The Utilization of Carbon Dioxide to Prepare TiC x O y Films with Low Friction and High Anti-Corrosion Properties

: Recycling carbon dioxide (CO 2 ) for weakening the greenhouse e ﬀ ect is still an outstanding question. Although many chemical methods have been designed for CO 2 conversion, they is still a need to develop new ways for CO 2 recycling. Plasma methods were employed to convert CO 2 into energy molecules, with the addition of H 2 , H 2 O and so on. Non heavy elements, like Ti, Cr, Si and Mo and so forth, were employed to take part in a reactive process, which might be very interesting for special scientiﬁc interest. In this work, magnetron sputtering method was used not only for igniting the plasma but also for providing Ti elements involved in reactions, via the selected Ti target. One can conﬁrm that the TiC x O y ﬁlms were successfully grew via sputtering a Ti target in CO 2 atmosphere with Ar as dilute gas, which proved that CO 2 is a key player in the matter of the involvement of excited CO 2 + , CO + , CO 3 − and so on, in the growth process reacting with Ti ions. The TiC x O y ﬁlms exhibit the highest hardness (20.3 GPa), lowest friction coe ﬃ cient (0.065) and the best corrosion resistance. The growth of the TiC x O y ﬁlms are not only a new strategy for consuming CO 2 but also a good way for reusing it for preparing TiC x O y ﬁlms with high hardness for anti-corrosion and reducing friction. Moreover, reducing CO 2 emissions via energy saving (through reducing friction and corrosion resistance) and recycling existing CO 2 are both important for mitigating the greenhouse e ﬀ ect. Z.M. and B.Z.; formal analysis, Z.M. and B.Z.; investigation, K.G.; resources, Z.W. and Q.J.; data curation, Z.W.; writing—original draft preparation, K.G. and Z.W.; writing—review & editing, K.G. and B.Z.; supervision, J.Z.; project administration, B.Z.; funding acquisition, J.Z. All authors have


Introduction
In the atmosphere, carbon dioxide (CO 2 ) acts rather like a one-way mirror in the roof of a greenhouse, which allows sunlight to enter but prevents heat from escaping. CO 2 is emitted in a number of ways such as burning of oil, coal, gas, petrol and deforestation and so forth [1,2]. The more CO 2 accumulates, the higher temperatures rise. To solve global warming problems and to recycle CO 2 as a resource, there is a crying need for methods to capture, storage and recycle of CO 2 [3][4][5][6]. At present, there are lots of methods for carbon dioxide capturing and conversion, such as electrocatalytic reduction by heterogeneous materials, photocatalytic conversion of CO 2 to CH 4 [6], hydrogen reduction by introduction of N 2 , H 2 and H 2 O will introduce newborn charged functional groups, which highly depend on the incoming gases in the reaction system [17]. One should notice that all the mentioned plasma conversion processes are related to gas resource, a solid material was not employed, which is probably because a gas resource, like N 2 , H 2 and H 2 O, benefit the synthesis of energy molecules or related chemical intermediates. However, no energy molecule is fond of atoms that compose solid elements in nature. Fascinatingly, one can guess what will happen if we bring solid elements into a plasma reactive process, like Ti, Cr, Si, Mo and so forth.
It is worth noting that reactive magnetron sputtering technology is a useful way to grow low friction solid films, such as TiN x , CrN x , TiC x and CrC x films and diamond-like carbon films as well as fullerene-like hydrogen carbon films [18][19][20][21][22][23][24][25][26], which show not only low friction or even superlubricity but also high anti-corrosive properties via magnetron sputtering method [27][28][29][30][31]. Further, the so-called metallic oxycarbides MeO x C y , produced by magnetron sputtering, have attracted the interest of materials scientists. A virtual certainty is that the introduction of oxygen allows the tailoring of "pure" metal carbides such as the band-gap, bandwidth, electronic and mechanic and friction properties [32][33][34]. Bringing oxygen into metal carbides films has a strong influence on the films' structure. A.C. Fernandes et al. studied the influence of the O/C ratio on the structure of TiC x O y films and they drew a phase diagram which can be divided into 3 different regimes. A carbide zone (I), a transition zone (II) and a oxide (III), corresponding to the crystal structure of TiC, poorly crystallized fcc TiC and TiO phases mixed into an amorphous matrix and an amorphous structure, respectively [32,33]. As a matter of fact, most of the work on MeO x C y films are connected to optic and electronic properties, while very few are related to mechanic and friction properties [32][33][34]. M.T. Mathew et al. investigated the friction, corrosion and tribocorrosion properties of TiC x O y films, both in artificial sweat solutions and bio-fluids [35,36]. Their results showed that the corrosion behavior of TiC x O y films is more correlated to bulk inner-structure of the films [35,36]. However, those referred TiC x O y films are all obtained via magnetron sputtering in an oxide atmosphere, using the carbon target as a carbon source and the tribology of TiC x O y films in moist air or pure water has not been reported before.
As we know, employing CO 2 as a feeding gas to grow oxycarbide films not only solidifies CO 2 but also reduces CO 2 via energy saving from reducing friction. In consideration of the above properties, the CO 2 is selected as feed gas to deposit TiC x O y films at the first time in present work. A series of films with different components were prepared on silicon wafers by changing the sputtering current of Ti target. The composition and structure of the as-obtained films were analyzed by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The mechanical, frictional and anticorrosive properties were also investigated by Nano Indenter, Tribometer and electrochemical workstation, respectively. Furthermore, the reasons for the high hardness, low friction and good corrosion resistance of the as-obtained TiC x O y films are proposed based on the structure and property characterization results. This work points out a new strategy for the conversion and utilization of CO 2 via growth films for anti-wear and anti-corrosion purpose.

