J. Cent. South Univ.

(2024) 31: 2210－2224

DOI: https://doi.org/10.1007/s11771-024-5703-7

Microstructural, wettability, and corrosion behaviour of

TiO2 thin film sputtered on aluminium

Rajeev VERMA1*, Vijay KUMAR1*, Saurabh KANGO2, Amindra KHILLA2, Rajeev GUPTA3

1. Department of Industrial and Production Engineering, Dr B R Ambedkar National Institute of

Technology Jalandhar, Jalandhar-144008, Punjab, India;
2. Department of Mechanical Engineering, Dr B R Ambedkar National Institute of Technology
Jalandhar, Jalandhar-144008, Punjab, India;
3. Department of Physics, School of Engineering, University of Petroleum & Energy Studies,
Dehradun 248007, Uttarakhand, India
© Central South University 2024

Abstract: The study investigated the application of radiofrequency (RF)-sputtered TiO2 coatings at various temperatures
to enhance the hydrophobicity and corrosion resistance of Al6061 alloy. The research aimed to establish a correlation
between the coating process and the resulting surface properties. Surface roughness and wettability were quantified with
a surface profilometer and goniometer. Additionally, chemical boiling and salt spray corrosion tests were conducted to
evaluate any topographical changes during these procedures. The analysis further involved the use of field-emission
scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques
to characterize the deposited coatings. The findings indicated that the TiO2 coating applied at 500 ℃ exhibited the
highest water contact angle and superior corrosion resistance compared to other temperatures. Surface characterization
confirmed that this specific TiO2 coating at 500 ℃ effectively delays corrosion due to its hydrophobic behavior, making
it durable for industrial applications.

Key words: aluminium; TiO2 coating; radiofrequency-sputtering; hydrophobic; corrosion

Cite this article as: Rajeev VERMA, Vijay KUMAR, Saurabh KANGO, Amindra KHILLA, Rajeev GUPTA.
Microstructural, wettability, and corrosion behaviour of TiO2 thin film sputtered on aluminium [J]. Journal of Central
South University, 2024, 31(7): 2210−2224. DOI: https://doi.org/10.1007/s11771-024-5703-7.

demanding applications where structural integrity is

1 Introduction paramount. Its potent strength coupled with its
lightweight properties makes it an ideal choice for
The Al6061 alloy is widely regarded as one of endeavors that necessitate both durability and
the most versatile and popular aluminium alloys weight reduction. While, Al6061 alloy possesses
available for industrial and domestic use. Its moderate corrosion resistance, particularly when
exceptional strength, decent corrosion resistance, compared to some other aluminium alloys, it does
and machinability make it a preferred choice for a form a protective oxide layer when exposed to
broad spectrum of applications across multiple oxygen [2]. This layer acts as a barrier inhibiting
industries [1]. Al6061 alloy exhibits outstanding further oxidation and preserving the underlying
strength properties, making it suitable for metal. This corrosion resistance ensures that
Received date: 2023-10-06; Accepted date: 2024-01-11
Corresponding author: Vijay KUMAR, PhD; E-mail: vknitj94@gmail.com; ORCID: https://orcid.org/0000-0001-5415-2745; Rajeev
VERMA, PhD, Assistant Professor; E-mail: vermar@nitj.ac.in; ORCID: https://orcid.org/0000-0001-5566-9834
J. Cent. South Univ. (2024) 31: 2210－2224 2211

components made from Al6061 alloy maintain their sensors, due to its exceptional heat resistance
longevity and durability even when subjected to properties, enabling it to endure high temperatures
harsh environmental conditions. effectively. TiO2 coatings are recognized for their
In its untreated state, Al6061 aluminium alloy self-cleaning attributes, and when applied, they can
does not naturally exhibit hydrophobic properties. It enhance the performance of the underlying material.
has a wide range of applications across various Additionally, TiO2 possesses antibacterial properties
industries, including aerospace, where it is used for and is biocompatible, making it particularly suitable
the fabrication of airframes, wings, and fuselages, for applications in the medical field [15, 16]. TiO2
the marine industry for boat hulls and fittings, and has demonstrated several advantages for the
the electrical industry for producing electrical development of hydrophobic surfaces. This includes
conduits, enclosures, and heat sinks [3]. reducing surface energy by altering the electronic
Hydrophobic and hydrophilic surfaces each have structure, creating surface defects and vacancies,
advantages and disadvantages depending on their increasing surface roughness, and decreasing water
intended use. Hydrophobic surfaces offer benefits adsorption.
such as repelling liquids, preventing water Physical vapor deposition (PVD) is a
absorption, maintaining dryness, self-cleaning vaporization coating technique that operates in a
capabilities, improved corrosion resistance, reduced vacuum environment and involves the deposition of
ice formation, and enhanced biomedical material at the atomic level. In PVD, the starting
applications by preventing the adhesion of material to be deposited, known as precursor, is in
biological substances like proteins and cells to solid form [17, 18]. The process can be broken
medical devices and implants [4 − 6]. For down into four key steps: (i) evaporation of the
applications of Al6061 alloy mentioned earlier, material to be deposited by transforming through a
hydrophobic surfaces would be advantageous due to high-energy source like an electron beam or ions,
their anti-corrosion, anti-icing, and self-cleaning which results in release of atoms from the surface;
properties. Aluminium surfaces are typically (ii) transport of the vapor to the substrate to be
hydrophilic, meaning that they naturally attract and coated; (iii) reaction between the metal atoms and
cause water droplets to spread and adhere to the the appropriate reactive gas (such as oxygen,
surface. Therefore, creating a hydrophobic surface nitrogen, or methane) during the transport stage; (iv)
on Al6061 alloy would enhance its usability. deposition of the coating at the substrate surface.
A hydrophobic surface can be created either by PVD is a highly effective technique for applying
modifying the surface topography or by depositing a thin film coatings, typically ranging from 1 to
superficial layer of a low-surface energy 10 μm in thickness [19, 20]. Various PVD methods
hydrophobic compound [7]. One common method exist including sputtering deposition, electron beam
for creating hydrophobic surfaces is chemical PVD, thermal evaporation deposition, cathodic arc
modification, which involves altering surface evaporation, and pulsed laser deposition. Among
chemistry [8, 9]. Additionally, altering the surface these methods, radiofrequency (RF) sputtering
topology through surface roughening involves offers several advantages over other PVD
the creation of micro or nano-patterned textures, techniques, such as better control over film
or nanoparticle deposition, involving the composition, higher deposition rates, the ability to
formation of a nanoparticle film significantly maintain low substrate temperature, uniform film
enhancing the material􀆳s hydrophobicity [10]. coverage, enhanced adhesion and density, reactive
Recent developments involve combining surface sputtering capability, and scalability [21]. When
roughening and surface modification, often using combined with surface roughening techniques, thin
techniques such as chemical vapor deposition film coatings produced through RF sputtering can
(CVD) [11], self-assembled monolayers [12], enhance the hydrophobicity of coatings like TiO2
etching [13], and electro-spinning [14]. when applied to substrates such as Al6061 alloy.
Titanium dioxide (TiO2) is a naturally existing The creation of surface roughness on coatings
substance that is widely employed in a range of is a virtue achieved through various techniques
applications, including fuel cells, solar cells, and documented in the literature. One such technique is
2212 J. Cent. South Univ. (2024) 31: 2210－2224

