FLUID DYNAMIC BEARING, SPINDLE MOTOR, AND DISK DRIVE DEVICE

Description

TECHNICAL FIELD

The present invention relates to a fluid dynamic bearing filled with a lubricating oil composition containing an ionic liquid with Read-Write Error hardly generated, and to a spindle motor including the fluid dynamic bearing. Moreover, the present invention relates to a disk drive device including the spindle motor.

BACKGROUND ART

Various lubricants such as grease and oil are used for pivot assemblies to be used in fulcrum parts of actuators of hard disk drives (HDDs) and bearings incorporated in spindle motors for smooth operation of these components and smooth driving of these devices.

For example, a rolling bearing to be incorporated in an actuator of a hard disk drive has been proposed, the rolling bearing filled with grease formed by compounding, as a thickener, a diurea compound having at least one of an alicyclic hydrocarbon group or an aliphatic hydrocarbon group in the skeleton into a base oil containing an aromatic ester oil (Patent Document 1).

An ionic liquid is a salt in a liquid state including only ions (anions, cations). Since ionic liquids have characteristics such as low vapor pressure (nonvolatile), high thermal stability, flame retardancy, low viscosity, and high ionic conductivity and can design various physical properties by a combination of a cation and an anion, ionic liquids are expected to be applied to various technical fields including electrolytic solutions and solvents.

From the characteristics including the low vapor pressure, the high thermal stability, and the low viscosity described above, application to the above-described lubricant has also been studied, and for example, there is a proposal of a lubricant composition with an ionic liquid added for the purpose of maintaining low friction property for a long time under a high load condition (Patent Document 2).

CITATION LIST

Patent Documents

Patent Document 1: JP 2006-236410 A

Patent Document 2: JP 2019-065256 A

SUMMARY OF INVENTION

Technical Problem

One of the causes of the occurrence of read-write errors in HDDs is volatilization and evaporation of a lubricant component filled in the bearing incorporated in the actuator or spindle motor. When the volatilized or evaporated lubricant component is cooled and condensed on the surface of a magnetic disk or a magnetic head and undergoes phase transition to a liquid or a solid, making the magnetic disk and the magnetic head attract to each other disabling normal reading and writing, these are considered to be a cause of the read-write error. Although it is conceivable to suppress the volatilization/evaporation amount by selecting, for example, a component such as a low-volatile base oil against the volatilization and evaporation of the lubricant component due to the temperature rise during the HDD driving as described above, it is difficult to completely eliminate the volatilization and evaporation of the component.

So far, in a known lubricant added with an ionic liquid, including the lubricant composition described in Patent Document 2, no proposal has been made in consideration of evaporation of a component contained in the lubricant, for example, a component such as a base oil.

An object of the present invention is to provide a fluid dynamic bearing filled with a lubricating oil composition containing a base oil and a specific ionic liquid, and to provide a spindle motor with evaporation being suppressed in the lubricating oil composition provided in the fluid dynamic bearing by incorporating the fluid dynamic bearing in the spindle motor, even if components of the lubricating oil composition evaporate or volatilize, adhesion of the evaporation/volatile component to a magnetic disk and the like is suppressed, and thus occurrence of a read-write error in an HDD can be suppressed, and a disk drive device including the spindle motor.

Solution to Problem

One aspect of the present invention relates to a fluid dynamic bearing filled with a lubricating oil composition containing a base oil and an ionic liquid,

• • the ionic liquid being an ionic liquid including:
  • at least one cation selected from the group consisting of tetraalkylammonium cations represented by Formula (B), and
  • at least one anion selected from the group consisting of a borate anion represented by Formula (C-1), a borate anion represented by Formula (C-2), and a borate anion represented by Formula (C-3):

where in Formula (B), R⁵, R⁶, R⁷, and R⁸ each independently represent a linear or branched alkyl group having from 1 to 18 carbon atoms,

where in Formula (C-1), R⁹, R¹⁰, R¹¹, and R¹² each independently represent a linear or branched alkyl group having from 1 to 22 carbon atoms.

The present invention also relates to a spindle motor including the fluid dynamic bearing.

Moreover, the present invention relates to a disk drive device equipped with the spindle motor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example of a main component structure of a spindle motor of the present invention.

FIG. 2 is a schematic view illustrating an example of a structure of a drive device (disk drive device) of the present invention.

DESCRIPTION OF EMBODIMENTS

As described above, when a component contained in a volatilization/evaporation component of a lubricant composition adheres to a recording disk and the like, the adhesion may lead to a read-write error, and thus it is desirable to suppress volatilization/evaporation of the lubricant component as much as possible.

