Properties of Diamond-like Coatings in Tribological Systems Lubricated with Ionic Liquid

Abstract

The paper shows the effect of using a lubricant in the form of an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆), on the tribological properties of a hydrogenated diamond-like coating (DLC) doped with tungsten a-C:H:W. The coatings were deposited on 100Cr6 steel by plasma-enhanced chemical vapor deposition PECVD. Tribological tests were carried out on a TRB³ tribometer in a rotary motion in a ball–disc combination. 100Cr6 steel balls were used as a counter-sample. Friction and wear tests were carried out for discs made of 100Cr6 steel and 100Cr6 steel discs with a DLC coating. They were performed under friction conditions with and without lubrication under 10 N and 15 N loads. The ionic liquid BMIM-PF₆ was used as a lubricant. Coating thickness was observed on a scanning microscope, and the linear analysis of chemical composition on the cross-section was analyzed using the EDS analyzer. The confocal microscope with an interferometric mode was used for analysis of the geometric structure of the surface before and after the tribological tests. The contact angle of the samples for distilled water, diiodomethane and ionic liquid was tested on an optical tensiometer. The test results showed good cooperation of the DLC coating with the lubricant. It lowered the coefficient of friction in comparison to steel about 20%. This indicates the synergistic nature of the interaction: DLC coating–BMIM-PF₆ lubricant–100Cr6 steel.

1. Introduction

One way to improve the properties of materials is to modify the surface layer by applying hard, thin coatings in vacuum processes, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD). An example of this type of coating is diamond-like DLC coatings, which have enjoyed unwavering interest among scientists since they were first produced over thirty years ago by Aisenberg and Chabot [1]. These coatings are amorphous, mainly hydrogenated, thin-film materials with different properties depending on the method used and the conditions of production. They can be deposited on virtually any material, including metals, ceramics and polymers. The most popular types of DLC coatings include: a-C form (hydrogen-free amorphous carbon), a-C:H form (amorphous carbon with hydrogen), a-C:H:Me form (Me = W, Ti, amorphous carbon doped with metal and hydrogen) or a-C:H:Si form (amorphous carbon doped with Si and hydrogen) [2,3].

DLC coatings are distinguished by high hardness, chemical inertness, biotolerance and resistance to thermal stress, brittle fractures and electromagnetic radiation. In addition, they are characterized by a low coefficient of friction and adhesive wear [4,5]. The high hardness is due to sp3 bonds typical of diamonds, while the low coefficient of friction is due to sp2 bonds typical of graphite. They also have excellent corrosion resistance in alkali and acidic environments, and are suitable for highly loaded friction pairs with poor lubrication [6]. The hardness of hydrogenated DLC coatings is around 10–20 GPa, while those without hydrogen can reach up to 80 GPa. The friction coefficient values for these coatings can be reduced to 0.1, even in dry friction. This is due to their very good self-lubricating properties [7,8,9,10,11,12]. For this reason, they are used in many industries. They can be mounted on woodworking tools, cutting tools, bearings, engine components, shaft journals, milling cutters, sliding and rolling guides and gears [13,14].

