Influence of deposition conditions on mechanical properties of a-C:H:SiOx films prepared by plasma-assisted chemical vapor deposition method

Accepted Manuscript Influence of deposition conditions on mechanical properties of a-C:H:SiOx films prepared by plasma-assisted chemical vapor deposition method

A.S. Grenadyorov, А.А. Solovyev, K.V. Oskomov, V.S. Sypchenko PII: DOI: Reference:

S0257-8972(18)30594-2 doi:10.1016/j.surfcoat.2018.06.019 SCT 23470

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

6 March 2018 11 June 2018 13 June 2018

Please cite this article as: A.S. Grenadyorov, А.А. Solovyev, K.V. Oskomov, V.S. Sypchenko , Influence of deposition conditions on mechanical properties of a-C:H:SiOx films prepared by plasma-assisted chemical vapor deposition method. Sct (2017), doi:10.1016/j.surfcoat.2018.06.019

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ACCEPTED MANUSCRIPT Influence of deposition conditions on mechanical properties of a-C:H:SiOx films prepared by plasma-assisted chemical vapor deposition method A.S. Grenadyorova, А.А. Solovyeva,b, K.V. Oskomova, V.S. Sypchenkob a

Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, Akademichesky Ave. 2/3, Tomsk, 634055 Russia National Research Tomsk Polytechnical University, Lenina Ave. 30, Tomsk, 634055

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b

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Russia

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Abstract

A series of a-C:H:SiOx films was deposited on polished silicon and glass substrates by

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plasma-assisted chemical vapor deposition combined with pulsed bipolar substrate bias from

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mixtures of argon and polyphenylmethylsiloxane vapors. Different Ar pressures and substrate bias voltages were applied for the synthesis of a-C:H:SiOx films having different mechanical

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properties. Detailed characterization of the mechanical properties of a-C:H:SiOx films was

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made using the nanoindentation. Hardness and elastic modulus were used for the evaluation of the endurance capability (H/E) and resistance to plastic deformation (H3/E2). The structural properties of the deposited films were analyzed by Fourier transform infrared spectroscopy

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(FTIR) and Raman spectroscopy. It was shown that the Ar pressure and substrate bias varia-

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tion can change the film properties and the growth rate and these changes are not linear. So, depending upon application, deposition conditions are to be optimized. In all of the examined coatings, increase of Ar pressure and amplitude of negative pulse of substrate bias lead to improvement in mechanical properties. According to the results of FTIR and Raman spectroscopy; this improvement is due to an increase in the sp3 bonded carbon content and decrease of hydrogen content in the films. Keywords: a-C:H:SiOx films, plasma CVD, substrate bias, FTIR spectroscopy

ACCEPTED MANUSCRIPT 1. Introduction For a long time, a great interest of researchers has attracted diamond-like coatings (a-C: H, DLC) that have high hardness, transparency in the IR wavelength range, chemical resistance [1–4]. Due to these properties, such films can be used in optical devices for the protection of optical elements. In spite of attractive properties of DLC, there are some barriers for

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direct applications of DLC. A serious drawback of DLC is high residual (internal) stresses (> 6 GPa), which is a problem for the formation of high-quality films with a thickness of more

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than 1 μm, since this is accompanied by cracking and peeling of the films [5, 6]. Another

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drawback is that, sp3 bonded carbon in the DLC film gets transformed to sp2 bonded carbon at high temperature (>400 °C). In this connection, a method is required that ensures the for-

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mation of optical coatings with low internal stresses at a substrate temperature of not more

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than 100–200 °C. It has been known that incorporation of different elements into amorphous matrix of DLC coatings leads to noticeable changes in their properties. Incorporation of SiOx

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nanoparticles in DLC films enhances its adhesion and stability with steel substrates [7]. The

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presence of the glass-like network in such film distinguishes it from the conventional DLC film and it is usually called as a-C:H:SiOx, SiOx-containing DLC film or diamond-like nanocomposite.

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a-C:H:SiOx films, formed from plasma in vapors of organosilicon precursors (siloxanes,

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silosans, etc.), have low internal stresses (less than 1 GPa) due to the presence of Si–C bonds (0.189 nm), which are longer than C–C bonds (0.154 nm) [8]. Fourfold coordination increases with SiOx doping which increases sp3/sp2 ratio of carbon bonding and reduces graphitization in the a-C:H:SiOx. In addition, a-C:H:SiOx films have high mechanical properties, good optical transparency in the visible and infrared spectral range, chemical inertness [10–13]. These films comprise two amorphous interpenetrating network structures: carbon, stabilized by hydrogen (a-C:H), and silicon, stabilized by oxygen (a-Si:O). The ability to control the ratio of

ACCEPTED MANUSCRIPT sp3 and sp2 bonded carbon in a film and the amount of SiOx phase makes it possible to vary the physico-mechanical properties of the synthesized films in a wide range. In [9, 12] the influence of the silicon content on the mechanical properties of aC:H:SiOx films formed in a DC discharge was investigated. It was shown that an increase in the silicon content to 22 at.% leads to a decrease in the hardness of the film from 14–16 to 9–

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12 GPa. Santra et al. [13] investigated the properties of a-C:H:SiOx films obtained with various proportions of the hexamethyldisiloxane and hexamethyldisilazane precursors, using a

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radio-frequency (RF) and direct current (DC) substrate bias. It is shown that the hardness of

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the films is 8-10 GPa with an elastic modulus of 84-105 GPa. In another paper by the same authors [14] it was shown that there is an optimal flow rate of hexamethyldisiloxane at which

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the hardness of the films is maximal and equal to 13 GPa. Recently, Jedrzejczak et al. [15]