Films Preparation
The films were deposited on Si wafers (100) by a magnetron sputtering system. Titanium was used as the sputtering target. The gas of CO 2 was used as a feed gas. The possible reactions pathway that favor CO 2 conversion, using Ar as a dilution gas, are shown in Equations (1)-(3) [37].
Thus, these charged ions will react with metal, like Ti, to grow TiC x O y films possibly. Prior to depositing the films, the substrates (Si wafers) were ultrasonically cleaned in ethanol for about 30 min and were rapidly transferred into the vacuum chamber. Then the oxidation layer of Si substrates was removed by Ar + (the negative bias of 900 V, pulsed frequency of 60 kHz and duty cycle of 0.6). The detailed deposition parameters for the TiC x O y films are presented in Table 1 and the samples are marked as T1-T5 corresponding to the settled current, respectively.

Characterization Methods
The Nano-indenter DCM system, XRD (Bruker D8Discover25, AXS, Karlsruhe, Germany), XPS (Al-Ka radiation), SEM (JSM-6701F, JEOL, Tokyo, Japan) equipped with Energy dispersive spectrometer (EDS, JSM-5601LV, JEOL), electrochemical workstation (µ-AutolabIII, Metrohm, Herisau, Switzerland) were employed to characterize the composition, structure and properties of the as-obtained films. During the electrochemical tests, Ag/AgCl and Pt were respectively used as reference and counter electrodes. The electrochemical tests were performed in a 3.5 wt.% NaCl solution at room temperature and the tested area was 0.196 cm 2 .

Friction Test
The reciprocatively ball-on-disc tribometer (MFT-R4000) was employed to measure the friction properties of the films. The ceramic balls (Al 2 O 3 , Φ5 mm) were used as the friction couples. The friction process was tested in a 3.5 wt.% NaCl water solution environment (the load of 10 N, sliding stroke of 5 mm and frequency of 10 Hz).

