Vapor Phase Lubrication Research Articles

Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction.

When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.

Empirical relationship between interfacial shear stress and contact pressure in micro- and macro-scale friction

This study examines the empirical relationship between frictional shear stress and pressure in macro- and micro-scale contact and sliding. Two types of friction tests are reported; the macro-scale tests deal with kinetic friction between stainless steel surfaces in a vapor phase lubrication condition, and the micro-scale tests measure kinetic friction at interfaces formed between MoS2 basal planes and surfaces of alumina or stainless steel specimens, using a quartz crystal microbalance microtribometer. A numerical model is used to calculate the contact areas in the macro-scale tests. The results from both friction tests confirmed that the interfacial shear stress in the contact area due to kinetic friction is proportional to the average contact pressure, and the constant of proportionality is close to the coefficient of friction (COF). These observations add to the validation of the Amontons’ law.

Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces

Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapor phase lubricants and antiwear additives. An atomic-level understanding of phosph...

Viscoelastic Lubricant Deformation and Disk-to-Head Transfer During Heat-Assisted Magnetic Recording

One of the challenges in heat-assisted magnetic recording (HAMR) is the formation of write-induced head contamination at the near-field transducer. A possible mechanism that has been proposed for this contamination is the transfer of lubricant from the disk to the head due to temperature driven evaporation/condensation. Most previous studies on lubricant depletion due to laser heating have assumed the lubricant to be a viscous fluid and have modeled its behavior using the traditional lubrication theory. However, perfluoropolyether lubricants are viscoelastic fluids and are expected to exhibit a combination of viscous and elastic behavior at the time and length scales of HAMR conditions. In this paper, we use a modified Reynolds lubrication equation for the viscoelastic fluid that employs the linear Maxwell constitutive model. We use this modified lubrication equation to develop a model that predicts the disk-to-head lubricant transfer during HAMR writing. This model simultaneously determines the thermocapillary stress driven deformation and evaporation of the viscoelastic lubricant film on the disk, the diffusion of the vapor phase lubricant in the air bearing, and the evolution of the condensed lubricant film on the head. We investigate the effects of lubricant type (Zdol versus Ztetraol), head/disk temperature, initial lubricant thickness, and laser spot size on the lubricant transfer process. Simulation results show a significant difference between the rates of transfer for Zdol (timescale of nanoseconds) versus Ztetraol (timescale of microseconds). The amount of transfer increases with the disk temperature and the initial lubricant thickness.

Modeling of the surface coverage and application to the calculation of friction on surfaces contaminated by particles

This paper deals with the modeling of the coverage of surfaces contaminated by fine particles, the objective being the prediction of the skid resistance of road surfaces when it rains after a long dry period. The research methodology is based on the identification of particles' flows in the tribological circuit composed of the particles (3rd body) and the tire and the road (1st bodies). Experiments are conducted in laboratory where sliding friction is measured between a rubber pad (simulating a tire tread block) and a sandblasted aluminum surface (simulating a microtextured road surface) covered by particles. The test program includes particle concentrations representative of deposits of particles on the road surface at different dry periods and different particle size fractions. The test protocol consists in repeating passages of a rubber pad on the test surface and visualizing the particles’ movements by means of high-speed cameras. Two particles' flows are identified: particles ejected from the contact and those raised by the rubber pad then fall back to the surface. Similarities are established with the removal and deposition of gas species in vapor-phase lubrication. An analytical model was derived to express the surface coverage as a function of the number of passages and two parameters (values between 0 and 1) called respectively the ejection and recirculation ratios. The proposed model is included in a linear rule-of-mixtures equation for the calculation of friction. Calculated friction coefficients compare favorably to experimental data and the model's parameters are determined. A master curve for different particles' sizes is obtained when relating the surface coverage to the mass of particles. Relationships between the ejection and recirculation ratios, the particles' characteristics and the surface texture are presented. Discussions are made in terms of transposition of the model to real road surfaces.

