Architecture of superthick diamond-like carbon films with excellent high temperature wear resistance

Tribology International 81 (2015) 129–138

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Architecture of superthick diamond-like carbon films with excellent high temperature wear resistance Junjun Wang a,b, Jibin Pu a, Guangan Zhang a, Liping Wang a,n a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’ Republic of China University of Chinese Academy of Sciences, Beijing 100039, People’ Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 24 June 2014 Received in revised form 22 August 2014 Accepted 24 August 2014 Available online 2 September 2014

A plasma-enhanced chemical vapor deposition system was used to deposit super thick diamond-like carbon (DLC)-based films ((Six-DLC/Siy-DLC)n). The aim of this work is to investigate the properties of super thick films to verify that increasing the thickness of DLC films offers the possibility of improving their properties at high temperatures. The investigation revealed that superthick (Six-DLC/Siy-DLC)n film exhibited excellent tribological property up to 500 1C. One reason is that a thin layer that consists of nanocrystals SiC is formed on the top of wear track. Another is that the stress mostly concentrates near the top surface. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Diamond-like carbon film High temperature Tribological property

1. Introduction The mechanical systems that operate under harsh conditions such as high speed, high load, and extreme temperature have been a strong driving force for the creation of effective lubricious materials [1]. When service conditions in tribological applications become severe, solid lubricants may be the only choice for controlling friction and wear since liquid lubricants degrade rapidly under these conditions [2–4]. Diamond-like carbon (DLC) film as a solid lubricant has currently attracted significant attention because DLC film exhibit excellent properties, such as high hardness, chemical inertness, low friction coefficient, and high wear resistance [5–9]. However, studies have shown that the DLC film loses its lubricous property at approximately 300 1C resulting in serious wear [10–16]. Thus, seeking effective way to modify the DLC films for acceptable high temperature performance is of important significance. A possible approach for improving the tribological property of DLC film involves the introduction of another element which can enhance the thermal stability of the DLC film. According to R. Hatada et al. [17], silicon-doped DLC (Si-DLC) film was prepared on silicon wafer substrate using a plasma source ion implantation method. The friction coefficient for the 13 at.% Si that contained film was 0.05. After it was annealed at 723 K, the friction coefficient of the 13 at.% Si that contained film was 0.03. The improved tribological performance of the Si-DLC film at high

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Corresponding author. Tel./fax: þ 86 9364968080. E-mail address: [email protected] (L. Wang).

http://dx.doi.org/10.1016/j.triboint.2014.08.017 0301-679X/& 2014 Elsevier Ltd. All rights reserved.

temperatures was attributed to the formation of SiO2 wear particles. C.W. Zou et al. [18] prepared Cr-doped DLC films on WC–Co cemented carbide substrates. They found that when the sliding distance reached 90 m at 400 1C, the undoped DLC coating fails with a higher friction coefficient, while the low Cr-doped DLC coating was still able to keep the stable friction behavior without failure, which indicated that low Cr doping considerably improves the excellent wear resistance capacity of the DLC coatings, possibly due to the combined protection of low stress and relatively high hardness. J. Choi et al. [19] studied the thermal stability and tribological properties of Si-DLC films. The results show that the Si-DLC films with 21 at.% Si content annealed at 500 1C presented a low wear rate as well as low friction, whereas the 29 at.% Si-DLC film exhibited a high friction due to the creation of cracks on the worn surface related to the SiC-like nature. The 11 at.% Si-DLC film annealed at 500 1C show the lowest friction coefficient at the cost of significant wear in the graphitized film. The formation of a thick silicon oxide layer on the Si-DLC film could be favorable for low friction and wear. A. Abou Gharam et al. [20] studied the tribological behavior of W containing DLC (W-DLC) ran against 319 Al at temperatures of up to 500 1C. Results showed that a low COF of 0.2 at 25 1C was observed, whereas between 100 and 300 1C, a high average steady-state COF of 0.60 was recorded. At 500 1C, the COF decreased to 0.12. The formation of transferred material layers on counterface and tungsten oxide layer on DLC coating top surface was the governing mechanism for the low COF. Recently, A. Banerji et al. [21] studied friction, adhesion and wear mechanisms of W containing DLC (W-DLC) coatings for elevated temperature applications of Ti–6Al–4 V alloy. They discovered that W-DLC coatings offered low and stable COF values of 0.11–012 at

