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Tribological Performance and Transfer Behavior of Lubricating Oils at Head-Disk Interface under


Tribology Letters, Vol. 19, No. 4, August 2005 (? 2005) DOI: 10.1007/s11249-005-7447-3

299

Tribological performance and transfer behavior of lubricating oils at head-dis

k interface under volatile organic contamination
P. Conga,b, T. Kuboa, H. Nanaoa, I. Minamia and S. Moria,*
b a Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Fudan University, Shanghai 200433, P. R. China

Received 13 December 2004; accepted 22 May 2005

Tribological performance of head-disk interface (HDI) under volatile organic contamination was investigated using a contact start/stop (CSS) tester. Slider and disk surfaces were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOFSIMS) after CSS tests. The CSS test results indicated that the friction forces were high and unstable under contamination. Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The transfer amount of lubricating oil was revealed to be dependent on the chemical structure of the terminal group in the lubricating oil. Piperonyl (–CH2)phe@(O)2@ CH2) terminated AM3001 lubricating oil was lost more easily than two hydroxyl (–OH) terminated Tetraol lubricating oil, probably because of the weak attractive force of the piperonyl groups with carbon overcoat. TOF-SIMS chemical images indicated that the transferring behavior of the lubricating oil onto the slider surface during CSS tests was dependent on the chemical structure of volatile organic contaminants. The lubricating oil became built up on the slider surface when the dioctyl sebacate (DOS) pollutant used. In contrast, the lubricating oil distribution on the slider surface was uniform under a polydimethylsiloxane (PDMS) vapor. The di?erent transfer behavior of lubricating oil onto the slider surface may be resulted from the changeable surface properties of slider and disk because of the coexistence with gaseous contaminants. KEY WORDS: volatile organic contamination, HDI, PFPE lubricating oil, transfer behavior, tribological characteristics, CSS test, TOF-SIMS, stability, reliability

1. Introduction Along with demands for decreasing dimensions, lower ?ying height, and increasing storage capacity, the in?uence of volatile organic contamination on the performance of head-disk interface (HDI) has become increasingly noticeable. It has been found that organic vapors that are emitted from disk lubricants, bearing greases, seals, adhesive components, and residual machining oils, etc. can dramatically change the ?ying characteristics of head, engendering signal performance problems and HDI failure [1]. Segar [2] and Golden [3] studied the tribological e?ect of adhesive outgassing. They found that all pressuresensitive adhesives had a detrimental e?ect on stiction. Yamamoto et al. [4] reported the deleterious e?ect of siloxanes on HDI. High stiction was reported in the presence of high concentrations of di(2-ethylhexyl) phthalate (DEHP) and a hydrocarbon lubricant that was held at signi?cantly higher temperature [5]. Smallen et al. [6] also found that a good relationship existed between the organic contaminant thickness on disks and the stiction forces measured in drives. The stiction force increase was explained by the propensity of ?ying slider to collect liquids, which subsequently migrate into HDI
*To whom correspondence should be addressed. E-mail: mori@iwate-u.ac.jp

during rest and ?ying times [7]. On the other hand, Volpe et al. [8] found that dioctyl phthalate (DOP) and tetramethylpentadecane (TMPD) had little e?ect on HDI stiction as long as the partial pressure was maintained below the threshold for condensation. Aside from high ?y-stiction, volatile organic contaminants can also cause wear and smearing of the slider and disk, and corrosion of the pole tip and disk. Yamamoto [9] and Fukushima [10] reported that outgassed DOP and siloxanes caused deleterious wear of HDI. Siloxanes were found to cause greater wear through their transformation to silica. Jesh [11] found that harmful e?ects of volatile organic chemicals were dependent on their chemical structure. Acrylic acid vapor caused remarkable increases in stiction, ?y stiction and head smearing, whereas acetic acid and DEHP did not markedly a?ect HDI performance [11]. Polydimethylsiloxane (PDMS) caused high stiction and disk wear, but one methacrylate adhesive caused adhesive stiction and debris [12] as a result of the chemical transformation of the HDI during CSS tests. Sulfate contamination during drivelevel contact start/stop (CSS) testing at 42°C and 80% RH caused both disk and head corrosion [13]. Notwithstanding, some adsorbed organics can have bene?cial e?ects on the HDI by being lubricious and lowering the friction, and serving as a protective lubricant in lubricant-depleted regions. Koka [5] also
1023-8883/05/0800–0299/0 ? 2005 Springer Science+ Business Media, Inc.

