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In heat-assisted magnetic recording (HAMR) technology, it is necessary to heat the magnetic layer of disks at a temperature that exceeds the Curie temperature (Tc). When the disk is laser irradiated, the lubricating film and diamond-like carbon (DLC) protective film on the disk surface are heated at high temperatures (300 °C–500 °C). If the lubricant film, DLC thin film, or organic contamination on the disk surface evaporates or thermally decomposes upon laser heating, it adheres as a smear on the slider surface. In view of the negative effect exerted thereof, the solution to this problem is, therefore, critical for the successful application of HAMR. Furthermore, recently developed helium-enclosed hard disk drives are currently utilized in industries. It is, thus, expected that the aforementioned novel HAMR technology will be employed in helium-enclosed drives. In this study, the effects of DLC thin films on the smear characteristics upon the laser heating of disks in air and He are experimentally investigated, and the manner with which these films on the disk surface affects the smear characteristics is clarified. Moreover, the technical countermeasures necessary to reduce the smear with respect to the hydrocarbons from the DLC thin films in HAMR are elucidated and proposed in this article. Simply put, the technical countermeasures depend on the heating temperature, and the use of a mixture of He and air or oxygen gases is found to be effective. This method, however, may not be significantly useful when the heating temperatures exceed 500 °C.

The formation of head smears during the heat-assisted magnetic recording is a significant problem that must be addressed to ensure the reliability of the head-disk interface. In this study, the formation of a head smear by diamond-like carbon (DLC) films on magnetic disks was analyzed through simulated heating experiments. DLC films with a thickness of 100 nm were prepared and heated on a hot plate in an environmental gas chamber. A smear was observed on a silicon wafer placed 3 mm above the DLC surface film after the heating of the film. The smear amount was quantified based on the intensity of the carbon peak of the smear, which was measured by X-ray photoelectron spectroscopy. In air, the carbon concentration decreased with the heating temperature, while in nitrogen, it increased. Further, the carbon concentration of the smear formed in air was lower than that in nitrogen. A comparison of the smears formed by DLC films with different hydrogen concentrations showed that a greater amount of the decomposition gases was generated from the DLC films with a higher hydrogen concentration and that this value increased at lower temperatures. This result suggests that DLC films with low hydrogen concentrations generate smaller amounts of the decomposition gases during heating than those with high hydrogen concentrations. In addition, the results of Raman spectroscopy suggested that the DLC films get degraded during heating, resulting in the generation of hydrocarbon gases because the films contain carbon and hydrogen. Moreover, since peaks related to fragments with atomic masses of 250–500 atomic mass units were observed in the time-of-flight secondary ion mass spectrometry spectra of the smears, it can be concluded that the smears consist of hydrocarbons with relatively higher molecular weights.

A heat-assisted magnetic recording (HAMR) is being implemented to increase the density of hard disks. However, adhesion of smear to the head slider is a drawback. We investigated a smear generation mechanism under laser irradiation in different environments (air and helium) using a disk coated with perfluoropolyether lubricant D-4OH. The amount of smear was smaller in air than in helium at all temperatures. An analysis of smears at 258°C using time-of-flight secondary ion mass spectrometry (TOF-SIMS) suggested that the end group was oxidatively decomposed and the hydrocarbons were burned. And the differences in the end group/main chain ratio and the hydrocarbon ratio between the two environments were smaller at 540°C than at 258°C. It suggested that the lubricant molecules were thermally decomposed owing to the high temperature. It was observed that there was a small difference in the smear amounts between the two environmental gases at 540°C due to the thermal decomposition of the lubricants, but the presence of oxygen was effective in reducing the amount of smear when the heating temperature was 200–300°C.

