Voltage assisted asymmetric nanoscale wear on ultra-smooth diamond like carbon thin films at high sliding speeds

Abstract

The understanding of tribo- and electro-chemical phenomenons on the molecular level at a sliding interface is a field of growing interest. Fundamental chemical and physical insights of sliding surfaces are crucial for understanding wear at an interface, particularly for nano or micro scale devices operating at high sliding speeds. A complete investigation of the electrochemical effects on high sliding speed interfaces requires a precise monitoring of both the associated wear and surface chemical reactions at the interface. Here, we demonstrate that head-disk interface inside a commercial magnetic storage hard disk drive provides a unique system for such studies. The results obtained shows that the voltage assisted electrochemical wear lead to asymmetric wear on either side of sliding interface.

Full text

Voltage assisted asymmetric nanoscale wear on
ultra-smooth diamond like carbon thin films at high
sliding speeds
Sukumar Rajauria1*, Erhard Schreck1, and Bruno Marchon1
1HGST, a Western Digital Company, Recording Sub System Staging and Research, San Jose, CA 95135 USA.
*corresponding.sukumar.rajauria@hgst.com
ABSTRACT
The understanding of tribo- and electro-chemical phenomenons on the molecular level at a sliding interface is a field
of growing interest. Fundamental chemical and physical insights of sliding surfaces are crucial for understanding
wear at an interface, particularly for nano or micro scale devices operating at high sliding speeds. A complete in-
vestigation of the electrochemical effects on high sliding speed interfaces requires a precise monitoring of both the
associated wear and surface chemical reactions at the interface. Here, we demonstrate that head-disk interface in-
side a commercial magnetic storage hard disk drive provides a unique system for such studies. The results obtained
shows that the voltage assisted electrochemical wear lead to asymmetric wear on either side of sliding interface.
Introduction
Wear is broadly classified in two categories: physical wear and progressive wear. Physical wear is further classified as adhesive,
fatigue and abrasive wear which all lead to the formation or transfer of material across the sliding interface.
1–6
At macro or
micro scale it follows Archard’s law describing fracture and plastic deformation with wear volume being proportional to both
the applied load and sliding distance.
7–9
Such phenomena lead to catastrophic wear and are rare in nanoscale devices where the
progressive chemically assisted wear is likely to be dominant. Recently observed nanoscale wear showed that stress-assisted
chemical reactions occur through an atom-by-atom process.
10–14
In chemical assisted wear, environmental species like oxygen
and humidity play a critical role. For instance, oxygen can readily chemisorb on carbon surfaces, leading to surface oxides
such as carbonyl groups, thereby depleting the carbon surface during thermal desorption.
15–21
In addition to environmental
species, the chemical potential across the sliding interface is expected to affect the surface chemical reaction rates. On the
macroscale level many studies were conducted to understand the impact of chemical potential on surface oxidation,
22–24
but at
the nanoscale a quantitative understanding of its mechanism and impact on wear appears to be little understood.
A complete investigation of the electrochemical effects on high sliding speed interfaces requires a precise monitoring of
both the associated wear and surface chemical reactions at the interface. The head-disk interface inside a commercial magnetic
storage hard disk drive provides a unique system for such studies. In a hard disk drive, the head has an embedded micro-scale
heater which produces through thermal expansion a well-defined mechanical protrusion. This allows to adjust the head-to-disk
interference level within sub-nanometer precision over a contact area of several square micrometers. The relative sliding speed
between head and disk ranges from 10-40
m/s
. It is worth noticing here that while the vertical spacing is of the same order
of various AFM based studies, the sliding speed is nearly six orders of magnitude higher, thus allowing a unique set-up for a
systematic study of nanoscale wear at high sliding speeds. Interfering surfaces of the head and the disk are coated with an
amorphous diamond like carbon which has exceptional mechanical properties like low friction and wear rate.25–27
In this letter, we report the precise monitoring of both the carbon overcoat wear and the corresponding interfacial current on
the nanoscale high sliding speed interface. Carbon overcoat wear is monitored and calibrated using the embedded micro-heater
power. The interface current between the head and the disk monitor the rate of electrochemical oxidation of the carbon overcoat.
