Thermal flying height (TFC) control technology has been used in the past few years to achieve a lower flying height for higher storage areal density. In this paper, a slider with dual thermal protrusion, one produced by the laser used to heat the disk in HAMR and the other one used for TFC, is programmed with feedback control to reduce the flying height modulation (FHM) due to disk waviness and to attenuate the external disturbances. A numerical simulation is conducted to investigate the feasibility of the system. The results show that FHM of the dual protrusion slider is reduced significantly when the protrusion heights and locations are optimized. External disturbance induced FHM can be suppressed as well. This scheme can benefit HAMR because it will provide a more consistent flying height and more freedom to compensate external disturbances.

The articular cartilage function is to allow the bones in a joint to move without causing excess friction and damage. When this cartilage becomes damaged, the supportive and lubricating mechanisms break down, leading to injuries which can be permanent or take extended periods of time for recovery. Because of its importance in general health and body mobility, the unique lubricating properties of cartilage have been studied for many decades. Many current theories exist to characterize the biphasic and triphasic nature of cartilage; however, an important reason that cartilage is so effective is its viscoelastic nature, which allows elastic and dissipative mechanisms to exist simultaneously. It is desired to derive the material properties of cartilage in order to better understand its mechanical effectiveness. Utilizing a CETR-UMT-3 Tribometer, stress relaxation experiments will be performed on freshly harvested equine cartilage plugs that remain hydrated in a fluid bath. Viscoelastic models, such as the Prony series and fractional derivative, are applied to the experimental data to determine the storage and loss moduli of the sample explants. The storage and loss information characterizes the mechanical response of cartilage, and provides insight into the effectiveness and longevity of biological joints. A comparison will be made between joints that experience similar loads, but undergo different relative motions, to determine if the mechanical properties of cartilage are tailored to joint function. Osteoarthritic cartilage will also be explored for deviations in viscoelastic behavior compared to healthy cartilage. Ultimately, it is hoped that a viscoelastic characterization of articular cartilage will lead to insight into the precursors of osteoarthritis, more advanced prosthetics, and biomimetric applications such as the integration of flexible surfaces in mechanical systems.

An ultrathin (<4 nm) film of amorphous carbon ( a -C) is used in contemporary disk drives to protect the magnetic medium of the hard disk from corrosion and mechanical wear due to intermittent impact of the low-flying magnetic head. Because of increasing demands for much higher magnetic storage densities (i.e., >10 Tbits/in 2 ), the a -C film thickness must be decreased to <2 nm. However, the tribological and mechanical properties of such thin a -C films are not well understood and, moreover, are extremely difficult to determine experimentally. The objective of this study was to obtain insight into the tribological behavior of ultrathin a -C films by performing molecular dynamics (MD) simulations. MD results of the hardness and friction properties of nanometer-thick a -C films are interpreted in terms of the ratio of tetrahedral-to-trigonal carbon atom hybridization. A critical thickness for the effective protection of the magnetic medium by the a -C film is estimated from MD results. The results of this study elucidate the nanomechanical and nanotribological properties of ultrathin a -C films used as protective overcoats in extremely-high-density magnetic recording.

A model is presented that accounts for contact with a molecularly thin lubricant layer confined between a rigid sphere and flat substrate. The sphere is moving at a high lateral velocity inducing high shear rates and a significant increase of lubricant viscosity. Interfacial slip is modeled based on experimental and molecular dynamics data and includes effects of lubricant morphology and chemistry. The model is used to interpret experimental measurements of viscosity and slip and express them as functions of shear rate. These data can then be used in a typical head-disk interface in magnetic storage.

In most industrial applications, a thin film of lubricant improves the tribological behaviour of the load carrying conjunctions. Modern hard disk drives are no exception, where one of the challenges is the progressive demand for higher data storage capacity, while decreasing the physical size of the device. This paper investigates the load carrying capacity of an ultra thin lubricant film formed between the hard disk drive platter and its flying read-write head slider. Hydrodynamic wedge action is responsible for the entrainment of a film of perfluoropolyether into the conjunction, but does not account for the main share of the load carrying capacity of the very thin discretized film, several molecular diameters of the fluid in thickness. It is shown that the main mechanism of pressure generation is in fact the solvation effect. Additionally, starvation of contact may well occur, further exacerbating the thinness of a lubricant film. There is often a cavitation region at the contact exit, which is followed by film reformation, changing the density distribution of lubricant on the surface and consequently the attainable data storage density. This is caused by the observed clumps of lubricant in the reformation zones (a phenomenon referred to as lubricant mogul) which can also cause transient motions of the read-write head.