The Mechanical and Frictional Performance of the Films
In order to investigate the mechanical performance of the TiC x O y films, the Nano-indenter DCM system was employed to analyze the hardness (H) and elastic modulus (E). Figure 1a illustrates the variation of the H and E for the TiC x O y films deposited at different target currents. From T1 to T5, the harnesses of TiC x O y films are 13.0 (T1), 17.3 (T2), 20.3 (T3), 19.4 (T4) and 16.3 (T5) GPa, respectively. Noticeably, a point of E inflexion turns out at target current of 0.80 A (T2) and shows a little change beyond the current of 0.80 A. It can be determined that the film owns the highest mechanical performance when the target current is 0.95 A (T3). Moreover, the values of H 3 /E 2 for the TiC x O y films prepared at different target currents exhibit a similar change trend with hardness. These results elucidate that T3 probably has the most prominent friction properties [38,39].
To acknowledge this speculation, the friction coefficients were assured via a ball-on-disc tribometer in a 3.5 wt.% NaCl water solution. Figure 1(b) shows the average friction coefficients (µ) of the TiC x O y films deposited at different target currents. When the target currents change from 0.65 to 1.25 A, the average friction coefficients of the TiC x O y films are 0.091, 0.085, 0.065, 0.071 and 0.080, corresponding to T1, T2, T3, T4 and T5, respectively. The friction coefficients, likewise, show an extreme low value at the target current of 0.95 A (T3). The friction variation is contrary to the shift tendencies of hardness and H 3 /E 2 . This result implies that T3 has great mechanical performance and a low coefficient of friction ( Figure 1).

The Electrochemical Corrosion Performances of Films
The open circuit potential and polarization curve tests were carried out to evaluate the corrosion resistance of the obtained samples on the electrochemical workstation. Figure 2a,b shows the polarization curves of the TiCxOy films deposited at different sputter current in 3.5 wt.% NaCl solution. Obviously, the lowest corrosion current density of 0.024 μA/cm 2 can be obtained for T3. Noticeably, with the sputter, currents increase from 0.65 to 0.80 A and the corrosion current density decreases three orders of magnitude (1.83-0.033 μA/cm 2 ). However, when the sputter current beyond 0.95 A, the corrosion current density increases along with the increase of sputter current, which are 0.34 μA/cm 2 at 1.10 A and 257.42 μA/cm 2 at 1.25 A, respectively, indicating that the corrosion resistance becomes worse. The corrosion current density of T5 (1.25 A) increases by five orders of magnitude compared with those of T3 (0.95A) and T2 (0.80 A).   [40,41], respectively. Therefore, it can be speculated that C and O atoms are

The Electrochemical Corrosion Performances of Films
The open circuit potential and polarization curve tests were carried out to evaluate the corrosion resistance of the obtained samples on the electrochemical workstation. Figure 2a,b shows the polarization curves of the TiC x O y films deposited at different sputter current in 3.5 wt.% NaCl solution. Obviously, the lowest corrosion current density of 0.024 µA/cm 2 can be obtained for T3. Noticeably, with the sputter, currents increase from 0.65 to 0.80 A and the corrosion current density decreases three orders of magnitude (1.83-0.033 µA/cm 2 ). However, when the sputter current beyond 0.95 A, the corrosion current density increases along with the increase of sputter current, which are 0.34 µA/cm 2 at 1.10 A and 257.42 µA/cm 2 at 1.25 A, respectively, indicating that the corrosion resistance becomes worse. The corrosion current density of T5 (1.25 A) increases by five orders of magnitude compared with those of T3 (0.95A) and T2 (0.80 A).

The Electrochemical Corrosion Performances of Films
The open circuit potential and polarization curve tests were carried out to evaluate the corrosion resistance of the obtained samples on the electrochemical workstation. Figure 2a,b shows the polarization curves of the TiCxOy films deposited at different sputter current in 3.5 wt.% NaCl solution. Obviously, the lowest corrosion current density of 0.024 μA/cm 2 can be obtained for T3. Noticeably, with the sputter, currents increase from 0.65 to 0.80 A and the corrosion current density decreases three orders of magnitude (1.83-0.033 μA/cm 2 ). However, when the sputter current beyond 0.95 A, the corrosion current density increases along with the increase of sputter current, which are 0.34 μA/cm 2 at 1.10 A and 257.42 μA/cm 2 at 1.25 A, respectively, indicating that the corrosion resistance becomes worse. The corrosion current density of T5 (1.25 A) increases by five orders of magnitude compared with those of T3 (0.95A) and T2 (0.80 A).   [40,41], respectively. Therefore, it can be speculated that C and O atoms are   [40,41], respectively. Therefore, it can be speculated that C and O atoms are possibly replaced with each other which induced the lattice distortion, where the peak position drifts slightly out of its original position, like silver-doping induced lattice distortion in TiO 2 nanoparticles [42]. More interestingly, when the target current was 0.8 A, the XRD pattern of TiC x O y films shows two peaks at about 52 • (201) and 56 • (221), which can be attributed to the standard XRD pattern of hexagonal corundum-type Ti 2 O 3 (PDF, No. 43-1033) [43]. It should be noticed that the ratio of these two peaks varied with the increase of the target current. The peak at 52 • (201) becomes gradually weak and the peak at 56 • (221) disappeared while the target current increased, which is probably to be influenced by C/O substituting effects. It can be speculated from EDS results that (Table 2), compared with other films, T3 (TiC 0.19 O 1.87 ) shows an abrupt decrease of C atoms content and an increase of O atoms content. Besides, the ratio of C/O trends is correlated with the variation of XRD peaks.

