- Email: [email protected]
In situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface
Wear 225–229 Ž1999. 690–699
In situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface Andrei Khurshudov ) , Peter Baumgart, Robert J. Waltman IBM, Storage System DiÕision, 5600 Cottle Road, San Jose, CA 95193, USA
Abstract A thin layer of lubricant is a critical element of the headrdisk interface needed to improve its tribological durability and to prevent media corrosion. Local thinning of lubricant with its subsequent breakdown often leads to the immediate failure of the interface. This paper is devoted to the in situ quantitative analysis of nano-scale lubricant migration on the surface of a thin-film disk using the Optical Surface Analyzer ŽOSA.. The calibration procedure, which enables quantitative measurements, is discussed and the technique’s capabilities are demonstrated using specially prepared samples. Two cases of sliderrdisk interaction are analyzed: low-speed, when the slider is dragged over the disk surface, and high-speed, when the slider is flown over the same track for several days. Lubricant migration phenomena, such as depletion and pooling, are investigated quantitatively to analyze the origination of carbon wear and the mechanisms of interfacial failure. A model of high-speed sliderrdisk interaction involving the dynamic formation of a liquid bridge at the interface is proposed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Lubricant migration; Head–disk interface; Optical surface analyzer
1. Introduction Several different techniques have been used in the past to investigate phenomena on the surface of magnetic rigid disks such as lubricant migration and degradation, carbon wear, and particle generation. These techniques include the Kelvin probe w1x, X-ray photoelectron spectroscopy ŽXPS. w2x, secondary ion mass spectroscopy ŽSIMS. w3x, photonic probe w4x, ellipsometry w5x, and infrared spectroscopy ŽFTIR. w6x. All of these techniques have some limitations such as low speed and resolution, or are ex situ techniques. The Optical Surface Analyzer ŽOSA. was introduced several years ago in an attempt to achieve high-speed, highresolution in situ measurements of thin-film lubricant migration and degradation, as well as carbon thickness changes w7x. This technique was successfully used in studies of lubricant migration and interfacial wear. This paper extends research in the field of head–disk interface tribology using OSA. It concentrates on the role
) Corresponding [email protected]
author.
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q 1-408-256-2410;
E-mail:
of ‘mobile’ lubricant in providing durability, lubricant pick-up by the air bearing, lubricant migration, and discusses the possibility of a lubricant bridge formation at the head-disk interface.
2. Experimental method and equipment An OSA Žsee Fig. 1. uses P- and S-polarized light to measure thickness changes in both, lubricant and carbon layer of a thin-film disk. Polarized light interacting with the disk surface results in a combination of absorption, reflection, and scattering. The amount of reflected and scattered light is measured using two photodetectors: PrS scattered Žwith an integrating sphere. and PrS specular. OSA w7x is designed in such a way that S- and P-polarized light reflectivity will vary in different ways as a function of thickness of disk lubricant and carbon overcoat. Thinning of the lubricant increases the intensity of reflected S-polarized light, but decreases the intensity of reflected P-polarized light. The opposite is true for an increase of lubricant thickness. Both, S- and P-reflected light intensities increase in the case of carbon film thinning. Wear particle formation or surface roughness increase leads to a
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 8 1 - 0
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Fig. 1. Schematics of the OSA and the disk surface changes, which could be detected with OSA.
decrease in both, S- and P-polarized light specular components due to increased light scattering. This is summarized in Fig. 2 called the 2-D histogram. The data in the first quadrant correspond to the complete wear of the lubricant plus some wear of the carbon overcoat. Data in the second quadrant correspond to lubricant thinning. Wear and contamination particles and roughness increase can be detected in the third quadrant, and lubricant pooling will generate data points in the fourth quadrant. The central part of the 2-D histogram consists of data points, shown as a circle in Fig. 2, with values within the noise level of the system. More distant points mean stronger signals and allow more accurate interpretation. A special software w8x enables trace-back of data-points on the 2-D histogram to their original locations on the S- and P-images and to find the exact location of wear particles or areas of carbon wear and lubricant migration on the disk surface. Data from both S- and P-images are needed simultaneously to interpret the OSA images. OSA allows qualitative in situ monitoring of interfacial changes w7x. However, a quantitative analysis is much more desirable. In order to enable quantitative analysis, the following calibration procedure was used. A set of thin-film disks with both nitrogenated and hydrogenated carbon
Fig. 2. 2-D histogram based on THE data of S- and P-polarized light images and its interpretation. Marked area in the center indicates intensities within the noise level.
coatings was prepared and the disks were half-dipped into a lubricant bath containing perfluoropolyether lubricant ˚ ŽPFPE.. Thus, several steps of lubricant layers up to 20 A thick Žas measured by FTIR. were prepared. In the lubricated part of the disk, additional lubricant was removed in narrow bands by wiping with a cloth, saturated in solvent. The thickness removed was measured with FTIR. The OSA images of the half-lubricated disk surface with two wiped bands are shown in Fig. 3. It can be seen that lubricant thinning Žwiped areas. increases the intensity of S-polarized light and decreases the intensity of P-polarized light. In this study S-polarized reflectivity as a function of lubricant thickness was used to calibrate the OSA signal because S-polarized light is, in general, more sensitive to lubricant thickness changes, while P-polarized light is more sensitive to the carbon thickness changes w7x. In order to investigate tribological changes at the head–disk interface, two types of tests were performed ˚ of using fully lubricated thin-film disks with about 20 A PFPE film. In the first test, low-speed Ž0.6 mrs. dragging of the slider was utilized to prevent formation of an air bearing. Under these conditions, the normal force is known and equal to the suspension pre-load Žabout 40 mN in this case.. This type of testing allows better control of sliderrdisk interaction by eliminating local lubricant or COC damage during occasional high-speed asperity contacts, which exist at nominal drive operating speeds. Drag testing leads to a gradual wearing out of the protective layer of lubricant and carbon overcoat under a known contact force. In this test, the carbon-coated sub-ambient pressure slider with an elongated central pad was used. In the second test, a sub-ambient pressure carbon-coated slider with a central pad near the trailing edge was kept flying over the same track of the disk at 8 mrs for 7 days. Elimination of lateral slider movements Žseek. allows speeding-up interfacial processes Žlube migration, wear, etc.. while keeping the contact conditions Žvelocity, spacing, sliderrdisk interference, contact force, etc.. similar to that in a drive. The contact force between the flying slider and the disk is usually unknown.