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showed that tribological properties of a-C:H:SiOx films, produced using the RF plasma assisted chemical vapor deposition (PACVD) method, depend not only on the amount of incorpo-

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rated silicon and oxygen but mainly on the parameters of the process of synthesis, in particu-

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lar from values of the negative self-bias potential. Barve et al. [8] investigated the effect of high-frequency substrate bias on the properties of a-C:H:SiOx films formed in a mixture of methane and hexamethyldisiloxane. It is shown that an increase in the amplitude of the RF

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bias voltage from –25 to –225 V leads to an increase in hardness and elastic modulus up to 12

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GPa and 90 GPa, respectively. Batory et al. [16] illustrated influence of negative substrate bias and hexamethyldisiloxane precursor flow rate on the mechanical properties of aC:H:SiOx films. He used RF plasma and worked in relatively higher amplitude of self-bias values ranging from −200 to −800 Volts. Depending on the deposition conditions, the hardness varied from 5 to 17 GPa, elasticity index (H/E) from 0.08 to 0.15, resistance to plastic deformation (H3/E2) from 0.07 to 0.32 GPa. It has been found that there is an increase in the diamond-like nature of the films with increasing substrate bias [17], but the chance of graphitization also increases with continuous increase in the magnitude of bias values [18].

ACCEPTED MANUSCRIPT The most commonly used technique for the preparation of a-C:H:SiOx films is plasma assisted chemical vapor deposition by decomposition of organosilicon precursors in RF (13.56 MHz) capacitively coupled discharge. The negative self-bias voltage developed on the substrate holder electrode is one of the most relevant parameters affecting the microstructure and properties of the films, since it determines the impact energy of ions bombarding the

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growing film surface. Low or high bias voltages result in soft, polymer-like or graphite-like aC:H networks in films; whereas, harder a-C:H networks, with diamond-like properties, are

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obtained at intermediate bias voltages. In RF discharge the relationship between bias voltage

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and power for a given system depends on the electrode. The greater the asymmetry between the powered and grounded electrodes, the higher the developed self-bias voltage. As a conse-

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quence, the deposition on large area substrates makes the industrial application of RF PACVD

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processes difficult. Use of bipolar pulsed plasma technology has recently attracted a great deal of interest since it can use higher power levels in comparison with RF and does not require

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matching networks [19]. In particular, the use of bipolar pulsed plasmas has been successfully

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applied in the sputter deposition of insulating films [20]. To the best of our knowledge, investigations on a-C:H:SiOx films deposited at pulsed bipolar substrate bias during deposition have not been reported so far. Therefore, it is necessary to compare the efficiency of the mid-

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frequency bipolar substrate bias in comparison with the RF PACVD process.

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In this present case, all the films were deposited using plasma of Ar and polyphenylmethylsiloxane (PPMS) vapors and a systematic investigation about the effect of varying Ar pressure and amplitude of negative pulse of substrate bias on the properties of the deposited aC:H:SiOx films have been done with the help of different characterization techniques.

2. Experimental details The plasma-assisted chemical vapor deposition (PACVD) system used in the present study is shown in Fig. 1. Si (100) and glass substrates 0.2 and 1.2 mm thick (size 2×2 cm2)

ACCEPTED MANUSCRIPT were used for deposition. The substrates were fixed to a conductive substrate holder located 300 mm from the plasma generator. As a result of large source-to-substrate distance the film thickness uniformity was better than 5% on the area of 100 × 100 mm2. The substrates were cleaned with acetone and alcohol, using ultrasonic vibration followed by drying in an air jet. Cleaned substrates were loaded into the deposition chamber followed by evacuation up to a

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pressure of 0.015 Pa. Thereafter, argon gas was introduced into the chamber and samples were cleaned by argon plasma for 6 minutes prior to film deposition. During plasma cleaning

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the argon pressure was maintained at 0.28 Pa and asymmetrical bipolar pulsed bias voltage

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with frequency of 100 kHz was applied to the substrate. The voltage waveform consisted of a variable negative pulse whose peak amplitude was varied from –100 up to –650 V followed

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by positive pulse with duration of 4 µs and peak amplitude, which was 20% of the negative

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pulse, as measured by a high-voltage probe connected to an oscilloscope.

Fig. 1. Schematic diagram of the deposition system. Films deposition was in vapors of a liquid polyphenylmethylsiloxane precursor (PPMS). The precursor ejection head (the Laval nozzle) was located near the filament, such that on evaporation the precursor molecules came into the path of thermionic electrons. The head was heated during operation by IR radiation and electrons from the hot cathode to a temperature of 430 °C, which did not change during the experiment. The precursor vaporized due to low

ACCEPTED MANUSCRIPT pressure and high temperature near the filament in the electron atmosphere, which enhanced ionization of vapor molecules by collision. The filament current was maintained at about 44 A. The thermionic electrons emitted from the filament were drawn towards ground at zero potential by applying a voltage to the floating filament with respect to ground. The substrate temperature during the deposition pro-

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cess was measured by a K-type thermocouple and did not exceed 150 оС. The thickness of the films was approximately 1 μm.

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As results of the deposition processes, two sets of a-C:H:SiOx films were manufactured.

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The first series of experiments was made by varying the argon pressure PAr in the working chamber from 0.025 to 0.28 Pa. The discharge current Id was 6 А and discharge voltage Ud

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can be adjusted from 80 to 200 V depending on the argon pressure. The amplitude of the neg-

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ative pulse of the bipolar bias voltage Ubias applied to the substrate was –100 V, since without substrate bias, the film has a polymer-like structure [11]. The second series of experiments was made with a change in the negative pulse voltage amplitude from –100 to –650 V. In this

age Ud was 200 V.

