The tribological performance of DLC-based coating under the solid–liquid lubrication system with sand-dust particles

Combination of solid and liquid lubricants to meet emission or environmental requirements of future tribological systems while providing the levels of desired friction and wear performance have received considerable research attention in the near term. The aim of the present work was to investigate the tribological behavior of oil-lubricated (PAO, PFPE, SO, IL and MAC) DLC coated surfaces under the conditions without and with sand-dust particles. The effects of applied load, frequency, and sand-dust particles on the tribological performance of DLC coating were systemically studied. The analysis results showed that solid–liquid lubricating coatings including SO and IL exhibited excellent anti-friction (～0.026) but relative poor wear-resistance performances under the conditions without and with sand-dust environments. But for PFPE and PAO, they demonstrated the worst tribological behaviors with high friction coefficient and wear rates. The added sand-dust particles lead to the wear rates to the one order of magnitude large than that without sand-dust conditions for all the selected liquid lubricants. The viscosity, contact angle and work of adhesion played an important part in affecting the tribological performances. The lubrication regimes in Stribeck curve for the five kinds of liquid lubricants were affected obviously by the sand-dust particles in different way. The formed transfer films on the coating surface and pin have much influence on the tribological behavior and the transition between lubrication regimes.     

Laplace pressure measurement on laser textured thin-film disk

To increase the recording density of hard disk drives (HDD), head and disk surfaces must be very flat. This will make the friction between them large when liquid bridges are formed. This is a result of Laplace pressure in the liquid bridge. Therefore, the study of Laplace pressure in real HDD interface is of an interest for head-disk interface engineers. However, Laplace pressure of perfluoropolyether (PFPE) lubricant on carbon coated thin-film disk surface was not clear until now.We measured Laplace pressure between transparent flat pins and carbon coated thin-film disks with laser texturing. Using laser textured disks, we could control the distance between two surfaces precisely by the bump height. The friction coefficient between the pin and the disk surfaces was determined when the interface was fully wet by liquids. It was 0.16 and 0.1 for water and a PFPE lubricant. The Laplace pressure was then calculated using the friction force and liquid wet area when the interface was partially wet by a liquid. The liquid wet area was measured by the observation of the contact point through the transparent pins.The results showed that the Laplace pressure at the lowest bump height (11 nm) was about 2.8 MPa for the PFPE lubricant. Results agreed well with calculated curves. We consider that PFPE acts as liquid down to 11 nm.     

Application of diamond-like carbon coatings to elastomers frictional surfaces

Nitrile-butyl rubber-like materials were coated with amorphous hydrogenated diamond-like carbon (DLC) coatings in order to modify their surface and tribological properties. Measurements of water contact angle were performed by the sessile drop method and showed that the coated samples are more hydrophobic with water contact angles up to 116°. The surface free energy of the elastomers was calculated by the acid-base regression method considering polar and dispersive contributions and the results were correlated with changes in the surface chemistry measured by X-ray photoelectron spectroscopy. It has been found that the lower presence of oxygen functional groups on the elastomer surfaces led to lower surface free energies, even though the polar contribution was not predominant. We also found that the DLC coatings led to a significant decrease of the surface free energy (up to 16%) and that there is a good correlation between the surface free energy values and the corresponding water contact angle values. The coefficient of friction was also measured and presented a significant decrease after coating with DLC.     

Kelvin probe study on the perfluoropolyether film on metals

The effect of mineral oil on surface potentials of copper and aluminum coated with a perfluoropolyether (PFPE) film is studied with a non-vibrating Kelvin probe. The probe measures the contact potential difference (CPD) between the probe surface and the coated copper and aluminum surfaces. The PFPE film was applied to the Cu and Al surfaces by a dip-coating technique. The Kelvin probe signals are especially sensitive to the interface between the coated and uncoated regions of the metal surfaces. It is found that the PFPE film causes the surface potentials of Cu and Al to drop by 150 and 98 mV, respectively. Immersion of the PFPE-coated surfaces in mineral oil causes the surface potential difference of the PFPE/Cu and PFPE/Al to increase, and removing the mineral oil re-establishes the surface potential.     

