Enhanced high-temperature tribological performance of fluorinated tetrahedral amorphous carbon (ta-C:F) coatings in sliding applications

This study investigated the dry sliding behaviour of fluorinated tetrahedral amorphous carbon (ta-C:F) coatings against uncoated 52100 steel at temperatures ranging from 25 °C to 300 °C. The ta-C:F coatings demonstrated significant reductions in both the coefficient of friction(COF) during the running-in stage and at steady state, particularly within the temperature range of 25 °C to 200 °C, surpassing the performance of well-established a-C:H coatings. SEM analyses revealed the formation of transfer layers on the contact surface of 52100 steel when tested against ta-C:F coatings at temperatures up to 200 °C, while none were detected at temperatures ≥250 °C. Raman spectroscopy indicated a transition from sp3 to sp2 carbon structures in the carbonaceous transfer layers with increasing temperature, and XPS scans confirmed an increase in fluorine (F) concentration within these layers, correlating with reduced COF. The comparative analysis at 120 °C emphasized the intrinsic advantages of ta-C:F coatings in high-temperature applications, demonstrating a nearly 50% lower COF (0.08) when compared to traditional boundary-lubricated steel-to-steel sliding contacts. These findings have significant implications for enhancing the efficiency and durability of various mechanical systems, particularly in industries like automotive and manufacturing.


Introduction
The tetrahedral amorphous carbon (ta-C) coatings characterized by a high sp 3 hybridized C content exhibit remarkable stability at elevated temperatures [1][2][3][4].Several studies, including those by Ferrari et al [5] and Alam et al [6], have confirmed the retention of a high sp 3 fraction in ta-C coatings up to temperatures of 600 °C-800 °C.For instance, Ferrari et al [5] observed that the sp 3 content remained as high as 87% up to 700 °C, while Alam et al [6] reported a marginal 2% decrease in sp 3 content when heating ta-C coatings to 650 °C.These observations underscore the potential suitability of ta-C coatings for tribological applications that operate under hightemperature conditions [1][2][3][4].
Orwa et al [7] used a combination of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) and Raman spectroscopy to determine the sp 2 content within ta-C coatings after annealing at temperatures up to 1100 °C.Their investigations revealed a gradual increase in sp 2 contents until reaching 1000 °C, beyond which a substantial structural transformation occurred, leading to a significant increase in sp 2  bonding.Chen et al [8] used TEM in conjunction with EELS to determine the sp 3 content of ta-C coatings.They found that annealing at 600 °C did not result in substantial alterations in sp 2 content.However, significant structural modifications in ta-C occurred at 1000 °C.Additionally, Grierson et al [9] conducted a comprehensive study of the thermally induced conversion of sp 3 to sp 2 carbon-carbon bonds in ta-C films.They utilized analytical methods including near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy.Their findings revealed that the bonding structure remained relatively stable up to 600 °C, but a significant transition from sp 3 to sp 2 bonding occurred at 1000 °C.Despite these valuable insights, there remains limited literature on the tribological behaviour of ta-C coatings at elevated temperatures.Further research is necessary to gain a comprehensive understanding of the high-temperature tribological properties of ta-C coatings, and to assess their potential advantages in applications such as forming dies and engine components.
The incorporation of fluorine into DLC coatings has the potential to significantly reduce the coefficient of friction (COF) while simultaneously maintaining low wear rates [10][11][12][13].The existing body of literature suggests that the introducing of fluorine into DLC coatings can decrease surface energy through the passivation of carbon dangling bonds and/or formation of transfer layers composed of fluorocarbons [14][15][16][17][18][19][20].First-principles simulations predicted that high repulsive forces occur between two F-passivated carbon (F-F) surfaces in contact with each other, surpassing the forces generated between C-C, H-H, and C-F bonds [20,21].For example, in tribological experiments, ta-C coatings incorporating fluorine exhibited a significant reduction in COF when sliding against an aluminum alloy within the temperature range of 200 °C to 400 °C [22].Similarly, ta-C:F coatings, when in contact with non-hydrogenated diamond-like carbon (NH-DLC, a-C:H) coatings containing less than 2 at.% hydrogen, displayed a low running-in COF of 0.1 up to temperatures of 300 °C [23].In a separate study [24], wear and friction behaviour of ta-C coatings against Si 3 N 4 counterfaces over a temperature range of 100 °C to 500 °C.The results showed a low COF of 0.06 up to 400 °C, with an increase observed at 500 °C.
This study has a specific aim: to systematically examine the tribological behaviour of ta-C:F coatings during dry sliding against tool steel within a temperature range from 25 °C to 300 °C.The research is driven by the significant demand for low friction in numerous industrial applications.Notably, ta-C:F coatings are being considered as potential replacements for the conventional hard coatings, CrN and TiN, commonly used in highspeed steel tools.These tools often face challenges related to high COF during both standard and elevatedtemperature operations.The potential applications of ta-C:F coatings extend beyond high-speed steel tools and could include various fields such as engine components, bearings, and piston rings, where maintaining low friction levels is critical.Therefore, the study also encompasses a comparison between ta-C:F coatings and conventional steel-to-steel lubricated contacts under engine operating temperatures, specifically at 120 °C.This comparison aimed to evaluate the potential of ta-C:F coatings as a protective barrier against wear and friction, potentially replacing traditional lubricated friction over extended periods and also providing protection against wear and friction under critical operation conditions such as oil starvation.This study also aims to examine the mechanisms controlling the COF and wear in ta-C:F coatings at high temperatures using Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), SEM, and EDS to analyze carbon phase changes, surface morphology, and chemical composition shifts before and after the high temperature sliding tests.

