S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
75–83
NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL
PROPERTIES OF THIN TiO2, WO3 AND THEIR COMPOSITES
LAYERS, DEPOSITED BY SPRAY PYROLYSIS
PREISKAVE MEHANSKIH LASTNOSTI Z NANOTRDOTO TANKIH
TiO2, WO3 IN NJUNIH KOMPOZITNIH PLASTI, NANE[ENIH S
PR[ILNO PIROLIZO
Sabina Cherneva1, Rîumen Iankov1, Nenad Radic2, Bosko Grbic2,
Maria Datcheva1, Dimitar Stoychev3
1Bulgarian Academy of Sciences, Institute of Mechanics, Acad. G. Bonchev str., bl.4, 1113 Sofia, Bulgaria
2IChTM, University of Belgrade, Department of Catalysis and Chemical Engineering, Njegoseva 12, 11000 Belgrade, Serbia
3Bulgarian Academy of Sciences, Institute of Physical Chemistry, Acad. G. Bonchev str., bl.11,1113 Sofia, Bulgaria
stoychev@ipc.bas.bg
Prejem rokopisa – received: 2015-07-10; sprejem za objavo – accepted for publication: 2016-01-05
doi:10.17222/mit.2015.216
The aim of the present work is to determine the indentation hardness (HIT) and indentation modulus (EIT) of pure TiO2 and WO3
thin films, as well as thin films composed of different TiO2 and WO3 proportions and deposited by spray pyrolysis on a
stainless-steel (OC 404) substrate. Since the HIT and EIT of the films are properties expected to depend on the phase-chemical
composition, morphology, structure and their changes when increasing the WO3 content in the TiO2-WO3 composite film, the
correlation between the mechanical and structural properties is also addressed. The obtained results show that HIT and EIT
strongly depend on the concentration of the co-deposited WO3. The determined values of HIT and EIT noticeably decrease (in
comparison with HIT and EIT of the pure (100 %) TiO2 layer) when very low concentrations of WO3 (up to 2.5 % of W) are
co-deposited. At higher concentrations of the co-deposited WO3 (more than 2.5 % of W), the HIT and EIT values increase almost
linearly with an increase of the WO3 in the precursor. The observed non-proportional behavior of HIT and EIT is associated with
specific changes of the structure and a development of defects in the deposited TiO2-WO3 composite phase, as well as with the
increase in the amount of the formed separate WO3 phase (with increasing of WO3 (H2W3O12) in the working solution)
surrounded by solitary TiO2 particles.
Keywords: inorganic compounds, chemical synthesis, electron microscopy, elastic properties
Namen predstavljenega dela je dolo~iti trdoto vtiska (HIT) in modul vtiska (EIT) v tankih filmih iz ~istega TiO2 in WO3, kot tudi
tankih filmov, sestavljenih iz razli~nih delov TiO2 in WO3, nane{enih s pr{ilno pirolizo na podlago iz nerjavnega jekla (OC 404).
Ker se pri~akuje, da sta lastnosti filma HIT in EIT odvisni od kemijske sestave faz, morfologije, strukture in njenih sprememb, ko
pove~ujemo dele` WO3 v TiO2-WO3 kompozitnem filmu, se to nana{a tudi na odvisnost med mehanskimi lastnostmi in
lastnostmi strukture. Dobljeni rezultati ka`ejo, da sta HIT in EIT mo~no odvisna od koncentracije nane{enega WO3. Dolo~ene
vrednosti HIT in EIT se opazno zmanj{ajo (v primerjavi z HIT in EIT plasti iz ~istega (100 %) TiO2) ko se nanese WO3 z nizko
koncentracijo (do 2,5 % dele` W). Pri nanosih WO3 z vi{jo koncentracijo (nad 2,5 % dele` W), vrednosti HIT in EIT nara{~ata
skoraj linearno z pove~evanjem dele`a WO3 v osnovi. Opa`eno neproporcionalno obna{anje HIT in EIT je povezano s
specifi~nimi spremembami v strukturi in z razvojem napak v nane{eni TiO2-WO3 kompozitni fazi, kot tudi s pove~anjem
koli~ine nastale WO3 faze (pri pove~evanju WO3 (H2W3O12) v delovni raztopini), ki jo obkro`ajo posamezni TiO2 delci.
