Volume 6 Preprint 47
Metal Surfaces with Ultrahydrophobic Properties: Perspectives for Corrosion Protection and Self-Cleaning
M. Thieme, R. Frenzel, V. Hein and H. Worch
Keywords: aluminium, anodisation, oxide layer, micro-roughness, ultrahydrophobicity, bionics, stability testing
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Volume 6 Paper C113
Metal Surfaces with Ultrahydrophobic
Properties: Perspectives for Corrosion
Protection and Self-Cleaning
M. Thieme1, R. Frenzel2, V. Hein1, H. Worch1
Technische Universität Dresden, Institut für Werkstoffwissenschaft,
2 Institut für Polymerforschung Dresden e.V., Hohe Str. 6, D-01069
This contribution describes the generation of ultrahydrophobic
aluminium surfaces in view of improving the corrosion prevention
behaviour of this material group. The morphological and chemical
prerequisites for ultrahydrophobicity are fulfilled by a treatment which
comprises an anodisation step under intensified conditions and a
chemical modification. The paper focusses on i) findings gained for
the special anodisation procedure and for the produced oxidic layers,
such as the composition, micro-structure and micro-hardness, ii) the
chemical modification and the wetting properties achieved in
dependence on the morphology of the substrates, and iii) results of
first tests directed on the evaluation of the resistance of different
coating systems towards water and light exposure.
Keywords: aluminium, anodisation, oxide layer, micro-roughness,
ultrahydrophobicity, bionics, stability testing
Water shows a remarkably ambivalent behaviour with respect to
corrosion. On the one hand, it is an essential prerequisite for
generating passive layers well-known for iron, nickel, chromium,
titanium, aluminium and many other elements and alloys. On the other
hand, water molecules play a key role in the course of the active
dissolution as the transfer of metal ions from the solid into aqueous
electrolyte solutions, which may subsequently be connected with the
precipitation of solid, non-protecting layers. One of the traditional
ways for preventing metal surfaces from such an attack comprises the
formation of separating layers between substrates and media. This
strategy is well-known in corrosion science under the designation
‘passive corrosion protection’. In the case of aluminium, which
exhibits a very high thermodynamic tendency for the formation of its
oxide, the surface is usually anodised forming oxidic layers with
thicknesses from nanometers up to about 0,1 mm. Normally, this type
of conversion layers are very hydrophilic; porous sub-layers absorb
water and with it also dissolved deleterious substances. Moreover,
corrosion-stimulating solid particles may deposit on the surface.
On this background, it can be expected that the protecting effect of
anodised layers on Al and its alloys will be enhanced, if the separation
is combined with water-repelling properties and, even better, with
ultrahydrophobic properties, which are connected with self-cleaning
effects, too. This property profile is an originally natural strategy,
which was evolved in order to minimise deleterious effects of water
itself and of micro-biological attack. Clearly, such a strategy has found
considerable attention, stimulated by the fundamental investigations
of Barthlott and Neinhuis [#ref01,02] into the water-repelling
properties of the leaves of the lotus plant (Nelumbo nucifera) and a
great number of other herbs. They created the term ‘Lotus-effect®’ and
promoted a bionics-based development in technology. Meanwhile,
impacts are recognised in various fields, such as ceramics, plastics and
paintings for facades. In each case, certain morphological and
chemical features must be generated.
We have transferred ultrahydrophobicity to metals such as aluminium
and titanium. In contrary to pathways using structured hydrophobic
coatings of higher thickness, we produced the required micro2
roughness directly on the metals' surfaces utilising quite different
approaches [#ref03]. This was followed by a thin-film coating
(chemical modification) affecting neither the morphology nor the
optical look. Alkyl silanes without or with fluorine-substituted chain,
ω-functionalised alkyl silanes, a fluorine-containing co-polymer, and
polyelectrolyte/surfactant complexes were successfully employed
[#ref03], alkanephosphonic acids as well. Efforts are now being made
to improve the mechanical stability of the systems developed.