Diamond-like carbon (DLC) coatings have undergone a significant amount of research over the last several years and shows promise as a coating material in reducing the friction between moving components of machinery and various other applications. This study investigated the reduction of friction coefficients on these surfaces by applying an external electric field under the assumption that thermal plasma would be generated under an inert gas atmosphere and that the number of hydrocarbons generated by thermal decomposition could also be controlled. Results show that the friction coefficient of DLC heated to 200 °C by laser irradiation was indeed controlled by an external electric field. Although the friction coefficient without the bias voltage between the DLC and leaf spring was approximately 0.35, applying a positive bias voltage to DLC of + 10 and + 20 V corresponding to an electric field of 80 and 160 kV m−1 decreased to 0.1. Meanwhile, the negative bias voltage did not show a reduction in the friction coefficient. Raman spectroscopy of the pin surface after the friction test indicated that the hydrocarbons generated by the thermal decomposition of DLC were transferred onto the pin surface by applying a positive bias voltage of + 10 and + 20 V. Meanwhile, we measured the current from DLC heated using a hotplate on the counter surface of the Si wafer insulated with the DLC using a ceramic ring by applying a bias voltage to the DLC. The current from the DLC forward Si wafer was changed by the applied electric field. When a positive voltage is applied to the DLC, the current flows from the DLC to the Si wafer, and when a negative voltage is applied, the current flows from the Si wafer to the DLC. This dependence on the electric field intensity shows the opposite tendency to that of the friction coefficient on the electric field intensity. Therefore, the hydrocarbon ions generated by the thermal decomposition of the DLC are attracted to the pin surface as the counter surface side by the electric field when an appropriate positive bias voltage is applied to the DLC. Subsequently, liquid hydrocarbons are transferred and deposited on the pin surface, and the friction coefficient decreases owing to the lubrication effect of the adhered hydrocarbons. If this voltage becomes too large, thermal electron emissions from the DLC will be suppressed, and the amount of hydrocarbon generation will decrease. In contrast, when a negative voltage is applied to the DLC, the thermal electron emissions are increased by the work function reduction due to the electric field; however, the hydrocarbon ions are attracted to the DLC and redeposited to the DLC. Finally, we estimated that the number of hydrocarbons transferred to the pin surface could be controlled by the external electric field, and these hydrocarbons changed the friction coefficient of the DLC.

We propose a novel electromagnetic energy-harvesting device (EHD) for structural health monitoring systems of transportation infrastructures. The EHD is embedded in the road surface and uses the tread force of cars as the input force when tires of cars pass over it. Because the input force is very fast, the proposed EHD can generate a large amount of energy. The footprint of the device is 20 × 20 mm, its height is 7.5 mm, and its volume is 2.4 cm3. We measured the energy generated when a bicycle passed over the EHD 34 times at various speeds between 5 and 15 km/h. Subsequently, we obtained the regression curve from the results, which showed the relationship between the bicycle speed and generated energy, and estimated the electric energy generated by car at higher speeds. The results showed that, even though the size of the EHD was small, electric energies of 100 μJ, and 1.0 mJ could be generated at car speeds of 17 km/h, and 52 km/h, respectively.

A novel vibration-sensing device based on the giant magnetostrictive effect for a self-powered structural health monitoring system was developed. This device can produce a large voltage because the magnetostrictive rod, in principle, can generate a large variation in magnetic flux density, even when it is subjected to small and low-frequency vibrations of the bridge girder. Therefore, the device could function as a power generator to supply power to a storage battery, when it is not being used as a vibration sensor, to drive the electric circuit. A field test for the developed device was conducted. The experimental results confirmed that the device worked as a vibration sensor with considerable accuracy in detecting the natural frequency of the bridge, even when subjected to the intermittent load of passing cars.

The formation of head smear from DLC films on magnetic disks was confirmed by a simulated heating experiment. DLC films of 100-nm thickness were prepared and heated on a hot plate in an environmental gas chamber. Smear on a silicon wafer placed 3 mm above the DLC surface was observed after heating of the DLC film. The smear amount was quantified using the carbon intensity of the smear measured by XPS. In air, the carbon concentration decreased with the heating temperature; however, the carbon concentration increased with temperature in nitrogen. The carbon concentration of the smear in the air environment was smaller than that of nitrogen. As the result of comparison of smears on DLCs with various hydrogen concentrations, more decomposed gas is generated from the DLC film as the amount of hydrogen contained in the DLC increases, and this value increases at lower temperatures. This result suggests that DLC films with low hydrogen concentrations generate a smaller amount of decomposition gas than those with high hydrogen concentrations during heating. In addition, Raman spectra suggest that the DLC degraded by heating and the hydrocarbon gas-like materials were generated from the DLC film because the DLC films include carbon and hydrogen. Because fragments of 250–500 atomic mass units were observed in the TOF-SIMS spectra of smears, the smear is estimated to consist of a hydrocarbon of relatively high molecular weight.