In particular, with an increase in capacity and density of recording information of an HDD and an increase in processing speed in recent years, a fly height (distance between a magnetic head and a magnetic disk) of a disk drive device has narrowed to about several nm, and there is an increasing concern about defects that may be caused by evaporation of the lubricant component and adhesion associated with the evaporation. Since the fly height is reduced, a space between the magnetic head and the disk can be brought into a negative pressure state, and in this case, the surrounding gas is compressed/condensed toward the space between the magnetic head and the disk, so that even a small amount of evaporative/volatile components may be liquefied, leading to adhesion to the disk and the like. In recent years, with an increase in the recording capacity per HDD, the number of disks in the device has increased, and disk drive devices having 9 or more 3.5-inch-diameter disks have been put on the market. In such a device, a spatial volume within the device is further reduced. In such an environment having a small spatial volume and a fly height on the order of several nanometers, even a very small amount of contamination may lead to a read-write error.

A disk drive device including an internal space filled with a gas (for example, helium or the like) having a lower density than air has also started to spread. In such a disk drive device, the air pressure inside the device may be less than one atmosphere. In this case, it is more difficult to suppress evaporation/volatilization of the lubricant component. In particular, in the case of an HDD employing a heat-assisted magnetic recording (HAMR) system, a next-generation recording technology, the temperature of a head part of an actuator can locally reach a temperature as high as 400° C. As a result, since an internal temperature of the HDD rises, evaporation and volatilization of the lubricant component are more likely to occur than before, and there is an increasing possibility of causing a defect related to disk reading and writing.

A lubricating oil composition applied to the fluid dynamic bearing according to the present invention is characterized in that a specific ionic liquid is blended as described later. The blending of the lubricating oil composition can be expected to suppress an evaporation amount of the composition even in application in a higher temperature environment as in a disk drive device employing a thermally assisted magnetic recording system, can be expected to suppress the adhesion of the components to a magnetic disk and the like even when the components are evaporated or volatilized, and can contribute to the suppression of the occurrence of a read-write error of the HDD due to the component, such as evaporation.

Hereinafter, this will be described with details.

Fluid Dynamic Bearing

First, a preferred embodiment of the fluid dynamic bearing according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic view for illustrating a fluid dynamic bearing and a spindle motor including the fluid dynamic bearing according to an embodiment of the present invention. Note that the embodiments described below are exemplary embodiments of the present invention, and the present invention is not limited to the embodiments.

As illustrated in FIG. 1, a spindle motor 1 is used as a motor for driving a data storage device including a magnetic disk, an optical disk, or the like used for a computer. As a whole, the spindle motor 1 includes a stator assembly 2 and a rotor assembly 3. Although the spindle motor 1 in FIG. 1 is a shaft rotating motor, the present invention is also applicable to a shaft fixed motor.

The stator assembly 2 is fixed to a cylindrical part 5 provided to a housing 4 (base plate) constituting a casing of the data storage device in such a manner that the cylindrical part 5 protrudes upward. A stator core 8 wound around with a stator coil 9 is fitted and attached to an outer circumferential part of the cylindrical part 5.

The rotor assembly 3 includes a rotor hub 10, and the rotor hub 10 is fixed to an upper end part of a shaft part 11 and rotates together with the shaft part 11. The shaft part 11 is inserted into a sleeve 7 being a bearing member and is rotatably supported by the sleeve 7. The sleeve 7 is fitted and fixed inside the cylindrical part 5. A lower cylindrical part 10a of the rotor hub 10 rotates inside the housing 4, but a back yoke 13 is mounted on an inner circumferential surface of the lower cylindrical part 10a, and a rotor magnet 14 is further fitted and fixed inside the back yoke 13 and is magnetized to a plurality of poles of N and S poles.

When the stator coil 9 is energized, a magnetic field is formed by the stator core 8, and this magnetic field acts on the rotor magnet 14 disposed in the magnetic field to rotate the rotor assembly 3. On an outer circumferential surface of an intermediate cylindrical part 15 of the rotor hub 10 of the rotor assembly 3, a recording disk, such as a magnetic disk (not illustrated), constituting a storage unit of the data storage device, is mounted, and is configured to be rotated or stopped by the operation of the spindle motor 1, so that information writing and data processing are performed by a recording head (not illustrated).

In the spindle motor 1 of such an embodiment, a fluid dynamic bearing 6 is provided at a part where the sleeve 7 rotatably supports the shaft part 11.

A large-diameter first recess part 16 opening downward is provided at a lower end part of the sleeve 7, and a small-diameter second recess part 17 is further formed at a top surface of the first recess part 16. A counter plate (thrust receiving plate) 18 is fitted into the large-diameter first recess part 16 and fixed to the first recess part 16 by, for example, welding, bonding, or the other means, so that the inside of the sleeve 7 is in an airtight state.

A thrust washer 19 is fitted, press-fitted and fixed to a lower end part of the shaft part 11, and the thrust washer 19 is disposed in the second recess part 17 of the sleeve 7 to rotate together with the shaft part 11 while opposing the counter plate 18 and a top surface of the second recess part 17.