In order to obtain specific functional properties, DLC coatings are doped with various elements, e.g., adding nitrogen or silicon contributes to increased thermal resistance, copper and titanium increase adhesion and tungsten improves residual stresses [15,16]. Tungsten reduces residual stresses several times. Without the addition of tungsten in DLC coatings, they are about 1.4 GPa, and after doping they decrease below 1 GPa. Up to 3% content, tungsten reduces the hardness of the coating, but above this value, both Young’s modulus and hardness increase [17,18]. The use of lubricants to improve the tribological properties of friction pairs is widely known. Their task is not only to maintain the appropriate operating temperature, but also to reduce wear and increase corrosion resistance. Due to the very good anti-wear properties of diamond-like coatings in friction conditions without lubrication, it is difficult to find such an alternative for them. Currently, there are few literature reports on extending the service life of DLC coatings by using lubricants. In this aspect, ionic liquids seem to be the most promising [19,20,21]. Ionic liquids are organic substances characterized by a melting point lower than the boiling point of water [22,23]. A special type of ionic liquids are those that melt at room temperature (i.e., 25 °C). Currently used ionic liquids are characterized by thermal and electrochemical stability, low vapor pressure, electrical conductivity and non-flammability. Due to the variety of their properties, they can be used in various places. The physicochemical properties of ionic liquids mainly depend on the structure of the cation and the type of anion. They can also be modified by mixing them together or mixing them with typical solvents [24,25,26]. Ionic liquids are used in cooling technologies (sorption cooling), separation technologies (gas separation or extractive distillation), analytics (electrophoresis, solvents for protein crystallization), as functional liquids (gas storage or surfactants), as electrolytes (batteries, sensors, metal deposition or fuel cells), in synthesis and catalysis (synthesis of nanoparticles), but also in tribology (as lubricants) [27,28,29,30]. Ionic liquid additives effectively reduce friction coefficient values compared to typical lubricants [31,32]. They can be, depending on the configuration (percentage of ionic liquid in solution), even about 0.06 compared to 0.12 for polyalphaolefins [33,34]. Due to their structure and very good parameters, ionic liquids are an alternative to classical lubricants [35,36].

So far, the focus has been on the study of the effect of ionic liquids on diamond-like coatings not doped with tungsten. The novelty of this work is the comparison of the effect of a small load change (10 N and 15 N) on the action of ionic liquid in the friction node of a 100Cr6 steel ball and a diamond-like coating doped with tungsten. The previously described BMIM-PF₆ ionic liquid has not been used during tribological tests in friction contact with W-DLC coatings. Therefore, in this work, an attempt was made to investigate the tribological properties of tungsten-doped diamond-like coatings lubricated with BMIM-PF₆ ionic liquid.

2. Materials and Methods

2.1. Materials

The tests were performed using samples—42 mm diameter and 6 mm high discs made of 100Cr6 steel with and without DLC coating. 100Cr6 steel is characterized by high wear resistance and high fatigue life. The diamond-like coating was applied using physical vapor deposition (interlayer) and plasma-assisted chemical vapor deposition (diamond-like carbon coating) techniques.

Table 1. Chemical composition of 100Cr6 steel [37].

100Cr6 [% Content]
Fe	C	Mn	Si	P	S	Cr	Ni	Cu
95.8–96.7	0.95–1.1	0.25–0.45	0.15–0.35	max. 0.025	0.025	1.3–1.65	max. 0.3	max. 0.3

provides the chemical composition of 100Cr6 steel.

Proper surface preparation is the most important step before applying the coatings. Before applying the coating, the discs were ground and polished using a grinder-polisher (Pace Technologies, Tucson, AZ, USA) on SiC abrasive papers. Papers with increasing grain gradation were used, in the following order: 120, 240, 600, 1200 µm. The grinding process was started with gradation 120 and continued until the last abrasive paper with gradation 1200 was used. After surface preparation, the selected amplitude parameters were as follows: Sa = 0.33 µm, Sq = 0.42 µm, Sz = 3.81 µm. The DLC film was applied using PECVD (plasma-assisted chemical vapor deposition) method. The coating process was divided into two stages. The first was the application of a chromium interlayer (improving the adhesion of the coating to the material), and the second was the application of the actual DLC coating doped with tungsten.

Before deposition, the samples were cleaned in an ultrasonic cleaner in ethanol. After that, diamond-like coatings were applied by plasma-assisted physical vapor deposition, using sputtering at a temperature of ˂250 °C. The specimens were first heated in a vacuum chamber, then cleaned using argon ion etching to ensure a contaminant-free surface. Once the optimal surface conditions were achieved, a high negative voltage was applied to the coating material source. This caused an electric discharge, which led to the sputtering of the material by positive argon ions. As a result of this process, the atomized metal (tungsten) in the gas phase was deposited together with a gas containing the non-metallic component of the hard carbon coating. After deposition of the appropriate coating, it was tested and the following roughness parameters were obtained: Sa = 0.44 µm, Sq = 0.57 µm, Sz = 4.8 µm.

