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Hindawi Publishing Corporation
Advances in Tribology
Volume 2012, Article ID 303071, 11 pages
doi:10.1155/2012/303071
Research Article
Parametric Investigations at the Head-Disk Interface of
Thermal Fly-Height Control Sliders in Contact
Sripathi V. Canchi,1David B. Bogy,1Run-Han Wang,2and Aravind N. Murthy2
1Computer Mechanics Laboratory, Mechanical Engineering, University of California, Berkeley, CA 94720, USA
2HGST, a Western Digital Company, San Jose, CA 95135, USA
Correspondence should be addressed to Sripathi V. Canchi, sripathi.canchi@cal.berkeley.edu
Received 3 August 2012; Accepted 12 November 2012
Academic Editor: Bo Liu
Copyright © 2012 Sripathi V. Canchi et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
Accurate touchdown power detection is a prerequisite for read-write head-to-disk spacing calibration and control in current hard
disk drives, which use the thermal fly-height control slider technology. The slider air bearing surface and head gimbal assembly
design have a significant influence on the touchdown behavior, and this paper reports experimental findings to help understand
the touchdown process. The dominant modes/frequencies of excitation at touchdown can be significantly different leading to very
different touchdown signatures. The pressure under the slider at touchdown and hence the thermal fly-height control efficiency as
well as the propensity for lubricant pickup show correlation with touchdown behavior which may be used as metrics for designing
sliders with good touchdown behavior. Experiments are devised to measure friction at the head-disk interface of a thermal fly-
height control slider actuated into contact. Parametric investigations on the effect of disk roughness, disk lubricant parameters,
and air bearing surface design on the friction at the head-disk interface and slider burnishing/wear are conducted and reported.
1. Introduction
In order to realize higher magnetic storage densities in
hard disk drives, it is necessary to reduce and control
the read-write head-to-media spacing, or, equivalently, the
physical spacing/clearance separating the head from the
disk. Current hard disk drive (HDD) products operate
with subnanometer clearances using the thermal fly-height
control (TFC) technology, where the TFC heater locally
deforms the region around the read-write head of the slider
bringing it closer to the disk. The head-to-disk clearance can
therefore be adjusted by changing the power supplied to the
TFC heater.
A touchdown test is used to calibrate the TFC heater
power to the clearance. The heater power required to make
the head contact the disk lubricant surface, that is, the touch-
down power (TDP), is first determined, and the heater power
is then reduced to retract the head away from the disk in
order to achieve a target clearance. Accurate TDP detection
is therefore a key enabling step to using the TFC technology.
Inaccurate TDP detection can severely compromise the drive
performance: if the actual clearance is too high, the recording
performance suffers, and if the actual clearance is too low, it
increases the probability for head-disk contact, which leads
to unwanted head wear, thus compromising drive reliability.
Research studies on the slider-disk contact interactions
and the resulting slider vibrations have been of interest to the
HDD community for a long time. Research investigations on
the traditional (non-TFC) slider dynamics were motivated
by the need to design low flying sliders while mitigating the
contact-induced slider vibrations and slider-lubricant inter-
actions, and extensive literature exists on these topics [1–8].
After the introduction of TFC sliders, several researchers have
tried to understand and explain the contact and touchdown
behavior of TFC sliders owing to its importance in HDD
spacing calibration. It has been shown through experiments
and simulation that in addition to the slider air bearing
surface (ABS) design the disk lubricant and suspension
design play an important role in the slider touchdown
process [9]. Numerical and analytical simulations accounting
for the nonlinear forces at the head-disk interface (HDI) have
also successfully explained the strong vibration dynamics of
2Advances in Tribology
the slider close to the TDP and the subsequent suppression
of vibrations for powers higher than the TDP [10–12], and
they qualitatively support the experimental results reported
for certain ABS designs [12,13]. A full understanding of the
touchdown behavior of TFC sliders is still lacking, and it
continues to be an active topic of research.
