Friction and wear behaviors of polycrystalline diamond under vacuum conditions

Int. Journal of Refractory Metals and Hard Materials 50 (2015) 43–52

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Friction and wear behaviors of polycrystalline diamond under vacuum conditions Yihui Zhao a,b, Wen Yue a,b,⁎, Fang Lin a, Chengbiao Wang a,b, Zongyi Wu c a b c

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 Beijing Huayou Guanchang Environment and Energy Science & Technology Development Co., Ltd., Beijing 100020, China

a r t i c l e

i n f o

Article history: Received 4 September 2014 Received in revised form 21 November 2014 Accepted 26 November 2014 Available online 27 November 2014 Keywords: Polycrystalline diamond Vacuum Friction and wear Wear mechanism

a b s t r a c t In this work, the friction and wear behaviors of the polycrystalline diamond (PCD) sliding against Si3N4 ball were evaluated under both ambient air and vacuum by a ball-on-disk tribometer. The microstructures of the PCD were characterized by scanning electron microscopy, X-ray diffractometry, and Raman spectroscopy. The results showed that the friction coefficients obtained under vacuum are much higher than those under ambient air, and it mainly attributed to serious adhesion under vacuum because of the absence of the absorbed layer formed under ambient air. The high friction heat and the catalyzing of Co resulted in the graphitization of PCD under vacuum. The diamond grains shedding off from the PCD surface can be easily found under vacuum, whereas no observable abrasion was formed under ambient air. Both the strong adhesion between counter surfaces and the weak bonding strength among diamond grains contributed to the serious wear under vacuum. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Polycrystalline diamond (PCD) sintered under high temperature and high pressure, composed of diamond micro-powders and Co which act as a “binder phase” [1]. PCD possesses extremely high wear resistance, high hardness and excellent toughness, and it is used widely in the manufacture of machining tools and geological drill bits [2–4]. Researchers have studied the tribological behaviors of PCD in order to make it more widely and effectively applicable under extreme environmental conditions [5–8]. Deng et al. [5] pointed out that the friction coefficient of the PCD decreases with the increase of temperature. At the temperature of 700 °C, the binder phase Co extrudes out, and surface graphitization of diamond is to be expected. Huang et al. [6] found that both the abrasive and the adhesive wear mechanisms take place under a dry machining condition, whereas the mechanisms dominating the tool flank wear are abrasion under a wet machining condition. The working environment significantly affects the tribological properties of PCD. However, few researches have been performed on friction and wear behavior of PCD under a vacuum condition. The tribological properties of diamond materials would be significantly affected by vacuum conditions [9–14]. The friction of natural ⁎ 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.2014.11.008 0263-4368/© 2014 Elsevier Ltd. All rights reserved.

diamond and chemical vapor deposition (CVD) diamond films has been investigated under ambient air and vacuum condition. Diamond sliding on diamond was known to have a low friction coefficient of between 0.05 and 0.15 in air, but a much higher friction coefficient of between 0.5 and 1 under vacuum [9]. Bowden and Tabor [10] proposed that friction arises from atomic bonding between diamond surfaces. Grillo and Field [11] announced that the termination of dangling bonds by adsorbed water molecules and other adsorbates, e.g. oxygen and nitrogen, would account for the low friction coefficient of diamond under ambient air. Conversely, the desorption of the adsorbates can lead to a higher friction coefficient under vacuum conditions. Miyoshi et al. [12] reported that the equilibrium friction coefficients of a natural diamond sliding on chemical vapor deposition (CVD) diamond films are 0.02–0.04 in humid air or dry nitrogen, but the friction coefficients are 1.5–1.8 under vacuum. Consequently, the wear rates of the CVD diamond films are considerably higher under vacuum than in humid air and in dry nitrogen. Gardos and Ravi et al. [13,14] found that the friction coefficients for CVD diamond self-mating films under vacuum increase to as high as 0.8. They attributed this high friction to the adhesion between the surfaces of the diamond films. Based on a previous work, the natural diamond and CVD diamond films exhibit significantly different tribological properties under vacuum. However, the friction and wear behaviors of PCD under vacuum have not been further studied. In the present work, the friction and wear behaviors of the PCD were evaluated using a ball-on-disk tribometer under atmosphere and

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Diamond grain

Binder phase Co

Fig. 1. Optical photo of PDC.