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A. KhurshudoÕ et al.r Wear 225–229 (1999) 690–699
Fig. 3. S- and P-polarized images of the same area of a partially lubricated disk and their correspondent cross-sections. The disk lubricant was partially wiped in radial direction using a solvent Žsee two vertical bands.. The width of the image in radial direction of the disk is 3 mm. The top of the image is closer to the OD Žouter diameter. of the disk. The image corresponds to 3608 scan of the disk.
Both the friction force and acoustic emission ŽAE. signals w9x were continuously monitored during the test. AE signal was high-pass filtered below 600 kHz to obtain information only about the slider body vibrations due to asperity impacts and to eliminate the effect of lowfrequency signals caused by mechanical noise of the sys-
tem, vibrations of the suspension, air bearing vibrations, etc. The OSA was used to take images of the disk surface at predetermined times during the test. In the case of low speed testing, the slider was always lifted from the surface to enable OSA measurements, and loaded again on the
A. KhurshudoÕ et al.r Wear 225–229 (1999) 690–699
same track after the measurement. The slider positioning accuracy was about 0.5 um.
3. Results and discussion 3.1. Calibration Fig. 4 presents the results of the OSA sensitivity calibration. In this figure the change in S-polarized light reflectivity is presented as a function of lubricant thickness for both hydrogenated and nitrogenated carbon overcoats. Fig. 4 indicates that there is a linear relationship between S-polarized reflected light intensity and the thickness of the lubricant and that this relationship is independent of the chemistry of the carbon overcoat. With the help of linear data fitting, this relationship could be described analytically as follows S-polarized light reflectivitys K = Lubricant thickness,
Ž 1. where K is the proportionality constant equal in our tests to 0.0022 when the lubricant thickness is given in angstroms. It should be mentioned that this constant K strongly depends on the type of the lubricant and may also vary from one specific equipment to another. 3.2. Low-speed drag testing Fig. 5 presents OSA images of the same area of the disk at different cycle Žrotation. numbers. This specific disk ˚ of partially bonded film of PFPE initially had about 20 A ˚ ˚ free. on the CH x overcoat. lubricant Ž15 A bondedq 5 A
693
From the first two images Ž3000 cycles. we observe that ˚. there is already some lubricant pooling Žup to 14 A outside the contact area of the outer rail of the slider Žcloser to the outer diameter of the disk.. Another location ˚ . is observed is the where even larger pooling Žup to 54 A center of the track just under the central rail of the slider. The slider used here has a negative camber Žcross-crown. of about 30 nm. No lubricant was observed on the slider air-bearing surface ŽABS. after the test. Some lubricant was observed on the vertical face side of the slider at the trailing edge. Therefore, these narrow linear zones of accumulated lubricant under the central rail are most likely a result of trailing edgerdisk contacts when the slider was pitching back and forth. This may happened both during the constant speed sliding and during the slider deceleration to a complete stop just before the OSA images were taken. When the slider contacts the lubricated disk with its trailing edge, some lubricant pick-up occurs and this lubricant could be later found on the vertical face side of the slider at the trailing edge. When the slider touches the disk surface again, some part of this lubricant drops on the disk surface resulting in the observed lubricant pooling Žsee Fig. 5.. More importantly, there is some lubricant depletion ˚ . observed in the first two images under the inner Ž; 13 A rail of the slider. When averaged over the entire track, this ˚ but at some lubricant depletion is equal to only about 5 A, ˚ which is more than half of local spots it is as high as 13 A, the total lubricant thickness. The second set of images, taken at 40,000 cycles, shows approximately the same picture, but the scale of observed phenomena is larger. The lubricant pooling is clearly in˚ at creasing from 3000 cycles and reaches up to 90 A 40,000 cycles. The location of this highest lubricant pool is approximately the same. The lubricant depletion was also
Fig. 4. Calibration results: S-polarized light reflectivity vs. lubricant thickness for CH x and CN x overcoats.
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Fig. 5. S- and P-polarized light images of a disk surface after 3, 40 and 43 K cycles of low-speed drag testing. Lubricant pooling, depletion, and carbon wear are observed. The width of the images is 3 mm, the width of the slider is about 1.6 mm. Sliding direction: from right to left. The top of the picture is closer to the OD of the disk.
continuing under the same inner rail with the maximum ˚ . also at exactly the same location lube depletion Ž; 20 A as at 3000 cycles. Lubricant pooling under the central rail is less repeatable since the slider could also smear the drops of lubricant on the disk surface. The final two images in Fig. 5 show interfacial failure with carbon wear and particle generation under the inner
rail of the slider at 43,000 cycles. It is worth noticing that interfacial failure occurred under the same rail where lubricant depletion was previously detected. Another thing to notice is that some wear particles can be found as far as 1–1.7 mm away from the wear tracks closer to the outer edge of the disk. The test was performed at relatively low 300 rpm. Still, the centrifugal force was high enough to
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˚ of lubricant Ž15 A˚ bondedq 5 A˚ free. on CN x . Fig. 6. COF, AE, lube pooling and depletion vs. drag cycles for the disk with initially 20 A
move some of these large particles away from the wear track. Fig. 6 shows changes in the AE intensity, the coefficient of friction ŽCOF., averaged over the entire disk track lubricant pooling and depletion values Žat the locations discussed above. as a function of drag cycles.
