Tribology International 136 (2019) 455–461

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Tribology International
journal homepage: www.elsevier.com/locate/triboint

Design of “double layer” texture to obtain superhydrophobic and high wear- T

resistant PTFE coatings on the surface of Al2O3/Ni layered ceramics
Hengzhong Fan, Yunfeng Su, Junjie Song, Hongqi Wan, Litian Hu, Yongsheng Zhang∗
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China

A R T I C LE I N FO A B S T R A C T

Keywords: Solid lubricating polytetrafluoroethylene (PTFE) coating was designed to have a “double layer” texture after
Al2O3/Ni layered ceramics being sprayed on the untextured and textured surface of Al2O3/Ni layered ceramics. Surface texture of PTFE
Bonded PTFE coating coating with smaller diameter and higher density showed excellent superhydrophobicity. Laser caused de-
Double texture fluorinated and enhanced hardness of the PTFE coating. Meanwhile, Solid lubricants embedded in micro-dimples
Superhydrophobic and wear-resistant
of substrate surface act as “mechanical self-locking” roles, which increase the surface contact area and provided
a secondary lubricating effect, thus significantly increase the wear life of the coatings. Contact angle and wear
life of the optimized "double layer" texture PTFE coatings could reach 160° and 1.6 × 104 s, which are about 2.0
and 5.0 times higher comparing to untextured smooth PTFE coatings surface, respectively.

1. Introduction coating maintained its superhydrophobicity after 300 cycles of abrasion

resistance testis at 2 kPa. It remained super-hydrophobic even after
With the rapid development of marine engineering, ship-related being out or after impact tests. Despite remarkable achievements in
industry and other high-tech fields, requirements for surface lubrication (multi-)functional coatings with excellent performances, many pro-
technology significantly increased. Lubricating coatings must not only blems still need to be solved in order for these coatings to be used for
have excellent friction and wear properties, but also good corrosion practical applications. For example, functional coating preparation is
resistance, self-cleaning capability, super-hydrophobicity, drag reduc- tedious and complex, and the cost of raw materials is high. Most studies
tion and other special properties suitable for harsh conditions (e.g., focus on the surface modification of super-hydrophobic coatings, while
water, marine environment, etc.) [1–5]. Because of their low interface only few reports discuss multifunctional abilities of solid lubricating
bonding strength and wear resistance as well as absence of self-cleaning coatings.
and anti-fouling capabilities, conventional lubrication coatings wear Bonding solid lubricating coatings is the most economic and work-
out and peel off rapidly upon friction, which diminishes their protective able fabrication process comparing to other methods [9,10]. These
properties. A lot progress was made during the recent years to improve types of coatings have excellent anti-friction and wear-resistance per-
interfacial bonding strength and to achieve multifunctionality of such formances and are widely used in many fields such as aerospace,
coatings. Li et al. [6] used spraying method to prepare super- ground mechanical equipment, and large aircraft carriers [11–13].
hydrophobic nanocomposite coatings consisting of poly-organosilicon Because of its excellent chemical inertness, thermal stability, corrosion
and various contents of attapulgite. The hydrophobic stability of this resistance, hydrophobicity and low friction coefficient, polytetra-
coating was tested using sediment wear and high pressure. The coating fluoroethylene (PTFE) is often used as a self-lubricating material
remained super-hydrophobic after sediment wear. Milionis et al. [7] [14–16]. In our previous studies, we successfully prepared bonded
prepared a composite coating using acrylene-butadiene-styrene resin as MoS2 lubricating coating on the textured surface of Al2O3/Ni layered
binder and nano-silica as a filler. The coating maintained its super- ceramics and studied tribological properties of this composite. Wear life
hydrophobicity after 1700 cycles of linear abrasion at 20.5 kPa. Simo- of the coated Al2O3/Ni layered ceramics was significantly increased
vich et al. [8] also used spraying technique to prepare acrylic poly- because solid lubricant embedded in the surface micro-dimples im-
urethane coating with fluorosilane-modified nano-silica as a filler, hy- proved bonding strength of the coating and provided a second lu-
droxyacrylic acid as a resin and polyisocyanate as a curing agent. The bricating effect [17]. Moreover, bonded PTFE lubricating coatings,
resulting coating exhibited good wear resistance and adhesion. The extensively studied by other research groups, exhibit excellent friction

∗
Corresponding author. Tel: +86 9314968833; fax: +86 9314968019.
E-mail address: zhysh@licp.cas.cn (Y. Zhang).