Klju~ne besede: neorganske spojine, kemijska sinteza, elektronska mikroskopija, elasti~ne lastnosti
1 INTRODUCTION
The multi-functionality of titanium dioxide is of great
interest for both contemporary science and technology.1
It is the most widely used metal oxide for environmental
applications2, paints, electronic devices3, gas sensors4
and solar cells.5 Due to the broad range of applications
and the importance of nano-sized titanium a large num-
ber of preparative methods for its synthesis have been
reported, including: high-temperature processes6, sol-gel
techniques7, chemical vapor deposition8, solvothermal
processes9, reverse micelles10, hydrothermal methods11,
ball milling12, plasma evaporation13, sonochemical reac-
tions14, etc. Unlike many other techniques, spray pyro-
lysis represents a simple and cost-effective processing
method, which employs precursor solutions to form
different types of dense or porous mono- and multiphase
layers with a wide range of thicknesses. This method is
also extremely versatile due to the large number of
adjustable process parameters such as: substrate tem-
perature, composition and concentration of the precursor,
atomization technique, spray geometry, liquid- and
gas-flow rates, etc.15
In this regard, extensive research has been carried out
over the past few decades for characterizing the chemi-
cal, physical-chemical and surface/bulk-structural
properties of these layers synthesized by spray-pyro-
lysis.16–22 However, investigations of their mechanical
properties (such as microhardness and indentation hard-
ness, wear resistance, indentation modulus, adhesion,
Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83 75
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
UDK 620.17:622.023:620.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)75(2017)
cohesion, etc.) which are very important in functional
and operation exploitation aspects are practically absent.
As a very important addition it has to be pointed out
that the anatase nanocrystalline form of titanium dioxide
is of particular interest, because it has the highest
reactivity in photocatalysis and the best antimicrobial
activity.23–27 However, there is still a problem with the
ability of TiO2 to respond only to a small portion of the
solar spectrum (<5 %) due to its relatively wide band gap
(~3.2 eV).28 This invokes the necessity to create a new
generation of nano-sized photocatalysts based on TiO2,
being capable of utilising effectively both components
(UV and visible) of the sunlight.29,30 It is established that
doping TiO2 with different metals or nonmetals (such as
SnO2 31, WO3 32, ZrO2 33 and V2O5 34) is a modification
approach, used to extend the absorption range of TiO2 to
the visible region of solar light. The different dopant ions
introduce electron energy levels narrowing the TiO2 band
gap. In this aspect the TiO2-WO3 composite material35,36
seems promising for visible-region-induced photocata-
lysis, due to the suitable combination of the energy band
gaps for anatase and for WO3. That is why several inve-
stigations were focused on the synthesis and characteri-
zation of TiO2-WO3 composites.37–41 The energy band
gap of WO3 is ~2.4–2.8 eV and both the upper edge of
the valence band and the lower edge of the conduction
band of WO3 are lower than those of TiO2. Thus, the
TiO2-WO3 composite has a narrower energy band gap
(compared to that of TiO2) and shows enhanced photo-
catalytic activity with respect to its single components.
This coupling of TiO2 and WO3 favors the transition of
electrons from the valence to the conduction band and
hole transfers between the bands in the opposite direc-
tion. This also reduces the electron–hole recombination
rate in both semiconductors.42
It was also recently shown that the unique optical and
electric properties of TiO2 unveil the possibility for its
use also as photo-catalytic anticorrosion protection of
steels.43–49 Considering the investigation of T. Tsai and
co-authors50 it is expected that deposited on steel, TiO2
layers will create photocathode protection under the
influence of UV irradiation. This protection property is
based on the transfer of photo-generated electrons to the
metal substrate, as a result of which its electrode po-
tential becomes more electronegative than its corrosion
potential. Consequently, the titanium oxide (in the sys-
tem TiO2/steel) will act as a non-soluble anode, provid-
ing cathode protection of the steel. Obviously, the
protective layer of pure TiO2 cannot act as photo-gene-
rated cathodic protector in the dark. However, it can be
expected that doping TiO2 with WO3, SnO2, MoO3, etc.,
could solve this problem. These semiconductors (WO3,
SnO2, MoO3, etc.), which are characterized with a diffe-
rent energy level from those of TiO2, can store excess
electrons during UV irradiation and the stored electrons
can be later released in the dark period of the corrosion
attack.
Going back to the considered TiO2-WO3 system here,
it should be pointed out that the methods of preparation
of TiO2-WO3 systems and the characterization of their
catalytic and physicochemical properties are extensively
studied and discussed in the literature. Nevertheless, the
data for their physical-mechanical properties are very
few. Generally, the available data is obtained indirectly
using the reference data of chemically or metallurgically
synthesized powders that may not be proper for a deter-
mination of the properties of the materials deposited as
layers/coatings (by other methods) on a specific sub-
strate like metal, alloy, ceramics, etc.51 And to the best of
our knowledge, there are no studies of thin layers ob-
tained by spray-pyrolysis on foreign substrates. For this
reason, it is essential to perform studies characterizing
the mechanical properties of TiO2, WO3, as well as of
layers composed of TiO2-WO3 mixtures. A knowledge
about the mechanical properties is important from the
exploitation point of view. The layers of TiO2, WO3, and
TiO2-WO3 mixtures are exposed to a wide range of static
and dynamical mechanical loads, temperature variations
as well as corrosion, and other factors that lead to the
degradation of their strength characteristics.52 The inte-
raction of the layers with the substrate should also be
investigated. Exceptionally, it is important to consider
the formation of composite TiO2-WO3 layers on a steel
substrate by spray-pyrolysis, because the process is
taking place at high temperatures and it is possible to
have chemical or structural interactions in the volume of
the layer, as well as diffusion transitions on the
TiO2-WO3/Substrate interface. Therefore, it is highly
likely that the mechanical properties of the layer and
those of the interface may differ significantly.