This contribution refers to the major technological pathway elaborated
that comprises an anodisation step under intensified conditions, where
formation of an oxidic layer and micro-roughening are simultaneously
done. The paper focusses on i) findings gained for the special
anodisation procedure and for the produced oxidic layers, such as the
composition, micro-structure and micro-hardness, ii) the chemical
modification and the wetting properties achieved in dependence on
the morphology of the substrates, and iii) results of first tests which
are directed on the evaluation of the resistance of different coating
systems towards water and light exposure.
Materials: Pure aluminium EN AW-Al 99.5 (EN AW-1050) was used as
well as the materials Al 99.9 (1090; Merck) and Al Mg1 (5005). The
size of the sheet specimens was 26x38 mm2.
Pretreatment: All the specimens were initially undergone a cleaning
pre-treatment (abbr. ‘P’), consisting of alkaline pickling (1 M NaOH,
ambient temperature, 10 min) and neutralising (1 M HNO3, 60 s);.
Anodisation: For micro-roughening, the specimens were anodised
under the following conditions which are designated as ‘intensified’
(abbr. ‘Asi’): stirred solutions of H2SO4 and Al2(SO4)3 with a total
sulphate concentration of 2.3 mol L-1 (c(Al3+)max = 0.36 mol L-1),
temperature T = (40±1) °C, current density j = 28-30 mA cm-2,
duration t = 1.2 ks. The specimens were rinsed and water-immersed
for at least 3.6 ks.
Other well-known anodisation procedures were applied for
comparison [#ref04,05]: i) the usual sulphuric acid anodisation (18-
20 °C, 15 mA cm-2, 1.5 ks; ‘Asu’), ii) conditions for generating greater
thickness and hardness of the layer (1.7 M H2SO4, 1 °C, 22 mA cm-2,
0.9 ks; ‘Ash’), iii) conditions for generating a barrier-type oxide layer
(0,5 M B(OH)3 + 0,05 M Na2B4O7, ca. 22 °C, 2.5 mA cm-2, max. 40 V;
‘Ab’). Additionally, alternative conditions for achieving
ultrahydrophobicity (0.5 M H2SO4, 10 mA cm-2, 10.8 ks; [#ref07]) were
tested. Thermal treatments (‘T’) up to 450 °C were added in some
Mass changes were followed by weighing, where m1 - starting mass,
m2 – mass after anodisation, m3 – mass after thermal treatment. These
parameters were supplemented by the mass m4 after stripping the
layer in a small volume of 1 M H2SO4 and 0.7 M HCl, respectively
(40 °C, 2.4 ks). These values were corrected for data obtained with
specimens without oxide, because the stripping proved to be not fully
selective. Before stripping the specimens were immersed in water for 1
Chemical modification (‘cM’): The following solutions were employed:
hexadecyltrimethoxysilane (HTMS) (5 vol.% in aceton, 3 h),
tetradecanephosphonic acid (TDPA) (0.1 wt.% in ethanol-water
1+1, 12 h), both for routine testing,
perfluoroalkyltriethoxysilane (PFATES) (2 vol.% in t-
butylmethylether, 3 h),
[3-(2-aminoethyl)aminopropyl]trimethoxysilane (AAPS) (10 vol.%
in ethanol, 3 h) followed by coating with the fluoropolymer
difluoro-1,3-dioxole) (Teflon® AF; DuPont) (0.6 wt.% in
perfluoro-(2-perfluoro-n-butyl)-tetrahydrofuran (FC75; 3M)).
Characterisation: The morphology was examined by scanning electron
microscopy (Zeiss DSM 982 Gemini) preferably under specimen tilting.