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case, the Ar pressure was 0.025 Pa, the discharge current Id was 6 А, and the discharge volt-

Properties of the deposited coatings were studied by using different characterization

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techniques. Thickness of the deposited sample was measured by a Linnik microinterferometer

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MII 4 (LOMO, Russia). Hardness (H) and elastic modulus (E) of the deposited films were measured by NanoTest 600 device (MicroMaterials, Great Britain) at 2 mN load by Oliver– Pharr method [21]. The bonding structure and quality of the deposited films was characterized using Raman spectroscopy. The Raman spectra were recorded using Centaur U HR spectrometer (Nano Scan Technology, Russia) with 532 nm wavelength. Raman spectra were recorded with a laser power of 10 mW and within a spectral range of 800-1800 cm-1 with a spectral resolution better than 1.5 cm-1. For this purpose, films with the thickness of 1000±200 nm on a monocrystalline silicon substrate were used. The Fourier transform infrared spectra (Nicolet

ACCEPTED MANUSCRIPT 5700, USA) in the range of 500 to 4000 cm−1 were recorded in transmittance and reflectance mode with the 4-cm−1 resolution. The morphology of the produced films was studied with use of atomic force microscope Solver P47 in semi-contact mode. A detailed description of the characterization of optical properties of the as-received films will be the subject of a separate publication.

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3. Results and discussion 3.1 Deposition rate of a-C:H:SiOx films

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During the plasma-assisted deposition of the a-C:H:SiOx films two factors define the

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deposition rate and thickness of the films. First is the re-sputtering rate of the film due to ion bombardment on the substrate and the second is the number density of plasma [8]. Growth of

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the film is interplay between these two processes [8, 16]. The dependence of the deposition

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rate on Ar pressure and negative pulse voltage amplitude is shown in Fig. 2. Since deposition rate of the film is found to decrease substantially with the PAr and Ubias increasing, it means that films re-sputtering significantly increased. This is due to the increase in the number of

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Ar+ ions and their energy at the PAr and Ubias increasing, respectively.

Fig. 2. Deposition rate of a-C:H:SiOx films as a function of Ar pressure (a) and negative pulse voltage amplitude (b).

3.2 Nanoindentation technique

ACCEPTED MANUSCRIPT The hardness (H) and elastic modulus (E) of a-C:H:SiOx films have been studied on monocrystalline silicon substrates. H and E are strongly dependent on chemical bonding and microstructure. Observed variations in the hardness and elastic modulus from nanoindentation measurements for the deposited films are shown in Fig. 3. H and E show a linear increasing trend with the increasing of Ar pressure from 0.025 to 0.28 Pa (Fig. 3a). Hardness increases

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almost threefold from 3.3 to 9.8 GPa. The argon pressure growth in the vacuum chamber leads to increase in argon ions concentration near the substrate. When a bias voltage is applied

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to the substrate, then ion bombardment of the substrate increases. Ions bombard the growing

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film causing a number of processes: ion etching, heating, phase transformations, atomic displacement, etc. Due to these processes, the content of the sp3 bonded carbon in the film in-

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creases, as further confirmed by the results of the Raman spectra study. In addition, an in-

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crease in hardness can be connected with a decrease in the hydrogen content in the film as described in [8].

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The increase in amplitude of negative pulse of substrate bias also leads to an increase in

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the hardness and elastic modulus of the film from 3.3 to 13.6 GPa and from 35.8 to 121.6 GPa, respectively (Fig. 3b). The bias voltage applied to the substrate leads to increase in energy of ions bombarding the film and energy transferred to the film. At a bias voltage of about –

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500 V, the dependences of H and E on Ubias go into saturation, which indicates the attainment

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of the optimum value of the amplitude of the negative pulse of the substrate bias and, consequently, the energy of the bombarding ions.

ACCEPTED MANUSCRIPT Fig. 3. Dependence of the hardness and elastic modulus of a-C:H:SiOx films on the Ar pressure (a) and negative pulse voltage amplitude (b). It is observed in Fig. 3 that increase in both PAr and Ubias has a positive effect on the increase in hardness and elastic modulus. Nevertheless, the simultaneous increase in these parameters to a maximum is not desirable, since the process of film etching will predominate

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over the deposition of a-C:H:SiOx film. It is established that the optimal parameters are Ubias = –500 V and PAr = 0.09 Pa. In this case, a-C:H:SiOx films have a hardness of 13-15 GPa and an

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acceptable deposition rate of 15-20 nm/min.

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It is generally accepted that wear resistance of a solid can be adjusted by tailoring its elastoplastic properties, either by increasing H or decreasing E [22]. The H/E parameter ex-

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plains elastic–plastic characteristics and is an indirect measure of wear resistance. To predict

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the wear resistance of the films, the endurance capability (H/E) and resistance to plastic de-

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formation (H3/E2) was evaluated from obtained data and are depicted in Fig. 4.

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Fig. 4. Values of H/E and H3/E2 ratios of a-C:H:SiOx films deposited using different Ar pressures (a) and negative pulse voltage amplitudes (b). According to the Johnson analysis [22], H3/E2, is proportional to the load at which the material starts deforming plastically under the action of a rigid sphere of radius r. Therefore, to obtain a material highly resistant to plastic deformation H should be high and E low, consistently with the H/E parameter. This association allows the applied load to be dissipated over a larger area. On the other hand, if high plastic deformation is required, low H3/E2 values are desirable [22].