https://doi.org/10.1016/j.triboint.2019.04.004
Received 31 January 2019; Received in revised form 25 March 2019; Accepted 1 April 2019
Available online 05 April 2019
0301-679X/ © 2019 Elsevier Ltd. All rights reserved.
H. Fan, et al. Tribology International 136 (2019) 455–461

and wear behavior. However, functionalization (e.g. to achieve super- Table 1

hydrophobicity and antifouling) of bonded PTFE as a lubricating Geometric parameters (diameter, center distance and density for samples) of
coating on the ceramic surface is rarely studied. Superhydrophobicity of different patterns.
PTFE coating can be achieved by two methods: by changing roughness Pattern Diameter d/μm Center distance l/μm Density
of the coating either mechanically and chemically, or by adding a ε/%
surface modifier to reduce surface energy [18,19]. Laser surface tex-
Micro-dimples 50 198 5
turing (LST) is a new strategy for obtaining various micro-patterns on
50 114 15
the material surface, which can provide precise control over the shape 50 89 25
and dimensions of micro-patterns. It is also considered to be a simple 50 75 35
and economical technique among other micro-surface patterning 50 66 45
100 149 45
methods [20,21]. Thus, in this work, we propose PTFE-based lu-
150 224 45
bricating coating sprayed on the surface of micro-structured Al2O3/Ni 200 299 45
laminated ceramic composites to obtained materials with excellent
tribological as well as superhydrophobic and anti-fouling properties. Micro-grooves 50 142 35
Based on the previous studies [17,22,23], solid lubricating PTFE Micro-meshes 50 130 35
Micro-squares 50 200 35
coating was designed to have a “double layer” texture after being
sprayed on the untextured and on textured surfaces of Al2O3/Ni layered
ceramics. Al2O3-based laminated composites are ideal candidates for 2.2. Texture design and preparation of substrate and coating surfaces
high-tech mechanical equipment (such as wear-resistance moving ma-
chine components) because of their high bearing capacity, excellent Surface of the polished substrate was subjected to the laser micro-
strength and toughness. To improve tribological performance of PTFE texture processing. Micro-dimple structure was selected as a pattern.
coatings on layered Al2O3/Ni ceramics, this study simulated harsh Diameters of these micro-dimple diameters were about 100 μm.
service conditions to test wear ability of surfaces of Al2O3/Ni layered Distances between centers of these micro-dimple were 177 μm. Texture
ceramics in high-tech environments to analyze their potential use as density of the processed surface was 25%. Samples with laser-treated
wear-resistance moving components of the machinery. The effects of surfaces were sonicated in acetone, after which prefabricated PTFE was
patterns and geometric parameters of the texture on (super)-hydro- uniformly sprayed on the substrate surface using a Lotus No. 2 spray
phobicity of the coating were investigated. Influence of “double layer” gun under 0.2 MPa of compressed nitrogen. For comparison, PTFE
texture of the substrate and coating surfaces on the wear resistance and coating was also sprayed on the untextured substrate. After spraying,
service life of the coating was studied as well. The overall design con- the coatings were cured at 150 °C for 30 min and then at 170 °C for
cept is illustrated in Fig. 1. 60 min.
Bonded PTFE coating was further textured by same laser equipment.
Surface texture and geometric parameters of the coating designed in
2. Material and methods this study are summarized in Table 1. Four texture patterns (micro-
dimples, micro-grooves, micro-meshes and micro-squares) as well as
2.1. Preparation of substrate material geometric parameters (diameter, center distance and density) of micro-
dimple patterns, were designed and fabricated. Bonded polymeric PTFE
Substrate material used in this study was Al2O3/Ni layered ceramics coating is very different from the ceramic substrate. Therefore, laser
material (Case 3 sample H). Parameters of this layered ceramics (such parameters could not be processed under the same conditions, thus,
as number of layers, transition interface and texture interface) were further debugging and optimization was required. Final processing
optimized in our previous work [22]. These layered ceramics consisted parameters were: 1064 nm wavelength, 8 W power, 10 ns pulse width,
of 4 layers of Al2O3, 3 layers of Ni and 6 transition layers. Thicknesses 10 kHz pulse frequency, 5 mm/s scanning speed and processing cycle.
of Al2O3, Ni and Al2O3+Ni transition layers were 759, 75 and 55 μm, Fig. 1 illustrates design concept of these “double texture” structures
respectively. Ni content in the transition layer was 30%. Texture surface formed on the surface of the Al2O3/Ni-laminated ceramics. The “double
density at the interface of the transition layer was 26.5%. Layered texture” coating was only on the surface of the top Al2O3 layer. Thus,
ceramic substrate material was prepared by dry powder laying, then by each texture (whether it is on the substrate or on the coating itself) will
their cold pressing and hot-pressing sintering, followed by surface not pierce through the adjoining layers below. Thicknesses of the PTFE
polishing to 0.10–0.20 μm roughness. coating and texture depths were determined using stylus profilometer
(KLA-Tencor) and a 3-D profilometer (VHX-5000, Keyence).

