Adhesion Between Surfaces Separated by Molecularly Thin Perfluoropolyether Films

The relationship between the adhesion of surfaces separated by a molecularly thin liquid film and the surface energy of the film was investigated. AFM-based force–distance curves were measured on a series of carbon surfaces coated with hydroxyl-terminated perfluoropolyether (PFPE) films. The surface energy of the PFPE films was varied by altering either the total film thickness or the bonding ratio of the film by changing the concentration of the PFPE film in the solution and/or the pull-rate during dip-coating. A linear relationship between adhesion force and surface energy was observed. Adhesion was found to vanish at non-zero values of surface energy. The experimental results indicate that the adhesive force between macroscopic bodies separated by molecularly thin liquid films is linearly proportional to the excess surface energy of the film.     

Spreading characteristics of thin liquid films of perfluoropolyalkylethers on solid surfaces. Effects of chain-end functionality and humidity

Spreading characteristics of thin liquid films of perfluoropolyalkyl ethers (PFPE) on silica surfaces and thermally bonded PFPE surfaces have been measured by scanning microellipsometer as function of molecular weight, chain-end functionality, and humidity. The effects of molecular weight are much smaller than those of chain-end functionality. Changes in spreading profiles with time show that interactions of chain-end groups with solid surfaces have dramatic effects on the structure and mobility of thin polymer films on solid surfaces. Wetting autophobicity is clearly manifested by PFPEs with functional chain-end groups on silica surfaces.On leave from Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul, Korea.On leave from Department of Chemical Engineering, Sogang University, Seoul, Korea.     

Evaluations of tribological characteristics of PFPE lubricants on DLC surfaces of magnetic disks

This article presents measurements of adhesion and friction of perfluoropolyether (PFPE) lubricant films dip-coated on magnetic disks covered with diamond-like carbon (DLC) film. We have developed a custom-built pin-on-disk type micro-tribotester to perform the tribological measurements. The adhesion tests were performed by pulling down/up a 1.5-mm-diameter glass ball on a stationary disk surface, and the friction tests were carried out by sliding the glass ball on a rotating disk surface without changing head-disk interface conditions from the adhesion tests. Experiments were performed for the different kinds of 2- and 6-nm-thick PFPE lubricants (polar: Zdol4000 and Zdol2000; nonpolar: Z03) under lightly loaded and slow sliding conditions to minimize disturbance against the molecular layered structure. The adhesive forces were found to decrease with increasing film thickness in the order of Z03 > Zdol2000 > Zdol4000 (decreasing rate), which closely corresponds to the order of monolayer thickness, and then to saturate to almost the same calculated values. As for the friction forces of 2-nm-thick films, Zdol2000 featured extraordinarily large friction in comparison with Zdol4000 and Z03, while Zdol4000 was slightly larger than Z03. The largest friction of Zdol2000 reveals that the 2-nm-thick Zdol2000 formed a monolayer that served as an immobile layer. With the increase in film thickness, the friction force of Zdol2000 decreased, indicating that extra lubricant molecules served as a mobile layer, while that of Z03 remained unchanged as the lowest value. By extrapolating the loading force versus friction force relationship into a negative loading force region, it is found that the friction force of Z03 tended to zero at zero net load (loading force plus adhesion force), while those for Zdol2000 and Zdol4000 exhibited finite values, indicating formation of an immobile layer, which shows similar characteristics to those of adhesive rubber material. The dewetted surface is found to feature violently changing friction force only at the first stage of sliding, and it then becomes stable after several sliding passes.     

Density Variation in the Ultrathin Liquid Perfluoropolyether Films on Solid Surfaces

Variation of density in the ultrathin liquid perfluoropolyether films on solid surfaces has been studied by Monte Carlo simulations of Lennard-Jones systems. The liquid film is 8.75 nm thin film consisting of polymer molecules assuming the structure of perfluoropolyether Z having molecular weights of 3840, 2500 and 1700 g/mol and interacting among themselves by Lennard-Jones potential. The substrate is assumed to be continuous without atomic structure and exerting Lennard-Jones potential on liquid molecules in the ultrathin film. The system temperature is considered to be 25 °C and the liquid molecules also have the gravitational potential. It is found that the bead density decreases towards the surface in a thinner sublayer in the ultrathin liquid film above the surface and the thickness of this sublayer just above the surface may increase with the increase of molecular weight of the of polymers in the film. Repulsive potential of the surface further decreases the bead density near the surface. The results are compared with the experimental results of the pefluoropolyether lurbricants by X-ray reflectivity (Shouji, et al., 1998).     