Roughness and roughness parameters were assessed by atomic force
microscopy (AFM; Digital Instruments Dimension 3100 in tapping
mode equipped with Pointprobe or UltraSharp silicon cantilevers;
Nanosensors). Additionally, the confocal laser scanning microscopy
(Zeiss LSM 510) was used. Metallographic cross-sectioning was done
for studying the micro-hardness (Vickers diamond, 50 mN, 20 single
measurements on one specimen) and the micro-structure of different
oxide types. The latter was accomplished using the etching technique
after Weck and ac metal deposition from a bath with SnSO4 and CuSO4
Structural information on the atomic level was derived from X-ray
diffraction (XRD; Siemens D 500) and from nuclear magnetic resonance
spectrometry (NMR) probing the
nuclei (Bruker MSL 300, magic
angle spinning) in the powdered layer material scratched off from the
Water content and binding or adsorption of the modification
substances were studied by infrared spectrometry (FT-IRRAS; PerkinElmer FTS 2000 equipped with an autoimage microscope and a
motorised xyz-stage). Additionally, quantitative analysis of the
hydrogen content was accomplished by nuclear reaction analysis (NRA
[#ref08]) employing the reaction 1H (15N, αγ)
resonance energy 6.385 MeV, max. dosis ca. 3e+13
6 mm2). Basic
information on the composition of the anodic layers was gained from
chemical analyses of the dissolved oxidic layers using complexometry
with back titration [#ref12] (mAl,ox – mass of Al in the layer), ICP-MS
(Perkin-Elmer ELAN 5000; mAl,ox*, mMg,ox*) and ion chromatography
(Metrohm, equipped with suppressor and conductivity detector;
mSO4,ox). The stripping had been optimised before in view of selectivity
and completeness. Small acid volumes of 1 M H2SO4 and 0.7 M HCl,
respectively, were used (40 °C, 2400 s; m4 – mass after stripping).
The wetting properties were characterised by dynamic contact angle
(DCA) measurements (Krüss DSA10) based on 3-5 sites on each
Stability tests: i) Water spraying using a home-made setup, which
directed a fine water beam (ca. 1 ml s-1) in an angle of 45° onto the
centre of the specimens. An area of about 40 mm2 was affected.
Durations of 4 h, 24 h and 100 h were employed including specimens
with HTMS, PFATES and Teflon® AF coatings.
ii) Constant-climate tests according to DIN 50017-KK (40 °C, 100 wt.%
relative moisture under dewing, exposure 360 h) including TDPA,
PFATES and Teflon® AF coatings, 8 specimens each.
iii) Simulating weathering tests according to DIN EN ISO 11341, Zyklus
A, comprising a change of moistening and drying (period length 2 h)
combined with Xenon lamp radiation (exposure 360 h; Institut für
Korrosionsschutz Dresden GmbH). Likewise, TDPA-, PFATES- and
Teflon® AF-coated specimens (4 of each) were tested.
Results and Discussion
Characteristics of the intensified anodisation process
Electrochemical features: Chronopotentiometric curves of the anodic
process are characterised by an initially steep potential rise up to a
maximum at about 12 V, which is reached after a few seconds pointing
to the fast buildup of the barrier film (see below). In the following a
potential plateau is formed at a level of 9-10 V. The slight variation of
the potential heights reflect the different conductivity of the solutions
according to the varying H+-to-Al3+ ratio. The curves are similar to
those for the usual anodisation conditions where the potential levels
are higher by about 2 V. This indicates that the oxide forming
proceeds qualitatively in a similar manner.
Mass analysis: Further insight into the anodisation process was gained
from the analysis of the mass parameters in dependence on the
anodisation time t (Fig. 1a). The value (m2-m1)/A, which is connected
with the anodisation process itself, passes a shallow maximum for
anodisation times of about 0.3 ks and moves into the negative range
beyond 0.6 ks, i.e. the specimens are undergone an increasing mass
loss in the course of anodisation. The specific oxide mass (m2-m4)/A
grows up to a limiting value of about 2.5 mg cm-2 for anodisation
times of more than 1.2 ks, disregarding the potential constancy from
about 10 s on. The overall metal loss described by (m4-m1)/A obeys
Faraday’s law very correctly. This means that the transformation of Al
into Al(III) proceeds exclusively on electrochemical basis.