The objective of this investigation was to determine
the indentation hardness (HIT) and indentation modulus
(EIT) of the layers of TiO2 and WO3 deposited by spray-
pyrolysis on stainless steel (OC 404), as well as to study
the influence of the process parameter concentration of
the co-deposited WO3 in the mixed TiO2-WO3 layers.
Since HIT and EIT are properties that depend on the
structure, the structural and morphological changes and
phase-chemical content/composition of the layers were
carefully investigated, especially with regard to the in-
crease in the WO3 content (from 1 % to 75 % of mass
fractions) in the working solution and in the deposited
composite TiO2-WO3 layers, respectively.
2 THEORETICAL ASPECTS OF NANO-INDEN-
TATION AS A METHOD FOR THE MECHANI-
CAL CHARACTERIZATION OF THIN FILMS
Instrumented-indentation testing (IIT or so-called
nano-indentation) has been developed over the past
decade for the purpose of probing the mechanical
properties of very small volumes.53–57 IIT is ideal for
mechanically characterizing thin films, coatings, and
surface layers. In addition IIT is an attractive method to
S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
76 Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
characterize the mechanical properties, because in most
cases it requires little sample preparation efforts and has
high measurement precision.
Basically, IIT uses a high-resolution actuator to
control the penetration into the test surface by the
indenter and a high-resolution sensor to continuously
measure the penetration depth. One of the benefits of this
method is that the contact area under load can be
calculated in most cases from the load-displacement data
alone, meaning that the residual impression does not
have to be viewed directly using complicated imaging
techniques, thus making it far easier to measure pro-
perties on the sub-micron scale. Indentation hardness
(HIT) and indentation modulus (EIT) are the properties
most frequently determined by IIT.58
The fundamental relation from which the elastic
modulus EIT can be estimated is the well-known relation
between the true projected contact area Ac, the initial
unloading slope S and the reduced elastic modulus Er:59,53
E
S
Ar c
=
π
2
(1)
where  is a constant that depends on the geometry of
the indenter tip. For indenters with a triangular cross-
section like the Berkovich tip  = 1.034. The true
projected area is determined using the true contact depth
hc and employing the approximation given below with
coefficients obtained after calibration using indentation
data from a standard fused-silica sample:
A C h C h C h C h C h C hc ≈ + + + + +0
2
1 2
1 2
3
1 4
4
1 8
5
1 16
c c c c c c
/ / / /
The indentation hardness is defined through the ratio
of the applied load P and the corresponding true pro-
jected contact area:
H
P
AIT c
= (2)
The indentation (elastic) modulus EIT of the test
material is calculated using the relation:
E v
E
v
E
i
i
IT
r
= − −
−⎡
⎣⎢
⎤
⎦⎥
−
( )1
1 12
2 1
(3)
where 
 is the Poisson’s ratio for the test material, and
Ei and 
i are the indenter’s elastic modulus and
Poisson’s ratio, respectively.54 In our case we used the
elastic constants for diamond Ei = 1141 GPa and 
i =
0.07.
3 EXPERIMENTAL PART
3.1 Preparation of the samples
The spray-pyrolysis method has been applied for the
synthesis of TiO2-WO3 composite coatings on foils of
Sandvik OC 404 stainless steel (SS). A homemade
spray-pyrolysis apparatus for the synthesis of these com-
posites is presented in previously published studies.60 As
precursors, a 0.02-M TiO2 colloidal solution and 0.02-M
H2W3O12 were used. A colloidal solution containing TiO2
nanoparticles (d  4.5 nm) was synthesized in the
manner previously described by T. Rajh et al.61 A solu-
tion of H2W3O12 was prepared by dissolving the metal W
in H2O2 at 60 °C. These two precursor solutions are
mixed in different weight ratios in order to vary the
contents of WO3 and TiO2 in the composites. The
stainless-steel specimens (foil thickness 35 μm, 1.5 cm ×
10 cm), prior to depositions of the oxide layer, were
subjected to standard procedures of degreasing and
ultrasonic cleaning.
The typical twin-fluid spray pyrolysis system, using a
nozzle made of Pyrex glass, diameter of 0.2 mm, was
applied within a homemade computer-controlled device
that enabled nozzle movement with adjustable speed and
direction. The key preparation parameters of the syn-
thesized samples are presented in Table 1.