The tribological tests used BMIM-PF₆ ionic liquid (Sigma-Aldrich, St. Louis, MO, USA), the chemical composition of which is detailed in

Table 2. Chemical composition of BMIBM-PF₆ ionic liquid [38].

Parameter	Value
color	colorless
melting point	−8 °C
density at 20 °C	1.38 g/mL
viscosity at 20 °C	300–400 cP

.

2.2. Research Methodology

The coating thickness was determined by microscopic observation of metallographic sections at three different locations. Measurements were made using a scanning electron microscope (Phenom XL, Eindhoven, The Netherlands). An accelerating voltage of 15 kV and a magnification of ×10,000 were used. Linear elemental distributions supplemented the study. The surface morphology observation before testing was observed at ×1000 magnification. The test results are shown in Section 3.1.

The geometric structure of the surface (including the amplitude parameters) before the tribological tests was analyzed using a DCM 8 (Leica, Heerbrugg, Switzerland) confocal microscope with interferometric mode and a ×20 magnification objective in the confocal mode. The results are presented in Section 3.2.

An optical tensiometer (Attension Theta Flex, Espoo, Finland) was used to determine the wettability of 100Cr6 steel and the coating. The contact angle measurement was performed by precisely putting a 4 µL drop of the measuring liquids (distilled water, ionic liquid and diiodomethane) on the sample surface and at once measuring it. The drops were applied to randomly selected locations on the sample surface. Measurements for each liquid and surface were performed five times, from which average values were obtained. Figure 1 presents the measurement scheme for the contact angle value. The results of the study are shown in Section 3.3.

Tribological tests were performed on the TRB³ tribometer (Anton Paar, Baden, Switzerland) in rotational motion. The counter-samples were balls made of 100Cr6 steel. They had diameters of 6 mm. The friction coefficient was recorded during dry friction and friction with lubrication with BMIM-PF₆ ionic liquid. The parameters of the tribological test are presented in

Table 3. Parameters of tribological test.

Sample Discs	Ø 42 mm—100Cr6 Steel Ø 42 mm—100Cr6 Steel with DLC Coating
Counter-sample ball	Ø 6 mm—steel 100Cr6
Type of friction	Rotation
Lubricant	Ionic liquid BMIM-PF6
Radius	14 mm
Load	10 N, 15 N
Sliding speed	0.1 m/s
Sliding distance	1000 m
Test duration	10,000 s
Ambient temperature	22 ± 1 °C

. The friction tests were repeated four times for each friction node. A view of the friction pair is shown in Figure 2. The results of tribological tests are shown in Section 3.4.

A DCM 8 confocal microscope (Leica, Heerbrugg, Switzerland) was used to analyze the geometric structure of the surface after tribological tests. A 20× objective lens was used in confocal mode over an area of 1.78 mm × 3.58 mm. The wear area value (expressed in micrometers) was averaged based on the results of ten randomly selected 2D profiles. The average value was calculated using Leica software (Leica Map 7.3). The results of the tests are shown in Section 3.5.

The surface morphology of the samples after the tests was observed at ×1000 magnification using a scanning electron microscope (Phenom XL, Eindhoven, The Netherlands). Based on the observations, differences in the wear of the coating and steel were determined in two configurations: dry friction and ionic liquid lubrication. The results of the tests are presented in Section 3.6.

3. Results

3.1. Coating Thickness

Figure 3 presents the SEM view of the cross-sections along with the thickness measurement. The coating thickness was determined based on observations in five areas. The results obtained were averaged.

