Enhanced tribological behaviors of sintered polycrystalline diamond by annealing treatment under humid condition

International Journal of Refractory Metals & Hard Materials 80 (2019) 85–96

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Enhanced tribological behaviors of sintered polycrystalline diamond by annealing treatment under humid condition ⁎

Xiaohua Shaa,c, Wen Yuea,b, , Wenbo Qina, Chengbiao Wanga,b,

T

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a

School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China National International Joint Research Center of Deep Geodrilling Equipment, China University of Geosciences (Beijing), Beijing 100083, China c Ningxia Vocational Technical College of Industry and Commerce, Ningxia 750021, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Polycrystalline diamond Carbonaceous transfer film Tribochemistry Annealing treatment Tribological behaviors

The tribological behaviors of the 700 °C annealed sintered polycrystalline diamond (PCD) at various relative humidity (RH) levels were systematically investigated. The comparison of tribological behaviors between the 700 °C annealed PCD and the pristine PCD was made to further understand the tribological mechanisms. The results reveal that the friction inducing carbonaceous transfer film and oxidation and hydrolysis induced tribochemistry reaction dominant the tribological behaviors of the annealed PCD at various RH levels. The low coefficient of friction (COF) obtained in dry environments is attributed to carbonaceous transfer film on the worn Si3N4 surface, which was formed by the layers shearing action of massive tiny diamond grains exfoliated from the annealed PCD surface. The graphitization, oxidation and stress relaxation of the PCD induced by the 700 °C annealing treatment make the tiny diamond grains more easily to exfoliate and be grinded on the Si3N4 interface. It facilitates the formation of friction reducing carbonaceous transfer film, and finally results in the 30% lower COFs than those of pristine PCD at low RH levels (5%–50% RH). Meanwhile, an enhanced wear resistance of PCD can be achieved after 700 °C annealing treatment. The tribochemistry reaction induced by the oxidation and hydrolysis of Si3N4 governs the tribological behaviors of the annealed PCD at high RH levels (60%–99.9% RH). It reveals higher COFs accompanied with serious wear of Si3N4 ball and nearly no wear loss of annealed PCD. The produced SiO2 and silicic acid embeds into massive spalling pits on the annealed PCD surface, resulting in slighter wear of the PCD and Si3N4 than that of the pristine PCD/Si3N4. These results propose that the tribological behaviors of PCD under humid environment can be significantly improved by the 700 °C annealing treatment.

1. Introduction The polycrystalline diamond (PCD) is widely used in geological or petroleum drilling systems, such as thrust bearings and drill bits, due to its attractive mechanical and tribological properties (e.g. high hardness, good toughness, great thermal conductivity and distinguished wear resistance, etc.) [1–3]. Among these applications, the PCD is always required to work in different environments such as humid, thermal, acid-based and vacuum conditions, which may result in dissimilar tribological behaviors [4,5]. Researchers have proposed that the tribological behaviors of diamond related materials are tremendously sensitive to working environments. Konicek et al. [6] found that the self-mated ultrananocrystalline diamond (UNCD) reveled various tribological behaviors in different environments such as humid, inert atmospheres and

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vacuum. The diamond-like carbon (DLC) films is capable to give a low friction coefficient (< 0.01) in vacuum and inert environments, such as the highly hydrogenated DLC and boron-doped DLC, but much higher value when the oxygen or moisture is introduced into the test chambers [7,8]. Such a distinct performance is due to dissimilar tribological mechanisms in various testing environments, among which the relative humidity (RH) plays a crucial role. In highly humid air, condensed water molecules can give rise to capillary forces, which can increase friction [9]. Meanwhile, the test materials would experience a surface chemical-state change from hydrocarbons to oxygen-containing groups with the increase of the RH level [10,11]. Besides, the carbonaceous transfer film formed on the counterface is reported as a positive role for the tribological behaviors of diamond related materials [12]. Chen et al. [13] suggested that a near-frictionless lubrication state (the friction coefficient quickly evolving to a steady-state value of 0.001 at normal

Corresponding authors at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China. E-mail addresses: [email protected] (W. Yue), [email protected] (C. Wang).

https://doi.org/10.1016/j.ijrmhm.2019.01.007 Received 21 November 2018; Received in revised form 11 December 2018; Accepted 5 January 2019 Available online 07 January 2019 0263-4368/ © 2019 Published by Elsevier Ltd.

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load of 10 N) of hydrogenated amorphous carbon film was due to the formation of interfacial nanostructures, mainly a tribolayer, by the carbon rehybridization of sp3-to-sp2 transformation. The hydrocarbon fragments from a-C:H surface made bonds with the oxidized steel surface to form a highly-crystallized C-Fe-O bonding sublayer, then followed by the growth of the other sublayers with high fraction of sp2-C phase. The non-hydrogenated DLC shows low and stable coefficient of friction at high RH level due to the passivation of carbon dangling bonds in transfer layer [14]. Qin et al. [15] reported a low friction coefficient (~0.04) of PCD at dry environments, which was explained to coincide with the formation of carbonaceous transfer films in the run-in periods. In brief, different mechanisms such as dangling bonds passivation, tribochemical reaction and carbonaceous tribolayer dominant tribological behaviors of diamond related materials in various RH environments. The high temperature condition is widely existed in the sintering and drilling process of diamond related materials, which makes the materials suffer serious thermal damages, thus influences their tribological behaviors. Konca et al. [16] suggested that the DLC would present poor wear resistance at 400 °C for the serious oxidation. Grill et al. [17] and Knight et al. [18] reported that the hydrogenated DLC film would lose its diamond-like properties at high temperatures above 400–550 °C for severe oxidation and structural degradation induced lower toughness. Pu et al. [19] observed that the grain boundaries of nanocrystalline diamond (NCD) film were preferentially etched away by oxygen at high-temperatures, thus leaded to the decreased relative intensity of sp3 bonds. Li et al. [20] found that the polycrystalline diamond compact (PDC) annealed above 800 °C would be severely damaged with mixed mechanisms such as graphitization, oxidation and stress-induced micro-cracks. Therefore, the thermal damage might reduce the physical and chemical properties of diamond related materials on some degree. Simultaneously, researchers have proposed that heating treatment would facilitate the tribological behaviors of diamond related materials at the temperature of 650–750 °C. Yu et al. [21] indicated that surface graphitization of diamond was favorable in reducing the friction coefficient of chemical vapor deposition (CVD) diamond. Deng et al. [22] reported that the friction coefficients of PDC decreased with the increase of temperatures and reached the lowest value at 700 °C due to its surface graphitization. Liu et al. [23] found that the vacuum wear mechanism of PDC would transfer from adhesion to abrasive wear when annealed from 25 to 700 °C. Li et al. [24] proposed that the carbonaceous transfer films induced by tiny diamond grains could effectively reduce the friction coefficients of the annealed PCD. However, the tribological behaviors of high temperature annealed PDC at different RH levels and its corresponding mechanism has not been further studied. In this work, a systematic investigation on tribological behaviors of the 700 °C annealed PCD sliding against Si3N4 balls at different RH levels was carried out. Special efforts had been devoted to detect the tribological surface characteristics of the worn annealed PCD and Si3N4 surface, thus understand the tribological mechanisms of the annealed PCD at various RH levels.