etching, a process employed to selectively remove

materials from a surface with the aim of generating 2 Experiment
a specific pattern, texture, or shape. This process
involves the controlled chemical and physical 2.1 Materials
removal of the material using an etchant that has the The study utilized locally sourced Al6061
capability to dissolve or erode the target material. aluminium alloy from Jalandhar, Punjab, as the
Etching can be broadly classified into two substrate material. The alloy’s chemical
categories: wet etching and dry etching. Wet etching composition was determined through X-ray
entails immersing the target material in a liquid fluorescence spectrometry at JBS testing solutions
etchant, which reacts with the material 􀆳 s surface, in Jalandhar, revealing a composition of 1.1 wt%
leading to either dissolution or erosion. On the other Mg, 0.14 wt% Mn, 0.46 wt% Si, 0.16 wt% Fe, and
hand, dry etching involves the use of plasma to etch 98.11 wt% Al. These values aligned with the
the material. This plasma contains reactive species nominal chemical composition of Al6061 alloy. The
that chemically interact with the surface, while the acquired alloy was sectioned into specimen pieces
material removal is aided by the physical measuring 15 mm×15 mm using wire electrode
bombardment of ions or radicals. One specific wet discharge machining to avoid any microstructural
etching technique worth mentioning is the use of transformations. These specimens underwent a
boiling materials in HCl (hydrochloric acid). The polishing process using emery paper with varying
process creates surface roughness, which is crucial grades (P600, P800, P1000 and P1200) to eliminate
for achieving better adhesion for any superficial surface oxides and achieve a flat reference surface.
layer [22, 23]. Additionally, such etching can be Subsequently, the polished samples were cleaned
advantageous for subsequently creating micro/nano- with distilled water and underwent ultrasonic
patterns on the surface, thereby enhancing its cleaning in ethanol to remove any remaining debris
hydrophobic properties. from the surface. Finally, the samples were dried
The objective of this research is to develop a using a hot air blower. Figure 1 presents the
superhydrophobic aluminium surface by TiO2 morphological and elemental characterisation
coating through a radio frequency sputtering evidence acquired from FESEM analysis of Al6061
technique followed by HCl boiling. This surface after polishing. The sputtering targets used in the
modification is envisaged to improve the surface 􀆳 s study were TiO2 with 99% purity, procured from
non-wetting behaviour and corrosion resistance. In M/s Testbourne, UK.
the view of above objective, in this work, the focus
was on the application of RF-sputtered TiO2 2.2 DC/ RF- magnetron sputtering
particles onto Al6061 surfaces with a variation of The polished samples of Al6061 alloy were
sputtering parameters [24 − 26]. Additionally, some coated prudently with TiO2 using the DC/RF-
of the coated samples underwent a treatment phase magnetron sputtering method at the University of
involving immersion in a 3.5 wt% HCl solution and Petroleum and Energy Studies (UPES) in Dehradun,
boiling. The configured samples were then Uttarakhand, India. Table 1 outlines the DC/RF
characterized using techniques such as field- sputtering parameters employed for TiO2 coating
emission scanning electron microscopy (FESEM), and specimen configurations investigated in the
energy-dispersive spectroscopy (EDS), and X-ray study.
diffraction (XRD). To evaluate the surface The working chamber was thoroughly
properties, a digital surface profilometer was evacuated carefully to a base pressure of 6.1×10−6
employed to compare the surface roughness and mbar using a turbo-molecular pump. By evacuating
profile of the coated and treated specimens. the area, a clean and controlled environment was
Furthermore, the hydrophobicity of the prepared created for the trials that followed. During the pre-
samples was assessed through the measurement of uniform sputtering stage, which lasted for 5 min, the
water contact angles using the sessile drop method. target material 􀆳 s surface was further cleaned. Any
Lastly, the corrosion resistance of the TiO2 coatings remaining impurities or debris on the target surface
was investigated through salt spray testing. were successfully eliminated by this pre-sputtering
J. Cent. South Univ. (2024) 31: 2210－2224 2213

Figure 1 Al6061 alloy: (a, b) SEM images of the polished specimen; (c) Energy spectrum plot; (d) Line scan

Table 1 DC/RF sputtering specimen configurations and [27]. Additionally, a gas flow rate of 30 cm3/min
parameters for TiO2 coating on Al6061 alloy (sccm) of argon (Ar) was maintained, along with a
Specimen configuration sputtering pressure of 3 Pa. To produce a uniform
Parameter
A1 A2 A3 coating thickness, the substrates were rotated at a
Target TiO2 constant angular velocity of 10 r/min. These
Temperature/℃ 300 400 500 parameters were carefully selected and regulated to
Base pressure/Pa 6.1×10 −4 ensure a homogeneous deposition of the TiO2 target
Pre-sputtering/min 5 material onto the Al6061 alloy substrates. Figure 2
Sputtering pressure/Pa 3 displays the macro images of Al6061 alloy and DC/
Power/W 100 RF magnetron sputtered coating specimens.
Sputtering time/min 90
Gas Argon
3 −1
Flow rate/(cm ·min ) 30
Specimen rotation/(r·min−1) 10
A: TiO2 coated; A+ : TiO2 coated and HCl boiling surface
treatment.