We show that the interface current decay sharply with time indicating the chemical passivation of the surface carbon dangling
bonds that are created while sliding. This unique approach provides an in-depth understanding of the electrochemically assisted
wear at high sliding speeds which until now has never been applied to such devices.
Results
A typical head-disk interface setup features the head flying on top of the disk similar to the one studied in Ref.
6,28
The disk
is fabricated by depositing a magnetic multilayer film structure onto a glass substrate, then coated with 3
nm
amorphous
arXiv:1605.02278v1 [cond-mat.mes-hall] 8 May 2016

nitrogenated carbon (protective overcoat layer), and finally covered with a molecular layer of perfluoropolyether polymer
lubricant (
∼
1
nm
thick). The electrically conductive ceramic substrate head is also coated with 1.4
nm
of diamond like carbon
on top of a 0.3
nm
Silicon based adhesive layer, making the head and the disk interface a carbon-lubricant-carbon sliding
interface. The disk facing head surface is carefully shaped through etching such that while flying on top of the disk an airbearing
lift force is generated that keeps it afloat over the disk in the nanometer range.
29,30
The linear sliding speed in the described
experiments is set to 10
m/s
. The initial clearance (physical gap) between the head and the disk is typically 10
nm
. Clearance is
controlled precisely using the embedded micro-heater in the head.
31,32
We estimate the wear depth by continuous monitoring
of the micro-heater power and later verified the wear using AFM, scanning electron microscope (SEM) and Auger electron
microscopy (Auger).33 Contact between head and disk is detected using an acoustic emission (AE) sensor.34
Figure 1. Simultaneous measurement of interfacial current and overcoat wear: (a) Blue and Red dots represent the
rapid decay of the interfacial current between the head and the disk with -1
V
and +0.8
V
applied on head. Dashed line is a fit
to Eq. 1. Inset shows the associated charge transfer for the two polarities. (b) Blue and Red dots show the simultaneous in-situ
measurement of overcoat wear depth as a function of time. (c), (d) Shows the schematic cartoon of the wear on head and disk
overcoat for positive and negative voltage on the head. Clearance between head and disk is controlled precisely using the
embedded micro-heater in the head.
While in operation, a voltage is applied directly to the head overcoat while the disk was electrically grounded. Voltage
difference across the interface decrease the initial flying clearance, which is symmetric to 0
V.
Variation in initial flying
clearance across various applied voltages here is estimated to be around 3
%
of the initial flying clearance (see supplementary
Figure S1) and part-to-part variation in initial flying clearance is estimated to be around 5
%
. Interfacial current was measured
across the interface. The load is set using the micro-heater design. Air bearing simulations estimate the normal load to increase
by 0.25
mN/mW
of excess heater power (Air bearing simulation using HGST internal code). Uncertainty due to variation in
initial flying clearance is estimated to change the contact force by 10
%
. Excess heater power is defined as excess power applied
to the micro-heater after head-to-disk contact was detected. Figure 1a shows the interfacial current at two applied voltages
of +0.8
V
and -1
V
on two heads under a normal load condition of 2.5
mN
. For both polarities, the interfacial current at head
disk interface decay with time. For further insight, an in-situ monitoring of the wear is desired to gain better insight into the
impact of electrochemical oxidation during sliding. The head disk interface has a unique feature with the micro-heater power
calibrated precisely to measure the head overcoat wear depth in a continuous manner during the experiment (see Supplementary
Information for more detail).
6
Figure 1b shows the wear depth profile of the head carbon overcoat for the respective voltage
condition as a function of time. For positive voltage, the 0.3 nm thin carbon overcoat wears out within 3 second of intimate
head disk contact (see red dots in Figure 1b).
We associate the rapid decay in current to the electrochemical oxidation of the carbon overcoat, with voltage polarity
governing whether the head or disk is undergoing oxidation. It is worth mentioning that similar rapid decay in current has
previously been observed in macroscopic systems such as the carbon based fuel cell.