A gap between the sleeve 7 and the shaft part 11, a gap between the thrust washer 19 and the second recess part 17, and a gap between the thrust washer 19 and the shaft part 11 and the counter plate 18 communicate with one another, and a lubricating oil composition 12 described later is filled in the communication gaps. The lubricating oil composition 12 is injected from between the sleeve 7 and the shaft part 11.

A first radial dynamic pressure groove 20 and a second radial dynamic pressure groove 21 for generating dynamic pressure are formed at an inner circumferential surface of the sleeve 7 opposing the shaft part 11 to be spaced apart from each other in an axial direction. Due to the rotation of the shaft part 11, the radial dynamic pressure grooves 20 and 21 generate dynamic pressure causing the shaft part 11 and the sleeve 7 to be in a non-contact state in a radial direction. A first thrust dynamic pressure groove 22 and a second thrust dynamic pressure groove 23 are formed at the top surface of the second recess part 17 opposing an upper end surface of the thrust washer 19 and an upper end surface of the counter plate 18 opposing a lower end surface of the thrust washer 19, respectively. Due to the rotation of the shaft part 11, the thrust dynamic pressure grooves 22 and 23 generate dynamic pressure for stably floating the shaft part 11 in a thrust direction. Due to the operation of the dynamic pressure grooves, the shaft part 11 can stably rotate at a high speed in the non-contact state with respect to the sleeve 7. As the dynamic pressure grooves, known patterns such as herringbone grooves and spiral grooves can be used.

Disk Drive Device

FIG. 2 is a perspective view illustrating an overall configuration of a disk drive device 30 with the spindle motor according to the present embodiment.

As illustrated in FIG. 2, the disk drive device 30 according to the present embodiment includes a base member (base plate) 31 having a substantially rectangular box shape, the spindle motor 1 placed on the base member 31, a magnetic disk 32 configured to be rotated by the spindle motor 1, a swing arm 33 having a magnetic head 34 for writing information at a predetermined position on the magnetic disk 32 and reading information from an arbitrary position on the magnetic disk 32, a pivot assembly bearing device 35 for swingably supporting the swing arm 33, an actuator 36 for driving the swing arm 33, and a control unit 37 for controlling these components.

The disk drive device of the present invention can be a disk drive device including 9 or more 3.5-inch-diameter magnetic disks, for example. In such a device having a large number of disks, a spatial volume in the device is further reduced. The internal space of the disk drive device may be filled with a gas having a lower density than air. In such a disk drive device with its internal space filled with such a low-density gas, the air pressure inside the device may be less than one atmosphere. The disk drive device can employ a heat-assisted magnetic recording (HAMR) system as a recording system. In the disk drive device employing the heat-assisted magnetic recording (HAMR) system, the temperature of a head part of an actuator may locally reach a high temperature of 400° C.

The lubricating oil composition used in the present embodiment described later exhibits low evaporability and low volatility by employing a specific ionic liquid and further employing a specific base oil, and the volatilized and evaporated components exhibit low adhesion to a disk and the like. Thus, in the fluid dynamic bearing and the spindle motor using the lubricating oil composition, the evaporation and the like of the component of the lubricating oil composition are suppressed even in driving at a high temperature, and it is possible to suppress a disk read-write error of the disk drive device due to the adhesion of the component such as evaporation to the magnetic disk and the like.

Lubricating Oil Composition

The present inventors have focused on the addition of an ionic liquid in the lubricating oil composition applied to the fluid dynamic bearing. The present inventors have found that an ionic liquid including a specific cation and a specific anion suppresses an evaporation amount of the lubricating oil composition.

Hereinafter, the lubricating oil composition filled in the fluid dynamic bearing of the present invention will be described.

Ionic Liquid

The lubricating oil composition applied to the fluid dynamic bearing according to the present embodiment essentially contains a specific ionic liquid.

In the known art, in order to release static electricity generated between components due to rotational friction, conductivity is imparted to a lubricant as necessary, and addition of an ionic liquid is considered as such a method.

The ionic liquid used in the present invention plays a role of suppressing the evaporation amount of the lubricating oil composition in addition to imparting conductivity and suppressing hydrolysis of ester oil when the ester oil is used as the base oil.

The ionic liquid has at least one cation selected from the group consisting of tetraalkylammonium cations represented by Formula (B) described below and at least one anion selected from the group consisting of a borate anion represented by Formula (C-1), a borate anion represented by Formula (C-2), and a borate anion represented by Formula (C-3) described below.

Cation

The tetraalkylammonium cations used in the ionic liquid according to the present invention are represented by Formula (B).

In Formula (B), R⁵, R⁶, R⁷, and R⁸ each independently represent a linear or branched alkyl group having from 1 to 18 carbon atoms.

Preferably, R⁵, R⁶, R⁷, and R⁸ each independently represent a linear or branched alkyl group having from 5 to 18 carbon atoms.

Examples of the alkyl group having from 1 to 18 carbon atoms in Formula (B) include a methyl group, an ethyl group, a n-propyl group, a n-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, and an octadecyl group.

