2.1 Coatings deposition

Figure 1 shows the home-made direct current hybrid beams system used in this study, which is equipped with two Cr targets (purity, 99.99%) and one HCIS on the chamber wall. Both of the Cr targets and the HCIS face the substrates at an angle of 45°. The distance between the targets (or HCIS) and the substrates is approximately 20 cm.

Fig. 1.Full-screen View

Schematic diagram of the hybrid beams system.

The nc-CrC/a-C:H coatings were synthesized on tungsten carbides, stainless steel (austenitic 304) and silicon (100) wafers substrates at C₂ H₂, H₂, and Ar ambient under various H₂ flow rates. All the substrates were cleaned in acetone by the ultrasonic for 10 min, rinsed by deionized water and dried in a constant temperature cabinet of 60 ℃. Then the substrates were placed sideways on a rotating sample holder, which is located in the center of the chamber. Before the coating deposition, the experimental chamber was pumped down to the pressure of 7×10^-3 Pa and the chamber ambient was heated up to 250 ℃ by an accessional heater. The temperature of the experimental chamber was constantly monitored by a thermocouple located in the top center of the chamber. A high direct current bias voltage of -900 V was applied to the experimental substrates and the Cr target current was set as 80 A to offer high energy ions to clean the substrates. The deposition conditions of coatings are shown in Table 1. The C₂ H₂ and H₂ gases were introduced through the HCIS and ionized by the HCIS plasma. For all coating deposition processes, the substrates rotation speed was set at 4 rpm and the value of accessional heater was maintained at 250 ℃, while the pressure of the chamber was kept at 0.5 Pa by controlling the Ar flow rate. The H₂ flow rates were different for each deposition: 20, 40, 80, 120, and 150 sccm.

2.2 Characterization of coatings

The surface and cross-sectional morphologies of coatings were observed by a scanning electronic microscopy (SEM, FEI Sirion IMP). The surface topographies were probed by an atomic force microscopy (AFM, Shimadzu SPM-9500J3) employed in the tapping mode with a measuring area of 5×5 um². The chemical bonding states were investigated using an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) using Al K_α (1486.6 eV) X-ray radiation. A Laser Confocal Raman Micro-spectroscopy (Renishaw RM 1000) with Ar+ laser excitation (514.5 nm) was used to determine the content of the sp³ hybrid bond and G, D peak position. The microstructure was analyzed by a high-resolution transmission electron microscope (HRTEM, JEM-2100 HR) operated at 200 kV and a grazing incidence X-ray diffraction (GIXRD, D8 ADVANCE, and DAVINCI DESIGN) performed on an X-ray diffractometer with a Cu Kα radiation (0.154 nm) at a grazing incidence angle of 3°.

The hardness and elastic modulus were measured with a nano-indenter (MT-G200) equipped with a Berkovich diamond tip. The indenter load increased at constant displacement rate, until a depth of 100 nm was achieved. This maximum indentation penetration depth was only less than 1/10th of the coating thickness to minimize the influence of substrate on the measure results ^[21]. Furthermore, the scratch tests were operated by a HH-3000 tester in a continuous load of 0-50 N. The loading rate and moving speed were kept at 50 N/min and 4 mm/min, respectively. The tribological performance was evaluated at room temperature in ambient air (23℃, relative humidity of 30%) during dry sliding experiments conducted with a conventional ball-on-disc wear tester (MS-T3000). Here, the Si₃ N₄ ball (99.9%, purity) with the diameter of 3 mm was used as mated materials, and 2-3 measurements were made for each sample. A load of 500 gf and a sliding speed of 100 rounds per minute for 30 min were carried out. The radius of the friction trajectory was set as 3 mm. The images and corresponding chemical composition of wear tracks were investigated by an optical microscope (Zeiss Axio Lab. A1) and a EDAX genesis 7000 EDS, respectively. The wear rates of coatings were calculated by measuring the wear tracks using a 3D optical profilometer (Contour GT-K, BRUKER).