In this work, acoustic emission (AE) sensors and laser
doppler vibrometers (LDVs) are used in spin stand experi-
ments focused on touchdown detection and slider dynamics
studies. The similarities and differences in the touchdown
signature for different slider ABS designs are highlighted.
Parametric investigations are conducted to explain the
dependence of friction and head wear on disk roughness and
lubrication condition. In conjunction with recent work on
the interactions of TFC sliders with the disk lubricant during
touchdown/contact [14,15], these results help develop a
better understanding of the touchdown process of TFC
sliders and the complex interactions at the HDI.
2. Experiments
Experiments are conducted on a spin stand equipped with
an AE sensor to detect contact and an LDV to detect vertical
flexure motions of the head gimbal assembly. Three different
ABS designs mounted on the same suspension are used in
this study. The TDP (i.e., power required to achieve zero
clearance or contact with the disk lubricant) is determined
experimentally by supplying the TFC heater with a square
pulse lasting 70 ms with increasing power. The AE signal
standard deviation is monitored during each power pulse,
and the power at which the AE signal standard deviation
crosses a specified threshold (set to be 20% above baseline) is
recorded as the TDP.
3. Results and Discussion
3.1. Touchdown Behavior/Characteristics. The touchdown
plots for three different ABS designs are shown in Figure 1
where ABS-3 shows a favorable “sharp touchdown” while
ABS-1 shows an unfavorable “gradual touchdown,” and has
a slow rise in AE signal for increasing TFC power. ABS-2
has touchdown performance which falls between ABS-1 and
ABS-3. A sharp touchdown behavior is preferred as it gives
a well-defined estimate of the exact power at which contact
with the disk lubricant is achieved.
The sharp touchdown for ABS-3 is characterized by
strong individual spikes in the time history of the AE signal
while the gradual touchdown for ABS-1 shows a uniformly
increased AE signal during the TFC pulse as shown in
Figure 2. (The AE signal on these plots have been shifted by
1 V to show them clearly.)
In addition, tests in overpush (i.e., TFC power above the
TDP) reveal that ABS-3 with “sharp touchdown” shows an
“overshoot behavior” with very strong AE signal at powers
slightly above the TDP and a subsequent suppression of AE
signal when the power increases into overpush as seen in
Figure 3. In contrast, ABS-1 with the “gradual touchdown”
shows no “overshoot” behavior but a gradual increase in
0 20 40 60 80 100 120
0.8
1
1.2
1.4
1.6
1.8
2
2.2
TFC power (mW)
ABS 1
ABS 2
ABS 3
Normalized AE σ
Figure 1: Touchdown plots for the three different ABS designs.
Figure 2: Touchdown signature for the three different ABS designs
(plots offset by 1 V for clarity).
AE detected contact with overpush. ABS-2 has behavior in
between those of ABS-3 and ABS-1.
Simulations for these three ABS designs show that
the increasing sharpness of touchdown correlates with
decreasing pressure under the TFC protrusion at touchdown,
increased TFC efficiency, and lower TDP (Ta b l e 1 ). It is also
observed that lubricant pickup is higher for ABS-3 compared
to ABS-2 or ABS-1.
While an explanation of the physical mechanism behind
the different ABS touchdown signatures requires a further
comprehensive study, important correlations may be drawn
from these results. The strong individual spikes for ABS-
3 are evidence of a snap-in/snap-out behavior, which is
an indication of the spontaneous instability of the slider
Advances in Tribology 3
Figure 3: Contact behavior in overpush for the three different ABS
designs.
Tab le 1: Simulated results for the three different ABS designs.
ABS design
Simulation TFC
efficiency
nm/mW
Touchdown power
mW
Pressure at
touchdown
atm.