Fig. 1. Schematic of the preparation process of “double texture” coating on the surface of Al2O3/Ni laminated ceramics.

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2.3. Measurement of surface contact and sliding angles generated on the coating surface if the laser energy density is too high.
According to the high magnification photos in Fig. 2 (a'-c'), an obvious
Surface static contact angle and sliding angle were measured using heat affected zone with dense gas holes was observed on the texture
SA100 instrument (KRUSS Co., Germany). Testing drop volume was edges after laser irradiation. This can be attributed to the uneven
5 μL. Six points were randomly selected on each surface for measure- coating shrinkage caused by laser ablation. Based on the thermal dif-
ments. The average of these 6 measurements was reported as a final fusion length, material heat zone affected by the laser pulse can be
result. Spreading rate of water droplets on the coatings surface was also evaluated as L= kτp/(QC ) [25] (where k = 0.3 W/m⋅K is thermal
tested. Specifically, 5 μL of water was dropped on the coating, after conductivity, τp = 10 ns is the pulse width, Q = 2.1–2.3 kg/m3 is den-
which their contact angles ware measured dynamically until the water sity, and C = 0.96–1.05 J/Kg⋅K is specific heat). Calculated laser
droplets became completely spread. thermal diffusion length on the PTFE coating surface was ∼1.1–1.2 μm.
Thickness of PTFE coating and depths of micro-dimple “double tex-
2.4. Characterization of microstructure and composition of the coating tures” are shown in Fig. 3. It can be seen that the PTFE coating with a
surface thickness of about 30 μm (Fig. 3a). The substrate surface and coating
surface texture depth were ∼55 and 20 μm, respectively (see Fig. 3b).
Surface texture, microstructure and morphology were characterized Micro-dimples on the composite substrate surface were clear and uni-
using scanning electron microscope (SEM). The typical 3-D topo- form. Surface around the dimples remained smooth after polishing (see
graphical and 2-D surface roughness of the worn surfaces of both un- the top right corner of Fig. 3b).
textured and textured samples were determined using 3-D profilometer
(VHX-5000, Keyence). Double-shearing method was used to determine 3.2. Effect of surface texture on hydrophobicity of bonded PTFE coatings
interfacial bonding strength with a span length of 5 mm and a cross-
head rate of 0.05 mm/min. Double-shearing test was performed using 3.2.1. Effect of geometric parameters of texture on hydrophobicity of the
preparation method used for laminated materials with coatings
12 mm × 5 mm × 8 mm in dimensions. Interfacial bonding strength Fig. 4 illustrates how density and diameter of micro-dimples af-
was calculated using the formula τ = PMax/2S (PMax is the maximum fected water contact angle and sliding angles of PTFE coatings. Geo-
load in double shearing test, and S is the actual interfacial area) [24]. metric parameters of the texture significantly affected hydrophobicity
Peak positions of PTFE coatings before and after laser-treatment were of the PTFE coatings. Thus, texture can tune PTFE surface energy. As
obtained by XPS. Hardness changes in different areas of surface micro- the texture density increased, the contact angle of PTFE coating in-
dimples were measured using Nano Indenter II (MTS, USA) with creased, while the sliding angle decreased severely at micro-dimple
100 nm indentation depth. 2.5 Friction and wear properties of PTFE diameters equal to ∼50 μm. When the texture density approached 45%,
coatings. contact angle of the PTFE surface increased to 160°, and the sliding
Frictions tests were performed using bolt-disc contact and re- angle decreased to 4.9°. Comparing to untextured PTFE coatings, con-
ciprocating friction and wear tester (UMT-3, USA). The upper panel was tact and sliding angles of textured PTFE coatings were 2.0 times higher
a stainless-steel bolt (12Cr17Ni7, ϕ = 3 mm × 18 mm) with 200 HV and 3.5 times lower than those of untextured PTFE coatings, respec-
hardness and ∼0.02–0.05 μm surface roughness. The bottom panel was tively. Textured PTFE coatings exhibited excellent super-hydro-
sample of textured or untextured PTFE coating 25.0 × 25.0 × 3.5 mm phobicity. In addition, contact angle of the coating decreased sig-
in dimensions. Friction and wear properties were tested in deionized nificantly, and the sliding angle increased gradually as the micro-
water at 20 ± 2 °C and 18 ± 5% humidity. Test conditions were 50 N dimple diameters increased at micro-dimple density equal to 45%.
load, 5 Hz frequency and 4.95 mm reciprocating strokes. Thus, excessively large micro-dimple diameters did not improve PTFE
superhydrophobicity. According to optical photos of the contact angles,
3. Results and discussion static contact angle of PTFE coating before and after texturing was
different. The water droplets on the surface of untextured coating
3.1. Controllable fabrication of PTFE coating surface texture spread very obviously, while water droplets on the textured surface
maintained their round shapes.
Fig. 2 shows typical SEM images of three different textures on PTFE
coating surface. Textured patterns with controllable feature sizes, reg- 3.2.2. Effect of texture morphology on the coating hydrophobicity
ular morphologies and clear structures were obtained for PTFE coatings Fig. 5 illustrates effects of texture pattern (micro-dimples, micro-
by optimizing laser parameters. Generally, a heat affected zone is grooves, micro-squares and micro-meshes) on contact and sliding