Table 1: Key preparation parameters of spray pyrolysis
Tabela 1: Klju~ni parametri pri pr{ilni pirolizi
Initial temperature of
substrate (°C) 460
Nozzle to substrate distance
(cm) 4
Nozzle speed (mm/s) 1
Diameter of spraying spot on
the substrate (cm) 2
Concentration in precursor
solutions (M)
0.02 (TiO2 colloidal solution)
0.02 (H2W3O12)
Air-flow rate (L/h) 300
Precursor solution flow rate
(mL/h) 44
Number of nozzle passes 200
Duration of spraying (min) 50
The synthesized samples were named: TiO2(100),
TiO2(99)-WO3(1), TiO2(95)-WO3(5), TiO2(90)-WO3(10),
TiO2(75)-WO3(25), TiO2(50)-WO3(50), TiO2(25)-
WO3(75), and WO3(100), according to the content (w/%)
of single component in the working solution.
The thicknesses of the coatings were determined
according to the relation in Equation (4):
T
M
A
=

(4)
where A is the geometric area of the coated surface, the
mass (M) and the bulk density () of the coatings. The
mass of the coating (M) was determined by weighing
the foil before and after the spray pyrolysis.
The coating bulk density is calculated according to
Equation (5) and using the true density of anatase
(A = 3.9 g/cm3) and WO3 phase (B = 5.4 g/cm3), the
percentage of anatase (kA) and WO3 phase (kB) in the
precursor solution, and the powder sample porosity (P)
obtained by mercury intrusion porosimetry:62
  = + −( )( )A A B Bk k P1 (5)
S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
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Although the characteristics of these powders were
not entirely the same as that of the films, the porosity of
these powders should be considered valuable for an eva-
luation of the properties of pure TiO2 and WO3 coatings
as well as of TiO2-WO3 composites, such as bulk density,
thickness and surface area.
3.2 Structural characterization
The surface morphology, structure and elemental
microanalysis of the samples were characterized by
scanning electron microscopy (SEM) using a JEOL JSM
6390 electron microscope (Japan) equipped with an
ultra-high-resolution scanning system in a regime of
secondary-electron image (SEI), back-scattered electrons
(BEI) and an INCA energy-dispersive X-ray spectro-
meter (EDS).The accelerating voltage was 25 kV, I ~ 65
mA. The vacuum was 10–6 mm Hg.
3.3 Mechanical characterization
The indentation modulus EIT and hardness HIT of the
deposited TiO2, composite TiO2-WO3 and WO3 films
were determined via the instrumented indentation
technique. The tests were performed using NanoIndenter
G200 (Agilent Technologies) equipped with a Berkovich
three-sided diamond pyramid with centerline-to-face
angle of 65.3° and a 20 nm radius at the tip.63
The particular indentation method employed here is
described in 64. It prescribes a series of 10 loading/
unloading cycles in a single-indentation experiment. The
maximum prescribed load is 0.49 N with 20 s peak hold
time at the maximum load for each loading-unloading
cycle. As a result of the nano-indentation experiments,
load-displacement curves are obtained and HIT and EIT
are calculated as explained above using the Oliver &
Pharr approximation method53 and Equations 1 to 3.
Within this study the indentation hardness and modulus
were determined using the stiffness calculated by em-
ploying 50 % of the upper portion of the load-displace-
ment curve during each unloading cycle. Each sample
was subject to 25 indentation tests in order to have better
statistics.
For the realization of an adequate and correct assess-
ment of the mechanical properties of the considered thin
deposited layers, it is necessary to guarantee a very good
adhesion of the layers to the substrate, reduce the un-
certainty in the determination of the layer thickness as
well as to have a previous knowledge about the mate-
rial’s internal structure and the existing defects in the
layers. The quality of the adhesion of the coatings depo-
sited on the SS substrate was examined by observing
whether there is a detachment of the coating from the
substrate after a repeated bending of the foil at angle of
180° (EN ISO 2819-1994:2.9 "Bending test").
4 RESULTS AND DISCUSSION
4.1 Analytical and structural characterization of the
specimens
The coatings’ thicknesses, calculated according to
Equation (4), are about 1 μm for all the samples, as
presented in Table 2.
More details about the porous structure for pure TiO2
and WO3 powders, which are constituent parts of all the
composites, are given in 62.
XRD data obtained for the same systems in our
previous investigation confirm the formation of only the
anatase phase of TiO2, no reflections corresponding to
the rutile TiO2 phase were observed.62 The composites
with WO3 content greater than 10 % of mass fractions
exhibits diffraction peaks of monoclinic tungsten oxide.
The absence of reflections corresponding to WO3 for
samples with WO3 content below 10 % of mass fractions
reveals that clusters of WO3 are present either in the
highly dispersed form or in a concentration below the
detection limit of the XRD apparatus.
For confirmation of the presence of WO3 in the layers
deposited from the working solutions with a WO3 con-
tent below 10 % of mass fractions, which may not be
detected by XRD analysis, we realized the investigation
of all the TiO2-WO3 composite layers by EDX analysis
for sufficient time of exposure (120 s). The results from
the EDX analysis are shown in Tables 3 and 4.
