The present invention relates to magnetic discs for use in computer disc drives, and, more particularly, to application of the lubricant layer over the magnetic disc surface to enhance tribological performance of an air bearing slider.
Computer disc drives commonly use thin film media to store information in a high-density, high-reliability format. The magnetic layer in thin film media is typically provided by a cobalt alloy film. The cobalt alloy film may be applied at a thickness of around 500 Angstroms over a supporting substrate. The substrate may be nickel-phosphorous plated aluminum coated with a chromium underlayer for the magnetic media.
While thin film media offer important advantages toward higher storage density, the cobalt alloy films are significantly less durable than films composed of the magnetic and alumina particles used in particulate media. To enhance the durability of the disc, a protective layer of a very hard material is applied over the cobalt alloy film. A typical protective layer is an overcoat of sputtered amorphous carbon about 300 Angstroms thick. The amorphous carbon overcoat structure can be thought of as a hybrid between diamond and graphite. Other materials which have been used for overcoats include sputtered ceramic zirconium oxide and amorphous films of silicon dioxide. Overcoat surfaces have been textured to affect the tribology between the slider and the disc. A surface roughness of 50 to 200 Angstroms peak-to-valley is typical.
The overcoat surface is usually lubricated to further reduce wear of the disc due to contact with the magnetic head assembly. The lubricant is typically applied evenly over the disc in a molecularly thin film having a thickness from 10 to 50 Angstroms. Thicker films tend to be spun off by centrifugal forces. Slider-disc interaction, air shear and evaporation may also affect the amount of lubricant on a disc.
In computer disc drives which use air bearing sliders, magnetic discs have two discrete zones which may be defined based on how the slider of the magnetic head assembly travels over the disc surface. A "landing zone" is the zone where the slider containing the read/write transducer lands, rests while the disc drive is off, and takes off from when the disc drive is started up. A "data zone" is the zone where the slider flies over the disc and stores magnetic data. Lubricant is generally applied both to the landing zone and the data zone on the magnetic disc.
When the power is switched on in a disc drive, enough force has to be applied for the slider to overcome the static friction or "stiction" force on the slider. During the relatively low speeds encountered during takeoff and landing, the slider maintains almost constant contact with the disc. Lubricant over the landing zone is important for its contribution toward the stiction force and to minimize wear and drag force during takeoff and landing.
As the speed of the disc increases, the air in contact with the disc surface lifts the slider away from the disc surface such that the magnetic head assembly becomes airborne. During use of the disc drive, the magnetic head assembly is designed to fly over the disc surface without contacting the disc. However, the magnetic head assembly occasionally contacts the disc during use of the disc drive. These in-flight contacts between the slider and the media occur infrequently, but at high speeds. Most of these contacts are caused by collision of the slider with media asperities, third bodies such as corrosion products, or other contaminant particles. Lubricant on the data zone minimizes wear and/damage to the disc due to these occasional contacts between the flying magnetic head assembly and the disc.
Perfluoropolyethers (PFPEs) are currently the lubricant of choice for thin film recording media. PFPEs are long chain polymers composed of repeat units of small perfluorinated aliphatic oxides such as perfluoroethylene oxide or perfluoropropylene oxide. As a class of compounds, PFPEs provide excellent lubricity, a wide liquid-phase temperature range, low vapor pressure, small temperature dependency of viscosity, high thermal stability, and low chemical reactivity. PFPEs also exhibit low surface tension, resistance to oxidation at high temperature, low toxicity, and moderately high solubility for oxygen. Several different PFPE polymers are available commercially, such as Fomblin Z (random copolymer of CF.sub.2 CF.sub.2 O and CF.sub.2 O units) and Y (random copolymer of CF(CF.sub.3)CF.sub.2 O and CF.sub.2 O) including Z-DOL and AM 2001 from Montedison, Demnum (a homopolymer of CF.sub.2 CF.sub.2 CF.sub.2 O) from Daikin, and Krytox (homopolymer of CF(CF.sub.3)CF.sub.2 O). Fomblin Z and Y are prepared by photo-oxidation of tetrafluoroethylene and hexafluoropropylene, respectively, and are random copolymers of indicated units. Krytox and Demnum are synthesized via base catalyzed polymerization of perfluoropropylene oxide and trimethylene oxide, respectively. See U.S. Pat. Nos. 3,242,218 and 3,665,041.
The PFPE lubricant is normally applied by dipping the discs in a bath containing a few percent of lubricant in a solvent and gradually removing the discs out of the bath at a controlled rate. The solvent evaporates and leaves behind a layer of lubricant preferably from about 20 to 40 Angstroms thick.
It may be beneficial in some cases to bond some or all of the PFPE lubricant molecules to the surface of the disc. The bonded layer contains lubricant molecules which are chemically or physically bonded to the carbon overcoat on the disc. Bonded lubricant molecules cannot be removed by washing with a solvent. Bonding helps to reduce lubricant which may be lost due to spin-off, evaporation, or chemical displacement. It has also been theorized that bonding of the lubricant helps to lower stiction forces.
Different strategies have been employed for bonding the lubricant molecules to a disc surface to enhance lubricant performance. For instance, the molecules may be chemically bonded to the disc surface. The PFPE lubricant molecules may be terminated by reactive, functional end-groups, such as hydroxyl, carboxyl, or piperonyl. The end groups chemically react with the amorphous carbon overcoat to bond the lubricant to the disc surface. Similarly, polar end groups may be used to bond the lubricant to the disc surface. Lubricant bonding schemes may also involve thermal treatment or exposure to ultra-violet light.
A mobile layer of lubricant may be used on the disc surface, either with or without a bonded layer. The mobile layer contains molecules which are not bonded and can be easily washed away by an appropriate solvent.
With thin film magnetic media, decreasing head to media spacing is critical for higher storage densities. Present flying altitudes of magnetic head assemblies over the disc surface are usually in the 100-500 Angstrom range. Still lower flying altitudes are anticipated in the future. As flying altitudes are decreased, the tribology between the slider and disc surface becomes more and more important. While the tribology between a slider and a disc is a function of the properties of the substrate and all the deposited layers, the overcoat and the lubricant are of primary importance. The slider structure also greatly affects the tribology, and sliders are usually formed of fairly hard ceramics such as Mn-Zn ferrite, calcium titanate (CaTiO.sub.3) and Al.sub.2 O.sub.3 -TiC.
Stiction is one of the most important and complex tribological phenomenon for disc drives. High stiction forces can lead the disc drive to fail, as the motor may not be strong enough to overcome the initial stiction. The mount of stiction is a function of storage time as well as the normal force between the slider and the disc. The time dependency associated with the storage time is accredited to lubricant migration toward points of contact between the slider and the disc surface, resulting in meniscus forces which increase slowly over time. The time dependency of stiction may also be the result of increased elastic deformation of contacting asperities between the slider and disc surface, as well as the result of slow diffusion of ambient species, mainly water, into the lubricated junctions followed by displacement of lubricant from the meniscus.