Properties of coatings
Tetrahedral amorphous carbon doped with fluorine (ta-C:F) coatings were deposited onto 25.4 mm diameter WC-Co substrates using a laser-induced pulsed arc deposition technique equipped with a fluorine (F)doped graphite electrode and Laser-Arc plasma source.During deposition, the temperature was maintained below 100 °C and the coatings were deposited in a gas-free, vacuum atmosphere (p < 10 − 5 mbar).The fluorine (F)doped graphite cathode served as the fluorine source.The average surface roughness (R a ) of the metallographically polished WC-Co substrates was measured as 45.03 ± 8.89 nm using a 3D surface profilometer before deposition after final polishing.The composition of WC-Co substrate consisted of 94% WC and 6% Co, and was supplied by General Carbide Corporation, PA, USA.The WC-Co substrate was selected for its well-established properties, such as high hardness, wear resistance, and thermal stability, which are highly valued in the manufacturing industry for tools and dies.The selection of WC-Co was also influenced by its suitability as a substrate for ta-C and ta-C:F coatings in automotive applications, particularly for engine components like valves that operate at high temperatures.Prior to the deposition of coatings, all substrate surfaces were cleaned through Ar gas glow discharge.The fluorine (F) content within the ta-C:F coating was quantified through Rutherford backscattering spectroscopy (RBS), revealing a composition of 12 atomic percent (at.%).The coating's thickness was measured at 1.15 ± 0.13 μm.Mechanical properties, including hardness (H) and elastic modulus (E), were assessed using a Hysitron TI 900 Nanoindenter, resulting in H = 6.2 ± 2.0 GPa and E = 170.0± 20.5 GPa.
Hydrogenated DLC (a-C:H) coatings were selected as a reference coating for comparison due to their widespread use in industry and the well-studied mechanisms [25][26][27] by which they maintain low COF up to 200 °C.The deposition of these coatings was accomplished on 25.4 mm diameter WC-Co substrates employing an unbalanced magnetron sputtering system featuring one chromium and two graphite targets.Prior to the deposition of the a-C:H coating, a Cr interlayer with a thickness of 0.10 μm was deposited on the WC-Co substrate to enhance coating adhesion to the substrate.A butane (C 4 H 10 ) precursor gas was used as the source of hydrogen for the a-C:H coating.The hydrogen content within the a-C:H coatings amounted to 40%, as determined by elastic recoil detection (ERD) analysis.The thickness of the a-C:H coatings was 1.50 ± 0.03 μm.Mechanical properties, encompassing hardness (H) and elastic modulus (E), were measured using a Hysitron TI 900 Nanoindenter, yielding values of H = 11.40 ± 1.76 GPa and E = 103.00± 3.86 GPa, respectively.