∆ m A / mg cm
ε ox, r Al
m 2- m 4
m 2- m 1
m 4- m 1
q / C cm -2
q / C cm -2
Fig. 1. Characteristics of the anodisation under intensified T, j
conditions and medium stirring intensity for varied anodic charge
density q; a) specific mass changes ∆m/A and spec. metal loss acc. to
Faraday’s law (green line); b) oxide formation effectivity εox acc. to (1)
based on m4-m1, and mass ratio rAl acc. to (2); means of 2-8 single
Oxide formation effectivity: The degree of the formation of the oxidic
layer was characterised by the oxide formation effectivity εox as that
portion of the oxidised Al(III) which is bound in the layer (mAl,ox as
determined by complexometric titration following the selective
dissolution) ratioed by the total metal loss. The latter quantity may be
substituted by the charge density q as stated above:
ε ox =
zF mAl,ox / A
m4 − m1 M Al
Fig. 1b shows the course of εox vs. q. The oxide formation effectivity
has a maximum of εox ≈ 0.45 at the beginning of the anodisation
(possibly with the exception of the first point at 100 s), whereon it
decreases with increasing duration in a practically linear manner down
to about 0.2. Interestingly, experiments without stirring resulted in
even lower effectivities (∆εox = -(0.1-0.15) for t = 800-1200 s). Similar
results were obtained with Al 99.9 and Al Mg1. It should be noted that
the relatively low values of the effectivity may be disadvantageous
from an economic point of view, but this is a consequence of the
occurring reactions which are leading to the specific morphology (see
below) as the major target of the anodic treatment.
It is well-known that the oxide formation effectivity of the usual
anodisation process is much higher due to the lower temperature.
Comparative measurements gave values of nearly 80 wt.%.
Morphology and wetting properties
After anodisation under the intensified conditions according to the
‘Asi’ procedure stated, the surface is given a micro-roughness
characterised by non-regularly ordered mountain-like structure
elements of typically 1-2 µm in distance and height (Fig. 2a, b).
Additionally, a very fine, sub-µm-scale roughness can be seen.
Contrarily, the usual anodisation ‘Asu’ is characterised by a more or
less flat, shell-like shaped (oxidic) surface (Fig. 2c). Subsequent hotwater sealing may lead to a complete change of the respective
Fig. 2. SEM images of differently anodised surfaces; a) Al 99.5, ‘P’ +
‘Asi’; b) Al Mg1, ‘P’ + ‘Asi’ (35° tilted); c) Al 99.5, ‘P’ + ‘Asu’.
These features have been proved to be decisive in view of the issue,
whether or not the surface will display ultrahydrophobicity. Only on
micro-rough surfaces, advancing and receding angles of 150-160°
each were received, following an appropriate chemical modification
treatment (see below).
Whereas the ‘Asi’-produced surface was very uniform on Al 99.9, with
Al 99.5 hole-like features (dimension of about 10 µm, Fig. 2a)
occurred additionally. These regions are ascribed to oxygen gas
evolution at sites of impurities in the material [#ref06]. The wetting
properties were not affected.
Surprisingly, the treatment conditions given by Shibuichi and co-
workers [#ref07] did not produce a suitable roughness and, hence,
failed in achieving ultrahydrophobicity.
In view of generating micro-rough surfaces and ultrahydrophobic
properties, a closer examination of the influence of the anodisation
parameters led to the following conclusions:
The anodisation time had to exceed a certain minimum duration
to produce the micro-rough surface type. At T = 40 °C, j =
28 mA cm-2 a minimum duration of about 1.0 ks was found;
however, influences of the temperature and the stirring rate
were detected (Fig. 3). A sharp change in the microscopic shape
may occur in between only 0.1 ks during anodisation.
Fig. 3. SEM images (35° tilted) of anodised surfaces under increasing
stirring intensity (a-c); Al 99.9, ‘P’ + ‘Asi’, t = 1.0 ks.
The temperature showed a ‘treatment window’ which ranges
altogether from 35 °C to 50 °C at j = 28 mA cm-2, t = 1.5 ks.
This window becomes more narrow for t = 1.2 ks.
At 40 °C, the current density could be varied from 28 up to 42 mA cm2
(or even more) without changing the microscopic shape and, hence,
the wetting behaviour. Disadvantageous effects of a lower c.d. could
not be compensated for by prolonged duration, i.e. the charge density
is an important, but not the decisive quantity.