Table 3: Estimated by EDX analysis percent concentration (a/%) of
O, Ti and W in deposited by spray-pyrolysis thin TiO2-WO3 layers
Tabela 3: EDX-koncentracija (a/%) O, Ti in W v tankih TiO2-WO3
plasteh, nane{enih s pr{ilno pirolizo
Sample
O
a/%
Ti
a/%
W
a/%
Total
100 % TiO2 80.44 19.56 0 100
TiO2(99)-WO3(1) 81.16 18.22 0.62 100
TiO2(95)-WO3(5) 70.83 27.37 1.80 100
TiO2(90)-WO3(10) 77.57 20.04 2.39 100
TiO2(75)-WO3(25) 72.95 20.89 6.16 100
TiO2(50)-WO3(50) 79.47 10.07 9.83 100
TiO2(25)-WO3(75) 76.83 6.33 16.84 100
100 % WO3 76.41 0 23.59 100
S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
78 Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Table 2: Estimated surface area, bulk density and thickness of
TiO2-WO3 samples
Tabela 2: Povr{ina, gostota in debelina vzorcev TiO2-WO3
Sample Depositedmass, mg
Specific
surface
area, m2/g
Bulk
density,
g/cm3
Thickness,
μm
TiO2(100) 3.25 32.3 (5)* 2.27 0.95(5.8)**
TiO2(99)-WO3(1) 3.40 32.1 (6) 2.30 0.98 (5.6)
TiO2(95)-WO3(5) 3.90 31.1 (7) 2.42 1.07 (5.1)
TiO2(90)-WO3(10) 4.05 30.0 (7) 2.58 1.04 (5.3)
TiO2(75)-WO3(25) 4.80 26.7 (8) 3.04 1.05 (5.2)
TiO2(50)-WO3(50) 5.15 21.1 (10) 3.81 0.90 (6.1)
TiO2(25)-WO3(75) 7.05 15.5 (12) 4.58 1.02 (5.4)
WO3(100) 8.10 9.9 (15) 5.36 1.01 (5.4)
*Standard deviation of specific surface area (%)
**Standard deviation of thickness (%)
Table 4: Estimated by EDX analysis percent concentration (w/%) of
O, Ti and W in deposited by spray-pyrolysis thin TiO2-WO3 layers
Tabela 4: EDX-koncentracija ( w/%) O, Ti in W v tankih TiO2-WO3
plasteh, nane{enih s pr{ilno pirolizo
Sample Ow/%
Ti
w/%
W
w/% Total
TiO2 (100) 57.86 42.14 0 100
TiO2(99)-WO3(1) 56.85 38.21 4.94 100
TiO2(95)-WO3(5) 40.83 47.23 11.94 100
TiO2(90)-WO3(10) 47.00 36.35 16.65 100
TiO2(75)-WO3(25) 35.36 30.32 34.32 100
TiO2(50)-WO3(50) 35.40 14.27 50.33 100
TiO2(25)-WO3(75) 26.56 6.55 66.89 100
WO3(100) 21.99 0 78.01 100
It is seen from the obtained results, that at concen-
trations of WO3 in the precursor lower than 10 %, its
inclusion in the composite layers takes place. At the
same time the content of co-deposited WO3 and TiO2 in
the composite TiO2-WO3 thin films practically does not
correspond to the weight ratio of the two mixed precur-
sor solutions (Tables 3 and 4). A specific deviation is
observed, especially at the concentration interval of
1–10 % of mass fractions of WO3 in the working solu-
tion. It is interesting to point that for the solution con-
taining 5 % WO3 both the weight and atomic percentages
of co-deposited Ti are having their maximum along all
the investigated samples and are even higher than in the
case of the spray-pyrolysis deposited pure TiO2, while
the atomic percent of O has its minimum in this case.
The results of the SEM observations of the inve-
stigated TiO2-WO3 composite layers as well as pure TiO2
and WO3 layers at different magnifications are shown in
Figure 1. It is seen from the obtained results that the
surface structure of the pure TiO2 layer is very smooth
and compact, with no visible cracks (Figure 1a). The
addition of WO3 (H2W3O12) to the working solution
affected the structure of the obtained TiO2-WO3 compo-
sites and the layer surface morphology becomes well
populated with irregularities, which can be associated
with the irregular inclusion of WO3 particles into the
TiO2 matrix (Figures 1b to 1g). At the low concentra-
tions of WO3 in the working solution there is a systema-
tic appearance of macro-void formations. With increas-
ing the content of WO3 in the working solution and the
content of the WO3 in the composite layers, respectively,
the number of elevations and depressions on the surface
increases while the number of formed voids decreases.
The surface of the composite coatings becomes lacy.