ACCEPTED MANUSCRIPT Fig. 4 shows that H/E is about 0.1±0.01 and increases slightly with increase of PAr and Ubias. For DLC coatings, the H/E ratio variation is in the range of 0–0.1. The lower ratio (0) indicates the elastic–plastic behavior and the upper one (0.1) indicates the elastic behavior [23]. The minimum value of H/E ratio of a-C:H:SiOx film indicates that a higher fraction of the mechanical work is dissipated during plastic deformation and hence a large plastic strain

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is expected. On the other hand, the higher value of H/E ratio in films is related to higher sp3 bonding or an enhanced diamond-like structure [24].

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Resistance to plastic deformation in contrast to H/E ratio increases significantly with in-

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crease of PAr and Ubias during deposition. Thus, with an increase in PAr, a quadruple increase in H3/E2 from 27 to 122 MPa is observed, while with an increase in Ubias, a sixfold increase in

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H3/E2 from 27 to 169 MPa is shown. Thus, nanoindentation of a-C:H:SiOx films showed that

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an increase of both РAr and Ubias improves the mechanical parameters of the films, such as H,

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E, H/E and H3/E2.

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3.3 Raman spectroscopy characterization Raman shifts for all films deposited at different PAr and Ubias levels are recorded in the range of 1000 to 2000 cm−1 region for understanding the effect of these parameters on struc-

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tural properties (Fig. 5). At relatively high Ar pressures (0.2 and 0.28 Pa) and amplitudes of

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negative pulse of substrate bias (–300 and –650 V) the broad asymmetric bands in the 12001700 cm−1, typical for amorphous diamond-like films, are observed. These Raman spectra were deconvoluted into two Gaussian peaks: the G peak and D peak by curve fitting. The G peak, which may occur from C=C sp2 stretching vibration of olefinic or conjugated carbon chains, is attributed to the relative motion of sp2 hybridized carbon and the down shift of the G peak is related to bond angle disorder [25]. The G peak is caused by the bond stretching of all pairs of sp2 atoms in both rings and chains. The D peak is attributed to the disordered breathing motion of sixfold aromatic rings [25, 26]. Thus, the peak positions in the Raman

ACCEPTED MANUSCRIPT spectra, full width at half maxima (FWHM) of G peak and the intensity ratio of ID/IG are the

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most important parameters to understanding the bulk properties of the diamond-like films.

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Fig. 5. Raman spectra of a-C:H:SiOx films formed at different Ar pressures (a) and neg-

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ative pulse voltage amplitudes (b).

A lot of research has been done to establish the relationship between the Raman pa-

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rameter and the characteristics of carbon-based films [27–29]. Amorphous diamond-like films are characterized by structural and topological disorders. First originates from bond angle and

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length distortion, while second arises from the size and shape distribution of sp2 clusters [27].

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Raman parameters allow identifying the relationship between deposition parameters and kind of disorder in resulting films. It is considered that FWHM of G peak is mainly sensitive to

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structural disorder, while G peak position is sensitive to topological one [29]. The obtained Raman parameters show that with an increase in PAr and Ubias, the position

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of the D and G peaks are shifted towards low energies, the FWHM of G peak is increased and the ID/IG ratio is decreased (Table I). The shift of the peaks may indicate that topological disorder in a-C:H network increases. The change of the FWHM of G peak indicates the increasing of structural disorder and amount of sp3 C-C bonds in films. The intensity of the D peak decreases and it becomes less pronounced, which indicates a decrease in sp2 bound carbon in the film. Diamond-like nature of the films is enhanced; i.e. sp3 content of carbon bonding in the film is increased. This affects the increase in the mechanical properties of a-C:H:SiOx films, in particular a triple increase in hardness.

ACCEPTED MANUSCRIPT Nakao et al. [29] showed that Raman parameters and properties of Si-doped a-C-H films are strongly influenced by the content of Si. The latter was regulated by changing the ratio of C7H8 and Si(CH3)4 in the mixture of working gases. The hardness and ID/IG ratio of the films is monotonically decreased with increasing Si content. It is suggested that polymerlike structure may be formed and increase in amount as Si content increases, which is con-

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sistent with the hardness and Young’s modulus decreasing. In our case ratio of Si to C (Si/C) in the precursor (PPMS) did not change. Therefore, we believe that the Si/C ratio in the films

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changed insignificantly, despite the fact that it was not specifically measured. This is con-

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firmed by the fact that the hardness of the films obtained in this work increases with an increase in PAr and Ubias, while the ID/IG ratio decreases.

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Raman spectra of a-C:H:SiOx films obtained at relatively low Ar pressures (0.025 and

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0.09 Pa) and amplitudes of negative pulse of substrate bias (–100 and –250 V) do not have pronounced peaks typical for diamond-like films and photoluminescence background is increased. Additional overlapping peaks with D and G bands were obtained. Peak located at

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~1460 cm−1 may be attributed to the transpolyacetylene (trans-PA) chains [30]. Peak attributed to the trans-PA chains is overlapping with D and G peaks and it complicates the definition of the correct positions of these peaks. In order to resolve the D and G peaks in these films, it

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is necessary to use smaller excitation wavelengths, as shown in [30]. It should be noted that

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the photoluminescence background on Raman spectra indicates a significant content of hydro-

gen in the film. When the PAr and Ubias increasing, the photoluminescence decreases, the content of hydrogen atoms in the film decreases.

