Investigation and Evaluation of Structural Color
of TiO2 Coating on Stainless Steel
J. H. Lee1*, Y. H. Jun2
1
Department of Materials Engineering, K University, Seoul, 136-791, Korea
2
Electronic Materials Laboratory, Korea Institute of Science and Technology, Seoul,
136-791, Korea
+
Corresponding author. Tel: +82-2-xxx-xxx; Fax: +82-2-xxx-xxx
E-mail address: jhlee@xxxx.ac.kr
Abstract
Titanium dioxide (TiO2) thin films were deposited on the stainless steel substrates by
RF magnetron sputtering. In order to prevent film delamination and cracking caused by
thermal mismatch between the stainless steel and TiO2 thin film, Ti thin film was
initially deposited on the stainless steel as buffer layer. The color of the multilayer film
indicating (L*, a*, b*)=(52.93, -40.69, 7.69) deposited for 209 nm thickness was
visually observed as dark green color with the maximum peak of 31.54 % spectral
reflectance at a wavelength of 510 nm, corresponding to the simulation results . It was
found that the simulated results are relatively well matched with the experimental ones.
Keywords : TiO2, Optical properties, Films, Reflectance
1. Introduction
Ceramic coatings are very attractive to protect metals from corrosion because they
possess good thermal and electrical properties, and also more resistant to oxidation,
corrosion, erosion and wear than metals in high temperature environments. Among
many ceramic materials, TiO2 is perhaps one of the most widely studied oxides
because it exhibits high chemical and mechanical stability [1].
Shen et al. [2] synthesized the 316 L stainless steel by uniform TiO2 nano-particulate
coatings and claimed that the TiO2 coatings exhibited very good corrosion resistance
by acting as a protective barrier on the steel surface. Additionally TiO2 coated surface
can show a wide range of functionality with a colorful appearance because of its high
refractive index and the optical transmittance in the visible range [3-5].
So far a number of experimental researches have been successfully conducted in the
area of the structural and optical properties of TiO2 films, influenced by deposition
conditions such as substrate temperature, working pressure, and oxygen partial
pressure [6,7]. However, not many works have been undertaken to estimate the optical
properties with simulation program in advance.
The overall objective of this study is to understand and evaluate the phenomenon of
coloring on TiO2 thin films. In order to estimate and compare with the experimental
results, the simulation program, the Essential Macleod Program (EMP) [8], was
adopted. For this purpose, TiO2 thin films were deposited on stainless steel substrates
by RF magnetron sputtering method and optical characteristics in terms of different
film thickness were systematically investigated.
2. Experimental
2.1 Films Preparation
The cut stainless steel sheets(304SS, 10 × 10 × 0.2 ㎣) were ultrasonically cleaned
with acetone, absolute ethyl alcohol, and de-ionized water for 30 minutes. Before
deposition of TiO2 thin film, Ti thin film was initially deposited on the stainless steel
substrates using DC magnetron sputtering method to reduce the thermal stress caused
by thermal expansion mismatch between the stainless steel and TiO2 film. It was
known that the thermal expansion coefficients of the stainless, Ti and TiO2 thin films
are 16.0×10-6 /℃, 8.6×10-6 /℃ and 7.14×10-6 /℃ respectively [9,10]. The film
thickness of Ti was 200 nm. Following that, TiO2 thin films were deposited on Ti thin
film using RF magnetron sputtering method.
All the films were deposited under the following parameters: 50 sccm in Ar flow
rate, 12 mTorr in working pressure, 100 Watt in RF sputtering power for TiO2 target
and 120 Watt in DC sputtering power for Ti target at room temperature. These
deposition conditions were remained constant while deposition time was varied. The
deposition rate of TiO2 thin film was 2.9 nm/min..