Figure 2 Macro images of the TiO2 DC/RF sputtering

process. The primary sputtering process was started specimens: (a) Al6061 sustrate; (b) Sample A1;
after the pre-sputtering phase and deposition was (c) Sample A2; (d) Sample A3
performed for 90 min. For the RF sputtering
process, a power of 100 W was utilized. A literature 2.3 Chemical boiling
review and preliminary experimentation revealed In order to enhance the wettability of the TiO2
that the substantial achievable coating thickness for RF-sputtered surfaces, the specimens were
TiO2 via RF sputtering occurs at 300 ℃ and beyond immersed in a water bath containing HCl solution
2214 J. Cent. South Univ. (2024) 31: 2210－2224

(3.5 vol.% HCl in distilled water) maintained at following ASTM B-117 standard. A 3.5 wt.%
50 ℃ for 2 h [28]. This solution was meticulously sodium chloride (NaCl) solution was prepared and
prepared to induce roughness through a pitting introduced into a chamber designed for salt spray
action, thereby improving the surface morphology. fog generation [29]. The salt spray chamber was
Subsequent to the HCl treatment, the specimens filled with regular water in a manner that ensured
were exposed to the ambient environment the heater coil inside the chamber was properly
characterized by temperatures ranging from 25 to submerged. These prepared samples were then
27 ℃ and relative humidity levels between 45% and positioned on the rack within the salt spray chamber
50% for a period of 60 d. This was done to observe which is meant for the corrosion study. The chamber
any alterations in the substrate’s surface temperature and salt mist temperature were set at 35
morphology over time. and 47 ℃ , respectively. The chamber pressure was
The surface roughness and profile of the RF- held constant at 1×102 kPa simulating the typical
sputtered surfaces treated with HCl were atmospheric conditions. The test specimens
subsequently quantified using a surface profilometer underwent a cyclic exposure regimen as per
with a measurement range spanning from 160 to KUMAR et al [30], consisting of three stages:
210 mm. The roughness profile was recorded wetting, transition, and drying. The cyclic test was
utilizing the following specific parameters: the repeated seven times, resulting in exposure for 56 h.
cut-off value of 2.5 mm, λ-filter set at 2.5 mm, the Observations were made at 8 h intervals to monitor
minimum evaluation length of 12.5 mm, and the collection of salt fog and changes in the surface
operational speed of 0.6 mm/s. With the mentioned morphology of the corroded specimens.
surface modifications, the surface roughness 2.4.3 Surface characterisation
values (Rq) for the specimens A1, A2 and A3, The grown TiO2 samples underwent a
were obtained as 0.525, 0.605, 0.673 μm, comprehensive characterization process to
which substantially increased to 3.653, 3.976 and
investigate their structural and morphological
5.563 µm for A1+, A2+ and A3+ respectively.
properties under different surface treatment
conditions. FESEM was employed at various
2.4 Thin-film characterization
magnification levels, utilizing secondary electron
2.4.1 Water contact angle
imaging, a high-definition backscattered electron
The water contact angle (WCA) of the surfaces
detector (HDBSD), and energy-dispersive
under investigation was measured using a
spectroscopy (EDS). These techniques allowed for
goniometer equipped with micro-lens and a CCD
the examination of particle deposition on the sample
camera capable of capturing 75 frames per second.
surfaces, the arrangement of these deposited
The device sourced from Apex Instruments Co.
particles, and the surface morphology. Furthermore,
Private Limited, India, had a high contact angle
the development of compounds and their phase
accuracy of ±0.05° . To determine the WCA as per
the sessile drop method, a precise 9 µL water micro- structure was ascertained through XRD analysis,
droplet was carefully deposited onto the surface. covering a diffraction angle (2θ) range from 15°
This was achieved by employing a syringe that was to 80°. The XRD data were obtained using
connected to the goniometer, allowing for accurate PANalytical Empyrean XRD equipment with a Cu
droplet placement. The proprietary Apex Acam anode and Kα radiation (wavelength of 1.54060 Å)
software was employed to analyze and evaluate the at 40 mA and 45 kV. The full width at half
shape of the water droplet which enabled the maximum (FWHM) was used for peak analysis with
calculation of the static contact angle. To ensure a precise step size of 0.017° and 30 min of scanning
accuracy, WCA measurements were taken at three time to ensure sufficient data points.
distinct locations on the surface. These individual The average crystal size of the coated surface
measurements were then averaged to report the was calculated using the Scherrer methodology,
WCA of the surface. which considers the diffraction patterns from
2.4.2 Salt spray test various lattice planes, each having a different crystal
The corrosion performance of as-coated TiO2 size. This analysis allowed us to determine the
surfaces was studied using salt spray testing average crystal size of the particles on the coated
J. Cent. South Univ. (2024) 31: 2210－2224 2215

surface. Additionally, the dislocation density of the and comparing it to the total area of all the peaks in
particles on the surface of the coating was the XRD pattern. By conducting this comprehensive
quantified, providing insights into defects within the analysis of XRD data, including crystal size,
crystal structure of the coated surface. The micro- dislocation density, micro-strain, stress, and
strains of the coated surface were also evaluated. crystallinity, a thorough understanding of the
Micro-strain refers to the small atomic distortions structural characteristics and properties of the
and deformations occurring around dislocation lines coated surface was achieved.
due to the presence of extra planes of atoms. This
measurement provides information about the strain 3 Results and discussion
on the coated surface resulting from these atomic
distortions. To assess the mechanical behaviour and 3.1 Microstructure
stability of the coating, the uniform strain density The surface morphology and elemental
function was used to calculate the stress developed distribution of TiO2 coatings sputtered at
on the coating surface [31, 32]. The crystallinity of 500 ℃ (A3) using SEM images is illustrated in
the coated surface was determined by analyzing the Figure 3. It may be observed from Figure 3(a) that
peaks in the XRD pattern. Crystallinity was the TiO2 coating was homogeneously deposited on
quantified by considering the area under the peaks the aluminum substrate, free from any major coating

