Magnetic Thin-film Rigid Disks Research Articles

Studies on degradation mechanisms of lubricants for magnetic thin-film rigid disks

The decomposition mechanisms of the perfluoropolyether (PFPE) lubricants have been studied through calculation of chemical reaction kinetics and several experimental approaches. Four degradation mechanisms were considered: thermal and catalytic decomposition, triboelectrical decomposition and mechanical scission. The results show that the effect of catalytic reaction on the degradation of the PFPE lubricants is negligible in sliding conditions because of the short contact time, small contact area and low interface temperature. Illumination with ultraviolet light is found to accelerate the decomposition of the lubricant, reducing the head-disk interface durability and causing more gaseous fragments because low-energy electrons created by the illumination interact with the lubricant molecules, activating and breaking up the molecules. A decomposition mechanism of the lubricant from the effect of low-energy electrons is proposed. Mechanical shear is also found to have an important effect on the decomposition of the lubricant. The partial pressures of the gaseous fragments of the lubricant increase rapidly with increase in the sliding velocity. The temperature from ambient up to 70 °C has no obvious effect on the decomposition of the PFPE lubricant, although higher temperatures are expected to be detrimental. Dominant degradation mechanisms in the sliding conditions are triboelectrical reaction and mechanical scission.

Lubricant film thickness mapping using a capacitance technique on magnetic thin-film rigid disks

We describe a novel capacitance measurement technique which is used to determine lubricant film thickness on magnetic thin-film rigid disks. Using this technique, variations in lubricant thickness as small as 0.1 nm can be measured quickly and nondestructively. Film thickness measurements made with an ellipsometer are found to be comparable to the capacitance measurements. We compare the uniformity in film thickness of disks lubricated with a dip-coating process with that of disks lubricated with a drain-coating process and find that the drain coated disks have better film uniformity than those that are dip coated. Also, wear tracks in fully bonded and partially bonded perfluoropolyether lubricants are profiled. We find that lubricant depletion occurs more rapidly in fully bonded lubricant than in partially bonded lubricant and that some recovery of the partially bonded lubricant occurs while none is observed in fully bonded lubricant.

Effect of lubricant thickness and viscosity and rest time on long-term stiction in magnetic thin-film rigid disks

This study makes a first attempt to address long term stiction as a function of lubricant film thickness and its viscosity, and if stiction ever approaches a constant value. Three types of lubricants are chosen in the tests. The equilibrium rest time, defined as the time for static friction force to reach its steady-state value, is reached after the head slider rests on the disk surface for a long enough time. A thicker lubricant film increases the static friction force but shortens the defined equilibrium rest time. Higher viscosity increases the static friction force and equilibrium rest time. Comparison between unlubricated and lubricated disks shows that stiction due to solid-solid asperity contact is negligible. The effect of lubricant viscous flow on the meniscus force at intermediate humidities is discussed.

High shear rate viscosity measurements of perfluoropolyether lubricants for magnetic thin-film rigid disks

Lubricant rheology data relevant to the very high shear rates encountered in disk drive operation do not exits. In this study, the viscosity of eight perfluoropolyether lubricants is measured at shear rates up to 107 s−1 at ambient pressure. All eight lubricants display Newtonian behavior at all shear rates. No shear thinning was observed and the lubricants were found capable of sustaining extremely high tensile stresses, on the order of 1 MPa.

Wear and degradation mechanisms of magnetic thin-film rigid disks with different lubricants using mass spectrometry

Wear and degradation mechanisms of perfluoropolyether (PFPE) lubricants are studied using mass spectrometry and friction measurements during sliding in a high vacuum environment. Three kinds of lubricants with nonpolar and polar ends and with and without thermal treatment are applied over a thin-film disk with diamondlike film (DLC) overcoat. The results show that fluorocarbon fragments are generated from lubricants during a period of sliding with a low and stable coefficient of friction, followed by a sharp rise in frictional force and generation of gaseous products of DLC overcoat material. Increase in the friction is associated with lubricant depletion leading to intimate contact of the slider with the DLC overcoat. It is concluded that decomposition of PFPEs begins from the onset of sliding, and the DLC overcoat starts to wear simultaneously from the beginning of the sliding for untreated lubricants, but was well protected in the case of chemically bonded lubricants. Fully bonded lubricants better protect the DLC followed by partially bonded lubricants. Two bonded lubricants tested, exhibited similar performance.