Nanotribological properties of novel lubricants for magnetic tapes

Two classes of novel lubricants, perfluoropolyethers (PFPE) and ionic liquids (ILs), were deposited on metal film magnetic tapes. The adhesive force and coefficient of friction of lubricated and unlubricated tapes were investigated at the nanoscale with an atomic force microscope (AFM) as a function of various humidity and temperature conditions. Microscale tests with a ball-on-flat tribometer were also performed in order to study the length-scale effects on friction. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated by monitoring the friction force, surface potential and contact resistance with the AFM. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) experiments were conducted to determine the chemical species that affect intermolecular bonding and as an aid in interpreting how the lubricant film tribological properties vary with the environmental conditions. Z-TETRAOL, one of the PFPEs, was found to exhibit the lowest adhesion and friction among the lubricant films studied. The ionic liquid 1,1′-(pentane-1,5-diyl)bis(3-hydroxyethyl-1H-imidazolium-1-yl) di[bis(trifluoromethanesulfonyl)imide)] exhibited comparable nanotribological properties with the PFPEs. This is attributed to the presence of hydroxyl groups at its chain ends, which can hydrogen bond with the surface similar to PFPEs.     

The effects of texture height and thickness of amorphous carbon nitride coating of a hard disk slider on the friction and wear of the slider against a disk

In order to minimize the stiction force caused by contact of the extremely smooth surfaces of head sliders and disks in hard disk drives, texture is usually applied on the disk surface. For future contact/near-contact recording, the stiction-induced high friction between slider and disk will become a problem. Texture on the slider/disk interface will still be an expected method to reduce friction. Recently, it was suggested to texture the slider surface. A protective coating is usually required on the textured slider surface to reduce wear of the texture. The results showed that texture on the slider surface was effective in reducing the friction between head sliders and disks. On the other hand, the texture and coating on the slider surface increase the spacing between the read/write element and the magnetic layer of the disk. The necessary and effective texture height and coating thickness are still not clear. In the present research, island-type textures with different heights (3–18 mn) were formed on slider surfaces by ion-beam etching. Amorphous carbon nitride (a-CN_x) coatings of different thicknesses (0–50 nm) were coated on the textured slider surfaces as a protective overcoat. The friction and wear properties of these sliders were evaluated by constant-speed drag tests against hard disks coated with diamond-like carbon (DLC). The results show that 2 nm texture on a slider surface is sufficient for low (0.3–0.5) and stable friction of the slider against the disk in a drag test, and coatings thicker than 5 nm show similar wear resistances of the texture on slider surfaces.     

Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces

The effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si₃N₄) ball counterface. The coefficient of friction for the negative fingerprint textured surface (μ=∼0.08) is much lower than that of the untextured surface (∼0.2) and the honeycomb textured surface (∼0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation.     

Tribological Properties of Ultrathin DLC-Coated Nanotextured Surface on a PET Film

The wear resistance of two types of nanotextured surfaces, fabricated on polyethylene terephthalate films and coated subsequently with a diamond-like carbon (DLC) layer and then a self-assembled monolayer (SAM) film with perfluoropolyether main chains, was compared. Nanotextures with hole and pillar patterns were fabricated by a nanoimprint process. The hole nanotexture with DLC and SAM films showed better wear resistance than the pillar nanotexture, and that film showed higher oil repellency than the pillar nanotexture with DLC and SAM films. The von Mises and shear stresses of the pillar nanotexture were concentrated on the center and at the bottom of the asperity surface, and it was estimated from finite element method calculations that these stresses accelerate the wear of asperities.     

