1. Introduction

Nitride-based protective films have been extensively studied due to their excellent properties such as high hardness and resistance against wear, erosion, and corrosion. TiBCN is a promising nanocomposite film with good tribological properties, super high hardness, and high resistance to oxidation and corrosion ^[1-5]. It is synthesized by chemical vapor deposition (CVD) ^[1,2], cathodic arc plasma evaporation ^[3], thermal plasma CVD ^[4], and magnetron sputtering ^[5]. Depending on deposition conditions, the films of different microstructures, phase contents, morphologies, and grain sizes, can be produced. Multiphase coatings of TiBCN consist of TiN, TiC, TiB₂, TiB and amorphous BN, C and CN_x, etc. By taking advantage of the high hardness phase of TiB₂ and TiC, and the low friction coefficient phase BN and CN_x, it is possible to fabricate TiBCN coatings with excellent mechanical properties by adjusting the atomic concentration of Ti, B, C and N. Researchers

TiBCN films have been prepared on Si(100) substrate by reactive cathodic arc evaporation from the graphite/ boron-containing composite target and Ti target in N₂ and C₂H₂ atmosphere. The cathode current of the target influenced composition and structure of the TiBCN films ^[3]. Shimada et al. studied the composite TiBCN films deposited on WC-Co tools at 800^oC by varying the N₂ flow rate from 0 to 1000 mL/min and found that the N content in TiCN increased with the N₂ flow rate, but TiB₂ content decreased ^[4]. Lin et al. synthesized Ti-B-C-N nanocomposite films by sputtering compound target containing 80 mol.% TiB₂ and 20 mol.% TiC in Ar/N₂ plasma using a pulsed closed-field unbalanced magnetron sputtering system ^[5]. They found that when the N content was low (3 at.%), the coating was characterized as a mixture of nano-columnar grains of Ti(B,C) and small volume fractions of amorphous free carbon, BN and CN_x, while increasing the N content to 10at.%–13at.%, the N atoms would replace the B sites in Ti(B,C) leading to reduced Ti(B,C) phase and the formation of Ti(C,N), and an increase in the volume fraction of amorphous BN. When N content reached 36 at.%, the hard Ti(B,C) phase diminished completely, accompanied by the formation of a large volume fraction of amorphous BN.

Film compositions can be examined by several methods. X-ray photoelectron spectroscopy (XPS) is an efficient tool for determining the type of bonding states involving each element ^[6]. When the environment of the central atom becomes more electropositive, the peaks shift to lower energy; while it shifts to higher energy if the atom is bound to more electronegative neighbors ^[6]. XPS provides a relative amount of different phases, such as the carbon-containing phase as a function of the total carbon content ^[7]. It also provides depth profiles of elements by layer-etching (which takes much time and energy), but it is unable to provide accurate concentration of light elements, whereas they can be detected with Rutherford scattering spectroscopy (RBS), including non-elastic RBS, by using especially MeV protons to do nondestructive depth profiling with better sensitivity, smaller straggling and greater penetrability for light element detection.

In this work, TiBCN films were deposited by multi-cathodic arc ion plating at different flow rates of N₂. The samples were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic field microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), RBS and XPS, to observe the influence of N₂ flow rate.

2. Experimental section

All the samples were prepared by cathode ion plasma deposition, which are described in a previous report ^[8]. The Si (100) and cemented carbide substrates were ultrasonically cleaned by acetone, alcohol and de-ionized water. The sputtering targets were TiB₂ and metal Ti, with a target-sample distance of 100 mm.

The chamber was pumped to 1×10⁻³ Pa and the deposition substrate was kept at 300℃. The samples were prepared as follows. First, the substrates were cleaned for 30 min by Ar⁺ sputtering at a bias voltage of −800 V before Ti ion etching to smooth the substrate surface. Then, Ti interlayer was deposited to improve adhesion and reduce stress at the coating- substrate interface. Finally, TiBCN films were deposited on the Ti interlayer. The deposition parameters are listed in Table 1. To ascertain the film homogeneity, all the substrates were rotated at a speed of 4 rpm. We note that a low flux of Ar gas could be fed into the chamber to enhance the efficiency of glow discharge during the process of Ti-ion etching. Particularly, C₂H₂ gas was set at a stable flow rate of 150 sccm at N₂ flow rates of 40–350 sccm during deposition using the TiB₂ target, while the Ti cathode was turned off. Ar gas was fed in to maintain work pressure at 0.59 Pa for depositing TiBCN films. The samples were allowed to cool down to room temperature.