Fig. 3. Surface morphology of PCD.

vacuum condition. Analytical instruments were used to clarify the reason of high friction coefficient and high wear under vacuum. It aims to obtain a further understanding of the tribological mechanism of PCD under vacuum. 2. Experimental details

Si3N4 ball was selected as the counter sample in a tribotest, its dimension is a diameter of 9.525 mm and a roughness (Ra) of 15– 20 nm. Physical properties of the PCD and the Si3N4 ball are listed in Table 1. 2.2. Tribotests

2.1. Materials The specimens used in this work are commercial polycrystalline diamond compacts (PDCs) from Zhongnan Diamond Co., Ltd. The dimensions of PDC are approximately 45 mm in diameter and 2.9 mm in thickness. The PDC consists of a WC-8 wt.% Co cemented carbide substrate with a PCD layer sintered onto the circular face of the substrate. The PCD layer is composed of a coarse grain (half-content diameter, D50 = 25 μm) diamond with Co binder. The PCD was polished to make the surface roughness (Ra) reach a value of 3–4 nm. The optical images and the cross-sectional image of the PDC specimen are shown in Figs. 1 and 2. It can be seen that the thickness of the PCD layer is 540 μm, and the thickness of the cemented carbide substrate is 2370 μm. Fig. 3 shows the surface morphology of PCD. It can be seen that the PCD layer is compact and smooth. The binder phase corresponding to the bight regions distributes along the grain boundaries of diamond which corresponds to the dark ones.

2370µm

WC/Co substrate 540 µm

The tribotests were performed under ambient air (relative humid, 30 ± 5%) and vacuum (7.0 × 10−4 Pa) using a MSTS-1 multifunctional space tribometer test system [15]. The upper sample was a Si3N4 ceramic ball, and the PDC acted as the lower sample. Prior to the tribotest, both the ball and PDC were rinsed with hexane, and then ultrasonically cleaned in fresh hexane, finally by ultrasonic cleaning with acetone for 30 min. During the tribotest, the Si3N4 ceramic ball was fixed, while the PDC disc was rotated. The loads were given as 15 N, 30 N, and 45 N and the corresponding mean contact pressure were 1.24 GPa, 1.56 GPa, and 1.79 GPa, respectively. The applied speed of revolution was 100 r/min and the turning radius was 10 mm. The duration of tribotest was 30 min. The friction forces were measured by a dynameter and then converted into friction coefficients. The surface topography of the wear tracks was measured using a NanoMap-D three-dimensional White Light Interferometer. 2.3. Microanalysis The surface structure and topography of the PCD and Si3N4 balls were studied using a CS3400 scanning electron microscope (SEM). Oxford EDX-450 energy dispersive X-ray (EDX) analysis was used to investigate the chemical compositions. The phase composition of the PCD was measured by D/max-2550X X-ray diffraction (XRD) (Cu Kα: 40 kV, 200 mA), with the angle of incidence of the X-rays set to 2°, a scanning speed of 4°/min, and scanning angle from 20° to 90°. Renishaw 2000 Raman spectrometer with 514.5 nm wavelength of Ar laser was performed to study the C–C bonding structures of the PCD surfaces before and after the tribotest.

Table 1 Physical properties of PCD and Si3N4.

PCD

300 µm

Fig. 2. Cross-sectional image of PDC.

Materials

Density (g/cm3)

Young's modulus (GPa)

Thermal conductivity (W/(m·K))

Hardness (GPa)

Poisson's ratio

PCD Si3N4

3.3–3.7 3.4

810 260–320

700 1.67–2.09

50–60 30–32

0.070 0.250

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Fig. 4. Variation of friction coefficient of the PCD under ambient air and vacuum.

