Mechanical properties of ZrO2-Al2O3 nanostructured PVD coatings
evaluated by nanoindentation
A. Portinha1, V. Teixeira1*, J. Carneiro1, S. N. Dub2, R. Shmegera2
1
2
University of Minho - Physics Department, Campus de Azurém, 4800 Guimarães, Portugal
Institute for Superhard Materials, Department No.11, 2 Avtozavodskaya St., Kiev, 04074 Ukraine
*Corresponding author: fax: +351-253510401, tel.: +351-253510465/400, email: vasco@fisica.uminho.pt
Keywords: nanolayers, ceramic coatings, nanoindentation, mechanical properties
ABSTRACT
ZrO2-Al2O3 thin nanostructured coatings and stabilised ZrO2-Y2O3 coatings have been deposited
by magnetron sputtering in rotating mode with two metallic targets in order to stabilise at room
temperature the high temperature phases of zirconia.
The structural studies were carried out using X-Ray Diffraction (XRD) analysis and for coatings
with a content of 2wt% of Al2O3 the tetragonal phase of Zirconia can’t be stabilised. Scanning
Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were used to characterise the
coatings morphology and microstructure.
The mechanical behaviour of ZrO2-Al2O3 thin films under point loading conditions was studied
by nanoindentation using Berkovich indenter with 130 nm tip radius. Low load tests were used to
study the effect of the deposition parameters on the hardness and elastic modulus of coatings.
The hardness of the films reaches 15.3 GPa for a ZrO2-Al2O3 nanostructured coating sputtered
with -70V applied bias. High peak load tests were used to estimate the film adhesion to steel
substrates and for 120mN peak load the film showed goog adhesion properties.
The residual stresses were also determined using the Stoney equation and the highest
compressive residual stress found was about -3 GPa corresponding to the sample with highest
hardness.
1
1. INTRODUCTION
Zirconia is a technological material that has lots of applications such as, optical coatings [1],
hard and protective applications at high temperatures [2-4], thermal barrier coatings [4],
chemical sensors [5-6], makings Zirconia coatings a very interesting material for the next
generation coating for wear resistant applications. ZrO2 crystallizes in three distinct polymorphs
depending on the temperature and pressure conditions. ZrO2 monoclinic phase is stable at room
temperature, the tetragonal phase is stable between 1170 ºC to 2370 ºC and cubic phase stable for
2370 to 2680 ºC [2, 7-11]. When the applications for these coatings involve operations in a range
of temperatures that phases transformations occurs, it is necessary to stabilise the high
temperature phase at room temperature because this phase transformation is accompanied by a 3
to 5% volume expansion, which can produce high residual stresses and micro cracks leading to
delamination and spallation of coatings.
Phase stabilization is made by doping ZrO2 with other oxides such as Y2O3, CeO2, MgO, CaO
and Al2O3. [2,7-13].
Alumina has a higher young modulus than zirconia (390 GPa approximately twice than ZrO2 )
and in the stabilizing process of tetragonal phase of zirconia it is suggested to forms a rigid
matrix around the zirconia crystals, which causes a local compressive stress and hinders this
mechanism of the martensitic phase transformation [14], it can be in form of nanocomposite
coating where the zirconia nanocrystals were embedded in an amorphous alumina matrix or in a
nanolaminated structure which can improve the behaviour of
these coatings at higher
temperatures in which the diffusion barrier is important, but also can improve the corrosion and
wear protection [12,13,15-17].
2
Magnetron sputtering is a deposition technique that allows a good control of the mechanical
(hardness, elastic modulus, residual stresses, adhesion and so on) and physical properties,
crystallinity, and adhesion of the coatings, in order to improve the behaviour of coatings during
operation at room and/or higher temperatures [15-21].
The study of mechanical properties had an important development in the last decade due mainly
to the nanoindentation technique which combined with finite element studies can offer
capabilities to measure hardness and elastic modulus of nanocoatings with a few nanometers
thickness at room and high temperatures (400-500ºC) [22-29]. This technique is a method to
evaluate mechanical properties that gives real values without substrate material influence since
the maximum deep of indentations remains lower than 10-15 % of the coatings thickness[30].
In this work we gave especial attention to the micro structural and mechanical characterization of
ZrO2-Al2O3 thin nanostructured coatings and doped with Y2O3, through the XRD studies,
residual stresses and nanoindentation measurements.
2. EXPERIMENTAL
Preparation
ZrO2-Al2O3 nanostructured coatings and ZrO2-Al2O3-Y2O3 coatings have been deposited by
reactive magnetron sputtering in rotating mode at 4 rpm constant rotation speed with two
metallic targets in order to stabilize at room temperature the high temperature phases of zirconia.
For the zirconium target was used a DC power supply and for aluminum target was RF.
As substrates steel, glass and Inconel 738 LC (Ni-alloy) were used. The alloy substrates were
polished until 1 micron diamond suspension, and all substrates were ultrasonically cleaned
3
before the deposition process. Both targets and substrates were pre-cleaned in argon atmosphere
for 10 minutes. All coatings were deposited at constant target-substrate distance (60 mm). In the
table 1 are presented the main deposition parameters.
Characterization
All coatings were examined by Scanning Electron Microscopy (SEM) and Atomic Force
Microscopy AFM to characterise the coatings morphology and microstructure. By Energy
Dispersive X-Ray spectroscopy (EDX) was determined the coatings chemical composition.
