**1. Introduction**

Coating is one of the approaches that can improve the tribological properties economically. In recent years, the development of the coating methods has opened up new possibilities to enhance the surface properties. Coatings can be generally divided into "soft coatings" and "hard coatings" [1]. Soft coatings, including soft metal (e.g., lead, indium) and lamellar solids (e.g., graphite and molybdenum disulfide (MoS2)), provide good shearing characteristics and thus result in a reduction of friction. Hard coatings (e.g., diamond-like carbon (DLC), titanium nitride (TiN)) can improve protection against wear and present low wear rates.

The unique properties of elastomers, such as low modulus of elasticity, high Poisson's ratio, and high degree of resilience with low hysteresis, make elastomers very suitable for the application as seals. However, high and erratic friction under dry and starved lubrication conditions could increase the friction and wear rates. As a consequence of surface damage, the lifetime of seals can be shortened greatly [2]. An approach to reduce the friction under dry and starved lubrication conditions is to deposit DLC on rubber. A lot of studies, from deposition techniques to DLC composition on various rubber materials, such as nitrile butadiene rubber (NBR), hydrogenated nitrile butadiene rubber (HNBR), fluoroelastomer (FKM), and ethylene propylene diene monomer rubber (EPDM), has been done by a

Japanese group of Nakahigashi [3,4], Takikawa et al. [5–7], a Dutch group of Pei and Bui et al. [8–12], and other researchers [13,14]. MoS2 as a solid lubricant is mostly employed with hard surfaces (e.g., metals, ceramics) [15–17]. As to the combination of the two coatings, Wang et al. [18] has deposited MoS2 on Steels with a supporting DLC film and it showed the MoS2/DLC compound film reduced the friction force in humid environment. Recently, Zhao et al. [19] has deposited the MoS2/DLC multilayer coatings on Si wafer and steel in high humidity for aerospace industries and it showed a moderate improvement of the tribological properties. The influence of space irradiation on MoS2/DLC composite film on Si and steel was investigated by Wu et al. [20]. It showed a reduction of the wear rate after irradiation, which could be related to the increase of hardness. Noshiro et al. [21] has studied the friction properties of sulfide/DLC coating with a nanocomposite or –layered structure on Si wafer, which shows better tribological properties than DLC film. Previous work has focused only on either the composite MoS2/DLC coating on metals or DLC and MoS2 separately as coating on elastomers. Therefore, more work is needed to investigate the potential of application of composite coatings on elastomers.

In this research, the tribological properties of DLC, MoS2, and combined coatings of MoS2 and DLC were investigated on four elastomers. Coated elastomers were tested in model tests and after that the results were verified in component-like tests. The influence factors of tribological behaviors are discussed. The aim of this study is to investigate the potential of tribological application of composite coatings of MoS2 and DLC on elastomeric substrates for industrial seals, especially under starved lubrication conditions. In addition, the study provides a guideline to evaluate the coatings.

#### **2. Experimental Details**

#### *2.1. Test Materials and Coatings*

Four classical sealing materials were tested; i.e., fluoroelastomer (FKM), nitrile butadiene rubber (NBR), hydrogenated nitrile butadiene rubber (HNBR), and thermoplastic polyurethane (TPU). Among these four elastomers, FKM is the softest material, having a shore—a hardness of 84; followed by NBR (85) and HNBR (86). Due to its special chemical composition, TPU is the hardest material with a shore—A hardness of 95. For ball on disc tests, the samples were 20 mm × 20 mm square rubber sheets with a thickness of 2 mm, which were produced by the compression molding process. However, slight differences could be found on the surface under the microscope among TPU, HNBR, and FKM. For TPU a totally different molding die was used and the surface was polished. This is explained in more detail in Section 3.1.1 (microscopic analysis). For ring on disc tests, special samples were used, which are structurally similar to seals [22]. In order to remove contamination on the substrate and also inside the rubber (e.g., plasticizers [23]), all of the samples were cleaned using the standard cleaning procedures [9]. The difference between set and actual values can be explained with sputtering duration (Table 1). As a result of about three times longer sputtering duration time of the DLC 300 nm than the DLC 150 nm, the actual thickness of the DLC 300 nm is over three times thicker. The thickness of MoS2 coating is proportional to the sputtering time. The thickness varied due to the influence of different sputtering parameters.

