EFFECT OF WETTING ON FRICTION M. Kalin*, M. Polajnar *Corresponding author:

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1 EFFECT OF WETTING ON FRICTION M. Kalin*, M. Polajnar *Corresponding author: Laboratory for Tribology and Interface Nanotechnology, University of Ljubljana, Ljubljana, Slovenia When it comes to design of mechanical systems in a specific application, determination of the lubrication regimes and oil film efficiency is crucial. In this case, primarily physical phenomena of contact interactions can be predicted with current engineering models, such as for viscosity, film thickness (with all subsequent surfacerelated consequences), lubrication regimes, etc. It is evident that these phenomena include also the solid-liquid interactions. Accordingly, it is of key importance for the engineering design to understand the surface-oil physical phenomena in the lubrication performance of the DLC-coated systems. This work describes how different base oils and other more pure model lubricants influence the oil film formation and lubrication behaviour due to physical interactions with DLC coatings. Phenomena such as wetting and surface energy are discussed and slip at the oil-dlc interface is evidenced. The work shows how the Stribeck curve and lubrication regimes are changed due to use of DLC coatings with properties different from those in conventionally used materials, and this affects also the lubrication design. A newlydeveloped inverse calculation technique with ready-to-use diagrams in combination with conventional models, which allow tailoring of the lubrication conditions for DLC coatings is presented. A paramount effect on friction was shown and reported in last years, reaching up to 40 % reduction. INTRODUCTION In the last decade or two, biomimetic studies have shown (1,2) that wetting, surface energy and other solid-liquid interfacial phenomena, which influence the interactions between the surfaces and surrounding liquids can govern the surface performance of many species in nature and even provide their particular functionalities such as selfcleaning, superhydrophobicity, excellent adhesion, reduced friction drag, etc. Since solid-liquid interactions are present in every lubricated tribological contact it seems quite obvious that these properties are important also for the tribological performance. In particular, solid - liquid interactions are especially pronounced in HD/EHD lubrication regimes, where full lubrication film is formed and there are no solid-solid interactions between the surface asperities. In spite of this obvious correlation, these parameters are often neglected, and they are not yet considered in most of engineering tribological models and theories. One of the reasons is that their influence on friction and wear is not yet well understood and that there exists no generally accepted model to encounter these phenomena in engineering design. What is more, there is the lack of an understanding also about the correlation between those parameters (i.e wetting and surface energy), which we discussed in two other studies where we investigated wetting with oils and water, specifically for DLC coatings (3) and also for a broad range of several classes of materials and fluids (different types of oils and water) (4). In this study it is shown how physico-chemical background of interactions between DLC coatings and lubricant influence on friction performance of DLC coatings in EHL regime. 4-67

2 EXPERIMENTAL Materials and oil Three different hydrogenated DLC coatings and steel were used for a comparison of the tribological, surface and interface properties when lubricated with PAO oil. The steel DIN 100Cr6 / AISI was used as a reference engineering material due to the fact that it is frequently used in tribological studies. Three hydrogenated DLC coatings with different structures and chemical compositions were selected in order to provide different wettabilities and surface energies. Namely we used non-doped, hydrogenated, amorphous DLC (denoted as a-c:h) and two doped, hydrogenated, DLC coatings. One of these two was a nitrogen-doped DLC (denoted as N-DLC), while the second was doped with fluorine (denoted as F-DLC). Details about coatings used are reported elsewhere (3). The selected oil was a synthetic, poly-alpha-olefin (PAO) oil, with a kinematic viscosity of 55.9 mm 2 /s and 9.0 mm 2 /s at 40 C and 100 C, respectively. The surface tension is mn/m and the corresponding: polar component is 6.58 mn/m while the dispersive component is mn/m. These data are adapted from (3). No additives were included in the base oil. Surface energy and wetting The surface energy of the selected materials was determined with the sessile drop technique by measuring contact angles and using Owens-Wendt-Rabel-Kaelble (OWRK) method (5): All the surfaces were before contact-angle measurements cleaned with high-purity ethanol and dried in a stream of hot air. At least 10 drops of each model liquid were deposited on each surface in order to ensure statistical relevant evaluation. The procedure how the surface energies are obtained is described in detail elsewhere (3,4). Wetting performance with used lubricating oil was determined by employing two different parameters, i.e. contact angle θ and spreading parameter SP. We used two different parameters for wetting evaluation because contact angle θ was proved as adequate parameter for wetting evaluation in case of adhesional-wetting, while spreading parameter SP in case of spreading-wetting. About these two types of wetting we discuss in detail in recent study (4). Prior to the contact-angle measurements the surfaces were rinsed with high-purity ethanol and dried in a stream of hot air. Contact angle measurements were performed using a contact-angle goniometer (CAM 101, KSV Instruments, Helsinki, Finland) at ambient temperature (22±2 C). Stable steady-state contact angle was determined after when reaching steady-state value (usually after about 15 seconds). For each surface several measurements of contact angles were done. The spreading parameter is determined by using equation (2) and data on individual components of free surface energy from the same samples, as described in detail in references (3, 4). (1) (2) 4-66