Fig. 2. SEM images of PTFE coating with micro-dimples (a), micro-grooves (b) and micro-mesh (c) together with their corresponding high-magnification images (a′,
b′ and c′, respectively).

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Fig. 3. Thickness of the PTFE coating (a) and 3D profilometer results showing depth of the textures (b).

angles of PTFE coatings. Results showed that textured patterns also

significantly affected the hydrophobicity of the PTFE coatings. Texture
pattern affected PTFE hydrophobicity: excepted micro-squares, contact
angles of the coatings with micro-dimples, grooves and mesh textures
all exceeded 150°, which demonstrates that PTFE became super-hy-
drophobic. Comparing to the smooth PTFE surface (which had water
contact angle equal to 79°), micro-dimples texture increased water
contact angle of PTFE by 2.0 times. Rolling angles of the coating sur-
faces with all texture patterns were below 5°, which is 3.5 times lower
than that of the untextured PTFE coating (equal to 17°).
Spreading rate of water droplets on the coating surface was also
tested. Coatings with four different structural morphologies were used.
Surface morphologies and spreading rates of water droplets for all four
samples are shown in Table 2 and Fig. 6, respectively. Surface texture
played a significant role in water droplet spreading. Coatings with
texture surface (samples C and D) had more influence on contact angle
and water droplet spreading than substrates texture (Sample B). Surface
texture of the PTFE coating suppressed water droplets spreading: they Fig. 5. Effects of micro-patterns on the hydrophobicity of PTFE coatings.
did not move for a prolonged period of time. This surface effect con-
siderably increased hydrophobicity of the coating and also inhibited Table 2
spreading of water droplets. Substrate and coating with four different surface structural morphologies.
Sample Surface structural morphologies
3.3. Effect of surface texture on tribological properties of bonded PTFE
A Untextured substrate surface and untextured bonded PTFE surface
coatings B Untextured substrate surface and textured bonded PTFE surface
C Textured substrate surface and untextured bonded PTFE surface
To further investigate how texture affected tribological properties of D Textured substrate surface and textured bonded PTFE surface
the coatings, we tested the wear life of four PTFE coatings (Samples A-D
mentioned in Section 3.2) in water (see Fig. 7). Wear life of the PTFE
coatings on the Al2O3/Ni layered ceramics substrate in water was clo- the substrate surface affected wear life of bonded PTFE coating mainly
sely related to the surface morphology of the substrate, surface mor- because substrate texture significantly improved bonding strength at
phology of the coating surface and interfacial bonding properties of the the coating/substrate interface. By comparing results for Samples B and
coating and the substrate. Specifically, surface textures of substrate and D with Samples A and C, we found that wear life of the textured PTFE
bonded PTFE significantly extended wear life of the coating. Texture of coating was longer. At the same time, PTFE wear life was affected to a

Fig. 4. Effects of micro-dimple (a) densities and (b) diameters on hydrophobicity of PTFE coatings.

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Fig. 6. Static contact angles of surfaces with different textures and micro-
structures.
Fig. 8. The SEM of wear track morphologies of PTFE coatings on different
surfaces of Samples A-D (a-d, respectively).

found for samples with “double texture” (see Fig. 9d).