Probably, this effect is due to the fast hydrolysis of the
tungsten salts leaving holes behind them that create
micron-sized concavities characterizing the "pure" WO3
layers (Figure 1h). These results are fully consistent
with the quantitative analysis of the surface topography
and surface roughness obtained for the same systems
using AFM.62
As shown in Figure 1, the surfaces of the composite
layers are decorated by agglomerated grains having a
considerable surface roughness. Increasing the content of
WO3 in the TiO2-WO3 composites leads to the formation
of numerous irregularities in their surface. The layers
with a higher WO3 content exhibit a rough surface text-
ure with high šmountains’ and deep švalleys’ generated
by the fusion of particles at the inter-particle contacts. As
shown in62 there are differences in surface irregularity
when forming the TiO2-WO3 composites. The surface
roughness values increase significantly with an increase
of the WO3 content in the composite, reaching to
316 nm. The change of the surface roughness suggests
that the small TiO2 grains (with average diameter of
about 4.5 nm) fill the voids and pores between the WO3
agglomerates, promoting the surface flattening.
4.2 Mechanical characterization
In order to confirm the high adhesion of the formed
coatings we performed adhesion tests with repeated
S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83 79
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 1: SEI images of samples: a) TiO2(100), b) TiO2(99)-WO3(1),
c) TiO2(95)-WO3(5), d) TiO2(90)-WO3(10), e) TiO2(75)-WO3(25),
f) TiO2(50)-WO3(50), g) TiO2(25)-WO3(75), and h) WO3(100).
Slika 1: SEI-posnetki vzorcev: a) TiO2(100), b) TiO2(99)-WO3(1),
c) TiO2(95)-WO3(5), d) TiO2(90)-WO3(10), e) TiO2(75)-WO3(25)
(eII BEC posnetek), f) TiO2(50)-WO3(50) (fI in fIV – BEC posnetek;
fIII: EDS-spekter in izra~unane vrednosti v spodnji tabeli – Ti in W,
dobljena v to~ki 3 – prikazani na Sliki 1 fII), g)TiO2(25)-WO3(75), in
h) WO3(100)
bending of coated foils at an angle of 180° (according
EN ISO 2819-1994:2.9 "Bending test"). A good adhe-
sion of the TiO2-WO3 coatings to the SS substrate was
found for all samples, and the attrition of coatings was
negligible (less than 1 %). This observation ensures that
during the indentation tests there is no separation of the
coating from the substrate. This result gives us confi-
dence in excluding the separation of the coating from the
analysis of the mechanical properties.
As a result of the nano-indentation measurements,
the load-displacement curves for all samples were ob-
tained and analysed for a determination of the indenta-
tion modulus and hardness of just the foil coatings.
Figures 2 to 5 present the results from calculated inden-
tation hardness and indentation modulus for all the eight
samples at a load of approximately 1.89 mN and inden-
tation depths below 250 nm (25 % of the average film
thickness).
The two main factors that may influence the HIT and
EIT of the analysed composite thin films are their che-
mical content and their surface morphology and struc-
ture. As shown in Figure 1 as well as in Tables 3 and 4,
the changes of the chemical content (the ratio between
concentrations of TiO2 and WO3 phases) in the
TiO2-WO3 composite layer have a significant influence
on the surface morphology, bulk structure, defects and
porosity. This observation suggests that the change in the
ratio between the concentrations of the two components
and of their ingredients (O Ti, W) in the composite layer
could have an important influence on the HIT and EIT
values. This was the reason to investigate the influence
of the change in the chemical content and the subsequent
structural and phase changes in the spray-pyrolysis depo-
sited TiO2-WO3 layer on its mechanical characteristics.
First we consider the variation of the mechanical
properties depending on the weight percent of the two
precursors in the working solution. Figures 2 and 4 show
that the indentation hardness of the pure WO3 and pure
TiO2 films is approximately of the same value. The
slightly higher hardness of the pure WO3 coating may be
attributed to the observed less porosity. At small con-
centrations of WO3 in the working solution (from 0 % to
5 %), there is a rapid drop in the indentation hardness of
the obtained composite films. With a further increase of
the WO3 concentrations, the indentation hardness
increases gradually to reach its maximum for the pure
WO3 layer. The behaviour of the indentation modulus is
S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
80 Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 5: Indentation modulus EIT as a function of Ti, W, and O
content, (w/%)
Slika 5: Modul vtiska EIT v odvisnosti od vsebnosti Ti, W in O, (w/%)
Figure 3: Indentation modulus EIT as a function of the concentration
of Ti, W and O in the spray-pyrolysis-deposited TiO2-WO3 layers,
(a/%)
Slika 3: Modul vtiska EIT v odvisnosti od koncentracije Ti, W in O v
plasteh TiO2-WO3 nane{enih s pr{ilno pirolizo, (a/%)
Figure 4: Indentation hardness HIT as a function of Ti, W, and O
content, (w/%)
Slika 4: Trdota vtiska HIT v odvisnosti od vsebnosti Ti, W in O, (w/%)
Figure 2: Indentation hardness HIT as a function of the concentration
of Ti, W and O in the spray-pyrolysis-deposited TiO2-WO3 layers,
(a/%)
Slika 2: Trdota vtiska HIT v odvisnosti od koncentracije Ti, W in O v
plasteh TiO2-WO3, nane{enih s pr{ilno pirolizo, (a/%)
similar to that of the indentation hardness. When adding
a small amount of WO3 (from 0 % to 5 %) to the work-
ing solution, the indentation modulus of the composite
films first decreases with increasing the content of WO3
and reaches its minimum for sample TiO2(95)-WO3(5).