3 Tribological tests The tribological tests were performed using a mini-traction-machine (MTM, PCS Instruments, London, UK) and standard steel (DIN 100Cr6) specimens, i.e. discs with diameter 46 mm and ball with radius mm. The steel surfaces had the same level of R a roughness, i.e., 0.006±0.002 µm and 0.020±0.002 µm, for the discs and balls, respectively, measured with a stylus-tip profilometer (T8000, Hommelwerke GmbH, Schwenningen, Germany). On some of these disc and ball samples were later deposited DLC coatings as described above. We used steel/steel, steel/dlc (disc coated with DLC) and DLC/DLC (disc and ball coated with DLC) contacts, lubricated with PAO oil. All the surfaces were cleaned with ethanol and dried in a stream of hot air prior to the tests (i.e. the same cleaning procedure as before contact angle measurements). Each test was performed at ambient temperature (22±2 C) and repeated several times. The normal load was 5 N, resulting in 0.52 GPa of Hertzian pressure. A constant slide-to-roll ratio (ratio between the sliding velocity and the entrainment velocity) of 50 % was used. The entrainment velocity selected was from 0.8 m/s to 1.6 m/s in steps of 0.1 m/s. In addition to coefficient of friction, during the tests also electric contact resistivity (ECR) was measured and was always (for all contacts and through all entrainment velocities) about 100%, indicating that both surfaces were fully separated with a lubricating film, indicating a negligible amount of solid-solid asperity contacts during the tests and presence of EHD lubrication regime. RESULTS Surface energy and wetting The surface energies of studied surfaces are reported in Fig.1. It can be observed that steel provides the highest surface energy, i.e. around 43 mj/m 2. On the other hand, F- DLC coating provides the lowest surface energy, that is over two-fold lower compared to steel surface, i.e. around 18 mj/m 2. Furthermore, the comparison in surface energy between steel, N-DLC and a-c:h show that these three surfaces posses similar dispersive component of surface energy, i.e. around 32 mj/m 2, but different total surface energy. This means that polar component of surface energy is the key component that distinguish these three surfaces in surface energy. Steel has thus about two-fold higher polar component of surface energy compared to N-DLC coating, and about three-hold higher compared to a-c:h coating. F-DLC coating with the lowest total surface energy has on the other hand neglegible polar component of surface energy, i.e. about 1 mj/m 2 and thus 12-fold lower compared to steel surface. Neglegible polar component of surface energy for F-DLC coating shows very nonpolar nature of this surface, what means that this coating cannot form strong polar interactions with counter surfaces or liquids. 4-67

4 Figure 1: Surface energies of studied surfaces. Steady-state contact angles values of PAO oil on used surfaces are reported in Fig.2. It can be observed that contact angles for steel, N-DLC and a-c:h are very low, namely from 11.3 to 14.5 and also within the error of measurement, indicating that with contact angle values these three surface cannot be ranked in terms of which surface provides better or poorer wetting with used lubricating PAO oil. On the other hand contact angle of PAO oil on F-DLC is pronounced higher, namely around 57, what is consistent with surface energy results for this coating reported in Fig.1. Figure 2: Steady-state contact angle values of PAO oil on studied surfaces. Calculated values of spreading parameter SP are reported in Fig.3. The highest value of SP is observed on steel surface, followed by N-DLC and a-c:h. These three surfaces have the positive value of spreading parameter (values of SP from 5.2 mj/m 2 up to 11.8 mj/m 2 ) what means that used PAO oil spreads over these three surfaces. On the other hand, spreading parameter for used PAO oil on F-DLC coating is negative, namely mj/m 2. This results for F-DLC coating is in well agreement with the highest contact angle (see Fig.2), the lowest surface energy and dispersive nature of this coating (see Fig.1). 4-67

5 Tribological tests Figure 3: Spreading parameter values of PAO oil on used surfaces. Coefficient of friction results from tribological tests are reported in Fig.4. It can be observed that steel/steel contact provides the highest coefficient of friction through all entrainment speeds. Irrespective of the DLC coating type, steel/dlc contacts provide slightly lower coefficient of friction compared to the steel/steel contact, while the DLC/DLC contacts always provide the lowest coefficient of friction. Introduction of the DLC coating into the contact thus reduces the coefficient of friction compared to conventional steel/steel contact. This reduction is even more pronounced if both surfaces within the contact are coated with DLC. It can be also observed that coefficient of friction decrease with increase of entrainment speed what is attributed to the thermal effect of the liquid shear thinning.