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are found to be similar to those for the friction of the natural diamond and the CVD diamond studied by Grillo and Field [16]. In addition, it can be seen that the friction coefficient curves obtained from sliding in vacuum fluctuate significantly, whereas those curves in air are relatively smooth. Fig. 5 shows the three-dimensional surface topographies of the wear surfaces of PCD. The spalling pits and conical uplifts are irregularly distributed in the wear track of PCD under vacuum. Moreover, the number of spalling pits and conical uplifts increases as the loads increase. It implied that diamond grains in PCD shed off during the sliding process under vacuum. But three-dimensional surface topography of the PCD hardly changes under ambient air. Fig. 6 shows the comparison of the wear scars of Si3N4 balls after sliding tests under ambient air and vacuum. The wear scars of Si3N4 balls obtained from ambient air tests (Fig. 6(a), (b) and (c)) are very small, while those obtained from vacuum tests (Fig. 6(d), (e) and (f)) are much larger and the size of wear scar increases as higher applied load. A possible reason is that the Si3N4 is fractured under the function of high compression and shearing stress caused by adhesion during vacuum tests. 3.2. SEM and EDX analysis

3. Results 3.1. Tribological behaviors Fig. 4 shows the variation of friction coefficient of the PCD in ambient air and vacuum conditions. The average friction coefficients under vacuum are 0.88, 0.98 and 1.12, under loads of 15 N, 30 N and 45 N, respectively. In contrast, the friction coefficients are 0.07, 0.08 and 0.11 obtained in ambient air under applied loads. It showed that the friction coefficients under vacuum are obviously higher than those in air. The high friction coefficient of diamond materials sliding in vacuum is mainly attributed to the serious adhesion [9–14]. The friction coefficients appeared to rise as the load increased. The trends

The typical morphologies for the wear track of PCD are illustrated in Fig. 7. It showed that there is no obvious distinction after sliding test under ambient air (Fig. 7(a), (b) and (c)), whereas after test under vacuum (Fig. 7(d), (e) and (f)), obvious wear tracks are observed on the PCD surface. Inspection of the wear tracks under vacuum revealed that the white speckles are deposited on the wear track. The enlarged view of worn surface and the corresponded EDX analysis results under vacuum are shown in Fig. 8. It is identified that silicon which corresponded to the white areas in the SEM images is distributed on the surface of PCD, and more silicon is detected at a higher load condition. The results confirm that the Si3N4 coming from the counter ball is significantly adhered to the PCD surface under vacuum.

(a) Air, 15 N

(b) Air, 30 N

(c) Air, 45 N

(d) Vacuum, 15 N

(e) Vacuum, 30 N

(f) Vacuum, 45 N

Fig. 5. Three-dimensional surface topography of the wear track produced on the PCD under ambient air and vacuum.

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(a) Air 15 N

(b) Air 30 N

(c) Air 45 N

(d) Vacuum 15 N

(e) Vacuum 30 N

(f) Vacuum 45 N

Fig. 6. Wear scars of the Si3N4 balls after sliding operation under ambient air and under vacuum at load of 15 N, 30 N, and 45 N.

As shown in Fig. 8, spalling pits are detected on the wear tracks under vacuum, and the increased applied load caused the more spalling pit on the worn surface of the PCD. Fig. 9(b) and (c) shows the enlarged view of worn surface morphology of the PCD under vacuum with a load of 45 N. Layered fracture morphologies are observed clearly in the spalling pit. The chemical composition of inside and outside the spalling pit was examined by EDX as shown in Fig. 9(d). The results reveal that Co content in the spalling pit is significantly higher than that outside

the spalling pit. It indicates that the diamond grains shed off in the region with high content of Co under vacuum. 3.3. X-ray diffraction analysis The phase structure of the worn surface of PCD was assessed using XRD, and the patterns are shown in Fig. 10. Under ambient air, only diamond and Co are detected and no obvious difference is detected

(a) Air, 15 N

(b) Air, 30 N

(c) Air, 45 N

(d) Vacuum, 15 N

(e) Vacuum, 30 N

(f) Vacuum, 45 N

Fig. 7. SEM images of the wear surface of the PCD under ambient air and vacuum.