Sliding friction and wear tests on ta-C:F at elevated temperatures
The experimental approach encompassed three distinct sets of tribological tests: The first set consisted of conducting dry sliding tests to evaluate the friction and wear performance of ta-C:F coatings at elevated temperatures when in contact with 52100 steel at temperatures up to 300 °C.
The second set consisted of sliding tests on hydrogenated amorphous carbon (a-C:H or H-DLC) to measure their COF and wear resistance as a function of temperature.This was conducted to establish a benchmark for comparing the performance of ta-C:F coatings against a more established grade of DLCs and to gain insights into the friction and wear mechanisms specific to ta-C:F.
The third set of tests, highly relevant to practical engine applications, involved comparing the dry sliding coefficient of friction (COF) of ta-C:F coatings at 120 °C with the lubricated sliding COF of tool steel against tool steel.The aim was to evaluate the potential advantages of using these coatings in dry conditions as an alternative to boundary-lubricated sliding, particularly within the operational temperature range of various engine components.
The determination of the coefficient of friction (COF) between ta-C:F (and a-C:H) coatings while sliding against spherical 52100 steel counterfaces was conducted using a high-temperature ball-on-disk tribometer.The sliding tests were conducted at six different temperatures, namely 25 °C, 100 °C, 120 °C, 200 °C, 250 °C, and 300 °C.At each temperature, the tests were carried on using a constant sliding speed of 0.12 m s −1 and a normal load of 5.00 N, for 5000 revolutions.The radius of the wear track on ta-C:F surfaces was about 2 mm.The calculation of the average coefficient of friction (COF) during the steady-state stage (μ s ) was performed after 2000 revolutions, which is a typical duration for reaching a steady-state condition in these tests.The average μ s values reported for each temperature were obtained by averaging the results of five tests.The consistency observed in the COF responses across these five tests led us to conclude that a steady state is achieved after 2000 revolutions.
Wear rates were determined by calculating the volume of material removed from the ta-C:F and a-C:H wear tracks.The average volumetric wear rate for each coating was calculated by measuring the wear track area at four different locations.This measurement was performed using a Wyko NT1100 white light interferometer (WLI) operating in vertical scanning interferometry mode, as previously described in our earlier work [28,29].The measurement area for determining the wear rate was defined as 450 μm by 600 μm.
This study also included sliding tests under controlled lubricated conditions as indicated above.Specifically sliding tests on M2 steel disks against M2 steel balls of 6 mm diameter were conducted under lubricated conditions at 120 °C using 5W30 engine oil as the lubricant which typically operates within a temperature range of up to 150 °C.The sliding tests were conducted in accordance with the guidelines established in the ASTM International standard, ASTM G-181.The lubricant was applied before the tests and from the calculated film thickness and composite surface roughness; the λ ratio remained below 1, indicating boundary lubrication [30,31].The comparison of dry sliding of the ta-C:F coating with lubricated sliding of steel is important as it evaluates the effectiveness of the two different types of lubrication methods under similar loading conditions.This allows for the determination of the potential benefits of using the coating as an alternative lubrication method in high-temperature applications.

Surface characterization
SEM and energy-dispersive x-ray spectroscopy (EDS) analyses were conducted on the transfer layers formed on the 52100 steel counterfaces contact surfaces using an FEI Quanta 200 FEG SEM equipped with an EDAX SiLi detector.An EDS mapping technique was used to estimate the areal distribution of ta-C:F transferred to the counterfaces as this technique shows the distribution of specific elements, such as carbon and fluorine, which are constituents of the ta-C:F material.Raman spectra were acquired from wear tracks on the coatings and steel surfaces using a Horiba Raman microspectrometer.A 50 mW Nd:YAG solid-state laser emitting at a wavelength of 532.0 nm provided the excitation source.
X-ray photoelectron spectroscopy (XPS) analysis of the transfer layers on the 52100 steel counterfaces was performed using the Kratos Axis Ultra XPS instrument which provided detailed information on the chemical composition of elements within these layers.Survey scans were performed, utilizing a 160 eV pass energy, over a 27.84 μm 2 surface.