In this study, we experimentally investigate the lubricant depletion as well as the temperature distribution caused by pulsed laser irradiation heating and discuss the fundamental characteristics. It was determined that the maximum temperature increased as the duty ratio increased. However, the rate of the maximum temperature change due to the duty ratio was comparatively small. Therefore, it was suggested that the maximum temperature was almost constant when the duty ratio was more than 50% under experimental conditions. The thermal distribution or the thermal gradient also depended on the duty ratio and increased as this ratio decreased. It was also determined that the depletion of the lubricant film due to pulsed laser heating was smaller than that due to continuous laser heating. Moreover, it depended on the duty ratio and increased as the duty ratio increased. In addition, it depended on the full width at half maximum (FWHM) of the temperature distribution due to pulsed laser irradiation heating and increased as the FWHM is increased. Therefore, it is also suggested that the lubricant depletion issue will not be as critical in actual heat-assisted magnetic recording system because the FWHM would be very small under practical circumstances.

Structural and molecular weight changes to lubricant picked up the following laser heating in heat-assisted magnetic recording (HAMR) were analyzed using time-of-flight secondary ion mass spectroscopy. The average molecular weight of the disk lubricant increased by laser heating at temperatures over 500 °C and that of the picked up lubricant increased at temperatures over 400 °C. The presence of end-group fragments with low molecular weight (CH3O+) increases in both the disk and picked up lubricants, whereas at high temperature the concentration of the end-group component (C3H7O2+) decreases in the disk lubricant and increases in the picked up lubricant. Overall, the results presented herein suggest that the chemical and thermal stability of the terminal ends of lubricants should be improved in order to decrease depletion of the lubricant film and the lubricant pickup by the laser heating in HAMR.

