Mobile Lubricant Research Articles

Physics of the Sub-Monolayer Lubricant in the Head-Disk Interface

This review presents a series of studies which have demonstrated that the diffusion characteristics of rarefied mobile lubricant films used in modern magnetic disks can be evaluated by a novel diffusion theory based on continuum mechanics, and that the meniscus force of the rarefied film is the major interaction force at the head-disk interface. The limitations of the conventional diffusion and disjoining pressure equations are first shown, and diffusion and disjoining pressure equations for rarefied liquid films are proposed, showing that the diffusion coefficient is in good agreement with the experiment. The experiment, in which glass spheres with radii of 1 and 2 mm collided with magnetic disks of different film thicknesses, showed that attraction similar to the pull-off forces of a static meniscus was measured only at the separation. Furthermore, mathematical analysis of the elastic meniscus contact between a sphere and a plane with a submonolayer liquid film showed that the maximum adhesion force is equal to the meniscus pull-off force and that the contact characteristics become similar to those of the JKR theory as the liquid film thickness decreases. A basic physical model of submonolayer liquid film is also proposed to justify the continuum mathematical equations.

Atomistic Insights into Interfacial Optimization Mechanism for Achieving Ultralow-Friction Amorphous Carbon Films under Solid-Liquid Composite Conditions.

The combination of fluid lubricants and textured amorphous carbon (a-C) can provide an ultralow friction state, which can improve the reliability and service life of dynamic machinery. However, the coupling effects of the contact pressure and oil content on the friction-reducing efficiency is still lack of study, and the corresponding friction mechanism is also not fully understood, which cannot be achieved by experiment due to the limitation of in situ characterization. In this study, using the reactive molecular dynamics simulation, the insight into the evolution of interfacial structures induced by both contact pressures and oil contents on a-C surface was systematically investigated to explore the fundamental mechanism. In particular, the friction difference between textured and untextured a-C films was evaluated comparatively. Results indicate that the tribological performance strongly depends on the interfacial lubrication state, which is jointly determined by the oil content and contact pressure; the best operating condition to achieve ultralow friction coefficient (0.002) is obtained, and the evolution of friction coefficient with oil content and contact pressure is highly dominated by the lubricant mobility, cross-linking between mating a-C surfaces, or competition/synergy of the H stress state from the lubricant with interfacial passivation. Furthermore, the difference in friction reduction between textured and untextured systems is unveiled; with the increase of contact pressure, the role of texturing a-C surface in antifriction changes from positive to negative effect, which is related to the transformation of interfacial hybridized structure and anomalous flow of lubricant. These results can significantly enhance the understanding of composite lubrication systems through computation and also provide a roadmap for the R&D of the advanced lubrication system according to the working conditions.

Designing a Highly Stable Slippery Organogel on Q235 Carbon Steel for Inhibiting Microbiologically Influenced Corrosion.

Microbiologically influenced corrosion (MIC) accelerates the corrosion and degradation of metal materials due to the settlement of microorganisms on the surface. However, environmentally friendly and efficient methods to fabricate antifouling and anticorrosion surfaces are still lacking. Inspired by Nepenthes, a slippery liquid-infused porous surface (SLIPS) has been proven to be an efficient way to inhibit settlement of microorganisms on the metal surface and the following MIC due to the existence of a mobile defect-free lubricant layer. However, the stability of the lubricant layer and substrate of the SLIPS prevented its long-term antifouling and anticorrosion application. Herein, a highly stable slippery organogel was fabricated by depositing a homogeneous mixture of PDMS (base and curing agent), silicone oil, triethoxyvinylsilane, and SiO2 on Q235 and curing in an oven. Triethoxyvinylsilane was not only able to cross-link with the curing agent of PDMS through hydrosilylation but also able to interlink the organogel and Q235 through condensation between the -OH of the metal surface and hydrolyzed siloxane. As a result, the adhesion force between the organogel without triethoxyvinylsilane and the substrate (0.45 MPa) increased to 1.50 MPa for the organogel with triethoxyvinylsilane and SiO2. Also, the tensile strength of the organogel without SiO2 (0.97 MPa) increased to 3.88 MPa for the organogel with 2 wt % SiO2 because of the high elastic modulus of SiO2, which was important to improving its stability under external force. In addition, the organogel showed stable oil distribution and slippery performance after spinning at 4000 rpm for 30 s. Then, the bacterial settlement demonstrated that the organogel could effectively inhibit Pseudoalteromonas sp. settlement on the substrate under both static and dynamic conditions. Finally, an electrochemical test indicated that the MIC could be effectively mitigated by the organogel. This study provides an efficient method to fabricate a highly stable slippery surface on a metal surface for its potential application in mitigating MIC.