Table 2 shows the material and thickness of the investigated coatings. Five different materials (i.e., DLC, MoS2, and three hybrid combinations of DLC and MoS2 with various proportions) were deposited as coatings on the substrates. These two materials were not combined as multilayers, but rather in a composite. The proportion of MoS2 in the composite increases from Hybrid\_A to Hybrid\_C. Based on our previous work, the set value of 300 nm was selected as the standard thickness for the coatings and the set values of the thickness were defined based on the deposition rate [24]. In order to investigate the influence of the thickness on the tribological properties, 150 nm thick coatings were also obtained through controlling the deposition process time. In order to measure the thickness, several samples were partially covered with tapes during the coating process. After removing the

tapes, the thickness was measured with a contact stylus profilometry (Dektak 150 surface profiler, Veeco, Plainview, NY, USA).

In order to improve the adhesion of the coatings, prior to deposition a pre-treatment process was carried out by using a high vacuum experimentation bell jar system (Leybold Univex 450, Leybold Vacuum GmbH, Cologne, Germany) [25,26]. Substrates were fixed on the rotary table ( ϕ = 560 mm) with a distance of 12 mm to the target.

The cylindrical pulsed laser deposition (PLD) evaporator was used as a target. The pre-treatment was performed at 3 kV DC acceleration voltage with 15 sccm Ar and 5 sccm O2 gas flow. The chamber pressure was around 8.8 × 10−<sup>4</sup> mbar. After pre-treatment, the coatings were deposited by means of the pulsed DC magnetron sputtering method. A graphite target (electrographite, 99.5% purity) was used as a sputtering source for DLC coatings. For MoS2 its purity is 99.5%. Both targets were purchased from Sindlhauser Material GmbH (Kempten, Germany).

The parameters of the pre-treatment and deposition process are shown in Table 1. For pure DLC film, the ratio of C2H2/Ar was 0.19, due to the existence of C2H2, a-c: H film was generated [27,28]. For the hybrid coatings, only Ar was used as a source gas [28]. For the hybrid coatings, graphite and MoS2 were ejected individually from two sputtering sources. Different hybrid variants were generated by varying sputter power. Remarkably, differences of the micro-structures can be observed on the coating when the substrates were deposited at different temperatures [29]. To avoid the thermal influences on substrates and coating processes, the pre-treatment and deposition processes were performed under constant ambient temperature (23 ◦C). However, due to plasma flow the temperature of the sample surface can increase up to 40 ◦C. After the deposition process, the samples were stored in Petri dishes in a box.

An optical microscope (Stereo Microscope SZX 12, Olympus, Tokyo, Japan) was employed to analyze the wear scars of the counterparts. The surface roughness was measured in three different regions of each sample with a three-dimensional focus variation microscope (InfiniteFocus, Alicona, Graz, Austria). The surface morphology and wear tracks of coated rubber were characterized with a scanning electron microscope (SEM, VEGA-II, TESCAN, Brno, Czech Republic).

In order to characterize the chemical composition of the coatings, X-ray photoelectron spectroscopy (XPS) analysis were carried out using a Thermo Scientific spectrometer with a micro-focused monochromatic Al K α source (1486.6 eV, Thermo Fisher Scientific Inc., Waltham, MA, USA). All measurements were conducted with the radiation source operated at 12 kV and a beam current of 1.16 mA in a high vacuum below 10−<sup>7</sup> mbar. A hemispherical analyzer was applied to detect the accelerated electrons. The electrons were collected from a spot area of 300 μm, which is vertical to the analyzer. To prevent charging and electron charge compensation of the samples, a flood gun was used.