Table 1 Physical properties of PCD and Si3N4. Materials

Hardness (GPa)

Young's modulus (GPa)

Density (g/cm3)

Thermal conductivity (W/(m·K))

Poisson's ratio

PCD Si3N4

40–50 14–16

810 300

3.3–3.7 3.4

700.0 16.2–29.5

0.07 0.25

work, the PCD layer contains big size diamond grains (BD grains, sizes of ~15–25 μm) and tiny diamond grains (sizes of 0–5 μm) with the halfcontent diameter of 25 μm (D50 = 25 μm) [24,25]. The Co binder phase distributes along the boundary of diamond grains. Furthermore, the β-Si3N4 sphere, similar to hard silicified rocks, with a diameter of Φ 6 mm was set as a mating specimen in tribotests. The physical properties of PCD and Si3N4 ball are listed in Table 1 [26,27]. 2.2. Annealing tests The PDC specimens were annealed by a SX-8-10 muffle furnace in ambient air, and the temperature was detected by a thermocouple with a deviation of ± 20 °C. Prior to the annealing treatment, the PDC specimens were ultrasonically rinsed with acetone for 30 min and then alcohol for 15 min, finally dried by an air blower at room temperature. During the annealing treatment, the temperature raised by a rate of about 20 °C/min, and remained at 700 °C for 30 min. The annealing temperature was chosen based on our previous experiments [24]. As for the PCD specimens, the transition of diamond to amorphous carbon and graphite would occur when anneals at 700 °C. It induces the formation of friction reducing carbonaceous transfer film. Moreover, a markedly enhanced wear resistance of PCD could be achieved. Therefore, the 700 °C was selected to pursue a further understanding of annealed PCD's tribological mechanism in various RH levels. All the annealed specimens were cooled by the process of air-cooling after annealing treatment. 2.3. Tribotests The tribological behaviors of the PCD were investigated by a CSMTRN ball-on-disc rotated tribometer located inside a humidity controlled reactor chamber. The schematic diagrams of the humidity control system and reactor chamber are shown in Fig. 1. The RH levels were introduced by controlling the flow of dry and humid nitrogen through a breaker containing deionized water. The RH levels of reactor chamber can be accessibility controlled from 5% to 99.9% RH. It was measured with an AZ8706 hygrometer, which was accurate within 1%. In order to simulate the working conditions of the PCD tools effectively, containing dry and humid environments, the testing RH levels from 5% to 99.9% RH were achieved in the reactor chamber. The annealed PDC was used as the disk and the Si3N4 ball was used as the mating ball. During the tests, the mating ball was fixed while the PDC disc was rotated with a frequency of 400 r/min and a turning radius of 4 mm, corresponding to a liner sliding velocity of 107.47 mm/s. A normal load of 20 N was applied, yielding an initial Hertzian contact pressure of 1.89 GPa. The duration of the tests was 30 min, during which steady friction coefficients can be achieved. Before the friction test, the annealed PDC specimens and Si3N4 balls were rinsed with acetone and then dried by N2 gas blowing. The friction forces were measured by a dynameter in the tribometer system and the friction coefficients were calculated by Amonton's law.

2. Experimental details 2.1. Materials The commercial sintered PDC used in this work is sintered at Zhongnan Diamond Co., Ltd. by a high pressure (5–6 GPa) and high temperature (1300–1400 °C) technique. It is composed of a PCD layer and tungsten carbide substrate with cobalt binder (WC-16 wt% Co). The dimensions of the PDC disc are 45.0 mm in diameter and 2.9 mm in thickness. The thicknesses of the PCD layer and WC-Co substrate are 0.53 mm and 2.37 mm, respectively. The PCD is polished by mechanical grinding and precision mirror grinding to make the surface roughness average (Ra) reach a value of ~4 nm. As presented in our previous

2.4. Surface analysis The NanoMap-D 3D surface profilometer was used to measure wear tracks of the annealed PCD, while the Olympus BX51M optical 86