One‐Step Fabrication of Universal Slippery Lubricated Surfaces

AbstractAmong the various liquid‐repellent surfaces studied in recent times, lubricant‐impregnated surfaces (LISs) with a mobile lubricant top surface exhibit distinctive liquid repellency. For the fabrication of LISs with stable lubricant impregnation, there are two surface requirements: i) a porous/textured structure and ii) a hydrophobic surface. These limitations require complex processes to be overcome and impede the fabrication of a variety of potential substrates. Herein, a stable and homogeneous lubricant thin layer fabricated by a simple one‐step heating process is introduced on a nonhydrophobic surface with no porous/textured structures. Since the reaction minimizes the interfacial energy, a surface with a stable mobile lubricant thin layer, named “lubricated surface” (LuS), is fabricated. The LuS exhibits outstanding liquid repellency, anti‐biofouling properties against bacteria, and anticorrosivity for metals, along with excellent self‐healing characteristics. Additionally, the LuS can be applied on various materials and structures, including completely flat, nonporous surfaces. These superior virtues of the LuS can open new pathways for the production of universal slippery lubricant thin films for various potential applications.

Poly(lactic acid) phase transitions in the presence of nano calcium carbonate: Opposing effect of nanofiller on static and dynamic measurements

The effect of stearic acid-coated nano calcium carbonate (NCC) on transitions and chain mobility of poly(lactic acid) (PLA) was investigated. Dispersion state of NCC in polymeric matrix was explored using scanning electron microscopy and surface tension component measurements. Trends of PLA transitional phenomena were investigated using the results of dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC) in the nanocomposite systems based on PLA and NCC. In addition, two types of crystal structures and decreasing the glass transition temperature were distinguished using temperature-modulated DSC (TMDSC). Higher melting points of polymer crystals were found in TMDSC experiments due to low and dynamic heating rate compared to the conventional DSC. Dynamics of polymer chains, affected by NCC, were quantified using cooperativity length, ξ α, and the number of relaxing structural units, Nα, in the glass transition region. NCC particles hindered the cooperative motion of polymer chains at glass transitions and crystallization in TMDSC measurements, whereas the DMA results indicated that NCC particles may act as lubricant and simplified chain mobility.

Self-assembled monolayer of multiply-alkylated cyclopentenes on silicon via thiol-ene “click” reaction and its self-lubricating properties

Self-assembled monolayer (SAM) of multiply-alkylated cyclopentenes (MACEs), homologue of the famous hydrocarbon mobile lubricant multiply alkylated cyclopentanes (MACs), was fabricated as self-lubricating film on the surface of the thiol-functionalized silicon wafer (HS-Si) via facile thiol-ene “click” reaction, induced with UV-light or heating. The surface property analysis, including static contact angle and surface roughness, indicated the successful self-assembly. The microtribological tests of the different samples on a UMT-1 tribometer (CETR) in a ball-on-plate contact configuration showed that the SAMs possessed excellent self-lubricating property, in which the SAMH obtained via the heating-induced thiol-ene “click” reaction had the lower friction coefficient and longer tribological durability than the SAMUV one prepared under UV-irradiation. The proposed self-lubricating strategy is expected to be used in the lubrication of micro-/nano-electromechanical systems (MEMSs/NEMSs).