The first lubricant depletion value at 3000 cycles is ˚ which is equal to the thickness of the free about 5 A, ˚ of free lubricant. As can be seen in Fig. 6, about 5 A lubricant were completely removed during first 3000–5000 cycles. Lubricant depletion varies somewhat around the ˚ It can also disk track with an average value of about 5 A.
Fig. 7. S- and P-polarized light images of the disk surface after 16,000 cycles of low-speed drag testing. Lubricant depletion is observed. The width of the images is 3 mm, the width of the slider is about 1.6 mm. Sliding direction: from right to left. The top of the picture is closer to the OD of the disk.
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be seen from Fig. 6, that the AE rms signal was initially high and fluctuating, but decreased to its lowest values at approximately the same time as the free lubricant was removed. It is not quite clear how closely these two processes are related, but it is possible that free lubricant stayed at the interface for just long enough to assist the initial interfacial burnishing. After the initial quick lubricant removal, the depletion was almost a linear function of the drag cycles resulting in the continuous thinning of the lubricant. When the average depletion reached about 75% of the initial lubricant thickness, the lubricant film started to break down locally, causing rapid interfacial failure Žbetween 40,000 and 43,000 cycles., which was also accompanied by an increased AE signal and a decrease in the COF. The AE
intensity increase was caused by a direct interaction between the slider and disk surfaces at the spots of lubricant break-down. The AE is, in general, more sensitive to the interfacial changes w9x than friction. The decrease in COF could be observed about 3000–5000 cycles after the first lubricant breakdown Žas measured by AE at ; 39,000 cycles and by OSA at ; 40,000 cycles., and was caused by formation of a large number of wear particles on the disk surface. These particles increased sliderrdisk separation and quickly decreased both apparent and real contact areas between the disk and the slider thus decreasing stiction and friction forces. Lubricant pooling saturates with the number of drag cycles. Friction was clearly insensitive to lubricant migration on the disk surface and was, probably, controlled by
Fig. 8. S- and P-polarized light images of the disk surface after 5 and 120 h of on-track flying at 8 mrs using sub-ambient pressure slider. Lubricant pooling is observed. The width of the images is 3 mm, the width of the slider is about 1.1 mm. Slider flying direction: from right to left. The top of the picture is closer to the OD of the disk.
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˚ of lubricant Ž15 A˚ Fig. 9. COF, AE, lube pooling vs. time for on-track test with sub-ambient pressure slider flying over the disk with initially 20 A ˚ free. on CN x . bondedq 5 A
much stronger adhesive forces caused by the liquid meniscus formed at the sliderrdisk contacts. It is possible, that the observed local lubricant pools also caused some lubricant depletion at other locations without changing the
resultant meniscus force. The total test duration before failure was about 43,000 cycles, but the location of later lubricant breakdown was already observed during the first OSA measurement at 3000 cycles. This shows not only the
˚ of lubricant Fig. 10. COF vs. fitted lubricant thickness increase for on-track test with sub-ambient pressure slider flying over the disk with initially 20 A ˚ bondedq 5 A˚ free. on CN x . Ž15 A
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sensitivity of OSA, but also that the traces of interfacial failure can be found at very early stages of sliderrdisk interaction. Fig. 7 shows images of a similar disk with fully bonded ˚ of the same lubricant after 16,000 drag cycles—just 20 A before this interface failed at about 18,000 cycles. From this figure we observe lubricant depletion only, and no lubricant pooling. The only difference between the disks in ˚ of free lubricant on top Figs. 5 and 7 is the presence of 5 A of the bonded layer in the former case. It shows that some mobility of the lubricant is needed to provide higher durability since only a mobile lubricant is capable of curing damage to the bonded part. Free lubricant is less capable of carrying the slider load and is easily displaced from the sliderrdisk contacts. But, even when it was displaced from the contact area at the early stages of the test, it was still capable of migrating back onto the damaged spots and delaying interfacial failure under given experimental conditions. If the rate of lubricant displacement is equalized by the rate of its backward migration, then an interface with practically unlimited life could be obtained.
this phenomenon. For example, an increasing bridge should increase the meniscus force, which will increase the friction force. If the bridge size is constant, then the friction should stay constant. Fig. 9 presents the acoustic emission rms signal, the COF and the height of the observed lubricant pool as measured in situ by OSA. In order to minimize the scatter, the data-points for the pool height were fitted with a second order polynomial. The lubricant thickness was saturating with time. The AE signal saturated soon after the beginning of the test. Friction was decreasing until about 20 h into the test due to interfacial burnishing. Starting from about 20 h, friction was increasing and stabilized after about 90 h of testing. It is worth mentioning that the AE signal does not generally correlate with frictional because they originate from different frequency ranges. The AE signal is sensitive to high frequency phenomena. Measured friction force contains low frequency signal. Both these signals may correlate if a strong low-frequency process causes both of them. If the lowfrequency component of the AE signal is strong enough, it may saturate the high-pass filter and become detected.