Figure 3 SEM images (a, b) and HDBSD (c) of TiO2 coating RF-sputtered at 500 ° C (A3) and line scanning of TiO2
surface (d), EPS plot (e) and EDS elemental mapping (f)
2216 J. Cent. South Univ. (2024) 31: 2210－2224

defects like porosities or micro-cracks. RF as indicated by the strength of peaks. This is also
sputtering in argon gas has a uniform coating indicated by the crystal size shown in Figures 4(b)
structure and thickness. Some equiaxed crystals and (c). As the sputtering temperature increased to
were observed at the surface hinting at sufficient 400 and 500 ℃, the crystallinity of grains improved,
adatom mobility required to adherent TiO2 film, as potentially resulting in larger and more well-defined
shown in the SEM images at 4600× magnification crystal structures. As a result of the well-defined
refer to Figure 3(b). The findings imply that the crystal growth ensued at higher sputtering
examined sputtering parameters generated the temperatures shown in Figure 4(a), the micro-strain
necessary energetic particle flux, ensuring sufficient in the crystalline structure declined as shown in
kinetic energy for the particles involved in the
coating process.
As shown in Figure 3(c), the high-definition
BSE detector (HDBSD) image although exhibited
some dark patches indicating delusion of some
porosity, if closely monitored which was a
shadowing effect due to some spheroidal peaks
formed by raised particles resulting in surface
roughness. The surface roughness plays a key role
in affecting the wettability of the sputtered surfaces,
which has been examined next. Additionally, the
deposited particles’ surface was cross-examined by
the EDS line scanning of the TiO2 coatings, refer to
Figure 3(d). This EDS line scanning signifies the
elemental composition providing a valuable insight
of the deposited elements. It was revealed that all
the reported elements were uniformly distributed
along the specified length of the surface of the
sample. The elemental map exhibited homogeneous
elemental distribution throughout the scan area.
Closer inspection reveals that carbon deposition on
the specimen’s surface was somewhat greater than
that of titanium and oxygen. The carbon deposition
on the surface of the substrate indicates some
intrude reactions taking place during the RF
sputtering coating. However, the successful
deposition of titanium and oxygen particles on the
specimen surface confirms the effectiveness of the
RF sputtering technique in achieving the desired
coating. The acquired peak positions (2θ), full-
width at half maximum (FWHM-β), and the
corresponding hkl miller indices were employed to
determine the crystallographic characteristics
(Figure 4(a)). The lattice parameters a=9.769 Å, b=
2.921 Å and c=4.675 Å were used to determine the
orthorhombic crystal structure of the TiO2 particles.
The XRD pattern in Figure 4(a) illustrates that the Figure 4 (a) X-ray pattern of the coating􀆳s configurations
sputtered film at 300 ℃ had a lower degree of investigated; (b) Crystal size of different coating
crystallinity, or smaller crystal grains were observed configurations; (c) Micro-strain in crystal
J. Cent. South Univ. (2024) 31: 2210－2224 2217

Figure 4(c). Therefore, the diffraction patterns show morphology, the specimens were etched in a
that using RF sputtering power encourages 3.5 wt% HCl solution at 50 ℃ for 2 h as explained
crystallinity. This impact is explained by the rise in in Section 2.3. Further, the chemical boiling
surface mobility brought on by greater RF treatment aids in determining how coating behaves
temperatures, which are required for the to corrosive conditions and offers details on possible
development of a highly crystalline layer. Since the chemical reactions and mechanisms of deterioration
magnetron sputtering technique uses high RF that might take place when exposed to HCl solution.
power, it follows highly powered inert Ar ions Due to the strong acidic nature of the HCl solution,
giving the adatoms translational kinetic energy. the coated surfaces released metal ions into the
By facilitating the adatoms 􀆳 surface diffusion and solution due to the corrosion. As a result, a sponge-
encouraging momentum transfer to the expanding like surface with clear coating segregation with
surface, this increased energy eventually causes the platelets of length ranging from 30 to 210 µm was
creation of a highly crystalline layer. developed post-treatment, as shown in Figure 5(a);
Figure 5(b) at higher magnification shows the micro-
3.2 Chemical boiling cavities formed on the granular structure
To increase the surface roughness of the TiO2- contributing to the mico-nano roughness and
coated surface and to create a multi-modal stimulating the air trapping effect, which further