3.4. Fourier Transform Infrared Spectroscopy characterization In order to study the local bonding of C, H, Si, and O sites of the a-C:H:SiOx films, the FTIR spectra were recorded from 500 to 4000 cm−1. The corresponding FTIR spectra are shown in Fig. 6 and 7. Bands belonging to the class of organosilicon compounds are found in

ACCEPTED MANUSCRIPT the films. The transmission spectra show strong Si–O absorption peaks that appeared around the wave number from 600 to 1100 cm−1 [31]. The mode at 740 and 980 cm−1 is due to Si–C stretching [32]. The absorption band at 880 cm−1 is attributed to (SiH2)n bending [31]. The band at 1270 cm–1 belongs to the valence vibrations of Si–CH3. Presence of non-graphitic bonding of carbon is confirmed by C=C stretching peaks appearing in the 1445 cm−1 region.

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The band between 1570 and 1590 cm−1 is associated with the stretching vibrations of C=O bonds [9]. An absorption band has appeared at 2180 cm−1 which corresponds to Si–H and Si–

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H2 stretching. A broad spectrum due to C–H stretching, in particular sp3-CH and sp2-CH, has

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occurred at 2750–3100 cm−1 [33]. The substance class is alkanes and alkyl moieties. The sp3–

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CH stretching mode is located at a wave number of 2850 cm–1, and sp2–CH at 3000 cm–1.

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Fig. 6. FTIR transmittance spectra of the series of a-C:H:SiOx films deposited on silicon substrates at different Ar pressures (a) and negative pulse voltage amplitudes (b).

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Few general features in the spectra can be noted with changing pressure and bias and taken as important observations. It is clear that the bands corresponding to Si–O groups become more intense with an increase in Ar pressure and they become sharp (Fig. 6,а). The stretching vibrations of Si–C and C–H become more intense and pronounced. The intensity of the C–H bonding is an indication of the bonded hydrogen content in the films. With increasing of Ar pressure, the number of functional groups in the film increases, which may indicate a greater ionization ratio of PPMS molecules and are due to the increased ion bombardment. Thus the films became more interpenetrated, i.e. a more cross-linked structure.

ACCEPTED MANUSCRIPT With an increase in the negative pulse voltage amplitudes, a decrease in the intensity of the stretching vibrations of Si–O at 600 cm–1 and at 1070 cm–1 is observed (Fig. 6,b). There is the possibility that at low substrate bias PPMS is not completely decomposed and Si–O bonds are not sputtered off from surface by ions with low bombarding energy. At higher biases bombarding energy of the ions increases and more efficient dissociation of the PPMS precur-

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sor and reduction of the oxygen concentration in the film take place as confirmed by the FTIR results (noticeably lower intensities of Si–O bands). There is also a decrease in the content of

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Si–CH3 and C–H in the film and the formation of new Si–C bond which appears at ∼980 cm−1

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in FTIR spectra. As a result film hardness is increased. Decrease of hydrogen amount with increase of the ion beam energy in hydrogenated DLC films was already reported [34].

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For carbon-containing films, in particular for a-C:H:SiOx, the C–H absorption band is

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very important because it contains information on the number of sp3 and sp2 bonded carbon atoms that define the mechanical properties of these films. For this purpose, the Fourier spec-

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tra of obtained films have been studied in the wavenumber range 2840–3100 cm–1 in more

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detail (Fig. 7).

Fig. 7. FTIR absorption spectra of the series of a-C:H:SiOx films deposited on silicon substrates at different Ar pressures (a) and negative pulse voltage amplitudes (b) in the 2840– 3100 cm–1 wavenumbers range. This absorption spectrum is mainly formed by the valence bands of C–H2 (symmetric and asymmetric) and C–H3 (symmetric and asymmetric). The different types of sp3 and sp2

ACCEPTED MANUSCRIPT hybridized bonds corresponding to wavenumbers in the range of 2840–3100 cm–1 are presented in Table 2. Symmetrical C–H vibrations correspond to wavenumbers at 2850 cm–1 and 2875 cm–1, while wavenumbers at 2920 cm–1 and 2960 cm–1 correspond to asymmetric C–H vibrations. It is known that C–H vibrations at wavenumbers greater than 3000 cm–1 correspond to graphite-like bonds (sp2). It can be noted that FTIR absorption spectra in Fig. 7 begin

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to diverge at wavenumbers greater than 3000 cm–1 with an increase in both the Ar pressure and negative pulse voltage amplitudes. This indicates a decrease in sp2 bonded carbon in the

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film, which is consistent with the results of Raman spectroscopy.

3.5. Surface morphology study

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The surface morphology of the a-C:H:SiOx films is observed using AFM in semi-

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contact mode on single-crystal silicon substrates. As shown in Fig. 8 and 9, the film surface is smooth and uniform. This might be indicating that there was no macroparticles or pinholes on

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the surface of the film; i.e. it shows a good uniformity of the film.

Fig. 8. Typical three-dimensional AFM images of the a-C:H:SiOx films deposited at different Ar pressures. (a) at 0.025 Pa, (b) at 0.09 Pa, (c) at 0.2 Pa, (d) at 0.28 Pa.

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Very less surface roughness is more important for tribological performance which also influences the mechanical properties of the films. However, the surface quality varies with the pressure of Ar introduced (Fig. 8). The image reveals that film deposited at 0.025 Pa give the best surface quality, i.e. having least roughness. The root mean square roughness of the films,

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Rq, was evaluated from the 1×1 μm2 total image areas and is indicated in images. At higher argon pressure, it appears that film tends to form granules. An increase in PAr in a vacuum

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chamber leads to an increase in the roughness of the a-C:H:SiOx. During the process of film

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deposition, the ion bombardment of growing surface causes desorption of C or H atoms with creation of dangling bonds. These dangling bonds could interconnect and generate ordered

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clusters with high sp2 content in the surface layer [35]. These ordered clusters have structure

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closer to the thermodynamically stable graphite phase compared with typical amorphous diamond-like structure. Therefore such clusters form preferentially on the surface and lead to

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substantial roughening of the surface [36, 37]. The roughness increases because the sputtering

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by Ar ions increased with increasing Ar pressure, hence, the amount of dangling bonds and ordered clusters also increased. Besides, at the relatively high Ar pressures Ar ions will lose energy via collisions with gas molecules. Peng et al. [36] showed that there is a threshold val-

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ue of the ion energy, which determines the surface roughness of diamond-like carbon films. If

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the ion energy is about of 50 eV, the film surface is very rough on a nanoscale, while above it the roughness decreases significantly.