3.3. High-speed on-track testing Fig. 8 shows OSA images of the same area of the disk as Fig. 7 at different time of testing when a sub-ambient pressure slider with a central pad near the trailing edge ˚ of PFPE Ž15 was flying over the disk lubricated with 20 A ˚A bondedq 5 A˚ free. for 160 h. It is a known fact that sub-ambient pressure sliders often show some lubricant pick-up on the ABS surface or in the ABS cavity after testing. Fig. 8 presents in situ evidence of the air bearinginduced suction action on the lubricant film. The lubricant ˚ high on the disk surface after 5 h of testing. pooling is 2 A The continuing suction by the air bearing results in formation of a continuous pool of lubricant along the track as ˚ after 120 h. This is direct evidence that high as about 20 A the air bearing suction is sufficiently strong not only to move the lubricant laterally, but also move it vertically up to several nanometers. Finally, some lubricant molecules get separated from the disk and sucked into the negative pressure pockets on the slider surface. Another phenomenon we would like to introduce in this paper is the formation of a continuous lubricant bridge between the disk and the slider flying at high speed. By the term ‘continuous’ we mean the bridge, which exists for all or most of the time needed for the slider to complete a revolution. A continuous vertical bridge is most likely a series of random events occurring at high frequency between the different points of the slider and the disk. We used in situ measurements of the lubricant film thickness with OSA and correlated them with the measured COF. The reasoning in this case is that if a liquid bridge exists at some point of the test, then the adhesive force, and therefore, the friction force should correlate with the scale of
Fig. 11. Model of interaction between the lubricated disk and low-flying sub-ambient pressure slider at the beginning of the test Ža., when the disk burnishing just starts, and after the burnishing is completed and the liquid bridge is fully established Žb..
A. KhurshudoÕ et al.r Wear 225–229 (1999) 690–699
Starting from about 50 h in Fig. 9, there are several instances when frictional and AE signals respond in the same way: drop and increase together. This means that there is a common low-frequency Žof the order of a few kHz. interfacial process causing both of them. Introducing a continuous lubricant bridge between the slider and the disk formed between 20 and 50 h of testing, it is possible to explain these synchronized changes in both signals. If there is a liquid bridge at the sliderrdisk interface, then the contact force and, therefore, friction are controlled by the meniscus force. If the bridge breaks down temporarily then both, meniscus force and friction will drop. Since the AE signal amplitude is a strong function of the contact force, it should stay high when the liquid bridge is intact and drop in case of its breakdown, following the friction. This is exactly what we observe in the test shown in Fig. 9. Fig. 10 presents the correlation of the fitted Žsee Fig. 9. lubricant thickness increase values from Fig. 9 with the COF measured at the time when the OSA image was taken for the part of test when the liquid bridge was presumably established Žafter 20 h of testing.. A fairly good linear correlation can be observed. It supports the previous hypothesis that a lubricant bridge formed between the slider and the disk beginning from about 20 h of testing controls the magnitude of friction. Fig. 11 shows a possible model of the sliderrdisk interface evolution in the test discussed above. Before the burnishing is completed and while lubricant pooling is insignificant Žsee Fig. 11a., the contact force at the interface depends on disk topography, slider flying height, air-bearing characteristics, and the suspension pre-load. After the liquid bridge is established, the contact load becomes dependent on the force of the meniscus formed at the interface.
4. Conclusions 1. The technique of the in situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface was demonstrated. It was shown that this technique is highly sensitive to angstrom level interfacial changes. OSA allows not only to investigate in great detail critical interfa-
699
cial phenomena such as lubricant pooling and depletion, carbon wear and formation of wear particles, but also allows to observe directly the pooling of lubricant due to the sub-ambient pressure air-bearing. 2. The process of lubricant pooling, depletion, and its final breakdown at the head–disk interface was investigated using OSA during low-speed drag tests. It was shown that the free part of the lubricant was almost completely displaced during the first 7–15% of the total test time to failure. In spite of this, displaced lubricant was still working at the interface via the migration mechanisms, where it was curing damage to the bonded part of the lubricant layer. In the absence of lubricant pooling in the case of fully bonded lubricant, much shorter durability of the interface was observed. 3. Using in situ OSA measurement of lubricant thickness changes and their correlation with friction and AE signals, a model of dynamic liquid bridge formation between the disk and a sub-ambient pressure slider was proposed. A mechanism of the lubricant pick-up by the slider ABS surface was introduced. References w1x S. Yee, M. Stratmann, R.A. Oriani, Application of a Kelvin microprobe to the corrosion of metals in humid atmosphere, J. Electrochem. Soc. 138 Ž1991. 55. w2x M. Mate, V.J. Novotny, Molecular conformation and disjoining pressure of polymeric liquid films, J. Chem. Phys. 94 Ž1991. 8420. w3x A. Benninghoven, F.G. Rudenauer, H.W. Werner, Secondary Ion Mass Spectroscopy, Wiley, New York, 1987. w4x D. Jen, D. Gillis, Optical profiler for thin film disk, Adv. Inf. Storage Sys. 4 Ž1992. 219–230. w5x L.L. Nunnelley, M.A. Burleson, G.G. Fuller, On-line tribology measurements on lubricated rigid disks, IEEE Trans. Magnetics 26 Ž5. Ž1990. 2679–2681. w6x V.J. Novotny, T.E. Karis, N.W. Johnson, Lubricant removal, degradation, and recovery on particulate magnetic recording media, ASME Trans. J. Tribol. 114 Ž1992. 61–67. w7x S.W. Meeks, W.E. Weresin, H. Rosen, Optical surface analysis of the head disk interface of thin film disks, ASME J. Tribology 117 Ž1995. 112–118. w8x TriboScan 3000 Software from Candela Instruments. w9x A.G. Khurshudov, F.E. Talke, A study of sub-ambient pressure tri-pad sliders using acoustic emission, ASME J. Tribol. 120 Ž1998. 54–59.
In situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface Andrei Khurshudov ) , Peter Baumgart, Robert J. Waltman IBM, Storage System DiÕision, 5600 Cottle Road, San Jose, CA 95193, USA
Abstract A thin layer of lubricant is a critical element of the headrdisk interface needed to improve its tribological durability and to prevent media corrosion. Local thinning of lubricant with its subsequent breakdown often leads to the immediate failure of the interface. This paper is devoted to the in situ quantitative analysis of nano-scale lubricant migration on the surface of a thin-film disk using the Optical Surface Analyzer ŽOSA.. The calibration procedure, which enables quantitative measurements, is discussed and the technique’s capabilities are demonstrated using specially prepared samples. Two cases of sliderrdisk interaction are analyzed: low-speed, when the slider is dragged over the disk surface, and high-speed, when the slider is flown over the same track for several days. Lubricant migration phenomena, such as depletion and pooling, are investigated quantitatively to analyze the origination of carbon wear and the mechanisms of interfacial failure. A model of high-speed sliderrdisk interaction involving the dynamic formation of a liquid bridge at the interface is proposed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Lubricant migration; Head–disk interface; Optical surface analyzer
1. Introduction Several different techniques have been used in the past to investigate phenomena on the surface of magnetic rigid disks such as lubricant migration and degradation, carbon wear, and particle generation. These techniques include the Kelvin probe w1x, X-ray photoelectron spectroscopy ŽXPS. w2x, secondary ion mass spectroscopy ŽSIMS. w3x, photonic probe w4x, ellipsometry w5x, and infrared spectroscopy ŽFTIR. w6x. All of these techniques have some limitations such as low speed and resolution, or are ex situ techniques. The Optical Surface Analyzer ŽOSA. was introduced several years ago in an attempt to achieve high-speed, highresolution in situ measurements of thin-film lubricant migration and degradation, as well as carbon thickness changes w7x. This technique was successfully used in studies of lubricant migration and interfacial wear. This paper extends research in the field of head–disk interface tribology using OSA. It concentrates on the role
) Corresponding [email protected]
author.
Fax:
q 1-408-256-2410;
E-mail:
of ‘mobile’ lubricant in providing durability, lubricant pick-up by the air bearing, lubricant migration, and discusses the possibility of a lubricant bridge formation at the head-disk interface.
2. Experimental method and equipment An OSA Žsee Fig. 1. uses P- and S-polarized light to measure thickness changes in both, lubricant and carbon layer of a thin-film disk. Polarized light interacting with the disk surface results in a combination of absorption, reflection, and scattering. The amount of reflected and scattered light is measured using two photodetectors: PrS scattered Žwith an integrating sphere. and PrS specular. OSA w7x is designed in such a way that S- and P-polarized light reflectivity will vary in different ways as a function of thickness of disk lubricant and carbon overcoat. Thinning of the lubricant increases the intensity of reflected S-polarized light, but decreases the intensity of reflected P-polarized light. The opposite is true for an increase of lubricant thickness. Both, S- and P-reflected light intensities increase in the case of carbon film thinning. Wear particle formation or surface roughness increase leads to a
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 8 1 - 0
A. KhurshudoÕ et al.r Wear 225–229 (1999) 690–699
691
Fig. 1. Schematics of the OSA and the disk surface changes, which could be detected with OSA.
decrease in both, S- and P-polarized light specular components due to increased light scattering. This is summarized in Fig. 2 called the 2-D histogram. The data in the first quadrant correspond to the complete wear of the lubricant plus some wear of the carbon overcoat. Data in the second quadrant correspond to lubricant thinning. Wear and contamination particles and roughness increase can be detected in the third quadrant, and lubricant pooling will generate data points in the fourth quadrant. The central part of the 2-D histogram consists of data points, shown as a circle in Fig. 2, with values within the noise level of the system. More distant points mean stronger signals and allow more accurate interpretation. A special software w8x enables trace-back of data-points on the 2-D histogram to their original locations on the S- and P-images and to find the exact location of wear particles or areas of carbon wear and lubricant migration on the disk surface. Data from both S- and P-images are needed simultaneously to interpret the OSA images. OSA allows qualitative in situ monitoring of interfacial changes w7x. However, a quantitative analysis is much more desirable. In order to enable quantitative analysis, the following calibration procedure was used. A set of thin-film disks with both nitrogenated and hydrogenated carbon
Fig. 2. 2-D histogram based on THE data of S- and P-polarized light images and its interpretation. Marked area in the center indicates intensities within the noise level.
coatings was prepared and the disks were half-dipped into a lubricant bath containing perfluoropolyether lubricant ˚ ŽPFPE.. Thus, several steps of lubricant layers up to 20 A thick Žas measured by FTIR. were prepared. In the lubricated part of the disk, additional lubricant was removed in narrow bands by wiping with a cloth, saturated in solvent. The thickness removed was measured with FTIR. The OSA images of the half-lubricated disk surface with two wiped bands are shown in Fig. 3. It can be seen that lubricant thinning Žwiped areas. increases the intensity of S-polarized light and decreases the intensity of P-polarized light. In this study S-polarized reflectivity as a function of lubricant thickness was used to calibrate the OSA signal because S-polarized light is, in general, more sensitive to lubricant thickness changes, while P-polarized light is more sensitive to the carbon thickness changes w7x. In order to investigate tribological changes at the head–disk interface, two types of tests were performed ˚ of using fully lubricated thin-film disks with about 20 A PFPE film. In the first test, low-speed Ž0.6 mrs. dragging of the slider was utilized to prevent formation of an air bearing. Under these conditions, the normal force is known and equal to the suspension pre-load Žabout 40 mN in this case.. This type of testing allows better control of sliderrdisk interaction by eliminating local lubricant or COC damage during occasional high-speed asperity contacts, which exist at nominal drive operating speeds. Drag testing leads to a gradual wearing out of the protective layer of lubricant and carbon overcoat under a known contact force. In this test, the carbon-coated sub-ambient pressure slider with an elongated central pad was used. In the second test, a sub-ambient pressure carbon-coated slider with a central pad near the trailing edge was kept flying over the same track of the disk at 8 mrs for 7 days. Elimination of lateral slider movements Žseek. allows speeding-up interfacial processes Žlube migration, wear, etc.. while keeping the contact conditions Žvelocity, spacing, sliderrdisk interference, contact force, etc.. similar to that in a drive. The contact force between the flying slider and the disk is usually unknown.