Figure 5 (a, b) SEM images of TiO2 coated

surface (A3+), after HCl boiling at 50 °C for 2 h;
(c) Line scanning; (d) EDS elemental mapping;
(e) EDS plot
2218 J. Cent. South Univ. (2024) 31: 2210－2224

enhanced the non-wetting behaviour of water precipitates, potentially decreasing the surface
droplets on the surface. durability [32].
The intergranular precipitation may be evident
as a result of the accelerated corrosion at elevated 3.3 Wettability behaviour
temperatures during the chemical boiling. The As surface roughness is the predominant factor
precipitation leads to the dwindling of the elements in ascertaining the non-wetting behaviour, the
as evident from comparing the EDS elemental roughness profile of the deposited coatings
composition of the untreated (Figure 3(e) and was determined. Surface roughness assessment
treated surfaces (Figure 5(e)). Whereas, there is an involves representing a material 􀆳 s surface texture
increase in Al element which may be ascribed to the and roughness profile through a graphical depiction
aluminium leaching during boiling. Since the known as a profile curve. This curve illustrates
specimens were not masked for boiling, the variations in the surface along a predefined line,
aluminium from the uncoated faces would have highlighting peaks and valleys. Figure 6 provides
settled on the surface with prolonged boiling. the surface roughness profile curves of DC/RF
During boiling, aluminium was deemed to react, sputtered TiO2 specimens A2 and A3 respectively.
forming aluminium oxide while liberating Table 2 and Figure 8 present the roughness
dihydrogen gas. This aluminium oxide may be parameters of the untreated and HCl-boiling treated
observed as a precipitate at the grain boundary in surfaces. Examining the roughness profile curves
the form of an aggregate and was confirmed by the and roughness values, it may be deduced that the
line scan, refer to Figure 5(c). However, prolonged sputtered TiO2 was distributed uniformly across the
chemical etching in conjugation boiling for 2 h coating surface exhibiting low standard deviation in
leads to the development of cracks of the aluminium Ra and Rq values. It is worth noting that the
oxide along the grain boundaries of these roughness value dwindled more than 18% for the

Figure 6 Roughness profile curves of DC/RF sputtered TiO2 specimens: (a) A2; (b) A3; (c) A2+; (d) A3+

Table 2 Roughness parameters of the untreated and HCl boiling treated TiO2 surfaces
Sample Ra/μm Ra,sd/μm Ra,max/μm Ra,min/μm Rq/μm Rq,sd/μm Rq,max/μm Rq,min/μm
A2 0.557 0.024 0.582 0.518 0.697 0.023 0.720 0.657
A3 0.434 0.087 0.551 0.320 0.566 0.087 0.750 0.427
A2+ 2.235 0.302 3.682 2.401 3.291 0.387 4.620 3.120
A3+ 3.156 0.397 3.763 2.607 4.080 0.446 4.821 3.470
J. Cent. South Univ. (2024) 31: 2210－2224 2219

specimen sputtered at 500 ℃ compared to the one at transformed the surface from hydrophilic to
400 ℃ . These show that increasing sputtering hydrophobic with maximum sessile WCA up to
temperature leads to a smoother finish. This may be 109°.
due to an increase in the kinetic energy of the However, after etching in an HCl solution, a
incident particle while sufficient electrons and ions sponge-like structure was formed on the coated
bombard the substrate enhancing the adatom surface, enhancing the non-wetting behavior of the
diffusion at higher temperatures [33, 34]. surface. When etched in an HCl solution, micro-
Whereas, Figure 6(c) represents the roughness nano cavities were formed, leading to the spongy
profile curve of the coating after HCl boiling for structure appearing on the surface, as compared to
2 h. When compared with untreated coatings, the the untreated TiO2 coating, as shown in Figure 5. It
profile of HCl boiled illustrates the increase in
may be conferred that the surface roughness values
roughness with regularly higher undulations and the
for the considered coating configurations increased
considerable rise in Ra and Rq values (2.235 and
with the rising sputtering temperature, and this
3.291 μm for A2+, whereas 3.156 and 4.080 μm for
increase was further enhanced after etching in the
A3+ respectively). A plateau was observed at an
HCl solution, as demonstrated in Figure 7. The
evaluation length of 7.5 mm, which could have been
increased roughness facilitated the transformation of
a result of encountering an uncracked oxide
precipitate at the grain boundary of the boiled the surface from hydrophobic to superhydrophobic
surface as evidenced in Figure 5(c). with a maximum WCA of 151°.
TiO2 coatings exhibited different wettability
characteristics, depending on their surface
properties. Generally, pure TiO2 particles were
hydrophilic in nature, indicating a strong affinity for
water and a tendency to reduce the contact angle of
water droplets on the surface, causing them to
spread out. However, TiO2 sputtering coating at
specific temperatures demonstrated decent increase
in non-wetting behaviour compared to TiO2
particles. This action further intensified with the
increasing sputtering temperature, as illustrated in
Figure 8(a). It is considered that as the coating
temperature increased, the oxide layer settled down, Figure 7 Average root means square surface roughness
creating a more pillar-like structure, which (Rq) of different surface treatment conditions

Figure 8 Graphical representation of WCA: (a) TiO2 coating before and after HCl boiling; (b)Image of WCA on water
droplet during sessile drop test, Sample A3+
2220 J. Cent. South Univ. (2024) 31: 2210－2224

3.4 Salt-spray corrosion TiO2 surface.

A corrosion study was conducted on the coated Following 56 h of salt spray testing, the surface
and uncoated surfaces to assess the effectiveness of contained corrosion products. To visualize the
the reported coating in protecting the underlying surface underneath, an ultrasonic cleaning
substrate from corrosion. In this study, salt spray procedure was employed to remove loose particles
corrosion testing was employed to evaluate the resulting from oxidative corrosion. Subsequently,
corrosion resistance of the reported surface. With the specimens were dried for 20 min at 50 ℃ to
practical application in mind, this test subjected the eliminate any remaining moisture. After drying, the
coated surfaces to a highly corrosive environment surfaces were subjected to FESEM examination.
(3.5% NaCl) for 56 h, accelerating the corrosion When compared for the same specimen
process and assessing the protective capabilities. configuration (A3), the SEM images of the corroded
Visual inspection of the coated and uncoated surface, as displayed in Figure 9(a) revealed that the
specimens was done at regular intervals of 8 h pristine aluminium was more susceptible to the
during the salt-spray cyclic test. The bare corrosive environment than the RF-sputtered TiO2
aluminium specimen turned blackish after 8 h of the (Figure 9(b)) coating. The pristine aluminium
test, which turned greyish and later dull white after exhibited excessive pitting marks and oxidation
7 cycles of testing. During the test, the specimen signs because of continuing contact in a corrosive
showed high corrosion and the corrosion products environment. Surface morphology indicates
kept leaving the surface. Whereas, a pale rusty red formation of an oxide layer due to salt water
colour was observed on the coated surface with low reaction with aluminium. The growing Al2O3 layer
signs of corrosion products forming on the coated with the exposure time is believed to passivate the