2.2 Simulation
For optical characterization, the Essential Macleod Program (EMP) was adopted and
the calculated results were compared with measured optical properties. EMP is a
comprehensive software package for the design, analysis, manufacture and trouble
shooting of thin film optical coatings [8,11]. The EMP simulation method was
processed through the following steps; First, construction parameters such as refractive
index and extinction coefficient of Ti and TiO2, which were calculated by using
ellipsometry measurement, were input. Second, simulation with variable parameters
such as wavelength ranges (380 - 780 nm) and number of layers [304SS|Ti|TiO2|Air]
was designed. Finally, analysis of each layer effect with a variable thickness of Ti and
TiO2 thin films from 10nm to 300nm respectively was performed for optical properties
whether it is appropriated for optimal simulation with system modification.
2.3 Characterizations
The film thicknesses were measured using a surface profiler meter (α-step,
TENCOR P-2). To examine the surface morphologies of TiO2 thin films Scanning
probe microscope (SPM, Nanoscope Ⅲa) was used. The refractive index n and the
extinction coefficient k were evaluated by ellipsometry measurement using
ellipsometer (Elli-SE) in the visible range from 380 to 780 nm with a step width of 5
nm. The color and lightness of the TiO2 thin films were estimated using a
spectrophotometer (CM-3600d) with a light source of D65 according to the 1931 norm
of the Commission International de l’Eclairage(CIE). The angle of incidence was 8°.
Color properties for the TiO2 thin film can be described using the L*a*b* scale: L*
attributes the lightness, a* axis points the red (positive) to green (negative) and b*
points the yellow (positive) to blue (negative). And white point is positioned on the
center (a*, b*) = (0, 0). The quantity of color is measured using the chromaticity
a *2  b*2
[12]. The spectral reflectance was calculated with the same apparatus.
3. Results and discussion
3.1 Simulation
EMP was adopted in order to analysis optical properties of [304SS|Ti|TiO2|air]
multi-layers in advance. To extract an accurate analysis from EMP simulation, the
refractive index and the extinction coefficient are necessary. Fig. 1 shows the refractive
index n and extinction coefficient k of each film as a function of the wavelength in the
visible range, which were obtained by the ellipsometry measurement. The measured
value of TiO2 thin film were n = 2.1053 and k = 0.0086, which were slightly different
from the theoretical value of n = 2.2789 and k = 0.0002 at a wavelength of 633 nm [8].
The difference between the refractive index and extinction coefficient in simulation
and experiment can be explained in terms of the oxygen vacancy and packing density.
TiO2 thin films have a known character that they are apt to lose oxygen in some
conditions and exhibit suboxides [13].
TiO2 → TiO2-x + O2↑
(1)
As oxygen gas was not flowed into the chamber during TiO2 deposition, TiO2 thin
films showed a bit of oxygen vacancy. It was known that a large number of vacancies
and voids can be incorporated into thin films during the deposition processes so that
the films are generally porous. The presence of excess vacancies and voids greatly
influences the physical properties [14-16].
Ti thin film showed the experimental value of n = 2.97118, k = 2.58534 and the
theoretical value of n = 2.93792, k = 3.59569. The difference between extinction
coefficients might be due to the change of the density in Ti thin film.
Fig. 2 shows the investigation on the trace of CIE 1931 chromaticity with different
film thickness of TiO2 thin films. As shown in Fig. 2, the value of CIE 1931
chromaticity diagram was sensitively changed from orange color (0.45, 0.37) for 40
nm thickness to dark-violet color (0.36, 0.23) for 170 nm thickness by increasing the
thickness of TiO2 thin film. On the other hand, the values of chromaticity x, y were
elliptically shifted (x = 0.34 - 0.48, y = 0.19 - 0.47) over 170 nm thickness of TiO2 thin
film and the colors were also changed rotationally. However, Ti thin film was not
shown any optical changes with a variable thickness in the simulation program.