In Situ Studies of Wear Mechanisms in Magnetic Thin-Film Disks

Contact start-stop (CSS) tests and continuous drag tests are used for detailed in situ analysis of wear mechanisms in magnetic thin-film rigid disks. Coefficient of static friction and disk surface reflectance are measured during the contact start/stop tests and coefficient of kinetic friction, acoustic emission rms, acoustic emission count, and disk surface reflectance are all measured during the continuous drag tests. It is the first time that friction, acoustic emission and optical reflectance sensors are used simultaneously for wear studies. In situ measurements are used to determine precursors to failure of magnetic thin-film disks, to compare wear processes in CSS and drag tests, and to correlate CSS and drag tests. Disk wear is seen to follow a pattern of lubricant depletion and/or localized polishing of overcoat asperities. The sliding distance associated with CSS tests is less damaging to disks than continuous drag sliding distances. A combination of sensors used in this study, in particular an optical sensor used to measure changes in disk reflectance, has been demonstrated to be valuable in understanding the failure mechanisms. Presented at the 52nd Annual Meeting in Kansas City, Missouri May 18–22, 1997

Effect of bonded lubricant films on the tribological performance of magnetic thin-film rigid disks

Polar perfluoropolyether lubricants are widely used for topical lubrication of thin-film rigid disks. However, little is known about the role of bonded lubricant on tribological performance. In this study, as-applied lubricant films, after thermal treatment and after thermal treatment and washing to remove unbonded fraction, have been studied. The bonded fraction of lubricant films on coated disks made with AlMg and glass substrates was measured at various applied lubricant thicknesses, soaking times and thermal treatments, with silicon wafer as a reference. Friction and wear performance of lubricated disks against Al 2O 3TiC microsliders was measured. Results show that bonded lubricants thickness increases initially with increasing soaking time, baking temperature and baking time, and then levels off. Bonded lubricant thickness also increases initially with increasing applied lubricant thickness and levels off above a certain film thickness. A critical lubricant thickness exists above which coefficients of static and kinetic friction increase rapidly and durability decreases rapidly, and this thickness increases with disk roughness. A rise in static and kinetic friction above a critical value of film thickness is associated with meniscus forces. The drop in the disk durability above a critical level may result from the observed stick/slip phenomena and erratic friction at lubricant thicknesses which is thick as compared to disk surface roughness. It is further noted that lubricant film with mobile and immobile fractions is important for the disk to have a long life.

Tribological Behavior of Amorphous Carbon Nitride Overcoats for Magnetic Thin-Film Rigid Disks

Abstract Amorphous carbon nitride coatings of thickness of 5 and 30 nm were deposited onto 65 and 95 mm magnetic thin-film rigid disks surfaces using single-cathode and dual-cathode magnetron sputtering systems containing nitrogen-argon plasmas. Under optimum deposition conditions, amorphous carbon nitride coatings can be synthesized on ultrasmooth thin-film disks with no significant pinholes at thickness down to 5 nm, with hardness 22–28 GPa (compared to 7–12 GPa for amorphous carbon), and r.m.s. roughness as low as 0.25 nm. These amorphous carbon nitride coatings were shown to have better contact-start-stop performance and three-to-four times better pin-on-disk contact durability compared with amorphous carbon overcoats under identical testing conditions. Amorphous carbon nitride appears to be a promising candidate overcoat material for replacing amorphous carbon in the next-generation magnetic thin-film rigid disk systems.