22,23
In an acidic environment, the current
across the fuel cell decays rapidly with time across all potentiostatics. The electrochemical oxidation of carbon in fuel cell
is written as:
C+2H2O→CO2+4H++4e−
. Empirically, the measured oxidative electrochemical current is adequately
2/9

Figure 2. Head overcoat wear depth: (a) Shows the wear depth profile as a function of sliding distance under an applied
bias of -1 V and +1 V on the head overcoat. (b) Shows the wear depth profile of the head overcoat wear as a function of
interfacial voltage on the head under different environment conditions.
represented by a simple power law expression:
i=kt−n+io(1)
where
t
is the time,
i
is the specific current,
k
is the rate parameter which is a function of both the temperature and potential,
n
is
the time decay exponent, and
io
is the ohmic current. This decay in current with time is mainly attributed to two competing
parallel reactions involving one to passivate the surface and another to oxidize the carbon producing carbon dioxide CO
2
. Eq.
(1) fits the interfacial current data in Figure 1a well. This good agreement demonstrates the surface passivation dominated
electrochemical oxidation of the carbon overcoat is an important mechanism at the high sliding speed interface. In a head-disk
interface both the head and disk have a carbon overcoat. The head is coated with FCAC (field cathodic arc carbon) carbon
on top which is more resistant than the nitrogenated carbon present on the disk.
35–37
The voltage polarity solely determines
which surface is undergoing electrochemical oxidation. The inset of Figure 1a shows the associated net charge transfer in the
electrochemical reaction for the negative and positive voltage. Charge associated mass transfer is driven more by the negative
voltage. We associate it with electrochemical oxidation on the disk as nitrogenated disk carbon is tribochemically less stable
than FCAC head carbon. The associated wear or weight loss due to electrochemical oxidation is given by:
Weightl oss =qM
4F(2)
where
q
is the integrated total charge transfer across the interface,
M
is the molecular weight, and
F
is the Faraday constant.
The factor of four assumes the number of electrons transferred in the head overcoat reaction is similar to the fuel cell oxidation
case. Typically the area of contact on the head overcoat due to the heater bulge is around 10
µm2
. For positive voltage, Eq. 2
estimates the weight loss of head overcoat to 8.75.10
−18
kg which corresponds to a wear depth of around 0.3
nm
. This is in
excellent agreement with the measured wear depth (see red dots in Figure 1b), thus confirming the electrochemical oxidation of
carbon overcoat as the dominant wear mechanism. For negative voltages, no head overcoat wear is observed (see blue dots in
Figure 1b). Here the carbon overcoat on the disk undergoes electrochemical wear, and the head carbon remains intact, and
leading to almost no wear under the same loading force conditions. The comparative volumetric loss for two polarities is more
for positive bias, which results in high wear volume on disk overcoat. However, the overcoat wear on disk is diluted over a
large area leading to a negligible wear depth in comparison to a localized wear on head where it could be quantified using
AFM, SEM and Auger (see supplementary Figure S3 and Figure S4).33,38,39
Figure 2a shows the head overcoat wear as a function of sliding distance on the same head-disk interface. No head overcoat
wear is observed for the first 1400
m
sliding distance under an applied bias of -1
V.
As the polarity is reversed to +1
V,
the wear
rate increases sharply. This behavior exemplifies clearly that the voltage leads to asymmetric wear on head overcoat.
Environmental effects on electrochemical wear
To gain further insight in the involved electrochemical process, we now turn to measurements in a controlled environment. The
measurements are performed in an enclosed humidity controlled continuous flow set-up (as shown in Figure 3a). The chamber
3/9

is connected to high purity gases (Nitrogen or dry Air) and the percentage of oxygen and humidity is monitored using gas
sensors placed inside.
Figure 2b shows the head overcoat wear depth as a function of head and disk interfacial voltage under atmospheric nitrogen
condition with different relative humidity. The head overcoat undergoes the same wear cycle at each interfacial voltage and
environment condition. The head overcoat wear depth is measured on an unused and pristine location on a disk after sliding
on it for 3000
m
in intimate contact. It shows the importance of humidity on the head overcoat wear profile. At high relative
humidity(RH) of 45
%
, the head overcoat wear is not symmetric to the interfacial voltage. Positive voltage leads to high overcoat
wear on the head, and negative voltage significantly suppresses the head overcoat wear. It is consistent with the overcoat wear
as observed in Figure 1b. The impact of interfacial voltage polarity on the head overcoat reduces as the humidity content in the
environment decreases. At low RH of 7
%
the overcoat wear is very high and insensitive to voltage polarity. We attribute the
high wear to the slower tribochemical passivation of the dangling -OH bond on the surface at low humidity conditions.