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conducted. Both water contact angle values of the right and left sides of the deionized water droplet were measured, and an average value was taken. Prior to the above analysis, both the worn Si3N4 balls and annealed PCD samples were ultrasonically rinsed with acetone and then with alcohol for 30 and 15 min, respectively, and then dried by flowing nitrogen in order to remove the absorbent accumulated after friction tests. 3. Results 3.1. General characterization of the annealed PCD The comparison of the microstructure and chemistry phase of the PCD specimens before and after 700 °C annealing treatment is exhibited in Fig. 2. It is identified that the pristine PCD layer contains BD grains and tiny diamond grains (Fig. 2 (a)). The Co binder phase corresponded to the bright regions distributes along the boundary of BD grains. Fig. 2 (b) reveals the water contact angle measuring image of PCD surface with the water contact angle of 65.7 ± 2.9°, indicating a hydrophilic surface [28,29]. The XRD pattern of the pristine PCD exhibits obvious diamond and Co peaks (Fig. 2 (d)). The Raman spectrum of the pristine PCD presents the only one peak approximately located at about 1332 cm−1, showing the existence of sp3-bonded diamond (Fig. 2 (h)) [30]. The microstructure and chemistry phase of PCD changes greatly after 700 °C annealing treatment. Massive spalling pits and tiny holes appear on the annealed PCD surface (Fig. 2 (e)), especially around the boundary of BD grains, which makes the surface Ra increased to a value of ~20 nm (Fig. 2 (g)). The Raman spectrum of the annealed PCD presents peaks approximately located at about 1332 cm−1 and 1560 cm−1, showing the existence of disordered diamond and graphite character, respectively (Fig. 2 (h)) [31,32]. It suggests that the graphitization of PCD occurs during the 700 °C annealing treatment. The XRD pattern in Fig. 2 (d) exhibits diamond, graphite and cobalt oxide (Co3O4, CoO) peaks. Moreover, the thermal stress was reported to appear in the interface of diamond grains and the Co binder during the 700 °C annealing process, making the diamond grains flaky [20]. As the PDC surfaces are heated, thermal stress will appear at the interface of diamond grains and Co binder due to the difference between the thermal expansion coefficients of the diamond (3.2 × 10−6/K) and Co (14.4 × 10−6/K) [33]. When the thermal stress reaches a critical value these diamond grains will exfoliate from the PDC surface, leads to formation of cracks. Finally, fine intergranulardiamond particles exfoliate and spalling pits appear [20,24]. The water contact angle measuring image of the annealed PCD surface shown in Fig. 2 (f) exhibits an increased water contact angle of 74.6 ± 3°, which suggests that the hydrophilicity of the PCD surface deduces after 700 °C annealed treatment. It may be attributed to the rough surface and the increase of sp2bond character [34]. In general, the graphitization, oxidation and stress relaxation occurred during the annealing treatment reduce the adhesive strength of diamond grains, and inducing spalling pits and tiny holes on the annealed PCD surface. During the 700 °C annealing treatment in air, the oxygen and oxygen containing species play a great role in the composition and mircrostructure changes of the PCD. It is the oxygen and oxygen containing species that would alter the surface and subsurface chemistry with O species. The PCD would experience a surface chemical-state change from hydrocarbons to oxygen-containing groups [10,11] and the grain boundaries would be preferentially etched away by oxygen at high-temperatures, thus leaded to the decreased relative intensity of sp3 bonds [19], reduced adhesive strength of diamond grains, graphitization, oxidation, stress relaxation.

Fig. 1. The schematic of (a) the relative humidity control system of CSM-TRN tribometer and (b) parameters of frictional tests at different RH levels.

microscopy was performed to measure the wear scars of Si3N4 balls. In this work, the wear rate was used to evaluate the wear performance of specimens. The wear rate of annealed PCD discs was calculated by the Archard wear equation: (1)

V = k·F ·s 3

where V (mm ) is the wear volume of specimens, obtained from the cross-sectional area of the wear track measured by the NanoMap-D 3D surface profilometer, F (N) is the normal load, s (mm) is the sliding distance, and k is the wear rate per unit load and per unit distance. The wear rate of Si3N4 balls was also calculated by the Archard wear equation. The wear volume of the mating ball Vball was calculated by the radius of the wear scar measured by the optical microscope, and using the equation:

Vball = πh [3r 2 + h2]/6

(2)

where h = R−(R2−r2)1/2, R is the radius of the Si3N4 ball (3 mm), r (mm) is the radius of the wear scar, and h (mm) is the height of the sphere of the worn Si3N4 ball. The CS3400 scanning electron microscope (SEM) was used to observe the surface structures and topographies of PCD discs and mating balls. The chemical compositions of the pristine and worn annealed PCD surfaces were investigated by the Oxford EDX-450 energy dispersive X-ray spectrum (EDS) and D/max-2550× X-ray diffraction (XRD) (Cu Kα: 40 kV, 200 mA). The compositions and structures of the materials on the worn surfaces of PCD discs were detected by the LabRAM HR Evolution Raman spectrometer with 514.5 nm wavelength of Ar + laser. The chemical analysis of worn Si3N4 balls and annealed PCD discs was performed by PHI Quantera X-ray photoelectron spectroscopy (XPS). As for the experiment, Al Ka (hv = 1486.6 eV) was chosen as the X-ray excitation source, and the beam spot was 100 μm in size. The worn surfaces were sputtered for about 5 nm in depth by Ar ion. The fine spectral flux was 55 eV and the step was 0.1 eV, and the binding energy of 284.8 eV for C1s was set as a reference for charge correction. The OCA-20 contact angle system was performed to measure the contact angle on the PCD surface before and after annealing treatment. The sessile drop method with deionized water was

3.2. Friction and wear behaviors The typical trends of the coefficient of friction (COF) of the annealed PCD tested at different RH levels are shown in Fig. 3 (a). It reveals an obvious run-in period and steady-state. The COF fluctuates strongly in 87

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Fig. 2. The SEM images (a and e) and three-dimensional topographies (c and g) of the pristine and 700 °C annealed PCD surface, respectively, (b and f) the typical water contact angle measuring images of the pristine and 700 °C annealed PCD surface, respectively, (d) the XRD patterns detected on the pristine and 700 °C annealed PCD surface, (h) the Raman spectra of the pristine and 700 °C annealed PCD surface.