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25 1C while at 400 1C and 500 1C, extremely low COF (0.07–0.08) values were recorded with minimum titanium adhesion to the coating surface. The low COF and high wear resistance were attributed to the formation of a WO3 layer on the W-DLC surface. It has been proven in many studies that the reason for tribological property degradation of DLC film is due to graphitic transformation and oxidization on the top of film surface [17,19,20,22,23]. Thus, we suspect that another possible way of improving the tribological property of DLC films at high temperatures is increasing their thickness. So far, no studies have been published about such method because the high intrinsic stress and mismatch in chemical bonding between the films and the substrates often cause poor adhesion, which limits the film thickness to a range between 1 and 5 μm [24–26]. A novel plane hollow cathode plasma-enhanced chemical vapor deposition (PHC-PECVD) method was developed in our previous work [27], allowing super-thick Si-DLC-based films to be deposited. These super-thick films (Six-DLC/Siy-DLC)n exhibited ultrahigh load-bearing capacity, ultralow internal stress, and high hardness. The aim of this work was to verify how DLC films are used in high temperature application by increasing thickness. In pursuit of this goal, microstructure, hardness, and tribological properties under in-situ high-temperature environment are conducted.

2. Experimental 2.1. Film deposition (Six-DLC/Siy-DLC)n films with different thicknesses were deposited on 304 stainless steel and P (1 0 0) Si substrates by a PHC– PECVD method. The (Six-DLC/Siy-DLC)n film deposition consists of the following steps: (1) The substrates were cleaned ultrasonically in ethanol and acetone baths in succession and then dried with nitrogen. The substrates were placed in a vacuum chamber and then are pre-sputtered for 15 min with a constant flow of Ar gas fed into the chamber. (2) A silicon transition layer was deposited to enhance the interfacial adhesion. (3) A multilayered film consisting of (Six-DLC/Siy-DLC)n was fabricated by repeated synthesis of Six-DLC (low silicon-doped DLC layer) and Siy-DLC (high silicon-doped DLC layer, x oy), n was the number of (Six-DLC/SiyDLC) layer. The thickness of (Six-DLC/Siy-DLC)n coating was

dependent upon the n. No external heating of the substrate was employed and the maximum temperature during the deposition was about 180 1C. The compositions of the constituents are as follows: 83.9 at.% C, 7.30 at.% Si, 1.20 at.% O and 7.60 at.% H for SixDLC; 77.8 at.% C, 11.0 at.% Si, 1.7 at.% O and 9.50 at.% H for Siy-DLC. More detailed description of the film deposition processes can be found in a previous paper [27,28]. 2.2. Film characterizations The film thickness was monitored by scanning electron microscopy (SEM, JSM- 5310, JEOL) observation. The composition was measured by time-of-flight elastic recoil detection analysis (TOFERDA) and energy-dispersive X-ray spectrometry (EDX). The structures of the films were investigated using high-resolution transmission electron microscopy (HRTEM) and Raman spectrometer with an excitation wavelength of 514 nm of an Argon ion laser. HRTEM and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) were operated on FEI Tecnai G2 F20 FE-TEM at 200 kV. The adhesion of the sample was tested by a scratch tester (CSEM Revetest) equipped with diamond tip of radius 200 μ m. The normal load was increased from 0 to 100 N at the loading rate of 100 N/min and scratching speed of 10 mm/min. During the scratch test, acoustic emission and friction force were continuously monitored. A calibrated Hysitron Triboindenter with a Berkovich indenter was employed to determine the film hardness (H) and Young's modulus (E). A maximum load of 500 mN was used in order to assure that the indention depth was within the 10% of the film thickness. Six repeated measurements were made for each specimen. A high temperature ball-on-disk tribometer (UMT-3) was employed for the evaluation of wear and friction behavior of the (Six-DLC/Siy-DLC)n films at 30, 100, 200, 300, 400, and 500 1C. These tests were carried out in humid air (relative humidity of 65 75%). The counterpart materials were heat-resistant steels balls (2CrWMoVNbB) with a diameter of 9 mm. All tests were conducted at a rotation rate of 300 and a constant normal load of 10.0 N for 60 min. Following the sliding experiments, focused ion beam (FIB) (Carl Zeiss NVision 40 CrossBeam) milled cross sections of the wear tracks were prepared to investigate structures. A non-contact 3D surface profiler (model MicroMAXTM, made by ADE Phase Shift, Tucson, AZ, USA) was used to capture 3D images on a wear track for measuring the wear

Fig. 1. SEM cross section of the (Six-DLC/Siy-DLC)n films with the total thickness of 41.4 μm (a) as-deposited and (b) annealing after 500 1C.