21
The
interfering surfaces are sliding at high speeds (almost six orders of magnitude faster than AFM), potentially leading to high
flash temperatures and posing a greater challenge to stability of carbon under dry conditions.
Figure 3. Environment control: (a) Shows the set-up schematic of the high sliding speed head-disk interface in an
environmental control chamber. (b) Black, Red and Blue dots represent the rapid decay of the interfacial current in dry
Nitrogen, dry Air and wet Nitrogen environment conditions. The measurements are done at a normal load of 3.75 mN with -1.3
V applied on the head overcoat. Dashed line is fit to Eq. 1. (c) Shows the associated charge transfer for the three environment
conditions.
To understand the complex electrochemical reaction, we investigate the effect of environmental conditions on the passivation
process. The involved electrochemical reaction involves two competing parallel reactions: one to oxidize the carbon to produce
carbon dioxide CO
2
leading to current in the circuit and another to passivate the surface leading to reduction in current. Figure
3b shows the interfacial current time decay at the head disk interface at -1.3
V
applied voltage on the head under a normal
load of 3.75
mN
for three environmental conditions: dry nitrogen (black), dry air (red) and wet nitrogen (blue). The two dry
conditions have RH¡6
%
and wet air has RH=40
%
. Compared to the two dry (nitrogen and air) conditions, the current under the
wet nitrogen condition starts at lower value and saturates earlier, demonstrating the effectiveness of water in passivating the
surface. Among the two dry case (nitrogen and air), air has 20
%
atmospheric oxygen. As observed, molecular oxygen also
contributes to surface passivation. Figure 3c shows the integrated charge transfer across the interface for the three environmental
conditions. It shows that both humidity and molecular oxygen passivate the carbon overcoat with the former being more
effective.
Friction under electrochemical wear
In addition to an in-situ monitoring of wear and interfacial current, a measurement of the frictional properties is also desired to
gain further insight into surface modification during sliding. The friction force between the head and the disk is also measured
under a normal load condition. A calibrated strain gauge was instrumented to measure the friction force (in the sliding direction)
between the head and the disk interface. The frictional force at an interface is written as,
F=µL+FA
, where
F
is the total
friction at the interface,
µ
the non-dimensional coefficient of friction,
L
the applied normal load, and
FA
the adhesion component
of friction.
40–42
Figure 4 shows the delta friction force as a function of normal load when applied -1
V
and +1
V
on the head
4/9

overcoat. Here, we have subtracted the friction contribution at low load (=1.25
mN
) from all other load conditions to measure
delta friction force. Dashed line is the linear fit with slope being the coefficient of friction. For positive voltage cycle on
head, the friction coefficient is 0.4 in comparison to 0.2 for negative voltage cycle on the head. This further demonstrates the
importance of the electrochemical activity on the carbon overcoat during sliding is determining the friction properties and long
term durability of the overcoat.
Figure 4. Frictional properties under an applied voltage:
Shows the delta friction force as a function of normal load under
a repetitive cycle of applied +1 V and -1 V on head. Dashed line is the linear fit with slope being the friction coefficient.
Chemical marking of contact location
As a practical application of surface passivation, it was used to chemically mark the disk overcoat surface. Intentional contacts
at two distinct tracks (radius 21
mm
and 23
mm
on disk) were made for 3 seconds under a normal load of 3.75
mN
at the head
disk interface. The head is held at -1
V
with respect to the disk. Figure 5b shows the decay in the interfacial current with the
same head at two distinct tracks. Once the head passivates the first track on disk, the initial current on the new track is similar to
the initial current of the previously passivated track. This further shows that for negative head voltages, the surface passivation
dominated the electrochemical activity on the disk leaving the head in pristine condition. We used the same head to scan probe
the electrical conductivity of the complete disk. Figure 5c shows the interfacial current as probed by the same head at different
tracks. The electrical current on the passivated track is found to be significantly lower than the untreated area of the disk surface.