deviation of the wear rate values [24]. Almost no wear loss can be observed at high RH levels from 60% to 99.9% RH. Overall, the annealed PCD wears seriously in relatively dry air and mildly in humid environments. The comparison between wear rates of the 700 °C annealed and pristine PCD was performed to evaluate the wear resistance changes. It is identified that the wear rate of the 700 °Cannealed PCD reveals the same turning trend as that of the pristine one. Meanwhile, the wear rates of the 700 °Cannealed PCD at low RH levels are about an order of magnitude lower than those of the pristine one. No much difference was observed at high RH levels. In brief, a great enhancement for the wear resistance of PCD was obtained after the 700 °C annealing treatment. The wear rates and optical topographies (see Fig. S2) of the wear scars formed on Si3N4 mating balls are demonstrated in Fig. 5. It reveals that the diameters of the wear scars decrease with the RH level firstly and increase then, resulting in the minimum wear rate of 0.75 × 10−10 mm/Nmm at 30% RH. Meanwhile, inordinately multicolor transferred substance can be detected on the worn surface operated at low RH levels (5%–50% RH), revealing decreased covering areas (see Fig. S2). The ones formed at 5% and 10% RH exhibit especially conspicuous continuity. Moreover, little black substance distributes along the direction of sliding ploughs on the wear scars acquired from 60% to 99.9% RH. The worn Si3N4 surface turns much rougher at higher RH levels, leading to bigger deviation of wear rate values. It is obvious that two different mechanisms dominant the wear behaviors of Si3N4 at low and high RH levels. Besides, the transfer substance observed on the Si3N4 interface may be related to the COF

the run-in period and stabilizes after ~5 min. The advent of the run-in period may be correlated to the initial interactions between the asperities of two mating surfaces, which leads to plastic deformation and the creation of third-bodies [35,36]. As shown in Fig. 3 (b), the mean steady-state COFs were compared with those of the pristine PCD, which was reported in our previous work [15,37]. The mean steady-state COF of the 700 °C annealed PCD exhibits a sustainable rising trend versus RH levels, which increases from 0.030 at 5% RH to 0.107 at 99.9% RH. It is interesting to notice that the COFs of the 700 °C annealed PCD differ greatly from those of the pristine specimen. The COFs at low RH levels (5%–50% RH) reduce by ~30% to those of the pristine PCD with the same increased trend. Instead, the COFs at high RH levels (60%–99.9% RH) are slightly higher than those of the pristine PCD with the opposite turning trend. 85%. The pristine PCD reveals decreased COFs at the humidity level ranges of 50% RH to 80% RH for the water molecule lubricating film formed in the sliding interfaces [37]. In brief, the 700 °C annealing treatment is of significant benefit to the friction behaviors of PCD in relatively dry environments, which may be attributed to the annealing-induced structure and chemical phase changes. In order to estimate wear behaviors of the annealed PCD discs and Si3N4 balls, the wear rates were calculated. The wear rates and corresponded three-dimensional surface topographies of wear tracks formed on the annealed PCD surfaces (see Fig. S1) are presented in Fig. 4. It reveals that the wear rate of annealed PCD discs decreases gradually as the RH level increases. The annealed PCD discs wear severely at low RH levels (5%–50% RH), revealing deep wear tracks. Furthermore, several spalling pits can be found on the wear tracks, leading to the large

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Fig. 4. Wear rates of the worn PCD discs and the corresponded three-dimensional surface topographies (the inset). Wear rates: the black scatters reveal the wear rates of the 700 °C annealed PCD, and red scatters show those of the pristine PCD [15,37]. The 700 °C annealed PCD and pristine PCD were tested with same experiment parameters except for different initial Hertzian contact pressures (1.89 and 2.39 GPa, respectively), but it reveals a similar wear trend.

Fig. 3. Friction coefficients of the annealed PCD/Si3N4 tested at different RH levels: (a) integrated friction coefficient curves in sliding process, (b) the mean friction coefficients during the steady stage (The black scatters reveal the mean friction coefficients of the 700 °C annealed PCD/Si3N4, and red scatters show those of the pristine PCD/Si3N4 [15,37]. The 700 °C annealed PCD and pristine PCD were tested with same experiment parameters except for different initial Hertzian contact pressures (1.89 and 2.39 GPa, respectively), but it reveals a similar friction trend.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 5. Wear rates of the worn Si3N4 balls and the corresponded optical images. Wear rates: the black scatters reveal the wear rates of the worn Si3N4 mating balls sliding against the 700 °C annealed PCD, and red scatters show those of the worn Si3N4 mating balls sliding against the pristine PCD [15,37]. The 700 °C annealed PCD/Si3N4 and pristine PCD were tested with same experiment parameters except for different initial Hertzian contact pressures (1.89 and 2.39 GPa, respectively), but it reveals a similar wear trend of Si3N4 balls. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

trend shown in Fig. 3. Comparing with the Si3N4 sliding against the pristine PCD, the wear rates present little lower values for the specimens sliding against the annealed PCD tested at high RH levels.