Structural and mechanical properties
No characteristic features of any oxyhydroxide or hydroxide
compounds could be gained by X-ray diffraction measurements on
anodised specimens, independent of the anodisation type.
NMR revealed that the Al atoms are surrounded by O in a non-uniform
manner. AlO4, AlO5 and AlO6 coordination was found at the same time,
thus excluding the presence of a certain crystalline compound. Rather
an amorphous structure should be present. The same holds for the
powder material after a thermal treatment at 450 °C, 3.6 ks. XRD
detected traces of bayerite γ-Al(OH)3.
Infrared measurements pointed to a difference between ‘Asi’- and
‘Asu’-generated layers comparing the region below 900 cm-1, in which
the Al-O binding is displayed (Fig. 4). This reflects a different atomic
binding for Al, even after tempering (see below).
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber ν ' / cm -1
Fig. 4. FT-IRRAS results of specimens after different anodisation (plus
Images of cross-sections indicated an average layer thickness of about
10 µm with a profile height of 2 µm. Stripping of the oxidic layer led to
a specific mass loss of 2.3 mg cm-2 corresponding to a density of
about 2.3 g cm-3, which is similar to the tabulated value of 2.42 g cm-3
for Al(OH)3, but considerably lower than for Al2O3 (3.90 g cm-3
Under high magnification, SEM images of cross-sections revealed the
porous, anisotropic nature of the major part of the layer. This finding
could be unambiguously visualised by ‘decorating’ the pores with a
highly contrasting metal filling (Fig. 5a). Additionally, the cross-
sections were etched to achieve a greater differentiation of the bulk of
the layer. Fig. 5b demonstrates that the outer part of the layer
underneath the micro-rough surface appears uniform, whereas the
inner part shows a fibre-like micro-structure, which means that the
chemical stability of the oxidic substance is locally different. This
statement was true also for ‘Asu’-type oxides (Fig. 5c).
Fig. 5. SEM images of cross-sections; a) ‘P’ + ‘Asi’ plus cathodic
deposition of Sn/Cu into the pore grounds of the oxidic layer; b) ‘P’ +
‘Asi’, cross-section etched; c) ‘P’ + ‘Asu’, cross-section etched; d) ‘P’
+ ‘Asi’ + Cu evaporated for contrasting, micro-hardness indent in the
oxidic layer; substrate-layer interfaces marked by arrows.
Micro-hardness indents were nearly regularly shaped without any
cracks (Fig. 5d). This points to a certain degree of compressibility,
which is obviously larger in the lateral direction, as is expressed by the
marked asymmetry of the indents. The micro-hardness amounted to
250-290 HV 0.005 / 5 for ‘Asi’-produced layers on Al and Al Mg1.
This was probably somewhat lower than for ‘Asu’ (296±34 for T =
23 °C, 343±33 for 16 °C). The standard deviation on a single specimen
was in the range ±10-15 %. Of course, the hardness of ‘Ash’-type
specimens was highest (403±30). Thermal post-treatments may lead
to an enhancement of the micro-hardness, especially with Al Mg1.
Interestingly, the subsequent application of coating systems (see
below) influences the micro-hardness of the oxidic layer. Whereas an
enhancing tendency was found for specimens treated in non-aqueous
media containing silanes, the micro-hardness was considerably
diminished by employing TDPA solution, which contain water in a high
concentration (e.g. 253 HV 0.005 Æ 168 HV 0.005 for an Al Mg1
specimen). However, TDPA solution did not reduce the mechanical
stability following a thermal treatment. Thus, re-hydratisation
phenomena can be limited or even prevented.
Analytical results referring to the oxidic layers
Al content: The content of Al in the oxidic layer was ratioed by its total
mass in order to get a first information on what compound the layer
substance might represent:
m2 − m4
For instance, AlOOH (= Al2O3.H2O) would correspond to a mass ratio rAl
= 0.45, Al(OH)3 (=Al2O3.3H2O) to rAl = 0,35. The experimental results
(Fig. 1b) gave values of rAl = 0.34-0.41 with an increasing tendency in
the course of anodisation. This means the composition would lie
between the hydroxide and the oxyhydroxide, if an oxidic compound
without further components was preliminarily assumed.