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Fig. 9. Typical three-dimensional AFM images of the a-C:H:SiOx films deposited at dif-

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ferent negative pulse voltage amplitudes. (a) at –100 V, (b) at –200 V, (c) at –300 V, (d) at – 650 V.

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The inverse relationship is observed with an increase in Ubias, where was a decrease in

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roughness (Fig. 9). The AFM image of a-C:H:SiOx film deposited at –100 V negative pulse voltage amplitude shows the Rq roughness value of 0.39 nm, which is more than Rq value of

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0.19 nm observed in film deposited at –650 V substrate bias. When the energy of ions that bombard the surface of the film, exceeds the critical value for atomic displacement, the ions

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could penetrate into the film and the ion energy will be dissipated into a relatively large vol-

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ume. Wherein the surface morphology remains relatively unaffected by the penetration of the ion, which leads to denser and more diamond-like film structure. Therefore, there is a transi-

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tion from a rough surface with many sp2-rich clusters at low ion energy to an atomically

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smooth surface at high energy of ions.

4. Conclusions The results indicate that the application of asymmetric bipolar pulsed substrate bias is a promising technology for the PACVD of a-C:H:SiOx films, and provides an alternative to conventional RF or DC PACVD processes. The bipolar pulsed plasma technology is attractive because it can be easily implemented in industrial systems. Furthermore, it offers new additional process parameters (pulse frequency, duration and amplitude of the positive and negative pulses) that can be varied in order to optimize the functional properties of the deposited

ACCEPTED MANUSCRIPT films for a given application. In this work a-C:H:SiOx films of thickness 1000 nm were deposited on dielectric substrates under different Ar pressure (0.025 to 0.28 Pa) and amplitude of negative pulse of bipolar substrate bias (–100 V to −650 V). It can be stated that based on the results of the investigation the variations of the mechanical properties of a-C:H:SiOx films depend both on the Ar pressure and amplitude of negative pulse of bipolar substrate bias. In par-

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ticular, increasing the Ar pressure during the formation of a-C:H:SiOx films leads to an increase in hardness from 3.3 to 9.8 GPa, the elastic modulus from 35.8 to 87.8 GPa and plastic

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deformation resistance from 27 to 122 MPa with practically unchanged endurance capability

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of 0.1 ± 0.01. The increase in the amplitude of the negative pulse of the bipolar bias voltage also provides a hardness growth from 3.3 to 13.6 GPa, the modulus of elasticity from 35.8 to

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131.6 GPa and plastic deformation resistance from 27 to 169 MPa with minimum changes in

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endurance capability of 0.1 ± 0.01. Taking into account the process of film re-sputtering during its growth, for practical applications the optimal parameters of deposition are PAr = 0.09

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Pa and Ubias = –500 V.

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The observed changes in the properties of the deposited films have been explained by the structural variations in the deposited films. DLC nature of the films is enhanced with the Ar pressure and substrate bias. Results from various characterizations show that, conversion

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of sp2 sites to sp3 sites have an important impact on the properties of the a-C:H:SiOx films.

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The FTIR spectra of the films show the existence of an a-C:H network and a-Si:O network which are connected to each other through Si–C bonds. The hydrogen contained in the film decreases with Ubias amplitude and leads to new Si–C bonding. Mechanical properties of synthesized a-C:H:SiOx films not only depend on the sp3 bonded carbon content but also on the chemical structure of functional groups, that are formed by Si atoms and the degree of dissociation of the organosilicon precursor. High dissociation rate of PPMS precursor was the main reason for the increased hardness of samples deposited at higher bias voltages.

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Acknowledgements This work was performed in terms of the State task of the Institute of High Current Electronics and within the framework of Tomsk Polytechnic University Competitiveness Enhancement Program. The authors are thankful to N.V. Ryabova for her help in the discussion

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of the obtained IR Fourier spectra. The authors are thankful to Tomsk Regional Center for Collective Use of the TSC SB RAS for the provided NanoTest 600 nanoindentator, AFM

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Solver P47 atomic force microscope and Nicolet 5700 IR Fourier spectrometer.

References

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[1] B. Mednikarov, G. Spasov, T. Babeva, J. Pirov, M. Sahatchieva, C. Popov, W. Kulisch, Optical properties of diamond-like carbon and nanocrystalline diamond films, J. Optoelectron. Adv. Mater. 7 (2005) 1407–1413. [2] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R Reports. 37 (2002) 129–281.

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D

[3] R.D. Mansano, R. Ruas, A.P. Mousinho, L.S. Zambom, T.J.A. Pinto, L.H. Amoedo, M. Massi, Use of diamond-like carbon with tungsten (W-DLC) films as biocompatible material, Surf. Coatings Technol. 202 (2008) 2813–2816. [4] K. Bewilogua, D. Hofmann, History of diamond-like carbon films - From first experiments to worldwide applications, Surf. Coatings Technol. 242 (2014) 214–225.

CE

[5] R. Paul, S.R. Bhattacharyya, R. Bhar, A.K. Pal, Modulation of residual stress in diamond like carbon films with incorporation of nanocrystalline gold, Appl. Surf. Sci. 257 (2011) 10451–10458.

