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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.

Understanding the dynamics and instability of a thermal fly-height control (TFC) head slider close to and at touchdown (TD) on a magnetic recording disk is essential for developing a head slider with a clearance of ~ 0.5 nm to enhance the recording density of hard disk drives. This paper presents a theoretical simulation of a practical TD test, performed by continuously increasing the head protrusion according to the TFC power, and demonstrates ideal TD behaviors to be established in design guidelines for an effective head slider–suspension–disk system. The effects of the air-bearing stiffness and magnitude of disk waviness (DW) on the TD behaviors and their repeatability were extensively investigated. Comparison with the experimental TD behaviors of effective head sliders demonstrated that the different TD behaviors at different radial disk positions could be explained by the numerical TD simulation of the DW-excited response of a single-degree-of-freedom slider model. Possible reasons for the discrepancies between the experimental and simulation results in the light contact state were analyzed in detail, and a method of designing the head–disk interface conditions to achieve a head spacing of ~ 0.5 nm was developed. In addition, the deviation from the ideal TD behavior according to the DW magnitude was examined. The TD behavior of a head slider on a disk with a perfectly bonded lubricant was theoretically clarified, and the potential for realizing a head slider with reduced clearance was assessed.

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.

In magnetic hard disk drives, it is important to evaluate the replenishment effect of a submonolayer lubricant film under a more severe condition that the head–disk spacing has to be reduced from the current 0.7 nm to ~0.5 nm. In contrast to the prevailing conventional diffusion equation validated for multilayer liquid film, the author has already proposed a new diffusion equation more suitable for submonolayer film by intuitively incorporating the density reduction effect in the submonolayer liquid film. This paper presents a rigorous derivation of the disjoining pressure (DP) from Lennard–Jones potential (LJP) and formulated the diffusion equation incorporating the DP. The difference in the rigorous DP and diffusion equation from the previous versions is negligibly small except in a small film thickness less than the van der Waals (vdW) distance. The theoretical relationship between the vdW distance in the DP and the molecular force equilibrium distance in the LJP is elucidated. Rigorous derivations of the DP and diffusion equation for multilayer liquid film from the LJP are also presented. The superiority of the submonolayer diffusion equation over the conventional equation in the submonolayer film regime is demonstrated by comparing their theoretical diffusion coefficients with Waltman’s experimental data.

Reducing the fly-height between the head and disk to less than 1 nm with high reliability is necessary to improve the recording density of magnetic disks. Therefore, the mechanism of the head touchdown (TD) phenomenon, particularly the surfing state after the TD, needs to be understood. Assuming that the contact sliding on the sub-nanometer asperities, covered with a monolayer lubricant film, generates the lubrication film forces and reduces the surface forces, the present study shows that numerical simulations of a one-degree-of-freedom slider model can be used to understand various TD phenomena, including surfing conditions. The three different TD behaviors of a commercial head slider were explained by the differences in the rate of generation of the lubrication film force after the TD and initial surface force. The parametric studies demonstrated that surfing states could be generated at a separation of more than three times the standard deviation of the asperity height by increasing the surface force and magnitude of the disk-waviness, thereby suggesting the possibility of quasi-contact recording.

The dynamics and stability of a flying head slider at proximity to and touchdown on a magnetic recording disk are attracting considerable research attention, because it is necessary to reduce the head clearance from the lubricant film to ~ 0.5 nm to achieve a higher recording density. As a succeeding study continuing the theoretical investigation of slider dynamics at touchdown, this study theoretically analyzes further details of the different behaviors of a practically applied head slider. The experimental touchdown behaviors at inner, middle, and outer radius positions in a commercially available hard disk drive are introduced. From the improved theoretical analyses, it is determined that different behaviors result from differences in air-bearing stiffness at the inner, middle, and outer radius positions. Moreover, it is found that the first half of the period for which a loss-of-contact signal is obtained corresponds to a light asperity contact state between the head and disk surfaces, whereas the latter half of this period corresponds to a flying state. It is thought that the former light asperity contact state can be well lubricated even at a high bond ratio, and that the light contact state can be used for future contact recording.

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.

A slider with an air-bearing surface that has a small spherical pad around the read/write elements on the trailing center pad was developed to reduce the meniscus and friction forces and slider clearance loss. While the slider which has a spherical pad of a radius less than 10 mm, the meniscus force is reduced to less than 10% of the air-bearing lift force, thus, flying height modulation is decreased and instability are suppressed. Evaluation using an air-bearing surface model with a spherical pad showed that slider clearance was increased by 1 nm at a 5-nm nominal flying height. Evaluation using touchdown-takeoff testing of a spherical pad slider fabricated by depositing carbon on the center pad air-bearing surface by means of the lift-off resist technique showed that the spherical pad slider had a very small friction force and acoustic emission output up to a 5-nm interference height. It thus provides instability-free sliding in the near-contact regime.

Based on an introduced optimal trajectory planning method, this paper mainly deals with the accuracy analysis during the function approximation process of the optimal trajectory planning method The basis functions are composed of Hermit polynomials and Founer series to improve the approximation accuracy Since the approximation accuracy is affected by the given orders of each basis function, the accuracy of the optimal solution is examined by changing the combinations of the orders of Hermit polynomials and Fourier series as the approximation basis functions As a result, it is found that the proper approximation basis functions are the 5th order Hermit polynomials and the 7th-10th order of Fourier series

The dynamic indentation characteristics of 1- and 2-mm-radius hemispherical glass sliders when colliding with stationary magnetic disks under various lubricant conditions were investigated to clarify the dynamic interfacial forces between flying head sliders and magnetic disks. The collision times were ~15 and ~30 μs, respectively, and independent of the impact velocity. For a 1-mm-radius slider (Ra roughness = 1.71 nm), a clear adhesion force nearly equal to the static pull-off force was observed at the instant of separation when the lubricant thickness was from 1 nm without UV (0.69 nm mobile lubricant thickness) to 3 nm with UV (1.89 nm mobile lubricant thickness). The dynamic adhesion force was maximum when the slider had separated from the disk surface by about 2 nm and dropped from the maximum to zero when the separation reached more than 5 nm. When the mobile lubricant thickness was 0.43 nm, a clear adhesion force was not observed. For a 2-mm-radius slider (Ra roughness = 0.34 nm), a clear adhesion force, similar to the static pull-off force, was observed at the instant of separation at almost all lubricant thicknesses and impact velocities tested except at a small mobile lubricant thickness of 0.43 nm with impact velocities greater than 1.1 mm/s. The dynamic adhesion force dropped from the maximum to zero when the distance traveled from the maximum reached more than 5 nm. These results suggest that the dynamic adhesion force of 1- and 2-mm-radius sliders originates from meniscus formation rather than van der Waals force.