ABS-1 0.108 96 60
ABS-2 0.119 91 38
ABS-3 0.145 69 27
during touchdown. It is highlighted that the circumferential
locations of the spikes do not remain fixed upon repetition of
the touchdown test and hence are not caused by disk defects
as such. The snap-in/snap-out process causes a higher level
of head-disk lubricant interaction, which correlates well with
the higher amount of lubricant pickup for ABS-3. It may
also be reasoned that the lower pressure at the TFC bulge for
ABS-3 makes it more susceptible for lubricant transfer from
disk to the slider in comparison to ABS designs with higher
pressure at the TFC bulge, such as ABS-1 and ABS-2.
3.2. Analogous Experimental Results. A separate set of exper-
iments with ABS-2 reveals that the touchdown can be sharp
or gradual depending on the disk RPM (or the linear
velocity) as shown in Figure 4. Specifically, a lower disk RPM
increases the touchdown sharpness and a higher disk RPM
degrades the touchdown sharpness. It is also observed that
the touchdown performance degrades for a burnished slider
(as shown by the dashed lines in Figure 4), where the slider
is burnished in a controlled fashion by increasing the TFC
power above the TDP on a separate disk track. The results
for the 7200 RPM case on Figure 4 highlight the possibility
of false TDP detection: for the same 20% AE threshold,
the unburnished case shows a gradual AE rise until about
95 mW, but the sharp AE rise at 102 mW is the actual TDP.
However, the burnished case reads a false TDP at 93 mW
owing to the gradual rise in AE that occurs before the sharp
AE rise marking touchdown. The time history of the AE
signal is similar to that observed with the different ABS
designs; namely, strong individual AE spikes appear for the
3600 RPM case with sharp touchdown, and a uniformly
increased AE signal appears for the 7200 RPM case with
gradual touchdown (Figure 5). These analogous results for
ABS-2 provide a way to probe the same HDI under different
disk RPM to understand the changes that occur in the AE and
LDV signals for “sharp” and “gradual” touchdown signatures
as well as in overpush.
3.3. LDV Spectrum and AE Signal Content. Experiments are
conducted with ABS-2 to simultaneously capture the AE
signal and the LDV signal in order to identify the frequencies
that correspond to the flexure and slider vertical motions
and to see how they appear in the AE signal. The tests are
conducted with the power increased above the TDP (i.e., into
overpush). For this ABS-2 design, the simulated air bearing
frequencies at 5400 RPM and 1 nm minimum spacing are
142 kHz (roll), 167 kHz (pitch-1), and 324 kHz (pitch-2).
For each of the overpush tests conducted at different
RPM, Figure 6 shows the profile of the applied TFC voltage
and the time history of the LDV signal on one plot, and the
corresponding joint time frequency spectrum of the LDV
signal on a separate plot. As seen on the figures, the TFC
voltage increases from zero to the target overpush value
(between 50 ms to 100 ms), remains at this value (between
100 ms to 200 ms), and decreases back to zero (between
200 ms to 250 ms). During the overpush regime, the slider
vibrations are excited due to contact, and the dominant
excitation frequencies are identified on the plots showing
the joint time frequency spectrum. It is observed that for
3600 RPM the lower frequencies (notably 139, 148, and
165 kHz which are close to simulated roll and pitch-1 air
bearing frequencies) are dominant, while for the 7200 RPM
the higher air bearing frequency 321 kHz (corresponding
to pitch-2 air bearing frequency) is dominant. This result
indicates that the mechanism/nature of touchdown and
contact at the HDI is significantly altered by the disk RPM.
Specifically in this case, the excited vibration modes at
contact are shifted from the lower frequency pitch-1 and roll
modes at lower disk RPM, to the higher frequency pitch-2
modes at higher disk RPM. The pitch-1 nodal line passes
through the trailing end of the slider, and its excitation in
general is associated with stronger motions of the entire
slider body. Such excitation may, in fact, be responsible for
the stronger “sharp touchdown” signature. In contrast, the
pitch-2 nodal line lies closer to the leading end of the slider,
and its excitation only causes the vertical bouncing motion of
the trailing end, resulting only in localized contact at the TFC
bulge location, and, therefore, leading to the weaker “gradual
touchdown” signature. It is surmised that, in an analogous
fashion, the touchdown process excites different vibrations
modes for the three different ABS designs presented in
Section 3.1, hence causing the very different sensitivities at
touchdown.