Survey scans were acquired within an energy range of 0–1350 eV using a pass energy of 200 eV, a step size of 1.0 eV, a dwell time of 50 ms, and 2 scans. High resolution scans were obtained using a 50 eV pass energy, 0.1 eV step size, a dwell time of 50 ms, and 8 scans. For C 1*s*, Mo 3*d* and S 2*p*, binding energy ranges and total number of energy steps are as follows: 279–298 eV, 181 steps; 222–240 eV, 181 steps; 157–170 eV, 181 steps; respectively.

The spectra were referenced to the alkyl C 1*s* photoelectron peak at 284.8 eV, characteristic of the alkyl moieties (C–C/C–H). Peak positions for qualitative analysis are consistent with the corresponding assignment positions found in literature [30].

Spectra were analyzed using the Thermo Avantage software (Version 5932). The ratio of Lorentzian/Gaussian is 0.3. A standard Shirley background is used for the reference samples spectra. The spectra were fitted with Powell algorithm with a convergence of 10−6. The maximum error for peak energy and full width at half maxima (FWHM) is ±0.1 eV. The sensitivity factors (SF) used for calculation are provided by the equipment supplier.



**Table 2.** Material and thickness of the coatings.


The determination of surface energy was carried out in a self-developed contact angle device. Distilled water and diiodomethane were applied as liquids to determine the polar and dispersive part of the surface energy, respectively. For each measurement, a drop of 2.5 μL volume was used. Each measurement was repeated three times. Owens et al. [31], Rabel [32] and Kaelble [33] method was applied for calculating the surface energy.

#### *2.2. Test Procedures*

The tribological properties were investigated by means of model tests and component-like tests. The model tests were performed on a micro tribometer with a ball on disc configuration (UMT-2, Bruker, Billerica, MA, USA). The development of the sample geometry for the component-like test was reported by Hausberger [22]. The tests were performed on a precision rotary tribometer (TE-93, Phoenix Tribology Ltd., Kingsclere, UK). Each test was repeated three times. All of the tests were conducted at room temperature (22 ◦C) with a relative humidity of 50% ± 10%. About one month after the coating process, the tribological tests were performed.

#### 2.2.1. Ball on Disc Tests

Commercial 100Cr6 stainless steel balls of 6 mm diameter (HRC 60–62) were used as counterparts. The counter body slid on the elastomer at 100 mm/s with 1 N normal load. The radii of the run tracks were 5 mm and 7.5 mm. The total length of the tracks was 3.143 × 10<sup>5</sup> m. In order to obtain a better understanding of the function of the coatings, the tests were performed under dry and lubricated conditions. For the lubricated tests, approximately 7 mg Mobil SHC Grease 460WT (Viscosity of Oil, ASTM D 445 [31] cSt @ 40 ◦C = 460) was smeared equally over the whole surface [32]. The average thickness of the grease can be calculated. Its amount was chosen so that the thickness of the grease layer was approximately 0.02 mm.

#### 2.2.2. Ring on Disc Tests

Ring-shaped counterparts of 34CrNiMo6 were used in the ring on disc test. They possessed an average roughness (*R*a) of 0.035 μm. The ring-like sample was so constructed that there was only a line contact between the sample and counterpart [22]. The tests were conducted with 50 N normal load at room temperature (23 ◦C) and the speed of revolution was 118 rpm. The aim of this research is to improve the tribological properties of seals under starved lubricated conditions. In order to simulate starved lubrication condition in component-like tests, approximately 2 mg Mobil SHC Grease 460WT was smeared on the contact edge of the samples. For uncoated samples the tests lasted 168 h. For the coated samples, the tests were stopped automatically when the abort condition was reached. The abort condition was set according to the coefficient of friction of the uncoated samples. The principle of the ring on disc test is illustrated in Figure 1.

**Figure 1.** Principle of ring on disc test on TE-93.

The counterpart was fixed on the counterpart holder. The load, which was produced by a pneumatic pump, acted on the sample through the thrust bearing and counterpart. An electric motor was mounted on the top of the machine and drove the sample against the counterpart in a rotational movement. The torque, which was generated through friction, was measured by a torque sensor. Furthermore, the temperature near the contact area and in the middle of the counterpart was also measured during the test.