Figure 9 SEM images of surfaces after salt-spray corrosion study: (a) Pristine Al6061; (b) TiO2-A3; (c) HDBSD-1000×;
(d) Line scan-A3; (e) EDS plot-A3; (f) EDS elemental mapping-A3
J. Cent. South Univ. (2024) 31: 2210－2224 2221

further corrosion of the underlying surface. The

galvanic cell that develops during corrosion
involves the formation of pits acting as the anode,
with the metal serving as the cathode. This setup
leads to the migration of anions towards the pits and
consequently results in the localized production of
metal cations. The aluminium cations create an
excessive positive charge while attracting chlorine
ions forming the Al2O3 which in turn reacts with
water molecules present in a moist test chamber to
form hydrogen gas. The film-breaking mechanism
caused due to pitting corrosion makes continual
detachment of the oxide film formed.
Whereas, the coated specimen exhibited much
less corrosion, and the polishing marks were visible.
However, there were some trivial oxidation clusters
visible, and specimen surface was free from any
major pitting marks. The delay in corrosion of the
aluminium substrate indicates the effectiveness of
TiO2 coating for corrosion protection against hostile
saline environments.
The EDS mapping after the salt spray revealed Figure 10 A3+ specimen: (a) SEM images of corroded
the dwindling of all the elements except Al when surface: (b) Line spectrum after salt spray corrosion
compared to the as-deposited TiO2 surface
(Figure 3). Since the specimens were not masked, to oxide formation during salt spray testing, with the
the increase in Al may be its diffusion from the foam-like structure formed during chemical etching
substrate side to the surface during the corrosion playing a crucial role in trapping these oxides.
testing. 52 h of prolonged oxidation later led to the
development of cracks in the oxide clusters due to 3.5 Application for surface treatment
the thermal coefficient mismatch experienced The Al6061 alloy is commonly utilized due to
during the corrosion cycles. While comparing the its outstanding resistance to corrosion, weldability,
EDS elemental composition of the salt-sprayed machinability, and specific strength advantage for
specimen to the chemical boiled (Figure 5), it may domestic and industrial applications including
be noticed that there was retention of the elements aerospace as discussed in preceding Section 1.
that were present in larger proportion. However, However, with daily and prolonged usage, this
there is a lesser amount of O and Al present aluminum alloy is prone to corrosion. Applying a
indicating lesser oxidation concerning the boiled TiO2-coating enhances its non-wetting behavior,
specimen. ensuring that the Al6061 alloy remains in a dry
During salt spray testing, the foam-like condition. This coating further improves corrosion
structure depicted in Figure 5(b) was disrupted. resistance, as discussed in Section 3.4. The
Simultaneously, the size of micro-cavities increases, enhancement in the corrosion resistance, achieved
as observed in Figure 10(a). This augmentation through the application of a TiO2 coating,
enhances the surface􀆳s air-trapping capacity, leading contributes to an increased lifespan of the Al6061
to an improvement in non-wetting behavior alloy substrate. As evidenced by SEM images, the
compared to TiO2 coating, as illustrated in application of TiO2 coating and chemical etching
Figure 3(a). Additionally, the chemical composition significantly enhances the lifespan of the base
analysis of the reported corroded surface through substrate. Although, RF-sputtering incurs additional
line scanning reveals an increase in the percentage costs, the overall cost associated with applying these
of oxide layer (see Figure 10(b)). This is attributed coatings and surface modifications is remarkably
2222 J. Cent. South Univ. (2024) 31: 2210－2224