Micromagnetic transverse correlation length in thin film magnetic recording media, medium noise, and tracking

We compare the cross correlation coefficient of signal and noise voltages read on magnetic thin film rigid disks by a magnetic recording head at accurately controlled different lateral positions, and deduce from the results a transverse correlation length λ which is a measure of the characteristic lateral dimension of the magnetic microfeatures of the medium. By analyzing correlation data from uniformly magnetized media and from media with saturated and unsaturated written transitions we can determine λ as a function of the medium's state of magnetization. Values of λ inferred from our measurements on typical media are less than 0.5μm, ranging down to several hundred angstroms in some instances. We have applied the short correlation length as the basis for detecting and controlling lateral head displacements.

Micromechanical properties of amorphous carbon coatings deposited by different deposition techniques

Amorphous carbon coatings about 20 nm thick are commonly used as an overcoat on magnetic thin-film rigid disks and tape and disk head surfaces to improve their wear performance. In this study, we deposited amorphous carbon coatings with thicknesses ranging from 20 to 400 nm on single-crystal silicon substrates by four deposition processes: cathodic arc, ion beam deposition, r.f.-plasma-enhanced chemical vapor deposition, and r.f. sputtering. R.f.-sputtered SiC coatings were also deposited for comparison. The hardness, elastic modulus, and scratch resistance of these coatings were measured by nanoindentation and microscratching using a nanoindenter. The cathodic arc carbon coatings followed by sputtered SiC coatings exhibited the highest hardness, elastic modulus, scratch resistance/adhesion, and residual compressive stresses. The critical load, a measure of the scratch resistance/adhesion of the coating, increases with thickness. The cathodic arc coatings of lower thicknesses (~ 30 nm) exhibited instant damage when the normal load exceeded the critical load, whereas thick coatings (greater than or equal to 100 nm) exhibited gradual damage through the formation of tensile cracks. The sputtered carbon coatings exhibited damage to the coating at very low loads and ploughing of the tip into the coating occurred right from the beginning of the scratch.

Friction and wear studies of magnetic thin-film rigid disks with glass-ceramic, glass and aluminum-magnesium substrates

Glass-ceramic and glass substrates are beginning to be used in the construction of magnetic'thin-film rigid disks for their rigidity, shock (dent) resistance, and smoothness over commonly used Ni-P coated aluminum-magnesium substrates. Coefficients of static friction (stiction) and kinetic friction in clean ambient (class 100) and durability in various environments were measured for disks constructed from two types of glass-ceramic substrates and for the substrates themselves and for glass disks. For comparison purposes, the same tests were also conducted for commercially available aluminum-magnesium (A1-Mg) disks. The results show that, although stiction differences can exist between different types of lubricated finished glass-ceramic disks, differences between lubricated substrates and corresponding unlubricated, finished disks are minor. The wear performance of lubricated glass-ceramic disks with low stiction is also clearly better than that of lubricated glass-ceramic disks with high stiction and is comparable to that of lubricated glass and A1-Mg disks. The wear performance for all carbon-coated disks tested is the best in nitrogen or argon environments, the worst in vacuum, and intermediate in humid or oxygen environments. For the glass disks coated with silicon dioxide, the presence of water vapor in the operating environment improved its wear performance. Adhesion forces due to meniscus formation at the head-disk interface, which cause high stiction, were calculated for all lubricated disks and substrates. The trends of the calculated values agree with that of the experimental results for coefficients of static friction.

Tribological performance of magnetic thin-film glass disks: its relation to surface roughness and lubricant structure and its thickness