Effect of Rheology and Slip on Lubricant Deformation and Disk-to-Head Transfer During Heat-Assisted Magnetic Recording (HAMR)

The high-temperature laser heating during heat-assisted magnetic recording (HAMR) causes the media lubricant to deform and transfer to the head via evaporation/condensation. The ability of the lubricant to withstand this writing process and sufficiently recover post-writing is critical for robust read/write performance. Moreover, the media-to-head lubricant transfer causes a continuous deposition of contaminants originating from the media at the head near field transducer, challenging the reliability of HAMR drives. Most previous studies on the effects of laser exposure on lubricant depletion have assumed the lubricant to be a viscous fluid and have modeled its behavior using traditional lubrication theory. However, Perfluoropolyether lubricants are viscoelastic fluids and are expected to exhibit a combination of viscous and elastic behavior at the timescale of HAMR. In this paper, we introduce a modification to the traditional Reynolds lubrication equation using the linear Maxwell constitutive equation and a slip boundary condition. We study the deformation and recovery of the lubricant due to laser heating under the influence of thermocapillary stress and disjoining pressure. Subsequently, we use this modified lubrication equation to develop a model that predicts the media-to-head lubricant transfer during HAMR. This model simultaneously determines the deformation and evaporation of the viscoelastic lubricant film on the disk, the diffusion of the vapor phase lubricant in the air bearing, and the evolution of the condensed lubricant film on the head. We investigate the effect of viscoelasticity, lubricant type (Zdol vs Ztetraol), molecular weight, slip, and disjoining pressure on the lubricant transfer process.

Mechanochemical Association Reaction of Interfacial Molecules Driven by Shear.

Shear-driven chemical reaction mechanisms are poorly understood because the relevant reactions are often hidden between two solid surfaces moving in relative motion. Here, this phenomenon is explored by characterizing shear-induced polymerization reactions that occur during vapor phase lubrication of α-pinene between sliding hydroxylated and dehydroxylated silica surfaces, complemented by reactive molecular dynamics simulations. The results suggest that oxidative chemisorption of the α-pinene molecules at reactive surface sites, which transfers oxygen atoms from the surface to the adsorbate molecule, is the critical activation step. Such activation takes place more readily on the dehydroxylated surface. During this activation, the most strained part of the α-pinene molecules undergoes a partial distortion from its equilibrium geometry, which appears to be related to the critical activation volume for mechanical activation. Once α-pinene molecules are activated, association reactions occur between the newly attached oxygen and one of the carbon atoms in another molecule, forming ether bonds. These findings have general implications for mechanochemistry because they reveal that shear-driven reactions may occur through reaction pathways very different from their thermally induced counterparts and specifically the critical role of molecular distortion in such reactions.

Surface Chemistry Dependence of Mechanochemical Reaction of Adsorbed Molecules-An Experimental Study on Tribopolymerization of α-Pinene on Metal, Metal Oxide, and Carbon Surfaces.

Mechanochemical reactions between adsorbate molecules sheared at tribological interfaces can induce association of adsorbed molecules, forming oligomeric and polymeric products often called tribopolymers). This study revealed the role or effect of surface chemistry of the solid substrate in mechanochemical polymerization reactions. As a model reactant, α-pinene was chosen because it was known to readily form tribopolymers at the sliding interface of stainless steel under vapor-phase lubrication conditions. Eight different substrate materials were tested-palladium, nickel, copper, stainless steel, gold, silicon oxide, aluminum oxide, and diamond-like carbon (DLC). All metal substrates and DLC were initially covered with surface oxide species formed naturally in air or during the oxidative sample cleaning. It was found that the tribopolymerization yield of α-pinene is much higher on the substrates that can chemisorb α-pinene, compared to the ones on which only physisorption occurs. From the load dependence of the tribopolymerization yield, it was found that the surfaces capable of chemisorption give a smaller critical activation volume for the mechanochemical reaction, compared to the ones capable of physisorption only. On the basis of these observations and infrared spectroscopy analyses of the adsorbed molecules and the produced polymers, it was concluded that the mechanochemical reaction mechanisms might be different between chemically reactive and inert surfaces and that the chemical reactivity of the substrate surface greatly influences the tribochemical polymerization reactions of adsorbed molecules.

A Model for Lubricant Transfer from Media to Head During Heat-Assisted Magnetic Recording (HAMR) Writing