4Advances in Tribology
Figure 4: Touchdown plots for ABS-2 at different disk RPM
(dashed lines for a burnished slider).
0 10203040506070
0
0.5
1
1.5
2
2.5
Time (ms)
AE signal (V)
−1
−0.5
3600 RPM
5400 RPM 7200 RPM
Figure 5: Touchdown signature for ABS-2 at different disk RPM
(plots offset by 1 V for clarity).
The components of the AE signal at the different frequen-
cies observed in the LDV signal are plotted in Figure 7 to
observe how they change as a function of the TFC power.
(The cumulative effect of adding all these components would
result in the plot shown in Figure 4.) At 3600 RPM the
touchdown is marked by the sharp rise in the 148 kHz
component and there are no components that show gradual
rise. At 5400 RPM, the 148 kHz component shows a gradual
rise, but touchdown is marked by a sharp rise in the 321 kHz
component. At 7200 RPM, there are no components with a
sharp rise, and the 321 kHz component shows a gradual rise.
These results indicate that at 3600 RPM the contact
is dominated by the slider’s pitch-1 and roll motions
(together with any suspension-related motions that give rise
to frequency peaks in the 65–100 kHz region.) At 7200 RPM,
contact is mainly dominated by the vibration of the slider
at the pitch-2 frequency. At 5400 RPM, the interaction is a
combination of the above two modes: as the TFC powers
increase, a gradual rise in the 148 kHz component occurs
first, but a strong vibration in the pitch-2 mode (321 kHz)
eventually marks touchdown.
These results are in agreement with recent studies that
show that at close spacing and at the onset of lubricant-
contact, the in-plane shear forces and friction can destabilize
the slider for certain ABS designs resulting in vibrations
dominantly occurring at suspension and lower air bearing
frequencies (60–200 kHz in our case), while stronger contact
with the disk causes slider vibrations with higher frequency
content (above 200 kHz) [9].
3.4. Friction Measurements in Contact. Friction forces at
the HDI become important during contact conditions and
may in fact play a dominant role in HDI performance and
slider dynamics. Friction-induced slider wear, as well as disk
lubricant redistribution and disk overcoat damage, needs
to be examined carefully to explore future designs that can
accommodate a certain level of head-disk contact.
Experiments are devised to measure the friction forces
in the downtrack direction during contact and overpush
conditions by instrumenting a strain gage on the fixture
which holds the head gimbal assembly on the spin stand.
The voltage signal from the strain gage may be converted
into force measurement by determining the calibration
constant for the strain gage. Such a calibration is performed
using the usual technique: by noting the strain gage voltage
signal corresponding to different standard forces, which are
applied by suspending standard weights from the fixture, and
subsequently fitting a linear curve.
Once the TDP is determined on the test track, the TFC
is powered with a voltage profile having 100ms dwell time
at the maximum power. It is noted that strain gages have a
low bandwidth, and several experiments reveal that a dwell
time of at least 100 ms is necessary to allow the strain gage
to respond to the friction force and give good, repeatable
measurements. All experiments are conducted with ABS-2
on a reference “standard disk” unless specified otherwise.
3.4.1. Friction, AE, and Slider Bouncing in Contact. Figures
8(a) and 8(b) show the TFC voltage profile and the resulting
slider bouncing (displacement and velocity), AE detected
contact, and the friction force, for 10 mW and 20 mW
overpush, respectively. It is evident that slider bouncing
and AE signal remain high throughout the overpush region
for 10 mW overpush case, and they get suppressed (after
an initial overshoot region) for the 20 mW overpush case.