Scratch-related magnetic signal degradation can occur during magnetic storage hard disk drive operation when the read-write heads contact the spinning multilayer disks. To investigate this phenomenon controlled nanoscratch experiments were performed on perpendicular magnetic recording media using various indenters of different radii of curvature. Various loading conditions were used to cause permanent scratches that were measured using atomic force microscopy. The nanoscratch experiments were simulated using finite element analysis (FEA) that included the detailed nanometer scale thin-film multilayer mechanical properties. The permanently deformed field in the sub-surface magnetic recording layer was extracted from the FEA results. The residual scratch widths measured on the surface of the magnetic storage disk were directly compared with the residual sub-surface widths of the region on the magnetic recording layer where extensive permanent lateral deformation was present. It was found that the sub-surface widths of the deformed regions were significantly larger than the surface scratch widths. Thus, sub-surface thin-film layers, such as the magnetic recording layer could be damaged without observable damage to the protective top surface carbon overcoat. The exact location and extent of damage to the magnetic recording layer depends on the scratch load, size of scratch tip, and the friction at the interface. Such permanent deformation in magnetic recording layer could lead to demagnetization, which has been reported in the literature.

In a probe-based ferroelectric recording system a protective coating of lubricant provides low friction and a low wear rate as well as allows the continuous contact of the probe tip with the storage medium. SO5, a member of the commercially available electric connector lubricants polyphenyl ether (PPE), was experimentally proven effective as a lubricant in supporting this probe/media interface and has enabled the demonstration of the probe-based ferroelectric recording technology. The unique stability of SO5 lubricant in an electric field and in the presence of electrons makes this liquid particularly suitable for ferroelectric recording storage devices. A broad range of analytical techniques were used to identify this lubricant as a mixture of alkylated diphenyl ethers having varying molecular weight. A thin layer of this lubricant was uniformly coated on the probe media surface through a dip-lubing procedure. Its thermal stability and wettability was investigated in terms of the requirements for the successful application of this unique ferroelectric recording technology. The impact of this lubricant film on the read/write capability, durability and tribological performance was also explored.

A compliance operator, E ji , is often utilized to evaluate elastic deformation in the simulation of transient and steady state elastohydrodynamic lubrication of conformal-contact systems. The values of the compliance operator represent the elastic responses of all nodes when only one node is under a unit load. The accuracy of compliance operator values, computational cost, and storage size are important issues. Based on our previous study on steady-state conformal-contact elastohydrodynamic lubrication systems, an advanced method of selective-fine-mesh with selective storage is suggested, and a special technique of combined selective-fine-mesh with selective storage mapping is proposed. These two techniques enable an efficient elasticity procedure for the simulation of transient conformal-contact elastohydrodynamic lubrication systems.

Patterned media is being considered as a means for achieving a storage density beyond 1Tb/in 2 on hard disks by reducing cross talk and overcoming thermal stability problems. In this investigation, the flying characteristics of a femto-type slider over bit patterned media (BPM) is investigated. The discrete bits of the disk are modeled as cylindrical protrusions on the disk surface. The steady-state flying characteristics over BPM at ultra-low head disk spacing is simulated.

Frictional characteristics of mesoporous SiO 2 thin films were evaluated with different pore sizes. The films were manufactured by sol-gel and self-assembly methods to have a porous structure. The pores on the surface may play as the outlet of wear particle and the storage of lubricant so that the surface interactions could be improved. The pores were exposed on the surface by chemical mechanical polishing (CMP) or plasma-etching after forming the porous films. The ball-on-disk tests with mesoporous SiO 2 thin films on glass specimen were conducted at sliding speed of 15rpm and a load of 0.26N. The results show considerable dependency of friction on pore size of mesoporous SiO 2 thin films. The friction coefficient decreased as increasing the pore size. CMP process was very useful to expose the pores on the surface.

Oblique impact of a slider with a rotating disk in hard disk drives was analyzed using the finite element method. A three dimensional, thermomechanical, impact model was developed to study the mechanical and thermal response during the impact of a spherical slider corner with the disk. The model was validated by comparing finite element results with analytical solutions for homogeneous glass disk under simple conditions. Impact penetration, stress and incurred flash temperature were obtained for various normal impact velocities.

A compliance operator is often utilized to evaluate the elastic deformation during simulation of elastohydrodynamic lubrication of conformal-contact systems. The values of the compliance operator represent the elastic deformations of all nodes when only one node is under a unit external load. The accuracy of compliance operator values, computational cost, and storage size are important issues. It is found that values only in a small region surrounding the loading node should be used and those in other regions can be discarded. Moreover, the mesh refinement in thickness direction is found to be more important than that along the longitudinal and width directions. A simple formula is suggested as the relationship between the values of a compliance operator and refinement steps. These two approaches enable an efficient elasticity procedure for the simulation of conformal-contact Elastohydrodynamic lubrication systems.