One of the challenges in heat-assisted magnetic recording (HAMR) is the creation of write-induced head contamination at the near-field transducer. A possible mechanism for the formation of this contamination is the transfer of lubricant from the disk to the slider (lubricant pickup) due to temperature-driven evaporation/condensation and/or mechanical interactions. Here we develop a continuum model that predicts the head-to-disk lubricant transfer during HAMR writing. The model simultaneously determines the thermocapillary shear stress-driven deformation and evaporation of the lubricant film on the disk, the convection and diffusion of the vapor phase lubricant in the air bearing and the evolution of the condensed lubricant film on the slider. The model also considers molecular interactions between disk–lubricant, slider–lubricant and lubricant–lubricant in terms of disjoining pressure. We investigate the effect of media temperature, head temperature and initial lubricant thickness on the lubricant transfer process. We find that the transfer mechanism is initially largely thermally driven. The rate of slider lubricant accumulation can be significantly reduced by decreasing the media temperature. However, as the amount of lubricant accumulation increases with time, a change in the transfer mechanism occurs from thermally driven to molecular interactions driven. A similar change in transfer mechanism is predicted as the head–disk spacing is reduced. There exists a critical value of head lubricant thickness and a critical head–disk spacing at which dewetting of the disk lubricant begins, leading to enhanced pickup.

Mechanochemistry at Solid Surfaces: Polymerization of Adsorbed Molecules by Mechanical Shear at Tribological Interfaces.

Polymerization of allyl alcohol adsorbed and sheared at a silicon oxide interface is studied using tribo-tests in vapor phase lubrication conditions and reactive molecular dynamics simulations. The load dependences of product formation obtained from experiments and simulations were consistent, indicating that the atomic-scale processes observable in the simulations were relevant to the experiments. Analysis of the experimental results in the context of mechanically assisted thermal reaction theory, combined with the atomistic details available from the simulations, suggested that the association reaction pathway of allyl alcohol molecules induced by mechanical shear is quite different from chemically induced polymerization reactions. Findings suggested that some degree of distortion of the molecule from its equilibrium state is necessary for mechanically induced chemical reactions to occur and such a distortion occurs during mechanical shear when molecules are covalently anchored to one of the sliding surfaces.

Nano-lubrication: A review

Lubrication is essential for proper and reliable operation of mechanical systems as it is vital to reduce friction and wear of moving components to assure high quality performance. Particularly, with continued development of precision devices the need to acquire effective lubrication technology for micro-systems continues to rise. The challenge lies in the fact that conventional lubrication methods cannot be applied to micro-systems due to viscosity effects. Thus, driven by the needs of precision devices, stringent operating conditions and environmental preservation, numerous alternatives to conventional liquid lubricants have been proposed. In this paper, various lubrication methods to replace or improve conventional liquid lubrication and their characteristics are reviewed. Fundamental mechanism and tribological characteristics of minimum quantity lubrication (MQL), vapor phase lubrication (VPL), and self-assembled monolayer are discussed.

Wear Reduction of Borosilicate Glass Microballs Using Vapor-Phase Lubrication With n-Pentanol

ABSTRACTThe effect of n-pentanol vapor-phase lubrication on the wear mechanism of borosilicate glass was investigated. Glass microballs with a diameter of ∼60 μm were slid against a silicon wafer under a normal load of 100 μN up to a sliding distance of ∼210 km. It was shown that wear volume of the microball could be reduced by ∼18 times using n-pentanol vapor-phase lubrication. The wear mechanism was assessed using 3D laser microscopy and scanning electron microscopy. Abrasion was identified as the main mechanism of wear for the borosilicate glass under n-pentanol vapor-phase lubrication condition. In addition, despite the fact that lubrication was supplied to the sliding interface as a vapor, it was sufficient to prevent agglomeration of wear debris, which aided in decreasing abrasion.

Tribochemical synthesis of nano-lubricant films from adsorbed molecules at sliding solid interface: Tribo-polymers from α-pinene, pinane, and n-decane

The mechanochemical reactions of adsorbed molecules at sliding interfaces were studied for α-pinene (C10H16), pinane (C10H18), and n-decane (C10H22) on a stainless steel substrate surface. During vapor phase lubrication, molecules adsorbed at the sliding interface could be activated by mechanical shear. Under the equilibrium adsorption condition of these molecules, the friction coefficient of sliding steel surfaces was about 0.2 and a polymeric film was tribochemically produced. The synthesis yield of α-pinene tribo-polymers was about twice as much as pinane tribo-polymers. In contrast to these strained bicyclic hydrocarbons, n-decane showed much weaker activity for tribo-polymerization at the same mechanical shear condition. These results suggested that the mechanical shear at tribological interfaces could induce the opening of the strained ring structure of α-pinene and pinane, which leads to polymerization of adsorbed molecules at the sliding track. On a stainless steel surface, such polymerization reactions of adsorbed molecules do not occur under typical surface reaction conditions. The mechanical properties and boundary lubrication efficiency of the produced tribo-polymer films are discussed.