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showed that the addition of palmitic acid to an environment that already contained DEHP negated the detrimental e?ects of the DEHP. A major way of volatile organic contamination affects tribological performance is that the adsorbed organic vapors build up a liquid-like condensation on the HDI. That buildup increases meniscus forces and thereby induces high ?y-stiction [7,14,15], adversely a?ecting head ?yability. In fact, collection of liquids has been observed to originate from water vapor, organic vapors [5], and lubricants on the disk [16]. For a headdisk system with an ultra-low ?ying height, liquid accumulation on the slider surface should have a strong in?uence on slider-disk interaction and HDI tribology. However, most of the published literature has emphasized stiction force measurement along with observations of the pick-up of liquids and wear phenomena. This study investigated tribological performance of HDI under model compounds of organic contamination using a CSS tester. Time-of-?ight secondary ion mass spectroscopy (TOF-SIMS) was used for chemical identi?cation of the accumulated and transferred materials on the ?ying slider surface following ?yability tests. This study seeks the relationship of the mass transfer onto the slider surface to the change of tribological characteristics. Results will yield useful information pointing to e?ective methods to reduce harmful e?ects of organic vapors, improving HDI stability and reliability.

2 DLC coating Al2O3-TiC 1

Positions by TOF-SIMS analysis Area: 180 ?m × 180 ?m

pad

Figure 1. Schematic diagram of the ABS of the slider used in the study.

2. Experimental details 2.1. Tribological test procedure Two kinds of thin-?lm magnetic recording disk were used in the study. Both disks were constructed from a substrate coated with a magnetic layer, followed by a diamond-like carbon (DLC) overcoat, and lubricated with per?uoropolyether (PFPE) lubricating oil. Table 1

gives detailed information about the disks. The lubricating oils on the surfaces of disks 1 and 2 were Fomblin AM3001 and Fomblin Z Tetraol, respectively. A stiction free slider was used in all experiments. A schematic diagram of the air-bearing-surface (ABS) view of the slider is shown in ?gure 1. The body material of the slider was Al2O3–TiC ceramic. Some positions and all pads on the ABS were coated with DLC ?lm. All tribological tests were conducted on a commercially available single spindle CSS tester (KT-701; Koyo Precision Instruments) in a clean room at 21–23 °C. Lateral forces that acted on the slider were measured by precision strain gauges that were attached to the beam on which the slider suspension was mounted. Measurement results were collected and displayed by an attached computer with a data acquisition system. The spindle motion was controlled by the computer, which was programmed to select the duration and periodicity of spinning and idling intervals. In this study, the disk was ?rst accelerated from 0 to 10,000 rpm in 5 s (take-o?). After spinning at 10,000 rpm for 1 s (?ying), the disk was decelerated from 10,000 rpm to 0 rpm in 5 s (landing) and kept stationary for another 1 s (rest). A load force of 7 mN was supplied via the suspension and adjusted by a micrometer. Figure 2 is a typical variation of friction forces during take-o? (a) and landing (b) processes in one CSS test cycle. During each CSS cycle, the slider took o? from

Table 1. Magnetic recording disks used in the study. disk 1 Lubricating oil chemical structure terminal group R-(OCF2CF2)p(OCF2)q-O-R disk 2 ACF2 CH2 OACH2 ACH ACH2 OH A
OH

commercial name molecular weight kinetic viscosity, cSt thickness, nm Carbon overcoat Substrate Surface roughness Ra, nm Diameter, mm

