Research Article | Open Access
Structure and Friction Behavior of Cr/a-C:H Nanocomposite Films
CrN and Cr/a-C:H nanocomposite films were deposited on Si substrates by the magnetron sputtering technique. The structure, chemical state, and friction behavior of the Cr/a-C:H films prepared at various CH4 content were studied systematically. The CrN film shows strong (111) and (220) orientation, while the Cr/a-C:H films consist of the nanocrystalline Cr or Cr particles embedded in an amorphous hydrocarbon (a-C:H) matrix and show weak diffraction peaks, which is in accordance with the XPS analysis results. The typical Raman D and G peaks are observed, indicating that the separated amorphous carbon or C phase appears in the Cr/a-C:H films. However, no chromium carbide was observed in all the as-deposited samples. From the SEM graphs, all the deposited films depicted a dense and compact microstructure with well-attached interface with the substrate. The average friction coefficient of the Cr/a-C:H films largely decreased with increasing CH4 content.
Transition metal nitride films have found a wide technological use as hard, protective, and wear resistant coatings for cutting tools. Of transition metal nitrides, chromium nitride (CrN) films show a significantly higher adhesion to the substrate along with a high ductility, which is desirable for mechanical applications [1–5]. However, there are increasing numbers of applications where the mechanical and chemical properties of the binary nitrides are not sufficient. To further improve the tribological properties of CrN, alloying with another element to form a ternary hard coating has been explored [6–11]. Most frequently, a low friction coefficient is required, which helps to reduce friction losses and to increase load support capability. Nanocomposite coatings consisting of nanocrystalline ceramic or metal particles embedded in an amorphous hydrocarbon (a-C:H) matrix are able to combine high fracture toughness and wear resistance with a low coefficient [12, 13]. An example of these a-C:H based nanocomposite films is the TiC/a-C:H systems in which a correlation between coating mechanical properties and metal concentration has been made by Meng and Gillispie  and Pei et al. [14, 15].
Cr/a-C:H systems are, probably, similar to TiC/a-C:H systems, the most promising nitrides for protective coatings due to the formation of diamond-like structure, which can eventually decrease the friction coefficient. Cr/a-C:H system is a relatively less studied coating but with promise to improve tribological behavior compared to CrN. However, the structure, chemical state, and friction behavior of the Cr/a-C:H films were poorly studied systematically.
In this paper, we report the deposition and characterization of Cr/a-C:H nanocomposite films using medium frequency magnetron sputtering system at various CH4/N2 ratios. The friction coefficient of the nanocomposite films was also observed in conjunction with the detailed examinations of the microstructures.
2. Experimental Process
The CrN and Cr/a-C:H nanocomposite films were deposited on silicon (111) wafer using a medium frequency (20 kHz) magnetron sputtering technique. For the deposition system used in this study, a planar (280 mm × 80 mm × 8 mm) Cr target (99.8%) was mounted on the chamber wall.
The Si substrates were cleaned ultrasonically in acetone and deionized water and then dried under flowing gas. The sputtering chamber was evacuated to a pressure of 4.0 × 10−3 Pa with a turbomolecular pump before introducing sputtering gas. Then the Si substrates were sputtering-cleaned for 10 min at 1 Pa argon pressure with the substrate bias at −700 V. The 2-hour deposition process was carried out at a 40 sccm Ar flow rate and a 160 sccm N2 and CH4 mixture flow rate with a −100 V substrate bias. The power was kept about 1 kW. The substrate was positioned 50 mm away from the planar target. A series of Cr/a-C:H nanocomposite films with different Cr, C, and N ratios were prepared and the details were given in Table 1.
In this study, X-ray diffraction (XRD) was performed on Philips X’perts diffractometer at Cu-Kα wavelength. To obtain the diffraction peak profiles of the thin films along the vertical direction, grazing angle X-ray diffraction (GAXAD) with a grazing angle of 1° was applied for phase identification and qualitative texture characterization. SEM (JSM-5600Lv) was utilized to observe the microstructure and measure the film thickness. Raman spectroscopy was done using a JOBIN-IVON LabRam HR800 with a laser wavelength of 633 nm (He-Ne-Laser). The XPS measurements of the films were performed on a Perkin-Elmer PHI-5702 multifunctional X-ray photoelectron spectroscope, using Al-Kα radiation (photon energy 1476.6 eV) as the excitation source. The XPS spectra were collected in a constant analyzer energy mode, at a chamber pressure of 10−8 Torr and pass energy of 29.4 eV, with 0.125 eV/step. The energy calibration of the XPS is always using the contaminating carbon from the absorbed carbonous materials and the pump oil evaporation to sample surface. However, /a-C:H nanocomposite films cannot select contaminating C for energy calibration as the films contain carbon element. Therefore, we must adopt other means for energy calibrations. In this experiment, Au thin films about 2 nm thick were deposited on the tested surfaces for energy calibration of all the samples by thermal evaporation, and the binding energy of Au (Au 4f7/2 84.00 eV) was used as the charge-up correction.
