tanakaa, Tai Kumagai, Influence of sliding mating materials on the tribological behavior of diamond-like carbon films, Thin Solid Films 352 (1999) 145–150. [31] M.N. Gardos, K.Y. Ravi, Tribological behavior of CVD diamond films, J. Electrochem. Soc. 89 (1989) 475–493. [32] M.N. Gardos, B.L. Soriano, The effect of environment on the tribological properties of polycrystalline diamond films, J. Mater. Res. 5 (1990) 2599–2609. [33] U. Bögli, A. Blatter, S.M. Pirnenovy, E.D. Obraztsova, A.A. Smolin, M. Maillat, A. Leijala, J. Burger, H.E. Hintermann, E.N. Loubnin, Tribological properties of smooth polycrystalline diamond films, Diamond Relat. Mater. 4 (1995) 1009–1019. [34] R.W. Soukup, Historical aspects of the chemical bond, chemical relationality versus physical objectivity, Monatsh. Chem. 136 (2005) 803–818. [35] P. Cartes, M. Cea, A. Jara, A. Violante, Description of mutual interactions between silicon and phosphorus in Andisols by mathematical and mechanistic models, Chemosphere 131 (2015) 164–170.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of sliding mating materials on vacuum tribological behaviors of sintered polycrystalline diamond Xiaohua Sha a, Wen Yue a,b,⁎, Yihui Zhao a, Fang Lin a, Chengbiao Wang a,b a b
School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 12 May 2015 Received in revised form 16 July 2015 Accepted 19 July 2015 Available online 23 July 2015 Keywords: Polycrystalline diamond Vacuum Mating material Tribological behaviors
a b s t r a c t The comparative tribological behaviors of the sintered polycrystalline diamond (PCD) sliding against SiC, Si3N4, GCr15 and Al2O3 mating balls have been investigated by a ball-on-disk tribometer under vacuum conditions. Scanning electron microscope (SEM), energy dispersive X-ray spectrum (EDS) and Raman spectroscopy were performed to study the morphologies and chemical composition of the worn surfaces on the PCD and mating balls. The results showed that the coefficients of friction (COFs) of PCD/SiC and PCD/Si3N4 tribopairs were about ten times as high as those of PCD/GCr15 and PCD/Al2O3 tribopairs. The EDS and Raman spectroscopy results demonstrated that the higher friction and wear of PCD/SiC and PCD/Si3N4 tribopairs were attributed to serious adhesion caused by the formation of C–C and Si–C bonds at the contact interface. Some diamond grains were found in the wear scar of GCr15 mating ball, which indicated that the embed diamond grains spalling from the PCD surface resulted in abrasive wear by plowing mechanism. Sliding against Al2O3, the low ultimate surface roughness and chemical inertness maintained the super low COF and minimal wear of both the PCD disc and mating ball. These results proposed that the vacuum tribological behaviors of the PCD were significantly affected by mating materials. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The polycrystalline diamond (PCD), sintered under high temperature and high pressure, is composed of diamond micro-powders and a “binder phase” of Co [1,2]. It is one of the potential materials for space drilling bits and aerospace components, including thrust bearings, due to its extremely high hardness, toughness and thermal conductivity, and high wear resistance [3–7]. The much-needed comparative study for tribological mechanism on variant mating materials in vacuum can offer engineering reference for proper mate selection, and make it more effectively applicable in aerospace field [8,9]. The vacuum tribological behaviors of diamond materials vary when sliding against dissimilar mating materials [10–12]. Liu et al. [10] examined the friction and wear behaviors of diamond-like carbon films sliding against GCr15, bronze, ZrO2, Al2O3, SiC, WC, and Si3N4 mating balls under high vacuum conditions, and the contact radius and the contact pressure were introduced to explain the diverse friction behaviors. Yu et al. [11] studied the vacuum tribological behavior of chemical vapor deposition (CVD) diamond films sliding against Si3N4, and observed a
⁎ Corresponding author at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China. E-mail addresses: [email protected], [email protected] (W. Yue).
