Spreading Characteristics of Molecularly Thin Lubricant on Surfaces With Groove-Shaped Textures: Effects of Molecular Weight and End-Group Functionality
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Abstract:
Effects of molecular weight and end-group functionality on spreading of molecularly thin perfluoropolyether (PFPE) film over solid surfaces with groove-shaped textures have been studied by experiments and Monte Carlo simulations. In the experiments, lubricant spreading on a surface with groove-shaped textures was measured by making use of the phenomenon in which diffracted light weakens in the lubricant-covered region. It is found that grooves serve to accelerate spreading and this effect increases for deeper grooves, and also the accelerating rate becomes larger for a lubricant having a larger molecular weight or functional end-groups. In the simulations, the Monte Carlo method based on the Ising model was extended to enable us to evaluate the effect of molecular weight on the spreading of non-functional lubricant inside a groove. The validity of the newly developed simulation method was well confirmed from the agreement between the simulation and experimental results.Keywords:
Groove (engineering)
The physics behind lubricant transfer from disk to slider and lubricant accumulation on the slider in hard disk drives is explained. The effect of slider air bearing pressure on the lubricant transfer is discussed. It is found that the lubricant transfer is not affected by slider air bearing pressure. Lubricant molecular weight plays a dominant role in the lubricant transfer and lubricant accumulation. The amount of lubricant transfer and accumulation decreases dramatically with the increase in lubricant molecular weight. A thinner lubricant and higher bonding ratio of lubricant on disk surface reduce the lubricant transfer and accumulation obviously.
Air bearing
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Transparent-pin wear tests with in-situ microellipsometer measurement were performed on thin-film magnetic disks with different lubricants. We found that pin wear started when the decrease in the lubricant thickness at the pin slide-path leveled-off and that pin wear sometimes started when the lubricant thickness was still larger than that of the adsorbed lubricant. These mean that the free lubricant on the disk surface is not replenishing the real points of pin-disk sliding. Consequently, we considered that the replenishment of free lubricant to the real points of pin sliding is more important than the fact whether the average lubricant thickness exceeds the adsorbed lubricant thickness or not. Tests on disks with different lubricants show that the lower the lubricant viscosity, the greater the lubricant loss and the smaller the pin wear. We find that a low-viscosity-lubricant is better in replenishing free lubricant to the real points of sliding than high-viscosity-lubricant is.
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In Hard Disk Drives, lubricants are very important materials to reduce head and disk wear. Therefore, it is necessary to know the lubricant depletion under flying heads. Lubricant depletion due to flying heads has been studied experimentally. We developed simulation program to calculate numerically the change in lubricant thickness under a flying head on a thin-film magnetic disk from 10nm thick lubricant film. In recent HDDs, the lubricants thickness has become molecularly thin and polar lubricants have been used. In this paper, we took account of thickness-dependent lubricants diffusion and viscosity in our simulations to calculate a 1.2 nm thick polar lubricant film used in recent HDDs. The simulated results considering the thickness-dependent diffusion and viscosity showed that depletion was small in molecularly thin lubricant films. We considered it necessary to include thickness-dependent diffusion and viscosity in lubricant depletion simulation.
Viscosity index
Flying height
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The technology of heat-assisted magnetic recording (HAMR) has improved the storage density of hard disc drives. The PFPE molecules of lubricant layer adhered on the disc can transfer from the lubricant layer and form the lubricant bridge which can deteriorate the stability of read/write process. In this paper, the formation and breaking of lubricant bridge at the head–disc interface (HDI) affects HAMR stability and deserves to be investigated. Using molecular dynamic simulation, a full-atom model was built to evaluate the behaviour of the lubricant bridge. Moreover, the effects of lubricant temperature, heating-up time, disc rotation speed and bearing pressure on the HDI were studied. It has been found that the amount of transferring atoms sharply increased when the lubricant temperature was above 700 K. The loss rate of lubricant layer decreased gradually during the heating process and it took about 2.2 ns for the remaining lubricant to reach stability. Furthermore, transferring PFPE molecules can form the lubricant bridge. A shorter heating-up time makes the lubricant bridge thicker and more robust. And the duration of lubricant bridge is notably affected by heating-up time, rotation speed and bearing pressure. A shorter heating-up time leads to a longer duration of lubricant bridge, whereas a higher rotation speed and bearing pressure reduces the duration of lubricant bridge.
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Flying height
Dry lubricant
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The time to lubricant film breakthrough in an oil-lubricated spherical roller bearing has been measured using SKF Lubcheck for various settings of bearing load, outer ring temperature, shaft speed and oil type and viscosity. The bearings were lubricated once and then run to film breakthrough with the aim of estimating the lubricant losses under different conditions. The result of the experiments supports the theory of lubricant replenishment, and it is shown that high speed combined with high lubricant viscosity will shorten the time to film breakthrough.
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Viscosity index
Air bearing
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Magnetic head wear performance was studied usingvarious kinds of D-4OH lubricant thickness of hard disk(HD) media. Different characteristic behavior wasobserved in each lubricant thickness. The difference ofthe behavior was well related with media frictioncoefficiency. At the same time, surface free energychanged as well as it. According to lubricant coverageanalysis, it was found that the head wear behaviordepended on the lubricant coverage and head mediainteraction. Furthermore, molecular dynamics methodwas applied to observe lubricant molecular distributionand the coverage changed as a function of lubricantthickness.
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In this study, we investigated the effects of ultra-thin liquid lubricant films on light contact recording by using dynamic flying height (DFH) control sliders. In other words, lubricant pickups, lubricant wear and slider wear on different PFPE lubricant (Ztetraol2000, D-4OH, and QA-40) films were investigated and compared by using a SSA tester developed to observe the lubricant thickness mapping on the slider surfaces after the heads slide on the disk surfaces under the conditions of light contact recording. These lubricants have the same four OH functional end-groups but different backbones. As a result, the effects of ultra-thin liquid lubricant films on light contact recording by using dynamic flying height (DFH) control sliders were elucidated and it was suggested that they were affected by the conformation of the lubricant molecules. In addition, it was found that the experimental results have comparatively good correlation with the fundamental AFM evaluation results for each different lubricant film.
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Profiles of lubricant thickness on sliding traces are monitored by an ellipsometer during sliding tests. Various thickness profiles for lubricants are observed for two selected lubricant categories, perfluoropolyether as a liquid lubricant and tetracontane as a solid lubricant. The liquid lubricant shows either a swollen portion or a sunken portion in the profile. However, no swollen portion is observed for the solid lubricant, due to its removal from a sliding track as wear debris. Recovery of the lubricant to average thickness is observed for liquid lubricant after an elapsed time. However, solid lubricant does not show thickness recovery.
Dry lubricant
Liquid paraffin
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Lubricant on thin-film magnetic disks is divided into two parts. One is bonded lubricant that is fixed on disk surface and remains on disk surface after solvent rinsing. The other is mobile lubricant that can move on disk surface and is removed by solvent rinsing. It has been believed that mobile lubricant is imperative to design reliable hard disk drives. However, mobile lubricant would also be a cause of high friction between heads and disks. We started our studies on disks without mobile lubricant to find the possibility of disks with only bonded lubricant. In this paper, tribological characteristics of disks with and without mobile lubricant at the same lubricant thickness were compared using our transparent pin-on-disk wear tests. The results showed that pin wear was smaller on disks without mobile lubricant than those on disks with mobile lubricant in low load or slow speed conditions. The reason of this was considered to be the contact force increase by meniscus force due to lubricant bridge.
Dry lubricant
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