An Adhesion-Dominated Rolling Friction Regime Unique to Micro-scale Ball Bearings

We demonstrate that micro-scale rolling bearings exhibit friction and wear properties markedly different from their macro-scale counterparts. A microfabricated testing platform uses variable rolling element diameters or vapor-phase lubricated interfaces to independently test friction force with varying contact area and surface energy. A linear, consistent, relationship between friction force and contact area is observed among different rolling element diameters. When surface free energy is altered through the introduction of vapor-phase lubrication, an 83 % decrease in friction is observed. When coupled with observed ball material adhered to the raceway, there is strong evidence for adhesion-dominated rolling friction regime at the micro-scale.

Humidity Effects on In Situ Vapor Phase Lubrication with n-Pentanol

The effect of water vapor on n-pentanol vapor phase lubrication (VPL) was studied with a microelectromechanical system (MEMS) side-wall tribometer, a pin-on-disc tribometer, and attenuated total reflection infrared (ATR-IR) spectroscopy. The n-pentanol vapor pressure was fixed at 50 % relative to its saturation vapor pressure (P sat = ~2.2 Torr at room temperature), which is sufficient to maintain a monolayer of n-pentanol on a SiO2 surface in a dry Ar environment. As the relative humidity (RH) was increased from zero to 30 %, ATR-IR measurements showed that the water adsorption on the surface increases and the adsorbed pentanol thickness decreases by 60 %. These changes in the adsorption isotherm were manifested as higher, and more scattered friction coefficients observed during the MEMS tribometer operation. The maximum RH tolerance appeared to be 25–30 % RH above which the MEMS tribometer failed to operate reliably. In contrast, the n-pentanol VPL efficiency was not affected significantly during the macro-scale pin-on-disc tribometer tests. These results imply that the friction behavior of the asperity contacts in MEMS is more susceptible to co-adsorption of water than the friction behavior of macro-scale contacts.

Vapor phase lubrication using high molecular weight lubricant for friction reduction of metals

Liquid lubricants are widely used to reduce the friction and wear of mechanical components. In this research, silicone oil and canola oil were used to investigate the effectiveness of vapor phase lubrication in the lubrication of aluminum and copper. Specifically, the aim was to assess the friction reduction characteristics of a high molecular weight lubricant by vapor phase lubrication. Friction experiments were performed using a specially built test chamber for vapor phase lubrication. The silicone oil and canola oil were heated to produce the vapor phase. Then, the vapor phase lubricant was injected into the test chamber. Aluminum and copper plates were slid against stainless steel balls in the friction tests. The results of the friction experiments showed that the vapor phase lubrication using a high molecular weight lubricant was effective in reducing the friction of aluminum and copper. It was also observed that the friction coefficient of the silicone oil vapor phase lubrication temporally increased due to decomposition of the oil. The vapor phase technique should be optimized based on the type and characteristics of the lubricant. The results of this work demonstrate the possibility of vapor phase lubrication for the friction reduction of metals.

Effects of Vapor Environment and Counter-Surface Chemistry on Tribochemical Wear of Silicon Wafers

The effects of counter-surface chemistry and adsorption of water and alcohol from the environment on the tribological responses of silicon surfaces were investigated using atomic force microscopy. When scratching with SiO2 tips at contact pressures below the hardness of the materials, changing the environment yielded drastically different wear behaviors. In humid air, the adsorbed water molecules facilitated wear of the surface and material removal. In N2 environment, there was subsurface deformation but no wear, so the surface protruded outward in the rubbing region. In the ethanol vapor condition, the adsorbed alcohol molecules acted as a lubricant and prevented any discernible changes to the surface even at contact pressures above 1 GPa. These results extend upon previous studies of vapor-phase alcohol lubrication using even more protective longer-chain alcohols where failure was observed at much lower contact pressures in macroscale tests, probably due to high-pressure asperity contacts. Thus, the chemical environment can govern the response of silicon to mechanical rubbing. Rubbing with a diamond tip, however, yielded protrusions in all three environments, showing that the chemistry of the counter-surface also contributes to the tribological response; in this case, diamond is not tribochemically reactive toward Si surface. The protrusion formed by the diamond tip in ethanol vapor was only ~20 % the height of the one in humid air, even though the measured friction coefficients (and so the applied shear forces) were similar. These results clearly show that the surface chemistry at the tribological interface can substantially alter both the wear and subsurface damage processes.