Examples of the combination of R⁵, R⁶, R⁷, and R⁸ in Formula (B) include a combination where R⁵ is a linear or branched alkyl group having from 1 to 4 carbon atoms and R⁶ to R⁸ are each independently a linear or branched alkyl group having from 6 to 14 carbon atoms, a combination where R⁵ is a linear or branched alkyl group having from 11 to 16 carbon atoms and R⁶ to R⁸ are each independently a linear or branched alkyl group having from 6 to 10 carbon atoms, and a combination where R⁵ to R⁸ are each independently a linear or branched alkyl group having from 6 to 12 carbon atoms.

The total number of carbon atoms of R⁵, R⁶, R⁷, and R⁸ in Formula (B) may be, for example, 24 to 40.

Examples of the tetraalkylammonium cations represented by Formula (B) include a tetrahexylammonium cation where R⁵ to R⁸ are hexyl groups, a methyltri(octyl)ammonium cation where R⁵ is a methyl group and R⁶ to R⁸ are octyl groups, a (tetradecyl)tri(hexyl) ammonium cation where R⁵ is a tetradecyl group and R⁶ to R⁸ are hexyl groups, a tetraoctylammonium cation where R⁵ to R⁸ are octyl groups, and a tetradecylammonium cation where R⁵ to R⁸ are decyl groups.

Anion

The anion used in the ionic liquid according to the present invention is selected from the group consisting of a borate anion represented by Formula (C-1), a borate anion represented by Formula (C-2), and a borate anion represented by Formula (C-3).

The borate anion can be said to be a preferred aspect also from the viewpoint that recent regulations such as prohibition and restriction of use of fluorine-based compounds are in progress.

In Formula (C-1), R⁹, R¹⁰, R¹¹, and R¹² each independently represent a linear or branched alkyl group having from 1 to 22 carbon atoms.

In one aspect, R⁹, R¹⁰, R¹¹, and R¹² in the above-described (C-1) each independently represent a linear or branched alkyl group having from 1 to 22 carbon atoms, or each independently represent a linear or branched alkyl group having from 1 to 10 carbon atoms, or each independently represent a linear or branched alkyl group having from 6 to 10 carbon atoms, or each independently represent a linear or branched alkyl group having from 1 to 8 carbon atoms, or each independently represent a linear or branched alkyl group having from 1 to 6 carbon atoms, or each independently represent a linear or branched alkyl group having from 1 to 2 carbon atoms.

Examples of the alkyl group having from 1 to 22 carbon atoms in R⁹, R¹⁰, R¹¹, and R¹² include a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an eicosyl group, a heneicosyl group, and a docosyl group.

Examples of the combination of R⁹, R¹⁰, R¹¹, and R¹² in Formula (C-1) include a combination where R⁹ to R¹² are each independently a linear or branched alkyl group having from 1 to 8 carbon atoms, and a combination where all of R⁹ to R¹² are methyl groups.

As the ionic liquid used in the present invention, for example, combinations of cations and anions shown in the following (a) to (k) can be used.

• • (a)

A combination of at least one cation selected from the group consisting of tetraalkylammonium cations as represented by Formula (B), where R⁵, R⁶, R⁷, and R⁸ each independently represent a linear or branched alkyl group having from 5 to 18 carbon atoms, and at least one anion selected from the group consisting of a borate anion represented by Formula (C-1), a borate anion represented by Formula (C-2), and a borate anion represented by Formula (C-3).

• • (b)

A combination of at least one cation selected from the group consisting of tetraalkylammonium cations as represented by Formula (B), where R⁵, R⁶, R⁷, and R⁸ each independently represent a linear or branched alkyl group having from 5 to 18 carbon atoms, and at least one anion selected from the group consisting of a borate anion represented by Formula (C-1) and a borate anion represented by Formula (C-3).

• • (c)

A combination of at least one cation selected from the group consisting of tetraalkylammonium cations as represented by Formula (B), where a total number of carbon atoms of R⁵, R⁶, R⁷, and R⁸ is from 24 to 40, and at least one anion selected from the group consisting of a borate anion represented by Formula (C-1), where R⁹, R¹⁰, R¹¹ and R¹² are methyl groups and a borate anion represented by Formula (C-3).

• • (d)

A combination of a tetrahexylammonium cation and a borate anion represented by Formula (C-1) where R⁹, R¹⁰, R¹¹, and R¹² are methyl groups.

• • (e)

A combination of a tetraoctylammonium cation and a borate anion represented by Formula (C-1) where R⁹, R¹⁰, R¹¹, and R¹² are methyl groups.

• • (f)

A combination of a tetradecylammonium cation and a borate anion represented by Formula (C-1) where R⁹, R¹⁰, R¹¹, and R¹² are methyl groups.

• • (g)

A combination of a tetraoctylammonium cation and a borate anion represented by Formula (C-2).

• • (h)

A combination of a tetradecylammonium cation and a borate anion represented by Formula (C-3).