J. Wang et al. / Tribology International 81 (2015) 129–138

volume. Three profilometry traces were taken on each wear surface to obtain wear depths and cross-sectional areas. The wear rate of the film was defined as the wear surface volume divided by the load and the total distance traveled by the counterface ball.

3. Results and discussion 3.1. Microstructure of film Fig. 1a shows a SEM cross section of the as-deposited multilayer (Six-DLC/Siy-DLC)n films with the total thickness 41.4 μm. A cyclical layer consisting of a low silicon-doped DLC layer and a high silicon-doped DLC layer compose the DLC-based film. The color is related to the atomic number. The bright contrast layer corresponds to the Si-rich layer (Siy-DLC), and the dark contrast layer

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corresponds to the C-rich layer (Six-DLC). The image of the (SixDLC/Siy-DLC)n film with the total thickness of 41.4 μm annealing after 500 1C is shown in Fig. 1b. The figure shows signs that the film is breaking up. Notably, no spontaneous delamination of the films after the tribological test at difference temperatures was observed. The breaking up of the film may appear in the sample preparation for the observation using SEM. The interface between Six-DLC and Siy-DLC layer of the first cyclical layer fuse together. Interestingly, the rest of the cyclical layer clearly shows separated layers, indicating 41.4 μm multilayer (Six-DLC/Siy-DLC)n films with good thermal stability. The thickness of the entire multilayer, SixDLC layer, and Siy-DLC layer is 41 70.3, 0.9 70.1, and 0.3 7 0.1 μm, respectively. The increasing of thickness of Siy-DLC layers could be due to diffusion Si from Siy-DLC to Six-DLC. Cross-sectional TEM was employed to study the structure of the similar multilayer coating with a total thickness of 2.6 μm.

Fig. 2. Cross-sectional TEM image of the (Six-DLC/Siy-DLC)n film with a total thickness of 2.6 μm (a) as-deposited at low magnification and (b) annealed at 500 1C at low magnification. (c) As-deposited at high magnification and (d) annealed at 500 1C at high magnification. The insets in (a) and (b) show the corresponding SAED image.

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Parts a and c of Fig. 2 show bright field cross-sectional TEM micrograph at high magnification of the as-deposited multilayer coating. The top inset in Fig. 2a shows SAED pattern from the entire film. The diffuse cloudy SAED patterns of the multilayer (SixDLC/Siy-DLC)n film indicates the absence of crystalline phases. Fig. 2b shows that a large portion of the original Six-DLC and SiyDLC layers participate in the mutual reaction after annealing at 500 1C. For first cyclical layer, the initial thickness of Six-DLC layer is transformed into a wide intermixed layer, leaving only a very narrow 30 nm unreacted Six-DLC layer. These observations are consistent with the SEM results. In addition, the SAED pattern indicates that the coating annealing after 500 1C is also consistent with an amorphous nature. Within the first Six-DLC (from substrate) coating, many tree-like structures can be observed (brighter areas), the composition of which was detected through HAADF-STEM combined with EDX. The results are shown in Fig. 3. The columnar boundary areas are enriched in C as compared to the column bodies. These tree-like structures originate from the substrate and appear to grow and expand toward the Siy-DLC layer. However, the Siy-DLC layer itself is little changed.

The performance of the DLC films at different temperatures is greatly influenced by the nature and structure of the films. The structural transformation that took place in DLC films was investigated using Raman spectra. Fig. 4 shows the Raman spectra for the wear track of 41.4 μm multilayer (Six-DLC/Siy-DLC)n film and wear scar after sliding against a steel ball at different temperatures. These Raman spectra can be resolved using two Gaussian peaks that are associated with D and G peaks. The D band around 1350 cm  1 is attributed to A1g, which creates the vibration of a six-fold aromatic ring. Its intensity is strongly related to the presence of six-fold aromatic rings. The G band around 1550 cm  1 is due to the bond stretching of all pairs of sp2 atoms in both rings and chains [29–31]. The parameters of the Gaussian curves used to fit the D and G bands of Raman spectra obtained after the DLC films were tested, i.e., the position of the G band, the intensity ratio of D and G bands, and half the bandwidth in the Raman spectra, as shown in Table 1. In terms of wear track, the G-band position and the ID/IG is seen to increase as temperature and G FWHM decrease. The G-band position significantly increases from 1526 to 1544 cm  1 when the tested temperature exceeds

Fig. 3. (a) HAADF-STEM image of (Six-DLC/Siy-DLC)n film; (b) corresponding EDX analysis along the solid red line marked in (a). The scanning direction is from left to right.