Chemically strengthened glass substrates have recently been used in the construction of magnetic thin-film rigid disks for their rigidity, shock resistance, and smoothness over commonly used Ni-P coated aluminum substrates. The effects of disk roughness, lubricant film thickness and viscosity on coefficients of static and kinetic friction and on disk durability were investigated for disks constructed with glass substrates with three roughnesses, as well as for aluminum-substrate disks with one roughness for comparison purposes. Three nonpolar and one polar perfluoropolyether lubricants with wide range of viscosities were chosen in the experiment. Disks were lubricated using a dip-coating technique with 1 to 10 nm thickness. Coefficients of static and kinetic friction and durability of each of the disks were measured. It was found that above a critical lubricant-film thickness, coefficient of static (stiction) and kinetic friction increased rapidly with increasing lubricant-film thickness and decreasing lubricant viscosity. The critical film thickness was approximately proportional to the lubricant viscosity and disk roughness. The friction force increase was more rapid for a smoother disk. The disk durability increased with decreasing disk roughness and lubricant viscosity, and with increasing lubricant thickness. For glass-substrate disks, the polar lubricant provided best disk durability among the four lubricants tested. Failure mechanisms for glass-substrate and aluminium-substrate disks appear to be similar, however, aluminum-substrate disk exhibited better disk durability, as compared to glass-substrate disk. These experimental observations are attributed to the interplay of meniscus formation around contacting and near-contacting asperities, lubricant depletion and/or migration at contacting asperities, as well as ploughing of contacting asperities. For the first time, a kinetic meniscus model to account for time and viscosity dependence of meniscus formation is presented, which explains the experimental trends.

Tribological performance of thin film amorphous carbon overcoats for magnetic recording rigid disks in various environments

An ultrahigh vacuum tribotester equipped with Auger electron analyzer and mass spectrometer is used to study the friction and wear behavior of amorphous carbon coated magnetic thin film rigid disks. The tests include continuous sliding of Al 2O 3-TiC sliders against lubricated and unlubricated polished and textured disks. Operating environment is found to play an important role in the tribological performance of the carbon overcoat. Wear lives were longest in dry nitrogen and argon environments as compared with ambient environment (with oxygen and water vapor), in agreement with some of the previously reported results. The wear lives of the carbon coatings in vacuum were inferior to those in other environments. In comparison, surface roughness and lubrication play a less significant role in friction and wear for the disks tested. Based on this study, we conclude that poor wear life in vacuum results from intimate slider-to-disk contact. The long life in dry Ar and nitrogen as compared with Ar+O 2 and ambient environment, results from the absence of tribochemical oxidation prevalent in the oxidizing environment.

Nanoindentation hardness measurements using atomic force microscopy

An atomic force microscope (AFM), with a specially prepared diamond tip, has been modified to measure indentation hardness with an indentation depth as low as 1 nm. This indentation depth is much smaller than the depth of more than 20 nm that have been reported to date. The AFM indentation technique allows the hardness measurements of surface monolayers and ultrathin films in multilayered structures at very shallow depths and low loads. The nanoindentation hardness of single crystal silicon is measured using this technique. A subtraction technique is also described which allows the actual hardness measurements of rough surfaces such as magnetic thin film rigid disks.

Processing of diamond/alumina composites for low wear applications

A simple hot-pressing procedure for fabricating composites of diamond particulates in an alumina matrix at moderate applied pressures is described. Dense composites with up to 33 vol. % diamond particles are made by pressure-sintering at applied stress of 35 MPa in vacuum atmosphere. Preliminary wear tests of these composites on magnetic thin-film rigid disks show a low friction comparable to that of single crystalline diamond. Diamond/alumina composites can be an economical alternative to diamond or diamond coated materials for abrasion resistant applications.

Tribological studies of various magnetic heads and thin-film rigid disks

Tribological characterization of three different magnetic heads (Mn-Zn ferrite, CaTiO 3 and Al 2O 3-TiC) and two different magnetic storage thin-film rigid disks have been made. The abrasive wear characteristics of the two disks (only differing in protective overcoat; diamond-like carbon and yttria-stabilized zirconia, respectively) have been investigated using a precision dimple grinder in order to grind small and well defined wear scars on the disks. The scratch response of the magnetic heads and the rigid disks have been studied using a single tip scratch test equipment in situ in the scanning electron microscope. Finally, the friction characteristics of the six possible magnetic head/rigid storage disk couples have been tested using in situ sliding tests in a scanning electron microscope. The results from the abrasive test show that one of the disks (with a diamond-like carbon overcoat) only displays abrasive wear, while the other disk (with a zirconia overcoat) besides abrasive wear also displays extensive spalling of the magnetic coating. The scratch tests show that for all three magnetic heads the friction coefficient increases rapidly with increasing normal force until a constant value, independent of the normal force applied, is reached. For the two disks, the friction coefficient increases with increasing normal force within the entire normal force interval investigated (0–0.5 N). The in situ friction tests show that, for a given head material, sliding against the disk with the zirconia overcoat yields lower friction than sliding against the disk with the diamond-like carbon overcoat. It was also found that, for a given disk material, the Al 2O 3-TiC head yields the highest friction. All heads, with the possible exception of the Al 2O 3-TiC head, were found to be virtually unaffected by sliding against the disks. Further, it was found that scratching of the disks occasionally occurs for the following head/disk combinations: CaTiO 3/diamond-like carbon overcoat, Al 2O 3-TiC/diamond-like carbon overcoat and Al 2O 3-TiC/zirconia overcoat.