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(a) Vacuum, 15 N

Si-K

(b) Vacuum, 30 N

Si-K

(c) Vacuum, 45 N

Si-K

Fig. 8. EDX images (right) corresponding to the enlarged SEM images (left) of the worn surfaces after sliding operation under vacuum at the load of (a) 15 N (b) 30 N (c) 45 N.

among wear surfaces of PCD under different applied loads. From Fig. 10(b), it indicates that the wear surface under vacuum contains a small amount of graphite and Si3N4 besides diamond and Co. Apparently, the detection of Si3N4 and Co is consistent with above EDX results shown in Figs. 8 and 9. The observation of graphite implies that a transition from diamond to graphite occurred in sliding test under vacuum.

diamond. Under vacuum condition, the Raman spectra obtained from the outside of the spalling pit are also only exhibiting the peak of sp3-bonded diamond. However, inside the spalling pit, the Raman spectra present an obvious graphite peak (1582 cm− 1) and the disordered graphite peak (1390 cm−1). These phenomena and previous XRD results confirm that a transformation of sp3-bonded to sp2-bonded occurred on the wear surface of the PCD under vacuum.

3.4. Raman spectra analysis 4. Discussion Raman spectroscopy can be used to distinguish different forms of carbon [17,18]. Fig. 11 shows the Raman spectra for the PCD surface after sliding test under different conditions. The characteristic peaks identified in the Raman spectra include the following: (i) a peak approximately 1332 cm− 1 showing the existence of sp3-bonded diamond; (ii) a peak located at about 1390 cm−1 corresponds to non-diamond phases, including disordered graphite and sp2-hybridized carbon phases, and (iii) a peak around 1582 cm− 1 representing a graphite structure with sp2 bond character [19]. The Raman spectra obtained from ambient air condition show the only one peak of sp3-bonded

The friction coefficient under vacuum is about ten times as high as that under ambient air. In addition, both the PCD and Si3N4 ball seriously wear under vacuum, whereas there is no obvious wear under ambient air. A model is proposed to illustrate the wear mechanism as shown in Fig. 12. Under ambient air, water molecules and nitrogen molecules adhere to the frictional surface thereby forming an absorbed film which prevents direct contact between clean surfaces of the mating materials [20,21] as shown in Fig. 12(a). The XRD and Raman results show that there is no structural change in the PCD surface which demonstrates

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(a)

(b)

C

B

A

(c)

B

(d)

A

Fig. 9. (a) (b) and (c) Morphology of the worn surface of the PCD after sliding operation under vacuum with a load of 45 N and (d) surface chemical composition.

an effective prevention between the mating materials under ambient air. Such an adsorbed layer may be the main reason why the friction is lower and there is no obvious wear under ambient air. In addition, the absorbed film would preferentially form under lower loads, and its thickness would decrease with increasing contact pressure [16]. It means that the higher load induces a higher friction coefficient under ambient air. Under vacuum, two clean surfaces of the friction pairs were contacted directly due to the breakdown of surface absorbed films as shown in Fig. 12(b). As a result, Si3N4 is significantly adhered to the PCD surface under the high compression and shearing which

causes friction coefficients go up rapidly. The EDX results also present adhesion that occurs under vacuum and more Si3N4 adheres to the PCD surface with a higher load, hence Si 3N4 balls are seriously worn out. The distinct difference between PCD and other diamond materials is the wear behaviors. The spalling pits on the PCD surface could be detected under vacuum because of diamond grains shedding off. Moreover, the XRD results showed graphite forms on the worn surface under vacuum. Furthermore, Raman results confirmed that the transition from sp3 to sp2 occurred where diamond grains shed off.

Fig. 10. XRD patterns of (a) the worn surface of the PCD and (b) an enlargement of the 20–45° 2-theta region.