http://dx.doi.org/10.1016/j.ijrmhm.2015.07.025 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
mass of transferred layers on the worn track of diamond. Zeiler et al. [12] investigated the friction behaviors of CVD diamond sliding against different ceramic materials. And the results show that Al2O3 and Si3N4 exhibit excellent wear behaviors accompanied by very low friction, and SiC and ZrO2 show somewhat inferior wear and friction behaviors. The different tribological behaviors of diamond materials result from diverse tribological mechanisms. Adhesion was proposed explaining diamond friction in vacuum [13–16]. Grillo et al. [13] investigated the vacuum tribological properties of self-mated natural diamond, and suggested that adhesion plays a fundamental role in diamond tribological behavior in vacuum. Bowden and Tabor [14] proposed that vacuum friction resulted from atomic bonding between diamond surfaces. Under vacuum conditions, the surfaces are not passivated by adsorbed molecules and therefore the dangling σ bonds of the carbon atoms on diamond surfaces are free to join at the sliding interfaces. Such interactions can cause strong adhesion and hence high friction [15,16]. Besides, the effect of mate surface roughness on tribological behaviors of diamond materials has been investigated [17–19]. Schade et al. [17] announced that the self-mated fine-grained CVD diamond coatings with low surface roughness and good flatness were best for dry sliding, and they indeed revealed in some cases longer running-in times. Similar speculation on the role of roughness was made by Hayward et al. They concluded that running-in period depended on surface roughness, and that the friction would decrease only after the interface had been
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smoothed by abrasive wear [18,19]. Based on previous work, the diamond materials exhibit significantly various vacuum tribological behaviors by different tribological mechanisms when sliding against diverse mating balls. The study on friction and wear behaviors of the PCD sliding against Si3N4 ball indicated that the friction coefficients obtained under vacuum are much higher than those under ambient air, and Zhao et al. [2] attributed it to the serious adhesion between counter surfaces and weak bonding strength among diamond grains. Nevertheless, few researches have been performed on the influence of mating materials on the tribological behavior of PCD under vacuum conditions. Therefore, in the present paper, vacuum sliding wear tests were carried out using SiC, Si3N4, GCr15 and Al2O3 mating balls, to investigate the role of mating materials on the tribological behaviors of the PCD. The wear track and wear scar were examined using scanning electron microscope (SEM)/energy dispersive X-ray spectrum (EDS) and Raman spectroscopy to explore the vacuum tribological mechanisms of the PCD when sliding against different materials.
117
Cobalt catalyst
Diamond grains
2. Experimental details Fig. 2. SEM morphology of the PCD surface.
2.1. Materials The flat samples used in this work are commercial grade polycrystalline diamond compacts (PDCs), which contain coarse grain (half-content diameter, D50 = 25 μm) diamond, and WC-Co (16 wt.% Co) substrate. The diamond grains are typically placed adjacent to WC-Co substrate that provides a source for catalyst metal. The PDC disc was polished to make the surface roughness reach a value of 3–4 nm. The cross-sectional image and optical photo of the PDC are shown in Fig. 1. It can be seen that the dimensions of the disc are 45.0 mm in diameter, 2.9 mm in thickness, and 540 μm in thickness of the PCD layer and 2.37 mm in thickness of WC-Co substrate. Fig. 2 presents the surface morphology of the PCD. It is identified that the binder phase corresponded to the bright regions distribute along the grain boundaries of diamond corresponded to the dark ones. In this study, SiC, Si3N4, GCr15 and Al2O3 balls were used as mating samples. The physical properties of the PCD and mating balls are listed in Table 1 [2,9,20]. 2.2. Tribological tests The tribological tests were performed in vacuum (~7.0 × 10−4 Pa) by a ball-on-disk space tribometer (MSTS-1) [21]. The PDCs were used as the disks. The ball specimens were SiC, Si3N4, GCr15 and Al2O3 balls
540 µm
2370 µm
PCD layer
WC/Co substrate
with diameter of Φ 9.525 mm. During the tests, the mating ball was fixed while the PDC disc was rotating with a turning radius of 10 mm, and the duration of tribological test was 30 min. The friction forces were measured by the dynameter and then converted into friction coefficients throughout the tests. Prior to the tribological test, both the PDC discs and mating balls were rinsed with hexane, and then ultrasonically cleaned in fresh hexane, finally cleaned by ultrasonic with acetone for 30 min. The investigation aims to study the effect of mating materials on vacuum tribological behaviors of the PCD. Generally, the tribological behaviors of the PCD should be affected by the applied load and revolution speed. Therefore, the impact of applied load (30 N and 45 N) on the vacuum tribological behaviors of the PCD was investigated under a constant revolution speed of 100 r/min. Moreover, the applied load was constant at 30 N to evaluate the effect of revolution speed (100 r/min and 150 r/min) on the tribological behaviors of the PCD in vacuum. The selected parameters of the applied load and revolution speed were based on previous tribotests under diverse applied loads and revolution speeds.
2.3. Surface analysis The surface analysis of the PCD discs and mating balls tested under 30 N and 100 r/min was carried out to study the vacuum tribological mechanisms. Because the tribological behaviors of the PCD sliding against different mating materials under various applied loads and revolution speeds present a similar trend. The three-dimensional wear profiles across wear tracks of the PCD were measured by NanoMap-D three-dimensional white light interferometer. The wear scar morphologies of mating balls were observed using the Olympus BX51M optical microscopy and VK-X100K confocal microscope. The surface roughness of the mating balls before and after tribological tests were measured by VK-X100K confocal microscope. The CS3400 scanning electron Table 1 Physical properties and surface roughness of test specimens.