Fomblin AM3001 3200 85 1.25 a-C:H Al–Mg 0.4 88.9

Fomblin Tetraol 2000 2200 1.85 a-C:H:N Glass 0.4 63.5

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2.2. Model compounds of organic vapor contamination Dioctyl sebacate (DOS) and polydimethylsiloxane (PDMS) were selected as model compounds of organic vapor contamination because they have been commonly observed during outgassing measurements of disk drives. Table 2 lists the contaminants’ structures and some physical properties. The CSS tests were conducted at room temperature in an environmental chamber with a volume of about 0.02 m3. The ambient gas was air. A slider, a tested disk and about 5 ml contaminant in a beaker were loaded in the environmental chamber for 24 h before the CSS tests to ensure vaporization of the contaminant and condensation onto the slider and disk surfaces. During that time, the disk was stationary and did not contact with the slider. Organic vapors were also applied continuously during the entire CSS testing process. 2.3. Surface analysis Identi?cation of materials picked up on the ABS of the tested slider and the disk wear were conducted using TOF-SIMS (TFS-2100; Ulvac-PHI Inc.) after CSS tests. The primary ions of Ga+ were at 15 kV; secondary ions were extracted with ±3 kV bias on the samples. TOFSIMS spectra for the mass range of 0–3000 amu were acquired for 5 min. Position 1 (air?ow entrance, ?gure 1) and position 2 (air?ow outlet, ?gure 1), with respective areas of 180 ? 180 lm2 on the ABS of the slider, were analyzed. Our attention was given mainly to position 1 because it was observed easily by TOF-SIMS. Furthermore, all slider and disk specimens before and after CSS tests were observed by means of an optical microscope (SC35 TYPE 12; Olympus Optical Co.). 3. Results and discussion 3.1. Tribological performance under contamination for di?erent lubricating oils Figures 3 and 4 show the comparison of friction forces in air and in the presence of DOS vapor for AM3001 and Tetraol lubricated disks, respectively. The

Figure 2. A typical variation of friction force collected by a data acquisition system during take-o? (a) and landing (b) processes in one CSS test cycle.

the landing zone, ?ew for 1 s, and then landed back on the landing zone. No seeking operation was carried out. High static friction was observed at the beginning of the CSS test (?gure 2(a)) followed by a relatively low and stable dynamic friction at 70–550 ms. Subsequently, the friction decreased continuously and reached zero at the take-o? time of 1643 ms. Variation of the friction force during the landing process (?gure 2(b)) was opposite that during the take-o? process, which showed a high dynamic friction from 450–1000 ms. A higher static friction was observed at the landing time of 1020 ms. Four friction forces were used to assess the e?ects of vapor phase chemicals on the tribology of HDI in this study: the maximum static and dynamic friction forces during the acceleration and deceleration processes, which are expressed as Up-SMax, Up-FMax, DownSMax and Down-FMax (see ?gure 2).

Table 2. Physical properties of the contaminants used in the study. Dioctylsebacate (DOS) Chemical structure Polydimethylsiloxane (PDMS)

Speci?c gravity Melting point, °C Boiling point, °C Vapor pressure, Pa

0.91 – 170–172 2.4 ? 10)5

0.95 <)62 – –

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Figure 3. Friction forces, take-o? and landing times vs. CSS count in air and in the presence of DOS vapor for AM3001 lubricated disk. (load: 7 mN, disk rotation speed: 10,000 rpm).

take-o? (?gures 3(c) and 4(c)) and landing (?gures 3(f) and 4(f)) times versus the CSS count are also given in the corresponding ?gures. Regardless of the lubricating oil,

DOS visibly increased Up-FMax and Down-SMax. DOS apparently a?ected the tribological performance of the AM3001 lubricated disk more seriously than that of

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Figure 4. Friction forces, take-o? and landing times vs. CSS count in air and in the presence of DOS vapor for Tetraol lubricated disk. (load: 7 mN, rotate speed of disk: 10,000 rpm).