• • (i)

A combination of a tetrahexylammonium cation and a borate anion represented by Formula (C-1) where R⁹ and R¹¹ are methyl groups and R¹⁰ and R¹² are ethyl groups.

• • (j)

A combination of a tetrahexylammonium cation and a borate anion represented by Formula (C-1) where R⁹, R¹⁰, R¹¹, and R¹² are ethyl groups.

• • (k)

A combination of a tetrahexylammonium cation and a borate anion represented by Formula (C-1) where R⁹, R¹⁰, R¹¹, and R¹² are n-octyl groups.

The blending amount of the ionic liquid in the lubricating oil composition is not particularly limited, and can be appropriately selected according to the purpose.

For example, the content may be 0.01 mass % or more and 10 mass % or less, 0.03 mass % or more and 1 mass % or less, or 0.03 mass % or more and 0.5 mass % or less with respect to the base oil described later.

Base Oil

In the lubricating oil composition applied to the fluid dynamic bearing according to the present embodiment, the base oil is not particularly limited. A synthetic oil generally used as a base oil in a lubricating oil, including a mineral oil, a hydrocarbon-based synthetic oil, an ester-based synthetic oil, or an ether-based synthetic oil, can be used alone or in combination.

Among these, the ester-based synthetic oil can be preferably used from the viewpoint of easily dissolving an ionic liquid described above.

Examples of the ester-based synthetic oil (also simply referred to as ester oil) include monoester oil, diester oil, polyol ester oil, and aromatic ester oil.

In one aspect, the base oil used in the present embodiment contains at least one compound selected from the group consisting of aliphatic monoester compounds (monoester oils) and diester compounds (diester oils) having a specific alkyl chain length described later.

While volatilization of a component to be blended in the lubricating oil composition is considered to be even more problematic, the present inventors have further gone beyond a known issue of making the component low in volatility, and have conducted studies on the component based on a new idea that even if volatilization occurs, the volatile component is less likely to adhere to a disk and the like (even if the volatile component adheres, the component does not remain). As a component that realizes the above-described idea, the aliphatic monoester compound or diester compound with a certain alkyl chain length or longer has been adopted.

In one aspect, as the base oil used in the present embodiment, monoester oil and diester oil other than the following aliphatic monoester compound (monoester oil) and diester compound (diester oil) having a specific alkyl chain length may be used or used in combination.

Monoester Compound (Monoester Oil)

The monoester compound is represented by Formula (1):

where in the above-described Formula (1),

• • R²¹ is a linear or branched alkyl group having 10 or more carbon atoms in total, and preferably 23 or less carbon atoms in total. When R²¹ is a branched alkyl group, the number of carbon atoms in a side chain is 10 or more, and may be preferably 15 or less, and
  • R²² is a linear or branched alkyl group having 9 or more carbon atoms in total, and preferably 20 or less carbon atoms in total. When R²² is a branched alkyl group, the number of carbon atoms in a side chain is 7 or more, and may be preferably 8 or less.

In the specification, the number of carbon atoms in the side chain is the number of carbon atoms in the side chain part(s) of a branched alkyl group, and is not the number of carbon atoms counted from a carbon atom bonded to a carbonyl group (—C(═O)—) or an oxygen atom (—O—). The side chain is a branch part from the main chain (carbon chain that is the longest chain counted from the carbon atom bonded to the carbonyl group or the oxygen atom), and the number of side chains is not particularly limited.

In one aspect, one of R²¹ and R²² may be a linear alkyl group, and the other one may be a branched alkyl group.

Specific examples of the monoester compound (monoester oil) include, but are not limited to, the following compounds.

Diester Compound (Diester Oil)

The diester compound is represented by Formula (2):

where in Formula (2),

• • R²³ and R²⁵ each independently represent a linear or branched alkyl group having 8 or more carbon atoms in total, and preferably 10 or less carbon atoms in total. When R²³ and R²⁵ are branched alkyl groups, the number of carbon atoms in the longest carbon chain counted from the carbon atom bonded to E¹ or E² may be 9 or more, and preferably 9, and
  • R²⁴ is a linear or branched alkylene group having 4 or more carbon atoms in total and preferably 6 or less carbon atoms in total.

Here, it is preferable that R²³ and R²⁵ be both linear alkyl groups and R²⁴ be a branched alkylene group, or that R²³ and R²⁵ be both branched alkyl groups and R²⁴ be a linear alkylene group.

E¹ and E² each independently represent —C(═O)O— or —OC(═O)—.

In a preferred aspect, R²³ and R²⁵ may be an identical group.

Moreover, for E¹ and E², E¹ represents —C(═O)O— and E² represents —OC(═O)—, or E¹ represents —OC(═O)— and E² represents —C(═O)O—.

For example, in the compound, R²³ and R²⁵ represent an identical linear alkyl group, R²⁴ represents a branched alkylene group, E¹ represents —C(═O)O—, and E² represents —OC(═O)—.