Effect of Bonded Molecules on Replenishment of Lubricant-Depleted Area Created by Sliding on Molecularly Thin Lubricant Film

The effect of a bonded lubricant on the replenishment of a lubricant-depleted area was examined. The replenishment process for a monolayer perfluoropolyether lubricant with different bonding ratios was imaged in real time using two-stage imaging ellipsometric microscopy. The relationship between the diffusion coefficient and bonding ratio was evaluated by comparing the experimental results with those of lubricant diffusion simulation. The diffusion coefficient of the replenishment process agreed with that of spreading on a disk with bonded lubricant, which suggests that the replenishment process can be modeled by the diffusion of mobile lubricant molecules through bonded ones.

Diffusion Equation for Spreading and Replenishment in Submonolayer Lubricant Film

In the field of head–disk interface technology, evaluating the effects of lubrication of a submonolayer lubricant film is becoming increasingly important as the thickness of the mobile lubricant film is reduced to <0.3 nm. Although the spreading and replenishment speed of a submonolayer lubricant can be evaluated by conventional diffusion theory if a large effective viscosity value is selected, it is commonly considered inappropriate to use the conventional diffusion equation based on continuum mechanics for a submonolayer liquid film. This paper presents a new diffusion equation that formulates the averaged flow of a submonolayer liquid film with a reduced density compared to the bulk density. The Hamaker constant for disjoining pressure in the submonolayer liquid film is also modified to be proportional to the film thickness. The difference between the viscosity in the submonolayer film and the bulk viscosity is also taken into account. From the derived diffusion equation for a submonolayer film, the spreading of a lubricant boundary and replenishment process of a depleted lubricant surface are studied in comparison with the results obtained using the conventional diffusion equation. The replenishment process of submonolayer lubricant profiles in a depleted groove for the Z-tetraol lubricant with a 0.24-nm-thick mobile layer can be calculated from submonolayer diffusion theory by using a low viscosity value consistent with that measured by a blow-off experiment.

メニスカス弾性接触特性の解析

This paper presents a numerical analysis of adhesion contact due to meniscus between a sphere and a flat considering the elastic deformation of the contacting surfaces. Analytical model and basic equations for numerical analysis are explained first. Meniscus contact characteristics are presented for a 2-mm-radius glass contact on a magnetic disk coated with submonolayer mobile lubricant, and 2-μm and 20-nm-radius asperity contacts on mobile and bonded lubricant layer with a low elasticity. It was found that the sphere-flat contact with submonolayer liquid meniscus shows Derjaguin-Muller-Toporov theory-like behavior to Johnson-Kendall-Roberts theory-like one as the mobile layer thickness decreases from 1 nm to 0.3 nm. There is a critical liquid film thickness below which the meniscus length becomes zero so that the meniscus liquid cannot be transferred to the mating surface. This numerical method can be used for the rigorous analysis of various meniscus adhesion contact problems. Some of the experimental adhesion characteristics between a glass sphere and a magnetic disk are discussed from the aspect of the calculated results of elastic meniscus contact.

Optimization of molecularly thin lubricant to improve bearing capacity at the head-disk interface

A molecularly thin lubricant layer (of the order of 1---2 nm thick) has been shown to provide bearing forces at the interface between contacting solid surfaces under light loads and high shear rates. This phenomenon is important, for example, in the head-disk contact in magnetic storage hard disk drives to ensure that some of the contact is sustained by the lubricant layer and thus avoiding damage of the solid surfaces. The magnitude of the normal and tangential bearing forces that the lubricant layer can provide depends on temperature, viscosity of the lubricant, sliding velocity and radius of gyration of the lubricant molecules. This study shows that viscosity has the greatest effect on the load bearing capacity of the molecularly thin lubricant. Thus, by controlling the flash temperature and the ratio of molecularly thin lubricant-to-bulk viscosity, the bearing load carrying capacity of the layer can be controlled. This would allow for the contact to be sustained within the mobile lubricant layer, avoiding solid contact so as to protect the diamond-like carbon coating, and thus reduce wear and potential catastrophic failures.