692
A. KhurshudoÕ et al.r Wear 225–229 (1999) 690–699
Fig. 3. S- and P-polarized images of the same area of a partially lubricated disk and their correspondent cross-sections. The disk lubricant was partially wiped in radial direction using a solvent Žsee two vertical bands.. The width of the image in radial direction of the disk is 3 mm. The top of the image is closer to the OD Žouter diameter. of the disk. The image corresponds to 3608 scan of the disk.
Both the friction force and acoustic emission ŽAE. signals w9x were continuously monitored during the test. AE signal was high-pass filtered below 600 kHz to obtain information only about the slider body vibrations due to asperity impacts and to eliminate the effect of lowfrequency signals caused by mechanical noise of the sys-
tem, vibrations of the suspension, air bearing vibrations, etc. The OSA was used to take images of the disk surface at predetermined times during the test. In the case of low speed testing, the slider was always lifted from the surface to enable OSA measurements, and loaded again on the
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same track after the measurement. The slider positioning accuracy was about 0.5 um.
3. Results and discussion 3.1. Calibration Fig. 4 presents the results of the OSA sensitivity calibration. In this figure the change in S-polarized light reflectivity is presented as a function of lubricant thickness for both hydrogenated and nitrogenated carbon overcoats. Fig. 4 indicates that there is a linear relationship between S-polarized reflected light intensity and the thickness of the lubricant and that this relationship is independent of the chemistry of the carbon overcoat. With the help of linear data fitting, this relationship could be described analytically as follows S-polarized light reflectivitys K = Lubricant thickness,
Ž 1. where K is the proportionality constant equal in our tests to 0.0022 when the lubricant thickness is given in angstroms. It should be mentioned that this constant K strongly depends on the type of the lubricant and may also vary from one specific equipment to another. 3.2. Low-speed drag testing Fig. 5 presents OSA images of the same area of the disk at different cycle Žrotation. numbers. This specific disk ˚ of partially bonded film of PFPE initially had about 20 A ˚ ˚ free. on the CH x overcoat. lubricant Ž15 A bondedq 5 A
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From the first two images Ž3000 cycles. we observe that ˚. there is already some lubricant pooling Žup to 14 A outside the contact area of the outer rail of the slider Žcloser to the outer diameter of the disk.. Another location ˚ . is observed is the where even larger pooling Žup to 54 A center of the track just under the central rail of the slider. The slider used here has a negative camber Žcross-crown. of about 30 nm. No lubricant was observed on the slider air-bearing surface ŽABS. after the test. Some lubricant was observed on the vertical face side of the slider at the trailing edge. Therefore, these narrow linear zones of accumulated lubricant under the central rail are most likely a result of trailing edgerdisk contacts when the slider was pitching back and forth. This may happened both during the constant speed sliding and during the slider deceleration to a complete stop just before the OSA images were taken. When the slider contacts the lubricated disk with its trailing edge, some lubricant pick-up occurs and this lubricant could be later found on the vertical face side of the slider at the trailing edge. When the slider touches the disk surface again, some part of this lubricant drops on the disk surface resulting in the observed lubricant pooling Žsee Fig. 5.. More importantly, there is some lubricant depletion ˚ . observed in the first two images under the inner Ž; 13 A rail of the slider. When averaged over the entire track, this ˚ but at some lubricant depletion is equal to only about 5 A, ˚ which is more than half of local spots it is as high as 13 A, the total lubricant thickness. The second set of images, taken at 40,000 cycles, shows approximately the same picture, but the scale of observed phenomena is larger. The lubricant pooling is clearly in˚ at creasing from 3000 cycles and reaches up to 90 A 40,000 cycles. The location of this highest lubricant pool is approximately the same. The lubricant depletion was also
Fig. 4. Calibration results: S-polarized light reflectivity vs. lubricant thickness for CH x and CN x overcoats.
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Fig. 5. S- and P-polarized light images of a disk surface after 3, 40 and 43 K cycles of low-speed drag testing. Lubricant pooling, depletion, and carbon wear are observed. The width of the images is 3 mm, the width of the slider is about 1.6 mm. Sliding direction: from right to left. The top of the picture is closer to the OD of the disk.
continuing under the same inner rail with the maximum ˚ . also at exactly the same location lube depletion Ž; 20 A as at 3000 cycles. Lubricant pooling under the central rail is less repeatable since the slider could also smear the drops of lubricant on the disk surface. The final two images in Fig. 5 show interfacial failure with carbon wear and particle generation under the inner
rail of the slider at 43,000 cycles. It is worth noticing that interfacial failure occurred under the same rail where lubricant depletion was previously detected. Another thing to notice is that some wear particles can be found as far as 1–1.7 mm away from the wear tracks closer to the outer edge of the disk. The test was performed at relatively low 300 rpm. Still, the centrifugal force was high enough to
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˚ of lubricant Ž15 A˚ bondedq 5 A˚ free. on CN x . Fig. 6. COF, AE, lube pooling and depletion vs. drag cycles for the disk with initially 20 A
move some of these large particles away from the wear track. Fig. 6 shows changes in the AE intensity, the coefficient of friction ŽCOF., averaged over the entire disk track lubricant pooling and depletion values Žat the locations discussed above. as a function of drag cycles.