Fig. 4. Raman spectra of (a) wear track (41.4 μm (Six-DLC/Siy-DLC)n films) and (b) counterface after sliding against steel ball at different temperatures.

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Table 1 Raman G band position, G band full width at half maximum (G FWHM) and band intensity ratio ID/IG for wear track (41.4 μm multilayer (Six-DLC/Siy-DLC)n films) and counterface after sliding against steel ball at different temperatures. Wear track G position (cm 30 1C 100 1C 200 1C 300 1C 400 1C 500 1C

1508 1512 1513 1517 1526 1544

Counterface 1

)

G FWHM (cm

1

)

184 186 180 164 165 144

ID/IG

G position (cm  1)

G FWHM (cm  1)

ID/IG

0.32 0.34 0.34 0.38 0.57 0.95

1596 1589 1587 1589 1597 1599

102 102 104 107 103 103

0.86 1.10 1.20 1.10 1.20 1.10

Fig. 5. (a) Nanohardness and elastic modulus (b) H/E as a function of the annealing temperature of the multilayer (Six-DLC/Siy-DLC)n film with the thickness of 41.4 μm. Reported values and error bars represent averages and standard deviations, respectively, based on six different measurements on each of as-prepared films.

400 1C. At the same time, the decrease in G FWHM and increase in ID/IG become noticeable. The spectra for the wear scar after sliding against steel ball at different temperatures show that the value of ID/IG tested room temperature is 0.86. Thereafter, an increase to 1.2 is obtained at 100 1C. This value is maintained up to 500 1C. However, the G peak position and G FWHM has no obvious regular change. X. Tang et al. [32] proposed that the structure of RF magnetron sputtered (a-C:H) films with hydrogen content in the medium range consists mainly of sp2 bonded planar carbon clusters. These clusters were connected through sp2 and sp3 bonds that form a matrix of sp2 dominated clusters. Some of the cluster boundaries had hydrocarbons. The boundaries extend to form hydrocarbon networks that also connect the sp2 matrix clusters together. Wild and Koidl [33] suggested that hydrogen desorption and hydrocarbon from the edges of the sp2 dominated clusters took place at high temperature. They also suggested that hydrogen effusion occurs in a-C:H films at elevated temperatures from 300 to 600 1C depending on the deposition parameters used for the coating deposition. Thus, the increase in ID/IG ratio indicated that the average cluster size decreases. However, Tang et al. suggested that, at relatively low temperatures, the reduction of the cluster size was only due to hydrocarbon desorption from the edges of the sp2 clusters. No structural modification of the sp2 carbon clusters occurs. At higher annealing temperatures, they suggested that a structural modification occurs in (a-C: H) in which the sp2 clusters combine and grow rapidly (i.e. graphitization). In this work, even though sp2 is not dominated in Si-DLC-based film, we believe that a substantial sp2 exists in the Si-DLC structure and that the proposed model is still suitable. Thus, we conclude that the conversion of sp3 bonds to sp2 bonds takes place in the wear track when the tested temperature exceeds 400 1C.

