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574 Lubricant Additives: Chemistry and Applications

dialkyldithiophosphate (ZDDP) and zinc dialkyldithiocarbamate (ZDTC). The products of hydro-

peroxide decomposition by these SecAOXs form an antiwear  lm on metal surfaces [74]. Dissolved

catalytic metal ions M

and M

n + 1

decompose hydroperoxide in a charge shuttle reaction [75] as

shown in Figure 22.34c. The superscripts refer to the valence state of the metal ion. The rate con-

stants in the elementary reactions were estimated by approximating isothermal oil oxidation life

test results on a pentaerythritol tetraester oil [73] (Figure 22.35a). The values of the model rate

constants are given in Table 22.21. Unless stated otherwise, model calculations and isothermal

measurements were performed at 150°C. As the activation energies for the individual elementary

reactions are unknown, the activation energies for nonmetal-catalyzed reactions were approximated

by the overall activation energy of 120 kJ/mol determined from the measured lifetimes of oil with

and without antioxidants. The activation energy for metal-catalyzed elementary reactions was set to

RH R

+ H

ROO

ROOH + R

(a)

(b)

ROO

+ RH

+ O

ROOH

ROH + R

+ RH H

O + R

+ RH

+ HO

FIGURE 22.33 Oil oxidation initiation (a) and propagation (b) elementary reactions. RH stands for the

hydrocarbon oil. H is a proton and R the rest of the molecule.

ROOH

ROH

+ PriAOX

ROO

+ PriAOX

ROOH + SecAOX

Inert products

ROOH + M

n+1

ROOH + M

ROO

+ H

+ M

+ ΟH

−

+ M

n+1

(a)

(b)

(

)

FIGURE 22.34 Oxidation inhibition by PriAOX (a), SecAOX (b), and metal M catalysis of hydroperoxide

decomposition (c).

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Lubricants for the Disk Drive Industry 575

10 kJ/mol to  t the overall activation energy of 70 kJ/mol determined from the measured lifetimes

of oil containing antioxidant and catalytic metal ions.

Model-calculated oil lifetime is taken to be the peak in the ROOH concentration (Figure 22.36),

which corresponds to the sudden consumption of oil molecules RH shown in Figure 22.36a. Note

that the radical products actually continue on to further reactions, which are not included in the pres-

ent model. Low-molecular-weight products and water evaporate at elevated temperature. Hydroxyl

groups form hydrogen bonding, which increases the oil viscosity.

The model-calculated oil lifetime curves from the abovementioned rate constants and acti-

vation energies are shown in Figure 22.35b. The slopes of the curves with pure oil and oil con-

taining antioxidants are in reasonable agreement with the experimental data in Figure 22.35a. A

more complicated temperature dependence is predicted for the oil containing PriAOX and SecAOX

1E+5

1E+4

1E+3

1E+2

1E+1

(a)

(b)

100 150 200

Oil lifetime (h)

Pure oil, no metal

PriAOX = SecAOX = 0.06, no metal

PriAOX = SecAOX = 0.06, M

n+1

= 0.001

1E+5

1E+4

1E+3

1E+2

1E+1

Oil lifetime (h)

Temperature (°C)

50 100 150 200

Pure oil, no metal

1% Equimolar ZDTC/DDA, no metal

2% Equimolar ZDTC/DDA, metal

FIGURE 22.35 Isothermal oxidation lifetime of a pentaerythritol tetraester oil and oil containing equimolar

amounts of primary and secondary and metal catalyst (included as ball bearings) (a) and model-calculated oil

oxidation lifetime for equimolar PriAOX and SecAOX concentrations showing the qualitative effect of metal

catalyst (b).

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576 Lubricant Additives: Chemistry and Applications

and metal catalyst. The increase in the hydroperoxide decomposition rate by metal catalyst [75] is

apparently more signi cant at lower temperatures [76].

Model-calculated depletion of PriAOX and SecAOX is shown in Figure 22.36b. The end of

life for the oil corresponds to the simultaneous depletion of both the PriAOX and the SecAOX.

This suggests that oil life could be extended by replenishing the antioxidant before the end of oil

lifetime. The qualitative ability of the model to match the experimental effect of increasing metal

ion concentration on the PriAOX depletion with time in oil containing PriAOX and SecAOX is

shown in Figure 22.37. Dissolved catalytic metal ions decrease the oil life by increasing the rate of

antioxidant consumption, hence decreasing the induction period of time until the onset of rapid oil

oxidation.

