Friction and wear are two key factors that are detrimental at the nanoscale, because they are linked with surface forces. It is important to understand their mechanisms and to find ways to overcome them. At the macroscale, lubricants are classically used to do so and are well understood. However, those lubricants cannot be readily used at the nanoscale and other lubrication methods must be found. In this paper I will focus on two new potential lubricants that were tested in Microelectromechanical Systems (MEMS): ionic liquid films and alcohol vapor. They are both very different and are not used in the same situations but they both show great potential. In fact, thanks to the alcohol vapor lubricant a device did not show signs of wear after 11 days of operation, whereas it failed within minutes in the absence of alcohol. No signs of wear were seen after 100 cycles of operation of a device with ionic liquid film but were observed in an uncoated device.

1. Mitigating nanotribology issues using green
biodegradable lubricants
Antoine Galand
University of Pennsylvania, Philadelphia, Pennsylvania, USA
Executive summary
Friction and wear are two key detrimental factors at the micro- and nanoscale, because they
are linked with surface forces that are predominant at those scales. Understanding their mecha-
nisms is important but finding ways to overcome them is even more important to make a lot of
technology ideas a reality. At the microscale, lubrication is well understood and classically used
to reduce friction. Today, lubricants are also used at the micro- and nanoscale but they are not
efficient enough. At every scale, most lubricants have an impact on the environment and lead to
waste, emission of green house gases and increase operation costs. There is a need for lubricants
at the small scale and there is a need for green and sustainable solutions at both scales.
Dugger, Kim et al have studied alcohol vapor lubrication as a means to achieve reliable and
lasting operation of microelectromechanical systems (MEMS) [1]. According to their result the
adsorption of pentan-1-ol molecules on their substrates has interesting consequences. It shifts the
water adsorption equilibrium and water molecules start desorbing, drastically reducing friction and
wear in Si and SiO2 systems. It also leads to the formation on an oligomeric liquid build-up which
further lubricates the systems. Using that method they managed to get a MEMS device working
for 11 days without showing any signs of wear, whereas it previously failed within minutes.
I think a biodegradable lubricant would be a good answer to the waste problem as it would
decompose and could be used as biomass. To capitalize on the alcohol adsorption results, I want
to study the mechanism of this equilibrium shift and of the oligomer formation. By studying that
oligomer, I believe I could assess its sustainability or try and design a lubricant that would be as
efficient but be biodegradable or at least photodegradable.
If successful, this lubrication method would drastically reduce energy losses at the micro- and
nanoscale and would reduce wear, enabling the fabrication of devices that would fail within min-
utes of operation otherwise. This would lead to the realization of great technology ideas and would
drive the MEMS industry forward. Indeed, the reduction of friction, wear and adhesion would al-
low the use of micro- and nano-gears or rotary hubs in the designing of complex systems.