The friction force measured by the strain gage, however,
continues to increase with the amount of overpush indicating
a higher level of interference and contact for larger overpush
powers even though the AE detected contact and slider
Advances in Tribology 5
(a) (b)
(c)
Figure 6: Applied TFC voltage profile, vertical velocity time history, and the joint time frequency spectrum of the vertical velocity for ABS-2
at different disk RPM.
dynamics get suppressed. (It is noted that the amplitude of
the AE signal in the suppressed state is noticeably higher than
the baseline AE signal with no TFC power, implying a certain
amount of contact).
3.4.2. Effect of Disk Roughness. Disk roughness plays an
important role in HDI performance. The combined slider
and disk roughness affect the nominal physical spacing at
the HDI, the magnitude of interaction forces (intermolec-
ular/adhesive, etc.), and the actual area of contact, thereby
influencing the magnitude of contact and friction forces. A
parametric study is conducted with three disk types: disks A,
B, and C with decreasing roughness, in that order, and with
surface roughness parameters tabulated in Table 2 ,whereRq
is the root mean square roughness, Rpis the maximum peak
height, and Rvis the maximum valley depth. These disks are
coated with ZTMD lubricant of nominal 12 ˚
A thickness.
First, several tests are conducted using the same slider
to determine the TDP on a standard disk and on each
of the disks A, B, and C. Tabl e 2 presents the change in
the TDP (i.e., δTDP) on each of the disks A, B, and C
compared to the TDP on a standard disk. The roughness
of the standard disk is similar to that of disk A. This
difference in TDP is converted into a clearance gain value
(i.e. a gain in clearance from that on a standard disk) using
a conversion factor of 0.119nm/mW, which is the TFC
efficiency estimated for ABS-2 from simulations. Figure 9
shows the same information in graphical form and highlights
the linear relationship between the disk roughness (Rp
or Rq) and clearance gain. Since the thermal protrusion
comes into contact with the peaks of the roughness, the
relationship between the clearance gain and Rp(Figure 9(b))
is of importance, and it is seen that for every 1 nm decrease
in Rpthere is a 0.8 nm actual gain in clearance at the HDI
for the range of surface roughness values considered in these
experiments.
Next, the dependence of friction on the disk roughness
is investigated by conducting a “friction test” on each of
the three disk types. A new (unburnished) slider is flown
on a fresh test track, the TDP is determined, and the TFC
heater is then supplied with the power profile with a 100 ms
6Advances in Tribology
0 102030405060
0.8
1
1.2
1.4
1.6
1.8
2
TFC power (mW)
AE component (a.u.)
65 kHz
74 kHz
78 kHz
88 kHz
98 kHz
139 kHz
148 kHz
165 kHz
321 kHz
345 kHz
3600 RPM
(a)
0204060
80
TFC power (mW)
0.8
1
1.2
1.4
1.6
1.8
2
AE component (a.u.)
65 kHz
74 kHz
78 kHz
88 kHz
98 kHz
139 kHz
148 kHz
165 kHz
321 kHz
345 kHz
5400 RPM
(b)
0 20406080100
TFC power (mW)
0.8
1
1.2
1.4
1.6
1.8
2
AE component (a.u.)
65 kHz
74 kHz
78 kHz
88 kHz
98 kHz
139 kHz
148 kHz
165 kHz
321 kHz
345 kHz
7200 RPM
(c)
Figure 7: AE signal components in the touchdown plot for ABS-2 at different disk RPM.
dwell time (as shown in Figure 8). The peak TFC power is
increased from TDP to a maximum of TDP + 50 mW in
5 mW increments, and it is then similarly decreased back
to TDP. The average friction measured by the strain gage at
each power step is tabulated. All tests are conducted on the
same disk track. The measured friction values are plotted as
those for the “unburnished” case. The same slider, which is
now deemed “burnished” because of the overpush testing, is
flown on an adjacent track and the friction test is repeated to
obtain friction values for the “burnished” case. Figure 10(a)
shows a representative plot for the strain gage measured
friction values as a function of overpush power supplied to
the TFC heater.