#### **3. Results and Discussion**

#### *3.1. Characteristics of Coatings*

After deposition the thickness of coatings was measured. The chemical composition was investigated with XPS measurements. The microstructures of the surfaces were analyzed with roughness and compared among different substrates. Furthermore, the surface energy of the substrates and coatings were identified.

#### 3.1.1. Thickness of the Coatings

For each coating, the thickness was measured at six different positions of the two samples. Table 2 shows the set and actual average thickness. The difference between set and actual values can be explained with sputtering duration (Table 1). As a result of about three times longer sputtering duration time of the DLC 300 nm than the DLC 150 nm, the actual thickness of the DLC 300 nm is over three times thicker. The thickness of the MoS2 coating is proportional to the sputtering time. The thickness varied due to the influence of different sputtering parameters.

As reported in [7], the application of C2H2 accelerates the deposition rate, which leads to a higher thickness than the set value.

#### 3.1.2. Chemical Composition

The chemical composition, the assigned peak energies, full width at half maxima (FWHM), and sensitivity factor (SF) of each peak are given in Table 3 and were obtained with XPS analysis.


**Table 3.** Spectral fitting parameters.

In Table 4, the chemical composition of each coating is listed. In order to avoid the influence of the different elastomeric substrates, coatings were deposited on silicon for the XPS analysis.

In both DLC coatings, the portion of C 1*s* is about 90% with no detectable silicon signal corresponding to a homogeneous carbon layer formation. The dominating carbon species are C–C/C–H bonds at 284.8 eV (Figure 2a) which are unambiguous assigned to the atomic structure of the used DLCs. The beneficial properties of DLC in tribology depend mainly on the similar hardness and Young's modulus as diamonds [28,40]. Besides, C–O and –COO signals were also found and are attributed to the surface oxidation during the coating process and storage [41] and is in good agreemen<sup>t</sup> with the results obtained in [42].


**Table 4.** Chemical compositions (C, O, Mo, S, and N) of the coatings.

In pure MoS2 coatings, the high nitrogen and carbon amounts are attributed to atmospheric contaminations (CO2, hydrocarbons, N2, etc.) during sample transport or storage or manufacturing of the samples. However, the S 2*p* doublet at 162.0 eV (ΔeV = 1.18) in combination with the doublet at 229.0 eV and 232.1 eV is unambiguous assigned to MoS2 (Figure 1c,d). A second doublet in the Mo 3*d* spectra is attributed to Mo with environment as in MoO3 [43]. On the subject of oxidation of molybdenum disulfide to molybdenum (VI) oxide, different reports were found [15,44]. In general, the oxidation rate is extremely low at ambient temperature and in the absence of a high concentration of moisture [15]. The oxidized layer at the outmost surface appears to protect the bulk material from further oxidation. However, different oxidation rates at ambient condition were investigated, and it was found the crystallite orientation plays an important role in the oxidation process [45]. Oxidation leads to a higher friction coefficient, enhanced wear rate, and hence a shorter wear life [36,46]. The ratio of MoS2/MoO3 in the reference coatings indicates that oxidation had occurred but the major portion is still MoS2. For the hybrid samples (Figure 2d–f), the amount of MoS2 is increased from Hybrid\_A to Hybrid\_C. As a result, the ratio of total S/Mo decreases from Hybrid\_A (1.3) to Hybrid\_C (1.0). The higher the concentration of MoS2, the higher the oxidation rate and as a result the lower the MoS2/MoO3 ratio. It is suggested that the increase of MoS2 amount in the coatings accelerates the oxidation rate and is highest for the Hybrid\_C sample.

The S 2*p* spectrum appears as two overlapping doublets. This means, different types of sulfur ligands, such as bridging terminal S2<sup>2</sup><sup>−</sup>, and bridging S2− species exist in the coating [37]. In addition, our results were in agreemen<sup>t</sup> with Benoist et al. observations as a higher oxygen content lead to a decrease in the S2− sulphur component whereas the S2<sup>2</sup>− pair increase [43].