The samples were measured by X-ray diffraction (XRD, D8 ADVANCE) with Cu Kα ray (λ=0.1541 nm). XPS(, XSAM800 KRATOS) using Al Kα ray (1253.6 eV) was employed to investigate the samples’ binding energy and elemental composition. The microstructure was examined by cross-view high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). Field emission scanning electron microscopy (SEM, S4800) was used to measure the cross-section micrograph. The surface topography was probed on an atomic force microscope (AFM, Shimadzu SPM-9500J3) operated in the tapping mode with a probing area of 2 μm×2 μm. The samples were measured by RBS using 1.62 MeV proton beams, with a detection angle of 165° using a planar silicon detector with a resolution of 14 keV at FWHM.

3. Results and Discussion

XRD patterns of the TiBCN films grown on Si(100) substrate are shown in Fig.1. The main peak at 40.17° is attributed to hexagonal Ti(101) (JCPDS No. 5-0682) ^[9] from the interlayer. The peaks at 35.1° and 38.36° are from interlayer Ti(100) and (002) or from TiN_0.3(100) and (002) in the TiBCN films. The two groups of peaks, which are very close (0.1° apart) to each other, are attributed to Ti and TiN_0.3, respectively. The attribution to TiN is based on chemical bonding of N-Ti (397.1 eV) and TiN (455.8 eV). The grain size of Ti/TiN crystals is 24.5–32.7 nm, calculated by Scherer formula ^[10] using the peak (40.17°).

Fig. 1.Full-screen View

XRD patterns of TiBCN coatings deposited on Si (100) substrate at different N₂ flow rates.

Intensity of the main peak increased with the N₂ flow rate up to 250 sccm, where it began to decrease. Above N₂ flow rates of 250 sccm, the grain size was smaller. This can be attributed to the corporation of N₂ into the growing film, which leads to a lower surface mobility ^[11]. The peak at around 35° can also be ascribed to TiB₂/Ti(B,C) (100) ^[7] and supported by Ti(B,C) (188.4 eV) from the following XPS B 1s spectra.

Figure 2 shows RBS spectra for the films deposited at N₂ flow rate of 40–350 sccm, with the simulated spectra (solid line) by SIMNAR. The RBS results of Ti, B, C and N concentrations in the samples are given in Table 2. The Ti concentration of TiBCN films, which was separated by subtracting Ti interlayer, almost keeps at 11% with varying N₂ flow rates. The B and C contents decreased with increasing N₂ flow rate from 18.9% to 9.4% and from 65.6% to 48.9%, respectively, while the N content increased with the N₂ flow rate from 3.9% to 29.3%. The atomic stoichiometry and depth profile of the films were obtained by fitting of the RBS data using SIMNRA. In Fig. 2(a), a small peak at the left side is assigned to B from TiB₂ layer, and the ratio B: Ti is about 2:1. A broad nearby peak (left) of low intensity is attributed to Ti_xB_y with the ratio of Ti:B (x:y) from 9:1 to 8:2, which alludes that the boron atoms diffuse from TiB₂ layer into Ti interlayer, forming a gradient layer. The results are in accordance with the cross-sectional SEM images (Fig.3), which reveal the three layers of TiBCN, TiB₂-Ti_xB_y, and Ti interlayer. Actually, high RBS cross-section is only available between 1.62 MeV proton and Ti atom; while the other target atoms have just high non-Rutherford cross-sections, with low resolution of mass difference for B, C and N with respect to incident proton, resulting in serious overlapping signals of these light elements. However, identifiable and prolific light elements in the sample can induce drastic non-Rutherford scattering cross-sections by colliding with incident proton at ~ MeV, which combined with SIMNRA fitting give their relative concentrations.

Fig. 2.Full-screen View

RBS spectra of TiBCN films deposited at N₂ flow rates of 40–350 sccm.

Fig. 3.Full-screen View

Cross-section SEM of TiBCN films on Si (100) substrate deposited at N₂ flow rates of 150–350 sccm

Figure 3 shows the cross-section SEM image of the TiBCN films. Although the deposition pressure was kept at 0.59 Pa in deposition of all the films, the micro-particle density on the surface is different. The flux of material emitted by either explosion or evaporation is composed of ions, neutral atoms, electrons and micro-droplets of the material. In arc ion plating, metal ions and micro-droplets are the main species responsible for the mass loss of the cathode, and unavoidably the particles would be deposited on the substrates ^[12]. The number of holes and microparticles reduce with increasing nitrogen pressure up to 300 sccm, which could be attributed to the poison effect of TiB₂ targets surface. During arc evaporation, if successive reactive gas is present in the deposition chamber, then the cathode material can be poisoned by reactive gas resulting in formation TiN, which has a high melting point, covering the target surface. The surface became rougher and exhibited worse morphology with increasing nitrogen pressure up to 350 sccm. This could be attributed to the excessively reactive nitrogen gas weakening the ion bombardment ^[13]. Cross section SEM of TiBCN-150 demonstrates that there are three layers: Ti interlayer, TiB₂-Ti_xB_y, and TiBCN, which are confirmed by the RBS spectra and their fitting.