Alternatively, R²³ and R²⁵ represent an identical branched alkyl group, R²⁴ represents a linear alkylene group, E¹ represents —OC(═O)—, and E² represents —C(═O)O—.

Specific examples of the diester compound (diester oil) include, but are not limited to, the following compounds.

Polyol Ester Oil

Examples of the polyol ester oil include full esters of polyhydric alcohols [triols (for example, trimethylolpropane), tetraols (for example, pentaerythritol), hexaols (for example, dipentaerythritol), and the like] with linear and/or branched fatty acids having from 4 to 22 carbon atoms.

Specific examples include trimethylolpropane triheptanoate, trimethylolpropane tricaprylate, trimethylolpropane tripelargonate, pentaerythritol tetraheptanoate, pentaerythritol tri(2-ethylhexanoate), pentaerythritol tetraoleate, and neopentyl polyol.

Aromatic Ester Oil

Examples of the aromatic ester oil include esters of aromatic polycarboxylic acids such as phthalic acid, trimellitic acid, and pyromellitic acid with aliphatic monoalcohols having from 4 to 16 carbon atoms.

Specific examples include ditridecyl phthalate, trioctyl trimellitate, tri-2-ethylhexyl trimellitate, tridecyl trimellitate, tetraoctyl pyromellitate, and tetra-2-ethylhexyl pyromellitate.

A ratio of the base oil to a total amount of the lubricating oil composition applied to the fluid dynamic bearing of the present invention can be a balance excluding a blending amount of the ionic liquid described above and a blending amount of other additives that can be blended as necessary.

Additives

In addition to the essential components described above, the lubricating oil composition can contain an additive normally used in lubricating oil compositions as necessary within a range not impairing the effects of the present invention.

Examples of the additive include extreme pressure additives, antioxidants, metal cleaners, oiliness agents, anti-wear agents, metal deactivators, corrosion inhibitors, rust inhibitors, viscosity index improvers, pour point depressants, conductivity-imparting agents, dispersants, anti-foaming agents, and hydrolysis inhibitors.

When these additives are blended, the blending amount can be, for example, from 0.5 mass % to 5 mass %, or from 1 mass % to 3 mass % with respect to the lubricating oil composition as the total amount of the additives.

Specific examples of the additives include, but are not limited to, the following.

Well-known additives containing sulfur, chloride, phosphorus, and the like can be used as the extreme pressure additives, and examples of the extreme pressure additives include: phosphorus compounds such as phosphate esters, phosphite esters, and phosphate ester amine salts; sulfur compounds such as sulfides and disulfides; chlorine compounds such as chlorinated paraffin and chlorinated diphenyl; and metal salts of sulfur compounds such as zinc dialkyldithiophosphate and molybdenum dialkyldithiocarbamate.

Examples of the antioxidant include phenolic antioxidants, diphenylamines, phosphorus-based antioxidants, and sulfur compounds such as phenothiazine. These antioxidants may be used alone or in combination of two or more.

Among them, a phenolic antioxidant, particularly a hindered phenolic antioxidant selected from the group consisting of octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], 2,2-thio-diethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and octyl-3,5-di-t-butyl-4-hydroxy-hydrocinnamic acid is preferable from the viewpoint of disk adhesion. In addition, it is desirable to avoid the use of alkylated phenyl-α-naphthylamine from the viewpoint of disk adhesion.

Examples of the anti-wear agent include phosphates, phosphites, and acid phosphates.

However, from the viewpoint of disk adhesion, the use of an amine salt of acid phosphate commonly used as an anti-wear agent is desirably avoided.

Examples of the rust inhibitor include dodecenyl succinic acid half ester.

Examples of the metal deactivator include benzotriazole-based compounds and thiadiazole-based compounds.

Examples of the viscosity index improver include polyalkyl methacrylates, polyalkyl styrenes, and polybutene.

Examples of the pour point depressant include the aforementioned viscosity index improvers such as polyalkyl methacrylates, polyalkyl styrenes, and polybutene.

Examples of the conductivity-imparting agent include nonionic surfactants and phenyl sulfonic acid.

Examples of the dispersant include polyalkenyl succinimides, polyalkenyl succinamides, polyalkenyl benzylamines, and polyalkenyl succinate esters.

Examples of the hydrolysis inhibitor include alkyl glycidyl ether type epoxy compounds, glycidyl ester type epoxy compounds, alicyclic epoxy compounds, and carbodiimides.

The present invention is not limited to the embodiment and specific examples described in the present specification, and various changes and variations can be made within the scope of the technical idea described in the claims.

EXAMPLE

The present invention is described below in more detail with reference to examples. However, the present invention is not limited to the examples.

Using the ionic liquids of Examples 1 to 8 and Comparative Examples 1 to 3 having cations and anions shown in Table 1 and the base oil A, a read-write error occurrence test was performed in the procedure described later. In the following description, the example number of the ionic liquid is also treated as the example number of the evaluation of each test.