3.3. Wear surface microanalysis 3.3.1. SEM and EDS analysis The SEM and EDS analysis were carried ot study the microstructure and chemical composion of the worn 700 °C annealed PCD and Si3N4 to further explore the tribological mechanism. The wear scars of Si3N4 mating ball formed at 5% and 90% RH were chosen as the typical ones to get the topographic features of the low and high RH levels, respectively (Fig. 6). The SEM topography and corresponded EDS mapping images of the worn Si3N4 tested at 5% RH (see Fig. S3) reveal that the C and O elements distribute mainly on the wear scar, demonstrating that the multicolor substance presented in Fig. 5 is a carbonaceous transfer film. The enlarged view exhibits that the wear scar is completely covered with transfer film, which forms on the sliding interface layer upon layer (Fig. 6a). Massive spot-like covering substance are observed on the transfer film. The EDS spectra illustrate that the different layers of the transfer film are enriched with different content of C element,

which reveals bigger value in the upper layer (detecting position A). It confirms that the carbonaceous transfer film becomes thicker and thicker by the layers shearing action of the massive tiny diamond grains during the sliding operation. The SEM topography and corresponded EDS mapping images of the worn Si3N4 operated at 90% RH (see Fig. S4) reveal that the sliding interface is fully covered by the O element, accompanying with bits of C, Si and N elements. It is interesting to notice that bits of the C element can be detected on the worn Si3N4 surface, which is extremely different from the bare ones sliding against the pristine PCD reported in our previous work [37]. This is corresponding to the distributed black substance observed on the worn Si3N4 surface operated at high RH levels (Fig. 5), which might be due to the 89

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Fig. 6. (a1-a3) The SEM topographies and EDS results of detecting positions on the worn Si3N4 surface operated at 5% RH; (b1-b4) the SEM topographies and EDS results of detecting positions on the worn Si3N4 surface operated at 90% RH.

exfoliated diamond grains from the annealed PCD surface during sliding operation. The selected regions from I to III in Fig. 6 (b1) clearly show the spalling progress. The tiny diamond grains exfoliating from the annealed PCD embed in the smooth thin film on Si3N4 surface, and gradually wrapped by the film, forming the small bump shown in region I. Then the other exfoliated diamond grains plough the sliding interface and destroy the formed bump, producing broken hollow in region II. At last, the thin film around the boundary of broken hollow peels, and the irregular spalling area in region III appears. The destroy of the completely hydrated film on the sliding interface induces more Si3N4 exposed to be further oxidized and hydrolyzed, finally leads to the serious wear of mating ball in humid environments. Overall, the diamond grains exfoliated from the annealed PCD facilitate the wear of Si3N4 in high humid environments. The SEM topography of the annealed PCD wear track operated at 90% RH is exhibited in Fig. 7. It is identified that massive spalling pits around BD diamond grains are filled with bright substance after sliding

plough and scraping of the tiny diamond grains spalling from the 700 °C annealed PCD surface. The magnified views of the wear scar present that the worn Si3N4 surface is covered with a thin smooth film, and massive irregular spalling areas and furrows appear on it (Fig. 6b). The EDS spectra of the irregular spalling area demonstrate that both of the smooth thin film and spalling area contain C, N, O and Si elements. The content of O element detected on the smooth thin film is extremely much higher than the one of the spalling areas, while those of C, N and Si element are lower. It suggests that the thin film contains mainly silicon oxide and some substance transferred from the annealed PCD appears on the irregular spalling areas. Some chemical reaction may occur on the Si3N4 surface during sliding operation in high humid environments, leading to the smooth thin film. Some research work has reported that the oxidation and hydrolysis of the Si3N4 would occur in humid environments, accumulating silicic acid friction surface and forming a thin hydrated film [38]. The spalling areas on the thin film would be attributed to the destroying of the 90

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Fig. 7. (a) The SEM topography of the annealed PCD track operated at 90% RH, (b) the enlarge image of the annealed PCD track, (c-g) the corresponded EDS mapping images of the annealed PCD track shown in (b), (h) the chemical composition of detecting position on the annealed PCD shown in (b).

environments, especially at 5% RH and 10% RH, reveal strong intensities of G peak accompanied with the weak D peak. It implies that some carbon rehybridization from sp3 to sp2 bonding and microcrystalline graphitic sheets and disorder carbon formation are found on the wear scars of Si3N4 spheres, which leads to the low friction coefficient of the annealed PCD/Si3N4 tribopair in dry environments [40]. With the increase of RH level, the G peak becomes weaker while the defect D peak turns stronger, suggesting that the transfer film mainly contains disordered carbon at higher humidity [41,42]. Moreover, extremely weak peaks are observed in humid environments, demonstrating wear debris with disordered carbon accumulated during sliding operation. Several other peaks with Raman Shift from 400 to 1200 cm−1 are the chemical characteristics of the Si3N4 mating balls.

operation. The EDS mapping and spectrum analysis were performed to further investigate the composition of the bright substance, as shown in Fig. 7 (c-g). It reveals that the bright substance contains enriched O, Si and little Co elements. The elemental analysis of detecting position B shown in Fig. 7 (h) only presents the composition of C and Co elements, demonstrating the typical feature of the annealed PCD disc. Moreover, the content of Co element on the bright substance is higher (detecting position A). This is due to the Co binder phrase around the grain boundaries, which accelerates the spalling of thin diamond grains during sliding operation [24]. Furthermore, the high content of O and Si elements in detection position A illustrates that the bright substance is the silicon oxide. During sliding operation in high humid environments, the silicon oxide generates in the chemical reaction of Si3N4 ball, which occurs at the contact interface, and fills into the spalling pits on the annealed PCD surface. The filling of silicon oxide plays a positive role in the result of almost no wear of annealed PCD in high humid environments.

3.3.3. XPS analysis The XPS analysis was further performed to explore the changes of chemical states of typical elements on the worn Si3N4 and annealed PCD surface. The survey spectra of the worn Si3N4 (see Fig. S5 and S7) show C 1 s, N 1 s, O 1 s, O 2 s, Si 2 s, Si 2p and O KLL peaks of substrate silicon nitride and top layer signals. The intensity of C 1 s peak decreases with the increasing RH level, which corresponds to the inordinately covered carbonaceous transfer film observed on worn Si3N4 surfaces, and the other peaks reveal an opposite trend. The C 1 s spectra (see Fig. S6 (a)) at 5% RH present a main CeC (or CeH) peak, which is corresponding to the sharp G peak with high intensity in Raman results. Besides, the CeO, C]O and COOR peaks appear at 40% and 90% RH, suggesting that the transferred carbon substance were oxidized on some degree. The Si 2p spectra (see Fig. S6 (b)) reveal the SiO2 and Si(OH)4 at