The XRD Results of Films
Coatings 2020, 10, 533 6 of 13 possibly replaced with each other which induced the lattice distortion, where the peak position drifts slightly out of its original position, like silver-doping induced lattice distortion in TiO2 nanoparticles [42]. More interestingly, when the target current was 0.8 A, the XRD pattern of TiCxOy films shows two peaks at about 52° (201) and 56° (221), which can be attributed to the standard XRD pattern of hexagonal corundum-type Ti2O3 (PDF, No. 43-1033) [43]. It should be noticed that the ratio of these two peaks varied with the increase of the target current. The peak at 52° (201) becomes gradually weak and the peak at 56° (221) disappeared while the target current increased, which is probably to be influenced by C/O substituting effects. It can be speculated from EDS results that (Table 2), compared with other films, T3 (TiC0.19O1.87) shows an abrupt decrease of C atoms content and an increase of O atoms content. Besides, the ratio of C/O trends is correlated with the variation of XRD peaks. Furthermore, cross-section SEM images and EDS element mapping of the as-deposited films were obtained, as shown in Figure 4. With the increase of the target current, the thickness of the film increases ( Figure 4). Specifically, the thicknesses are 0.95, 1.58, 1.66, 1.97 and 2.75 μm, corresponding to T1, T2, T3, T4 and T5, respectively. Besides, the SEM image of T1 shows a porous and column structure ( Figure 4a) and with the increase of the target current, the column structure disappears and becomes dense (T2, T3 and T4) (Figure 4b-d). However, when the target current reached to 1.25 A, the column structure presents again (Fugure 4e), which might be induced by a faster growth rate that inhibits the migration and adjustment of the particles and ions involved in growth [44]. For the EDS element mapping, only T1 shows a gradient change from bottom to surface (Figure 4a) and T2 to T5 give uniform element distribution (Figure 4b-e), the gradient transition of the composition reflects that the target poisoning happened due to the low current leading to a low sputter yield gradually (Figure 4a) [45,46]. For a detailed study, the composition data are summarized in Table 2. The Ti content has almost no change until the target current of 1.1 A(T4). The sudden change of Ti content accompanies an abrupt increase of the films' thickness, which implies that the sputtering mode changed from transition to metal mode [45,46].  Furthermore, cross-section SEM images and EDS element mapping of the as-deposited films were obtained, as shown in Figure 4. With the increase of the target current, the thickness of the film increases ( Figure 4). Specifically, the thicknesses are 0.95, 1.58, 1.66, 1.97 and 2.75 µm, corresponding to T1, T2, T3, T4 and T5, respectively. Besides, the SEM image of T1 shows a porous and column structure ( Figure 4a) and with the increase of the target current, the column structure disappears and becomes dense (T2, T3 and T4) (Figure 4b-d). However, when the target current reached to 1.25 A, the column structure presents again (Fugure 4e), which might be induced by a faster growth rate that inhibits the migration and adjustment of the particles and ions involved in growth [44]. For the EDS element mapping, only T1 shows a gradient change from bottom to surface (Figure 4a) and T2 to T5 give uniform element distribution (Figure 4b-e), the gradient transition of the composition reflects that the target poisoning happened due to the low current leading to a low sputter yield gradually (Figure 4a) [45,46]. For a detailed study, the composition data are summarized in Table 2. The Ti content has almost no change until the target current of 1.1 A(T4). The sudden change of Ti content accompanies an abrupt increase of the films' thickness, which implies that the sputtering mode changed from transition to metal mode [45,46].