Friction tests were performed using a micro-tribometer (UMT-2MT, CETR Co., USA). Reciprocating friction tests were performed using a 3 mm steel ball loaded under 1 N, reciprocated with a frequency of 100 rpm and amplitude of 5 mm, for up to 10 min. The tests were performed in ambient atmosphere with temperature of 18°C, humidity of 40%. The friction coefficient was determined by the average of continuously examined data.
3. Results and Discussions
Figure 1 shows the evolution on the XRD spectra of CrN and Cr/a-C:H films deposited under various CH4 contents. According to the index, CrN, Cr2N, and Cr peaks of principal diffraction in chart are compared with file PDF 76-2494, 35-0803, and 06-0694 (PCPDFWIN Version 2.3, JCPDS-ICDD (2002)). The films deposited without CH4 show strong peaks corresponding to CrN (111) and (220) (with 37.60° and 63.51°) orientation and weak peaks corresponding to Cr2N (111), (113), and Cr (110) (with 42.6° and 74.7° and 44.4°) (Figure 1 C1). This means that the film is mainly composed of CrN phase. Since the introduction of CH4 in the reactive gas atmosphere, the intensity of CrN diffraction peaks reduce dramatically and become wider (Figure 1 C2). This spectrum makes much less well-defined diffraction peaks and makes interpretation difficult. There are broad peaks in the spectrum at approximately 37.5° and 44.0°. The former could correspond to Cr2N (110) or CrN (111) and the latter is attributable to CrN (200) (with 43.7°), Cr2N (111), and/or Cr (110). Further increasing the CH4 content and reducing the N2 content, the diffraction peaks of the films (Figure 1 C3–C6) are still very weak, but a diffraction of weak Cr (110) peak is detected. The large content carbon in the Cr/a-C:H films may denitrify the CrN to Cr, and finally the nanometer Cr grain embedded in amorphous hydrocarbon (a-C:H) matrix.
Raman spectroscopy is widely used for the analysis of structural and phase disorder information in carbon and carbon related materials, at any wavelength, depending on several factors, such as disorder in bond length and in bond angle, clustering of the sp2 phase, presence of sp2 rings or chains, and sp2-to-sp3 ratio . In order to investigate the bonded structure of carbon in the Cr/a-C:H films, we have studied the Raman spectra of the Cr/a-C:H films to distinguish different bonding types of carbon. Figure 2 shows the Raman spectra of the CrN and Cr/a-C:H films produced at various CH4 contents. For the CrN films, no Raman peaks show in the wavenumber range of 1000–2000 cm−1. For the films in sample C2 prepared at 10% CH4 content, no obvious Raman peaks are shown in the wavenumber range of 1000–2000 cm−1, and this reflects a much lower amount of graphite-like or diamond-like bonds in the films. However, the observable D peaks at ~1350 cm−1 appeared in sample C3, with further increase of the CH4 content to 30% (sample C4) in the reactive gas; the intensity of G peaks at 1580 cm−1 are enhanced, indicating that the large content separated amorphous carbon phase appear in the films. By comparing Raman spectra of Cr/a-C:H films deposited at different CH4 content, the structure of these films can be distinguished. The relative intensity of the D-band to the G-band becomes noticeable with the increase of CH4 content. In a word, the “D” peak is mainly existent in Raman spectra when CH4 flow rate is low (samples C2 and C3), consequently implying the presence of sp2 rings. The “G” peak arises when the CH4 content reaches around 30%, and the peak value is gradually enhanced with increasing the reaction gas discharge. At the same time, decreases with increasing sp3 content. The sample C6 (50% CH4 content) possessed the lowest ratio, corresponding to the highest sp3 bond content. The conversion of sp2 bonds to sp3 bonds of the films is further confirmed from XPS analysis, as discussed later.
XPS analysis of the deposited films was performed to further investigate the chemical bonding features of carbon and nitrogen sites in the deposited films. And the typical XPS spectra of C 1s and N 1s of the CrN and Cr/a-C:H films deposited at different CH4 content s are shown in Figures 3 and 4.
The XPS C 1s peaks of the as-deposited films are shown in Figure 3. It can be seen that the C 1s spectra show an asymmetric and broad peak, and shift shifts towards higher binding energy with increase of CH4 content from 0 to 50%. The asymmetric broad feature of the C 1s spectra indicates the existence of different type of bonds in the deposited films. The dashed lines in Figure 3 show the C 1s binding energy position of different binding state of carbon; the peaks at binding energy of 282.8 eV  reflect the binding states of carbon in chromium carbide. From the XPS C 1s peaks, it is found that the Cr atoms are not bonded obviously with C atoms in the Cr/a-C:H films. This result is also in accord with the XRD result without the observation of any chromium carbide diffraction peaks. The binding energy at about 288 eV can be confidentially assigned to C–O species probably adsorbed on the surface of the samples, being the relative intensity very low. The C 1s spectra also could be fitted with three components at approximately 284.5, 285.5, and 286.5 eV, respectively. By comparing these peaks with referenced DLC films, the peaks at binding energy of 284.5 and 285.6 eV reflect the two binding states of sp2 carbon and sp3 carbon atoms, respectively . The peak centered at 284.5 eV is also assigned to sp2 bonding amorphous carbon phase and the surface contaminants . The shifts of binding energy to higher values with CH4 content increase have significantly been affected by the conversion of sp2 site to sp3 site C. This result is also in accordance with the result of the Raman analysis. The C 1s peaks are also deconvoluted to about 286.5 eV, which are assigned to bond of C–N . This suggested that the carbon in the film might be bonded with the nitrogen and will be further discussed below.