TABLE 1
Ionic liquid

	Cation	Anion

Example 1	Tetrahexylammonium cation	Borate anion represented by the
		following Formula (C-1-1)
Example 2	Tetraoctylammonium cation	Borate anion represented by the
		following Formula (C-1-1)
Example 3	Tetradecylammonium cation	Borate anion represented by the
		following Formula (C-1-1)
Example 4	Tetraoctylammonium cation	Borate anion represented by the
		following Formula (C-2)
Example 5	Tetradecylammonium cation	Borate anion represented by the
		following Formula (C-3)
Example 6	Tetrahexylammonium cation	Borate anion represented by the
		following Formula (C-1-3)
Example 7	Tetrahexylammonium cation	Borate anion represented by the
		following Formula (C-1-4)
Example 8	Tetrahexylammonium cation	Borate anion represented by the
		following Formula (C-1-6)
Comparative	Methyl tri(octyl)ammonium	Bis(trifluoromethylsulfonyl)imide
Example 1	cation	anion
Comparative	Tetradecyltrihexylphosphonium	Borate anion represented by the
Example 2	cation	following Formula (C-4)
Comparative	Tetradecyltrihexylphosphonium	Borate anion represented by the
Example 3	cation	following Formula (C-3)

Base Oil A: 3-methyl-1,5-pentanediol di(n-undecanoate), CAS No. 1265799-70-9

(1) Read-Write Error Occurrence Test

A cover of a unused disk drive device was removed, about 20 mg of sample oil obtained by dissolving 10 mass % of the ionic liquid of the example or comparative example shown in Table 1 in the base oil A was applied to a periphery of an upper part of a control unit (control unit 37 in FIG. 2) on a back surface (an inner surface of the housing, not illustrated in FIG. 2) of the cover, and then, the cover coated with the sample oil was mounted back on the disk drive device. The same type of disk drive device was used for all samples. Two tests (N=2) were conducted for each test condition.

The heater was brought into contact with a cover surface (an outer surface of the housing, not illustrated in FIG. 2) side around an oil application part, and the temperature of the heater brought into contact was maintained at 120° C. for 48 hours. Thereafter, the heater was turned off, and the operation of the disk drive device was continued during a 48-hour hold (96 hours in total). During the operation, a computer was connected to the disk drive device, and measurement was repeatedly continued with condition monitoring software (e.g., CrystalDiskInfo, HD Tune, etc.) of the disk drive device. The stop (time point when the software determined that the device failed due to the occurrence of a read-write error) of the disk drive device during the operation was monitored by the computer connected to the disk drive device. A case where the disk drive device did not stop during a test time of 96 hours was defined as a case where the test passed. The results obtained are listed in Table 2 described below.

Evaluation Criteria

• • A: a case where the disk drive device did not stop during a test time of 96 hours
  • N: a case where the disk drive device stopped during a test time of 96 hours

When a component is volatilized/evaporated due to an increase in ambient temperature, an error of the device can be caused due to the part of the volatilized/evaporated component being condensed when the temperature decreases, and the condensed component adheres to, for example, a disk or a head of a disk drive device. That is, it can be said that an error is likely to occur at the timing when the temperature is decreased, and on the other hand, if no error occurs during the time when the temperature is decreased, it can be determined that the temperature increase level before the temperature is decreased is acceptable.

In this test, it is important that the steps from the removal of the cover of the disk drive device to the remounting are carried out in a clean room in order to avoid contamination from the outside. In implementation of this test, the test was carried out without applying the sample oil, and it was confirmed that the disk drive device did not stop even after the test termination time of 96 hours.

TABLE 2
Read-write error occurrence test/result

	Evaluation

	Example 1	A
	Example 2	A
	Example 3	A
	Example 4	A
	Example 5	A
	Example 6	A
	Example 7	A
	Example 8	A
	Comparative	N (device stopped
	Example 1	in 19 hours)
	Comparative	N (device stopped
	Example 2	in 31 hours)
	Comparative	N (device stopped
	Example 3	in 23 hours)

As shown in Table 2, in Examples 1 to 8, no read-write error occurred, whereas in Comparative Example 1, the device was stopped in 19 hours, in Comparative Example 2, the device was stopped in 31 hours, and in Comparative Example 3, the device was stopped in 23 hours, resulting in occurrence of the read-write error.

(2) Evaporation Amount Test

The ionic liquids of Examples 1 to 8 and Comparative Example 1 were added to the base oil A so as to have a concentration of 500 ppm, thereby preparing a lubricating oil composition (evaporation amount test sample). As Comparative Example 4, a lubricating oil composition (evaporation amount test sample) containing only the base oil A (not containing ionic liquid) was provided. The test sample was left standing in an oven maintained at 140° C. for 2000 hours under atmospheric pressure at a humidity of about from 40 to 60% RH. The mass of the sample before and after the test (standing) was measured, and the amount of reduction in mass after standing was calculated.