The Results of XPS
XPS spectra have a wide range of applications in film analysis, which can provide abundant physical and chemical information about the surface of materials. The XPS spectra of the TiC x O y films deposited at different target currents are presented in Figure 5. Primary peaks of C, O and Ti are all detected. The Ti2p spectra, the O1s spectra and the C1s XPS spectra, recorded from the TiC x O y films, are displayed in Figure 5a-c, respectively. Accordingly, TiC is represented by a C1s peak of 281.8 eV, so the C element exists in the form of titanium carbide in the TiC x O y films [47]. Moreover, the blue shift of C1s spectrum occurs for the TiC x O y film deposited at the target current of 0.95 A (T3), implying that some carbon diffused out from TiC to become isolated carbon and the Ti was oxidized further. Besides, The Ti2p 3/2 peaks of T1, T2, T4 and T5 shift to the low binding energies at about 458.5 eV (TiO 2 ) to further confirm the change of bonding structure variation with the adjusting of target current. The Ti2p and O1s peaks are decomposed into three peaks, according to References [48,49]. The Ti2p spectrum (Figure 5d

The Results of XPS
XPS spectra have a wide range of applications in film analysis, which can provide abundant physical and chemical information about the surface of materials. The XPS spectra of the TiCxOy films deposited at different target currents are presented in Figure 5. Primary peaks of C, O and Ti are all detected. The Ti2p spectra, the O1s spectra and the C1s XPS spectra, recorded from the TiCxOy films, are displayed in Figure 5a-c, respectively. Accordingly, TiC is represented by a C1s peak of 281.8 eV, so the C element exists in the form of titanium carbide in the TiCxOy films [47]. Moreover, the blue shift of C1s spectrum occurs for the TiCxOy film deposited at the target current of 0.95 A (T3), implying that some carbon diffused out from TiC to become isolated carbon and the Ti was oxidized further. Besides, The Ti2p3/2 peaks of T1, T2, T4 and T5 shift to the low binding energies at about 458.5 eV (TiO2) to further confirm the change of bonding structure variation with the adjusting of target current. The Ti2p and O1s peaks are decomposed into three peaks, according to References [48,49]. The Ti2p spectrum (Figure 5d) could be deconvoluted into three spin-orbit components under binding energies of 455.9, 456.7 and 458.5 eV and are identified with TiO, Ti2O3 and TiO2 fractions in the film, respectively [48]. One can conclude that Ti2O3 is the dominant surface state. Furtherly, O1s spectrum ( Figure 6e) can be decomposed into three bands at 528.3, 530.08 and 531.1 eV, in correlation with TiO, Ti2O3 and TiO2 fractions in the film, respectively [48]. The deconvoluted result of O1s and Ti2p are consistent with each other and the collected values of Ti2O3/(TiO + Ti2O3 + TiO2) ratio of TiCxOy films deposited at different target current are described in Figure 6f. From the samples of T1 to T5, the percentage of Ti2O3 binding structure in the TiCxOy films is 37

Conclusions
A new method that utilizes CO2 as precursor to deposit TiCxOy films by magnetron sputtering has been developed. One can confirm that CO2 is a key player in the matter of excited CO2 + , CO + , CO3 − and so on involvement in the growth process reacting with Ti ions. The obtained TiCxOy films hold high hardness, low friction as well as good anti-corrosion properties, which can be employed for protecting coatings for drills, engine parts, especially for those using under water lubrication state or moisture conditions that benefit from nano-TiOx debris as liquid lubrication additives during friction. And further, the hardness, friction coefficient and anti-corrosion properties can be adjustable via target current and maybe atmosphere ratio and components and so forth. Our results could be predicting that, besides the Ti target, Si, Cr, Mo, V and Mn and so forth, could be employed as a solid source for growth MeCxOy films. This method does not only solidify CO2 but also reduces CO2 emissions via energy saving by reducing friction and resisting corrosion. Thus, we will focus on the correlation of plasma components and films' structure as well as composition, to reveal the growth mechanism and its inner factors on the tribology and corrosion properties.