With further increasing the concentrations of WO3, the
value of the indentation modulus increases gradually.
However, in the case of pure WO3 film the value of the
indentation modulus is less than in the case of pure TiO2
film.
The relation between the change of HIT and EIT and
the chemical content of the investigated layers is also
depicted in Figures 2 to 5. The results show that the HIT
and EIT values of the TiO2-WO3 composite layer con-
taining the maximum weight and atomic percent of Ti
and minimum atomic percent of O are having the lowest
values, as compared to those of the other composite as
well as mono-component layers. In this case, the value of
HIT was four times lower in comparison with the hard-
ness of the pure TiO2 layer (HIT/TiO2(100) 5.1 GPa vs.
HIT/TiO2(95)-WO3(5) 1.3 GPa). It should be pointed out
that the determined concentration of Ti in the deposited
composite layer has its maximum value for sample
TiO2(95)-WO3(5), even higher than the one determined
for the spray-pyrolysis deposited nanosize 100 %
("pure") TiO2 layer (27.4 % of amount fractions vs.
19.6 % of amount fractions of Ti). A remarkable pro-
perty of the TiO2(95)-WO3(5) sample is observed from
the Raman spectrum and is discussed in 63. The conclu-
sion is that the Raman spectra of the TiO2(95)-WO3(5)
sample suggests the appearance of a tensile stress at the
TiO2-WO3 interface. Such a tensile stress may decrease
the hardness of the coating. In our case, the further
increase of W percent concentration in the composite
coatings has led to a practically proportional increase of
HIT. At higher concentrations (above 17 % of amount
fractions) the increase of the HIT value became more
rapid.
When comparing the variation of HIT with that of the
Ti, W and O atomic concentrations in the studied layers,
it can be concluded that the increase of the W concen-
tration in the composite layer is monotonic, while that of
Ti possesses a complex non-monotonic behaviour. With
the addition of WO3 into the working solution, first the
concentration of the Ti increases, reaching its maximum
for TiO2(95)-WO3(5) sample. In the case of the working
solution containing 10 % WO3 precursor, the concen-
tration of Ti in the deposited composite TiO2-WO3 layer
starts to decrease, reaching ~11 % of amount fractions.
Increasing the concentration of the WO3 precursor (up to
25 %) leads to an increase of the concentration of W in
the composite layers; however, this does not change
proportionally the concentration of Ti. Furthermore, the
values of HIT continue to increase, indicating the domi-
nant influence of the second component (WO3) in the
composite layer mechanical characteristics. This is also
indicated by the values of HIT at the approximately equal
atomic concentration ratio of Ti and W – Table 5.
The values of HIT and EIT for the spray-pyrolysis de-
posited layers of TiO2, WO3 and TiO2-WO3 composites
with weight ratio of the TiO2 and WO3 precursors 95:5
and 50:50 corresponding respectively to maximum con-
centration of Ti (27.5 % of amount fractions) and mini-
mum concentration of O (70.83 % of amount fractions)
and to the approximately the same content in % of the
amount fractions of Ti and W (10.07:9.83 % of amount
fractions) can be found in Table 5.
Considering the results discussed above, it can be
concluded that the mechanical characteristics HIT and EIT
mainly depend on the chemical content, structure and
porosity of the investigated TiO2–WO3 composite layers.
The comparison of the size changes in agglomerates
building the layers shows that the amorphous "pure"