Mg content: The analysis data based on ICP-MS gave practically the
same Mg/Al mass ratio as in the original material.
Water content: The anodic layers contain a noticeable content of water
depending on the type of anodisation, but also on the degree of the
preceding water immersion. The content of hydroxyl groups and/or
incorporated water molecules could be semi-quantitatively determined
by FT-IRRAS investigation based on the region of 3400-3500 cm-1.
The ‘Asi’ layers contained markedly more water than ‘Asu’ layers
(Fig. 4). Moreover, the measurements showed that the water
concentration in ‘Asi’-type layers was reduced by thermal treatment
down to the level of ‘Asu’ layers. However, a complete de-
hydratisation could not be achieved under the conditions chosen
(400 °C, 0.9-3.6 ks). This finding does not correspond to the
statements given for the thermal transformation of bayerite and
Based on gravimetrical control, the amount of the desorbed water
reached a limit of 3-4 wt.% in the temperature range of 350-450 °C.
However, this content is not a constant. Following a prolonged water
immersion of 20 h, a much greater water amount was indicated by
IRRAS and the thermally removed portion was higher (10 wt.%).
These results were contrasted by data from NRA referring to a depth of
about 1 µm. Here, two parts could be discriminated: i) easily
removable hydrogen, probably owing to physically adsorbed water, ii)
stationary concentrations. The total concentrations were calculated as
atomic ratios to rH,at = 18.8 at.% H (‘Asi’) and 7.4 at.% H (‘Asu’).
Assuming again a hydroxidic-oxidic compound of the formula
Al2O3.xH2O for the ‘Asi’-produced layer type, the following values may
be assessed for x: xtherm ≈ 0.22 and xNRA ≈ 0.65, respectively. The
discrepancy xtherm ≠ xNRA underlines that a great portion of the total
amount of incorporated water appears to be bound relatively strongly.
Sulphate incorporation: Sulphate was qualitatively indicated by IRRAS
(S-O binding, 950-1000 cm-1). Based on mass data, a mass ratio
analogous to (2) was defined. Values of rSO4 ≈ 0.17 were found, equal
to data for usual anodic layers formed in sulphuric acid media
[#ref05]. This means that the simplified formula Al2O3.xH2O of the
layer substance should no longer be acceptable.
Formation and composition of the oxidic layer
From all these findings ideas about the formation of the layer may be
derived. The principal composition of the layer type generated by
intensified anodisation is similar to the usually acid-produced oxide
[#ref05]. Thereafter, the composite layer consists of a very thin barrier
film at the metal-oxide interface and a much thicker sub-layer of
porous nature (Fig. 5a, b). The total potential decay is divided between
the resistance of the solid state conduction process in the barrier film
(thickness db ≈ 0.9 nm [#ref05]) and, to a presumably lesser extent,
the electrolyte resistance in the pores. The layer growth takes place
primarily at the interfaces of the barrier film connected with the
transport of Al3+ and O2- ions through the defective oxidic solid
according to the brutto reaction
2 Al + (3+x) H2O → Al2O3.(H2O)x + 6 H+ + 6 e-
At the same time, a transformation of the barrier layer into the porous
layer proceeds so that db remains constant. Obviously, the interior
structure of the porous sub-layer is formed under the influence of the
electric field (Fig. 5a, b). In the pores and at the outer interface,
equilibria of the form
Al3+ + x H2O ↔ Al(OH)x(3-x)+ + x H+
will establish regulating dissolution and precipitation.
Additionally, the ions SO42- and HSO4- from the electrolyte solution will
participate in the reactions and incorporate into the solid. Substituting
now Al2O3.xH2O by the extended formula Al2[O3-y-z/2 (SO42-)y (HSO4)z].xH2O and assuming y << z because of the dissociation equilibria of
H2SO4, it is possible to give a rough idea about the composition of the
layer substance. From the experimental mass ratio data (rAl ≈ 0.40, rSO4
≈ 0.17 and rH,at ≈ 0.19) and the corresponding molar masses the
coefficients z ≈ 0.24 and x ≈ 0.52 are calculated for layers produced
under ‘Asi’ conditions.