Influence of surface passivation on the friction and wear behavior of ultrananocrystalline diamond and tetrahedral amorphous carbon thin films

Highly $s{p}^{3}$-bonded, nearly hydrogen-free carbon-based materials can exhibit extremely low friction and wear in the absence of any liquid lubricant, but this physical behavior is limited by the vapor environment. The effect of water vapor on friction and wear is examined as a function of applied normal force for two such materials in thin film form: one that is fully amorphous in structure (tetrahedral amorphous carbon, or ta-C) and one that is polycrystalline with $<10$ nm grains [ultrananocrystalline diamond (UNCD)]. Tribologically induced changes in the chemistry and carbon bond hybridization at the surface are correlated with the effect of the sliding environment and loading conditions through ex situ, spatially resolved near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. At sufficiently high relative humidity (RH) levels and/or sufficiently low loads, both films quickly achieve a low steady-state friction coefficient and subsequently exhibit low wear. For both films, the number of cycles necessary to reach the steady-state is progressively reduced for increasing RH levels. Worn regions formed at lower RH and higher loads have a higher concentration of chemisorbed oxygen than those formed at higher RH, with the oxygen singly bonded as hydroxyl groups (C-OH). While some carbon rehybridization from $s{p}^{3}$ to disordered $s{p}^{2}$ bonding is observed, no crystalline graphite formation is observed for either film. Rather, the primary solid-lubrication mechanism is the passivation of dangling bonds by OH and H from the dissociation of vapor-phase H${}_{2}$O. This vapor-phase lubrication mechanism is highly effective, producing friction coefficients as low as 0.078 for ta-C and 0.008 for UNCD, and wear rates requiring thousands of sliding passes to produce a few nanometers of wear.

Wear of silicon surfaces in MEMS

Abstract High levels of friction and wear are problems which currently limit the development of sliding micro-electro-mechanical systems (MEMS) – devices which would otherwise offer significant technological advancement. The current paper focuses on the wear of MEMS silicon surfaces, and specifically looks at the effect of environment and surface preparation on wear behaviour. Included in the study is the assessment of two self-replenishing lubrication mechanisms; namely liquid and vapour phase lubrication. All tests were carried out using a tribometer which operated and measured friction and wear under conditions representative of MEMS. It is shown that friction and wear behaviour depend strongly on subtle changes of the silicon surfaces prior to testing. Greatest wear was measured when the surfaces were tested immediately after plasma-cleaning, while subsequent exposure to ambient air for 15 h reduced wear to negligible levels. Exposure of plasma-cleaned surfaces to water-saturated argon prior to testing prevented wear to a limited extent. Based on this, and TOF-SIMS analysis, it is suggested that the observed wear reduction after exposure to air is caused by tiny amounts of lubricious long chain hydrocarbon contaminants present in ambient air. Tests carried out with the specimens submerged in a liquid bath show that the presence of liquid water reduces friction and wear, but only if specimens have been plasma-cleaned beforehand. This behaviour is tentatively attributed to the hydrophilic nature of plasma treated silicon, reducing the corrosive action of water. When hexadecane or 1-pentanol was used as a liquid lubricant, friction was minimal, and wear was undetectable under all sliding conditions. This was the case even though the contact operated in the mixed lubrication regime, suggesting a boundary film is formed on the silicon surfaces by both of these organic liquids. Results of tests carried out with the lubricant being supplied in the form of pentanol vapour also showed no appreciable wear. A considerable difference in friction was found between liquid and vapour lubricated contacts; under the conditions tested, the coefficient of friction for vapour was 0.28, while for liquid it was 0.05.

MEMS lubrication with alcohol vapour

The advancement of microelectromechanical systems (MEMS) could enable numerous advanced mechanical and communication technologies. At present, commercial MEMS devices are limited to the ones that have no motion or limited motion without sliding contacts such as linear sliding, hinge motions, rotations, etc. The main limiting factors impeding the inclusion of the full range of mechanical motions are tribological issues associated with adhesion, friction and wear of the devices. To resolve these issues, various types of lubricants and coatings have been developed and applied to MEMS. But the coating approaches have not yet satisfied long term operation requirements necessary for MEMS devices with sliding contacts to be commercially viable. In this review, we introduce a new paradigm in MEMS lubrication: vapour phase lubrication (VPL) with alcohols under equilibrium conditions. Fundamental aspects of alcohol adsorption on silicon oxide surfaces in ambient conditions are addressed and their implications in the reduction of adhesion, friction and more importantly, the prevention of wear, are presented.