3.2. Mechanical performances DLC films have been applied as hard coatings and their mechanical properties (i.e. hardness, elastic modulus, and critical load) are therefore critical. Fig. 5 shows the nanohardness and elastic modulus of the super-thick multilayer coating at different annealing temperatures obtained using the Oliver–Pharr method [34]. The hardness of the film increases as the range increases from 15 to 19 Gpa annealing temperature up to 300 1C. Moreover, the hardness decreases as the temperature further increases from 400 to 500 1C. After annealing at 500 1C, the hardness is approximately 9 GPa. The elastic modulus is also found with the same change trend as hardness. The elastic modulus increases as the annealing temperature increases from room temperature to 400 1C but decreases as the temperature increases further. The increase in elastic modulus rises from 142 to 172 GPa at a lower annealing temperature at 400 1C and decreases from 172 to 112 GPa at a higher annealing temperature. Similar results are reported for Ti-doped DLC. K.W. Weng et al. [35] proposed that highly energetic ion bombardment destroyed the microstructure of DLC films and produced numerous defects in the process of ion implantation, which reduced the hardness of the film. However, the induced defects were repaired with low annealing temperature. As the annealing temperature further increases to 400 1C, the hardness of the DLC films decreases as the annealing temperature increases because the sp3 bonds are converted to sp2 bonds, which is consistent with the observation in Raman spectra of the super thick DLC films. To examine the dependence of elasticity of the film on annealing temperature, the hardness/elastic modulus (H/E) values of the films are compared as shown in Fig. 5b. For H/E, the lower the H/E value, the more plastic and more brittle the material

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behaves. Ideally, for plastic behavior, H/E ¼0, whereas ideally elastic behavior would be indicated by H/E ¼0.17 [36]. The reduced H/E values at high annealing temperature can be correlated to the changes of structure on the film. The adhesive strength of the (Six-DLC/Siy-DLC)n film with the thickness of 41.4 μm prepared with different annealing temperature is evaluated by gradually increasing the load of the diamond stylus. In this process, the critical load Lc is defined as the load where the initial crack forms, with sharp increases of the friction force and acoustic emission. Fig. 6 shows the critical load as a function of the annealing temperature of the DLC films. As shown in Fig. 6, the critical load of the as-deposited film is approximately 52 N and almost remains at this value after annealing, indicating that annealing temperature is almost not affected by adhesive stress.

in COF for all films during the run-in period for first 20–250 s is observed. Thereafter, a sudden decrease in COF is observed and a steady state is reached at the end of the test. The steady-state

3.3. Tribological performance A ball on-disk type friction tester in ambient air (relative humidity of 65 75%) was measured to test the friction coefficients of the DLC films deposited on steel substrates. The counterpart materials were heat-resistant steel balls with a diameter of 9 mm. All tests were conducted at a rotation rate of 300 and a constant normal load of 10.0 N for 60 min. Fig. 7(a) shows the COF values of DLC film 41.4 μm thick as a function of tested time at different testing temperatures. At low temperatures (up to 200 1C), a peak

Fig. 6. Critical load as a function of the annealing temperature of the multilayer (Six-DLC/Siy-DLC)n film with the thickness of 41.4 μm.

Fig. 8. (a) (c) (e) (g) SEM image and their corresponding EDX elemental mapping of steel ball after tested different temperature. (b) (d) (f) (h) SEM image and their corresponding EDX elemental mapping of wear track of 41.4 μm after testing at different temperature. The color is related to the element. Purple represent C, aqua represent O and red represent Si.

Fig. 7. (a) The COF values of 41.4 μm thick (Six-DLC/Siy-DLC)n film as a function of tested time and (b) the wear rates at different testing temperatures.

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value of COF is 0.167 0.02, 0.0617 0.004, and 0.060 70.004 for the coatings tested at room temperature at 30, 100 and 200 1C, respectively. The COF gradually increased after a sudden decrease for DLC thin films tested at temperatures of 300 1C and higher. After sliding for some time, a steady state is also obtained. At the same time, the fluctuations of the friction curves became apparent. Finally, the average COF is approximately 0.15 70.04, 0.247 0.02 and 0.29 70.02 at temperatures of 300, 400 and 500 1C, respectively. The wear rates of the 41.4 μm DLC films are shown in Fig. 7b. At room temperature, the wear rate is the lowest at 2.1  10  6 mm3/Nm. The wear rate then remains at 2.4  10  6 mm3/Nm at 100 and 200 1C. At temperatures of 300 1C, the wear rates increase to 8.2  10  6 mm3/Nm. Large increases in wear rate are observed at 400 1C, reaching 3.1  10  5 mm3/N. Notably, the wear rate decreases to 2.4  10  5 mm3/Nm at 500 1C. Fig. 8 shows the SEM and EDS elemental mapping (C, Si, and O) of 41.4 μm DLC