Experimental Study of Head-Disk Interface in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) has been proposed to achieve magnetic recording density. In HAMR systems, a laser is used to locally heat the magnetic disk to the Curie temperature, which is ~ 400°C-600°C, to assist in the data writing process. The high temperature working condition is a great challenge for the head-disk interface. Lubricant can be depleted by evaporation or decomposition. The protective carbon overcoat can be graphitized and oxidized. The surface quality, such as its roughness, can be changed as well. The near field transducer (NFT) structure is also vulnerable under the large number of heating cycles. All of those effects of the heating will present challenges for the reliability of the HAMR performance. In this paper, an experimental HAMR platform is introduced. The free space laser exposure of the media is calibrated based on the magnetization decay of special data tracks. Lubricant loss is determined from the optical surface analyzer. The lubricant changes as a function of different thermal load cycles are investigated as well. The disk topography is measured by an atomic force microscope after the mobile lubricant is removed. The hard overcoat's topography dependence on the laser exposure conditions is also studied. With regard to the media disk, we establish three distinct laser power thresholds for lubricant loss, disk surface degradation, and magnetization decay. As the laser power increases, the lubricant starts to change first before the magnetization starts to decay, indicating that lubricant could be the most vulnerable component to the laser heating in HAMR systems. Then, the disk overcoat's surface changes, becoming rougher after the magnetization starts to decay. Furthermore, the lubricant degradation is more serious under cyclic loading conditions. With regard to the transducer, the metal film of the NFT gets removed due to high energy laser ablation, and then the NFT loses its capability to focus the light, as required for HAMR systems.

Diffusion equation for spreading and replenishment in submonolayer lubricant film

In the field of head–disk interface technology, evaluating the effects of lubrication of a submonolayer lubricant film is becoming increasingly important as the thickness of the mobile lubricant film is reduced to <0.3 nm. Although the spreading and replenishment speed of a submonolayer lubricant can be evaluated by conventional diffusion theory if a large effective viscosity value is selected, it is commonly considered inappropriate to use the conventional diffusion equation based on continuum mechanics for a submonolayer liquid film. This paper presents a new diffusion equation that formulates the averaged flow of a submonolayer liquid film with a reduced density compared to the bulk density. The Hamaker constant for disjoining pressure in the submonolayer liquid film is also modified to be proportional to the film thickness. The difference between the viscosity in the submonolayer film and the bulk viscosity is also taken into account. From the derived diffusion equation for a submonolayer film, the spreading of a lubricant boundary and replenishment process of a depleted lubricant surface are studied in comparison with the results obtained using the conventional diffusion equation. The replenishment process of submonolayer lubricant profiles in a depleted groove for the Z-tetraol lubricant with a 0.24-nm-thick mobile layer can be calculated from submonolayer diffusion theory by using a low viscosity value consistent with that measured by a blow-off experiment.

Replenishment Speed of Depleted Scar in Submonolayer Lubricant

The evaluation of submonolayer lubricant mobility is becoming important in the field of nanotribology, in particular, in hard disk drives for realizing the near-contact or surfing–recording. This paper experimentally and theoretically investigates the replenishment speed of a depleted scar in a submonolayer lubricant caused by the head–disk contact. The theoretical analysis is based on continuum mechanics. The replenishment process of a submonolayer lubricant height profiles in a depleted scar caused by the head touchdown operation was experimentally measured for the Z-tetraol lubricant with a 0.24 nm mobile lubricant thickness and compared with the numerical simulation. It was found that the analytical replenishment process can fairly agree with the experimental one if the ratio of Hamaker constant to effective lubricant viscosity is properly determined. By using the validated basic equation, a simple but useful generalized formula is proposed to evaluate the replenishment speed in relation to the depleted scar width of the mobile lubricant, the lubricant thickness, and the ratio of Hamaker constant to effective lubricant viscosity.