Fig. 1. The cross-sectional image and optical photo (the inset) of the polycrystalline diamond compact.
Materials
Hardness (GPa)
Young's modulus (GPa)
Surface roughness (nm)
Thermal conductivity (W/(m·K))
Poisson's ratio
PCD SiC Al2O3 Si3N4 GCr15
40–50 25–27 15–17 14–16 6–8
810 440 370 300 210
3–4 30–40 300–350 20–30 20–30
700.0 33.0–33.4 25.2–32.9 16.2–29.5 45.2–51.3
0.07 0.17 0.27 0.25 0.30
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Fig. 3. The COFs of the PCD sliding against dissimilar mating balls under different loads and revolution speeds in vacuum. (a) 30 N, 100 r/min; (b) 30 N, 100 r/min and (c) 30 N, 100 r/min.
microscope (SEM) was used to observe the surface structures and topographies of the PCD discs and counter balls. The Oxford EDX-450 energy dispersive X-ray spectrum (EDS) was used to investigate the element chemical compositions of the worn PCD surfaces. The compositions and structures of the materials on the worn surfaces of mating balls and PCD discs were studied using the LabRAM HR Evolution Raman spectrometer with 514.5 nm wavelength of Ar+ laser. 3. Results 3.1. Friction and wear behaviors The research focuses on the effect of mating materials on vacuum tribological behaviors of the PCD. Besides, the impacts of the applied load and revolution speed on vacuum tribological behaviors of the PCD are also investigated. Fig. 3 shows the coefficients of friction (COFs) of the PCD discs sliding against SiC, Si3N4, GCr15 and Al2O3 mating balls under different loads and revolution speeds in vacuum. The impacts of mating materials on vacuum tribological behaviors of the PCD are presented in Fig. 3(a). It reveals that the average COFs of PCD/SiC and PCD/Si3N4 tribopairs are around 1.15 and 0.85, respectively, while those of PCD/GCr15 and PCD/Al2O3 tribopairs are about 0.08 and 0.05, respectively. It is clear that the COF curves of PCD/SiC and PCD/Si3N4 tribopairs fluctuant great, whereas the PCD/GCr15 presents steady curve. As for the PCD/Al2O3 tribopair, it presents an exceptional variation of COF during sliding operation. The initial COF of PCD/Al2O3 tribopair is about 0.04, increasing with sliding operation, peaking after about 5 min, and then decreasing slightly to a steady stage of around 0.05. The COF curve implies that the PCD/Al2O3 tribopair undergoes a long running-in period for 20 min. It is revealed that the COFs of the PCD depend on mating materials. The PCD presents higher COFs when
sliding against SiC and Si3N4, and exhibits super low COF in the case of Al2O3. The effects of the applied load on vacuum tribological behaviors of the PCD are shown in Fig. 3(a) and Fig. 3(b). It presents that the COFs appear to rise as the load increases. Similar results were found for the CVD diamond and natural diamond studied by Grillo and Field [16]. Moreover, the COFs of the PCD sliding against dissimilar mating balls follow the same trends under loads of 30 N and 45 N. The results imply that the applied load probably plays no role on the vacuum tribological behaviors of the PCD sliding against different mating materials. The roles of the revolution speed on the vacuum tribological behaviors of the PCD are shown in Fig. 3(a) and (c). The COFs of the PCD/SiC, PCD/Si3N4 and PCD/Al2O3 tribopairs decrease with the increasing speed of revolution, except for the PCD/GCr15 tribopair. Totally, the tendencies of COFs for different tribopairs do not occur, which means that the revolution speed may be independent of the tribological behaviors of the PCD sliding against dissimilar mating materials in vacuum. The three-dimensional wear profiles across wear tracks of the PCD discs sliding against different mating balls were carried out, and the results are presented in Fig. 4. When sliding against SiC (Fig. 4(a)) and Si3N4 (Fig. 4(b)), irregularly distributed spalling pits and conical uplifts were found on the worn surfaces of the PCD. A similar observation reported for PCD/Si3N4 tribopair under vacuum conditions attributed the high friction property obtained in vacuum to serious adhesion across contact interfaces [2]. In contrast, the worn regions on the PCD sliding against Al2O3 (Fig. 4(d)) present mild wear. Fig. 4(c) indicates that spalling pits and plowing grooves present on the surface of the PCD against GCr15 ball, which implies the occurrence of abrasive wear. The optical morphologies of worn surfaces on different mating balls after sliding operation under vacuum conditions are illustrated in Fig. 5, and the arrows indicate the sliding directions. The following features are
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(a) SiC
(b) Si3N4
(c) GCr15
(d) Al2O3
Fig. 4. Three-dimensional surface topographies of the wear tracks formed on the PCD discs.