AFM images of the films are illustrated in Fig. 4. The root-mean-square (RMS) roughness decreases from 152.1 nm to 131.1 nm with increasing the N₂ flow rate, as summarized in Table 2. Instead of drastic jumps, there are some decreases of RMS roughness associated with larger N₂ fluxes, in agreement with nanocrystal size in the TiBCN films of XRD results.

Fig. 4.Full-screen View

AFM image of TiBCN films on Si (100) substrate by PVD at N₂ flow rates of 150–350 sccm.

The selective area diffraction (SAD) patterns of Ti interlayer and TiBCN films are shown in Fig. 5. The SAD pattern of Ti interlayer in Fig. 5(a) was indexed in terms of Ti hexagonal structure, which is consistent with the XRD results, showing the strongest Bragg reflection {101} and weak reflections of {100} and {200}. In the SAD of the TiBCN coating (Fig. 5b), a diffused halo was observed, indicating the amorphous properties of TiBCN films.

Fig. 5.Full-screen View

Selected area electron diffraction patterns of (a) Ti interlayer and (b) TiBCN coatings.

The high-resolution TEM images of TiBCN films are shown in Fig. 6. Clearly there are some nanocrystals embedded in the amorphous matrix, corresponding to TiB(211), Ti_0.68N_0.22B_0.1(102), TiB₂ (101), BN (012), and TiN (101) and (200) in Figs. 6(a)–6(d), respectively. The size of the particles is 2–10 nm and the distance between two nanoparticles is about 10 nm. Due to the limited content of nanocrystals embedded into a large fraction of the amorphous phase, it is difficult to find evidence by XRD, though it is clear in HRTEM.

Fig. 6.Full-screen View

High-resolution TEM images of TiBCN nanocomposite coatings deposited at N₂ flow rates of 250 sccm. (a), (b), (c) and (d) show different areas of TiBCN coatings and present different nanocrystal phases.

XPS spectra of the TiBCN films at different N₂ flow rates are shown in Fig.7. The XPS peaks were de-convoluted to provide further information about the films. Gaussian fitting was performed to evaluate the contribution of various factors in the spectra, and the fitted curves are shown in color lines. The Gaussian curves contributing to the fitting are represented by a dashed line. The B 1s spectrum of TiB₂ in Fig. 7(a) is ascribed to two peaks which correspond to TiB₂ (187.59 eV)^[14] and B₂O₃ (192.6 eV), respectively. The B 1s of TiBCN-40 shows three peaks at 188.4, 189.7 and 191.6 eV, corresponding to Ti (B,C), BN and BNO, respectively. The peak at 188.4 eV indicates that the environment around the boron atom in the TiB₂ structure is modified with increasing C content ^[15]. BN is amorphous in nature, as proved by XRD. In the Ti 2p spectrum of TiB₂ (Fig.7b), the peaks at around 454.3, 455.2, 457.6 and 458.7 eV are attributed to TiB₂^[14], TiO, Ti₂O₃^[16] and TiO₂^[17], respectively. In the Ti 2p spectrum of TiBCN-40, the peaks at 454.9, 455.8 and 458.9 eV are of TiC^[5,18], TiN^[19] andTiO₂. After the Gaussian fitting, C 1s line is decomposed into two peaks which arise due to sp² C-C (284.5 eV) ^[15,21] and sp³ C-C (285.4 eV) ^[22] (Fig 7c). In Fig.7(d), two peaks of N 1s spectrum of TiBCN-40 are ascribed to the amorphous N-C (399.3 eV)^[6] and Ti-N (397.5 eV)^[20]. The comparative study of B 1s, Ti 2p, C 1s and N 1s spectra for TiB₂ and TiBCN-40 films indicates that adding of C₂H₂ and N₂ influence the content of elements to a great extent, which leads to the sharp dropping of Ti and B content and increasing C and N contents. The films composition varied greatly, after adding N₂ and C₂H₂, the TiB₂ became Ti(B,C), TiN and TiC, forming amorphous phases, like BN, CN_x, sp² C-C and sp³ C-C.

Fig. 7.Full-screen View

XPS spectra of B 1s (a) and Ti 2p (b) from TiB₂ and TiBCN-40, and C 1s (c) and N 1s (d) from TiBCN-40.