3.3.2. Raman spectrum analysis Raman spectroscopy was performed to investigate the chemical characteristics of the multicolor transfer film formed on the worn Si3N4 surface. The multicolor and dark areas shown in Fig. 8 (b) and (c) were chosen as the detecting positions. The Raman spectra exhibited in Fig. 8 (a) present obvious D (~1360 cm−1) and G (~1580 cm−1) peaks with different intensity, corresponding to amorphous-C and sp2 C]C bonded carbon, respectively [39]. It demonstrates that both of the transfer film formed at low RH levels and the dark substance observed at high RH levels are carbon rich substance. The Raman spectra obtained in dry 91

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Fig. 8. (a) Raman spectra of the worn Si3N4 mating balls operated at different RH levels and the original Si3N4 (the inset), (b) and (c) the topographies of the worn Si3N4 operated at 5% RH and 90% RH levels.

molecules, the extremely high pull-off force or adhesion results in the high shearing stress in the sliding interface. The high pull-off force promotes the striping of Si(OH)4 and SiO2. In this case, the more serious oxidation and hydrolysis of the Si3N4 intersurface at high RH levels would lead to more striping of SiO2 and Si(OH)4. It means that the rate of the exposing of bare Si3N4 surface is much quicker than that of the SiO2 and Si(OH)4 generation at high RH levels, thus leads to the increased Si3N4/(SiO2 + Si(OH)4) ratio. The survey spectra of the worn annealed PCD (see Fig. S7) reveal C 1 s, O 1 s, O 2 s and O KLL peaks with different intensity, referring the substrate carbon and top oxide layer signals. It is identified that the intensity of C 1 s peak decreases with the increasing RH level, and the other peaks exhibit opposite trend. Small signals of N 1 s, Si 2 s and Si 2p peaks were observed on the wear tracks operated at 90% RH, which may be due to the filled silicon oxide in the spalling pits of the worn annealed PCD surface (Fig.7). As shown in Fig. S8 (a), the intensities of CeO and C]O peaks increase significantly with the rising RH level. The new C-OH peak appears at 40% and 90% RH. The ratio of (CO + C=O + C-OH)/C-C was calculated to illustrate the oxidized degree of the worn annealed PCD surface (Fig. 10). It presents the increased value from 0.14 at 5% RH to 1.58 at 90% RH, suggesting that the annealed PCD would oxidize seriously with the appearance of water molecules in high humid environments. The Si 2p spectrum (see Fig. S8 (b)) of the wear track tested at 90% RH exhibits the Si(OH)4, SiO2 and Si3N4 peaks, which is mainly attributed to the bright substance filled in massive spalling pits on the annealed PCD surface (Fig. 7). It strongly proves that the bright substance observed on the worn annealed PCD surface is Si(OH)4 and SiO2, the transferred product of oxidation and hydrolysis of Si3N4 mating ball during the friction operation in high humid environments.

Fig. 9. The content ratios of the worn Si3N4 surface operated at 5% RH, 40% RH and 90% RH levels. Si 2p: the red pattern refers to the ratios of peak integral area of Si3N4/(Si(OH)4 + SiO2), and the blue pattern refers to those of Si(OH)4/ SiO2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

different RH revels, proving some oxidation and hydrolysis of Si3N4. Some research work has reported that the oxidation and hydrolysis of Si3N4 would occur in humid environments, forming SiO2 and Si(OH)4 [37]. The ratio of Si(OH)4/SiO2 was calculated to evaluate the degree of the oxidation and hydrolysis of the worn Si3N4 surface after sliding operation. As shown in Fig. 9, it turns an enhanced trend from 0.66 to 1.53 with the increase of RH level, confirming that the higher concentration water molecules leads to a larger Si-OH/Si-O ratio. Thus severe oxidation and hydrolysis of Si3N4 occurs, accompanying with increased wear loss. During the sliding operation, the shearing stress acts on the sliding interfaces. The SiO2 and Si(OH)4 can easily strip from the Si3N4 ball, thus leads to more bare Si3N4 to further react with water molecules. Therefore, the debris gradually forms and accumulates on the both sides of wear track. With the high concentration of water

4. Discussion The test results above suggest that the tribological behaviors of the annealed PCD/Si3N4 strongly depend on the RH levels, and the 700 °C annealing treatment of PDC plays a positive role on some degree. As is shown in Fig. 11, the mean friction coefficient reveals increased trend with the increase of RH level while the wear rate of 700 °C annealed 92