The Application of Scanning Tunneling Microscopy to Study Lubricant Distribution of Magnetic Thin Film Rigid Disk Surfaces

A special imaging mode of the scanning tunneling microscope (STM) based on charge trapping was used to probe the uniformity of lubricant distribution on magnetic thin film rigid disks at resolutions of better than 35 nm. Electron beam deposited STM tips were used to achieve this resolution. Coupons from untested rigid disks and those subjected to wear in dry helium and humid air were examined. We also examined coupons of an untested disk on which a scratch mark was made with a ferrite pin. It was found that the lubricant layer was nonuniform in the scratched region.

Role of Fractal Geometry in Roughness Characterization and Contact Mechanics of Surfaces

A proper characterization of the multiscale topography of rough surfaces is very crucial for understanding several tribological phenomena. Although the multiscale nature of rough surfaces warrants a scale-independent characterization, conventional techniques use scale-dependent statistical parameters such as the variances of height, slope and curvature which are shown to be functions of the surface magnification. Roughness measurements on surfaces of magnetic tape, smooth and textured magnetic thin film rigid disks, and machined stainless steel surfaces show that their spectra follow a power law behavior. Profiles of such surfaces are, therefore, statistically self-affine which implies that when repeatedly magnified, increasing details of roughness emerge and appear similar to the original profile. This paper uses fractal geometry to characterize the multiscale self-affine topography by scale-independent parameters such as the fractal dimension. These parameters are obtained from the spectra of surface profiles. It was observed that surface processing techniques which produce deterministic texture on the surface result in non-fractal structure whereas those producing random texture yield fractal surfaces. For the magnetic tape surface, statistical parameters such as the r.m.s. peak height and curvature and the mean slope, which are needed in elastic contact models, are found to be scale-dependent. The imperfect contact between two rough surfaces is composed of a large number of contact spots of different sizes. The fractal representation of surfaces shows that the size-distribution of the multiscale contact spots follows a power law and is characterized by the fractal dimension of the surface. The surface spectra and the spot size-distribution follow power laws over several decades of length scales. Therefore, the fractal approach has the potential to predict the behavior of a surface phenomenon at a particular length scale from the observations at other length scales.

Long-duration lubrication studies in simulated space vacuum: R. D. Brown, R. A. Burton and P. M. Ku, ASLE Trans., 7 (1964) 326; 9 figs., 17 tables, 13 refs.

An ultrahigh vacuum tribotester equipped with Auger electron analyzer and mass spectrometer is used to study the friction and wear behavior of amorphous carbon coated magnetic thin film rigid disks. The tests include continuous sliding of Al2O3-TiC sliders against lubricated and unlubricated polished and textured disks. Operating environment is found to play an important role in the tribological performance of the carbon overcoat. Wear lives were longest in dry nitrogen and argon environments as compared with ambient environment (with oxygen and water vapor), in agreement with some of the previously reported results. The wear lives of the carbon coatings in vacuum were inferior to those in other environments. In comparison, surface roughness and lubrication play a less significant role in friction and wear for the disks tested. Based on this study, we conclude that poor wear life in vacuum results from intimate slider-to-disk contact. The long life in dry Ar and nitrogen as compared with Ar+O2 and ambient environment, results from the absence of tribochemical oxidation prevalent in the oxidizing environment.