Tribological behaviour of ta-C:F at elevated temperatures
This section provides an analysis of the frictional and wear properties observed in ta-C:F coatings under different test temperatures.A comparison is also made with the behaviour of a-C:H coatings.Figure 1 shows the variations of the coefficient of friction (COF) as a function of the number of revolutions for ta-C:F coatings tested at temperatures of 25 °C, 100 °C, 120 °C, 200 °C, 250 °C, and 300 °C, when sliding against a 52100 steel counterface.Three separate sliding tests were conducted at each temperature under the same loading conditions, and the curves in the figure represent their typical COF profiles.When examining the frictional behaviour of ta-C:F coatings from 25 °C to 250 °C, a general pattern emerges as there is an initial high runningin COF (μ R ) that transitions to a steady-state, characterized by a stable COF (μ s ).At 120 °C, the COF decreased to its lowest value of μ s = 0.08 ± 0.01.This average, μ s , was obtained from the friction curve in figure 1   2.21 μm, with a noticeable accumulation of debris along its periphery.This suggests that severe wear operated at this high temperature, causing pronounced degradation of the coating and possibly damaging the substrate.
Figure 4 (c) displays the variations of specific wear rates of ta-C:F sliding against 52100 steel at different test temperatures.At 25 °C, the specific wear rate was 0.90 × 10 −5 mm 3 Nm −1 , which increased to 1.29 × 10 −5 mm 3 Nm −1 at 100 °C.Further increments were noted: 1.31 × 10 −5 mm 3 Nm −1 at 120 °C and 2.78 × 10 −5 mm 3 Nm −1 at 200 °C.Optical surface profilometry measurements indicated that the ta-C:F coating remained intact at these temperatures.At 250 °C, the wear rate continued to increase to 3.79 × 10 −5 mm 3 Nm −1 , but a substantial rise to 9.99 × 10 −5 mm 3 Nm −1 occurred at 300 °C where severe wear and partial coating removal were observed.Figure 4(c) also compares the wear rates of a-C:H and ta-C:F sliding against 52100 steel at different temperatures.The data shows that ta-C:F had lower wear rates than a-C:H at all temperatures.Overall ta-C:F coatings maintained low COF and wear rates against 52100 steel in dry sliding up to 250 °C, making them suitable for high-temperature tribological applications.

Characterization of transfer layers formed on 52100 steel contact surfaces
Transfer layers formed during sliding on the sliding surfaces of 52100 steel counterfaces were analyzed using secondary electron SEM to observe their morphological features.In figure 5(a) the contact surface is seen to be covered with transfer layers when 52100 steel counterface slid against ta-C:F at 200 °C.The EDS elemental mappings revealed that the transfer layer primarily consisted of carbon, with the presence of transferred fluorine from the coating, which is evident in figures 5(b), (c).Figures 5 (d), (e) indicated the presence of oxygen and iron   on the 52100 steel surfaces, suggesting surface oxidation.These observations were consistent on all 52100 steel contact surfaces when tested against ta-C:F at temperatures below 200 °C.At temperatures 250 °C, no transfer layer was detected on the 52100 steel contact surfaces.For example, at 300 °C, the contact surface of the 52100 steel (figure 5(f)) exhibited no signs of a transfer layer.F-rich carbonaceous transfer layers were found at the periphery of the sliding surface (figures 5(g)-(j)).The absence of stable C and F-rich tribolayers at the contact surface was the primary reason for the high COF observed at temperatures 250 °C.While F-rich carbonaceous transfer layers were formed at the periphery of the sliding surface, they were not maintained or accumulated on the contact surface, potentially being swept away during sliding.
SEM analysis was also employed to investigate the wear tracks formed on the ta-C:F surfaces.No clear evidence of counterface material transfer to the wear tracks was observed at temperatures ranging from 25 °C to 200 °C.For instance, figure 6(a) presents a segment of a wear track formed on a ta-C:F surface during sliding against uncoated 52100 steel, along with EDS mapping of carbon and fluorine (figures 6(b), (c)).
However, when tests were conducted at 250 °C and 300 °C (figure 7(a)), some evidence of counterface material transfer onto the ta-C:F sliding surfaces was observed, suggesting partial removal of the coating.The SEM image of the wear track formed at 300 °C illustrates the presence of transferred iron on the wear tracks.The EDS mappings of carbon and fluorine are shown in figures 7(b), (c), demonstrating that the ta-C:F coating was still partially intact within the wear track.As mentioned previously, in certain sections of the wear track, the coating was partially removed at 300 °C.The EDS spectrum of the regions highlighted in white in white square in figure 7(d) revealed a high concentration of transferred iron.ratio (figure 9).Also there was no change in the G peak position, indicating that no significant structural changes occurred elsewhere on the coating.The bulk structure of the ta-C:F coatings remained intact.Thus, the structural changes observed were specific to the wear tracks, demonstrating that the transformation in carbon allotropies was confined to these regions.The dominant peaks found between <700 cm −1 and 1305 cm −1 in the Raman spectra of transfer layers and wear tracks formed at 300 °C were assigned to α-iron oxide.At this elevated temperature, severe wear of the ta-C:F coating prevented the formation of a carbonaceous transfer layer, as stated before, resulting in increased friction.