The Effect of Carbon Overcoat Thickness on the Zdol Boundary Lubricant Film

Formation of a tribologically reliable interface between the read-write head and the computer disk in hard-disk drives is accomplished by the use of a thin, wear-resistant carbon overcoat in conjunction with a molecularly-thin perfluoropolyether (PFPE) lubricant film. The intermolecular interactions that develop between the PFPE lubricant and the carbon overcoat govern the adhesion, coverage, and physical properties of the lubricant, e.g. the lubricant structure and mobility. Consequently, the molecular interactions at the lubricant-carbon interface will contribute to the overall tribological performance of the disk-drive. Due to the ever-increasing demands for storage capacity, pressure exists to reduce the separation distance between the read-write head and disk surface. One means of reducing this separation distance is to use thinner protective overcoats on both the head and disk surfaces. In this study the interactions between Fomblin Zdol and both amorphous hydrogenated (CHx) and nitrogenated (CNx) carbon overcoats were investigated as a function of overcoat thickness from 0 to 100Å. The Zdol film structure was probed by titrating the magnetic alloy, the CHx and CNx surfaces with Zdol. The molecular weight dependence of the maximum bonded Zdol thickness on these surfaces is used to deduce structural information on the adsorbed Zdol film. In progressing from CHx to CNx to the magnetic alloy, we find the Zdol boundary layer film to be characterized by an increase in average distance between the PFPE backbone and the surface, or equivalently an increase in the average Zdol monolayer thickness. On the CHx overcoat, Zdol preferentially lies more parallel to the surface, whereas on the magnetic layer, Zdol is oriented more perpendicular to the surface. When these experiments were conducted as a function of carbon overcoat thickness, we found that interaction of Zdol with the field of the underlying magnetic film becomes important at carbon film thicknesses 30Å. The dependence of the Zdol adhesion on carbon overcoat thickness was quantified by determining the Zdol film thickness dependence of both the dispersive and polar components of the Helmholtz free energy. The Zdol bonding kinetics were also studied as a function of carbon thickness.     

Behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives

In this paper, we describe the behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives. In the experiments, the ultrathin liquid lubricant film behavior is investigated using Zdol and cyclotriphosphazene-terminated PFPE lubricant which have different end groups as a function of lubricant film thickness. The disks are examined with a scanning microellipsometer before and after contact slider experiments. It is found that the lubricant film thickness profiles almost do not change, when the lubricant film thickness is less than one monolayer. It can also be observed that lubricant film thickness instability due to dewetting occurs as a result of slider-disk contacts for the tested lubricants and the films undergo spontaneous redistributions, resulting in significantly nonuniform film thickness profiles, when the lubricant film thickness is thicker than one monolayer. In addition, it is found that the observed behavior of ultrathin liquid lubricant films for cyclotriphosphazine-terminated PFPE lubricant contrasts markedly with that for Zdol. The difference between cyclotriphosphazene-terminated PFPE lubricant and Zdol is only the functional end group. Therefore, it may be concluded that their unstable lubricant behavior depends on the chemical structure of functional end groups.     

Velocity-Dependent Adhesion with Lubricants on Thin-Film Disks

The well-known problem of stiction in a magnetic disk drive largely depends on the forces induced by the presence of a thin liquid film. It is commonly recognized that both adhesive and viscous effects contribute to the magnitude of the stiction force, but is is not known what relative roles the two effects have in a lubricated contact. In the present work, the nature of adhesive and viscous effects is investigated for the slider/disk interface under conditions of constant-speed sliding.

Friction measurements are conducted over a range of sliding speeds, 0.25-250 mm/s, with eight perfluoropolyether (PFPE) lubricants applied in various thicknesses, 0-6.6 nm, to carbon-coated magnetic thin-film disks. The lubricants were selected to cover a broad range of viscosities. For several sliding speeds and lubricant film thicknesses, the friction force is found to decrease significantly with increasing sliding speed for all lubricants. In several instances, large friction forces are observed at the lowest sliding speeds, indicating stiction-like behavior, whereas, at higher speeds, the friction is reduced to even below unlubricated friction levels. At the highest film thickness and sliding speed, the friction was found to increase with speed for some lubricants. The implications of these results on current models of lubricant-mediated adhesion are discussed.     