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Generally diamond transforms to graphite at appreciable rates only at high temperatures (1700–2000 K) [22]. However, graphitization occurs at relatively low temperatures when diamond is in contact with transition metals with intermediate reactivity of Fe, Ni, Co, and Cr, which can catalyze the conversion of diamond to graphite [23]. The EDX results present that the Co content is higher inside the spalling pit than that of outside. In addition, the degree of graphitization seems to rely on the friction coefficient. Liu and Meletis [24] claimed that both friction-induced annealing and shear-induced strain energy are contributing to the graphitization process. The flash

temperature rise between both surface asperities can be estimated by the following equation [25]: ΔT ¼

μ Fnv   4a K PCD þ K Si3 N4

ð1Þ

where ΔT is the flash temperature rise at contacted asperities, μ is the friction coefficient, Fn is the applied normal load, v is the sliding velocity, a is the contact radius of real contact area, and KPCD and K Si3 N4 are the

Air 15 N

Air 30 N

Air 45 N

Vacuum 15 N

A

A

49

B B

Fig. 11. Raman spectra of PCD surface after sliding test under different conditions.

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Vacuum 30 N

A

B A

B

Vacuum 45 N

A

B A

B

Fig. 11 (continued).

thermal conductivities of the PCD film and Si3N4, respectively. Eq. (1) shows that friction force (μFn) affects the flash temperature rise. In addition, higher friction force will induce higher strain energy in the surface layer [24]. Relaxation of this strain energy can be achieved by remarkable atomic migration at the interfacial region, which induces the sp3-to-sp2 structural transformation at a temperature far below the annealing temperature, according to the molecular dynamics simulation [26,27]. Therefore, it is reasonable that high friction coefficient and the catalyzing of Co result in the graphitization of PCD when the tribotest was performed under vacuum. A model shown

in Fig. 13 illustrates the mechanism of diamond grains shedding off from the PCD surface during sliding test under vacuum. In the regions with high content of Co, graphitization occurred in the boundary zones of diamond grains under friction heat and the catalyzing of Co, and hence the original D–D bonds among the diamond grains were broken down. Under the function of tractive force by adhesion, the graphitized diamond grains were pulled out as a weak bonding strength [7]. The number of spalling pits on the surface of wear track became larger as the load increases, and a rougher surface would also result in a higher friction force.

Water molecule Nitrogen molecule Oxygen and other molecules

In ambient air

(a)

Si 3N 4 PCD layer

Sliding direction Si 3N 4 debris

Co-WC substrate

In vacuum

(b) Fig. 12. Wear mechanism of Si3N4 sliding on PCD with the environment change from ambient air (a) to vacuum (b).

Y. Zhao et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 43–52

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Graphitization regions

Friction heat

Tractive force by adhesion PCD layer

Diamond grains shedding off

Spalling pits

Friction heat

Diamond grains

Binder phase Co

Fig. 13. The mechanism of diamond grains shedding off from the PCD surface during sliding test under vacuum.

5. Conclusions In this work, the friction and wear behaviors of PCD sliding against Si 3N 4 ball were investigated. The following conclusions were obtained: (1) The friction coefficients of the PCD under vacuum are much higher than that under ambient air. The average friction coefficients under vacuum are 0.88, 0.98 and 1.12, with loads of 15 N, 30 N and 45 N, respectively. In contrast, the friction coefficients under ambient air with applied loads are 0.07, 0.08 and 0.11, respectively. (2) The adhesion of Si3N4 and spalling pits is found on the surface of PCD under vacuum, while there is no observable abrasion on the PCD surface under ambient air. (3) The high friction coefficients of PCD under vacuum are attributed to the adhesion caused by desorption of the absorbed film formed on the frictional surface. (4) The reason of diamond grains shedding off under vacuum is that the diamond grains with weak bonding strength caused by graphitization were pulled out by adhesion tractive force.

Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51375466) and the International Science and Technology Cooperation Project of China (2011DFR50060) 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] R.H. Wentorf, Sintered superhard materials, Science 208 (1980) 872–880. [2] M.V. Sneddon, D.R. Hall, Polycrystalline diamond: manufacture, wear mechanisms and implications for bit design, J. Pet. Technol. 40 (1988) 1593–1601. [3] A. Lammer, Mechanical properties of polycrystalline diamonds, Mater. Sci. Technol. 4 (1988) 948–956.

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