A quadratic curve passing through the origin is fit
to the friction measurements for the “unburnished” and
“burnished” cases, and the slope of this curve at the 10 mW
overpush point is used to obtain the “friction (μN) per
milliwatt of overpush power” value. Figure 10(b) plots these
friction values measured on the three disk types (A, B, and
C) based on experiments conducted with three new sliders
Advances in Tribology 7
Tab le 2: Disk roughness parameters and its effect on TDP/clearance
gain.
Disk Rp
nm
Rq
nm
Rv
nm
δTDP
mW
Clearance gain
nm
A2.02 0.49 1.87 0.08 0.01
B1.89 0.36 1.47 0.97 0.12
C1.00 0.24 1.11 6.71 0.84
on each disk type. While there is no particular trend relating
the measured friction and surface roughness, the friction is
higher for the burnished slider compared to the unburnished
slider in all tests. Slider burnishing wears and smoothens
the slider surface, increasing the actual contact area between
the thermal protrusion and the disk, thereby resulting in the
slightly higher friction force.
In order to directly compare the friction values between
the three disk types, another set of experiments is conducted
by flying the same “burnished” slider on the three disk
types in succession. The slider is burnished in a controlled
fashion separately before use in this test. The friction against
the overpush power is plotted in Figure 10(c) using data
from two “burnished” heads. It is concluded based on these
results that within the range of disk roughness considered
in this work there is no significant effect of the disk surface
roughness on the measured friction.
3.4.3. Effect of Lubricant Parameters. Friction tests are con-
ducted to determine the effect of lubricant type/bonding on
the friction in contact. Disks with three different lubricant
type/bonding ratios are used: Lube A (61% bonded ratio,
10.5 ˚
A), Lube A (69% bonded, 10.5 ˚
A),andLubeB(82%
bonded, 12 ˚
A). Figure 11(a) shows the friction measured for
the three media for the “unburnished” and “burnished”
slider cases (based on three experiments each). The friction
values are comparable for the unburnished sliders on all
three disks types. While the friction values for the burnished
and unburnished sliders are comparable on the disks with
Lube A 61% and 69% bonded ratio, the friction for the
burnished slider on the disk with Lube B 82% bonded
ratio is relatively higher than that for the unburnished
slider. This result is consistent with results for the change
in TDP occurring because of a friction test, where the TDP
change after and before a friction test is a measure of slider
burnishing. As shown in Figure 11(b), the highest burnishing
(indicated by highest δTDP) occurs to a new “unburnished”
slider on the Lube B 82% bonded disk. As a result of greater
slider burnishing on this disk, the friction is higher when a
subsequent test is conducted with this burnished slider.
A direct comparison of friction values is reported in
Figure 11(c) based on tests conducted in succession on the
three different disks using two “burnished” sliders, and it
shows marginally higher friction values on the disk with Lube
B 82% bonded ratio.
Friction tests are conducted to understand the effect of
the mobile part of the lubricant on friction and slider bur-
nishing. The disk with Lube A 10.5 ˚
A 61% bonded fraction is
delubed by immersing it in a solution of Vertrel XF solution
Tab le 3: EffectofTFCefficiency (ABS/heater design) on friction.