Evaluation of Head Wear Durability Due to Asperity Contact Based on Replenishment Velocity Analysis of Sub-Monolayer Lubricant in Hard Disk Drives

In magnetic hard disk drives, it is important to evaluate head wear durability in near contact regime because the head-disk spacing has to be reduced to ～1nm to achieve a high density recording above 1 Tb/in2. This paper presents an evaluation method of the replenishment speed of a mobile lubricant scar caused by asperity contacts. The replenishment speed of sub-monolayer lubricant into a removed scar was analyzed by using the two-dimensional long wave equation on the basis of continuum mechanics. Then, a generic evaluation chart to estimate the removed lube scar width with 50% replenishment after one disk rotation was proposed in relation to the thickness and viscosity of the lubricant. Next, the effective lubricant viscosity of an actual thin lubricant layer was identified by comparing the measured lubricant replenishment process and its numerical simulation. From the analysis of contact pressure and width for roughness and defect asperities of disk surface in addition to the above results, the minimum possible head-disk spacing with 50% lube replenishment for reliable read-write operation and critical asperity contact conditions resulting in head wear and medium scratch are discussed.

A-2-4 磁気ディスク表面におけるナノ厚さ液体潤滑膜の動的摩擦特性(ヘッド・ディスク・インターフェイス(2),口頭発表)

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.Molecules retained by the disk after a solvent rinse is defined to be "bonded",while those removed from the disk is defined to be "mobile".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.Here by measuring the friction force at light loads and 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 nanomer-thick PFPE films.We found that mobile lubricant molecules generally tend to decrease friction while bonded lubricant molecules determine the speed dependence of friction.

Adhesion Properties of Monolayer Lubricant Films Coated on Magnetic Disk Surfaces: Contributions of Mobile and Bonded Molecules

Monolayer lubricant films on magnetic disk surfaces consist of bonded and mobile molecules whose different adsorption states may result in different adhesion properties, which directly impact the stability of ultra-low flying head-disk interfaces. By measuring the adhesion force and the surface coverage as a function of bonding ratio, we investigated the contributions of the bonded and mobile molecules to the adhesion of monolayer perfluoropolyether Zdol4000 and Z03 films. The experimental results, which agree with our theoretical results, indicate that bonded lubricant molecules decrease adhesion by increasing the surface coverage, whereas the ability of mobile lubricant molecules to reduce adhesion depends on their adsorption strength with the substrate. Mobile molecules with polar end groups tend to adsorb strongly on a disk, thereby increasing the surface coverage and decreasing the adhesion force.

Tribological Properties of MACs-APS Films

Multiply-alkylated cyclopentanes (MACs) with different molecular structure were deposited on single crystal silicon wafers coated with a thin aminopropyltrimethoxylsilane (APS) film as an adhesive layer to form MACs-APS films. The thickness, wetting behavior and nano-scale morphologies of the films were characterized by means of ellipsometry, contact angle measurement, and atomic force microscopy (AFM). The friction and wear behaviors of the thin films sliding against a Si3N4 ball were examined on a UMT-2MT tribometer in a ball-on-disk contact mode. The worn surfaces of the MACs-APS films and the counterpart Si3N4 balls were investigated with a scanning electron microscope (SEM). It was found that the water contact angles on the MACs-APS film increased with the MACs alkyl chain-length. The MACs-APS film exhibited higher load-carrying capacity and better friction reduction and anti-wear behavior as compared with the APS film. This is suggested to occur because the APS acts as a strongly bonded lubricant phase and MACs as a mobile lubricant phase in the MACs-APS film. The increase of the chain-length of the alkyl substituent in the MACs compounds resulted in improved tribological properties of MACs-APS film. It is suggested that the longer alkyl chains are much more flexible and can dissipate the mechanical energy during the shearing process more easily than the short chain compounds. MACs with the longer chains have stronger chain-chain interactions and the larger MAC molecules have stronger intermolecular interactions, resulting in the good tribological properties of MACs-APS film.