The linear analysis of the chemical composition of the DLC coating confirmed the chemical composition assumed in the process of its production. The linear analysis of the metallographic cross-section of the coating was presented, starting from the substrate material (100Cr6 steel) through the chromium interlayer and ending with the diamond-like coating. The tests showed that carbon, tungsten and chromium were present on the surface, with the highest concentration of chromium noted in the interlayer. The exact percentages of the determined elements were not compiled due to insufficient measurement accuracy. The analysis discussed was a qualitative analysis to confirm the homogeneity of the tested coating. Based on the metallographic cross-section, the coating thickness was determined to be 1.8 ± 0.2 µm. Figure 4 shows the surface morphology of 100Cr6 steel and DLC coating before tribological tests.

The surface morphologies of 100Cr6 steel and DLC coating do not differ significantly. It can be observed that both materials are highly developed. However, small, spherical inclusions (tungsten impurities) can be observed on the coating surface.

3.2. Geometric Structure

Figure 5 shows isometric images and surface profiles of 100Cr6 steel and DLC-coated steel.

Table 4. Amplitude parameters of the 100Cr6 steel surface without coating and with DLC coating before friction tests.

Parameters	100Cr6 Steel	DLC Coating
Sa [µm]	0.33	0.44
Sp [µm]	1.61	1.61
Sv [µm]	2.20	3.20
Sz [µm]	3.81	4.80
Ssk	−0.32	−0.54
Sku	3.03	3.76

summarizes the amplitude parameters of both samples before friction tests.

Based on isometric images and amplitude parameter values, it was found that both 100Cr6 steel and the coating are characterized by a similar geometric surface structures. This proves that the process of deposition of the DLC coating by chemical vapor deposition did not significantly change the geometry of the elements subjected to the above-mentioned treatments. In addition, the negative value of the Ssk parameter for both steel and coating indicates that both surfaces are characterized by a predominance of valleys over peaks. For both surfaces, the Sku parameter was also similar, oscillating within the limits of the normal distribution: 3. For the DLC coating, this value was 3.76, which has a positive effect on reducing friction during friction tests without lubrication.

3.3. Contact Angle

Figure 6 shows sample photos of droplets deposited on the surface of the 100Cr6 steel and DLC coatings. Surface wettability was determined using three liquids: distilled water, diiodomethane and BMIM-PF₆ ionic liquid used during tribological tests. Figure 7 presents the average values of contact angles for the tested surfaces.

The test results confirmed that both 100Cr6 steel and DLC coating are characterized by good wettability. This was evidenced by the obtained values of contact angles below 90° for demineralized water. The average contact angle of the DLC coating with distilled water was 24% smaller than for uncoated 100Cr6 steel, which indicates better wettability of the DLC coating compared to steel. The values of the average contact angles with diiodomethane and BMIM-PF₆ ionic liquid were comparable for both samples.

3.4. Tribological Tests

The coefficient of friction was monitored continuously throughout the entire tribological test. The coefficient of friction for 100Cr6 steel during dry friction initially indicated the running-in of materials, but then stabilized. In the case of ionic liquid lubrication, the course of the coefficient of friction was observed to be stable, without a distinct running-in stage. The coefficient of friction for the coating during dry tests, as well as with BMIM-PF₆ lubrication, did not show large fluctuations. In both cases, it was stable from the beginning to the end of the test. Figure 8 shows average values of friction coefficients for both load.

After conducting tribological tests, it was found that there was no significant effect of a load change of 5 N on the obtained friction coefficients. For the load of 10 N and 15 N, similar values of the above parameters were recorded. Analysis of the results of the tribological tests conducted at a load of 10 N indicated that the value of the average coefficient of friction for the DLC coating in the case of technically dry friction was 58% lower than for 100Cr6 steel. During friction using BMIM-PF₆, its value for the DLC coating was about 20% lower than for the sample without the coating, for the load of 15 N, the value of the average coefficient of friction for the DLC coating in the case of technically dry friction was 60% lower than for 100Cr6 steel. During friction using BMIM-PF₆, its value for the DLC coating was about 18% lower than for the sample without the coating.