**Figure 2.** *Cont.*

**Figure 2.** X-ray photoelectron spectroscopy (XPS) spectra: C 1*s* of diamond-like carbon (DLC) coatings (**a**) and hybrid coatings (**d**); Mo 3*d* of MoS2 coatings (**b**) and hybrid coatings (**e**); S 2*p* of MoS2 coatings (**c**) and hybrid coatings (**f**).

#### 3.1.3. Microscopic Analysis

The uncoated substrates, except HNBR, were analyzed with a microscope and presented in a previous work [24]. Generally, on a macroscopic level the uncoated HNBR, NBR, and FKM possess similar parallel, strip-like structures, whereas TPU presents completely different structures. Due to the different physical properties of elastomers, especially elasticity and viscosity, which can have an effect on the flow properties in the molding process, they behaved differently during the processing [47]. Although uncoated HNBR and FKM show similar macrostructures, on the microscopic level utterly different microstructures can be observed. The surface of uncoated HNBR is relatively smooth but with some small debris. However, the surface of uncoated FKM is much rougher and with dense particles. This can also be explained with *R*a and *R*z. In spite of the very similar *R*a value of uncoated HNBR and FKM, the *R*z value of uncoated FKM is more than 30% higher than that of uncoated HNBR (Table 5).


**Table 5.** The average roughness (*R*a), mean roughness depth (*R*z) of uncoated samples.

The surface of uncoated TPU was full of small strips. However, the strips were not as neatly arranged as those of HNBR and FKM. Also, compared to HNBR and FKM, the strips on TPU were much narrower and shallower. Another difference, which must be mentioned, is that, except for the strips, there were almost no small debris or particles on uncoated TPU.

In this section, for each substrate two coatings have been chosen and discussed. The two coatings were so chosen that, regarding the substrate, one of them showed the best tribological performance and the other showed the worst in the dry and lubricated ball on disc tests. In addition, one thin coating for HNBR was chosen to analyze the influence of the thickness.

The strip-like structures on the surface, which can be observed in Figure 3a, were produced because of the compression molding process. Not only can these structures be found in HNBR, but also in FKM and NBR. Figure 3c shows one other position from the same sample as Figure 3a. Not like the rough surface in Figure 3b, flake-like structures with small debris can be observed in Figure 3d. Moreover, cracks can be observed on the surface. As reported by Takikawa and Pei, cracks are typical surface structures of DLC coated rubber [5,12,29].

**Figure 3.** With 300 nm (**<sup>a</sup>**,**<sup>c</sup>**) and 150 nm DLC (**e**) coated hydrogenated nitrile butadiene rubber (HNBR); with 300 nm Hybrid\_A coated HNBR (**g**), fluoroelastomer (FKM), (**i**) and nitrile butadiene rubber (NBR) (**o**); with 300 nm MoS2 coated FKM (**k**), NBR (**m**) and thermoplastic polyurethane (TPU) (**s**); with 300 nm DLC coated TPU (**q**). High magnification (**b**,**d**,**f**,**h**,**j**,**n**,**p**,**r**,**s**,**<sup>t</sup>**) are shown to the right side of the respective low magnification (500×).

Compared with the 300 nm DLC coating, the 150 nm DLC coating looks smoother on the whole. However, small particulates can be observed on the surface (Figure 3f). Figure 3g shows the microstructures of 300 nm Hybrid\_A on HNBR. Scaly microstructures were observed and they look similar to the DLC coating to some degree (Figure 3b). As previously mentioned, Hybrid\_A is a composite coating, which possesses the least MoS2 among the three hybrid coatings. However, a small amount of MoS2 changed the microstructures considerably. The gaps between each piece of debris are smaller and the coating is noticeably smoother. This can be attributed to the much lower hardness of MoS2 compared to DLC [15,48]. It seems that MoS2 lowered the average hardness. Therefore, the coating can be better suited to the substrates roughness.