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to prevent the annealed PCD from directly contacting with the Si3N4 ball, acting as a “lubricant” of a low-shear-strength at the sliding interface to result in a much smaller friction coefficient (0.03) [43,44]. It is reported that the build-up of a friction-induced transfer film on the counterpart, which follows by easy shear within the interfacial material, is the most frequently observed friction-controlling mechanism for DLC films [37,45]. Bhowmick et al. [46] found that the formation of graphene transfer layers at the counterface was a necessary condition for the low friction in DLC/52100 steel tribosystem. The SEM topographies of Si3N4 wear scars presented in Fig. 6 (a) vividly exhibit the formation mechanism of the carbonaceous transfer film in dry environments. The carbonaceous transfer film is formed layer by layer with the thickening of the spot-like shearing action of the massive tiny diamond grains. The formation mechanism of the carbonaceous transfer film is presented in Fig. 12. During the sliding operation under low humidity, due to the extremely low concentration water molecules, a large number of dangling bonds on the annealed PCD interface expose in dry environments. These un-saturate dangling bonds on the topmost surface of PCD are with high surface energies, which can form covalent interaction [47]. Diamond surfaces saturated by the dissociative adsorption of water molecule can decrease the adhesion interaction at the sliding interface supported by ab initio density functional theory (DFT) calculations and molecular dynamics simulations [48]. However, the dangling bonds of PCD and Si3N4 surface would not be effectively passivated under low humidity, which results in strong interaction by forming covalent between the sliding interface. Due to the weak bonding strength, the tiny diamond grains are inclined to be pulled out and gradually exfoliate [24]. Some of the exfoliated tiny diamond grains are prefer to bonding with the Si3N4 in dry environments. The subsequent shearing action accumulates layer by layer, forming and thickening the carbonaceous transfer film. Some confirmatory experiments further testify this wear mechanism (see Fig. S9). Comparing with the pristine specimen, the 700 °C annealing treatment of PCD results in much lower COFs and enhanced wear resistance in dry environments. In order to study the difference of COF between the pristine PCD and 700 °C annealed one, the transfer film covering fraction and mean steady-state friction coefficient among RH levels were performed in Fig. 13. It reveals that the lower mean COFs generally accompany with higher transfer film covering fraction. This suggests that the carbonaceous transfer film would reduce friction of PCD in dry environments. Besides, the lower COFs of the annealed PCD compared with the pristine one exhibit higher covering fraction of the carbonaceous transfer film. This implies that the 700 °C annealing treatment of PCD facilitates the formation of the friction reducing carbonaceous transfer film, thus results in lower COFs in dry environments. Moreover, a great enhanced wear resistance of the PCD was achieved after 700 °C annealing treatment. It is the amount of the tiny diamond grains that makes the difference. It is reported that some BD grains with weak bonds on the PCD surface are inclined to be pulled out in unstable running-in period, especially in dry environments, thus lead to the wear of the PCD [15]. The reasonable explanation is that the tiny diamond grains are more easily exfoliated from the contact interfaces, and then some of the grinded and fluctuated ones will cause the initiation of cracks around BD grains and finally lead to the exfoliation of BD grains [24]. It suggests that the exfoliation of BD grains mainly causes the wear loss of PCD. As for the pristine PCD, there are more tiny diamond grains exist in tribological interface, which would always lead to larger wear rate of the specimen. On the contrary, some difference takes place for the annealed PCD. The graphitization, oxidation and stress relaxation occurred on the PCD surfaces establish massive exfoliation of tiny diamond grains during 700 °C annealing treatment, which leads to much rougher surfaces with fewer tiny diamond grains left (Fig. 2 (e)). The BD grains are too hard to be worn off and pulled out in the absence of massive tiny diamond grains, thus results in the enhanced wear resistance of the PCD after 700 °C annealing treatment.

Fig. 10. The content ratio of the worn annealed PCD surface operated at 5% RH, 40% RH and 90% RH levels. C 1 s: the ratios of peak integral area of (CO + C=O + C-OH)/C-C.

Fig. 11. The variation tendency of mean friction coefficients and wear rates of the annealed PCD versus different RH levels. Blue scatter points show the variation of the mean friction coefficient of the annealed PCD. Red scatter points present that of the wear rates of the annealed PCD. Error bars are standard errors and represent variation within a set of measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

PCD presents opposite one. Meanwhile, it is identified that the 700 °C annealing treatment facilitates the formation of carbonaceous transfer film, resulting in the low COF and enhanced wear resistance of PCD in dry environments. The higher COF accompanied with serious wear of Si3N4 ball and nearly no wear loss of annealed PCD in humid environments are contributed to the oxidization and hydrolysis of Si3N4 mating ball. In brief, different mechanisms dominate the tribological behaviors of annealed PCD/Si3N4 at various RH levels.

4.1. 700 °C annealing treatment facilitates the formation of friction reducing carbonaceous transfer film accompanied with enhanced wear resistance of PCD in dry environments The 700 °C annealed PCD/Si3N4 reveals low COFs in relatively dry environments, especially the value of 0.03 obtained at 5% RH. That's testified to be attributed to the carbonaceous transfer film forming on the wear scars of Si3N4 mating balls. The EDS, Raman and XPS results demonstrate that the multicolor transfer film shown in Fig. 5 mainly contains disordered carbon. Graphite was also detected in the wear scar performed at 5% and 10% RH. This transferred carbon-rich layer helps 93

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Fig. 12. The tribological mechanism of annealed PCD/Si3N4 operated at low RH levels.

accompanied with some irregular spalling areas. The Si3N4 reacts with massive water molecules, producing SiO2 and silicic acid. It implies that the tribochemical reaction governs the friction process of annealed PCD/Si3N4 in high humid environments. The tribological mechanism of annealed PCD/Si3N4 operated at high RH levels is presented in Fig. 14. Massive water molecules absorb on the sliding interfaces of the annealed PCD surface and Si3N4 mating balls, accumulating a condensed water layer. The flash temperature is inevitable at the asperities in contact within the real contact area and is much higher than the average temperature over the nominal contact [49,50]. It would induce the rate of graphitization of PCD and lead to more spalling diamond grains to form the friction reducing carbonaceous transfer film, thus severe wear and low friction of the specimen [51]. As for the humid testing condition, the absorbed water layer may prevent the generation of high flash temperatures, which reduce the formation of the friction reducing carbonaceous transfer film. Instead, the appearance of the massive water molecules leads to great oxidization and hydrolysis of the Si3N4 mating ball on the contact interface, forming a thin hydrolyzed silicic acid film shown in Fig. 14 (b). During further sliding operation, some more tiny diamond grains exfoliated from the annealed PCD [45]. Some of the exfoliated diamond grains embed into the formed silicic acid film and finally result in the irregular spalling areas, undergoing the process shown in Fig. 6 (a) from region I to III. It means that the exfoliated tiny diamond grains broke the formed hydrolyzed film and make more Si3N4 exposed to be oxidized and hydrolyzed by water molecules during subsequent sliding operation. Our previous work has reported that the PCD after 700 °C annealing treatment reveals thermal damage such as graphitization, oxidation and stress-induced micro-cracks, which leads to more easier spalling of tiny diamond grains around the boundary of big size diamond grains [24]. It means that some amount of tiny diamond grains would exfoliate continuously. So the formation and destroy of hydrolyzed film exist together. It is the dynamic process that results in the thin smooth film enriched with amount irregular spalling areas. At the same time, the