3.5. Assessment of Surface Geometric Structure of Samples

After the tribological tests, the wear traces were subjected to microscopic observation. The surfaces were observed using a confocal microscope. Figure 9 and Figure 10 show isometric views of the wear traces and their profiles after the tribological tests.

Table 5. Average value of the wear depth and wear area after dry friction.

Parameters	Value	100Cr6–10 N	DLC–10 N	100Cr6–15 N	DLC–15 N
Maximum depth	µm	3.39	0.73	3.51	0.82
Wear area	µm2	494.5	145.3	694.4	182.41

presents the values of wear indicators: the average values of the maximum wear depth and the wear area on the cross-section. The surface of material wear was developed by calculating the average measurement of 10 randomly selected 2D profiles. The software of the optical microscope (Leica Map 7.3) was used to analyze the results.

3.5.1. Load 10 N—Dry Friction

For 100Cr6 steel, a higher maximum value of the abrasion depth was recorded—3.39 µm, while the abrasion area was 494.5 µm². For the DLC coating, these indicators were as follows: abrasion depth—0.73 µm, abrasion area 145.3 µm². The width of the abrasion trace was 1.6 mm for 100Cr6 steel, and 0.7 mm for the DLC coating. The use of the DLC coating reduced the abrasion depth by over four times and the abrasion area by three times compared to the uncoated sample.

3.5.2. Load 15 N—Dry Friction

For 100Cr6 steel, a higher maximum value of the abrasion depth was recorded, 3.51 µm, while the abrasion area was 694.4 µm². For the DLC coating, these indicators were as follows: abrasion depth, 0.82 µm; abrasion area, 182.41 µm². The width of the abrasion trace was 1.9 mm for 100Cr6 steel and 0.6 mm for the DLC coating. The use of the DLC coating reduced the abrasion depth by around three times and the abrasion area by about four times compared to the uncoated sample.

Figure 11 and Figure 12 show isometric views and examples of surface profiles after tests with 10 N and 15 N loads after friction with ion liquid.

Table 6. Average value of the wear depth and wear area after friction with lubrication.

Parameters	Value	100Cr6–10 N	DLC–10 N	100Cr6–15 N	DLC–15 N
Maximum depth	µm	0.82	0.19	0.12	0.11
Wear area	µm2	55.73	17.15	12.21	12.89

presents the average values of the wear trace depth and area after tribological tests with lubrication. The surface of material wear was developed by calculating the average measurement of 10 randomly selected 2D profiles. The software of the optical microscope was used to analyze the results.

3.5.3. Load 10 N—Lubrication

For 100Cr6 steel, a higher maximum value of the abrasion depth was recorded, 0.82 µm, while the abrasion area was 55.73 µm². For the DLC coating, these indicators were as follows: abrasion depth, 0.19 µm; abrasion area, 17.15 µm². The width of the abrasion trace was 0.3 mm for 100Cr6 steel, and 0.7 mm for the DLC coating. The use of the DLC coating reduced the abrasion depth by over four times and the abrasion area by about three times compared to the uncoated sample.

3.5.4. Load 15 N—Lubrication

For 100Cr6 steel, a higher maximum value of the abrasion depth was recorded, 0.12 µm, while the abrasion area was 12.21 µm². For the DLC coating, these indicators were as follows: abrasion depth, 0.11 µm; abrasion area, 12.89 µm². The width of the abrasion trace was 0.6 mm for 100Cr6 steel and 0.7 mm for the DLC coating. The use of the DLC coating reduced the abrasion depth by 10%, but the abrasion area was similar to the uncoated sample.

Table 7. Amplitude parameters of the 100Cr6 steel surface and with DLC coating after tribological tests.