We believe that this surface passivation of the carbon overcoat can have significant applications for high speed lithography. It
is worth mentioning that recently AFM has been used to perform similar surface passivation of graphitic surfaces but AFM
operates at typically six orders of magnitude slower sliding speed.
43
Chemical analysis of the oxide formed on carbon overcoat
is still missing and requires more work.
Discussion
In summary, we have outlined a quantitative analysis of voltage assisted nanoscale electrochemical wear on carbon overcoat at
high sliding speed interfaces. At high sliding speeds, in-situ measurements were performed of the interfacial current and the
associated wear amount due to the electrochemical process. In addition, the effect of electrochemical activity on the interface is
further quantified by measuring the friction force and the friction coefficient. It is found that the voltage assisted electrochemical
activity greatly influences the interfacial wear and frictional properties. Positive voltage applied to the head leads to high
wear on the head overcoat but no head overcoat wear was observed for negative applied voltage. As a useful application, we
exploited the electrochemical passivation to mark the head-disk contact regions on the disk. The contact regions can be clearly
identified by the associated conductivity variations of the surface. We believe that the observed voltage assisted asymmetric
nanoscale wear will lead to additional experimental and simulation work, and will help to understand precisely the chemical
origin of the involved process.
5/9

Figure 5. Chemical marking of contact location: (a) Cartoon of disk depicting the intentional contact at two location,
Radius =21 mm and 23 mm. (b) Marking: dots represent the passivation of carbon overcoat at two locations. (c) Detection:
shows the interfacial current as probed by the same head at different tracks.
Our results are expected to have strong impact on fundamentally improving the carbon overcoat for various applications.
The effect of interfacial voltage on the sliding interface is expected to be of great importance for understanding and improving
the wear properties in nanoscale devices.
Methods
Sample preparation
Disk: the rotating disk is a commercial 2.5” CoCrPt:oxide based hard disk media fabricated onto a glass substrate. Outermost
thin film layer of hard disk media consist of 3
nm
amorphous nitrogenated carbon overcoat coated with a molecular layer of
perfluoropolyether polymer lubricant (∼1nm thick).
Head: the head is a commercially available with read and write elements fabricated on a ceramic substrate. Similar to the
disk, the head is also coated with a carbon overcoat with 1.4
nm
diamond like carbon on top of a 0.3
nm
Silicon based adhesive
layer. The head surface is carefully etched (known as air bearing surface (ABS)) such that while flying on top of the disk an air
lift force is generated that keeps it afloat in the nanometer distance over the disk.
Contact detection between the head and disk
Contact between the head and disk is monitored using a piezo-electric based acoustic emission (AE) sensor of the type PICO -
200-750
kHz
. It detects elastic propagating waves generated during the head-disk contact events.
34
Figure 6 shows a typical
contact detection between the head and the disk. Vertical clearance between the head and disk is set using the embedded
micro-heater inside the head. For protruding head making a contact with disk, AE signal increases sharply compare to
non-contact condition.
Interfacial current and friction measurements
Electrical measurement: In all interfacial measurement, the head-disk interface is first voltage biased then the contact is made
using the micro-heater. The voltage bias is done using a HP 3314A source and the corresponding interfacial current is measured
using Agilent 4155C. Interfacial current measurement are done under a positive normal load condition.
Friction measurement: Contact friction force is measured using a calibrated strain gauge mounted at the end of the
suspension. The strain gauge signal is measured and amplified using a Vishay 2311 signal conditioning amplifier.
References
1. D.H. Buckley, Surface Effects in Adhesion, Friction, Wear, and Lubrication 429-508 (Elsevier, Amsterdam, 1981).
2.
Wimmer, M. A., Sprecher, C., Hauert, R., Tager, G. and Fischer, A. Tribochemical reaction on metal-on-metal hip joint
bearings - A comparison between in-vitro and in-vivo results. Wear 255, 1007-1014 (2003).