When the mass reduction amount measured in Comparative Example 4 (containing no ionic liquid) was 100, a relative value of the mass reduction amount of each test sample was calculated. The results obtained are listed in Table 3 described below.

(3) Hydrolysis Test

Since an amount of the ester oil hydrolyzed is very small under a normal temperature and normal humidity environment, a highly accelerated life test (HAST test: Highly Accelerated Stress Test) was performed in accordance with JIS C60068-2-66, “Environmental testing—Part 2: Test methods—Test Cx: Damp heat, steady state (unsaturated pressurized vapor)”.

The ionic liquids of Examples 1 to 8 and Comparative Example 1 were added to the base oil A so as to have a concentration of 500 ppm, thereby preparing a lubricating oil composition (hydrolysis test sample). In the same manner as described above, as Comparative Example 4, a lubricating oil composition (hydrolysis test sample) containing only the base oil A was provided. The accelerated life test was performed by leaving the test sample in an oven maintained at 120° C. for 250 hours under a humidity of about 90% RH and 2 atmospheres. The mass of the sample before and after the test (standing) was measured, and the amount of reduction in mass after standing was calculated.

Ester as the base oil is hydrolyzed into an acid and an alcohol by heat and moisture (humidity). Since an acid and an alcohol generated by hydrolysis are more likely to evaporate than an ester as a hydrolysis source, either or both of the acid and the alcohol evaporate more preferentially than the ester.

In this accelerated life test, the sample where hydrolysis has occurred has a more remarkable reduction in mass than the sample where no hydrolysis has occurred, and the larger the mass reduction amount, the more the hydrolysis proceeds. The accelerated test was performed on the premise that most of the mass reduction of the sample was caused by hydrolysis.

(2) Similarly to the evaporation amount test, when a weight reduction amount measured in Comparative Example 4 (containing no ionic liquid) was 100, a relative value of the weight reduction amount of each test sample was calculated. Based on the obtained results, evaluation was performed according to the following criteria for determination. The obtained results are shown in Table 3 together with the relative value of the weight reduction amount.

Evaluation Criteria

• • E (Excellent): The relative value of the weight reduction amount is less than 95
  • A (Acceptable): The relative value of the weight reduction amount is 95 or more

The upper limit of the relative value that can be evaluated as A (Acceptable) is generally about 110.

TABLE 3
			Comparative
Evaluation		Example	Example

item	1	2	3	4	5	6	7	8	1	4

Evaporation	Measurement result	94.6	96.2	92.7	94.5	95.2	92.5	93.2	92.3	95.3	100
amount	(relative value)*1
characteristics
Hydrolysis	Measurement result	90.4	83.0	91.7	83.4	92.7	92.1	90.2	91.5	108.5	100
characteristics	(relative value)*1
	Evaluation*2	E	E	E	E	E	E	E	E	A	—
*1Measurement results Relative value when Comparative Example 4 is taken as 100
*2Evaluation (hydrolysis characteristics) E (Excellent): less than 95 A (Acceptable): 95 or more

As shown in Table 3, the samples using the ionic liquids of Examples 1 to 8 had excellent evaporation amount characteristics and excellent hydrolysis characteristics (E evaluation).

The sample using the ionic liquid of Comparative Example 1 was inferior in hydrolysis characteristics to Examples 1 to 8.

In the sample of Comparative Example 1, the evaporation amount characteristics similar to the evaporation amount characteristics of Examples were obtained; however, as shown in Table 2 described above, the read-write error occurred. This is considered to be one of factors suppressing not only evaporation of components but also adhesion of the components to a magnetic disk and the like when evaporation/volatilization occurs in the sample according to the example, leading to achievement of read-write error suppression.

The best embodiments have been described in detail above, but the present invention is not limited to the embodiments described above, and variations, modifications, and the like within a range in which the object of the present invention can be achieved are included in the present invention.

REFERENCE SIGNS LIST

1: Spindle Motor; 2: Stator Assembly; 3: Rotor Assembly; 4: Housing; 5: Cylindrical Part; 6: Fluid Dynamic Bearing; 7: Sleeve; 8: Stator Core; 9: Stator Coil; 10: Rotor Hub; 10a: Lower Cylindrical Part; 11: Shaft Part; 12: Lubricating Oil Composition; 13: Back Yoke; 14: Rotor Magnet; 15: Intermediate Cylindrical Part; 16: First Recess Part; 17: Second Recess Part; 18: Counter Plate; 19: Thrust Washer; 20: First Radial Dynamic Pressure Groove; 21: Second Radial Dynamic Pressure Groove; 22: First Thrust Dynamic Pressure Groove; 23: Second Thrust Dynamic Pressure Groove; 30: Disk Drive Device; 31: Base Member (Base Plate); 32: Magnetic Disk; 33: Swing Arm; 34: Magnetic Head; 35: Pivot Assembly Bearing Device; 36: Actuator; 37: Control Unit