Comparing Figure 3h,j shows that the coating roughness was influenced to some extent by the substrate properties. Moreover, under high magnification, small holes can be identified on FKM with a 300 nm Hybrid\_A coating (Figure 3j). That means the coating did not totally adhere to the substrate. This can be caused by the lower wettability of FKM compared to HNBR (details in Section 3.1.5). Small holes can also be observed on MoS2 coated FKM (Figure 3k). However, the coating from MoS2 looks much finer and smoother than the hybrid coating.

Generally, the surfaces of coated NBR are smoother compared to coated HNBR. Also, it should be emphasized that almost no debris could be found on the surface after coating. Moreover, as can be observed in Figure 3o, cracks which were caused by the removal of the sample from the deposition chamber, are rather neatly arranged on the surface, either parallel or perpendicular to the original microstructure of the substrate.

For TPU samples, they do not have the strip-like, neatly arranged microstructures like other substrates (Figure 3q,s). Because of its shallower and sparser microstructures, the roughness of the TPU substrate is correspondingly lower. Like the previous comparison, MoS2 coated TPU is also finer and smoother than the DLC coated TPU (Figure 3s).

From the above comparisons, several influence factors that contribute to the coating microstructures were found and discussed. Firstly, the substrate topography is one of the most important influence factors for the coating microstructure. That is because of the smaller thickness (150–300 nm) compared to the roughness of the substrate (Table 5). Secondly, the composition of the coating plays an important role as well. Generally, on DLC or DLC-included coating small debris can be observed. In comparison with DLC, the MoS2 coating is finer and smoother. Thirdly, the coating microstructures can be influenced by the material properties of the substrates in several ways. Coatings on a substrate like FKM, which has a lower wettability, show a higher possibility that the coating becomes porous and loose. Thermal properties (e.g., thermal expansion coefficient and thermal conductivity), are also influence factors. As shown in Table 6, FKM expands the most among the four materials, when the temperature increases by a given degree. This can lead to the scaly coating, which can be observed in Figure 3i,k.



However, the importance of these factors depends to a large extent on the ambient conditions of the coating process. In addition, the microstructure of the coatings is also affected by the deposition condition. For DLC coating, a-c: H was produced by using a plasma of Ar and C2H2, while for other coatings, only Ar was used. Generally, the coatings prepared with C2H2 look smoother than those without C2H2. This is in good agreemen<sup>t</sup> with the results in [7].

#### 3.1.4. Surface Roughness

The surface roughness of the coated samples was affected by the substrate surface and also the microstructures of coatings, which can be changed by removing the samples from the deposition chamber (Figure 3o). In addition, the surface microstructure can also affect the adherence of the coating [49]. Generally, a rougher surface can have a better adhesion with coating because more bonding connections can be created. However, the scale of dimensions of the surface microstructures must be less than the film thickness [50]. As can be seen from Figure 4, uncoated substrates have different degrees of roughness. FKM and HNBR possess a similar roughness (*R*a ≈ 1.0 μm), while TPU and NBR have an appreciably lower roughness value. As mentioned previously, two different molding dies were used to produce samples, one for FKM, NBR, and HNBR, the other one for TPU. Moreover, the surface microstructure can be affected by additives, which could come up on the surface.

**Figure 4.** Surface roughness (*R*a) of uncoated and coated substrates.

As can be seen from Figure 3a,e, the HNBR with 150 nm DLC is smoother than that with 300 nm DLC. However, for the other three materials with DLC coating, the thickness does not play an important role in the surface roughness. Compared to the uncoated FKM, the roughness of 300 nm MoS2 coated FKM was reduced drastically, whereas the roughness of DLC-containing coatings on FKM was increased to varying degrees. This could be attributed to the larger difference between DLC and substrates in hardness and brittleness [48]. The DLC coatings with a very low thickness could be broken into fractures easily, when the coated samples are removed from the deposition chamber with a small deformation. However, this phenomenon was not found on other materials. For hard material TPU, no obvious differences could be identified in roughness. On the one hand, due to its different processing, its surface is smoother than other materials. On the other hand, higher hardness prevents its deformation by removal.