Mechanisms of low COF
Quantitative compositional analyses of transfer layers that formed on the contact surfaces of 52100 steel balls sliding against ta-C:F at 25 °C, 100 °C and 200 °C were conducted using XPS scans (figures 10(a), (b)).The highintensity presence of carbon (C) and oxygen (O) in both samples signifies that these elements are the principal components of the transfer layers, which aligns with the findings from the Raman spectroscopy results.The XPS data provides additional support to the presence of fluorine (F) within the transfer layers.It was evident that the intensity of fluorine (F) in the carbonaceous transfer layers was significantly higher in the samples tested at 200 °C (figure 10(b)) compared to those conducted at 25 °C (figure 10(a)).This increase in F concentration was consistent with the observed reduction in the coefficient of friction (COF), as demonstrated in figure 11.As a result, once the transfer layers were formed, the sliding would occur between C-F bonds located on the surface of the transfer layers and the C-F bonds within the ta-C:F coating.
First principles calculations were used to investigate the forces operating between two F-terminated diamond surfaces to calculate the interactions between the F-terminated coating and the transferred layer formed on the sliding counterface [20,21,32].It was suggested that when two F-terminated diamond surfaces came into contact with each other, the negatively charged F atoms would be expected to exert repulsive electrostatic interactions, which is the primarily responsible for the observed low COF at elevated temperatures.In this study, the low COF and wear till 200 °C were attributed to the existence of F in the coating and the transfer layers as evidenced by SEM (figure 5(a)) and XPS (figure 9(b)).It was found that the F content in the transfer layers increased at 200 °C where lower μ s of ta-C-F is observed compared to tests at 25 °C.

Comparing dry sliding behaviour of ta-C:F with steel-to-steel lubricated contact
The results summarized in sections 3.1-3.3highlight the significant impact of ta-C:F coatings on reducing friction in high-temperature applications and identify potential mechanisms contributing to this behaviour.When subjected to dry sliding contact with steel, an initial wear process leads to the formation of transfer layers rich in fluorine (F) during the running-in phase.These transfer layers play a pivotal role in decreasing the coefficient of friction (COF) as wear stabilizes, resulting in a lower steady-state COF.These observations offer promising prospects for engineering applications of ta-C:F coatings, particularly as piston ring coatings within internal combustion engines.To gain practical insights into the frictional behaviour of ta-C:F coatings, we conducted a comparative analysis, with a specific focus on their performance at 120 °C-a temperature range of critical importance for various high-temperature applications, including those prevalent in engines and    boundary-lubricated conditions.This resulted in a dry COF for ta-C:F coatings at 120 °C that were 43% lower than the COF observed for steel sliding against steel under boundary-lubricated conditions.Notably, this COF value of 0.08 for ta-C:F coatings during dry sliding at 120 °C coincided with the COF recorded for the boundarylubricated steel/steel configuration at 25 °C.
The bar graph in figures 12(c) compares the specific wear rates for three distinct combinations.In the case of the boundary-lubricated steel-on-steel configuration at 25 °C, the wear rate recorded was the lowest among the tested tribo-pairs, measuring just slightly above 0.2 × 10 −6 mm 3 Nm −1 .This is attributed to the effect of boundary lubrication, which mitigates metal-to-metal contact.However, at 120 °C, the wear rate of steel-onsteel contact increased to 1.2 × 10 −6 mm 3 Nm −1 , indicating the diminished efficiency of boundary lubrication at elevated temperatures.The wear rate for the ta-C:F against steel at 120 °C in a dry condition was 1.4 × 10 −6 mm 3 Nm −1 .The observed wear rate for ta-C:F coatings against steel at 120 °C in dry sliding contact, while slightly higher by approximately 16% compared to boundary lubrication, is well within expected norms.This result aligns with the lower coefficient of friction (COF) exhibited by ta-C:F coatings during dry sliding during the critical running-in phase (m R ).The transfer layers, while initially formed during a period characterized by higher friction (m R ) subsequently prove to be highly effective in reducing both the COF (m s ) and wear rate to levels comparable those observed in boundary lubricated condition.
The comparative analysis at 120 °C underscores the advantages of ta-C:F coatings in high-temperature industrial applications.At such elevated temperatures, conventional lubrication methods face challenges due to the potential breakdown or evaporation of lubricants.Consequently, the quest for a coating material capable of delivering low COF and wear rates in this temperature range, even without traditional lubrication, becomesimportantand in this context the potential of ta-C:F coatings in enhancing the efficiency and durability of hightemperature systems is evident.By mitigating parasitic friction losses, enhancing scuffing resistance, and improving engine durability, these coatings may hold the potential to significantly benefit in the automotive components.