Tetraol lubricated disk. For AM3001 lubricated disk, DOS engendered lower Up-SMax (?gure 3(a)), shorter take-o? (?gure 3(c)) and landing (?gure 3(f)) times, but

higher Down-FMax (?gure 3(e)) compared to results in air. However, no e?ects of DOS vapor on Up-SMax (?gure 4(a)), Down-FMax (?gure 4(e)) and landing time

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(?gure 4(f)) were observed in the case of the Tetraol lubricated disk. The above results indicate that volatile organic contamination a?ects tribological and ?yability characteristics of HDI; the degree of in?uence is dependent on the type of lubricating oil on the disk. It has been reported that the organic condensate, much like an excessive amount of mobile disk lubricant or condensed moisture, gives rise to meniscus and viscous forces in the contact, thereby engendering a high ?ystiction [8] and long take-o? time. Nevertheless, our tests showed no harmful e?ects of organic contamination on the Up-SMax and take-o? time for either the AM3001 or Tetraol lubricated disk. One of reasons for this phenomenon may be that a special slider (?gure 1) that was designed for stiction-free operation was used in our study. The ABS of the slider after CSS tests was examined using TOF-SIMS to clarify the liquid accumulation and transfer at HDI. Figure 5 shows TOF-SIMS spectra of the slider surface after 3000 CSS cycles in the presence of DOS vapor for AM3001 lubricated disk. The major peaks were at m/z = 12, 31, 50, 69, 100, 119, corresponding to C, CF, CF2, CF3, C2F4, C2F5, respectively. They are typical peaks from the PFPE lubricants, resulting from the PFPE lubricating oil used on the disk surfaces and transferred onto the slider surface during CSS processing. Similar TOFSIMS spectrum of the slider surface after 3000 CSS cycles in the presence of DOS vapor was also obtained in the case of Tetral lubricated disk. TOFSIMS spectra verify that the transfer material on the slider surface was mainly PFPE lubricating oil on the disk. For quantitative calculation the mass transfer of the lubricating oils, we selected ion intensities C2F4+ that

originated from PFPE lubricants and Al+ that originated from the body material of the slider. The relative intensity of C2F4+/Al+ was used to evaluate the amount of lubricating oil transfer onto the slider surface. Figure 6 shows the relative intensity of C2F4+/Al+ of the slider surface (position 1, ?gure 1) after CSS tests in air and in the presence of DOS vapor. Piperonyl (ACH2)phe@(O)2@CH2) terminated AM3001 lubricating oil transfers much more easily than two hydroxyl (AOH) terminated Tetraol lubricating oil under contamination, probably because of the weak attractive force of the piperonyl terminal groups with the carbon overcoat. That result implies that enhancement of the binding force of the lubricants with the carbon overcoat for reduction the transfer of lubricating oils during takeo? and landing operations is one way to reduce harmful e?ects of volatile organic contamination on the tribological performance of HDI.

Figure 6. E?ect of DOS vapor on the transfer amount of di?erent lubricating oils onto slider surface.

Figure 5. TOF-SIMS spectra of the slider surface after 3000 CSS cycles in the presence of DOS vapor for AM3001 lubricated disk.

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3.2. Chemical structure of contaminant vapors and e?ects on HDI tribological characteristics Figure 7 shows the Up-SMax (a), Up-FMax (b), Down-SMax (c), and Down-FMax (d) in the presence of DOS and PDMS contaminant vapors (lubricating oil: AM3001). Because the variation range of the friction forces during the entire CSS tests could be used to evaluate the HDI stability, it was also expressed in the ?gures as an error bar. Compared to friction forces in air, both contaminants caused markedly higher Up-Fmax and Down-SMax and a larger variation range of friction forces. Although great changes in the average friction forces that depend on the chemical structure of gaseous contaminants were unobservable, PDMS vapor engendered a larger variation range of friction forces than DOS vapor did. This result implies that PDMS leads more severe mechanical instability of HDI. Di?erent adsorption and desorption behaviors of contamination molecules on surfaces may explain the friction results. All slider and disk surfaces were examined after CSS tests using an optical microscope. However, all surfaces were as clean as they were before CSS tests. Figure 8 shows TOF-SIMS ion images of the slider surface after

