The lubricity properties of disks can be measured in terms of their static coefficient of friction. Static coefficients of friction (stiction values) are typically measured using a standard contact-start-stop (CSS) test in which the peak level of friction is measured as the disk starts rotating from zero to a selected revolution rate. After peak friction has been measured, the disk is brought to rest, and the start-stop process is repeated for a selected number of CSS cycles. One important property of a disk that is required for good long-term disk and drive performance is that the disk retain a relatively low coefficient of friction after many CSS cycles or contacts with a read/write head.

The particular solvent chosen for the lubrication process can have a significant effect on the tribological performance of the lubricant. In Molecular Orientation of Polymer Lubricant Films : Its Tribological Consequence, Gao et al., Journal of Tribology, April 1998, Vol. 120, p. 369, three solvents were compared for their lubrication performance. The authors of this paper concluded that solvents can affect the molecular orientation of functional end groups of polymer lubricants, which in turn affects tribology performance. Lubricants with functional end groups are better for lubricant retention and wear resistance due to their better bondability to the surface. The perfluorohexane PF-5060DL was found to primarily orient a lubricant's functional end groups on thin films in the preferable orientation perpendicular to the disk surface (PF- 5060DL, although not identified in the Gao et al. 1998 paper, was the solvent used in the preferred process in that paper). Perfluorohexanes such as PF-5060DL, therefore, provide better lubrication (and tribology performance) by orienting the lubricant's functional groups perpendicular to the disk surface. However, hexaphenoxycyclotriphosphazenes such as X-1P are generally immiscible in PFPE solvents such as PF-5060DL.

U. S. Pat. No. 5,587,217 discusses coating a disk with a polyphenoxycyclotriphosphazene lubricant and a perfluoropolyether ("PFPE") oil mixture. In one example of a process for creating the luber solution, X-1P cyclophosphazene is dissolved in CFC-113, and the resulting solution is added to a fully diluted solution of Z-25 PFPE in PF-5060 perfluorohexane.

U. S. Pat. No. 5,908,817 discusses the use of phosphazenes as lubricants for thin-film magnetic recording disks. It notes that in combination with Z-DOL, X-1P reduces stiction and increases the stability of Z-DOL. However, because X-1P is virtually immiscible with PFPAE (or PFPE) lubricants, phase separation occurs at the optimal phosphazene: PFPAE ratios. This leads to nonuniformity of the lubricant on the disk (balling effect) which tends to affect the durability of the head/disk interface.

Although both Z-DOL/X-1P and Z-DOL/NF-100 provide significant reduction in frictional drag relative to Z-DOL alone, the NF-100 solution is more compatible than is X-1P with Z-DOL.

SUMMARY OF THE INVENTION In one aspect of the invention, a process is disclosed for making a mixture for lubrication of magnetic recording media, by mixing an amount of a hexaphenoxycyclotriphosphazene or a phosphazene derivative additive and an amount of a perfluorohexane or a fluorocarbon solvent in a container. The substances are either mechanically or thermally mixed in a manner to at least substantially dissolve the additive in the solvent to create a mixture. The amount of additive added is between about 0.3 grams and about 3 grams and the solvent is added in an amount which results in a mass- to-volume ratio (additive: solvent) of between about 1: 2 and about 7: 8 (grams : liter). In an alternative embodiment, between about 0.5 grams to 0.7 grams of the additive is combined with an amount of solvent which results in a mass-to-volume ration of between about 1: 2 and about 7: 8 (grams: liter). The container in which the additive and solvent are mixed can be made of glass, ceramics or metal.

The mechanical mixing is carried out at a temperature between about 30°C and about 100°C and for a period of between about 3 minutes and about 3 hours. In another embodiment, the additive and solvent are mechanically mixed at a temperature of about 60°C for about 60 minutes.

In another aspect of the invention, the mechanical mixing is carried out with an ultrasonic mechanical mixer, at a frequency between about 10 KHz to about 200 KHz, at a temperature between about 30°C to about 80°C, for a period of about 30

minutes to about 3 hours. In another embodiment, the ultrasonic mechanical mixing is carried out for 1 to 2 hours at a temperature between about 60°C and 80°C at a frequency between about 40 KHz to 80 KHz. The ultrasonic mechanical mixing may also be carried out at a frequency between about 10 KHz to 100 KHz for between about 10 minutes to 3 hours. In an alternative embodiment, the ultrasonic mechanical mixing may be carried out at between about 40°C and about 55°C for between about 30 minutes to about 1 hour.