To achieve a higher storage density in a hard disk drive, the fly height of the air bearing slider, as part of the magnetic spacing, has to be minimized. At an ultra-low fly height, the intermittent / continuous contact at the head–disk interface (HDI) is unavoidable and directly affects the mechanical and magnetic performance of the hard disk drive, and is of great interest. The HDI wear has a non-linear and time-varying nature due to the change of contact force and roughness. To predict the HDI wear evolution, an iterative model of Coupled Head And Disk (CHAD) wear, is developed based on the contact mechanics. In this model, a composite transient wear coefficient is adopted and multiple phases of the wear evolution are established. A comprehensive contact stiffness is derived to characterize the contact at the HDI. The abrasive and adhesive wear is calculated based on the extended Archard’s wear law. The plastic and elastic contact areas are calculated with a 3-D sliding contact model. Based on the CHAD wear model, for the first time, the coupling between head and disk wear evolutions is thoroughly investigated. Accelerated wear tests have also been performed to verify the disk wear effect on the slider wear. A wear coefficient drop with time is observed during the tests and it is attributed to a wear mechanism shift from abrasive to adhesive wear. A shift in the type of contact from plastic to elastic accounts for the wear mechanism change.

Preferential surface texturing is expected to significantly improve tribological performance of ultra-low flying magnetic storage head-disk interfaces (HDIs) by modifying the roughness and reducing the contact area preferentially, thereby reducing the relevant interfacial forces such as friction, contact and adhesive forces. Due to the different etch rates in the titanium carbide (top surface) and alumina (bottom surface) portions of the slider air-bearing surface (ABS), during reactive ion etching the surface heights possesses a distinct bimodal distribution. In order to accurately and realistically capture the interfacial phenomena of the ultra-low flying HDI with a preferentially textured slider ABS, a probability density function was proposed by linking two different Gaussian asperity distributions. The proposed bimodal asperity distribution was then directly incorporated into a previously developed rough surface contact model to calculate the corresponding interfacial forces. The results were then directly compared to a single Gaussian approximation (ignoring the bimodality) as well as a high order polynomial curve-fit approximation (encompassing the bimodality). Comparative study revealed that the proposed bimodal distribution method has a main advantage of being able to resolve the top and bottom asperity contributions separately, thereby providing the interfacial force estimates that are more physically accurate. Other simpler methods, by assuming a single continuous distribution over the entire surface, are not able to isolate the top and bottom asperity distributions and thus are more likely to overestimate the interfacial forces in sub-5 nm flying HDIs.

As the areal density of magnetic disk storage continues to increase and head-disk spacing decrease, contact between the recording slider and the rotating media becomes imminent. In order to predict contact forces, fly-height modulations, and off-track motions, dynamic models are typically used. A critical element of these models is the contact stiffness and damping arising from the interfacial interaction between the slider and the disk. In this paper, we review different models for predicting contact stiffness based on roughness and layered media and then we report experimental data of both contact stiffness and contact damping of typical head-disk interfaces. It is found that the contact stiffness models (based on roughness alone), over-predict the contact stiffness of actual head-disk interfaces by as much as an order of magnitude. Also, it is found that the contact damping ratio is typically few percent and its behavior is substrate dependent. In addition, the effects of a molecularly thin lubricant and humidity on contact stiffness and damping were experimentally investigated and no significant effects were found.

In view of the difficulty in measurement of temperature rise at the contact between sliding bodies with engineering scale roughness a good deal of theoretical work has been carried out in the last few decades. However, as surfaces become smoother and loading decreases in applications such as MEMS, NEMS and magnetic storage devices measurement of flash temperature becomes increasingly more difficult due to the nanometer scale asperity interactions. Consequently measurement of flash temperature at the nano-scale asperity contact has not yet been possible. The analysis of flash temperature rise under these circumstances is no less challenging since it must consider not only the small-scale asperity height distributions but also the surface forces those may operate at very small surface separations. The paper attempts to predict the flash temperature rise analytically using a fractal approach to describe the nano-scale asperity interactions at low loads and also taking into account the influence of relevant parameters including the surface forces. The results show in general that the contact surface temperature steadily increases with load, nano-scale roughness and surface forces. Interestingly, the fractal analysis presents a wide spectrum of solutions. While under certain combinations of fractal and material parameters extremely high contact temperature rise is predicted, under certain other parametric combinations extremely low temperature rise can be seen. The later parametric combination is certainly of much practical use.