Figure 8(a) shows the B 1s spectra at the N₂ flow rate of 40–350 sccm. The peak at 190.3 eV is BN^[5,23], and 189.2 eV is pure B^[14]. It can be seen that the B-N peak increases with the N₂ flow rate, while the B content decreases with increasing N₂ flow rate. The peaks at 284.5 and 285.4 eV in Fig. 8(b) are related to sp² C-C and sp³ C-C, respectively. Fig. 8(c) shows the N 1s spectra of TiBCN at the N₂ rate flow of 40–350 sccm. At the low N₂ flow rates, the two peaks can be assigned to N-C (399.3 eV) and Ti-N (397.5 eV). At higher flow rates of N₂, BN (398.2 eV)^[5,17] appears and N-Ti shifts to 397.1 eV. The B-N and N-C peaks increase with the flow rate of N₂, while TiN (397.1 eV) shows different trends. Ti 2p spectra at the N₂ flow rate of 40–350 sccm are shown in Fig.8(d). The peak at 456.7 eV, corresponding to Ti(C, N) ^[5,24], increases with the N₂ flow rate. The TiN position shifts to 455.6 eV ^[24]. The peak position at 458.5 eV is TiO₂ 2p_3/2, due to oxidation of the surface of the TiBCN layer.

Fig. 8.Full-screen View

XPS spectra of TiBCN prepared at N₂ flow rates of 40–350 sccm. (a) B 1s, (b) C 1s, (c) N 1s, (d) Ti 2p.

With increasing flow rate of N₂, the phases of TiN, Ti(C,N), amorphous carbon, amorphous BN and pure B changed differently. The content of sp³ C-C (285.4 eV) and TiN (397.1 eV) are plotted in Fig. 9, as a function of the N₂ flow rate. The sp³ content decreased to a minimum at 150 sccm, where it began to increase; whereas the TiN content increased to a maximum at 200 sccm and then decreased, being the same changing trend as that of TiN (455.6 eV) in Ti 2p spectra.

Fig. 9.Full-screen View

Contents of TiN and sp³ C-C evaluated from XPS measurement at the N₂flow rates of 40–350 sccm.

Amorphous phase BN, observed first at 150 sccm, increased with the N₂ flow rate; while pure boron, observed at 150 sccm, decreased with increasing N₂ flow rates.

It can be seen that the growth of TiBCN films is affected significantly by the gas flow rate. At 40 sccm, the Ti atoms mainly bond with B and C atoms, just a small fraction of them bond with N atoms. At 150 sccm, Ti atoms become to bond with N atoms, resulting in excess B being present in single-element phase and in less C-Ti bonds, which leads to free C atoms becoming hybridized. At maximum amount of TiN at 200 sccm, amorphous BN and CN_x are formed while pure B becomes less. With even more N₂ in the growing procedure, the large amount of C₂H₂ and N₂ gases in the chamber favors Ti atoms to bond with N atoms ^[4] and form TiN on the surfaces, but the collisions between C, N, B and Ti atoms never stop, thus, excess C and N atoms will collide with TiN crystallites on the surface and incorporate into TiN, resulting in Ti(C,N) at the expense of TiN.

There are two steps influencing sp³ content at different N₂ fluxes: (1) at low fluxes of N₂, Ti atoms bond with B and C atoms, and sp² C-C bonding is disordered due to incorporation of Ti and B, hence the large sp³ content; while at higher fluxes of N₂, Ti atoms become to bond with N atoms, then sp² C-C is recovered and sp³ C-C content decreases; (2) at fluxes over 200 sccm, there are less Ti atoms to bond with N atoms, excess N atoms will incorporate with sp² C-C, leading to increase of sp³ C-C containment.

4. Conclusion

TiBCN films deposited by multi-cathodic arc ion plating were characterized by RBS, XPS, XRD, SEM and AFM measurements. It was found that the TiBCN films consisted of nanocrystals including TiN, TiB₂, TiB, and Ti_0.68N_0.22B_0.1, with sizes less than 10 nm and amorphous matrix including BN, a-C, and CN_x. XRD showed main peaks from Ti interlayer and TiN_0.3 of the TiBCN films, with weak peaks from TiN_0.7C_0.3(200) and TiB(100). Cross-sectional SEM images showed the density of micro-particles on the surface decreasing with the N₂ flow rates. XPS showed bindings characteristic of B-N, Ti-N, Ti-O, B, sp² C-C, sp³ C-C, and N-C in TiBCN films. The TiN content increased first with the N₂ flow rates and then decreased, being favorable for the formation of Ti(C,N). The BN and CN_x contents increased at the expense of pure B content. The sp³ content decreased to a minimum and then increased slowly, indicating the bonding of N atoms with Ti atoms and excess N atoms, tending to form N-C bonding.