Nanotribological properties of silicon nano-pillars coated by a Z-DOL lubricating film

This paper reports a novel approach for improving the nanotribological properties of silicon (Si) surfaces by topographically and chemically modifying the surfaces. In the first step, Si (100) wafers were topographically modified into nano-pillars by using the photolithography and reactive ion etching (RIE) techniques. Various patterns, including nano-pillars of varying diameters and pitches (distance between pillars), were fabricated. Then, the patterns were coated with a Z-DOL (perfluoropolyether (PFPE)) lubricating film using a dipcoating technique, and this process was followed by thermal treatment. These modified surfaces were tested for their nanotribological properties, namely adhesion and friction forces, using an atomic force microscope (AFM). The results showed that the topographical modification and Z-DOL coating each independently reduced the adhesion and friction forces on the Si surfaces. However, the combination of the two surface treatments was most effective in reducing these forces. This is attributed to the combined effects of the reduction in the real area of contact due to patterning and the low surface energy of the Z-DOL lubricant. Further, it was found that adhesion and friction forces of the surfaces with combined modification varied significantly depending on the diameter of the pillars and the pitch. It is proposed that such a combination of surface modifications promises to be an effective method to improve the nanotribological performance of miniaturized devices, such as MEMS, in which Si is a typical material.     

Grafting Lubricant Molecular Chains to the Carbon Overcoat

It was conceived and demonstrated that irradiation of magnetic disks coated with PFPE (perfluoropolyether) lubricants terminated with a carboxylic group at one terminus with long-UV (254 nm) would lead to grafting, via a bona fide C–C chemical bond, of genuine PFPE molecular chains to the carbon overcoat all at one terminus with all the remaining chain segments being free to sway. The water contact angle of disks coated with PFPE lubricants terminated with end-groups having hydroxyl unit(s) (e.g., Z-dol and Z-tetraol) decreases gradually, after the initial contact of the droplet, reaching an asymptote in 20–30 s. The gradual temporal change is accounted for by shifting of the equilibrium disposition of hydroxyl units of the lubricant molecules from that determined by the interaction with the surface of the carbon overcoat to that determined by the interaction with surfaces of both the carbon overcoat and the liquid droplet. The water contact angle of disks prepared by the presently conceived photo-grafting method is high (>110 degrees) and shows no temporal change. In a preliminary spin-stand drag test, disks with PFPE chains photo-grafted by this method and also the heads (for the read/write process) with similar photo-grafted PFPE chains exhibited extraordinary durability.     

Low adhesion,non-wetting phosphonate self-assembled monolayer films formed on copper oxide surfaces

Self-assembled monolayer (SAM) films have been formed on oxidized copper (Cu) substrates by reaction with 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), and octylphosphonic acid (OP) and then investigated by X-ray photoelectron spectroscopy (XPS), contact angle measurement (CAM), and atomic force microscopy (AFM). The presence of alkyl phosphonate molecules, PFDP, ODP, DP, and OP, on Cu were confirmed by CAM and XPS analysis. No alkyl phosphonate molecules were seen by XPS on unmodified Cu as a control. The PFDP/Cu and ODP/Cu SAMs were found to be very hydrophobic having water sessile drop static contact angles of more than 140°, while DP/Cu and OP/Cu have contact angles of 119° and 76°, respectively. PFDP/Cu, ODP/Cu, DP/Cu, and OP/Cu SAMs were studied by friction force microscopy, a derivative of AFM, to better understand their micro/nanotribological properties. PFDP/Cu, ODP/Cu, and DP/Cu had comparable adhesive force, which is much lower than that for unmodified Cu. ODP/Cu had the lowest friction coefficient followed by PFDP/Cu, DP/Cu, and OP/Cu while unmodified Cu had the highest. XPS data gives some indication that a bidentate bond forms between the alkyl phosphonate molecules and the oxidized Cu surface. Hydrophobic phosphonate SAMs could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces.     