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films on the wear track surface and steel ball after they were tested at different temperatures. Interestingly, carbon was not discovered on the surface of film and was only presented on wear track after tribology testing at 500 1C. The surface (Fig. 9a and b) and wear track (Fig. 9c and d) structures of the films tested at 500 1C were analyzed using FIB-TEM. Analyses of the surface structures of the film tested at 500 1C show that a top surface layer formed. The layer is observed to be approximately 130 nm thick. A high-resolution image of this layer reveals that the layer has an amorphous structure. Combine with C, Si, and O mapping images (Fig. 8h), this amorphous top layer is the SiO2 layer. For the wear track, a thin dark layer of approximately 8 nm developed (Fig. 9 c). HRTEM reveals that this layer has a typical nanocomposite structure that consists of nanocrystals and amorphous phases. This area is too small to investigate using SAED. The phases are identified through interplanar spacing, as shown in Fig. 9d.

Fig. 9. Cross-sectional TEM image of the surface and wear track of (Six-DLC/Siy-DLC)n film with a total thickness of 41.4 μm after tested at 500 1C. (a) surface at low magnification; (b) surface at high magnification; (c) wear track at low magnification; and (d) wear track at high magnification.

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The observed interplanar spacing is 0.21 nm. Thus, SiC (111) is identified using element mapping. The low friction performance of hydrogenated DLC films in dry environments has been attributed to the hydrogen-terminated dangling bonds with weak van der Waals forces acting in the contact between surfaces. In environments that contain water vapor, molecules disturb this mechanism [37]. As the test temperature increase, the amount of water remaining adsorbed on the film surface decrease and hence the efficiency of the coverage of the film surface by water molecules decreases. Thus, the COF of DLC film tested at temperature of 100 and 200 1C is lower than that tested at room temperature. As for tests conducted at 300 and 400 1C, the average COF was 0.15 and 0.28, respectively, is higher than that tests conducted at 200 1C. The increase in COF is attributed to the change in the structure on the film surface and transfer film on the counterpart. The Raman spectra of the wear track reveal that DLC films do not survive the transformation from sp3 to sp2 (graphite-like) that occurs in this temperature. EDS analysis reveal that a transfer layer that consists of SiO2 is developed (Fig. 8 l and m). These layers are considered to be lubricious and prevented the steel and graphitized DLC film from making direct contact [17,19]. Experiments conducted at 500 1C show no obvious increase for COF, although the wear track is found to have a different composition than the wear tracks observed at 300 and 400 1C. This finding may be because the composition of the transfer layers is the same, which is the main reason behind the friction. At room temperature, the very low estimated wear rates agree with the excellent mechanical property. At 300 and 400 1C, structural changes are observed. The softer graphitic-like DLC phase leads to easier plowing by the hard counterpart resulting in higher wear rates. At 500 1C, SEM of wear track, the FIB cross section and EDS elemental mapping of DLC film reveal an 8 nm thick nanocomposite layer consisting of nanocrystals SiC and an amorphous SiO2 layer that formed at the wear track surface. Nanocrystals SiC which shows better thermal stability lead to decrease in wear rate. The above-mentioned results show that tribological properties decrease slightly as temperature increase. However, given that the superthick DLC films keep their excellent mechanical properties and sufficient thickness for wear, these types film a potential candidate for severe high-temperature applications. To compare the tribological performance of the thin and thick films under high temperature, the tribological behavior of the DLC films with different thicknesses at different tested temperatures is investigated. Fig. 10

shows the average steady-state COF values and wears depths of the DLC multilayer with different thicknesses at different tested temperatures. At room temperature, the COF is 0.1670.02 for all films. At intermediate 100 and 200 1C, the films present similar average COF values (0.06070.004) except for the 3.2 μm films (0.4407 0.005). When the testing temperature increase to 300 1C, the COF is 0.2470.07, 0.2070.05, 0.2370.06, and 0.1570.04 for 8.4 , 17.5, 29.1 and 41.4 μm DLC film, respectively, which are much higher than the values observed at intermediate temperatures. When tested at 400 and 500 1C, the average COF remains on a certain value, approximately 0.29. The wear depth of the 3.2 μm (Six-DLC/ Siy-DLC)n film after tests at 100 1C is 35 μm, which illustrates that the film is completely worn, whereas the wear depths of the 8.4 and 17.5 μm (Six-DLC/Siy-DLC)n films after tests at 100 1C are only 1.4 and 0.9 μm, respectively. These films are worn through until the tested temperature increased to 400 1C. For 29.2 and 41.4 μm (Six-DLC/Siy-DLC)n film the wear depth is 12.2 and 9.3 μm when tested 500 1C. Obviously, thick DLC films show better wear resistance at the same temperature. To better understand this phenomenon, finite element method (FEM) was used to simulate the stress distribution in the multilayer DLC film under the test condition using ANSYS Multiphysics. The system of the ball indentation onto a coated specimen sees reference [27]. The counter body was assumed as perfectly rigid, and the transition layer was not considered. The substrate was modeled as an isotropic, rate-independent solid with a bilinear elastic–plastic constitutive relation, assuming kinematic strainhardening and the von Mises yield criterion. The film was assumed an isotropic, linear elastic material. The Young’ moduli of the coating were measured by a Hysitron TriboIndenter. Table 2 lists the selection of parameters. Conductivity and specific heat were calculated from References [38–40] and thermal expansion from