ABS design
Simulation TFC
efficiency
nm/mW
Friction
unburnished
μN/mW
Friction
burnished
μN/mW
ABS-A (ABS-1) 0.108 9 17
ABS-B 0.111 24 31
ABS-C (ABS-2) 0.119 37 54
ABS-D (ABS-3) 0.145 56 67
to remove the mobile lubricant. The delubed disk has a
lubricant thickness of 6 ˚
A (bonded lubricant). Figure 12(a)
shows the measured friction on the lubed and delubed
disks for the unburnished and burnished slider cases. The
friction values for the unburnished as well as burnished
slider on the lubed disk are similar and comparable to the
friction value measured for the unburnished slider on the
delubed disk. However, the friction is substantially higher
for the burnished slider on the delubed disk. Figure 12(b)
shows that slider burnishing (indicated by δTDP after a
friction test) is higher for tests conducted on the delubed
disk implying that an unburnished slider is substantially
burnished on this disk type, and the friction is higher for
the subsequent test conducted with such a burnished slider.
These results highlight the important role of the mobile part
of the lubricant in reducing friction and slider burnishing,
thereby increasing the reliability of an HDI with contact.
3.4.4. Effect of TFC Efficiency. The thermal protrusion size
and shape make a significant difference in the slider’s touch-
down and contact behavior. The friction during contact for
different slider ABS/heater designs is plotted in Figure 13
for the unburnished and burnished cases, and the same
data is tabulated in Tabl e 3 togetherwitheachdesign’sTFC
efficiency estimated from simulations. It is observed that the
friction forces increase as the TFC efficiency increases and
may be explained by the following argument. For the same
amount of overpush power, a higher TFC efficiency slider
will have greater level of interference (because of a larger
protrusion). As a result, the effective contact area is larger
for the higher TFC efficiency case, reflecting in a higher
measured friction.
3.4.5. Effect of Disk RPM. The similarities between the
touchdown plot and contact signature of ABS-2 at different
disk RPMs to those of ABS designs with different TFC
efficiencies are highlighted in Section 3.2. Particularly, it is
shown that at a higher RPM ABS-2 behaves like a design with
low TFC efficiency (showing a gradual touchdown plot), and
at a lower RPM ABS-2 behaves like a design with high TFC
efficiency (showing a sharp touchdown plot).
The friction results from tests with ABS-2 at different
RPM are consistent with the above analogy and with the
results presented in Section 3.4.4.Figure 14 shows that the
friction increases as the disk RPM decreases; that is, when
ABS-2ismadetobehavelikeasliderwithhighTFC
8Advances in Tribology
0 100 200 300
0
1
2
3
4
5
Time (ms)
Magnitude
−1
Disp. σ(nm)
AE σ(V)
Vel. σ(mm/s)
Fric. (mN)
TFC (V)
(a)
0 100 200 300
0
1
2
3
4
5
Time (ms)
Magnitude
−1
Disp. σ(nm)
AE σ(V)
Vel. σ(mm/s)
Fric. (mN)
TFC (V)
(b)
Figure 8: Time history of TFC power, vertical displacement, vertical velocity, AE signal, and friction. (a) 10mW overpush, (b) 20 mW
overpush.
0
1
2
3
4
5
6
7
8
ABC
Disk
δTDP (mW) referenced to standard disk
(a)
0
0.2
0.4
0.6
0.8
1
0.75 1 1.25 1.5 1.75 2 2.25
Rp (nm)
y=−0.8098x+1.6487
Clearance gain (nm) referenced
to standard disk
−0.2
(b)
y=−
3.2784x+1.5145
−0.2
0
0.2
0.4
0.6
0.8
1
y
=−
3
.
278
4
x
+1
.
51
4
5
0.25 0.3 0.35 0.4 0.45 0.5 0.55
Rq (nm)
Clearance gain (nm) referenced
to standard disk
(c)
Figure 9: Effect of disk roughness on clearance.
Advances in Tribology 9
Overpush (mW)
Friction (mN)
Burnished Unburnished
Poly. (burnished) Poly. (unburnished)
y=−0.0005x2+0.0646x
R2=0.97
y=−0.0003x2+0.049x
R2=0.9696
−0.5
0
0.5
1
1.5
2
2.5
0 102030405060
(a)
0
10
20
30
40
50
60
70
ABC
Disk
Unburnished
Burnished
Friction (μN/mW overpush)
(b)
0
0.5
1
1.5
2
2.5
3
0 10203040505040302010 0
Overpush (mW)
Friction (mN)
Disk A
Disk B
Disk C
−0.5
(c)
Figure 10: Effect of disk roughness on friction.
efficiency by decreasing RPM, it exhibits the characteristic
sharp touchdown plot and higher friction.