The first lubricant depletion value at 3000 cycles is ˚ which is equal to the thickness of the free about 5 A, ˚ of free lubricant. As can be seen in Fig. 6, about 5 A lubricant were completely removed during first 3000–5000 cycles. Lubricant depletion varies somewhat around the ˚ It can also disk track with an average value of about 5 A.
Fig. 7. S- and P-polarized light images of the disk surface after 16,000 cycles of low-speed drag testing. Lubricant depletion is observed. The width of the images is 3 mm, the width of the slider is about 1.6 mm. Sliding direction: from right to left. The top of the picture is closer to the OD of the disk.
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be seen from Fig. 6, that the AE rms signal was initially high and fluctuating, but decreased to its lowest values at approximately the same time as the free lubricant was removed. It is not quite clear how closely these two processes are related, but it is possible that free lubricant stayed at the interface for just long enough to assist the initial interfacial burnishing. After the initial quick lubricant removal, the depletion was almost a linear function of the drag cycles resulting in the continuous thinning of the lubricant. When the average depletion reached about 75% of the initial lubricant thickness, the lubricant film started to break down locally, causing rapid interfacial failure Žbetween 40,000 and 43,000 cycles., which was also accompanied by an increased AE signal and a decrease in the COF. The AE
intensity increase was caused by a direct interaction between the slider and disk surfaces at the spots of lubricant break-down. The AE is, in general, more sensitive to the interfacial changes w9x than friction. The decrease in COF could be observed about 3000–5000 cycles after the first lubricant breakdown Žas measured by AE at ; 39,000 cycles and by OSA at ; 40,000 cycles., and was caused by formation of a large number of wear particles on the disk surface. These particles increased sliderrdisk separation and quickly decreased both apparent and real contact areas between the disk and the slider thus decreasing stiction and friction forces. Lubricant pooling saturates with the number of drag cycles. Friction was clearly insensitive to lubricant migration on the disk surface and was, probably, controlled by
Fig. 8. S- and P-polarized light images of the disk surface after 5 and 120 h of on-track flying at 8 mrs using sub-ambient pressure slider. Lubricant pooling is observed. The width of the images is 3 mm, the width of the slider is about 1.1 mm. Slider flying direction: from right to left. The top of the picture is closer to the OD of the disk.
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˚ of lubricant Ž15 A˚ Fig. 9. COF, AE, lube pooling vs. time for on-track test with sub-ambient pressure slider flying over the disk with initially 20 A ˚ free. on CN x . bondedq 5 A
much stronger adhesive forces caused by the liquid meniscus formed at the sliderrdisk contacts. It is possible, that the observed local lubricant pools also caused some lubricant depletion at other locations without changing the
resultant meniscus force. The total test duration before failure was about 43,000 cycles, but the location of later lubricant breakdown was already observed during the first OSA measurement at 3000 cycles. This shows not only the
˚ of lubricant Fig. 10. COF vs. fitted lubricant thickness increase for on-track test with sub-ambient pressure slider flying over the disk with initially 20 A ˚ bondedq 5 A˚ free. on CN x . Ž15 A
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sensitivity of OSA, but also that the traces of interfacial failure can be found at very early stages of sliderrdisk interaction. Fig. 7 shows images of a similar disk with fully bonded ˚ of the same lubricant after 16,000 drag cycles—just 20 A before this interface failed at about 18,000 cycles. From this figure we observe lubricant depletion only, and no lubricant pooling. The only difference between the disks in ˚ of free lubricant on top Figs. 5 and 7 is the presence of 5 A of the bonded layer in the former case. It shows that some mobility of the lubricant is needed to provide higher durability since only a mobile lubricant is capable of curing damage to the bonded part. Free lubricant is less capable of carrying the slider load and is easily displaced from the sliderrdisk contacts. But, even when it was displaced from the contact area at the early stages of the test, it was still capable of migrating back onto the damaged spots and delaying interfacial failure under given experimental conditions. If the rate of lubricant displacement is equalized by the rate of its backward migration, then an interface with practically unlimited life could be obtained.
this phenomenon. For example, an increasing bridge should increase the meniscus force, which will increase the friction force. If the bridge size is constant, then the friction should stay constant. Fig. 9 presents the acoustic emission rms signal, the COF and the height of the observed lubricant pool as measured in situ by OSA. In order to minimize the scatter, the data-points for the pool height were fitted with a second order polynomial. The lubricant thickness was saturating with time. The AE signal saturated soon after the beginning of the test. Friction was decreasing until about 20 h into the test due to interfacial burnishing. Starting from about 20 h, friction was increasing and stabilized after about 90 h of testing. It is worth mentioning that the AE signal does not generally correlate with frictional because they originate from different frequency ranges. The AE signal is sensitive to high frequency phenomena. Measured friction force contains low frequency signal. Both these signals may correlate if a strong low-frequency process causes both of them. If the lowfrequency component of the AE signal is strong enough, it may saturate the high-pass filter and become detected.
