Patent Application
20050037932
1. Field of the Invention
Embodiments of the present invention generally relate to lubricants. More particularly, embodiments relate to lubricants used in thin-film magnetic storage media for improving the reliability and durability of the media.
2. Description of the Related Art
Information storage systems, such as disk drives found in personal computers and other data processing devices, employ a thin-film magnetic storage medium, such as a disc, which is moved relative to a read/write head to provide information introduction and/or retrieval from the magnetic storage medium. In accordance with conventional practices, the surface of the disc is lubricated with a thin film of lubricant to reduce friction and wear of the media due to head-disc contact during drive operation, and particularly during start and stop cycles that correspond to the turning on and off of a computer.
While the computer is turned off, the head typically rests on the surface of the disc. During the start-up of a disk drive, the disc starts to rotate and the read/write head begins to slide against the surface of the disc. As the disc continues to spin up, the distance between the head and the disc increases as the head takes off and begins to fly above the surface of the disc. Upon reaching a predetermined rotational speed, the head flies at a predetermined distance from the surface of the disc where the head is maintained during reading and writing operations. The very close proximity of the head to the disc can cause further wear of the media. Upon terminating operation of the disk drive, the disc spins down and the fly height of the head decreases until the head begins to slide against the surface of the disc and come to a stop. During drive operation, head-disc contacts may lead to excessive wear of the protective overcoat and catastrophic drive failure.
The thin-film magnetic storage medium typically comprises a thin rigid Al alloy substrate, successively sputtered layers, and a topical lubricant layer. The sputtered layers can include an underlayer, a magnetic layer, and a protective overcoat. The protective overcoat is typically a carbon overcoat which protects the magnetic layer from corrosion and oxidation and reduces frictional forces between the disc and a read/write head. A thin layer of lubricant, applied to the surface of the protective overcoat, is used to enhance the tribological performance of the head-disc interface by reducing friction and wear of the protective overcoat.
The reliability and tribological performance of a head-disc interface generally is monitored in terms of dynamic and/or static coefficients of friction or stiction values the head experiences during start-stop cycles. Dynamic stiction values can be measured using a standard drag test in which the drag produced by contact of a read/write head with a disc is determined at a constant spin rate, for example, 1 rotation per minute (rpm). Static stiction values can be measured using a standard contact start-stop (CSS) test in which the peak level of friction is measured as the disc starts rotating from zero to a selected revolution rate, for example, 5,400 rpm. After the peak friction is measured, the disc is brought to rest, and the start-stop process is repeated for a selected number of start-stop cycles. The tribological integrity and durability of a head-disc interface is often summarized by the number of CSS cycles before failure of the media as measured by unacceptably high friction values. An unacceptably high value of friction is an indicator of imminent head-disk failure or “head crash”.
Among the many lubricants available, liquid perfluoropolyethers (PFPEs) are the most utilized for forming topcoat lubricants on magnetic recording media. PFPEs are used because they possess certain desirable attributes for disc drive applications. For example, PFPEs are chemically inert, exhibit low vapor pressure, low surface tension, and high thermal stability. To achieve adhesion of the lubricant to the magnetic media requires the inclusion of polar and reactive functional groups, particularly on the end of the lubricating compound. Conventional lubricant systems typically comprise a mixture of a perfluoropolyether, such as “Fombline® Z-DOL” (Z-dol) available from Solvay Solexis, and a catalytic blocking agent, such as the hexaphenoxy compound “X—1P” available from Dow Chemicals Co. The PFPE Z-dol has polar hydroxyl end-groups that provide increased adhesion of the lubricant to the disc while the catalytic blocking agent prevents catalytic decomposition of the PFPE during head-disc contact.
As storage densities increase, the distance between a flying head and the magnetic layer of the thin-film medium decreases. Decreasing the distance between the flying head and magnetic layer involves increasing the smoothness of the media surface as well as reducing the thicknesses of the intermediary lubricant topcoat layer and protective overcoat layer. While conventional lubricants having polar or functional end-groups exhibit adequate adhesion under certain conditions, these lubricants compromise the stability and durability of the lubricant topcoat at ultra-thin thicknesses below 15 angstroms (Å).
As lubricant layer thicknesses are reduced to below 15 Å, physical bonding between an ultra-thin film of lubricant and the protective overcoat becomes less stable. Chemical bonding between the lubricant and the protective overcoat becomes increasingly important for maintaining a stable and uniform ultra-thin lubricant topcoat, particularly under increasingly demanding environmental conditions.