2. Mitigating nanotribology issues using green biodegradable lubricants 2
Introduction and motivation
The laws of thermodynamics limit the ideal efficiency of any system to the Carnot efficiency.
However as high as that value can be, we have yet to reach it in most applications. Any device
operation with loss of energy is irreversible and thus this device cannot achieve Carnot’s efficiency.
Those statements are true at any scale. Both at the macro- and microscale those energy losses can
easily be associated with tribology issues such as friction or adhesion. Issues that are also respon-
sible for the wearing of those systems. To overcome friction, we usually either use lubricants or
design the systems in such a way that reduces friction drastically. In this paper, I will focus on
lubrication at the micro- and at the nanoscale and particularly on lubrication in microelectrome-
chanical systems (MEMS) devices.
From accelerometers triggering the airbags of a car to smart drugs to smartphones, nanotech-
nology and MEMS have a wide range of application and opportunities. Yet we are only seeing
the beginning of that technology revolution that is already changing our every day life. At the mi-
croscale lubrication is well understood and friction is not usually a technology limiting constraint.
However a lot of lubricants are oil-based and lead to waste after their utilization. Greener solutions
are required for a more sustainable industry. I believe that while looking for suitable lubricants for
MEMS device, we should already design them with respect to the principles of Green Engineering
and Chemistry.
The reliable operation of class IV MEMS - MEMS with rubbing parts - would enable great
new technologies such as nano and micro gears and would thus enable more complex systems.
The predominance of surface forces at this scale leads to abrasive wear and failure even in devices
designed with low friction materials. Superlubricity, such as observed when rubbing tungsten on
graphite, could be an opportunity to make such MEMS, but it requires a very specific material with
a very specific geometry [2]. Ionic liquid lubricants and alcohol vapor lubrication are two fairly
new methods that show great potential. The advantage of the ionic liquid method is that it can be
used in systems that require a conductive medium. Moreover ionic liquids are considered green
lubricants and their use would reduce air emission associated with hydrocarbon oxidation [3].
However they cannot be used in every system, a lot of devices require insulating materials,
for instance a capacitor requires two conductive plates separated by a dielectric. New and green
lubrication methods must be found for those systems. I found the alcohol vapor lubrication very
interesting. Dugger, Kim, et al found that it enabled the reliable operation of MEMS for days and
that the lubricants would break down in a liquid build-up, further lubricating the system [1]. My
goal here is to capitalize on this method to find such a lubricant that will break down in a ”second”
lubricant which would be biodegradable.
Alcohol adsorption
Alcohol adsorption is a potential new lubricant that can be used to drastically reduce friction
and wear in systems with moving and rubbing parts. Pentan-1-ol is used as a vapor-phase lubricant
(VPL). The main problem with VPL is the need for high temperature or catalytic precursors to coat
a substrate, however pentan-1-ol has an interesting vapor pressure at ambient temperature and can

3. Mitigating nanotribology issues using green biodegradable lubricants 3
thus readily be used.
The lubrication mechanism of this technique is simple. Alcohol vapor is fed into the device
at a given pressure. As long as the equilibrium is maintained, alcohol molecules will adsorb to
the surface and in turn water molecules will desorb. Water is indeed one of the main friction and
wear factor in Si and SiO2 systems. As mentioned earlier a liquid build-up is observed as a result.
Thanks to TOF-SIMS spectroscopy the authors determined that this build-up was oligomeric and
they found it increased the lubrication properties of their method. They referred to that build-up as
an ”in-situ lubrication”. However the formation mechanism is not understood yet.
Figure 1: isotherm thickness for pentan-1-ol. It appears that coverage is obtained at P/Psat=10%. [1]
Figure 2: a: Friction coefficient over time for 0%, 15% and 95% Psat. b: On the left a SEM image of the
device at 15% Psat after 108 cycles and on the right an unused device. In a, we can see that 15% is enough
to prevent failure. In b, we can observe a liquid build-up circled in white. [1]
When they stopped feeding the vapor, they observed that the alcohol molecules desorbed and
the system reverted to its original state. Hence, unlike typical solid coating, this lubrication method
does not affect the long term performance of the device and does not change its chemical proper-
ties or geometry. This reversibility also means that alcohol needs to be continuously fed into the
MEMS during its operation.
The authors found a significant reduction in friction and wear. They used this method on a
device. After 11 days of operation, it was still running and there was no sign of wear, whereas the
same system failed within minutes without the pentan-1-ol.
Biodegradable secondary in situ lubrication
Biodegradable materials and photodegradable materials have replaced a lot of long-lasting in-