low, particularly advantageous for mass production benefits across various industries. Future research
scenarios. Consequently, the proposed coating is should focus on optimizing RF-sputtering
highly recommended for practical applications, parameters, exploring advanced characterization
presenting considerable benefits for end customers. techniques, and assessing long-term durability.
Additionally, the scalability and multifunctional
4 Conclusions potential of TiO2 coatings especially when doped
with rare earth elements like zirconium, could be a
The study aimed to enhance the hydrophobic key focus. Additionally, the coatings 􀆳 application in
behaviour and corrosion resistance of the Al6061 diverse industries, assessment of environmental
substrate by deposition of coatings using RF- impact, and public awareness considerations are
sputtering at various temperatures. Drawing upon crucial for practical implementation.
the significant findings derived from the research,
the following conclusions have been summarized: Contributors
1) Surface roughness and surface profile were Rajeev VERMA: Conceptualization,
notably influenced by the sputtering temperature. visualization, writing−original draft; Vijay KUMAR:
Higher temperatures resulted in a more distinct Investigation, writing − original draft; Saurabh
crystalline structure and reduced micro-strain within KANGO: Data curation, formal analysis, writing −
the coatings. This phenomenon was attributed to the review editing; Amindra KHILLA: Formal analysis,
enhanced mobility of adatoms during sputtering at investigation; Rajeev GUPTA: Review & editing.
500 ℃.
2) Initially, the TiO2 coatings exhibited
Conflict of interest
hydrophilic properties, but as the sputtering
Rajeev VERMA, Vijay KUMAR, Saurabh
temperature increased, their behavior shifted
KANGO, Amindra KHILLA, and Rajeev GUPTA
towards hydrophobic due to an increase in surface
declare that they have no conflict of interest.
roughness.
3) Subjecting the coatings to a chemical
References
boiling process in a 3.5 wt% HCl solution created
micro-cavities and a sponge-like structure,
[1] PRADEEP KUMAR G S, SHINDE A, YADAV Y, et al.
enhancing non-wetting behavior. This effect was Investigations on slurry erosive on wear performance of
attributed to the increased roughness, resulting in HVOF-sprayed Cr2O3 coatings on aluminum alloy [J].
superhydrophobic surfaces. Journal of Bio- and Tribo-Corrosion, 2021, 7(3): 106. DOI:
4) Corrosion testing in a salt spray 10.1007/s40735-021-00543-2.
[2] ROUNIYAR A K, SHANDILYA P. Experimental study on
environment indicated that the TiO2 coating material removal rate of Al6061 machined with magnetic
exhibited improved corrosion resistance. Even after field assisted powder mixed electrical discharge machining
56 h of exposure to a highly corrosive environment, [J]. Journal of Physics: Conference Series, 2019, 1240:
minimal signs of corrosive damage were observed. 012018. DOI: 10.1088/1742-6596/1240/1/012018.
[3] CHRISTY T V, MURUGAN N, KUMAR S. A comparative
5) The TiO2 coating applied at 500 ℃ onto the
study on the microstructures and mechanical properties of
aluminium demonstrated the highest WCA and Al6061 alloy and the MMC Al6061/TiB2/12p [J]. Journal of
superior corrosion resistance. This synergic effect of Minerals and Materials Characterization and Engineering,
hydrophobic properties and enhanced corrosion 2010, 9(1): 57−65. DOI: 10.4236/jmmce.2010.91005.
resistance renders the TiO2 coating at 500 ℃ highly [4] KUMAR V, VERMA R, KANGO S. Micro-texturing of a
WC-10Co-4Cr-coated ASTM A479 steel to form a super-
durable for practical applications, particularly in
hydrophobic surface [J]. Trans Indian Inst Met, 2020, 73:
industries such as aerospace and marine, where 1015−1026. DOI: 10.1007/s12666-020-01918-8.
surfaces with excellent hydrophobicity and [5] SINGH S, KUMAR V, KANGO S, et al. Fabrication and
corrosion resistance are sought-after. evaluation of superhydrophobic surfaces using carbon soot
Overall, these findings highlight the potential and different adhesives [J]. International Journal of Surface
Engineering and Interdisciplinary Materials Science, 2022,
of RF-sputtered TiO2 coatings as effective means to 10(1): 1−13. DOI: 10.4018/ijseims.304807.
improve the hydrophobicity and corrosion [6] SATYARATHI J, KUMAR V, KANGO S, et al. Comparative
resistance of aluminium surfaces, offering practical study of fabricated superhydrophobic surfaces via LST with
J. Cent. South Univ. (2024) 31: 2210－2224 2223
carbon soot, chemical etching and auto-oxidation [J]. Surf carbon coatings on the Ti13Nb13Zr alloy [J]. Open
Topogr Metrol Prop, 2022, 10: 45017. DOI: 10.1088/2051- Engineering, 2020, 10(1): 536−545. DOI: 10.1515/eng-2020-
672X/ac9c76. 0043.
[7] KUMAR V, VERMA R, SHARMA V S, et al. Recent [19] DAVIDSE P D. Theory and practice of RF sputtering [J].
progresses in super-hydrophobicity and micro-texturing for Microelectron Reliab, 1967, 6: 336. DOI: 10.1016/0026-2714
engineering applications [J]. Surface Topography: Metrology (67)90159-x.
and Properties, 2021, 9(4): 043003. DOI: 10.1088/2051- [20] SHARMA S, VYAS S, PERIASAMY C, et al. Structural and
672x/ac4321. optical characterization of ZnO thin films for optoelectronic
[8] WU Ting, XU Wen-hua, GUO Kang, et al. Efficient device applications by RF sputtering technique [J].
fabrication of lightweight polyethylene foam with robust and Superlattices and Microstructures, 2014, 75: 378−389. DOI:
durable superhydrophobicity for self-cleaning and anti-icing 10.1016/j.spmi.2014.07.032.
applications [J]. Chemical Engineering Journal, 2021, 407: [21] ZHAO M J, CHEN Z Z, SHI C Y, et al. Modulation of carrier