In another embodiment, the mixing is carried out by an electronic stirrer, at a temperature between about 30°C and about 100°C, for a period of between about 10 minutes and about 3 hours. In an alternative embodiment, the electronic stirring is carried out at between about 40°C and about 55°C for between about 30 minutes to about 1 hour.

In still another embodiment, the mixing is carried out by a magnetic stirrer, at a temperature between about 30°C and about 100°C and for a period of between about 10 minutes and about 3 hours. In an alternative embodiment, the magnetic stirring is carried out at between about 40°C and about 55°C for between about 30 minutes to about 1 hour.

In another aspect of the invention, the mixing is carried out at thermally, at a temperature between about 40°C and about the boiling point of the perfluorohexane or fluorocarbon used for a period of between about 10 minutes and about 3 hours. In an alternative embodiment, the thermal mixing is carried out between about 30 minutes and about 2 hours.

The mixture is substantially dissolved when the additive concentration in the resulting mixture at a temperature between about 20°C to about 23°C is about 200 t40 ppm. Once substantially dissolved, the mixture may be transferred to another container for storage, which is stored at a temperature between about 20°C to about 23°C.

The mixture resulting from the mechanical or thermal mixing comprises a hexaphenoxycyclotriphosphazene or phosphazene derivative additive and perfluorohexane or a fluorocarbon solvent in a mass-to-volume ration of about 1: 2 to 7: 8 (grams additive: liters solvent).

In another aspect of the invention, a process is disclosed for replenishing a depleted hexaphenoxycyclotriphosphazene or phosphazene derivative additive in a luber bath during a manufacturing process of lubricating magnetic recording media, by adding a mixture of a hexaphenoxycyclotriphosphazene or a phosphazene derivative additive and a perfluorohexane or a fluorocarbon solvent to the luber. The additive and solvent are added together and mechanically mixed in a manner to at least substantially dissolve the additive in the solvent to create a mixture. The additive is substantially dissolved in the

solvent when the additive concentration of the mixture at a temperature between about 20°C to about 23°C is about 200 40 ppm. The substantially dissolved mixture is then added into an additive-depleted luber bath, such as an additive-depleted luber containing PFPE lubricants, in an amount effective to raise the additive concentration in the luber bath to between about 0.0001% and 0.01 % of the lubricant concentration in the luber bath.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the molecular structure of the PF-5060DL solvent (available from 3M, St. Paul, MN).

FIG. 2 shows the molecular structure of the X-1P additive (available from Dow Chemical, Midland, MI).

FIG. 3 shows the Contact-Start-Stop traces showing the tribology performance of a lubricant without the X-1P additive.

FIG. 4 shows the Contact-Start-Stop traces showing the tribology performance of a lubricant with the X-1P additive.

FIG. 5 shows the molecular structure of PFPE lubricant Z-DOL (available from Ausimond, Morristown, N. J.).

FIG. 6 shows the molecular structure of PFPE lubricant AM (from Ausimond, Morristown, N. J.).

DESCRIPTION OF THE SPECIFIC EMBODIMENTS I. DEFINITIONS The following terms used in this invention are defined as follows: "Additive-depleted"means that the concentration of the additive in the luber bath has been reduced below the initial additive concentration of about 0.0001% and 0.01 % of the lubricant concentration in the luber bath. If the additive in the luber bath is depleted during the manufacturing process, the additive should be replenished.

"AM"means the PFPE lubricant whose molecular structure is depicted in FIG. 6.

"Luber bath"is defined as solution used in the lubrication of magnetic recording media containing a lubricant, solvent and additive.

"Mechanical mixing"is defined as mixing conducted with the use of a mechanical instrument, as opposed to mixing accomplished by hand. Such mechanical instruments include ultrasonic, magnetic, or electronic stirrers.

"PF-5060DL"is defined as the perfluorohexane molecule shown in FIG. 1.