(b) Si3N4
(a) SiC
Sliding direction
Sliding direction
~ 3.6 mm
~ 3.4 mm
(d) Al2O3
(c) GCr15 Sliding direction
Sliding direction
~ 680 µm
~ 610 µm
Fig. 5. The wear scars of mating balls after sliding operation under vacuum conditions.
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observed when comparing the distinct tested mating balls. The wear scars of SiC (Fig. 5(a)) and Si3N4 (Fig. 5(b)) are about an order of magnitude larger in diameter than those of GCr15 (Fig. 5(c)) and Al2O3 (Fig. 5(d)). Comparing Si3N4 with SiC ball, it can be found that the wear scar of Si3N4 ball is slightly bigger, which may result from the different hardness of mating balls [22]. It is clearly showed in Fig. 5(c) that the wear scar with overlap issue and plastic deformation presents some obvious deep grooves for GCr15 mating ball. Moreover, black deposits are observed in the grooves, which may lead to the grooves by plowing mechanism. As for the Al2O3 (Fig. 5(d)) mating ball, the wear scar is relatively smaller in diameter and some debris are piled up at the leading edge. The three-dimensional views of the wear scars on mating balls were performed by the confocal microscope. It presents much larger wear scars in diameter for SiC and Si3N4 mating balls as shown in Fig. 6. It reveals in Fig. 6(c) that the worn surface of GCr15 mating ball presents evident grooves, which might be result from the abrasive wear. The Al2O3 ball behaviors the smallest wear scar. The results are consistent with the optical results shown in Fig. 5.
3.2. SEM and EDS analysis The SEM images of worn surfaces on the PCD discs sliding against different mating balls are shown in Fig. 7. The inset images reveal that the wear track of the PCD against SiC (Fig. 7(a1)) is similar to that of the PCD against Si3N4 (Fig. 7(b1)), while the wear track of the PCD against GCr15 (Fig. 7(c1)) is much alike with that of the PCD against Al2O3 (Fig. 7(d1)). Sliding against GCr15 and Al2O3, the PCD discs present much slighter and narrower wear tracks than the others, which is corresponding to the results revealed in the wear scars of mating balls. After sliding against SiC and Si3N4, a large number of white speckles are deposited on the wear tracks of the PCD, and the one against Si3N4 presents much wider wear track. For further investigation, the enlarged
views of worn surfaces and the corresponded EDS mapping analysis were performed. It is identified that the silicon, which corresponded to the white areas in the SEM images, distributed on the wear tracks of the PCD discs sliding against SiC and Si3N4. The results interpret that the SiC and Si3N4 coming from mating balls significantly adhered to the PCD surfaces during sliding operation in vacuum. By contrast, few adhered materials were observed for the ones against GCr15 and Al2O3. The SEM morphologies of the wear scars on mating balls are shown in Fig. 8, and the arrows indicate the sliding directions. It is observed that the worn surfaces of SiC (Fig. 8(a)) and Si3N4 (Fig. 8(b)) exhibit the severe adhesive wear, which is corresponding to spalling pits and conical uplifts. The GCr15 counterface (Fig. 8(c)) reveals the abrasive wear, while mild abrasive wear occurs on the worn surface of Al2O3 (Fig. 8(d)).
3.3. Raman spectrum analysis Raman spectroscopy is an effective way to characterize the composition and structure of materials with molecular (and bond) specificity [23–25]. The worn surfaces of different mating balls were analyzed by Raman spectroscopy aiming to ascertain whether adhesive materials can be found on the mating balls. The results are exhibited in Fig. 9. The Raman spectrum obtained from the wear scar of SiC (Fig. 9(a)), Si3N4 (Fig. 9(b)) and Al2O3 (Fig. 9(d)) presents characteristic peaks proving the existence of SiC [26], Si3N4 [27] and Al2O3 [25]. These confirm that no other materials are observed on the wear scars of SiC, Si3N4 and Al2O3 balls. In order to find out the composition of embed materials in the deep grooves on the wear scar of GCr15 mating ball (Fig. 5(c)), the detecting positions of Raman spectrum analysis were selected at the black scratch. The result shown in Fig. 9(c) reveals that an apparently diamond Raman single (~ 1332 cm− 1) occurs
(a) SiC
(b) Si3N4
(c) GCr15
(d) Al2O3
Fig. 6. Confocal micrographs showing three-dimensional views of wear scars on (a) SiC, (b) Si3N4, (c) GCr15 and (d) Al2O3 mating balls.