Clearly to distinguish from other colors of the experimental result, different 4 colors
such as gold, purple, blue and green were selected. Fig. 3 represents the simulated total
hemispherical reflectance in terms of the thickness of TiO2 thin films. It is seen that the
reflectance was strongly influenced by the thickness of TiO2 thin film while general
parameters such as refractive index and packing density are remained constant. In
addition, the maximum peak intensities of TiO2 thin films show around 32 % spectral
reflectance in the visible range of 380 - 780 nm at normal incidence. The peak position
determines the color of the multilayer film. As shown in Fig. 3(d), green color visually
observed from simulation demonstrated the maximum peak of 31.90 % at a
wavelength of 510 nm, which is corresponded to green color that has wavelength range
of 500 ~ 570 nm in the visible spectrum. Moreover, blue color observed from
simulation program showed the maximum peak of 31.49 % at a wavelength of 470 nm,
which is good agreement with blue color pattern indicating wavelength range of 450 500 nm.
3.2 Optical Characteristics
The photographs of TiO2 thin films deposited on the stainless steel prepared by RF
magnetron sputtering displayed in Fig. 4. The TiO2 thin films exhibited different 4
colors such as (a) dark goldenrod, (b) medium orchid, (c) medium blue and (d) dark
green, which were fabricated with various deposition times for 120, 174, 189 and 209
nm respectively. The observed colors in Fig. 4 can be expressed with the maximum
reflectance curve in the visual wavelength range since the peak position determines the
color of the multilayer film. Fig. 5 represented the spectral reflectance with different
thickness of TiO2 thin films. From Fig. 5(d), the prominent peak intensity was
observed near the wavelength of 510 nm corresponding to green color.
However, the maximum reflectance of around 27 - 30 % across the entire
wavelength range in Fig. 5 seems slightly smaller than the simulated value showing
around 32 % spectral reflectance at normal incidence. Fig. 6 demonstrates gold and
green patterns for TiO2 thin film to compare the simulated spectral reflectance with the
experimental one. In Fig. 6(d), the maximum peak positioned at a wavelength of 510
nm was 31.54 %, corresponded well to the simulated value of 31.90 % respectively.
This result was well-matched as compared with the one shown in Fig. 7(d). Another
feature seen in Fig. 6(a), gold color pattern slightly decreased about 3.26 % at the
maximum peak.
L*a*b* values for TiO2 thin film are shown in Table 1. Table 1 indicated that the
simulated lightness values were higher than experimental ones. Generally it was
known that the color reproduction on display system is not equal to original images
[17]. Therefore, all of the experimental results were visually observed darker than
simulated one. TiO2 thin film with 209 nm thickness was visually observed as darkgreen while the film with 189 nm thickness was shown as dark goldenrod. Distribution
of chromaticity indices indicating the chromaticity value of 41.41 in Fig. 7(d) was
almost the same as the simulated value of 42.73 in Fig. 7(d´). Normally green color
pattern has wide wavelength range from 500 to 570 nm on the CIE 1931 chromaticity
so that human eyes are more sensitive to green than other colors [12]. For this reason,
the experimental green color which has a* = -40.69 and b* = 7.69 was corresponded
well with simulated a* = -42.45 and b* = 4.91.
The golden color of the sample observed in the simulated images from the display
changed into dark-goldenrod in the measured L*a*b* scale. This difference also can be
explained by color gamut on the display and the sensitivity of the eye [18]. As seen in
Fig. 7(a), the experimentally measured chromaticity value of 36.78 is smaller than the
simulated value of 50.78. This is attributed to the fact that yellow color pattern has
narrow wavelength range of 570 - 590 nm so that its chromaticity could be deduced by
the influence of the lightness value [19].
4. Conclusions
The present study showed that optical simulations provide numerous important
implications on the performance for the experimental optical characteristics of TiO2
thin films such as the spectral reflectance, color and chromaticity. Different 4 colors
such as dark goldenrod, medium orchid, medium blue and dark green were fabricated
based on the simulation results. The maximum peak positioned at a wavelength of 510
nm was 31.54 % in the experimentally measured spectral reflectance, which is a good
agreement with the simulated value of 31.90 % for green color pattern corresponding
wavelength range from 500 - 570 nm. The maximum peak which determines the color
was positioned at a wavelength of 510 nm, corresponding to the experimental
chromaticity value of (a*, b*) = (-40.69, 7.69) and simulated value of (a*, b*) = (-42.45,
4.91). The optical properties with experimental result were relatively well matched
with simulated one. It can be concluded that the experimental optical properties of thin
film were extracted the required information from the simulation results in advance.