The thickness growth of the porous layer is increasingly limited by the
chemical dissolution at the outer interface, until a stationary state will
establish. The stirring effect may be understood considering the heat
production at the ground and in the pores of the layer. In case of an
unstirred solution the temperature rise will inevitably lead to a more
intensive dissolution, connected with stronger profiling (Fig. 3a) and a
decrease of the oxide forming effectivity compared to stirring.
Chemical modification and wetting properties
Extensive selection tests were carried out for choosing suitable
hydrophobic compounds. This led to the employment of various
compound groups, such as differently functionalised reactive
alkylsilanes, alkanephosphonic acids, fluoropolymers, and
polyelectrolyte/surfactant complexes, which can covalently react with
the hydroxylated surface, simply adsorbed or electrostatically bound
For routine preparation, the application of an easy silane was sought.
Fig. 6 displays a comparison of the wetting behaviour based on ‘P’-
treated (non-anodised) Al 99.9 and coatings of alkyltrialkoxysilanes,
which are expected to react with the solid (s) primarily according to:
s-OH + (R-O)3SiCnH2n+1 → s-O-Si(OR)2CnH2n+1 + R-O-H
The wetting angles, especially θr, increased with increasing chain
length up to n ≈ 16. Therefore, HTMS was chosen as a standard
θa,r / °
Chain length n
Fig. 6. Wetting angles θa (left bars) and θr (right bars) of ‘P’-treated
Al 99.9 following a modification using alkyltrialkoxysilanes
Wetting angle data for different pretreatments and HTMS modification
are compiled in Table 1. For the ‘Asi’-treated surface type,
ultrahydrophobicity was clearly achieved. The corresponding contact
angle hysteresis (θa - θr) was mostly negligible. Water droplets will roll
off the specimen even in case of a minute inclination. The most
striking differences to the non-roughened surface types are seen for
the receding angles. As expected, all the non-coated surface types
were completely wettable in a fresh state.
θa / °
θr / °
‘P’ + ‘Asi’ (sulphuric acid, T, j intensified, t
‘P’ + ‘cM’ (HTMS)
‘P’ + ‘Ab’ (borate buffer) + ‘cM’ (HTMS)
‘P’ + ‘Asu’ (sulphuric acid, T, j regular) +
‘P’ + ‘As’ (j intensified) + ‘cM’ (HTMS)
‘P’ + ‘As’ (T intensified) + ‘cM’ (HTMS)
‘P’ + ‘As’ [#ref7] + ‘cM’ (HTMS)
= 1.2-1.5 ks) + ‘cM’ (HTMS)
Table 1. Comparison of wetting properties of differently treated
surface types (data scattering: standard deviation for one specimen,
span widths for several specimens).
Altogether, ultrahydrophobicity was achieved employing HTMS,
chlorosilanes, PFATES, TDPA, Teflon and others based on microroughened oxidic surfaces, thus forming inorganic-organic hybrid
systems. The modification coatings can generally not be detected by
means of SEM. IRRAS investigations, however, indicated typical
features of the coatings additionally to those owing to the substrates
(Fig. 7): i) alkyl chains of HTMS and TDPA (C-H stretch, 28003000 cm-1), ii) C-F binding preferably of the alkyl chains with PFATES
Wavenumber ν ' / cm -1
Fig. 7. FT-IRRAS results of coated specimens following ‘P’+’Asi’;
orange – HTMS, light green – PFATES, pink – AAPS / Teflon® AF; brown
(single bands) – HTMS after water spray test, dark green – PFATES after
water spray test.