Material Parameter	Sa [µm]	Sp [µm]	Sv [µm]	Sz [µm]	Ssk	Sku
Dry friction 10 N
100Cr6	0.50	3.47	3.78	7.25	−1.20	8.72
DLC	0.49	2.06	3.09	5.15	−0.48	3.37
Dry friction 15 N
100Cr6	0.40	2.63	3.87	6.49	0.09	6.38
DLC	0.60	2.27	3.94	6.21	−0.67	3.84
Ionic liquid lubricated 10 N
100Cr6	0.26	1.86	1.74	3.59	−0.51	3.75
DLC	0.64	2.37	4.35	6.72	−0.44	3.19
Ionic liquid lubricated 15 N
100Cr6	0.29	1.23	2.18	3.41	−0.45	3.59
DLC	0.46	2.89	4.65	7.53	−0.49	4.09

shows average amplitude parameters of the wear traces on the 100Cr6 steel and DLC coating after all tribological tests.

After dry friction with a load of 10 N, the 100Cr6 steel without coating had more than twice the Ssk parameter value than the DLC coating after the test, which indicates a flat surface containing significant indentations. The Sku parameter value for the DLC coating was also about twice as low as for the 100Cr6 steel. After dry friction with a load of 15 N, similarly to 10 N, we observed an approximately two-fold reduction in the Sku parameter for the DLC coating.

After friction tests using lubricant with a load of 10 Nm, the differences between the amplitude parameters of both surfaces (Ssk and Sku) were comparable. The amplitude parameters of the tested surfaces after tribological tests with a load of 15 N with lubrication also confirmed small differences between the surface without or with a coating after lubrication with ionic liquid. Both results indicate good cooperation with the lubricant used.

3.6. Surface Morphology After Tribological Tests

Figure 13, Figure 14, Figure 15 and Figure 16 show the surface morphology of the samples and counter-samples after tribological tests. The tribological tests were performed at two loads: 10 N and 15 N and in dry friction conditions and with ionic liquid lubrication.

Due to the large differences in the sizes of the abrasion marks, the above-mentioned photographs are at different magnifications. After analyzing the photographs of the surface morphology of the samples and counter-samples, one can see significant differences between the abrasion marks. After dry friction, the abrasion marks on the coating were three times narrower compared to 100Cr6 steel. This relationship occurs for both loads and both friction conditions. Despite the widening of the abrasion mark width for the DLC coating, the abrasion mark was poorly visible, which indicates a very small loss of material after the friction-wear tests.

4. Conclusions

The following conclusions were reached based on the study:

The use of the DLC coating reduced the friction coefficient by about 57% compared to 100Cr6 steel for a load of 10 N and by about 60% for a load of 15 N during technical dry friction. The use of the DLC coating reduced the friction coefficient by about 20% compared to 100Cr6 steel for a load of 10 N and by about 18% for a load of 15 N during lubrication with ionic liquid. The use of BMIM-PF₆ ionic liquid as a lubricant additionally intensified the anti-wear effect. The lowest friction coefficients were obtained for the sample with a DLC coating applied using BMIM-PF₆ ionic liquid. This indicates synergy between the DLC coating and BMIM-PF₆ ionic liquid. The analysis of wear traces showed that for the samples (100Cr6 steel, DLC) lubricated with ionic liquid, no traces of wear were observed. For a 10 N load, the use of the DLC coating reduced the abrasion depth by over four times and the abrasion area by about three times compared to the uncoated sample and for a 15 N load, the use of the DLC coating reduced the abrasion depth by 10% but the abrasion area was similar to the uncoated sample (lubrication). The highest water contact angle value was recorded for 100Cr6 steel, while the lowest for the DLC coating. The contact angle measurements indicated a more hydrophilic nature of the DLC coating compared to 100Cr6 steel. The Sku values for clean steel during technically dry friction doubled and even tripled, which indicates very high surface wear during dry friction. The analysis of the Sku parameter for the DLC coating after all tribological tests indicates its practical invariability compared to the analysis of this parameter before friction. The use of ionic liquids, although currently quite expensive, can find applications in, among others, high-speed bearings, engines and compressors in extreme conditions (e.g., aviation or astronautics). They can be used not only in their pure form, but also as additives to various oils and greases to improve their durability.