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.     

Experimental Study on Lubrication Film Thickness Under Different Interface Wettabilities

This paper describes some experimental studies about the effect of interface wettability on hydrodynamic lubrication film thickness by a custom-made slider bearing tester. The lubricated contact pair consists of a fixed-incline slider and a transparent disc, and a thin lubrication film can be generated when the disc rotates. The film thickness was measured by interferometry. The wettability of different slider surfaces was evaluated by the contact angle of the lubricant on them. The relationship of film thickness versus disc speed was measured under different liquid–solid interfaces, and the results showed that slider surfaces with strong wettability to the lubricant could generate higher film thickness. Furthermore, case experiments were carried out to validate the hydrodynamic effect by tailored-slippage. By numerical simulations, the experimental findings were tentatively explained with the phenomenon of wall slippage.     

Autophobic Dewetting of Perfluoropolyether Films on Amorphous-Nitrogenated Carbon Surfaces

The dewetting of perfluoropolyether (PFPE) films on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image perfluoropolyether films on CNx-overcoated magnetic recording disks. An autophobic dewetting transition is observed to result when the PFPE film thickness applied to the disk surface exceeds a critical value. This critical dewetting thickness is linearly dependent on the PFPE molecular weight. Addition of the phosphazine, X-1P, to the PFPE film reduces the critical dewetting thickness compared to that of the neat lubricant. Dewetting in these molecularly-thin PFPE lubricant films is shown to occur at thicknesses where the total disjoining pressure is negative. The impact of this autophobic dewetting on the performance of a head--disk interface is inferred from take-off height measurements conducted as a function of PFPE film thickness. A steep reduction in the slider--disk clearance is observed when the PFPE film is present at thicknesses in excess of the critical dewetting thickness.     

Adhesion and Friction Studies of Nano-textured Surfaces Produced by Self-Assembling Au Nanoparticles on Silicon Wafers

This article presents the results of nanoscale friction and adhesion of nanoparticle-textured surfaces (NPTS) using atomic force microscope (AFM). The effects of coverage ratio, texture height, and packing density on the adhesion and friction of the NPTS were investigated. The nano-textured surfaces were produced by self-assembling Au nanoparticles (NPs) with diameters of 20 nm and 50 nm on the silicon (100) surfaces, respectively. Surface morphology of the NPTS was characterized by field emission scanning electron microscopy and AFM. The results show that the NPTS significantly reduced the adhesive force compared to the smooth surface. The adhesion of NPTS is mainly dependent on the coverage ratio of NPs rather than the texture height and higher coverage ratio resulted in smaller adhesive force. The reduced adhesion of textured surfaces was attributed to the reduced real area of contact. The friction of NPTS is mainly dependent on the spacing between asperities. The lowered frictional force was obtained when the spacing between asperities is less than the size of AFM tip, because of the effectively reduced real area of contact between the AFM tip and the NPTS surface.     

Tribological properties of nanoscale GeSb2Te4 film in the ambient air

In this paper, lateral force microscope (LFM) is used to investigate the frictional behavior of GeSb₂Te₄ film of 20 nm thickness deposited by magnetron sputtering. The effect of relative humidity, scanning rate, scanning area on friction is concerned. The JKRS (Johnson–Kendall–Roberts) theory considering the energy of adhesion between tip and GeSb₂Te₄ (at.%) film is introduced. Experimental results indicate that high humidity leads to high adhesive force, low humidity to low adhesion. When the tip's surface energy is unchanged, minute change of surface energy of GeSb₂Te₄ film will affect the adhesive force. Experiment also shows that variance of friction level goes unanimous at the different scanning area and the same normal load. Next, the effect of scanning rate on friction is attributed to the surface absorbate and liquid film. Finally, it suggests that relative humidity and protective coating should be considered for the promising ultra-high density probe storage medium.     