Currently, hard disk drives (HDDs) are widely used as external storage devices. In HDDs, flow induced vibrations (FIVs), i.e., the vibrations of carriage arms and disks due to the internal flow of HDDs, are serious contributors to head positioning errors. To develop higher capacity HDDs, reducing head positioning errors is inevitable. Many experimental and analytical studies have been conducted thus far to reveal the internal flow in HDDs and to understand the mechanisms of the disturbance torque exerted on head-stack assemblies for mitigating FIVs. Most studies have exclusively used Fourier spectral analysis for frequency domain analysis (e.g., [1]–[3]). The Fourier spectral analysis, however, must be used for signals under restricted conditions: the system must be linear, and the signal must be stationary and periodic. The internal flow of an HDD is an extremely complex phenomenon, and it is governed by non-linear Navier- Stokes equations. Therefore, the results of the Fourier spectral analysis are not always adequate. Further, if both fine frequency-resolution and high frequency spectra are required for Fourier spectral analysis, the time series data must have a long time-span and a fine time-step. Because it takes a significant amount of time to conduct a computational fluid analysis with such requirements, the results of the Fourier spectrum analysis tend to have a coarse frequency-resolution. The Hilbert-Huang transform (HHT) proposed by Huang et al. is a useful method for analyzing a nonlinear and non-stationary signal, and it has been applied to seismic signal analysis [4]. HHT consists of two parts: ensemble empirical mode decomposition (EEMD) and Hilbert transform (HT). EEMD is a way to decompose time series data into intrinsic mode functions (IMF) considering the envelope of the waveform. After EEMD is conducted, HT is executed, and the analytic signal is generated afterward. Then, the instantaneous frequency and amplitude are calculated from the analytic signal as time series data; therefore, it is possible to analyze the nonlinear and non-stationary signal. In this study, we applied the HHT to the time series of the torque exerted on the carriage arm, which is calculated using finite element analysis. Then, a frequency analysis using FFT and HHT was conducted, and their results were compared. 2. Computational fluid dynamics analysis of HDD A finite element (FE) model for fluid dynamics analysis was produced by referring to a 3.5-in HDD. The computational region for air flow was the inner space of the HDD, which was cut out between two disks. The FE model had a voice-coil-motor (VCM) carriage and a flexible printed circuit. The head suspensions were excluded from the FE model. The disk diameter was 95 mm, and the rotational speed of the disks was 7,200 rpm. The VCM surface was divided into five areas, and the disturbance torque exerted on each area was calculated. ANSYS CFX, a code for fluid analysis, was used for air-flow simulation. Large eddy simulation (LES) was employed as the turbulence model. The time step was $2.0 \times 10 ^{-5}\mathrm {s}$, and the number of time steps was 15,000. Thus, the time span of the simulation was 0.3 ms, and the last 5,000 time-series data were used for frequency analysis. At first, contour-plot animations of the flow velocity and vorticity were made and visually observed. When the carriage arm is at the position where the head is tracking the outermost track, several periodic flow patterns are observed: vortex shedding at the arm-tip at approximately 500 Hz, fluctuation of strong vorticities inside the arm hole at approximately 1000 Hz, and vorticities between the disk and arm and the outside of the disk at 700 to 1000 Hz. Next, a Fourier analysis was conducted using the same simulation results. Although the time series of the torque exerted on the carriage arm seems to have a regularity or periodicity involving such specific frequencies as seen in the animations, there was no specific peak in the Fourier spectra, as shown in figure 1. At low frequencies less than 1000 Hz, the magnitudes of the Fourier spectra are almost constant, and with increase in frequency, the magnitude of the Fourier spectra decrease. Then, the frequency analysis using HHT was performed. The disturbance-torque time series, which is identical to the data used for FFT analysis, were divided into 11 IMFs by the EEMD process. Each IMF was analyzed using HT. Figure 2 shows the HHT results. Fluctuations in frequency were observed at approximately 500 Hz and 800 Hz. The spectra at these frequencies are caused by flow-fluctuations such as vortices, as observed in the flow-animations. Comparing the HHT with FT spectra, the results confirmed that, in the Fourier spectra, the energy of the non-stationary phenomenon, whose frequency and amplitude are fluctuating at a specific frequency, diffuses over a wide frequency-band, and that forms the broadly expanded spectra, especially at low frequencies. Therefore, it is necessary for more precise head positioning to decrease torque-fluctuations due to non-stationary phenomena, such as a vortex shedding around an arm, which are not always observed in the power spectrum obtained by the FFT analysis.

In this study, the smear by the lubricant and siloxane pickup at the laser heating in the heat assisted magnetic recording (HAMR) was evaluated using our developed noncontact pin-on-disk test. The smear area on the glass pin surface was measured on the disks with three types of different lubricant films prepared by changing the ultraviolet exposure condition, rinse condition, and lubrication process. The amount of siloxane adsorbed on disk surface relates with the siloxane wettability obtained from the contact angle of siloxane for disk surface and that increased as the siloxane wettability increases. On the other hand, the lubricant pickup increases as the physical and hydrogen bonding fractions increase. Overall, these data suggest that the sufficiently low surface free energy is needed by the lubricant film with only the chemical bonding fraction to reduce the lubricant and siloxane smear at the laser heating of HAMR, because the lubricant fractions of physical and hydrogen bonding increase the lubricant pickup and the much amount of siloxane adsorbs on disks with the large surface free energy.