Acknowledgements
This research was supported by a grant from 'Center for Nanostructured Materials
Technology' under '21st Century Frontier R&D Programs' of the Ministry of Education,
Science and Technology, Korea.
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Table caption
Table 1
A comparison of color images between experimental and simulated TiO2 thin film
visually observed according to the L*a*b* scale.
Tables
Table 1
Simulation
Measurement
dark
Medium
Medium
Dark
goldenrod
orchid
blue
green
Color
Gold
Purple
Blue
Green
L*
77.99
48.23
51.90
69.15
50.28
35.98
42.02
52.93
a*
4.92
34.53
-27.14
-42.45
17.06
36.36
-24.75
-40.69
b*
50.54
-46.06
-35.40
4.91
32.58
-33.46
-23.76
7.69
Chromaticity
50.78
57.56
44.61
42.73
36.78
49.41
34.31
41.41
Figure captions
Fig. 1. (a) The refractive index n and (b) extinction coefficient k with TiO2 and Ti thin
film in the visible ranges using the ellipsometry
Fig. 2. The thickness effects of TiO2 thin film on CIE 1931 chromaticity diagram in the
Essential Macleod Program.
Fig. 3. Comparison of simulated spectral reflectance with different thickness of TiO2
thin films;
(a) 124nm, (b) 172nm, (c) 187nm and (d) 207nm
Fig. 4. Coloring of the TiO2 thin films with different deposition time prepared by RF
magnetron sputtering; (a) 120 nm, (b) 174 nm, (c) 189 nm and (d) 209 nm
Fig. 5. Wavelength distribution with different deposition time of experimental TiO2
thin films;
(a)120 nm, (b) 174 nm, (c) 189 nm and (d) 209 nm
Fig. 6. Comparison of spectral reflectance for TiO2 thin films; (a) experimental and (a´)
simulated gold color pattern and (d) experimental and (d´) simulated green color
pattern.
Fig. 7. Distribution of chromaticity indices, a* and b*, for experimental (a)-(d) and
simulated (a′)-(d′) in TiO2 thin films.
Figures
(a)
Refractive Index
3.5
(d)
(c)
3.0
2.5
(a)
2.0
(b)
1.5
400
500
600
700
Wavelength (nm)
(b)
Extinction Coefficient
4
(c)
3
(d)
2
1
(b)
(a)
0
-1
400
500
600
Wavelength (nm)
Fig. 1.
700
0.9
520
530
540
0.8
510
550
0.7
560
570
CIE y
0.6
0.5
500
580
130 140
590
120
110
150
600
100
90
130 140
80
620
40
120
70
650700
110
150
100 160
60
90
80
40
50 70 170
60
160
50
170
0.4
0.3
490
0.2
0.1
480
470
450 360
0.0
-0.1
0.0
0.2
0.4
0.6
0.8
CIE x
Fig. 2.
Reflectance (%)
60
40
(b') (c') (d')
(a')
20
0
400
500
600
Wavelength (nm)
Fig. 3.
700
(a)
(b)
(c)
(d)
Fig. 4.
Reflectance (%)
60
40
(b) (c)
(a)
(d)
20
0
400
500
600
Wavelength (nm)
Fig. 5.
700
40
Reflectance (%)
(a')
(d')
30
(a )
(d)
20
10
0
400
500
600
700
Wavelength (nm)
Fig. 6.
100
Yellow
(a')
(a)
b* (D65)
50
0
Green
(d)
(d')
(c)
(c')
-50
-100
-100
White
Red
(b)
(b')
Blue
-50
0
a* (D65)
Fig. 7.
50
100