Conclusions
This study conducted experiments involving ta-C:F coatings subjected to dry sliding against uncoated 52100 steel at various temperatures and compared with lubricated experiments at 120 °C. 2 SEM analyses revealed the presence of transfer layers on the contact surface of 52100 steel counterface when placed in sliding contact against ta-C:F at temperatures below 200 °C.However, at temperatures 250 °C, no transfer layer was detected on the 52100 steel contact surface.
3 Raman spectroscopy indicated an increase in the intensity of the D peak relative to the G peak in Raman spectra of both the wear tracks and the transfer layers with increasing temperature.This suggests a slidinginduced transformation from sp 3 to sp 2 carbon structures on the carbonaceous transfer layers.XPS scans demonstrated a rise in the concentration of fluorine (F) within the transfer layers as the test temperature increased from 25 °C to 200 °C.This increase in F concentration corresponded with a reduction in the μ R and μ s .
and from two concurrent friction curves acquired at the same temperature.A low steady-state average COF of 0.11 ± 0.01 was also observed during the tests conducted at 200 °C.At 250 °C, the COF increased but remained relatively stable with smaller oscillations around the mean, indicating a stable frictional behaviour.However, as the test temperature increased to 300 °C, the stability in the friction behaviour disappeared, and significant fluctuations occurred with increasing the COF and, ultimately the COF peaked at 0.95.The average steady-state COF (μ s ) and running-in COFs (μ R ) obtained from the three tests performed at each temperature are plotted in figures 2(a) and (b), respectively.The trend mentioned above is further clarified in this figure: In the temperature range of 100 °C-200 °C, ta-C:F demonstrated a consistent, low steady-state COF and also maintained values that were lower than its starting COF at 25 °C.The data in figure 2 includes results for a-C:H, providing a benchmark to assess the performance of ta-C:F.During sliding contact in the temperature range between 100 and 200 °C, the steady -state COF (μ s ) of ta-C:F was lower than that of a-C:H, as shown in figure 2(a).The a-C:H coatings are recognized for their ability to sustain low COF levels up to temperatures of 200 °C when subjected to sliding contact with aluminum [22].The data shown in figure 2(a) substantiates this behaviour of a-C:H when sliding against the steel counterface, and demonstrates a clear increase in COF values at temperatures exceeding 200 °C.The running-in coefficient of friction (μ R ) of ta-C:F exhibited a low value of around 0.3 from 25 °C to 200 °C, and at all temperatures it was lower than that of a-C:H (figure 2(b)).The data represented in figure 2 for a-C:H were obtained from the COF versus the number of revolution curves illustrated in figure 3.In this figure, the COF variations of a-C:H are displayed when in sliding contact against a 52100 steel counterface, within a temperature range of 25 °C-300 °C.Each curve indicates a distinct initial high μ R stage, transitioning smoothly into a stable μ s .This behaviour mirrors that of ta-C:F, but with higher COF values, with the exception at 25 °C.The main wear damage characteristics of ta-C:F coatings under various temperatures can be identified from the 3-D profilometer images like the ones presented in figure 4(a) and (b).At 200 °C [figure 4(a)], the wear track formed at 200 °C had a shallow profile with a depth confined to 0.89 μm, indicating that the ta-C:F coating with the original thickness of 1.15 μm remained intact at this temperature with only a few wear marks on the track.However, a contrasting behaviour was observed at 300 °C (figure 4(b)).The wear track deepened significantly to