In view of the criticality of the lubricant topcoat in magnetic recording media, there continues to be a need to enhance the adhesion of ultra-thin lubricant films to the magnetic media while maintaining the desired tribological properties. In particular, there is a need for lubricants capable of achieving stronger bonding and preferably chemical bonding to the magnetic medium. There is also a need for lubricants for use as topcoats in the manufacture of magnetic recording media capable of achieving stronger bonding, and preferably chemical bonding, to the underlying protective overcoat and maintaining a uniform and stable film at ultra-thin film thicknesses.
The invention generally provides a lubricant composition comprising a perfluoropolyether chain having one or more epoxide rings, a method of manufacturing a magnetic storage medium, and a magnetic storage medium. In one embodiment, one or more epoxide rings may be included in an end group on the perfluoropolyether chain. In another embodiment the end-group comprising one or more epoxide rings may be located at both ends of the perfluoropolyether chain. In another embodiment, a method of manufacturing a magnetic storage medium comprising applying a lubricant topcoat to a magnetic storage medium wherein the lubricant topcoat comprises a perfluoropolyether having one or more end-groups comprising an epoxide ring. Still another embodiment comprises a magnetic storage medium including a substrate, a magnetic layer formed over the substrate, a protective overcoat formed over the magnetic layer, and a lubricant topcoat formed over the protective overcoat wherein the lubricant topcoat comprises a perfluoropolyether having one or more end-groups comprising an epoxide ring.
So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
One embodiment of the lubricant compound (Ep-PFPE) comprises a perfluoropolyether chain having an end-group comprising at least one epoxide ring, the general structure having the formula:
(1) A-PFPE
in which A represents the end-group comprising at least one epoxide ring and PFPE represents the perfluoropolyether chain. Another embodiment of the Ep-PFPE lubricant compound comprises a perfluoropolyether chain having an end-group comprising at least one epoxide ring at both ends of the perfluoropolyether chain, the general structure having the formula:
(2) A-PFPE-A
in which A represents the end-group comprising at least one epoxide ring and PFPE represents the perfluoropolyether chain.
In regards to formula (1) and formula (2), the epoxide ring has the following general structure:
Although many sources of the three-membered epoxide ring structure exist, a very common source is epichlorohydrin.
Additionally, in regards to formula (1) and formula (2) the PFPE chain may include, but is not limited to, the following structures:
A preferred embodiment of the Ep-PFPE lubricant compound has the general structure:
X—R—(CF2—CF2—O)p—(CF2—O)q—R—Y
in which X represents an epoxide ring; Y represents a hydroxyl, alkyl, fluoroalkyl, or perfluoroalkyl; R represents a fluoroalkyl, perfluoroalkyl, alkyl, fluoroalkoxy, perfluoroalkoxy, or alkoxy; and q and p may be the same or different and may be between 1 and 500. Another preferred embodiment of the Ep-PFPE lubricant compound has the general structure:
X—R—(CF2—CF2—O)p—(CF2—O)q—R—X
in which X represents an epoxide ring; R represents a fluoroalkyl, perfluoroalkyl, alkyl, fluoroalkoxy, perfluoroalkoxy, or alkoxy; and q and p may be the same or different and may be between 1 and 500. Other embodiments of the present lubricant include partially fluorinated PFPE chains and can be linear or branched.
One example of the synthesis process for preparing Ep-PFPE comprises the following general equation:
The process comprises dissolving fractionated Z-dol, available from Solvay Solexis, in a hydrofluorocarbon “Vertrel® XF” available from DuPont in a ratio of about 1 gram Z-dol to about 10 ml Vertrel® XF. The molecular weight of Z-dol may be a molecular weight in the range of about 2000-6000 Daltons. For achieving a high yield of Ep-PFPE, epichlorohydrin is added in excess in a ratio of about 3 to 4 grams epichlorohydrin to about 1 gram Z-dol. The Vertrel® XF diluted Z-dol and epichlorohydrin are combined in a flask containing 4-dimethylaminopyridine (DMAP) catalyst. For this reaction, the amount of DMAP catalyst is about 1% by weight of Z-dol. Triethylamine, a base, is dropwise added to the solution as an acid acceptor for the HCl that is generated during reaction. It is to be noted that there are many catalysts that could be used in lieu of DMAP and there are many bases that could be used in lieu of triethylamine. The mixture is constantly stirred with a mechanical stirrer and the flask is water-bath cooled to control the temperature. There is also a condensation/reflux line connected to the flask primarily for maintaining the volatile Vertrel® XF in solution. The reaction is allowed to go to completion overnight or for about 8-12 hours.