AC

[6] J.C. Damasceno, S.S. Camargo Jr, Plasma deposition and characterization of silicon oxide-containing diamond-like carbon films obtained from CH4:SiH4:O2 gas mixtures, Thin Solid Films. 516 (2008) 1890–1897. [7] N. Kumar, S.A. Barve, S.S. Chopade, R. Kar, N. Chand, S. Dash, A.K. Tyagi, D.S. Patil, Scratch resistance and tribological properties of SiOx incorporated diamond-like carbon films deposited by r.f. plasma assisted chemical vapor deposition, Tribol. Int. 84 (2015) 124–131. [8] S.A. Barve, S.S. Chopade, R. Kar, N. Chand, M.N. Deo, A. Biswas, N.N. Patel, G.M. Rao, D.S. Patil, S. Sinha, SiOx containing diamond like carbon coatings: Effect of substrate bias during deposition, Diam. Relat. Mater. 71 (2017) 63–72.

ACCEPTED MANUSCRIPT [9] L.K. Randeniya, A. Bendavid, P.J. Martin, M.S. Amin, E.W. Preston, Molecular structure of SiOx-incorporated diamond-like carbon films; evidence for phase segregation, Diam. Relat. Mater. 18 (2009) 1167–1173. [10] D. Neerinck, Diamond-like nanocomposite coatings (a-C:H/a-Si:O) for tribological applications, Diam. Relat. Mater. 7 (1998) 468–471. [11] A.S. Grenadyorov, K.V. Oskomov, A.A. Solov’ev, S.V. Rabotkin, Deposition of Silicon–Carbon Coatings from the Plasma of a Non-Self-Sustained Arc Discharge with a Heated Cathode, Tech. Phys., 61(5) (2016) 690–695.

RI

PT

[12] A. Bendavid, P.J. Martin, C. Comte, E.W. Preston, A.J. Haq, F.S. Magdon Ismail, R.K. Singh, The mechanical and biocompatibility properties of DLC-Si films prepared by pulsed DC plasma activated chemical vapor deposition, Diam. Relat. Mater. 16 (2007) 1616–1622.

SC

[13] T.S. Santra, T.K. Bhattacharyya, P. Patel, F.G. Tseng, T.K. Barik, Structural and tribological properties of diamond-like nanocomposite thin films, Surf. Coatings Technol. 206 (2011) 228–233.

NU

[14] T.S. Santra, T.K. Bhattacharyya, F.G. Tseng, T.K. Barik, Influence of flow rate on different properties of diamond-like nanocomposite thin films grown by PECVD, AIP Adv. 2 (2012).

MA

[15] A. Jedrzejczak, L. Kolodziejczyk, W. Szymanski, I. Piwonski, M. Cichomski, A. Kisielewska, M. Dudek, D. Batory, Friction and wear of a-C:H:SiOx coatings in combination with AISI 316L and ZrO2 counterbodies, Tribol. Int. 112 (2017) 155–162.

PT E

D

[16] D. Batory, A. Jedrzejczak, W. Szymanski, P. Niedzielski, M. Fijalkowski, P. Louda, I. Kote-la, M. Hromadka, J.Musil, Mechanical characterization of a-C:H:SiOx coatings synthesized using radio-frequency plasma-assisted chemical vapor deposition method, Thin Solid Films 590 (2015) P. 299–305.

CE

[17] W.J. Yang, T. Sekino, K.B. Shim, K. Niihara, K.H. Auh, Microstructure and tribological properties of SiOx/DLC films grown by PECVD, Surf. Coatings Technol. 194 (2005) 128–135.

AC

[18] L.Y. Chen, F.C.N. Hong, Effects of SiOx-incorporation hydrocarbons on the tribological properties of DLC films, Diam. Relat. Mater. 10 (2001) 1058–1062. [19] V.O. Oskirko, N.S. Sochugov, A.P. Pavlov, A modular bipolar power supply for highpower ion-plasma installations // Instrum. Exp. Tech. 7(5) (2014) 594–600. [20] A.A. Solovyev, A. V. Shipilova, I. V. Ionov, A.N. Kovalchuk, S. V. Rabotkin, V.O. Oskirko, Magnetron-Sputtered YSZ and CGO Electrolytes for SOFC, J. Electron. Mater. 45 (2016) 3921–3928. [21] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology, J. Mater. Res. 19(1) (2004) P. 3–20. [22] B.B. Lopes, R.C.C. Rangel, C.A. Antonio, S.F. Durrant, N.C. Cruz, E.C. Rangel, Mechanical and tribological properties of plasma deposited a-C:H:Si:O films, in: J. Nemecek

ACCEPTED MANUSCRIPT (Ed.), Nanoindentation in Materials Science, InTech, 2012, pp. 179–202. DOI: 10.5772/50278. [23] A. Modabberasl, P. Kameli, M. Ranjbar, H. Salamati, R. Ashiri, Fabrication of DLC thin films with improved diamond-like carbon character by the application of external magnetic field, Carbon 94 (2015) 485–493. [24] S. Kumar, N. Dwivedi, C. Rauthan, Investigation of radio frequency plasma for the growth of diamond like carbon films, Phys. Plasmas 19(3) (2012) 033515.

PT

[25] S. Jana, S. Das, U. Gangopadhyay, A. Mondal, P. Ghosh, A clue to understand environmental influence on friction and wear of diamond-like nanocomposite thin film, Adv. Tribol. 2013 (2013) 352387.