127100. DOI: 10.1016/j.cej.2020.127100. density in indium-gallium-zinc-oxide thin film prepared by
[9] BAGHERI H, ALIOFKHAZRAEI M, FOROOSHANI H M, high-power impulse magnetron sputtering[J]. Vacuum, 2023,
et al. Electrodeposition of the hierarchical dual structured 207: 111640. DOI: 10.1016/j.vacuum.2022.111640.
(HDS) nanocrystalline Ni surface with high water repellency [22] XIAO Xin-yan, XIE Wei, YE Zhi-hao. Preparation of
and self-cleaning properties [J]. Journal of the Taiwan corrosion-resisting superhydrophobic surface on aluminium
Institute of Chemical Engineers, 2017, 80: 883 − 893. DOI: substrate [J]. Surface Engineering, 2019, 35(5): 411 − 417.
10.1016/j.jtice.2017.07.023. DOI: 10.1080/02670844.2018.1433775.
[10] SAJI V S. Superhydrophobic surfaces and coatings by [23] DU C, HE X, TIAN F, et al. Preparation of superhydrophobic
electrochemical anodic oxidation and plasma electrolytic steel surfaces with chemical stability and corrosion[J].
oxidation [J]. Advances in Colloid and Interface Science, Coatings, 2019, 9(6): 398. DOI: 10.3390/coatings9060398.
2020, 283: 102245. DOI: 10.1016/j.cis.2020.102245. [24] SINGH V, SINGLA A K, BANSAL A. Enhanced erosion
[11] BARTHWAL S, LIM S H. A durable, fluorine-free, and resistance of HVOF-deposited laser-textured TiC coating
repairable superhydrophobic aluminum surface with with PTFE [J]. Surface Engineering, 2023, 39(7 − 12): 816 −
hierarchical micro/nanostructures and its application for 822. DOI: 10.1080/02670844.2023.2260049.
continuous oil-water separation [J]. Journal of Membrane [25] SINGH V, SINGLA A K, BANSAL A. Influence of laser
Science, 2021, 618: 118716. DOI: 10.1016/j. memsci. 2020. texturing along with PTFE topcoat on slurry and cavitation
118716. erosion resistance of HVOF sprayed VC coating [J]. Surface
[12] LIU Jun-peng, JANJUA Z, ROE M, et al. Super- and Coatings Technology, 2023, 470: 129858. DOI: 10.1016/
hydrophobic/icephobic coatings based on silica nanoparticles j.surfcoat.2023.129858.
modified by self-assembled monolayers [J]. Nanomaterials, [26] VIKRANT S, KUMAR S A, ANUJ B. Wetting and erosive
2016, 6(12): 232. DOI: 10.3390/nano6120232. behavior of VC-TiC + CuNi-Cr based coatings developed by
[13] FOROOSHANI H M, ALIOFKHAZRAEI M, HVOF: Role of laser texturing [J]. Engineering Failure
ROUHAGHDAM A S. Superhydrophobic aluminium Analysis, 2023, 152: 107479.
surfaces by mechanical/chemical combined method and its [27] SIMIONESCU O G, ROMANIȚAN C, TUTUNARU O,
corrosion behaviour [J]. Journal of the Taiwan Institute of et al. RF magnetron sputtering deposition of TiO2 thin films
Chemical Engineers, 2017, 72: 220 − 235. DOI: https://doi. in a small continuous oxygen flow rate [J]. Coatings, 2019, 9
org/10.1016/j.jtice.2017.01.014. (7): 442. DOI: 10.3390/coatings9070442.
[14] SARKAR M K, BAL K, HE Fu-en, et al. Design of an [28] SHRIVASTAVA S, VERMA R, KUMAR V, et al. Evaluation
outstanding super-hydrophobic surface by electro-spinning of corrosion resistance of as-sprayed WC-Co-Cr on DH-36
[J]. Applied Surface Science, 2011, 257(15): 7003 − 7009. steel with addition of 3% GNPs [J]. Transactions of the
DOI: 10.1016/j.apsusc.2011.03.057. Indian Institute of Metals, 2024, 77(5): 1413 − 1421. DOI:
[15] KAPRIDAKI C, MARAVELAKI-KALAITZAKI P. TiO2- 10.1007/s12666-023-03045-6.
SiO2-PDMS nano-composite hydrophobic coating with self- [29] SHRIVASTAVA S, KUMAR V, VERMA R, et al. Flexural
cleaning properties for marble protection [J]. Progress in and corrosion behaviour of WC-10Co-4Cr+Graphene
Organic Coatings, 2013, 76(2−3): 400−410. DOI: 10.1016/j. sprayed on textured HSLA-steel [J]. Surface Engineering,
porgcoat.2012.10.006. 2023, 39(4): 457 − 472. DOI: 10.1080/02670844.2023.
[16] KAMEGAWA T, SHIMIZU Y, YAMASHITA H. 2232970.
Superhydrophobic surfaces with photocatalytic self-cleaning [30] KUMAR V, VERMA R, SHARMA V S, et al. Investigations
properties by nanocomposite coating of TiO2 and into lattice strain and hardness of WC-10Co-4Cr coating on
polytetrafluoroethylene [J]. Advanced Materials, 2012, laser surface textured 420 stainless steel [J]. Lasers Eng,
24(27): 3697−3700. DOI: 10.1002/adma.201201037. 2022, 53: 231−252.
[17] VALLI J. A review of adhesion test methods for thin hard [31] SINGH N K, VINAY G, ANG A S, et al. Cavitation erosion
coatings [J]. Journal of Vacuum Science & Technology A: mechanisms of HVOF-sprayed Ni-based cermet coatings in
Vacuum, Surfaces, and Films, 1986, 4(6): 3007−3014. DOI: 3.5% NaCl environment[J]. Surface and Coatings
10.1116/1.573616. Technology, 2022, 434: 128194. 10.1016/j. surfcoat. 2022.
[18] PIOTROWSKA K, GRANEK A, MADEJ M. Assessment of 128194.
mechanical and tribological properties of diamond-like [32] SINGH N, ANG A, MAHAJAN D, et al. Cavitation erosion
2224 J. Cent. South Univ. (2024) 31: 2210－2224
resistant nickel-based cermet coatings for monel K-500 [J]. Materials, 2021, 14(23): 7161. DOI: 10.3390/ma14237161.
Tribology International, 2021, 159: 106954. DOI: 10.1016/j. [34] VRAKATSELI V E, KALARAKIS A N, KALAMPOUNIAS
triboint.2021.106954. A G, et al. Glancing angle deposition effect on structure and
[33] KHASKHOUSSI A, CALABRESE L, PATANÉ S, et al. light-induced wettability of RF-sputtered TiO2 thin films [J].
Effect of chemical surface texturing on the superhydrophobic Micromachines, 2018, 9(8): 389. DOI: 10.3390/mi9080389.
behavior of micro-nano-roughened AA6082 surfaces [J]. (Edited by YANG Hua)

中文导读

铝合金上射频溅射二氧化钛涂层的微观结构、润湿性和耐腐蚀行为

摘要：在不同温度下，在 Al6061 合金表面射频溅射二氧化钛涂层，以改善其疏水性和耐腐蚀性，旨在

摘要：
了解射频溅射工艺对二氧化钛涂层性能的影响规律。使用表面轮廓仪和测角仪测量表面粗糙度和润湿
性，通过化学沸腾法和盐雾腐蚀法过程中形貌变化来研究耐腐蚀性能。使用场发射扫描电子显微镜
（FESEM）、能量色散光谱(EDS)和 X 射线衍射(XRD)技术来表征涂层。结果表明，与其他温度相比，
在 500 ℃得到的二氧化钛涂层具有最高的水接触角和优越的耐腐蚀性。表面表征证实了这种 500 ℃得
到的二氧化钛涂层由于其好的疏水性，有效地延缓了腐蚀，使其持久用于工业应用。

关键词：
关键词 ：铝；二氧化钛涂层；射频溅射；疏水性；腐蚀