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(a1) SiC
(a2) SiC
121
Si-K
Wear track
(b1) Si3N4
(b2) Si3N4
Si-K
Wear track
(c1) GCr15
(c2) GCr15
Fe-K
Wear track
(d1) Al2O3
(d2) Al2O3
Al-K
Wear track
Fig. 7. SEM images (a1, b1, c1 and d1) of worn surfaces of the PCD sliding against different mating balls and corresponded EDS mapping images (a2, b2, c2 and d2).
on detecting positions, implying that some diamond grains filled in the wear scar of GCr15 counterface [19]. It confirms that the abrasive wear at the interface of the PCD/GCr15 tribopair is caused by the plowing action of embed diamond grains spalled from the PCD disc. As this reason, the increased applied load results in more embedded diamond grains in the wear scar of GCr15 mating ball, leading to more serious abrasive wear (Fig. 3(a) and (b)).
4. Discussion The PCD performs quite different tribological behaviors when sliding against SiC, Si3N4, GCr15 and Al2O3 in vacuum. The COFs of PCD/SiC and PCD/Si3N4 tribopairs are about ten times as high as those of PCD/GCr15 and PCD/Al2O3 tribopairs, and the former tribopairs worn much more seriously. Corresponding to these, the four tribopairs present dissimilar tribological mechanisms.
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(a) SiC Sliding direction
(b) Si3N4
Sliding direction
Adhesive wear Adhesive wear
(c) GCr15
(d) Al2O3 Sliding direction
Abrasive wear
Sliding direction
Mild abrasive wear
Fig. 8. SEM micrographs of the wear scars on (a) SiC, (b) Si3N4, (c) GCr15 and (d) Al2O3 mating balls after sliding test.
4.1. Tribological mechanisms of the PCD/SiC and PCD/Si3N4 tribopairs The PCD/SiC and PCD/Si3N4 tribopairs present high and great fluctuated COFs. Moreover, as shown in Figs. 4 and 5, both the discs and mating balls wear seriously after sliding operation in vacuum. The SEM micrographs of the wear scars on SiC and Si 3 N 4 presented in Fig. 7(a) and (b) reveal obvious adhesive wear. The EDS mapping images (Fig. 8(a) and (b)) of worn surfaces on the PCD discs also confirm that adhesion occurs during sliding operation with SiC and Si 3N 4 mating balls. Sliding under vacuum conditions, two clean surfaces of the tribopair contacted directly due to the desorption of hydrogen and other adsorbate, creating dangling bonds on the surface of the PCD [12,28]. In this case, the formation of chemical bonds between the PCD and mating balls seems to be extremely ready to occur, which would lead to adhesion. In order to find out the structure of adhesive materials observed on the wear track of the PCD against Si3N4 ball, Raman spectra analysis was performed, and the result is presented in Fig. 10. The Raman spectrum obtained on the detecting positions exhibits the only one peak at around 733 cm− 1, which illustrates the formation of Si–C bond during sliding operation for the PCD/Si3N4 tribopair [26]. Presenting analogous adhesive characteristics, it may also occur the formation of Si–C and C–C bonds at the contact interface of PCD/SiC tribopair during sliding operation. A model is proposed to further illustrate the adhesion wear mechanism, as shown in Fig. 11. Such a serious adhesion caused by the formation of Si–C and C–C bonds at the contact interface of PCD/SiC and PCD/Si 3N 4 tribopairs is the main reason why the friction is higher and it presents much serious wear under vacuum conditions. The result that the PCD against SiC ball wears a litter slightly than that against Si3N4 may result from the higher hardness of SiC. Previous study has reported that the hardness of mating materials has an enormous effect on the wear behaviors of diamond materials [23,29,30]. Liu et al. [30] investigated the tribological properties of DLC films against both hard and soft mating materials and found that the harder the sliders, the lower their wear rates. It indicates that the observed phenomenon showed in Fig. 5 that the SiC mating ball wears a litter slighter than Si3N4 is attributed to its comparatively higher hardness.