Fig. 13. Comparison between transfer films fraction and the mean fiction coefficient in steady-state as a function of RH levels. Black lines show the variation of transfer film fraction and mean friction coefficient of the 700 °C annealed PCD. Red lines present those of the pristine PCD [15,37]. Error bars are standard errors and represent variation within a set of measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.2. The oxidation and hydrolysis of Si3N4 prevent the 700 °C annealed PCD from wearing severely in high humid environments The 700 °C annealed PCD/Si3N4 presents relatively high COFs in high humid environments. Meanwhile, it is noticeable that the annealed PCD almost reveals no wear loss after sliding operation, which means that PCD tools would be more life-extending under high humidity. The SEM, EDS and XPS results strongly confirm that the oxidation and hydrolysis of Si3N4 mating balls occur during sliding operation with the appearance of massive water molecules, forming a thin hydrolyzed film 94

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Fig. 14. The tribological mechanism of annealed PCD/Si3N4 operated at high RH levels.

SiO2 and silicic acid from the tribochemical reaction gradually fill in the spalling pits on the annealed PCD surface, even the latest ones appear during the sliding operation, and smooth the rough surface after annealing treatment. Meanwhile, due to the strong dangling bonds passivation by forming H-, OH-, and H2O-terminated surface under high RH condition, less surface unsaturated atoms can form covalent interaction across the sliding interface. Consequently the wear loss of the annealed PCD decreases with the increase of RH levels. It means that the oxidization and hydrolysis of the Si3N4 mating ball protect the annealed PCD at the expense of wearing it. Besides, condensed water molecules can give rise to capillary forces that increase friction in high humid environments [9]. Moreover, the higher (C-O + C=O + C-OH)/ C-C ratio obtained from the XPS results at high RH levels demonstrates that more oxidization of the annealed PCD occurs during sliding operation, causing an increase of CeO, C]O and C-OH bonds. Scott [52] has reported that the oxidation of hydrocarbon polymers during the friction process would be divided into three steps, which is similar to that of the annealed PCD. Firstly, the friction force broken the CeH bonds on the annealed PCD surface mechanically, and the water molecules (and O2 molecules in air) absorbed to form the C-OH or CeO groups. Secondly, the absorbed peroxides were transformed to COOH or remained as active radicals such as COO-. Thirdly, the CeH bonds in the neighbors donated electrons to form C]O bonds during further shearing. Overall, a change from CeH bonds to oxygen-containing groups takes place on the sliding interface of the annealed PCD during the sliding operation at comparatively high RH levels. In this case, the bonding strength increased from about 0.08 eV per bond (Van der Waals bonding of CeH) to about 0.21 eV per bond (hydrogen bonding at CeO and C]O), which leads to a continuous increase in the friction coefficient with the increasing RH levels [9,10]. Overall, the larger COFs at high RH levels are attributed to the increased capillary forces and bonding strength of the oxygen-containing groups. Some confirmatory experiments further verify this view (see Fig. S10). Comparing with the pristine specimen, lower COFs of the 700 °C

annealed PCD and slighter wear loss of Si3N4 balls were observed at high RH levels. The deduced hydrophilicity of the PCD surface after 700 °C annealed treatment might decrease capillary forces at contact interfaces, thus results in lower COFs at high RH levels. Besides, the easily exfoliated tiny diamond grains induce slight abrasive wear at high humid environments on some degree, which leads to mildly lower COF than the pristine PCD/Si3N4. Meanwhile, the filling action of silicon oxide in the massive spalling pits on the annealed PCD surface smooths the rough surface formed in annealing treatment during later sliding operation. This alleviates the material cutting and ploughing from the virgin rough annealed PCD, thus results in slighter wear of Si3N4 balls than sliding against the pristine specimens at high RH levels. The wear process of Si3N4 balls proves that the tribochemical reaction dominates the wear behavior in high humid environments. Meanwhile, the Si3N4 would wear continuously and tremendously when slide against the annealed PCD. It means that the PCD tools would work efficiently when it cuts ceramic materials in high humid environments. 5. Conclusions Based on this research, the following conclusions can be drawn: (1) The tribological behaviors of PCD can be significantly improved by the 700 °C annealing treatment. The friction reducing carbonaceous transfer film and oxidation and hydrolysis induced tribochemistry reaction dominant the tribological behaviors of the annealed PCD at low (5%–50% RH) and high RH environments (60%–99.9% RH), respectively. (2) The annealed PCD/Si3N4 exhibits low COFs and severe wear of annealed PCD in dry environments. Meanwhile, higher COFs accompanied with serious wear of Si3N4 ball and nearly no wear loss of annealed PCD are contributed to the tribochemistry reaction induced by the oxidization and hydrolysis of mating ball occur in high humid environments. 95

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(3) 700 °C annealing treatment facilitates the formation of friction reducing carbonaceous transfer film, leading to 30% lower COFs of the tribopair than those of the pristine one. The shearing action of exfoliated tiny diamond grains accumulates layer by layer, forming and thickening the transfer film. Meanwhile, an enhanced wear resistance of the PCD was achieved after 700 °C annealing treatment. (4) The produced SiO2 and silicic acid embeds into massive spalling pits on the annealed PCD surface in high RH environments, resulting in slighter wear of the disc and mating balls than those of the pristine PCD/Si3N4.

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Acknowledgments [26]

The authors would like to thank the National Natural Science Foundation of China (51875537, 41572359, 51375466), Beijing Natural Science Foundation (3172026), Beijing Nova Program (Z171100001117059), and the Fundamental Research Funds for the Central Universities (53200859604).

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ijrmhm.2019.01.007. [32]

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