Figure 1 .
Figure 1.Coefficient of friction (COF) curves as a function of the number of revolutions for ta-C:F coatings at 25 °C, 100 °C, 120 °C, 200 °C, 250 °C, and 300 °C, using 52100 steel as the counterface.These curves illustrate the characteristic trends in COF of ta-C:F with rising temperature.

Figure 2 .
Figure 2. Variation of (a) average steady-state COF (m s ) and (b) running-in COF (m R ) of ta-C:F and a-C:H coatings with the test temperature.

Figure 3 .
Figure 3. Coefficient of friction (COF) curves as a function of the number of revolutions for a-C:H coatings at 25 °C, 100 °C, 200 °C, 250 °C, and 300 °C, using 52100 steel as the counterface illustrating the characteristic trends in COF of a-C:H at different temperatures.

Figure 4 .
Figure 4. (a) 3-D surface profilometer image illustrating the wear track produced by the steel ball sliding against ta-C:F coating at 200 °C.(b) 3-D surface profilometer image displaying the wear track formed by the steel ball in contact with the ta-C:F coating at 300 °C.(c) Temperature-dependent specific wear rates of ta-C:F and a-C:H coatings after sliding against uncoated 52100 steel for 5000 revolutions.

Figure 5 .
Figure 5. (a) Secondary electron (SE-SEM) image of the uncoated 52100 ball surface taken after sliding against ta-C:F coating at 200 °C; the elemental EDS maps taken from the area shown in (a) are for (b) C, (c) O, (d) F, and (e) Fe.(f) Secondary electron (SE-SEM) image of 52100 ball surface after sliding against ta-C:F coating at 300 °C; the elemental EDS maps taken from the area shown in (f) are for (g) C, (h) O, (i) F, and (j) Fe.

Figure 8 (Figure 6 .
Figure 6.(a) Secondary electron (SE-SEM) image of a section of a wear track formed on ta-C:F surface when sliding against 52100 steel ball at 200 °C.The elemental EDS maps taken from the area shown in (a) are for (b) C and (c) F.

Figure 7 .
Figure 7. (a) Backscattered electron (BS-SEM) image of a section of a wear track formed on ta-C:F sliding against 52100 steel ball at 300 °C.The elemental EDS maps taken from the area shown in (a) are for (b) C and (c) F. (d) The EDS spectra of light coloured regions in plate (a) observed in the wear track.

Figure 9 .
Figure 9.I D /I G ratios are plotted as a function of temperature for wear tracks, regions outside of wear tracks, and transfer layers.

Figure 10 .
Figure 10.XPS spectrum of transfer layer formed on steel surface during sliding against ta-C-F at (a) 25 °C and (b) 200 °C.The intensity of F in the transfer layer at 200 °C is notably higher compared to the transfer layer formed at 25 °C.

Figure 11 .
Figure 11.Increase in F concentration with temperature in the transfer layer formed on the top of 52100 steel ball surfaces at 25 °C, 120 °C, and 200 °C, along with corresponding steady-state COF (μ s ) values.

Figure 12 .
Figure 12.(a) The variations of COF with number of revolutions for ta-C:F sliding against 52100 steel at 120 °C in dry, 52100 steel sliding against M2 steel at 25 °C and 120 °C in boundary lubricated conditions; (b) the average steady state COF values and (c) the specific wear rate.

1
The ta-C:F coatings reduced both the coefficient of friction (μ R ) during the running-in phase and at a steady state within the temperature range of 25 °C to 200 °C, surpassing the performance of well-established a-C:H coatings.Furthermore, the ta-C:F coatings exhibited excellent stability in their (μs) values, characterized by minimal fluctuations, particularly up to 200 °C.