The friction coefficients of 1.15 and 0.85 for PCD/SiC and PCD/Si3N4 tribopairs mean that the friction force hindering the relative motion of tribopairs is extremely high under vacuum conditions. The high friction force mainly comes from the serious adhesion action of sliding tribopair ball during sliding operation [31,32]. It implies that under vacuum conditions, the PCD applied in such as thrust bearings and drilling bits, sliding against SiC or Si3N4 is prone to be stuck and is thus hardly applicable due to the severe adhesion action during movement operation. 4.2. Tribological mechanism of the PCD/GCr15 tribopair Comparing the EDS results of SiC and Si3N4 shown in Fig. 7, the PCD/ GCr15 and PCD/Al2O3 tribopairs do not exhibit much adhesion, which result in a lower COF and slighter wear. Sliding against GCr15, the PCD presents low and stable COF, being accompanied by wild wear of both the PCD and mating ball. In addition, the SEM micrograph (Fig. 8(c)) of the wear scar on GCr15 mating ball presents an obvious abrasive wear. The result exhibited in Fig. 9(c) confirms that some diamond grains shedding from the PCD disc embed in the mating ball during sliding operation, which is responding to the appearance of spalling pits on the worn surface of the PCD shown in Fig. 4(c). Therefore, it is reasonable that the vacuum wear mechanism of PCD/GCr15 tribopair is mainly the secondary abrasive wear caused by the embed diamond grains in the counterface of GCr15 mating ball. A model is proposed to further illustrate the wear mechanism, as shown in Fig. 12. When sliding against GCr15 mating ball in vacuum, some of tinier diamond grains on the contact surface of the PCD are likely to shed off during sliding operation. Moreover, during subsequent sliding, some spalling diamond grains may embed in the GCr15 mating ball, simultaneously scratch the PCD surface in return. 4.3. Tribological mechanism of the PCD/Al2O3 tribopair Sliding against Al2O3 in vacuum, the COF of PCD presents an enormous variation shown in Fig. 3(a). It undergoes a long running-in period and gradually reaches a steady stage of around 0.05. Besides, both Al2O3 and the PCD wear mildly. These phenomena may be related to the changes of surface roughness for Al2O3 mating ball during sliding
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(a1)
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(a2) Detecting positions
(b1)
(b2) Detecting positions
(c1)
(c2)
Detecting positions
(d2)
(d1)
Detecting positions
Fig. 9. Raman spectra of wear scars on (a) SiC, (b) Si3N4, (c) GCr15 and (d) Al2O3 balls.
operation. Al2O3 is one of the rough surfaced ceramics. In this study, the surface roughness of Al2O3 ball is approximately 10 times as high as that of other mating balls, as shown in Table. 1. Researchers [17,23,33] demonstrated that the rough surface not only gives a high COF during the
longer running-in period, but also governs the overall tribological behavior. To further investigate the effect of mating ball surface roughness on the vacuum tribological behavior of PCD/Al2O3 tribopair, a worn Al2O3
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(a)
(b) Detecting positions
Fig. 10. Raman spectrum of adhered material observed on the worn surface of the PCD sliding against Si3N4 ball: black adhesion on the wear track and Si–C vibration standard.
(a)
(b) Si-C
C-C
Si-C
Fig. 11. Vacuum tribological mechanisms of the (a) PCD/SiC and (b) PCD/Si3N4 tribopairs: adhesion caused by the formation of Si–C and C–C bonds. The gray, purple and blue atoms correspond to C, Si and N elements, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ball obtained after sliding against a PCD disc for 30 min was tested against a new PCD disc, and the COF curve is presented in Fig. 13. It reveals that the COF of the PCD sliding against the worn Al2O3 keeps a steady-state for a value of around 0.05, which is accordance with the stable ultimate COF of the new Al2O3 ball. Besides, almost no evidence of running-in period was observed. Moreover, the surface roughness of Al2O3 mating ball before and after tribological tests was measured by confocal micrographs, presenting in Table 2. The surface roughness of mating ball reduces much from 300–350 nm to 40–50 nm after 30 min sliding, and nearly kept at a low value during the further sliding.
It illustrates that the uneven protruding surface asperities of Al2O3 mating ball are gradually polished during the long initial running-in period, leading to a dropping friction value. In this way, the low ultimate surface roughness of Al2O3 maintains the final super low COF and minimal wear of the PCD/Al2O3 tribopair. Under vacuum conditions, two clean surfaces of the tribopair were contacted directly due to the breakdown of surface adsorption. The analogous elements are prone to form chemical bonds for strong affinity [34]. Therefore, SiC and Si3N4 are significantly likely to adhere to the PCD surfaces by the formation of Si–C and C–C bonds at the contact
Fig. 12. Vacuum wear mechanism of the PCD/GCr15 tribopair: secondary abrasive wear caused by embedded diamond grains spalling from the PCD.
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Stage
Fig. 13. The COF curve of the PCD sliding against a worn Al2O3 ball (obtained after sliding against a new Al2O3 ball for 30 min).