This paper examines the adsorption properties of the perfluoropolyether (PFPE) lubricant SNH2 with an ionic end-group (i.e., amine salt with a diphenylether moiety) for magnetic disk surfaces in heat-assisted magnetic recording (HAMR) drives. For comparison, the well-known lubricant Z-tetraol was used as a reference material. Initially, strong interactions between the ionic end-groups of SNH2 and an applied electric field in the lubricant solution were established. The surface free energy and bonding ratio were also compared before and after ultraviolet (UV) light exposure to diamond-like carbon (DLC) disk surfaces onto, which the lubricants were coated, with their photoelectron currents measured during UV exposure. In addition, the structural characteristics of the SNH2 lubricant on the DLC surface were analyzed using a time-of-flight secondary mass spectroscopy. Furthermore, the thermal stabilities of the lubricant films were compared following laser heating. Overall, SNH2 showed higher adsorption to the DLC surface than Z-tetraol and interacted more strongly with the electric field in the lubricant solution. Furthermore, although SNH2 without UV exposure showed a large polar surface energy, this energy decreased considerably after UV exposure. This decrease is attributed to interactions between photoelectrons emitted from the DLC surface and the ionic bonds of the SNH2 molecules; the cationic segments of the SNH2 ionic bonds dissociate, whereas the anionic main-chains chemisorb on the DLC surface at the electron holes generated from the ejected photoelectrons. SNH2 after the UV exposure showed high thermal stability because of its strong bonding to the DLC surface. Overall, these results demonstrate that a PFPE lubricant with ionic end-groups may be an effective lubricant for HAMR applications.

We investigated the laser-heating-temperature dependence of the friction characteristics of magnetic disks coated with boundary lubricant films by using a pin-on-disk tester and discussed the fundamental characteristics observed. In the contact area of the glass pin and disk, we observed the area that showed a buildup and depletion of the lubricant film thickness. The amount of buildup and depletion increased with increasing heating temperature. It was found that the friction coefficient of each lubricant film initially decreased as the temperature increased. Thereafter, the minimum value of the friction coefficient of each test lubricant occurred at a specific temperature, resulting in an increase in the friction coefficient inversely as the temperature further increased. It was suggested that the specific temperature corresponds to the transition temperature at which the test lubricant film evaporates. In addition, it was also suggested that the thermal durability of the lubricant film improved because of ultraviolet treatment.

Molecular alignment is key to achieving an ultra-low friction coefficient. We prepared associated carboxylic acids using oleic acid and p-pentylbenzoic acid via hydrogen bonding, involving a benzene ring and an eight-membered cyclic carboxylic acid in the molecules. We investigated frictional properties when these were added into a base oil and found that the associated carboxylic acids decreased the traction coefficient in EHL and boundary lubrication, although the viscosity increased. An in situ micro Fourier transform infrared spectroscope showed that both the eight-membered cyclic carboxylic acid and the benzene ring were oriented parallel to the shearing force, leading to the low friction coefficient.

Viscoelastic (VE) materials are widely used in daily life. For the effective utilization of VE materials, it is necessary to know their viscoelasticity with accuracy over a wide range of frequencies, especially for high frequencies. Currently, the viscoelasticity of solid-state materials is directly measured only in the low-frequency range; for high frequencies, it is estimated based on the time-temperature superposition (TTS) principle. However, it is generally recognized that the TTS principle is suitable only for estimating the viscoelasticity of rheologically simple materials for temperatures within a limited range that is higher than the glass-transition temperature. In this paper, we propose a device that can directly measure the shear modulus and tan δ values of solid-state VE materials at high frequencies. We also propose a method for compensating for the shear deformation mode resonance of VE materials based on the mass of the moving part, to gain a more accurate understanding of viscoelasticity in the high-frequency range, and discuss the causes of errors in the compensation method. Finally, we report the VE properties of natural rubber (NR 65 IRHD) measured using the developed device and compensation method, and compare the measured results with those obtained using commercial VE measurement equipment.

The failure of moving mechanical components is often accompanied by hydrogen embrittlement caused by hydrogen diffusion in steel. One considerable mechanism for hydrogen diffusion is its ingress from lubricant decomposition. We investigated quantitatively the ingress of hydrogen generated by lubricant decomposition into bearing steels using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Deuterated lubricants were used to discriminate between the original hydrogen in steel and the hydrogen generated from lubricant decomposition. The highest concentration of deuterium was always near the raceway surface, and the concentration decreased gradually as the depth was increased. The diffused deuterium increased in a parabolic relation with the duration of friction. To protect steels against the ingress of hydrogen, we studied the effect of a diamond-like carbon (DLC) coating, which has a low diffusion coefficient of hydrogen. DLC reduced but did not completely prevent the ingress of hydrogen, which resulted from the reduction in the amount of hydrogen generated from the tribochemical and thermal decomposition of lubricants.