TiO2 layers (Figure 2a) are characterized by consider-
ably higher HIT and EIT values than those of the com-
posite TiO2–WO3 layers. The co-deposition of 0.6–2.4 %
of amount fractions (5–17 % of mass fractions) W leads
to a substantial increase of the porosity and the size of
the agglomerates building the TiO2–WO3 layers that
determine the dramatic decrease of HIT and EIT,
according to the Hall-Petch relationship.65 Increasing the
concentration of the co-deposited W (WO3) further,
decreases the porosity of the layers, as well as the size of
the agglomerates that build them, which leads to the
increase of HIT and EIT.62 The latter values are close to
the HIT and EIT measured for the spray-pyrolysis
S. CHERNEVA et al.: NANO-INDENTATION INVESTIGATIONS OF THE MECHANICAL PROPERTIES OF THIN TiO2, WO3 ...
Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83 81
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Table 5: Values of HIT and EIT at the characteristic points of the concentration ratio of TiO2 and WO3 precursors in the working solution, content
of Ti and W in deposited layers, respectively
Tabela 5: Vrednosti HIT in EIT pri zna~ilnih to~kah razmerja koncentracije TiO2 in WO3 osnov v delovni raztopini ter vsebnost Ti in W v
nane{enih plasteh
Weight ratio of the TiO2 and WO3
precursors in the working solution
TiO2
100 %
TiO2 95 %
WO3 5 %
TiO2 50 %
WO3 50 %
WO3 100 %
Content (a/%) of Ti and W in
spray-pyrolysis deposited layers
Ti 19.56
W 0
Ti 27.37 (max)
W 1.8
O 70.83 (min)
Ti 10.07
W 9.83
(approx. equal)
Ti 0
W 23.59
HIT (GPa) 5.1 1.3 (min value) 2.7 5.4 (max value)
Standard deviation of HIT (%) 7.8 % 11.99 % 16.68 % 16.3 %
EIT (GPa) 155 (max value) 75 (min value) 110 133
Standard deviation of EIT (%) 4.34 % 7.17 % 9.31 % 9.54 %
deposited "pure" WO3, which according to the SEM
microphotographs is more likely to exhibit a crystal
structure. Importantly, in all of the composite layers the
values of HIT and EIT are lower than the one of the "pure"
TiO2 and WO3 layers. Moreover, the obtained values are
described by a dependency that has a minimum at low
concentrations (1.5–2.5 % of amount fractions) of the
co-deposited W, after which HIT and EIT increase with the
concentration of W. This complex dependency suggests
that along with the influence of the changes in the
structure (the size of the crystallites that build the layers)
other factors could have an influence on HIT and EIT
when the concentration of co-deposited W (WO3,
respectively) increases in the composite layers. We can
assume that the TiO2 and WO3 molecules interact in the
TiO2-WO3 composite layer on an electron level. The
reasons for making such an assumption are given in
62,66–68.
4.3 Further discussion
It is interesting to note that HIT and EIT drop coincides
with the rise of the photoactivity of the corresponding
TiO2-WO3 systems.62 After reaching the maximum
values of the photocatalytic activity at 10 wt. % of WO3,
the drop of activity occurred with further increasing of
the WO3 content. The drop of photoactivity is followed
by a simultaneous increase of the HIT and EIT factors.
Obviously, materials properties that suits photocatalytic
activities (well-developed surface area, porosity, surface
defect, etc.) are a disadvantage for the mechanical cha-
racteristics of these coatings. Our previous investigations
by XPS62 have shown that metals are in their main
oxidation state, Ti4+ and W6+, but positive shift of the
binding energy of Ti 2p by 0.5 eV is observed, pointing
out that there is kind of interaction between TiO2 and
WO3 phase. Furthermore, the Raman investigation
reveals that TiO2-WO3 composites with up to 10 % of
mass fractions of WO3 are without free WO3 phase that
is incorporated within TiO2 forming Ti1–xWxO2 phase.
Probably, such a structure leads to the disturbance of
TiO2 lattice, making it less resistant to the mechanical
stress. Obviously, this increasing of the quantity of de-
fects in the TiO2-WO3 composite phase and the increas-
ing of the quantity of separately formed WO3 phase
(with increasing of WO3 (H2W3O12) in the working solu-
tion) surrounded by solitary TiO2 particles can be
another reason that will lead to an increase of the HIT and
EIT of the deposited by spray-pyrolysis layers.
5 CONCLUSION
In present work it was shown that the mechanical
properties of a deposited spray-pyrolysis composite’s
TiO2-WO3 layers strongly depend on the concentration
of WO3 (weight ratio between the two precursor solu-
tions (TiO2 colloidal solution and H2W3O12), respec-
tively) in the working solutions. For low concentrations
of the WO3 (up to 10 %) in the working solution, the
indentation hardness and modulus of the studied films
decrease, due to the increase of the porosity and size of
the building agglomerates. However, for higher concen-
trations of WO3 (more than 10 %), the increase of HIT
and EIT with the increase of the concentration of WO3
can be attributed to the decrease of the size of the
building agglomerates of the phase TiO2-WO3, as well as
to filling of the concavities and pores between the
separate WO3 agglomerates with small-size TiO2 grains
that flatten the surface of the composite layer. The ob-
served specific changes of HIT and EIT can also be
associated with the interaction between the TiO2 and the
WO3 in the TiO2-WO3 composites at the electron level.
Moreover, it was found that increasing the defects in the
TiO2-WO3 composite phase and increasing the quantity
of separately formed WO3 phases (with increasing of
WO3 (H2W3O12) in the working solution) surrounded by
solitary TiO2 particles can be another reason leading to
increasing of HIT and EIT of the spray-pyrolysis layers.
Acknowledgements
The authors gratefully acknowledge the financial
support by the National Science Fund of Bulgaria under
Projects: T 02-22 and DNTS/Germany 01/6; and Mini-
stry of Education and Science of the Republic of Serbia
– Project No. 172022.
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Materiali in tehnologije / Materials and technology 51 (2017) 1, 75–83 83
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS