22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Solution precursor plasma sprayed superhydrophobic surface
Y. Cai1, J. Mostaghimi1, T.W. Coyle1 and G. Azimi2
1 Centre for Advanced Coating Technologies (CACT), Department of Mechanical & Industrial Engineering, University
of Toronto, Toronto, ON, Canada
2
Department of Chemical Engineering, University of Toronto, Toronto, ON, Canada
Abstract: This work presents the first transition metal oxide superhydrophobic surfaces
fabricated by using the solution precursor plasma spraying technique. The water contact
angle measured on the surface is 155°. The dynamic impact of a water droplet is also
captured by a high speed camera. The topography and cross-sectional microstructures of
the coated surface are examined by scanning electron microscopy.
Keywords: solution precursor plasma spray, superhydrophobic surface
1. Introduction
Since the development of electron microscopy in the
last century, people have been able to study nanotextures
of plant surfaces and animal skins. It has been found that
a unique surface architecture provides a self-cleaning
ability to leaves (e.g., lotus leaves), which is shown in
Fig. 1, and allows the wings of insects to remain dirt-free
and shed water when they fly in the rain [1]. This type of
surface is called a hydrophobic surface. Hydrophobic
surfaces are characterized by a high water contact angle
(>90°) and a low roll-off angle. When the water contact
angle on the surface is higher than 150°, the surface is
called superhydrophobic.
Due to these properties,
hydrophobic surfaces have a wide range of potential
applications to benefit the environment in energy
conservation, including enhancement of condensation in
steam power plants in order to increase the efficiency of
electricity generation [2]; and promotion of nucleation for
pool boiling at low heat flux to enhance boiling heat
transfer [3]. Very recently, transition metal oxides have
been proposed as a means of creating hydrophobic
surfaces [4]. Due to their unique electron configuration
and high melting temperature (over 1500 °C), transition
metal oxide surfaces exhibit a high water contact angle
even when they are smooth and at elevated temperatures.
However, the link between this research and industrial
applications has not been firmly established due to the
difficulties of large scale fabrication, and complex
processing procedures.
Plasma spray deposition is a technique which has been
widely used in industry to produce coatings due to its
high deposition rate, near-net shape finishing, and most
importantly, its ability to process almost all materials.
Conventional plasma spray uses micro scale powder
particles as the feedstock for the material to be deposited.
Solution precursor plasma spray (SPPS) is a relatively
new technique that, rather than using powder particles,
uses a solution which decomposes during deposition to
form the coating. The coating formation mechanisms for
solution precursor plasma spray are different than for
conventional plasma spray, and lead to nano- and
O-21-2
Fig. 1. a) Lotus leaves, which exhibit extraordinary water
repellency on their upper side. b) Scanning electron
microscopy (SEM) image of the upper leaf side prepared
by ‘glycerol substitution’ shows the hierarchical surface
structure consisting of papillae, wax clusters and wax
tubules. c) Wax tubules on the upper leaf side [1].
submicron-structured coatings [5].
This type of
hierarchical structured surface is very desirable in
fabricating hydrophobic surfaces since it captures the
essence of self-cleaning leaves and wings in nature. To
our knowledge, there has yet to be a superhydrophobic
surface produced by the solution plasma spray method.
In this work, superhydrophobic coatings were fabricated
using axial-injection SPPS.
Transition metal salt
dissolved in the water and alcohol mixture was used as
the liquid feedstock to the plasma torch. The wetting
behavior of the coated surface is characterized by
measuring the static water contact angle, roll-off angle
and the dynamic impacts of water droplets on the surface.
Surface and cross-sectional microstructures were
analyzed by scanning electron microscopy (SEM).
1
2. Experimental methods
The solution used in the experiment was prepared by
dissolving the transition metal salt into a distilled water
and alcohol mixture. The solution was deposited by the
Axial III Series 600 plasma torch (Northwest Mettech
Corp., North Vancouver, BC, Canda). Argon was used as
the atomizing gas. The plasma gas contained a mixture of
hydrogen, nitrogen and argon. Type 304 stainless steel
was used as the substrate, and the substrate was preheated at 350 °C. The robot arm which carried the torch
moved in a raster pattern, and in total 15 passes were
performed for each sample. The robot arm was set to a
𝐢𝐢
and its vertical step
linear translation speed of 200
𝒎𝒎𝒎
size was 0.2 in.
The as-sprayed coatings were sectioned by the precision
diamond saw IsoMet 5000 (Buehler, ON, Canada). The
sectioned samples were then mounted into epoxy inside a
vacuum chamber at 30 mbar. A low viscosity epoxy
Jetset Epoxy (MetLab Corp, ON, Canada) was selected,
so the epoxy can penetrates into the pores of the
cross-section of the coating. The mounted coating's cross
section was subsequently polished by P320 silica
sandpaper, 45 µm, 15 µm, 6 µm, and 3 µm diamond
disks. Then the 1 µm and 0.05 µm suspensions were used
as the final stages of polishing. Between each polishing
step, the surface was cleaned in an ultrasonic bath and
dried by compressed air. The polished cross sections of
the coating surfaces were coated with carbon and the top
surfaces of the coatings were coated with gold. Scanning
electron microscopy (Hitachi SU 3500) was used to
characterize
the
surface
and
cross-sectional
microstructures. Image processing software was used to
analyze the image of the static water droplet in order to
determine the static water contact angle. The dynamic
impacts of water droplets on the coating were captured by
a high speed camera FASTCAM SA5 (Photron, CA,
USA) at 4000 frames per second.