After reaction, the Ep-PFPE is extracted and purified. First, the amine in the solution is neutralized with an acid, such as HCl. The solution is washed with water to remove excess epichlorohydrin and residual triethylamine. The remaining solution of Ep-PFPE in excess Vertrel® XF, is distilled to remove the Vertrel® XF. The remaining Ep-PFPE is then purified using liquid chromatography followed by a supercritical fluid extraction purification step. A clear liquid product of Ep-PFPE was identified by a FTIR infrared spectrometer.
Many other synthesis processes for preparing Ep-PFPE are also possible. In addition, synthesis of Ep-PFPE is not limited to using Z-dol and epichlorohydrin as the reactants. Synthesis of Ep-PFPE may be accomplished by combining other functional PFPEs, having one or more functionalized end-groups, with an epoxy compound. Other analytical tools, including gel permeation chromatography (GPC) and thermogravimetric analysis (TGA), may be used for identifying the Ep-PFPE product.
The present Ep-PFPE lubricant compounds, whether of formula (1) or formula (2) preferably have an average molecular weight of between about 2,000 Daltons and about 6,000 Daltons. The Ep-PFPE is preferably fractionated. Fractionation can be achieved by distillation under vacuum, supercritical fluid fractionation, chromatography, e.g., GPC, or other molecular weight separation techniques.
The Ep-PFPE lubricant may be applied over magnetic media by any technique known in the art, such as dip, vapor, spray, solvent, solvent-free, vacuum, and non-vacuum processes. The Ep-PFPE lubricant may be applied as one layer or as multiple layers. To provide desirable lubricating properties, the Ep-PFPE lubricant is preferably applied to a thickness between about 8 Å and about 30 Å. Ep-PFPE lubricant layer thicknesses of less than 8 Å and greater than 30 Å may also be used. After application to the media, the Ep-PFPE lubricant is only weakly bonded to the carbon overcoat primarily due to the weak interaction between the epoxide ring and the carbon overcoat. A post-treatment of the lubricant topcoat is performed using ultraviolet (UV) light irradiation to increase the bonding of the Ep-PFPE lubricant molecules to the protective overcoat. Other energy sources may be used such as electron beam, ion beam radiation, infrared (IR), hydrogen/proton beam (H-beam), plasma, heat, or other treatments known in the art.
As illustrated in the following examples, the lubricant of the present invention provides improved tribological performance over conventional lubricants, such as Z-dol, at ultra-thin film thicknesses. The present lubricant compounds provide good lubrication properties, while maintaining a uniform and stable film at ultra-thin film thicknesses.
The following tests demonstrate the capabilities of the present invention and such examples are offered by way of illustration and not by way of limitation.
Contact start-stop (CSS) tests were performed on samples comprising a disc having a carbon overcoat with various fractionated lubricants formed thereover. A first sample comprised discs lubricated with a mixture of fractionated Z-dol and X1—P additive. A second sample comprised discs lubricated with fractionated Ep-PFPE having an epoxide end-group at both ends of the perfluoropolyether chain, and a third comparative sample comprised discs lubricated with fractionated Z Tetraol. Each of the lubricants had a molecular weight (Mw) in the range of about 2000 Daltons to 4000 Daltons and a polydispersity (Mw/Mn) of about 1 to 1.04. All the samples had lubricant topcoat thicknesses of about 10 Å and were UV irradiated for 40 seconds.
Contact start-stop tests were performed in a conventional spin stand using 20,000 start/stop cycles under stressful conditions of high relative humidity of 80% and temperature of 300 C. The CSS cycles also included occasional back rotation. The discs were spun up to a speed of 7,200 rpm.
Potentiostatic corrosion tests were also performed on samples comprising a substrate having a carbon overcoat with various lubricants formed thereover. A first sample comprised a disc lubricated with a mixture of fractionated Z-dol and X1—P. A second sample comprised a disc lubricated with fractionated Ep-PFPE having an epoxide end-group at both ends of the perfluoropolyether chain, and a third comparative sample comprised a disc lubricated with fractionated Z Tetraol. All the samples were UV irradiated for 40 seconds. Each sample was immersed into a sodium chloride solution and was biased at a 900 mV potential for 10 minutes. The total amount of charge (“total corrosion charge”) passing through each system over this 10 minute period of time was measured and calculated. The total corrosion charge of the Ep-PFPE sample was satisfactory and comparable to the Z Tetraol sample. Both the Ep-PFPE sample and Z Tetraol sample showed better potentiostatic corrosion resistance than the Z-dol/X1—P sample. Not wishing to be bound by theory, it is believed that the potentiostatic corrosion resistance data shows that Ep-PFPE and Z Tetraol are more resistant to decomposition as compared to the Z-dol/X1—P.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