"Replenished"means that the concentration of the additive is returned to between about 0.0001% and 0.01 % of the lubricant concentration in the luber bath.

"Substantially dissolved,"when referring to mixing the additive and solvent, means that the additive concentration of the mixture at about 20°C to 23°C is about 200 40 ppm.

"Tribology"is defined to relate to interacting surfaces in relative motion, friction, wear and lubrication.

'IJltrasonic"means an instrument which supplies mechanical vibrations, such as the ultrasonic instrument manufactured by Branson.

"X-1P"is defined as the hexaphenoxycyclotriphosphazene molecule shown in FIG. 2.

"Z-DOL"is defined to mean the PFPE lubricant whose molecular structure is depicted in FIG. 5.

II. DETAILED DESCRIPTION OF INVENTION A. OVERVIEW The present invention includes the recognition that, given the tribological benefits of hexaphenoxycyclotriphosphazene molecules such as X-1P, there is a need for a process to combine hexaphenoxycyclotriphosphazenes or phosphazene derivatives with lubricants for use in lubricating thin-film magnetic recording disks in such a manner as to avoid phase separation. Hexaphenoxycyclotriphosphazenes or phosphazene derivatives are not generally soluble in perfluorohexanes such as PF-5060DL. The present invention provides a process whereby hexaphenoxycyclotriphosphazene or phosphazene derivatives can be dissolved in perfluorohexane or fluorocarbon for the initial preparation of, or for replenishing, a luber bath.

This permits the benefits of such additives as X-1P to be combined with tribologically favorable solvents such as PF-5060DL, for use in lubrication of magnetic recording media. The combination of the additive/solvent solution with a PFPE lubricant when applied to a disk reduces frictional drag on the disk, increasing its life in terms of CSS cycles.

B. LUBRICATION PROCESS In order to meet the stringent tribology requirements for low flying magnetic head-disk interfaces, adding a certain amount of additive to a PFPE lubricant is

beneficial to the lubrication process. In the lubrication process, a liquid polymer lubricant film mixed with an additive is generally applied to the disk surface using such techniques as the dip-coating technique (discussed in U. S. Pat. No. 5,587,217). In the dip-coating technique, the magnetic disks are vertically submerged into a solution tank containing a desired lubricant/additive mixture in a selected solvent. Typical lubricant concentration in the solution is 0.01% to 0.1% and typical additive concentration in the solution is 0.0001% to 0.01%. The disks are then either vertically pulled out of the solution at a steady speed or the solution level is drained at a constant speed. A small amount of solution remains on the disk surface due to viscous force. After the volatile solvent evaporates, a thin mixed layer of lubricant/additive film coats the disk surface.

When the X-IP/lubricant mixture is used in the manufacturing process, the X-1P is depleted significantly faster than the lubricant. There is a very small window for such replenishment because the ratio of X-1P to lubricant must be kept below 10% to reduce the risk of droplet formation. To add an X-lP/lubricant mixture, without increasing the lubricant concentration beyond the control limit, would require removing part of the luber solution and diluting the lubricant concentration, which unfortunately further depletes the X-1P. It is preferable to prepare an X-1P mixture without any lubricant so that the amount of X-1 P added to replenish the luber bath, relative to the existing lubricant in the luber bath, may be controlled. Mixing the additive and solvent together permits introduction of the mixture into an additive-depleted luber bath, to raise the concentration of the additive without simultaneously raising the concentration of the lubricant.

C. THE PROCESS OF THE INVENTION According to the invention, hexaphenoxycyclotriphosphazenes or phosphazene derivatives can be dissolved in a perfluorohexane or fluorocarbon solvent for lubrication of magnetic recording media.

In one aspect, a hexaphenoxycyclotriphosphazene or a phosphazene derivative additive is added to a container made of glass, ceramics or metal in an amount between about 0.3 grams to about 3 grams. In another embodiment, the hexaphenoxycyclotriphosphazene or a phosphazene derivative additive is added in an amount between about 0.5 grams to about 0.7 grams. A perfluorohexane or fluorocarbon solvent is also added to the container in an amount which results in a ratio of between about 1: 2 to 7: 8 (grams hexaphenoxycyclotriphosphazene: liter perfluorohexane). In another aspect of the invention, the mechanical mixing is carried out at a temperature

between about 30°C and about 100°C, for a period of between about 3 minutes and about 3 hours. In another embodiment, the additive and solvent are mechanically mixed at a temperature of about 60°C for about 60 minutes.