6/9

Figure 6. Contact detection:
Shows the normalized acoustic emission signal as a function of normal load between the head
and the disk.
3.
Mosey, N. J., Muser, M. H. and Woo, TK Molecular mechanisms for the functionality of lubricant additives. Science
307
,
1612-1615 (2005).
4.
Bhushan, B., Israelachvili J. N. and Landman, U. Nanotribology: friction, wear and lubrication at the atomic scale. Nature
374, 607-616 (1994).
5.
Gosvami, N. N. et al. Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts. Science
348, 102-106 (2015).
6.
Rajauria, S., Canchi, S. V., Schreck, E. and Marchon, B. Nanoscale wear and kinetic friction between atomically smooth
surfaces sliding at high speeds. Applied Physics Letters 106, 081604 (2015).
7. Archard, J. F. Contact and rubbing of flat surfaces. Journal of Applied Physics 24, 981-988 (1953).
8. Jia, K. and Fischer, T. Sliding wear of conventional and nanostructured cemented carbides. Wear 203, 310-318 (1997).
9.
Chung, K. and Kim, D. Fundamental investigation of micro wear rate using an atomic force microscope. Tribology Letter
15, 135-144 (2003).
10. Gnecco, E., Bennewitz, R. and Meyer, E. Abrasive wear on the atomic scale. Phys. Rev. Lett. 88, 215501 (2002).
11. Gotsmann, B. and Lantz, M. Atomistic wear in a single asperity sliding contact. Phys. Rev. Lett. 101, 125501 (2008).
12.
Bhaskaran, H. et al. Ultralow nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like carbon.
Nature Nanotechnology 5, 181 (2010).
13.
Jacobs, T. D. B. and Carpick, R. W. Nanoscale wear as a stress-assisted chemical reaction. Nature Nanotechnology
8
,
108-112 (2013).
14.
Hanggi, P., Talkner, P. and Borkovec M. Reaction-rate theory: fifty years after Kramers. Review of Modern Physics
62
, 251
(1990).
15. Robertson, J. Diamond-like amorphous carbon. Materials Science and Engineering: R: Reports 37, 129-281 (2002).
16.
Marchon, B., Carrazza, J., Heinemann, H. and Somorjai, G. A. TPD and XPS studies of O
2
, CO
2
and H
2
O adsorption on
clean polycrystalline graphite. Carbon 26, 507-514 (1988).
17.
Marchon, B., Heiman, N. and Khan, M. R. Evidence for tribochemical wear on amorphous carbon thin films. IEEE
Transactions on Magnetics 26, 168-170 (1990).
18.
Strom, B. D., Bogy, D. B., Bhatia, C. S. and Bhushan B. Tribochemical effects of various gases and water vapor on thin
film magnetic disks with carbon overcoats. Journal of Tribology-Transactions of the ASME 113, 689-693 (1991).
7/9

19.
Dai, Q., Yen, B. K., White, R. L., Peterson, P. J. and Marchon, B. Toward an understanding of overcoat corrosion protection.
IEEE Transactions on Magnetics 39, 2450-2452 (2003).
20.
Konicek, A. R. et al. Influence of surface passivation on the friction and wear behavior of ultrananocrystalline diamond and
tetrahedral amorphous carbon thin films. Physical Review B 85, 155448 (2008).
21.
Marino, M. J. et al. Understanding run-in behavior of diamond-like carbon friction and preventing diamond-like carbon
wear in humid air. Langmuir 27, 12702–12708 (2011).
22.
Kinoshita, K. and Bett, J. Electrochemical oxidation of carbon black in concentrated phosphoric acid at 135
o
C. Carbon
11
,
237-247 (1973).
23.
Gallagher, K. G. and Fuller, T. F. Kinetic model of the electrochemical oxidation of graphitic carbon in acidic environments.
Physical Chemistry Chemical Physics 11, 11557-11567 (2009).
24.
Zilibotti, G., Righi, M. C. and Ferrario M. Ab initio study on the surface chemistry and nanotribological properties of
passivated diamond surfaces. Physical Review B 79, 075420 (2009).