Lubricants that are used in miniaturized moving mechanical components generally serve under severe conditions, such as high temperature, high speed, and high load. Although alkyldiphenylethers (ADEs) are used as base oils for high-temperature greases, their tribological properties remain unclear. This study investigated the influence of the alkyl chains on the tribological properties of ADEs. Longer and more attached alkyl chains decreased the friction coefficient of ADEs under both reciprocating and continuous sliding conditions. Wear was found to be independent of the alkyl chain under reciprocating sliding conditions due to abrasion being caused by debris that was not readily removed. ADEs showed good anti-wear properties when used as either a lubricant or an additive under continuous sliding conditions. Much smoother surfaces in the friction track were observed in comparison to poly-&alpha, olefin. Fourier-Transform infrared spectroscopy analysis suggested that the ether groups were attracted to the worn steel surface, phenyl groups became consequently perpendicular, and the attached alkyl chains repelled other substances and prevented further wear of the surface. Moreover, ADEs showed a high adaptability with traditional additives.

A hybrid lubricant film was prepared from the perfluoropolyether (PFPE) lubricant (Moresco DDOH), with a single end group, and a PFPE film obtained by depositing the HT170 film via photoelectron-assisted chemical vapor deposition (PACVD). The frictional properties of the prepared film were evaluated using a pin-on-disk tester and compared with those of a Z-tetraol dip-coated film. This hybrid lubricant film showed a considerably high bonding ratio of ~ 100% after Vertrel rinsing and lower surface energy than those of Z-tetraol. The frictional coefficient of only the PACVD-treated HT170 film was considerably high. However, the hybrid lubricant film without the mobile fraction showed a friction coefficient equivalent to that of the Z-tetraol film having a mobile fraction of 40%.

The positioning accuracy of magnetic heads need to be improved to increase the recording capacity of hard disk drives. In our previous study, the experimental results confirmed that the head positioning error could be decreased by attaching linear protrusions on the leading edge of the carriage arms. However, the mechanism underlying the phenomenon has not been elucidated. In the current research, we evaluated the effect of leading-edge protrusions on the flow-induced carriage excitation force using the finite element (FE) method analysis and the Hilbert-Huang transform (HHT). We prepared two carriage-arm FE models, with and without linear protrusions, on the leading edges of the arm, and performed the FE analyses. Subsequently, we conducted a frequency analysis by applying the HHT to the simulated torque-disturbance time series of each FE model and the results were later compared. Our results show that the leading-edge protrusions were found to decrease the mean velocity between the arm and the disk, thereby decreasing the vorticity fluctuation in the arm hole, and to decrease the fluctuations in pressure on the outer sidewall of the carriage arm around the coil support.

The structural and molecular weight changes to lubricant picked up following laser heating in heat-assisted magnetic recording (HAMR) were analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The intensity of the ion mass fragments depended on the decomposition pathways and changed drastically, as did the average molecular weight of the picked-up lubricant as heating temperature increased. The fragment series formed by the dissociation of end groups with low molecular weights (CH3O+) showed a high intensity at temperatures under 300 °C, whereas at temperatures over 400 °C, the fragments series formed by the dissociation of the end-group component (C5H9O4F2+) increased. Overall, the results presented herein suggest that the chemical and thermal stability of the hydrocarbon terminal ends of lubricants should be improved in order to decrease depletion of the lubricant film and lubricant pickup by laser heating in HAMR.

In this study, we experimentally investigated the lubricant depletion caused by pulsed-laser irradiation heating and discussed the fundamental characteristics, comparing it with the results obtained by continuous laser heating. As a result, it was found that the decrease in the lubricant depletion due to pulsed-laser heating was smaller than that to continuous laser heating and depended on an increase in the duty ratio as it increased. In addition, the lubricant depletion depends on the full width at half maximum (FWHM) of the temperature distribution due to pulsed-laser irradiation heating and increased as the FWHM increased. Therefore, it is also suggested that the lubricant depletion issue will not be so critical in actual HAMR systems because the FWHM would be very small.