3.1 Surface and cross-sectional morphologies

Figure 2 displays the surface images of the as-deposited coatings synthesized at varying H₂ flow rates. The surfaces of the coatings contain obvious micro-droplets, which are characteristic of cathodic arc ion-plating. As the H₂ flow rate varies in the range of 20 to 150 sccm, the coating synthesized at the H₂ flow rate of 80 sccm exhibits the smoothest surface morphology with low distribution density and small size of micro-droplets, which can be attributed to the appropriate level of hydrogen ions etching ^[22]. The 3D AFM morphologies of the coatings are given in Fig. 3, and the variation of surface roughness measured by AFM is shown in Fig. 4. It is noted that the square average roughness (Rms) reduces from 63.8 to 16.7 nm and the arithmetic mean roughness (Ra) decreases from 48 to 13.2 nm when H₂ flow rate increases from 20 to 80 sccm. The Rms and Ra of the as-deposited coatings increase gradiently with further H₂ flow rate. The result is consistent with the SEM ones. It can be considered that the optimal H₂ flow rate smoothes the surface, but the surface roughness increases drastically with a large or less degree of H₂ flow rate. The representative cross-sectional image of the coating deposited at the H₂ flow rate of 40 sccm is displayed in Fig. 5, in which the coating exhibits a very dense structure and homogeneous morphology. In particular, the thickness of the nc-CrC/a-C:H coatings decreases from 1.88 to 1.38 um as the H₂ flow rate increases from 20 to 150 sccm.

Fig. 2.Full-screen View

Surface SEM images of the coatings.

Fig. 3.Full-screen View

(Color online) AFM images of the as-deposited coatings synthesized at different H₂ flow rates: (a) 20 sccm, (b) 40 sccm, (c) 80 sccm, (d) 120 sccm, and (e) 150 sccm.

Fig. 4.Full-screen View

Rms and Ra of the coatings as a function of H₂ flow rates.

Fig. 5.Full-screen View

Coating thickness with the representative cross-sectional SEM image.

3.2 Microstructure

Figure 6 exhibits the XPS C 1s and Cr 2p spectra of the coating synthesized at the H₂ flow rate of 80 sccm. As seen in Fig. 6(a), the analysis of the C 1s spectra was operated by resolving into five Gaussian peaks. The two peaks at the binding energy around 284.2 and 284.7 eV correspond to sp²-C and sp³-C bonding, which confirms the presence of amorphous carbon phase in coatings ^[4]. The peak at 283.2 eV corresponds to the carbon phase in chromium carbide and the carbon peaks at 286.4 and 288.6 eV can be attributed to adsorbed oxygen ^[23,24]. The Cr 2p^3/2spectra were fitted into three peaks ascribed to Cr-C (574.4 eV), Cr-O (Cr⁶⁺, 575.2 eV), and Cr-O (Cr³⁺, 576.7 eV) bonding states ^[25,26].

Fig. 6.Full-screen View

Gaussian fitting of (a) C 1s and (b) Cr 2p for the coatings deposited at the H₂ flow rate of 80 sccm.

In addition to the XPS analysis, Raman spectroscopy can promote a result of the more accurate data and in-depth research on account of the sensitivity of this tool to the diversified structures of carbon. As shown in Fig. 7(a), the presence of amorphous carbon in our coatings is definitely confirmed by the Raman result, which exhibits the characteristic peaks (D and G peaks) of a-C:H coatings ^[27]. In order to perform a more detailed analysis, Raman spectra of the coatings were deconvoluted into four peaks (D1, D2, G1, G2 peaks) using Gaussian function to calculate the peak positions and the ratio of peak areas [I_D/I_G=I(D1+D2)/I(G1+G2)] presented in the different as-deposited coatings ^[12,24]. Fig. 7(b) presents the curves-fitted Raman data. The sp³/sp² ratio can be characterized by the I_D/I_G ratio, and the increasing sp³ fraction in the amorphous carbon coatings will decrease the I_D/I_G ratio ^[28,29]. As the H₂ flow rate increases from 20 to 80 sccm, the I_D/I_G decreases continuously from 1.16 to 0.92 with the simultaneous decrease of the G1 peak position. With further H₂ flow rate, both of the I_D/I_G and G1 peak positions tend to increase. The Raman result shows that an optimal H₂ flow rate leads to an increasing formation of a sp³ phase and generates a transitional formation from the graphite-like carbon to the amorphous carbon in the a-C:H coatings ^[30].

Fig. 7.Full-screen View

(Color online) (a) Raman spectra and (b) I_D/I_G ratio and G1 peak position of the coatings.

The GIXRD diffractions measured in stainless steel samples reveal the structural evolution in the nc-CrC/a-C:H coatings, and the results are displayed in Fig. 8. For all coatings, the diffraction peaks can be assigned to Cr₃ C₂ phases via referring to the JCPDS database card No.35-0804 ^[12,13]. In particular, the signals of chromium carbide phases become feeble as the H₂ flow rate increases from 40 to 150 sccm, indicating that increased hydrogen dilution suppresses the formation of chromium carbide nanocrystals. For a more in-depth understanding of the microstructure, the cross-sectional HRTEM of the coating synthesized on a Si substrate at the H₂ flow rate of 80 sccm has been completed. The nanocomposite structure consisting of the uniform nanocrystals and amorphous carbon matrix can be clearly observed in Fig. 9, in which the sizes of nanocrystals are about 4-6 nm. Furthermore, the nanocrystalline phases can be identified as Cr₃ C₂ phases by the corresponding SAED pattern where four major reflections can be observed.