Table 2 The variation of surface roughness for wear scar on Al2O3 ball. Sliding time (min) Surface roughness (nm)
0 300–350
30 40–50
60 30–40
surfaces during vacuum sliding operation, leading to the high COFs [2]. On the contrary, The PCD/Al2O3 tribopair is hardly possible for chemical bonds forming due to the weak affinity between Al and C elements [34, 35]. The comparative experimental results shown in Fig. 14 reveal that the COFs of the PCD sliding against different mating balls in vacuum were about ten times as high as those in air. This may be attributed to the difference between the termination and desorption of dangling bonds on the PCD surface in air and vacuum, respectively. Overall, the low ultimate surface roughness and chemical inertness of Al2O3 maintain the final low and steady COF and minimal wear of the PCD/Al2O3 tribopair. 5. Conclusions In the present study to evaluate tribological behaviors of the PCD sliding against dissimilar mating materials in vacuum, it can be concluded as follows: (1) Under vacuum conditions, the PCD performs low stable COFs around 0.05 and 0.08 sliding against Al2O3 or GCr15, respectively, whereas those of the PCD sliding against Si3N4 or SiC are about 0.85 and 1.15, respectively. (2) The adhesion of SiC or Si3N4 caused by the formation of Si–C and C–C bonds at the contact interface, is observed on the surface of the PCD, which lead to their high average COFs. What's more, the slighter wear of SiC mating ball is attributed to its higher hardness than Si3N4.
(3) The low ultimate surface roughness and chemical inertness of Al2O3 mating ball result in the super low COF and minimal wear of the PCD/Al2O3 tribopair. (4) As for the PCD/GCr15 tribopair, the embed diamond grains spalling from the PCD surface resulted in abrasive wear by plowing mechanism.
Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51375466), International Science and Technology Cooperation Project of China (2011DFR50060) and Fundamental Research Funds for the Central Universities (2652015074, 2652015072) for the financial support. The authors are grateful to Prof. Haidou Wang and Dr. Guozheng Ma from National Key Lab for Remanufacturing, Academy of Armored Forces Engineering, for their help with the use of the vacuum tribometer. References [1] Y.X. Wu, H.X. Li, L. Ji, Y.P. Ye, J.M. Chen, H.D. Zhou, Vacuum tribological properties of a-C: H film in relation to internal stress and applied load, Tribol. Int. 71 (2014) 82–87. [2] Y.H. Zhao, W. Yue, F. Lin, C.B. Wang, Z.Y. Wu, Friction and wear behaviors of polycrystalline diamond under vacuum conditions, Int. J. Refract. Met. Hard Mater. 50 (2015) 43–52. [3] X. Yu, C.B. Wang, G.W. Jiang, H.S. Liu, M. Hua, Tribological mechanism and property of 9Cr18 friction pair at atmosphere and in vacuum, Vacuum 72 (2004) 461–466. [4] D. Miess, G. Rai, Fracture toughness and thermal resistance of polycrystalline diamond compacts, Mater. Sci. Eng. A 209 (1996) 270–276. [5] C.L. Liu, Z.L. Kou, D.W. He, Y. Chen, K.X. Wang, B. Hui, R. Zhang, Y.F. Wang, Effect of removing internal residual metallic phases on wear resistance of polycrystalline diamond compacts, Int. J. Refract. Met. Hard Mater. 31 (2012) 187–191. [6] A. Lammer, Mechanical properties of polycrystalline diamonds, Mater. Sci. 4 (1988) 948–956.
Fig. 14. The COFs of the PCD sliding against different mating balls under (a) ambient air and (b) vacuum.