6 Figure 4: Results of tribological tests for steel and self-mated contacts of: (a) N-DLC, (b) a-c:h and (c) F-DLC. DISCUSSION In results we observed that contacts with DLC coatings provide lower coefficient of friction compared to steel/steel contact (Fig.4). Furthermore, results of surface energy (Fig.1) show that DLC coatings also provide lower surface energy compared to steel surface, indicating weaker interactions with lubricating oils. The question is why should be poorer interactions on the solid-liquid interface connected with reduced coefficient of friction in EHL? The explanation arises from the nature of lubricating contact in EHL, where viscous friction within lubricating film governs the friction performance of the contacts. In some previous studies it was shown that solid-liquid slip reduce viscous friction (6-8) and it is reasonable to expect that slip occur also on DLC coatings, what has also already been reported (6, 8) and what also indicate our surface energy results. Namely, solid-liquid slip change the velocity profile in the way to reduce the viscous friction as presented in Fig.5.

7 Figure 5: Velocity profile in case: (a) no-slip and (b) solid-liquid slip. Lower surface energy of DLC coatings indicates poorer interactions with lubricating oil and thus also poorer wetting. Due to this fact we plotted wetting properties of surfaces used (contact angle and spreading parameter) against coefficient of friction of self-mated contacts of steel and DLC coatings as presented in Fig.6. Figure 6: Coefficient of friction (at an intermediate tested velocity of 1.2 m/s), the contact angle and the spreading parameter SP for steel, N-DLC, a-c:h and F-DLC coatings. From Fig.6 we can observe good correlation between spreading parameter and coefficient of friction, i.e. both parameters decrease in the same order of surfaces: steel, N-DLC, a-c:h and F-DLC. On the other hand expected increase of contact angle values it this order of surfaces is not so obvious, especially for steel, N-DLC and a-c:h coating. This discrepancy between contact angle and spreading parameter for wetting evaluation is attributed to the type of wetting. On steel, N-DLC and a-c:h surfaces lubricating oils (and used PAO oil in our study as well) exhibit spreading wetting that must be evaluated with spreading parameter instead of contact angle. About these phenomena we report in detail in our companion studies (3, 4). While we have in our study one oil it means that spreading parameter is function of surface energy only (see equation 2). As we evaluate surface energy of used surfaces also in terms of its polarity (we determine also polar in dispersive component of

8 surface energy), we calculated correlation coefficient for coefficient of friction vs. surface energy that are reported in Fig.7. Figure 7: Pearson product-moment linear correlation coefficients for coefficient of and total surface energy as well as its dispersive and polar component for steel and N- DLC, a-c:h and F-DLC coatings. Figure 7 shows almost perfect correlation between polar component of surface energy and coefficient of friction. This observation shows importance of the polar forces on the solid-liquid interface in terms of enhancing the solid-liquid slip and thus reduction of the coefficient of friction. As seen from Fig.1, the steel has the highest polar surface energy and this also results in the highest coefficient of friction (Fig.4). What is more, in the same order as decrease polar surface energy, i.e. steel, N-DLC, a-c:h and F-DLC, decrease also coefficient of friction (Fig.6). For this reason, the steel surface with the highest polar component of surface energy will form the strongest interactions with the adjacent layer of lubricating oil (due to permanent dipole) and so have the highest viscous friction, because the fluid velocity profile under common engineering conditions has (ideally) no slip, Fig.5a. On the other hand, DLC coatings that have a much lower polar component of surface energy compared to the steel surface will also provide much weaker bonding interactions with the adjacent layer of the oil. So, this first layer of oil will, due to the weaker interactions with surface, slip more easily over the surface when exposed to shear, which will then result in a reduced fluid velocity at the surface (Fig.5b) and so a lower viscous friction, which will determine the friction performance in the (E)HD lubrication regime. REFERENCES 1. Gorb S N: Biological attachment devices: exploring nature's diversity for biomimetics. Philos. T. Roy. Soc. A Bhushan B: Biomimetics: lessons from nature an overview. Philos. T. Roy. Soc. A Kalin M, Polajnar M: The correlation between the surface energy, the contact angle and the spreading parameter, and their relevance for the wetting behaviour of DLC with lubricating oils. Tribol. Int

9 4. Kalin M, Polajnar M: Wetting of steel, DLC coatings, ceramics and polymers with oils and water: the importance of surface energy, surface tension, contact angle and spreading. App. Surf. Sci Owens D K, Wendt R C: Estimation of the surface free energy of polymers. J. Appl. Polym. Sci Kalin M, Velkavrh I, Vižintin J: The Stribeck curve and lubrication design for nonfully wetted surfaces. Wear Choo J H, Spikes H A, Ratoi M, Glovnea R, Forrest A: Friction reduction in low load hydrodynamic lubrication with a hydrophobic surface. Tribol. Int Jahanmir S, Hunsberger A Z, Heshmat H: Load Capacity and Durability of H-DLC Coated Hydrodynamic Thrust Bearings. J. Tribol