#### 3.1.5. Surface Energy

One of the conditions for good wetting is that the surface tension of the substrate is higher than that of the still liquid coating material [51]. To eliminate the influences of substrates, coatings were also deposited on silicon. As shown in Figure 5, uncoated Si and coated Si possess higher surface energies than the other four substrates. Surface roughness plays an important role for the surface energy [52]. For four elastomeric substrates, surface energies were increased to varying degrees after coating. On the one hand, through comparison of the microstructures before and after deposition, it can be found that it changed significantly. Although the mean roughness (*R*a) of the substrates had not been changed in a very large way, the microstructures were totally modified after the coating process. This leads to a

modification of the surface energy. On the other hand, from the perspective of the material, the surface energy of elastomer substrates [53], DLC and MoS2 are also different. These two factors together affect the difference of the surface energy after coating.

Compared with an uncoated elastomer, silicon shows a much higher surface energy in both polar and dispersed parts. After coating silicon shows a similar surface energy to the elastomers. Generally, FKM has almost the lowest surface energy in all coatings. Except for the influence of its chemical structure, the surface microstructure of uncoated FKM is different from HNBR and NBR. Comparing to HNBR, which has a similar mean roughness (*R*a), FKM is much rougher and dense particles can be seen on the surface [24]. The film thickness has only a limited effect on surface energy.

**Figure 5.** Surface energy of uncoated and coated substrates.

#### *3.2. Tribological Tests*

In order to study the potential of DLC/MoS2 coatings on elastomers for tribological applications, the coatings were firstly tested in the model test under dry and lubricated conditions, so that the coatings could be evaluated comprehensively. Subsequently, the best and worst coatings were selected and investigated under starved lubrication condition in component-like tests.

#### 3.2.1. Coefficient of Friction

The coefficients of friction (COF) for uncoated and coated elastomers in dry and lubricated ball on disc sliding tests are shown in Figure 6. Under dry sliding ambient conditions, almost all of the coatings bring an advantage to the tribological properties. In particular for HNBR, with 300 nm DLC coating, the COF was reduced from 0.99 to 0.18 by 82%. For NBR the frictional reduction, which the 300 nm DLC or Hybrid\_A coating brought, was also significant; approximately 74%. For FKM and TPU, the decrease was not so appreciable. What was interesting was that for TPU the COF was slightly brought down by 300 nm Hybrid\_B coating. The values of these measurements are in good agreemen<sup>t</sup> with the values reported in the literature [3,12,54]. However, when the thickness of the coating was reduced to 150 nm, the COF increased by 11%, compared to the uncoated TPU. This can be explained by microscopic analysis. As can be observed in Figure 7, the 150 nm coating was already severely damaged (Figure 7b) and the elastomeric substrate had direct contact with the counterpart during the test, while the thick variant was still intact (Figure 7a). That means for this coating, the thickness plays an essential role with respect to the tribological properties. However, the thickness cannot bring a significant difference in every case. That depends on several factors, for example, the hardness of the substrate, the coating material, and the adherence of coating material on the counterpart, the

microstructures of the surface, the adherence between coating and substrate, lubrication conditions and so on. Adhesion and deformation are the two most important mechanisms that are responsible for the frictional behaviors of elastomers [55,56]. The high friction of uncoated HNBR and NBR under dry conditions show that not only deformation, which can be related to the relatively low hardness, but also adhesion, which can be seen as a dissipative stick-slip process on molecular level, are influential factors for the dry frictional behaviors [57–60]. This is in good agreemen<sup>t</sup> with Rabinowicz's studies, which indicated that low ratios of surface energy/hardness are associated with better surface interactions and also less adhesion [61,62]. Moreover, because of the high friction more dynamic energy would be expected to be transformed into heat, which could lead to an increase of temperature. Based on this conjecture the material's hardness will reduce with a higher temperature so that it could experience a higher wear rate [63].