As PriAOX and SecAOX interact with the complex oxidation mechanism in distinctly different

ways, and depending on the values of the elementary reaction rate constants k

and k

, there may

be synergistic effects between the two. For example, the maximum inhibition of oxidation could be

achieved when the peroxy radicals react with PriAOX to form hydroperoxide (Figure 22.34a) before

they can abstract a proton (Figure 22.33a), and the hydroperoxide is decomposed into inert products

by SecAOX (Figure 22.34b) before the hydroperoxide decomposes into alkoxy and hydroxy radi-

cals (Figure 22.33b). Synergistic effect of PriAOX and SecAOX in oil containing ball bearings is

shown in Figure 22.38a. The optimum mole ratio of primary/(primary + secondary) antioxidant for

maximum oil oxidation lifetime is close to 0.2, or when 20 mol% of the antioxidant is primary and

80 mol% of the antioxidant is secondary. The model calculation for the effect of the ratio on oil life

is shown in Figure 22.38b. There is a maximum (synergistic) oil life predicted both with and without

catalytic metals in the oil. The optimum ratio is closer to 50 mol% of PriAOX and SecAOX. The

presence of catalytic metal ions diminishes the bene t of the synergistic effect. Further re nement

of the individual rate constants and possibly incorporation of more detailed elementary reactions

could improve the model  t to the measured oil life.

TABLE 22.21

Elementary Reaction Rate Constants for the Reactions

in Figures 22.33 and 22.34

Rate Constant Value

1 × 10

−10

0.05

0.3

Note: The units are nondimensional based on initial nondimensional

oil proton concentration of RH = 1 and nondimensional dis-

solved oxygen concentration of 0.5. The rate constants were

determined to provide qualitative agreement with oil oxidation

life in hours at 150°C.

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Lubricants for the Disk Drive Industry 577

The kinetic model also qualitatively predicts the oxygen consumption during oil oxidation,

the effects of antioxidant concentration, oxygen pressure on induction time in isothermal pressure

differential scanning calorimetry, exothermal peak temperature in nonisothermal pressure differ-

ential scanning calorimetry, and the effect of heating rate on the exotherm temperature. These will

be covered in a subsequent publication.

22.4 CONCLUSIONS AND FUTURE OUTLOOK

Magnetic recording disk drives employ the state-of-the-art lubrication in lubrication science and

technology to achieve unparalleled reliability. Continued growth in storage capacity relies more

than ever before on fundamental understanding to provide key advancements in the critical areas

of lubrication: nanolubrication of the head disk interface and  uid bearing motor lubricants.

0.00

0.04

0.08

0.12

0 500 1000 1500 2000 2500 3000

Relative concentration

Nondimensional time

Primary antioxidant

Hydroperoxide

Secondary antioxidant

(a)

(

)

0.5

0.4

0.3

0.2

0.1

0.0

Nondimensional concentration

200 400 600

800

Nondimensional time

ROOH

ROH

ROO

FIGURE 22.36 Oil oxidation products and reaction intermediates during the model isothermal oxidation of

pure base oil (a) and the change of PriAOX and SecAOX and the hydroperoxide concentration peak with time

for (PriAOX + SecAOX) = 0.12 and PriAOX/(PriAOX + SecAOX) = 0.67 (b).

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578 Lubricant Additives: Chemistry and Applications

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 500 1000 1500 2000

Millimoles of DDA/mole oil

Time (h)

Four-ball bearings

5 ppm Iron

10 ppm Iron

50 ppm Iron

0.00

0.02

0.04

0.06

0.08

0 500 1000 1500 2000 2500 3000

Relative primary antioxidant

concentration

Time (h)

n +1

= 2.5×10

−3

−4

(a)

(b)

5×10

−4

FIGURE 22.37 The effect of dissolved metal catalyst on PriAOX concentration during aging at 150°C in

air measured by NMR spectroscopy with 1 wt% (ZDTC + DDA) in the ratio DDA/(ZDTC + DDA) = 0.57 in

a pentaerythritol tetraester oil (a) and model calculated relative PriAOX concentration at various nondimen-

sional initial amounts of dissolved metal ion M

n+1

where nondimensional PriAOX + SecAOX = 0.12 and the

ratio PriAOX/(PriAOX + SecAOX) = 0.5 (b).

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Lubricants for the Disk Drive Industry 579

The physical properties of the PFPE disk lubricants were described here. The bulk and thin  lm

properties, in combination with polar end groups, provide many years of lubrication by virtue of

viscosity enhancement and vapor pressure reduction of molecularly thin  lms. Advanced disk lubri-

cants are being designed to ful ll increasingly stringent requirements of environmental variations

[77,78] and friction at low  ying height [79,80]. Currently, advanced lubricants are being developed

with a high-level chemisorptive attachment to the disk overcoat to prevent transfer of liquid lubri-

cant to the slider and to enable reduced element spacing to the magnetic layer before the onset of

adhesive interaction.

Grease-lubricated ball bearing spindle motors reached their pinnacle with 10,000 rpm drives.

Over the course of developing sophisticated channeling greases for these high-performance, low-

vibration, long-life bearings, much was learned about grease chemistry, physical properties, and

the effects of contamination. Herein was described a comprehensive overview of grease yield stress

and the effects of grease contamination on yield stress and electrochemical oxidation of an ester

FIGURE 22.38 Oil oxidation lifetime as a function of the molar ratio of primary + secondary antioxidant

measured at 170°C in pentaerythritol tetraester oil containing ball bearings with total antioxidant amount

1 wt% (a) and calculated from the kinetic model with PriAOX + SecAOX = 0.12 with and without catalytic

metals (b).