Energetically and pressure driven phase transitions in molecularly thin films of n-alkanes

Molecular dynamics simulations were carried out on molecularly thin films of n-octane confined between topographically smooth solid surfaces. We focused on determining the effect of increasing solid surface-methylene unit energetic affinity and the effect of increasing pressure (normal load) of the film in inducing liquid-solid phase transitions. Simulations of films wide enough to accomodate three segmental layers showed an abrupt transition in the structural features at a critical value of the characteristic energy that quantified the affinity between solid surfaces and methylene units. This energetically driven transition was evident from the discontinuous increase of intermolecular order, a precipitous extension of the octane molecules and freezing of molecular migration and rotation. Increasing pressure had a similar effect in inducing a liquid-solid phase transition. The characteristics of the transition showed that it is a mild first-order transition from a highly ordered liquid to a poorly organized solid. These findings demonstrate that the solidification of nanoscopically thin films of linear alkanes is a general phenomenon (driven either energetically or by increasing pressure), and does not require the aid of commensurate surface topography. Our findings on relatively wider films (5 segmental diameters wide) show that the interfacial layer undergoes a similar first-order phase transition with increasing solid-methylene unit energetic affinity. This energy threshold is significantly higher than the one observed in thin film simulations.     

The effects of acid passivation,tricresyl phosphate presoak,and UV/ozone treatment on the tribology of perfluoropolyether‐lubricated 440C stainless steel couples

Research on the boundary lubrication performance of two perfluoropolyether (PFPE) thin films in the presence of passirated 440C stainless steel is presented. The study used a standard ball‐on‐disc (BoD) tribometer operating in dry nitrogen and a vacuum spiral orbit tribometer (SOT). Stainless steel surfaces were passivated using one of four techniques: a high‐temperature and low‐temperature chromic acid bath, a tricresyl phosphate (TCP) soak, or UV/ozone treatment for 15 min. After passivation, each BoD disc had a 400 Å film of Krytox® 16256 (PFPE) applied to it. The lives of these films were quantified by measuring the number of sliding cycles before an increase in friction occurred. The lubricated life of the 440C couple was not altered as a result of the various passivation techniques. The resulting surface chemistry of each passivation technique was examined using X‐ray photoelectron spectroscopy. The SOT was used to examine the effects of the TCP treatment on the lubricated life of another PFPE, Brayco® 815Z, under rolling conditions. None of the passivation techniques were found to increase dramatically the oxide film thickness or lubricated life.     

Molecular dynamics study of dynamic contact and separation between tip and disk surface

Molecular dynamics simulations were performed to study contact and separation between tip and lubricants on disk surface. The effects of contact indentation depth, indentation velocity, separation velocity, adhesive energy, lubricant molecular structure and lubricating film thickness on interacting force were analyzed. The results indicate that the tip force exerted by lubricants is velocity-dependent. The tip force increases with increasing indentation velocity and separation force reduces with increasing separation velocity. The damping of branched molecule and thick lubricating film is high. The high adhesive energy of tip material can produce high separation force which reduces bouncing vibration.     

Molecular Dynamics of a Thin Liquid Argon Layer Squeezed between Diamond Surfaces with a Periodic Relief

The behavior of a liquid argon layer compressed between absolutely solid diamond surfaces with a periodic atomic relief has been studied by the method of molecular dynamics. The film consists of one or two layers of molecules for which the model of absolutely elastic spheres has been used. The equilibrium and dynamic properties of the film dependently on the load and shear force applied to surfaces and the number of argon layers have been investigated. The performed modeling allows us to compare the behavior of systems with smooth and rough plates. The results show that most of the properties of ultrathin argon films confined between surfaces of both types are similar.     

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.     

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.     

Molecular dynamics study of the interfacial slip phenomenon in ultrathin lubricating films

The tribological performance of thin films of a liquid alkane has been studied through a molecular dynamics simulation with particular attention being paid to the phenomenon of interfacial slip. The model system for the simulation consists of two solid walls, with the lubricant molecules confined between them. Molecules of n‐decane (C₁₀H₂₂) were chosen to represent the lubricant molecules. The results of the simulation show: the average velocity of decane molecules in a Couette flow exhibits largely a linear distribution, but with a slip velocity at the solid‐liquid interface; when the simulations are performed at different temperatures, the slip ratios were found to vary with temperature; slip behaviour depends strongly on the solid‐fluid interaction; and slip ratios increase with decreasing film thickness, suggesting that slip in the thin films is a confinement‐related phenomenon.