3000 CSS cycles under di?erent environments (lubricating oil: AM3001). Chemical images of C2F4+ and Al+ represent the distributions of lubricating oil and body material, respectively. Figure 8 indicated that the accumulation state of the lubricating oil on the slider surface was dependent on the environmental contaminants. For a DOS environment, the transfer of the lubricating oil gathered; thereafter, variously sized clusters of the lubricating oil became visible. The lubricating oil was so thick at some positions that Al+ originated from the slider body material at those positions was undetectable by TOF-SIMS (?gures 8(f) and (i)). In contrast, distribution of the transferred lubricating oil on the slider surface was uniform under the PDMS environment (?gure 8(k)). Disk surface wear under di?erent environments was analyzed using TOF-SIMS. Although a wear region was observed under the PDMS environment, no wear was detected at the ?ying region in air and in the presence of DOS vapor. Figure 9 shows TOF-SIMS chemical images of the disk after 3000 CSS cycles under PDMS vapor. Loss of the lubricating oil (?gure 9(b)) was caused by the CSS test. However, the carbon overcoat failure (?gure 9(c)) and debris formation were not detectable.

Figure 7. Friction forces and their variations in the presence of DOS and PDMS vapors. (lubricating oil: AM3001, load: 7 mN, disk rotation speed: 10,000 rpm, CSS cycle: 3000; each point is the average of more than 19 test results).

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P. Cong et al./Tribological performance and transfer behavior of lubricating oils

Figure 8. TOF-SIMS ion images of the slider after 3000 CSS cycles in the presence of DOS and PDMS vapors. (lubricating oil: AM3001, load: 7 mN, disk rotation speed: 10,000 rpm).

Figure 9. TOF-SIMS ion images of the disk after 3000 CSS cycles in the presence of PDMS vapor. (lubricating oil: AM3001, load: 7 mN, disk rotation speed: 10,000 rpm).

On the other hand, (?gure 9) shows that the width of the loss region of the lubricating oil is about 0.08 mm, which is much smaller than the width of the 1 mm slider. This fact implies that the real contact area of the slider with the disk is quite small during the take-o? and landing

processes. However, the lubricating oil was detected everywhere on the ABS of the slider, indicating that the transfer of the lubricating oil is caused not only by contact, but also by interaction between the lubricating oil and molecules from volatile contaminants.

P. Cong et al./Tribological performance and transfer behavior of lubricating oils

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Figure 10. E?ect of gaseous DOS and PDMS contaminants on the transfer amount of lubricating oil onto slider surface. (lubricating oil: AM3001, load: 7 mN, disk rotation speed: 10,000 rpm, CSS count: 3000 cycle).

Figure 10 depicts the relative intensity of C2F4+/Al+ of the slider surface after CSS tests in air and in the presence of DOS and PDMS vapors. The transfer amount of the lubricating oil onto the slider surface increased to 9–11 times that in air. PDMS causes more

transfer than DOS does. That is to say, the presence of organic vapor pollutants promotes the transfer of lubricating oil. In addition, the accelerating e?ect is dependent on the type of contaminant. Figure 11 shows TOF-SIMS spectra of CO+ and + Si of the slider surface after 3000 CSS cycles in air (a) and in the presence of DOS (b) and PDMS (c) vapors. Sources of CO+ are environmental organic contaminants, DOS molecules and the transferring lubricating oil. Si+ can originate from the adhesive agent on the slider surface and PDMS molecules. The shape of spectra at about m/z = 28 under DOS vapor is similar to that in air, which demonstrates that the height of CO+ peak is higher than that of Si+ peak. However, the height of the Si+ peak increases remarkably, it is even higher than that of the CO+ peak in the case of PDMS contaminant (?gure 11(c)). Relative intensities of CO+/Al+ and Si+/ Al+ were calculated to demonstrate this result clearly. Figure 12 shows those results. Compared to the result obtained in air, no noteworthy change in the relative intensity of CO+/Al+ under DOS environment was found. However, the relative intensity of Si+/Al+ under PDMS vapor increased to nearly three times that in air. That result indicates that