3.3. High-speed on-track testing Fig. 8 shows OSA images of the same area of the disk as Fig. 7 at different time of testing when a sub-ambient pressure slider with a central pad near the trailing edge ˚ of PFPE Ž15 was flying over the disk lubricated with 20 A ˚A bondedq 5 A˚ free. for 160 h. It is a known fact that sub-ambient pressure sliders often show some lubricant pick-up on the ABS surface or in the ABS cavity after testing. Fig. 8 presents in situ evidence of the air bearinginduced suction action on the lubricant film. The lubricant ˚ high on the disk surface after 5 h of testing. pooling is 2 A The continuing suction by the air bearing results in formation of a continuous pool of lubricant along the track as ˚ after 120 h. This is direct evidence that high as about 20 A the air bearing suction is sufficiently strong not only to move the lubricant laterally, but also move it vertically up to several nanometers. Finally, some lubricant molecules get separated from the disk and sucked into the negative pressure pockets on the slider surface. Another phenomenon we would like to introduce in this paper is the formation of a continuous lubricant bridge between the disk and the slider flying at high speed. By the term ‘continuous’ we mean the bridge, which exists for all or most of the time needed for the slider to complete a revolution. A continuous vertical bridge is most likely a series of random events occurring at high frequency between the different points of the slider and the disk. We used in situ measurements of the lubricant film thickness with OSA and correlated them with the measured COF. The reasoning in this case is that if a liquid bridge exists at some point of the test, then the adhesive force, and therefore, the friction force should correlate with the scale of
Fig. 11. Model of interaction between the lubricated disk and low-flying sub-ambient pressure slider at the beginning of the test Ža., when the disk burnishing just starts, and after the burnishing is completed and the liquid bridge is fully established Žb..
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Starting from about 50 h in Fig. 9, there are several instances when frictional and AE signals respond in the same way: drop and increase together. This means that there is a common low-frequency Žof the order of a few kHz. interfacial process causing both of them. Introducing a continuous lubricant bridge between the slider and the disk formed between 20 and 50 h of testing, it is possible to explain these synchronized changes in both signals. If there is a liquid bridge at the sliderrdisk interface, then the contact force and, therefore, friction are controlled by the meniscus force. If the bridge breaks down temporarily then both, meniscus force and friction will drop. Since the AE signal amplitude is a strong function of the contact force, it should stay high when the liquid bridge is intact and drop in case of its breakdown, following the friction. This is exactly what we observe in the test shown in Fig. 9. Fig. 10 presents the correlation of the fitted Žsee Fig. 9. lubricant thickness increase values from Fig. 9 with the COF measured at the time when the OSA image was taken for the part of test when the liquid bridge was presumably established Žafter 20 h of testing.. A fairly good linear correlation can be observed. It supports the previous hypothesis that a lubricant bridge formed between the slider and the disk beginning from about 20 h of testing controls the magnitude of friction. Fig. 11 shows a possible model of the sliderrdisk interface evolution in the test discussed above. Before the burnishing is completed and while lubricant pooling is insignificant Žsee Fig. 11a., the contact force at the interface depends on disk topography, slider flying height, air-bearing characteristics, and the suspension pre-load. After the liquid bridge is established, the contact load becomes dependent on the force of the meniscus formed at the interface.
4. Conclusions 1. The technique of the in situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface was demonstrated. It was shown that this technique is highly sensitive to angstrom level interfacial changes. OSA allows not only to investigate in great detail critical interfa-
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cial phenomena such as lubricant pooling and depletion, carbon wear and formation of wear particles, but also allows to observe directly the pooling of lubricant due to the sub-ambient pressure air-bearing. 2. The process of lubricant pooling, depletion, and its final breakdown at the head–disk interface was investigated using OSA during low-speed drag tests. It was shown that the free part of the lubricant was almost completely displaced during the first 7–15% of the total test time to failure. In spite of this, displaced lubricant was still working at the interface via the migration mechanisms, where it was curing damage to the bonded part of the lubricant layer. In the absence of lubricant pooling in the case of fully bonded lubricant, much shorter durability of the interface was observed. 3. Using in situ OSA measurement of lubricant thickness changes and their correlation with friction and AE signals, a model of dynamic liquid bridge formation between the disk and a sub-ambient pressure slider was proposed. A mechanism of the lubricant pick-up by the slider ABS surface was introduced. References w1x S. Yee, M. Stratmann, R.A. Oriani, Application of a Kelvin microprobe to the corrosion of metals in humid atmosphere, J. Electrochem. Soc. 138 Ž1991. 55. w2x M. Mate, V.J. Novotny, Molecular conformation and disjoining pressure of polymeric liquid films, J. Chem. Phys. 94 Ž1991. 8420. w3x A. Benninghoven, F.G. Rudenauer, H.W. Werner, Secondary Ion Mass Spectroscopy, Wiley, New York, 1987. w4x D. Jen, D. Gillis, Optical profiler for thin film disk, Adv. Inf. Storage Sys. 4 Ž1992. 219–230. w5x L.L. Nunnelley, M.A. Burleson, G.G. Fuller, On-line tribology measurements on lubricated rigid disks, IEEE Trans. Magnetics 26 Ž5. Ž1990. 2679–2681. w6x V.J. Novotny, T.E. Karis, N.W. Johnson, Lubricant removal, degradation, and recovery on particulate magnetic recording media, ASME Trans. J. Tribol. 114 Ž1992. 61–67. w7x S.W. Meeks, W.E. Weresin, H. Rosen, Optical surface analysis of the head disk interface of thin film disks, ASME J. Tribology 117 Ž1995. 112–118. w8x TriboScan 3000 Software from Candela Instruments. w9x A.G. Khurshudov, F.E. Talke, A study of sub-ambient pressure tri-pad sliders using acoustic emission, ASME J. Tribol. 120 Ž1998. 54–59.