Analysis of wear track morphologies of coating surfaces after com-
plete friction failures are shown in Fig. 10. Sample A exhibited serious
furrows and abrasive wear on the substrate, which would correspond to
large fluctuations of the substrate service life as well as potential peel
and failure of the coating in the friction area. Despite its textured
coating surface, Sample B peeled off completely through the entire wear
track area and wear was observed on the substrate surface. Substantial
amounts of lubricant particles were stored in the micro-dimples of
Samples C and D. These lubricant particles could be dragged on the
surface of micro-dimples to generate continuous lubrication zone after
the coating peeled off, effectively alleviating substrate wear and pro-
viding secondary lubrication. Therefore, surface texture can sig-
nificantly improve anti-friction properties of both the substrate and the
Fig. 7. Wear life of PTFE coatings with different surface texture. coating. Double-shearing method was used to determine interfacial
bonding strength of untextured and textured substrate with polyimide-
epoxy. The values of untextured and textured surfaces were about 9.7
significantly lesser degree when only the substrate was textured.
and 11.0, respectively. The results demonstrated that the micro-dimples
Overall, wear life of Sample D was 5 times higher than that of Sample A
significantly affected the interfacial bonding strength of bonded PTFE
because of the synergistic effect of textures of both substrate and the
with substrates. This was due to the PTFE solid lubricants embedded in
coating. Additionally, textured coating exhibited excellent wear re-
the micro-dimples as “mechanical self-locking” structure were tightly
sistance.
attached to the substrate surface [26].
The phase composition changes before and after texturing of coating
3.4. Discussion surface were further analyzed by XPS, and the results are shown in
Fig. 11. As shown in Fig. 11a and b, peaks corresponding to 1s electrons
Fig. 8 shows wear track morphology of coatings with four different of C and F, which are the two main elements of PTFE, were observed
structural morphologies at 50 N load with 5 Hz sliding frequency after before the laser treatment. Peaks of C1s and F1s located at 295 eV and
60 min of friction in water. Bonded PTFE coating deposited on un- 689.6 eV, respectively, were attributed to the –CF2– structure of PTFE.
textured substrate (Sample A) completely peeled off, and obvious fric- Intensities of the these peaks of C1s and F1s were significantly de-
tion scratches were observed on the substrate surface (Fig. 8a). How- creased, and new peaks at 292 eV and 691.5 eV appeared as a result of
ever, PTFE coating with textured substrate or textured coating surface recombination of –CF- and –CF3 bonds after –CF2– bonds fractured. It
peeled off only partially. At the same time, solid lubricant in the micro- can be seen that the F content, calculated from the area of F1s peak
dimples of substrate migrated to the surface forming discontinuous shown in Fig. 9b, decreased after laser processing. O1s peaks were
lubrication, which contributed to friction coefficients ability to be and observed because the samples were exposed to the ambient atmosphere
to remain low for long periods of time (see Fig. 8c). “Double textured” and absorbed O2 (see Fig. 9c and d). Peaks at 531.599 and 532.386 eV
coating exhibited excellent anti-wear ability and showed only slight belonged to C=O and C–O bonds, respectively. Intensity of C=O peaks
signs of wear. This coating also showed the least wear comparing to increased and C–O peaks decreased after laser treatment (see Fig. 9c
other structural morphologies (see Fig. 8d). and d, respectively). Therefore, laser treatment caused defluorination
Fig. 9 shows the typical 3-D topographies and 2-D surface roughness and decreased content of surface F. As a result, total concentration of
of the worn untextured and textured surfaces after 60 min of friction. oxygen increased. This explains how laser treatment acted as a surface
Significance of texture on tribological properties is further confirmed by modifier of PTFE [27]. Vacancies were observed around C atoms after
the results in Fig. 9. Sample A with untextured surface completely laser-induced defluorination, which facilitated cross-linking of different
peeled off forming deep grooves (see Fig. 9a). Depths of wear were less PTFE molecules. Cross-linking greatly enhanced surface hardness of
on textured substrates or on textured coatings than on untextured PTFE, which was confirmed by the nano-indentation hardness tests
surfaces (see Fig. 9b–c). On contrary, best tribological properties were

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Fig. 9. The typical images of worn surfaces of Samples (a: sample A, b: sample C, c: sample B, d: sample D, respectively) obtained from 3D topography and by 2D
profilometer.

Fig. 12. Nanoindentation force-displacement curves and hardness (shown as

insert in the left corner) of PTFE coating.
Fig. 10. SEM images of the wear track morphologies of PTFE coatings on dif-
ferent surfaces (a: Sample A, b: Sample B, c: Sample C, d: Sample D).

Fig. 11. XPS spectra of original and laser treated PTFE coatings. Binding energies of (a) C1s, (b) F1s and O1s before (c) and (d) after laser treatment.

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