Figure 11. TOF-SIMS spectra of slider surface after 3000 CSS cycles in air (a), in the presence of DOS (b) and PDMS (c) vapors. (lubricating oil: AM3001, load: 7 mN, disk rotation speed: 10,000 rpm).

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volatile organics may relate with tribological performance at HDI. More study should be done on these ?elds.

4. Conclusions Tribological performance of the head-disk interface (HDI) under volatile organic contamination was investigated using a contact start/stop (CSS) tester. Chemical identi?cation of the accumulated and transferred materials on the ?ying surfaces following ?yability tests was examined using TOF-SIMS. The main conclusions can be drawn as follows: 1) Volatile contaminants engender high and unstable friction forces. Harmful e?ects can occur from the transfer of lubricating oil onto the slider surface under organic contamination. 2) The amount of transferred lubricating oil depends on the chemical structure of the terminal group in the lubricating oil. Piperonyl (ACH2)phe@(O)2@CH2) terminated AM3001 lubricating oil is lost more easily than two hydroxyl (AOH) terminated Tetraol lubricating oil, possibly because of the weak attractive force of piperonyl groups with the carbon overcoat. 3) Transferring behaviors of the lubricating oil onto the slider surface are dependent on the contaminant vapors species. This dependency may be explained by the changeable surface properties of slider and disk because of the coexistence with the contaminant vapors.

Figure 12. A comparison of the relative intensities CO+/Al+, Si+/ Al+ of the slider surface after 3000 CSS cycles in the presence of DOS and PDMS vapors. (lubricating oil: AM3001, load: 7 mN, disk rotation speed: 10,000 rpm).

great amount of PDMS molecules exists on the slider surface after 3000 CSS cycles. 3.3. Discussion Figure 10 shows that the amount of transferring lubricating oil is greater under PDMS environment than under the DOS environment. The transferring lubricating oil on the slider surface distributed uniformly in the case of the PDMS (?gure 8(k)) contaminant. A great amount of Si+ that originated from PDMS also existed on the slider surface (?gure 12). For the DOS environment, the transferring lubricating oil accumulated (?gures 8(e), (f), (h), and (i)). In contrast, DOS molecules either did not accumulate or did so only slightly on the slider surface after 3000 CSS cycle (?gure 12). These results imply that surface characteristics of the slider may be changed by the environmental contaminants, and further a?ect the transfer and distribution of lubricating oil on the slider surface. Therefore, chemical modi?cation of slider surface for reduction the transferred lubricating oil may be an e?ective method for low friction and stability HDI under volatile organic contamination. Nevertheless, no PDMS liquid droplets were detected by TOF-SIMS on the slider and disk surfaces after CSS tests. One explanation is that the time that elapsed between the test termination and surface analysis might have allowed di?usion of droplets across the surfaces [11,14]. Although the DOS vapor pressure is as small as 10)5 Pa, DOS vapor also markedly increases Up-FMax (?gure 7(b)), Down-SMax (?gure 7(c)) and DownFMax (?gure 7(d)) compared to results for air; it also has dramatic e?ects on the transferred amount of the lubricating oil onto the slider surface (?gure 10). Concentration of the vapor phase is not entirely responsible for these results. Adsorption and desorption thermodynamics, di?usion behaviors and chemical reactivity of

Acknowledgments The authors gratefully acknowledge ?nancial support from the Storage Research Consortium (SRC), and for supplying all slider and disk specimens.

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