Boundary Lubricant Film Properties versus Molecular Polarity of Perfluoropolyethers Containing a Pendant Chain

The thin film properties of some novel branched perfluoropolyether (PFPE) boundary lubricants are investigated as a function of molecular polarity and end-group bonding to the underlying carbon film. The PFPE main chain, comprised of n-perfluoroethylene oxide monomer units, is stiffer than the Fomblin Z main chain that is a copolymer of perfluoro-methylene oxide and -ethylene oxide monomer units. Terraced flow and bonding kinetics show decreased lubricant mobility compared to PFPEs based on a Fomblin Z main chain. Lubricant pick-up is reduced for the tetraol adduct due to the larger Hamaker constant and increased intermolecular cohesion.

The Influence of Ultraviolet Irradiation on the Surface Chemistry of Ztetraol Magnetic Hard Disk Lubricant: a Combined Temperature Programed Desorption and X-Ray Photoelectron Spectroscopic Study

Magnetic hard disks coated with ztetraol lubricant were characterized with temperature programed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Ztetraol was found to have two adsorbed states, with desorption temperatures of ~650 and ~950 K. The complete removal of the low temperature state by solvent extraction identified it as due to the mobile lubricant. Desorption of the CF3 mass fragment was observed only at high temperature, indicating that lubricant in this state was in contact with the surface, which allowed the assignment of this high temperature state to the bonded lubricant. UV irradiation was found not to alter the TPD of the unextracted lubricant film. In contrast, the TPD of the UV-bonded layer remaining after extraction was observed to have only one desorption state, stabilized at ~40 K higher temperature as compared with the naturally bonded layer. XPS of the mobile layer was accomplished using spectral subtraction of the C1s carbon overcoat peak (284.8 eV BE), and the perfluoroethylene and perfluoromethylene lubricant peaks (293.5 and 295.0 eV BE, respectively), as a function of UV exposure. No change in the mobile lubricant layer was found with increasing UV exposure. C1s XPS of the UV-bonded layer identified five surface species and assigned XPS peaks to each: the carbon overcoat peak at 284.8 eV BE, ether peak at 286.4 eV BE, organic acid peak at 288.7 eV BE, perfluoroethylene peak at 293.5 eV BE, and the perfluoromethylene peak at 295.0 eV BE. Changes in the relative intensity of the assigned peaks with increasing UV irradiation exposure time were observed. The integration of the assigned XPS peaks from the UV-bonded layer with increasing UV exposure was used to identify UV dependent changes in the bonded layer. A significant relative decrease in the perfluoromethylene lubricant component was observed with increasing exposure, with a simultaneous increase of both the ether and organic acid surface concentrations. Quantum chemical calculations using small molecular models of the ztetraol were used to elucidate the XPS and TPD observations. The calculations revealed that the lubricant is fragmented with irradiation, forming reactive end groups, a volatile CF2O, and a hydrolyzeable CFO terminated fragment all consistent with the XPS results. The mechanistic implications and the possible surface chemistry are discussed.

Lubricant Pickup of Ultra-Thin PFPE Lubricants With Different Backbone Structures

Lubricant pickups on different perfluoropolyether (PFPE)-lubricant (Z-tetraol, D-4OH, and QA-40) films were compared by using a slider surface analyzing (SSA) tester developed to observe the lubricant thickness mapping on the slider surfaces after the heads slide on the disk surfaces. These lubricants have the same four OH functional end-groups but different backbones. Z-tetraol showed the least lubricant pickup among the four lubricants. D-4OH and QA-40 showed considerable pickup at a large clearance of 30-mW back-off. The pickup volumes of each lubricant disk with a similar mobile lubricant thickness were different, and D-4OH showed the largest pickup volume among these lubricants. An MD simulation was performed to understand the difference in the lubricant pickup of the three types of lubricants. The intermolecular force between the slider and the lubricant molecules was different for these lubricants. This difference in the intermolecular force depended on the dipole-dipole interaction between the end-groups and the slider surface, and the dipole-dipole interaction was affected by the conformation of the lubricant molecules. Therefore, it is estimated that the lubricant pickup phenomenon is influenced by the conformation of the lubricant molecules, and D-4OH with a strong -OH function at the lubricant surface exhibited considerable lubricant pickup.