The quantitative determination of surface energies is only restrictedly
possible, because this quantity is not defined for a material-air
composite surface, as is in fact produced by the intensified
anodisation (cf. below). The maximum wetting angle values measured
for more or less flat surfaces plus modification correspond to surface
energies of γs,min = 6-8 mJ m-2 based on the equation of Li and
cosθ a = −1 + 2
exp(− ß (γ l − γ s ) 2 )
where γl - surface tension of water as the test liquid (72.8 mJ m-2) and
β - constant (β = 0.0001247 m2 mJ-1). It should be noted that this
level of the surface energy is equal to the value given for the
trifluoromethyl group (ca. 6 mJ m-2), whereas it is considerably lower
than for flat PTFE (18.5 mJ m-2 [#ref07])
For micro-profiled and chemically modified substrates, the profile-
bearing area tp to a resting water droplet may be assessed according
to the formula given by Cassie and Baxter [#ref10]:
cosθ rough = t p ⋅ cosθ smooth − (1 − t p )
A level of only 10 wt.% is obtained for ultrahydrophobic surfaces. The
corresponding force for the droplet to start rolling on such a surface is
estimated to be in the sub-µN region, what explains the behaviour
Water spray test: These tests comprised HTMS, PFATES and AAPS /
Teflon® AF as coatings based on a ‘P’ + ‘Asi’ treatment. In the
beginning, the fine water beam bounced back vividly from all
specimens involved. This was expected from the wetting angle data.
With HTMS, the contact area enlarged after 2 h only. After 100 h a
dramatic deterioration was observed (θa = 120-125° and θr < 10°).
Whereas the IR spectrum showed no change after 4 h, the examination
of the wetted area after 100 h revealed some alterations, the most
prominent of which was a new band at 1730 cm-1 that must be
ascribed to the formation of a C=O group (Fig. 7). The AAPS /
Teflon® AF coating showed a noticeably better behaviour, although at
termination of the test the bouncing effect had disappeared and the
DCA values were reduced (140°/10°). Most stability could be stated
with the PFATES-coated specimens. Here, the visual wetting behaviour
had only slightly changed connected with moderately reduced receding
angles (158°/159° Æ 142-153°/130°). The IR spectra were practically
identical with the original state (Fig. 7).
Constant-climate test: The tests over 360 h were passed without
noticeable deterioration of the wetting properties for PFATES and
Teflon® AF coatings. TDPA-modified surfaces retained
ultrahydrophobicity, but showed a decrease of the wetting angles by a
few degrees (2-5°).
Simulating weathering test: The wetting data received before and after
this test are displayed in Fig. 8. According to the degree of the wetting
angles’ decrease the resistance towards dewing/drying and light
radiation is ranking according to: TDPA << AAPS / Teflon® AF <
θ a,r / °
Fig. 8. Comparison of the wetting data (θa/θr) for differently coated
specimen types; yellowish bars - original state, bluish bars - after the
simulating weathering test.
Ultrahydrophobic aluminium surfaces were successfully generated
based on an intensified anodisation process, which aims at forming a
micro-roughened oxidic surface, and a subsequent chemical
modification using various hydrophobic compounds. Characteristic
data were determined for the specific oxide mass and the oxide
formation effectivity, which is remarkably lower than with the usually
employed anodisation. Information was gained on structural properties
and the composition, which is characterised by a markedly higher
portion of water. In first tests under water and light exposure, the
silane coating type with a fluorinated alkyl chain shows the highest
stability among the coating compounds investigated. This may be a
promising finding in view of further developments and testing.
Although the micro-hardness of the oxidic layers is fairly high, the
total hybrid system is relatively sensitive to scratching. This issue is
under investigation at present.
Acknowledgments: The authors are indebted to valuable experimental
contributions of R. Born, M. Ruhnow, T. Fuhrmann, U. Cikalo, R.
Süptitz (Technische Universität Dresden), A. Hennig (Institut für
Polymerforschung Dresden e.V.), E. Brendler (Technische Universität
Bergakademie Freiberg), U. Schaefer, D. Birnstein, D. Grambole
(Forschungszentrum Rossendorf e.V.). The financial support by the
Sächsisches Staatsministerium für Wissenschaft und Kunst (contract
no. 4-7533-70-821-98/3) and the Bundesministerium für Bildung
und Forschung (FKZ 03C0340B) is gratefully acknowledged.
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