4. Conclusion
The touchdown behavior of TFC sliders is investigated
through experiments. Certain sliders exhibit a sharp rise of
0
10
20
30
40
50
60
70
Lube A: 69%
bonded
Lube A: 61%
bonded bonded
Lube B: 82%
Disk
Friction (μN/mW overpush)
Unburnished
Burnished
(a)
0
0.5
1
1.5
2
2.5
3
3.5
Lube A: 61%
bonded
Lube A: 69%
bonded Lube B: 82%
bonded
Disk
Friction test: δTDP (mW)
Unburnished
Burnished
(b)
−0.5
0
0.5
1
1.5
2
2.5
3
0 10203040505040302010 0
Overpush (mW)
Friction (mN)
Lube A: 61% bonded
Lube A: 69% bonded
Lube B: 82% bonded
(c)
Figure 11: Effect of lubricant parameters on friction and slider bur-
nishing.
AE signal at touchdown when the power is increased in
milliwatt steps while others show a gradual rise making it
difficult to exactly define the TDP to milliwatt resolution. An
analogous behavior occurs when the disk RPM is changed
for a particular slider ABS. It is found that the dominant
modes/frequencies of excitation at touchdown are signifi-
cantly different in these cases leading to the very different
touchdown signatures. Particularly, the sharp touchdown
10 Advances in Tribology
Lubed Delubed
Disk
Unburnished
Burnished
0
10
20
30
40
50
60
70
80
Friction (μN/mW overpush)
(a)
Lubed Delubed
Disk
Unburnished
Burnished
−2
0
2
4
6
8
10
12
Friction test: δTDP (mW)
(b)
Figure 12: Effect of mobile lubricant on friction and slider
burnishing.
ABS-A ABS-B ABS-C ABS-D
ABS/heater design
Unburnished
Burnished
0
10
20
30
40
50
60
70
80
Friction (μN/mW overpush)
Figure 13: Effect of TFC efficiency (ABS/heater design) on friction.
case is characterized by strong individual contact events
as observed in the AE signal, and the dominant excitation
occurs at frequencies that correspond to the slider’s first pitch
and roll modes in addition to suspension related-frequencies.
In contrast, the gradual touchdown case is characterized by
a uniform rise in AE signal over the duration of the TFC
30
35
40
45
50
55
60
3000 4000 5000 6000 7000
Disk RPM
Friction (μN/mW overpush)
Figure 14: Effect of disk RPM on friction for ABS-2.
pulse, and the dominant excitation occurs at the slider’s
second pitch mode. The pressure under the TFC protrusion
at touchdown, the TFC efficiency, and the propensity for
lubricant pickup show correlation with touchdown behavior
and may be used as metrics for designing sliders with good
touchdown features. Experiments are devised to measure
the friction at the HDI during TFC-induced contact, and
several parametric investigations are carried out. In the range
of parameter values considered, the disk surface roughness
does not significantly affect the friction during contact. The
mobile part of the lubricant plays an important role in
reducing friction as well as slider burnishing. A burnished
slider shows a higher friction value than an unburnished
slider because of an increase in effective/actual contact area,
and, for the same reason, sliders with higher TFC efficiency
show higher friction compared to sliders with lower TFC
efficiency.
Acknowledgments
This work was supported by the Computer Mechanics
Laboratory at the University of California at Berkeley and
Hitachi Global Storage Technologies. The authors thank Q.
Dai, L. Dorius, XC. Guo, B. Marchon, and R. Waltman for
their support and helpful discussions during the course of
this work.
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