SC

RI

[26] K. Honglertkongsakul, P.W. May, B. Paosawatyanyong, Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation, Diam. Relat. Mater. 19 (2010) 999–1002. [27] C. Casiraghi, A. C. Ferrari, J. Robertson, Raman spectroscopy of hydrogenated amorphous carbons, Phys. Rev. B 72 (2005) 085401.

NU

[28] J. Choi, K. Ishii, T. Kato, M. Kawaguchi, W. Lee, Structural and mechanical properties of DLC films prepared by bipolar PBII&D, Diam. Relat. Mater. 20 (2011) 845–848.

MA

[29] S. Nakao, J. Choi, T. Sonoda, S. Kotake, Y. Yamada, Characterization of diamond-like carbon films prepared using various source gases by plasma-based ion implantation and deposition, Surf. Coat. Technol. (2018) In press.

PT E

D

[30] A. Tamulevičienė, V. Kopustinskas, G. Niaura, Š. Meškinis, S. Tamulevičius, Multiwavelength Raman analysis of SiOx and N containing amorphous diamond like carbon films, Thin Solid Films. 581 (2015) 86–91.

CE

[31] S.J. Park, K.R. Lee, D.H. Ko, Tribochemical reaction of hydrogenated diamond-like carbon films: A clue to understand the environmental dependence, Tribol. Int. 37 (2004) 913–921.

AC

[32] M. Park, C.W. Teng, V. Sakhrani, M.B. McLaurin, R.M. Kolbas, R.C. Sanwald, R.J. Nemanich, J.J. Hren, J.J. Cuomo, Optical characterization of wide band gap amorphous semiconductors (a-Si:C:H): Effect of hydrogen dilution, J. Appl. Phys. 89 (2001) 1130– 1137. [33] X.Z. Ding, F.M. Zhang, X.H. Liu, P.W. Wang, W.G. Durrer, W.Y. Cheung, S.P. Wong, I.H. Wilson, Ion beam assisted deposition of diamond-like nanocomposite films in an acetylene atmosphere, Thin Solid Films. 346 (1999) 82–85. [34] Y. Miyagawa, H. Nakadate, M. Ikeyama, S. Nakao, S. Miyagawa, Dynamic MC simulation for a-C:H deposition in methane plasma based on subplantation model, Diam. Relat. Mater. 12 (2003) 927–930. [35] H. Dai, H. Jiang, C. Zhan, N. Huang, Effects of ratio of mixed gases on the surface roughness and nano-hardness of a-C:H films, J. Korean Phys. Soc. 52 (2008) 124–127. [36] X.L. Peng, Z.H. Barber, T.W. Clyne, Surface roughness of diamond-like carbon films prepared using various techniques, Surf. Coat. Technol. 138 (2001) 23–32.

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[37] Y.H. Liu, J. Li , D.P. Liu, T.C. Ma, G. Benstetter, Properties and deposition processes of a-C:H films from CH4/Ar dielectric barrier discharge plasmas, Surf. Coat. Technol. 200 (2006) 5819–5822.

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Ubias

ωD

ГD

ωG

ГG

(Pa)

(V)

(cm-1)

(cm-1)

(cm-1)

(cm-1)

1

0.025

-100

-

-

-

-

-

2

0.09

-100

-

-

-

-

-

3

0.2

-100

1379

266.2

1529

165.4

0.99

4

0.28

-100

1334

90.9

1506

197.8

0.88

5

0.025

-200

1372

148.9

1529

160.5

0.98

6

0.025

-300

1360

271.7

1509

181.5

0.96

7

0.025

-650

1355

187.9

0.94

289.3

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ID/IG

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Sample

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PAr

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Table I. Results of analysis of Raman spectra of a-C:H:SiOx films

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where PAr – Ar pressure, Ubias – negative pulse voltage amplitude, ω – peaks position, Г – full

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width at half maximum (FWHM), ID/IG – Intensity of D band/Intensity of G band.

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Wavenumber

Type of vibrations

(cm–1) 2850±10

symmetric

sp3 С-H3

2875±10

symmetric

sp3 С-H2

2920±10

sp3 С-H3

2960±10

sp2 С-H

3000±10

sp2 С-H2

3023±10

asymmetric asymmetric

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sp3 С-H2

symmetric

asymmetric

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pressure (a) and negative pulse voltage amplitude (b).

pressures (a) and negative pulse voltage amplitudes (b).

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Fig. 4. Values of H/E and H3/E2 ratios of a-C:H:SiOx films deposited using different Ar

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Fig. 5. Raman spectra of a-C:H:SiOx films formed at different Ar pressures (a) and negative pulse voltage amplitudes (b).

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Fig. 6. FTIR transmittance spectra of the series of a-C:H:SiOx films deposited on silicon

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substrates at different Ar pressures (a) and negative pulse voltage amplitudes (b). Fig. 7. FTIR absorption spectra of the series of a-C:H:SiOx films deposited on silicon substrates at different Ar pressures (a) and negative pulse voltage amplitudes (b) in the 2840–

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3100 cm–1 wavenumbers range.

Fig. 8. Typical three-dimensional AFM images of the a-C:H:SiOx films deposited at different Ar pressures. (a) at 0.025 Pa, (b) at 0.09 Pa, (c) at 0.2 Pa, (d) at 0.28 Pa.

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Fig. 9. Typical three-dimensional AFM images of the a-C:H:SiOx films deposited at dif-

650 V.

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ferent negative pulse voltage amplitudes. (a) at -100 V, (b) at -200 V, (c) at -300 V, (d) at -

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Fig. 9.

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a-C:H:SiOx films were deposited by plasma-chemical method with bipolar substrate bias. Effect of Ar pressure and substrate bias on mechanical properties of films was shown. DLC nature of the films is enhanced with the Ar pressure and substrate bias.

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  