Discussion
One can confirm that the TiC x O y films are successfully grown via sputtering a Ti target in CO 2 atmosphere with Ar as a dilute gas, which proved that CO 2 is a key player in the matter of excited CO 2 + , CO + , CO 3 − and so on which will be involves in the growth process reacting with Ti ions [17,27,50].
The results investigated above illustrate that the T3 sample has the highest hardness, lowest friction coefficient (Figure 1) as well as the best corrosion resistance properties (Figure 2). Some of the typical data are summarized in Table 2. It can be concluded that the composition of all samples studied in the present work are nearly unchangeable. Both the lowest C and the highest O composite are present in T3 but the Ti/(Ti + C + O) ratio keeps constant until the target current increases to 1.10 A, where the Ti content increases to about 40 at.% (Table 2), which can be assigned to the sputtering modes variation from transition to metal mode [45,46] and the O/(Ti+C+O) ratio fluctuation can be ascribed to the contamination of the oxygen from open atmosphere during transportation [26,27,29]. Thus, it can be confirmed that the composition has no influence on the friction and anti-corrosion properties. However, the growth rates increase with the increment of the target current (Figure 4), which can be speculated from the cross-section element mapping (Figure 4), where all the films show uniformity growth except of T1. In other words, T1 shows a gradient distribution of O and C elements which means that a heavy poisoning sputtering target occurred [45,46], so the suppressing growth rate of TiC x O y films happened. The target current increased from 0.80 to 1.10 A, transition zone growth mode instead of poisoning mode dominated and the growth rate increased [45,46]. At the target current of 1.25 A, a nearly metal mode presented and the growth rate increased quickly [45,46]. But the components of the TiC x O y film is changeable and can be assigned to high activity of CO 2 plasma [17,27,50,51].
The reason for the favorable friction properties of the sample T3 can be found from the hardness and elasticity modulus themselves. The T3 sample has the highest value of H 3 /E 2 , which predicts high resistance to plastic deformation and low friction [52]. Leyland A. et al. [38] proposed that controlling the H/E ratio in frictional progress of nanocomposite coatings is extremely significant for optimal tribological behavior. The value of H/E has a critical role in determining yield pressure and crack propagation. As we known, the value of H 3 /E 2 variation trends of the TiC x O y films are going in the opposite direction of the friction coefficients as presented in Figure 1b. Thus, combining the XRD and EDS results together (Table 2), it seems that the introduction of Ti 2 O 3 phase enforces not only the hardness but also the elasticity and in turn, tunes the friction properties to a lower state. With regard to the optimal tribological behavior of coatings/films, the H 3 /E 2 ratio in frictional progress is extremely significant [39]. The high value of H 3 /E 2 means the improvement of the elastic recovery and toughness endow good film friction properties [38]. On the other hand, the TiO x nano debris might be working together to reduce friction further. As reported by Han Huang et al., the introduction of nano TiO2 into water-based nanolubricant can decrease the friction coefficient by about 30% to some extent [53].
According to the results of EDS and XPS, no clue can be found associated with the good anti-corrosion properties of the TiC x O y films. Interestingly, as can be seen from Table 2, T3 has the best anti-corrosion and friction property among all the samples. It is believed that the corrosion behavior of the TiC x O y films depends on the dense nanoparticle stacking structure, which relates to the Ti 2 O 3 phase [38].

Conclusions
A new method that utilizes CO 2 as precursor to deposit TiC x O y films by magnetron sputtering has been developed. One can confirm that CO 2 is a key player in the matter of excited CO 2 + , CO + , CO 3 − and so on involvement in the growth process reacting with Ti ions. The obtained TiC x O y films hold high hardness, low friction as well as good anti-corrosion properties, which can be employed for protecting coatings for drills, engine parts, especially for those using under water lubrication state or moisture conditions that benefit from nano-TiO x debris as liquid lubrication additives during friction. And further, the hardness, friction coefficient and anti-corrosion properties can be adjustable via target current and maybe atmosphere ratio and components and so forth. Our results could be predicting that, besides the Ti target, Si, Cr, Mo, V and Mn and so forth, could be employed as a solid source for growth MeC x O y films. This method does not only solidify CO 2 but also reduces CO 2 emissions via energy saving by reducing friction and resisting corrosion. Thus, we will focus on the correlation of plasma components and films' structure as well as composition, to reveal the growth mechanism and its inner factors on the tribology and corrosion properties.