(a)

(

)

1000

2000

3000

4000

5000

0 0.2 0.4 0.6 0.8 1

Lifetime (h)

DDA/(DDA+ZDTC)

500

1000

1500

2000

2500

3000

Relative oil life

Primary/(primary+secondary)

No metals

n+1

0.001

0 0.2 0.4 0.6 0.8 1

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580 Lubricant Additives: Chemistry and Applications

oil–based grease with a lithium soap thickener. The same principles apply to grease in general,

therefore may potentially be useful in wide ranging  elds of application where smooth high-speed

rotation is needed.

Spindle motors have evolved to  uid dynamic bearings, which provide much smoother rotation

with comparable or better stiffness than ball bearings. Stiffness is provided by the relative motion

between a shaft and a sleeve, one of which contains a pattern of grooves on its surface. The grooves

pressurize a several micron thick  lm of oil. This chapter highlighted two key properties of the

 uid bearing oil—viscosity and vapor pressure. Present  uid bearing oils have a low enough vapor

pressure so that the evaporation loss of oil is small over the lifetime of the drive. However, there is

a trade-off between the vapor pressure and oil viscosity such that the lowest operating temperature

of the drive is limited by the maximum power of the motor and oil viscosity. Attempts to use lower-

viscosity oils result in oils with a higher vapor pressure, and this appears to be a physical constraint

of oils in general. Detailed thermodynamic analysis of the relationship between oil vapor pressure

and viscosity was reviewed. The  ow and vaporization activation energies are linked to one another

by interaction forces between the oil molecules. There is, however, in principle, some chance of

reducing the viscosity without altering the activation energies if a molecular structure that has a

large positive rotational component of the  ow activation entropy could be found. Molecular struc-

tures examined have so far provided limited insight into how this may be accomplished. Another

approach is to develop a blend of two different oils—combining mixtures of polar and nonpolar

molecules. There is the potential to develop an oil blend that forms an azeotrope and also has a

lower viscosity and vapor pressure than the pure component oils. Much fruitful work remains to be

done here in the quest for a lower-viscosity and vapor pressure  uid bearing motor oil. The reward

for success will be a reduction in spin-up time at cold temperature and a lower minimum operat-

ing temperature for drives with  uid bearing motors. Current work on advanced  uid bearing oil

formulation is aimed at suppression of evaporation, aerosolization, and air ingestion at the air–oil

interface meniscus and to suppress cavitation in the oil  lm between the rotor and stator.

Thermally stable oil and grease additive packages were developed with the assistance of a

chemical kinetic model. The model provides interpretation of thermally accelerated oil oxidation

life tests and predicts the interaction between primary antioxidant and secondary antioxidant and

dissolved catalytic metal ions. Oil oxidation tests at low temperature take a long time, whereas cer-

tain tests such as isothermal and nonisothermal pressure differential scanning calorimetry can be

done in a short period of time. The kinetic model links the short-time high-temperature test results

with the long-time low-temperature tests and enables the test data to be used to predict ideal oil

oxidation life at the use temperature.

ACKNOWLEDGMENTS

Throughout the decade of work on drive industry lubricants, a substantial team effort enabled

the progress described in this chapter. Thanks are due to P. Kasai at the IBM Almaden Research

Center for technical discussions on lubricant end groups and NMR peak assignments. Ther-

mal analysis, dielectric spectroscopy, and rheological and viscosity measurements were done by

M. Carter, and NMR spectroscopy and interpretation were provided by J. Burns, in the Hitachi

Global Storage Technologies San Jose Materials Analysis Laboratory. Also in the Hitachi Global

Storage Technologies San Jose Materials Analysis Laboratory, I thank D. Pocker, for the XPS, and

F. Eng for IR microscopy, on grease residues. Thanks are due to R. Siemens, who carried out the

GPC measurements on PFPE lubricants. I am grateful to T. Gregory for his invaluable advice and

technical discussions on PFPE vapor pressure and development of the polydisperse lubricant evapo-

ration model. Many of the fractionated PFPE lubricants were graciously provided by R. Waltman

in the Hitachi Global Storage Technologies San Jose Development Laboratory. Many thanks are

due to the excellent summer students who contributed to this work: A. Brooks, J. Castro, A. Voss,

and A. Green eld. Much of the NMR characterization, viscosity, and density measurement and

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Lubricants for the Disk Drive Industry 581

evaporation data analyses were performed by D. Hopper over about 5 years part time and summers

working with us. H.S. Nagaraj  rst introduced me to ball bearing grease and then  uid bearing

oil for spindle motors. The grease mill was designed and built by J. Miller, who also prepared the

modi ed grease samples, and performed the electrochemical measurements on the greases. Invalu-

able insight and technical discussions on surface energy were provided by G. Tyndall at the IBM

Almaden Research Center. Finally, I thank my managers B. Marchon, J. Lyerla, O. Melroy, and

J. Kaufman for their encouragement and support throughout this work. Special thanks are also due

to A. Hanlon and C. Hignite for their guidance and advice.

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