In one aspect of the invention, these substances are mechanically mixed using an ultrasonic at a frequency of between about 10 KHz to 200 KHz for between about 3 minutes and about 3 hours at a temperature between about 30°C and about 80°C.

In another embodiment, the ultrasonic mechanical mixing is carried out for between about 1 hour and 2 hours at a temperature between 60°C and 80°C at a frequency between 40 KHz to 80 KHz. In an alternative embodiment, the ultrasonic mixing is carried out at between about 40°C to about 55°C for between about 0.5 hours and 1 hour.

In another embodiment, the mechanical stirring is carried out by an electronic or magnetic stirrer, at a temperature between about 30°C and about 100°C for a period of between about 10 minutes and about 3 hours. The electronic or magnetic mixing may also be carried out for between about 0.5 hours and about 1 hour at between about 40°C and about 55°C.

In another aspect of the invention, the additive and solvent are thermally mixed at a temperature between about 40°C and about the boiling point of the solvent used, for between about 10 minutes and about 3 hours. In an alternative embodiment, the thermal mixing is carried out for between about. 05 hours and about 2 hours.

Once mixed, whether mechanically or thermally, the mixture can be transferred to another container for storage at a temperature between about 20°C and about 23°C.

In another embodiment of the invention, the mechanically or thermally mixed solution may be added to an additive-depleted luber bath to replenish the additive, without at the same time replenishing the lubricant. When the additive concentration of the mixture is about 20040 ppm at a temperature between about 20°C and about 23°C, the mixture can be added to an additive-depleted luber bath containing PFPE lubricants.

The substantially dissolved mixture is then added into the additive-depleted luber bath in an amount effective to raise the additive concentration in the luber bath to between about 0.0001% and 0.01% of the lubricant concentration in the luber bath.

D. EXAMPLE 1 Studies conducted in support of the present invention have shown that the use of X-1P with a lubricant increases the number of CSS cycles through which a disk can last. The disks tested had a laser-bump height of about 20 nm and a nitrogenated

carbon of 5 nm with about 22% of nitrogen content. The nominal load on the magnetic head was 4 grams and 7200 RPM was used. The tests were conducted in an ambient environment. One disk was coated with a lubricant, but no X-1P additive. The other disk was coated with a lubricant containing between about 5% to 10% of X-1P additive on the disk. As can be seen in FIG. 3, the head-disk interfaces on a disk lubricated with a lubricant but no X-1P additive could not last more than 3000 CSS cycles. With X-1P additive added to the same lubricant in an amount to result in between about 5% to 10% of the lubricant on the disk, the head-disk interfaces could last more than 50,000 CSS cycles. See FIG. 4.

E. EXAMPLE 2 X-1P and PF-5060DL are added to a glass bottle in amounts between about 0.5 grams to 0.7 grams and about 800 ml to 1000 ml, respectively. These substances are mechanically mixed using an ultrasonic at a frequency between about 40 KHz to 80 KHz for between about one to two hours. The temperature of the ultrasonic is optimally between about 60°C to 80°C.

After mixing the X-1P and PF-5060DL as described above, the temperature of the mixture is then brought to between about 20°C to about 23°C. The X- 1P concentration in the mixture is then measured by a UV-Vis spectrometer, such as Hitachi UV-Vis 3100. Optimally, the reading should be about 200 40 ppm. If the X-1P concentration is not within this range, the mixing process is repeated, adding more X-1P or PF-5060DL in an amount necessary to bring the mixture within this range, according to industry practice.

Once the mixture falls within the range of about 200 40 ppm, it can be stored or used to prepare or replenish a luber bath. The X-1P/PF-5060DL mixture is preferably used to replenish luber baths using PFPE lubricants, such as Z-DOL or AM All references cited herein are incorporated herein by reference in their entirety and for all purposes to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.

Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. The specific embodiments described herein are for illustration only and are not intended to limit the invention in any way.