**Figure 6.** Coefficient of friction of uncoated and coated elastomers in dry and lubricated ball on disc sliding tests: (**a**) FKM; (**b**) HNBR; (**c**) NBR; (**d**) TPU.

**Figure 7.** Microscopic images of wear tracks: (**a**) 300 nm Hybrid\_B coated TPU; (**b**) 150 nm Hybrid\_B coated TPU.

Under lubricated conditions, the differences of COF among various coatings were not as evident as in dry tests. The lubricant has no significant impact on the COF. One reason for this is that lubricants facilitate the stick-slip process on the molecular level to some extent. Therefore, the adhesion part for friction can be decreased [64]. As to the deformation part, it was assumed to stay on a similar level as under dry conditions in two aspects. One aspect is that the lubricant film is very thin on the contact area hence the stiffness of the film is negligible. The other aspect is that the COF of various coatings is similar.

It should be noted that in some cases the lubricant even brought a slight, negative impact on the tribological properties for DLC coated HNBR and NBR. This can be explained with two main reasons. One important aspect is that because of the high viscosity of the grease used, more energy would be needed to overcome the fluid friction [64,65]. The COF under dry conditions was extremely low. In this case, the benefit of the lubricant was less than its disadvantage. That means more energy was needed to overcome the resistance, which was brought by the lubricant.

Based on the results of the ball on disc tests under dry and lubricated conditions, the best coatings were chosen and verified in the component-like test (ring on disc). For FKM with Hybrid\_A coating, its dry COF is slightly lower (2.5%) than Hybrid\_B 300 nm. However, its lubricated COF is about 13% higher than Hybrid\_B 300 nm. For HNBR with Hybrid\_A coating, it is clear that MoS2 brings a negative effect for the tribological performance. In the dry tests, the coatings with pure MoS2 or high content of MoS2 (Hybrid\_B and Hybrid\_C) were broken after the tests. Therefore, these coatings were not taken into consideration for the selection. In addition, as references, uncoated substrate and the worst coatings were also tested. The best and worst coatings are presented in Table 7. As can be seen from this table, the soft coating MoS2 provides the best tribological properties for the softest material FKM, whereas the hard coating DLC is the best choice for the hardest material HNBR, only among FKM, NBR, and HNBR. For NBR, which has a middle hardness, a hybrid coating is better than other coatings. Because of its totally different surface structures, TPU was not comparable with the other elastomers.


**Table 7.** The best and worst coating for each material from ball on disc tests.

For the ring on disc tests an abort condition was set up so that when the coating was worn or damaged, the test would be stopped immediately. As abort condition, an average COF of the uncoated substrate under stable running conditions was employed. As shown in Figure 8, at the beginning of the tests, for HNBR and TPU the COF of the best and worst coatings were almost at the same level. However, the COF of the uncoated substrate kept at a constant level after the running-in phase with a higher value, while the COF of the worst coating started to increase gradually. After just several hours, the friction was raised to the same level as the uncoated substrate. Compared to the worst coating, the best variant lasted significantly longer until the COF reached the abort condition. This means that the coating failed with increasing test time. Therefore, for HNBR, NBR, and TPU the trends of validation show a good correspondence with the results from the ball on disc tests. However, for FKM with the best coating, after the loading phase, its COF was already slightly over the abort condition, which represents the COF of uncoated FKM. It was found that the coating was already damaged. This implies that the combination of soft coatings like MoS2 and soft substrate like FKM is inappropriate for this line contact. Because of its low hardness, FKM showed a strong local deformation under line contact. According to Archard's wear law for adhesive wear [66], wear volume is inversely proportional to the hardness of a substrate. By this situation, in which the contact area is relatively small, the soft coating on a soft substrate could be worn quickly.

**Figure 8.** Comparison of the COF: uncoated and coated in Ring on disc test. (**a**) FKM; (**b**) HNBR; (**c**) NBR; (**d**) TPU.