The Effect of Wetting and Surface Energy on the Friction and Slip in Oil-Lubricated Contacts

This work shows the influence of solid–liquid interactions between engineering surfaces (steel and several types of DLC coatings) and lubricating oil (polyalphaolefin, PAO) on the coefficient of friction in the elastohydrodynamic lubrication (EHL) regime. Specifically, it confirms that the spreading parameter, rather than the contact angle, is the relevant parameter to evaluate the wetting behaviour of these surfaces with oils. Both the spreading parameter and the surface energy correlate very well with the friction in the EHL regime and can predict its behaviour. In particular, the polar component of the surface energy was found to correlate almost perfectly with the friction behaviour (a Pearson’s linear correlation coefficient of 0.999). By tailoring the wetting and surface energy—achieved by varying the DLC/DLC contacts with different types of DLC coatings—the coefficient of friction in the EHL regime was reduced by more than 30 % compared to steel/steel contacts. Poor wetting of the DLC coatings with a low surface energy is reflected in low values of the spreading parameter, which indicates easier slip of the lubricant over the solid surface due to shear action, and this leads to a lower viscous friction. A “Slip-inducing interaction model based on surface forces” is presented to explain why oil slip is promoted, particularly at surfaces with a low polar surface energy. The model suggests that a small number of permanent polar interactions, i.e. a larger proportion of intermittent dispersive interactions, results in less adhesive interactions between the predominantly non-polar liquid (oil) and the low polar surface (DLC), which enables easier slip at the solid–liquid interface.     

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Bj?rling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

Conformation and Fundamental Properties of Novel Lubricant TA-30 for Near Contact Magnetic Recording

A novel perfluoropolyether (PFPE) lubricant called TA-30 has been developed recently. We investigate the conformation of TA-30 on diamond-like carbon (DLC) thin films, by attempting the direct observation of a lubricant film by atomic force microscopy (AFM) using a fluoride probe. We investigate the fundamental properties of a TA-30 lubricant film, such as its spreading characteristics, and the film thickness dependence of surface energy. Considering these experimental results, we conclude that the conformation of TA-30 is considerably different from that of conventional Z-tetraol2000 whose molecular height is 1.7 nm and which was adsorbed on the DLC surface with the random coil. The TA-30 molecules are adsorbed rigidly to the DLC surface with double layers. The thickness of the first TA-30 layer is ~0.9 nm (similar to diameter of the PFPE backbone) and that of the second layer from the DLC surface is 1.4 nm. Since TA-30 has a lower film thickness than Z-tetraol2000 on the DLC surface, it can have two layers, even if the film thickness is approximately of the order of 1 nm, whereas Z-tetraol2000 does not cover the DLC surface and does not form the complete first layer. In addition, we conduct slider touchdown and takeoff hysteresis tests by using TA-30 and Z-tetraol2000. It is confirmed that the use of TA-30 can improve the head–disk interface (HDI) reliability at low-fly-height conditions.     

Friction mechanisms of Silicon wafer and Silicon wafer coated with diamond-like carbon film and two monolayers

The friction behaviour of Si-wafer, diamond-like carbon (DLC) and two self-assembled monolayers (SAMs) namely dimethyldichlorosilane (DMDC) and diphenyl-dichlorosilane (DPDC) coated on Si-wafer was studied under loading conditions in milli-newton (mN) range. Experiments were performed using a ball-on-flat type reciprocating micro-tribo tester. Glass balls with various radii 0.25 mm, 0.5 mm and 1 mm were used. The applied normal load was in the range of 1.5 mN to 4.8 mN. Results showed that the friction increased with the applied normal load in the case of all the test materials. It was also observed that friction was affected by the ball size. Friction increased with the increase in the ball size in the case of Si-wafer. The SAMs also showed a similar trend, but had lower values of friction than those of Si-wafer. Interestingly, for DLC it was observed that friction decreased with the increase in the ball size. This distinct difference in the behavior of friction in DLC was attributed to the difference in the operating mechanism. It was observed that Si-wafer and DLC exhibited wear, whereas wear was absent in the SAMs. Observations showed that solid-solid adhesion was dominant in Si-wafer, while plowing in DLC. The wear in these two materials significantly influenced their friction. In the case of SAMs their friction behaviour was largely influenced by the nature of their molecular chains.     