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[7] C.W. Knuteson, T.N. Sexton, C.H. Cooley, Wear-in behaviour of polycrystalline diamond thrust bearings, Wear 271 (2011) 2106–2110. [8] T.N. Sexton, C.H. Cooley, Polycrystalline diamond thrust bearings for down-hole oil and gas drilling tools, Wear 267 (2009) 1041–1045. [9] M.W. Cook, Wear-resisting properties and application examples of PCD, Ind. Diamond Rev. 4 (1996) 107–111. [10] X.F. Liu, L.P. Wang, Z.B. Lu, Q.J. Xue, Vacuum tribological performance of DLC-based solid–liquid lubricating coatings: influence of sliding mating materials, Wear 292–293 (2012) 124–134. [11] X. Yu, J. Ai, L. Yang, C.B. Wang, Exploring tribological behaviour of diamond film by hot-filament chemical vapour deposition on tungsten carbide for lunar exploration, Vacuum 100 (2014) 41–45. [12] E. Zeiler, D. Klaffke, K. Hiltner, T. Grögler, S.M. Rosiwal, R.F. Singer, Tribological performance of mechanically lapped chemical vapor deposited diamond coatings, Surf. Coat. Technol. 116–119 (1999) 599–608. [13] S.E. Grillo, J.E. Field, The friction of natural and CVD diamond, Wear 254 (2003) 945–949. [14] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, 2001. [15] Z. Feng, Y. Tzeng, J.E. Field, Friction of diamond on diamond in ultra-high vacuum and low-pressure environments, J. Phys. D. Appl. Phys. 25 (1992) 1418–1424. [16] J. Andersson, R.A. Erck, A. Erdemir, Friction of diamond-like carbon films in different atmospheres, Wear 254 (2003) 1070–1075. [17] A. Schade, S.M. Rosiwal, R.F. Singer, Influence of surface topography of HF-CVD diamond films on self-mated planar sliding contacts in dry environments, Surf. Coat. Technol. 201 (2007) 6197–6205. [18] I.P. Hayward, I.L. Singer, L.E. Seitzman, Effect of roughness on the friction of diamond on CVD diamond coatings, Wear 157 (1992) 215–227. [19] I.P. Hayward, Friction and wear properties of diamonds and diamond coatings, Surf. Coat. Technol. 49 (1991) 554–559. [20] T. Hisakado, H. Tani, Effects of elevated temperatures and topographies of worn surfaces on friction and wear of ceramics in vacuum, Wear 224 (1999) 165–172. [21] G.Z. Ma, B.S. Xu, H.D. Wang, X.H. Wang, G.L. Li, S. Zhang, Research on the microstructure and space tribology properties of electric-brush plated Ni/MoS2-C composite coating, Surf. Coat. Technol. 221 (2013) 142–149. [22] K.P. Shaha, Y.T. Pei, D. Martinez-Martinez, J.Th.M. De Hosson, Influence of hardness and roughness on the tribological performance of TiC/a-C nanocomposite coatings, Surf. Coat. Technol. 205 (2010) 2624–2632.
[23] D.S. Knight, W.B. White, Characterization of diamond films by Raman spectroscopy, J. Mater. Res. 4 (1988) 385–393. [24] C.A. Charitidis, Nanomechanical and nanotribological properties of carbon-based thin films, Int. J. Refract. Met. Hard Mater. 28 (2010) 51–70. [25] A. Bharatish, H.N. Narasimha Murthy, G. Aditya, B. Anand, B.S. Satyanarayana, M. Krishna, Evaluation of thermal residual stresses in laser drilled alumina ceramics using Micro-Raman spectroscopy and COMSOL Multiphysics, Opt. Laser Technol. 70 (2015) 76–84. [26] M.I. Baraton, M.S. El-Shall, Synthesis and characterization of nanoscale metal oxides and carbides: II. Micro-Raman and FT-IR surface studies of a silicon carbide powder, Nanostruct. Mater. 6 (1995) 301–304. [27] W.B. Qin, W. Yue, C.B. Wang, Understanding integrated effects of humidity and interfacial transfer films formation on tribological behaviors of sintered polycrystalline diamond sliding against silicon nitride, RSC Adv. 5 (2015) 53484–53496. [28] Awadesh K. Mallik, S.A. Shivashankar, S.K. Biswas, High vacuum tribology of polycrystalline diamond coatings, Sadhana 34 (2009) 811–821. [29] V. Podgursky, T. Hantschel, A. Bogatov, E. Kimmari, M. Antonov, M. Viljus, V. Mikli, M. Tsigkourakos, W. Vandervorst, J.G. Buijnsters, A.T. Raadik, P. Kulu, Rippling on wear scar surfaces of nanocrystalline diamond films after reciprocating sliding against ceramic balls, Tribol. Lett. 55 (2014) 493–501. [30] H.W. Liu, Akihiro Tanakaa, Tai Kumagai, Influence of sliding mating materials on the tribological behavior of diamond-like carbon films, Thin Solid Films 352 (1999) 145–150. [31] M.N. Gardos, K.Y. Ravi, Tribological behavior of CVD diamond films, J. Electrochem. Soc. 89 (1989) 475–493. [32] M.N. Gardos, B.L. Soriano, The effect of environment on the tribological properties of polycrystalline diamond films, J. Mater. Res. 5 (1990) 2599–2609. [33] U. Bögli, A. Blatter, S.M. Pirnenovy, E.D. Obraztsova, A.A. Smolin, M. Maillat, A. Leijala, J. Burger, H.E. Hintermann, E.N. Loubnin, Tribological properties of smooth polycrystalline diamond films, Diamond Relat. Mater. 4 (1995) 1009–1019. [34] R.W. Soukup, Historical aspects of the chemical bond, chemical relationality versus physical objectivity, Monatsh. Chem. 136 (2005) 803–818. [35] P. Cartes, M. Cea, A. Jara, A. Violante, Description of mutual interactions between silicon and phosphorus in Andisols by mathematical and mechanistic models, Chemosphere 131 (2015) 164–170.