4. Mitigating nanotribology issues using green biodegradable lubricants 4
ert materials in most industries. We used to design for ”immortality”, we believed that the more
inert a material was, the better it was. However we realized that inert materials resisting to almost
everything are now piling up and are becoming waste. The cost of getting rid of those materials is
high and their accumulation has a drastic impact on the environment. It is true for solid materials
such as construction materials or devices but also for solvents and gases. Some gases will resist
most conditions and are piling up in the atmosphere where they start depleting ozone, for exam-
ple. Some molecules are hard to break down, that is true for alkanes because of their stability for
instance. Most commonly used lubricants in MEMS are perfluoropolyether (PFPE) based. PEFE
is such an example of a very resistant and durable polymer. The lighter PEFE are also volatile and
can escape to the atmosphere and they have a rapid sorption to sewage sludge [4, 5].
In Dugger, Kim et al the exact nature of the oligomeric build-up is not specified [1]. However
I believe this lubrication method has a great potential. I also believe that biodegradable lubricants
would be a great step towards fabricating green and viable class IV MEMS [6]. I believe assessing
the nature of the oligomeric build-up is the next step in that direction. What happens to this build-
up when the device is cleared of the vapor? What are the functional groups in the monomers?
What impact does it have on the environment, how is it discarded?
Figure 3: On the left: design concept of a system using a VPL with a biodegradable liquid build-up.
On the right: Pressure control
The objective is to design such a lubricant. It must be a VPL, with a good vapor pressure at
room temperature to be readily used, to break down into a liquid build-up coating and lubricating
the substrate, which would be biodegradable or at least photodegradable. The feeding system must
be designed to feed the vapor at a constant pressure. Hence, the vapor would not escape the system
as it would only be replenished as it breaks down in a biodegradable liquid.
Long term outlook and conclusion
At any scale, the use of green lubricant will reduce the emission of green house gas, ozone
depleting molecules and will globally reduce waste. It will reduce the cost in every industry by re-
ducing the amount of waste they have to deal with -logistic, discarding, processing. Biodegradable
materials enable further reduction of waste and can help fertilize the ground or be used as biomass
materials. The use of such lubricant would have a significant impact in sustainability and would
enable further improvement.

5. Mitigating nanotribology issues using green biodegradable lubricants 5
At the nanoscale, as far as MEMS and NEMS are concerned, finding a good lubricant is key to
to make a lot of technology ideas a reality and to help spawn new ones. The main expectations of
MEMS/NEMS with rubbing part are the use of gear and rotary hubs to make complex mechanical
devices. Those would enable the fabrication of micromotors, microrobots, more sensitive gyro-
scope. For an out of scale example of the power of gears, even though it is well known, gears can
be used to amplify angular velocity to amplify the energy generation of a wind turbine. At the mi-
croscale, gear structures have been observed in insects, such as Issus Coleoptratus [7]. Nano- and
microrobots or systems would benefit from the tribology phenomena happening at the nanoscale
such as adhesion: a ”nano-fly” could walk on the ceiling like a real one because of the size of
its ”paws”. Those properties can be useful to some applications and can be turned into amazing
technologies.
Further more as mentionned those lubricants would completely clear out of a device when
the vapor is not supplied. This shows that this lubrication method would not change the mechan-
ical properties and geometry of a device. It can be ”washed”. Whereas the typical lubricant used
rely on a solid coating such as diamond which cannot be removed, it is harder to replenish when
it wears off, as we want an even coating. With VPL, devices only use the minimum amount of lu-
bricant needed and replenishes the coating as soon as alcohol molecules desorb from the substrate.
Thus as long as the equilibrium is maintained, the substrate will be lubricated with just the right
amount of lubricant, there won’t be any excess piling up and altering its operation.
Reducing friction also means reducing energy loss and heat generation in systems. It would
enable the design of devices with smaller or no cooling systems leading to smaller and lighter
applications. We could get closer to the Carnot efficiency and be more energy efficient in the op-
eration of devices.
Hence, I believe that there is indeed research and commercial opportunities to be realized if
I can study and design such a lubricant - a lubricant with a biodegradable secondary in situ lubri-
cant.

6. Mitigating nanotribology issues using green biodegradable lubricants 6
References
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