Fretting of WC/a-C:H and Cr2N Coatings Under Grease-Lubricated and Unlubricated Conditions

The fretting phenomenon was investigated experimentally in contacts between coated and uncoated steel rod and ball specimens generating a circular Hertzian contact. A fretting wear test rig equipped with a video camera was used to observe the effects of fretting on coated steel surfaces in both grease-lubricated and unlubricated environments. Tungsten carbide reinforced amorphous hydrocarbon (WC/a-C:H) and chromium nitride (Cr₂N) coatings were tested and compared. Fretting wear volumes and surface profiles are presented for both grease-lubricated and unlubricated conditions. Videos of a coated ball fretting against a transparent sapphire flat were recorded and screen captures are presented. The role of normal load, lubrication, frequency, and amplitude of motion on the fretting wear of coatings is discussed. The lubricant released from the grease was observed to flow through channels in the stick zone of the fretting contacts. Both coatings were found to reduce fretting wear. WC/a-C:H was more effective at reducing wear under unlubricated conditions. WC/a-C:H decreased fretting wear more than Cr₂N when delamination was avoided in grease-lubricated contacts.     

Effect of tetraalkylphosphonium based ionic liquids as lubricants on the tribological performance of a steel-on-steel system

A series of asymmetrical tetraalkylphosphonium ionic liquids were synthesized and evaluated as a new kind of lubricant for the contact of steel/steel using an Optimol SRV oscillating friction and wear tester in ambient condition. The phosphonium ionic liquid shows excellent tribological performance when being used as the lubricating oil, and is superior to the conventional high temperature lubricants X-1P and perfluoropolyether (PFPE) in terms of anti-wear performance and load-carrying capacity. The chemical compositions of the boundary film generated on different contact surfaces were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM/EDS) and X-ray photoelectron spectrometer (XPS). The friction–reduction and anti-wear mechanism of tetraalkylphosphonium as the lubricant were proposed to originate from the active elements P in the tetraalkylphosphonium ionic liquids reacting with the fresh surface to form a reaction film onto specimen surface, an extreme-pressure film with lower shearing strength, which leads to lower friction coefficient, and good wear resistance.     

Electric contact resistance between finely polished nickel plates with PFPE coating and its relation to friction

The electric contact resistance between mechanically polished nickel plates, with one side dip-coated in perfluoropolyether (PFPE), was evaluated under statically loaded and sliding conditions to investigate the isolation property of the PFPE coating, which may have an important role in thin-film lubrication on relatively large, minutely rough, plane surfaces. The nickel plate surfaces had a roughness of 1–3 nm and the PFPE coating had a thickness of 4–6 nm. Two types of relationship between load and contact resistance were observed. One was estimated to be for a uniform coating of the oil film, the other for a patchy one. Friction tests revealed that a uniform coating of oil film would be preferable to a patchy one to prevent solid–solid contact, if the nominal thicknesses of the oil films were similar.     

Influence of surface texture on boundary lubricated sliding contacts

The friction and wear behaviour of boundary lubricated sliding surfaces is influenced by the surface texture. By introducing controlled depressions and undulations in an otherwise flat surface, the tribological properties can be improved. Lubricant can then be supplied even inside the contact by the small reservoirs, resulting in a reduced friction and a prolonged lifetime of the tribological contact.In the present paper, well-defined surface textures were produced by lithography and anisotropic etching of silicon wafers. The wafers were subsequently PVD coated with thin wear resistant TiN or DLC coatings, retaining the substrate texture. The size and shape of the depressions were varied and evaluated in reciprocating sliding under dry and boundary lubricated conditions.