3. Results and discussion
3.1. Surface and cross-sectional microstructure
Examining the microstructure of the surface of the
coating, a hierarchical microstructure was observed as
expected. Fig. 2 shows the SEM image of the surface.
Micro-scale irregular clusters ranging from 5 µm to
10 µm in size were uniformly formed on the surface. On
the clusters, nanoscale particles were also observed.
In SPPS, the coating is in general formed by the
stacking of irregular particles and molten splats, therefore,
micro-scale agglomerates are commonly observed. The
nanoscale particles may have resulted from the
condensation of vaporized particles. Another possibility
is the nanoparticles passed through the boundary layer
due to the thermophoresis force. Notice that this type of
microstructure is very similar to the hierarchical
structured hydrophobic surfaces in nature.
2
Fig. 2. Scanning electron microscope images of the top
surface showing micro-scale clusters (a) and the fine
nanoscale particles on the clusters (b).
Fig. 3 shows an SEM image of the cross-section. The
coating is porous and a feathery structure is observed.
The particles observed in the coating have irregular
shapes which is an indication of incomplete melting. The
coating appears to have been formed mainly by the
sintering of un-melted (or re-solidified) particles. As the
coating was built-up by the deposition of these particles,
the porosity was enhanced by a shadowing effect which
led to the formation of the feathery structure. The coating
was denser close to the substrate than near the surface.
This may be the result of the coating closer to the
substrate experiencing more torch passes, and therefore
more sintering, than the top of the coating.
Fig. 3. Scanning electron microscope images of the
cross-section.
O-21-2
3.2. Wetting behavior of the coating
A water contact angle of 155°, which indicates the
coating is superhydrophobic, was observed. On a smooth
transition metal oxide surface, the measured water contact
angle was 100° [4]. Thus, the hierarchical microstructure
of the coating formed by the SPPS method enhanced the
water contact angle by 55%. The roll-off angle, another
important characteristic of the wetting behavior, is so
small that the water droplet rolls off at the slightest tilt.
Fig. 4 shows the water droplets on the coating and the
measured water contact angle.
4. Conclusions
This work shows a promising method of fabricating
superhydrophobic coatings using precursor solutions as
feedstock in a plasma spray deposition process. It offers a
fast, simple and low cost method to fabricate a
hydrophobic surface in a large scale. The coated surface
has a hierarchical structure which is similar to lotus leaves.
The water contact angle measured is 155° and the roll-off
angle is less than 5°. The dynamic impact tests show a
great water repellent property of the coating.
5. Acknowledgments
First, the authors would like to thank the support from
our sponsor, NSERC. Also YC thank Dr. Larry Pershin,
Dr. Jeffery Harris, Mr. Sal Boccia, and Mr. Tiegang Li for
providing him the technical expertise required to develop
and implement the SPPS idea of the work.
Furthermore, YC wants to thank Mr. Pedro Isaza, Mr.
Alireza Dalili, Mr. Joel Castonguay, Mr. Gengxu Yan,
and his mother for their supports and helps.
Fig. 4. Water droplets on the coated surface. The droplet
size is 2.5 mm.
The dynamic impact of a single droplet and the
coalescence between two droplets were examined using a
high speed camera. For a single droplet impacted on the
coating at a speed of 1.4 m/s, the droplet completely
rebound. When a second droplet landed beside a static
droplet at the same velocity, it deformed and interacted
with the first droplet. Then the two droplets combined
into one and the combined droplet was also recoiled. The
combination of the hierarchical surface structure and the
intrinsic hydrophobic material gives the coating an
excellent water repellent property. Fig. 5 shows the
dynamic impacts of water droplets on the as-sprayed
coating.
6. References
[1] H. Ensikat, P. Ditsche-Kuru and P. Neinhuis.
Beilstein J. Nanotechnol., 152, 2 (2011)
[2] N. Milijkovic and E.N. Wang. Mat. Res. Soc., 397,
38 (2013)
[3] A. Betz, J. Jenkins, C. Kim and D. Attinger. Int. J.
Heat and Mass Transfer, 733, 57 (2013)
[4] G. Azimi, R. Dhiman, H. Kwon, A. Paxson and
K. Varanasi. Nature Mat., 315, 12 (2013)
[5] Y. Shan, Y, Wang and T. Coyle. Appl. Thermal
Engng., 690, 59 (2013)
Fig. 5. The dynamic impact of water droplets on the
coated surface. Scale bar: 2 mm.
O-21-2
3