Fig. 8.Full-screen View

(Color online) GIXRD patterns of the coatings.

Fig. 9.Full-screen View

Cross-sectional HRTEM image of the coating deposited at the H₂ flow rate of 80 sccm.

3.3 Mechanical and tribological properties

Mechanical performance of the as-deposited coatings intrinsically depends on the bonding states and chromium carbide phases. A higher ratio of sp³/sp², optimal chromium carbides structure can preferentially form a three-dimensional, interconnected structure to increase the hardness and elastic modulus of coatings ^[3]. Fig. 10 exhibits the variation of the hardness and elastic modulus as a function of H₂ flow rate. It is noted that the coating deposited at the H₂ flow rate of 40 sccm has the highest hardness and elastic modulus of 28.1 GPa and 534.2 GPa, respectively, while the coating synthesized at the H₂ flow rate of 150 sccm owns the lowest hardness as well as elastic modulus of only 19.1 GPa and 378.5 GPa, respectively. The variation of hardness and elastic modulus can be attributed to the comprehensive influence between sp³ carbon and chromium carbide nanocrystals. In particular, the suppressed formation of hard nanocrystals is possibly the dominant reason for the decrease on hardness and elastic modulus, since the decreased chromium carbide nanocrystals cause less interfaces or increscent mean grain separation ^[31].

Fig. 10.Full-screen View

Hardness and friction coefficient of the coatings.

The scratch tests of the as-deposited coatings on the cemented carbide substrates were operated to characterize the critical load. In the adhesion test, damage and cracks first appeared and progressed in the amorphous phase, which led to the coating spallation and exposed the base material, thus causing an increasing friction coefficient ^[32]. The critical loading (L_C) was measured by the acoustic emission signal and the inflection point in the frictional force curves. The load at which the first crack occurred was referred to the lower critical load L_C1 and the load corresponding to coating delamination was called the higher critical load L_C2^[33]. As shown in Fig. 11, the coating shows the maximal critical load, L_C1, of 12.5 N and L_C2 of 21 N at the H₂ flow rate of 80 sccm. Actually, during the indenter sliding, the adhesion failures are significantly influenced by the shear stresses occurring in the interface induced by the normal applied load and the tangential load generated. The increased H₂ dilution leads to a significant reduction of the compression residual stress that will retard the failure occurrence, thus giving a rise to the critical load, L_C^[34]. However, the adhesion strength is lower for the higher H₂ flow rate and, in this work, that can be attributed to the decrease of coating thickness ^[35].

Fig. 11.Full-screen View

(Color online) Critical loads of scratch test with the inserted optical microscope image of coating failure in the scratch test.

The influence of H₂ flow rate on the friction and wear performances of the as-deposited coatings was examined by a ball-on-disk tribometer. Figure 12 displays the friction coefficient and wear rate of the coatings. It is clearly seen that the friction coefficient of coatings shows a decreasing tendency followed by a subsequent increase as the H₂ flow rate increases from 20 to 150 sccm and minimizes at 80 sccm at about 0.16. Meanwhile, the specific wear rate Ws exhibits the smallest value of 4.17×10^-6 mm³/N·m when the H₂ flow rate reaches 80 sccm. The variations of friction coefficient and wear rate can be due to the hydrogen dilution significantly affecting the tribological performances by dominating the sp³/sp²ratio, or the roughness ^[36]. The measured wear tracks with EDS analysis are displayed in Fig. 13, which presents the tribological performance of the coatings. For the EDS analysis, the change of chemical content along with the axial direction of wear tracks agrees with the wear rate behavior. Furthermore, the wear microgrooves observed on the wear tracks are most likely caused by trapped wear debris particles during sliding motion. A notable smooth surface is exhibited on the wear track displayed in Fig. 13(c), which suggests the degree of abrasive wear becomes less dominant and the asperities are polished.

Fig. 12.Full-screen View

Friction curves of the coatings sliding against Si₃ N₄ balls in ambient air.

Fig. 13.Full-screen View

(Color online) Optical images with corresponding EDS analysis of wear tracks.a 20 sccm, b 40 sccm,c 80 sccm, d 120 sccm, e 150 sccm