25.
Mate, C. M., McClelland, G. M., Erlandsson, R. and Chiang, S. Atomic-scale friction of a tungsten tip on a graphite
surface. Phys. Rev. Lett. 59 1942 (1987).
26. Robertson, J Ultrathin carbon coatings for magnetic storage technology. Thin Film Solids 383, 81-88 (2001).
27.
Ferrari, A. C. Diamond-like carbon for magnetic storage disks. Surface and Coatings Technology
180-181
190-206 (2004).
28.
Suh, A. Y., Mate, C. M., Payne, R. N. and Polycarpou, A. A. Experimental and theoretical evaluation of friction at
contacting magnetic storage slider-disk interfaces. Tribology Letters 23, 177-190 (2006).
29. CMLAir 8.3 (2015): CMLAir Bearing Design Program. URL http://cml.berkeley.edu/ (Accessed: 12th January 2016).
30.
Zheng, J. and Bogy, D.B Investigation of flying-height stability of thermal fly-height control sliders in lubricant or solid
contact with roughness. Tribology Letters 38, 283-289 (2010).
31.
Zeng, Q., Yang, C.-H.. Ka, S. and Cha, E. An experimental and simulation study of touchdown dynamics. IEEE Transactions
on Magnetics 47, 3433-3436 (2011).
32.
Canchi, S. V., Bogy, D. V., Wang, R. H. and Murthy A. N. Parametric Investigations at the Head-Disk Interface of Thermal
Fly-Height Control Sliders in Contact. Advances in Tribology 2012, 303071 (2012).
33.
Chen, Y.-K., Murthy, A. N., Pit, R. and Bogy, D. B. Angstrom scale wear of the air-bearing sliders in hard disk drives.
Tribology Letters 54, 273-278 (2014).
34.
Bhushan, B., Wu, Y. and Tambe, N. S. Sliding contact energy measurement using a calibrated acoustic emission transducer.
IEEE Transactions on Magnetics 39, 881-887 (2003).
35.
Wei, B., Zhang, B. and Johnson, K. E. Nitrogen-induced modifications in microstructure and wear durability of ultrathin
amorphous-carbon films. Journal of Applied Physics 83, 2491-2499 (1988).
36.
Khun, N. W., Liu, E. and Krishna, M. D. Structure, adhesive strength and electrochemical performance of nitrogen doped
diamond-like Carbon thin films deposited via DC magnetron sputtering. Journal of Nanoscience and Nanotechnology
10
,
4752-4757 (2010).
37.
Dwivedi, N. et al. Understanding the role of nitrogen in plasma-assisted surface modification of magnetic recording media
with and without ultrathin carbon overcoats. Scientific Reports 5, 7772 (2015).
38.
Juang, J.-Y., Forrest, J. and Huang, F.-Y. Magnetic head protrusion profiles and wear pattern of thermal flying-height
control sliders with different heater designs. IEEE Transactions on Magnetics 47, 3437-3340 (2011).
39.
Su, L. et al. Tribological and dynamic study of head disk interface at sub 1-nm clearance. IEEE Transactions on Magnetics
47, 111-116 (2011).
40. Bowden, F.P. and Tabor D. The friction and Lubrication of Solids 90-98 (Claredon, 1950).
41. Mo, Y. F., Turner, K. T. and Szlufarska, I. Friction laws at the nanoscale. Nature 457, 1116-1119 (2009).
42.
Berman, A., Drummond, C. and Israelachvili, J. Amontons’ law at the molecular level. Tribology Letters
4
, 95-101 (1998).
43. Garcia, R., Knoll A. W. and Riedo E. Advanced scanning probe lithography. Nature Nanotechnology 9, 577–587 (2014).
Acknowledgements
The authors thank O. Ruiz for air bearing simulations, and S. Canchi, A., Murthy, N. Wang and V. Sharma for technical
assistance.
8/9

Author contributions statement
S.R. and E.S. designed the experiments with contribution from B.M.. S.R. performed the experiments and analyzed the data.
S.R., E.S. and B.M. wrote the manuscript.
Additional information
The authors declare they have no competing financial interests.
9/9