Table 2 Material properties used in FEM simulations.

Substrate Six-DLC Siy-DLC

Thickness (μm)

Ex

PRXY

KXX

D

C

ALPX

3000 0.8 0.3

193 82 150

0.3 0.25 0.25

66.6 0.57 0.31

7800 1800 2200

460 800 650

1.05E-5 3.5 E-5

Note: Ex: Young's modulus(GPa). PRXY: Poisson’s ratio. KXX: Conductivity (W/ (m 1C)). D: Density (kg m  3). C: Specific heat (J/ (kg 1C)). ALPX: Thermal expansion (1/1C).

Fig. 10. (a) The average steady-state COF values and (b) wear depths of the DLC multilayer with different thickness at different tested temperatures.

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Fig. 11. Contour plots of the von Mises stress and shear stresses for a (Six-DLC/Siy-DLC)n/DLC-coated system with different thicknesses: (a) von Mises stress of 8.8 μm; (b) shear stresses of 8.8 μm; (c) Mises stress of 44.4 μm; (d) shear stresses of 44.4 μm.

Reference [41]. Despite material properties were not very precise. Simulations improve our understanding of the wear behavior. Fig. 11 presents the FEM simulation results. For 8.8 μm (SixDLC/Siy-DLC)n/DLC-coated system, the maximum stress is located at the interface between the films and substrates when 10N force are applied on the counterpart ball and 4501C temperature on the substrate. The common outcome is the formation of fractures near the interface due to the large amount of stress. Hard DLC film is unable to perform its function of providing resistance to wear adequately. In contrast, the stress contours of the 44.4 μm (SixDLC/Siy-DLC)n-coated system are sparser near the interface and are mostly concentrated near the top surface. Thus, the stress gradually changes from the top surface to the interface. Meanwhile, the maximum value of stress is decreased. Therefore, these thin films are more likely to fail compared with thick DLC film.

are drawn as follows: (i) The total thickness of superthick DLC films almost unchanged after annealing up to 500 1C in the air. After annealing at 400 and 500 1C, the hardness of superthick DLCbased films became approximately 15 and 9 GPa, respectively. The critical load of the as-deposited superthick DLC-based film is approximately 52 N. (ii) The friction coefficient measured against a steel ball is approximately 0.31 at 500 1C. The wear rates of films first increase as the temperature increased to 400 1C due to the softer graphitic-like DLC phase formed on the surface. Then, the wear rates of films decreased as the temperature was increased further to 500 1C due to a thin nanocomposite layer that consists of nanocrystals SiC. (iii) Thin films are more likely to fail compared with the thick DLC film due to large amount of stress located on the interface between the substrate and films. Given that the super-thick DLC films keep their excellent mechanical properties and sufficient thickness for wear, these films are considered as potential candidates for severe high-temperature applications.

4. Conclusion The PHC-PECVD method allows deposition of superthick (SixDLC/Siy-DLC)n films with thickness of 44.4 μm. Chemical bonding was examined by Raman and X-ray photoelectron spectroscopy techniques. The structures were investigated by scanning electron microscopy and transmission electron microscopy. Mechanical and tribological properties were evaluated using nanoindentation, scratch and ball-on-disk sliding friction testing. The major results

Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (Grant 11172300). The authors gratefully acknowledge Mr. Yaonan Zhang and Guohui Zhao for supporting the numerical simulation at Lanzhou branch of supercomputing CAS.

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