The N 1s spectra of various CH4 contents CrN and Cr/a-C:H films are shown in Figure 4. For the sample C1 (Figure 4 C1), the binding energy at 396.2 eV was due to CrN and higher binding energy at 397.4 eV was identified with Cr2N bond nitrogen (agreeing with the published binding energy values [20, 21]) with clear predominance of CrN phase in the film. These results mean that CrN and Cr2N coexist in the sample C1 as the composition calculation result and XRD result. For the samples prepared at mixed CH4 and N2 gas atmosphere, the N 1s peak could be reconstructed by three peaks at 400.2 eV, 398.6 eV, and 396.2 eV. The binding energy of 398.6 eV and 400.2 eV is attributed to nitrogen atoms bonded sp3C (sp3C–N) and sp2C (sp2C–N) atoms , but the difference will not be discussed here. We consider these two binding energies as C–N bonding state. The binding energy of N 1s shifts towards higher value, and the peaks become more asymmetric and wider with a tail at high energy regions when the CH4 content increases. It can be seen that the fraction of the Cr–N bond corresponding to the peaks at 396.2 eV decreases due to the shift of the N 1s binding energy to higher energy with increasing CH4 content, which indicated that the peak at 396.2 eV becomes weaker and finally disappears in N 1s spectra for samples C5 and C6. This is corresponding to the denitridation of CrN to Cr in the relative higher CH4 content atmosphere as the XRD results. The fraction of the peak of C–N bond increases with increase of CH4 content, which suggests that the fraction of N bonded to carbon atoms increases with the CH4 content and became dominant at higher CH4 content.
Figure 5 shows the fracture cross-sectional SEM micrographs of the CrN and Cr/a-C:H films. As shown in figures, the deposited Cr/a-C:H films possess a dense and compact microstructure with well-attached interface to the substrate. For the CrN film deposited at 0% CH4 content, dense consecutive columnar grains with diameter of about 100 nm are observed (Figure 5(a)). The columnar structures are almost invisible in the cross-section layer of the film obtained at 10% CH4 content so that the fractured cross-section looks nearly featureless (Figure 5(b)). The layers of the Cr/a-C:H films fabricated at 30% and 50% CH4 content are also without columnar structure (Figures 5(c) and 5(d)). Instead, some shallow dimples are observed on the fractured cross-sections, implying that plastic deformation occurred during the fracture of the film. These denser without super structure cross-section images of the Cr/a-C:H films are corresponding to the poor crystallization as confirmed by the XRD results above.
Figure 6 shows the friction coefficients of the CrN and Cr/a-C:H films sliding against a steel ball. The average friction coefficient of the films largely decrease from 0.825 to 0.45 with increasing CH4 content. The decrease in friction coefficient with the increase of CH4 content could be caused by the amorphous C:H or C phase and the formation of tribochemical reaction in the wear process, which often takes place in many carbon-contained ceramics, for example, C decomposed in the tribology process producing graphite tribolayer. These products of graphite are known to function as a self-lubricating layer. The formation of tribolayer would be more promoted with larger C content Cr/a-C:H films. The increase of the self-lubricating layer resulted in the decrease of the friction coefficient in these film systems.
CrN and Cr/a-C:H films were deposited on Si substrates by the magnetron sputtering technique. The composition of films can be controlled in large ranges through varying CH4 content. The structure, chemical state, and friction behavior of the films were studied systematically. The CrN film showed strong (111) and (220) orientation with well crystallization. Cr/a-C:H films are consisted of the nanocrystalline Cr or Cr particles embedded in an amorphous hydrocarbon (a-C:H) matrix with poor crystallization. The large content amorphous carbon or C may prevent the CrN crystallization and denitrify the Cr to Cr. However, no chromium carbide diffraction peaks were observed in all the as-deposited samples. The sp3-C content in films increase with the CH4 content increase, and the carbon in the films is bonded with the nitrogen in large CH4 content. The deposited films depict a dense and compact microstructure with well-attached interface with the substrate. The /a-C:H films show smooth without super structure cross-section images corresponding to the poor crystallization. The average friction coefficient of the Cr/a-C:H films largely decreased with an increase in C content. This behavior can be attributed to the tribochemical decomposition of a-C:H or C in the tribology processes, which enables formation of graphite tribolayer, playing a role as self-lubricant. In addition, studies based on structure and friction behavior of /a-C:H nanocomposite films, we will extend the research scope of such nanocomposite films are considered as potential protective surfaces on precise components or cutting tools in the future work.
Conflict of Interests
The authors declare that there is no conflict of interests including direct or indirect financial relations with any of the trademarks and companies mentioned in this paper.
The authors are grateful to National Science Foundation of China (Grant no. 51305433 and no. 11104126) for financial support.
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