Performing X-Ray diffraction (XRD) measurements with a powder X-ray diffractometer with Cu
Kα radiation in order to study phase composition, preferential crystal orientation and grain size.
The mechanical properties were evaluated by Nanoindentation with a Nano Indenter II (MTS
Systems, Oak Ridge, USA) using Berkovich indenter. 5 indents were performed for each sample
and for 3 diferent loads (5mN, 10mN and 30mN). For a qualitative evaluation of the adhesion
indentations at 120mN were performed.
Residual stresses were calculated using the Stoney equation after measuring the curvature radius
before and after deposition with a laser beam.
3. RESULTS AND DISCUSSION
3.1 - Coatings Microstructure
In this work are presented two different nanocomposite coatings of stabilised zirconia. (ZrO2)
nanocrystaline grains in an amorphous alumina matrix and a ZrO2 Y2O3 Al2O3 mixed oxides.
All coatings had a transparent aspect after deposition. Film thickness, evaluated by SEM, is
presented in table 1. The microstructure is similar in all films and it was observed a dense
4
columnar growth structure and a very smooth surface showing thin tiop columns. However, it
was also observed a few small hillocks , we think due to the loose of atomic mobility during
deposition, some surface defects on the substrate that is responsible for imperfect growth of some
columns, and due to the high quantities of material sputtered consequence of the high currents
used and some target surface oxidation that when it breaks allows high remotion of material from
target surface. The film semi quantitative composition was observed by EDX and is presented in
table 1.
Figure 1 exemplifies a typical tree-dimensional micrograph of the sample ZrAl7 (sputtered at
350 ºC and -70 V as negative bias) obtained by AFM showing the surface nanomorphology. This
thin size of top columnar growth, smooth and compact surface was characterized for the lower
surface roughness presented in the table 2.
It was observed a decrease in surface roughness with the increase of substrate deposition
temperature and also with increase the negative bias. This effect is due the higher atomic
mobility that improves the coalescence between grains and the higher structural densification
improved by the bombardment during film growth. ZrO2Y2O3Al2O3 coatings have higher
roughness values [31].
3.2 - Structural Characterization
ZrO2 is an interesting material and has many technological applications, however for high
temperature applications it is necessary suppress the tetragonal to monoclinic transformation and
get at room temperature the high temperature phases. This transformation can be suppressed
alloying zirconia with trivalent oxides such Y2O3, CeO2, Gd2O3 and Al2O3 or retain the
5
crystallite grain sizes under the critical size (30nm) [15,32]constrained on an amorphous matrix
like Al2O3.
With the aim to get the phase composition and other structural information the coatings have
been analyzed by XRD, figure 2 presents the XRD patterns for the ZrO 2Al2O3 nanostructured
coatings. All coatings crystallize in the tetragonal phase, however for the coating with 1.7 %Wt
Al2O3 the maim phase present is the monoclinic. Other coatings were produced and the limit
of %Wt Al2O3 concentration for stabilize the ZrO2 is between 2 and 5.
When the substrate deposition temperature increases from 100 ºC to 350 ºC significant changes
in the nanocrystaline structure orientation are observed as well for changes in the applied
negative bias. For the coating deposited at 100 ºC no preferential crystalline orientation was
observed, the ratio between intensities of the two main diffraction directions (111) and (200) are
≈ 1, but increasing the deposition temperature it is well observed a pronounced preferential
increase in the direction (200) and reduction of (111) direction. The ratio between the intensities
of these two diffraction peaks is presented in table 2. On the other hand the increase in negative
bias improves a preferential growth in the direction (111).
For the coatings doped with Y2O3 and Al2O3 no monoclinic phase were observed when
increasing the negative bias from -50 V to -100 V and there are no significant changes in the
structural crystalline orientation, that have preferential growth in the (200) direction. Only small
changes are seen in peaks intensities.
Using a Pseudo-Voigt function the main peaks were fitted and determined for each peak the
Gaussian and Lorentzian components that represent the micro strain and grain size
respectively [33]. The grain size was determined with the Scherer equation and table 2 has
shown the grain sizes for various coatings, an increase in deposition temperature increase the
6
grain size as expected due the better atomic mobility and coalescence during the film growth.
The increase in negative bias reduces the grain size. The crystallite size of
ZrO2Y2O3Al2O3
coatings was higher than in the ZrO2Al2O3 nanocomposite coatings for same deposition
conditions and reaches to 36 nm.
3.3 – Residual Stresses
Residual stresses σ were calculated using the curvature radius [34] (measured with a microdisplacement-laser transducer) technique based on the Stoney equation [35]:
 
 1
Ests2
1
   
6(l - s )t f  Ra Rb 
(1)
where Es/(l-νs) is the biaxial modulus of the substrate, t s and tf are the substrate and coating
thickness respectively; R a is curvature radius after deposition and R b is the curvature radius
before deposition. The residual stresses for the coatings are listed in the table 2 and for
coatings prepared at lower temperature are positive and for the others is negative indicating a
compressive state in the plan parallel to de surface. For ZrO2Al2O3 samples was observed an
increase in compressive residual stresses, however the sample prepared at 350ºC presents a
tensile stress that can be due the some relaxation after deposition. The highest compressive
stress (-3.4 GPa) were observed for the sample deposited with -70 V Bias and 350 ºC that
implies a more densification of the film structure due to some bombardment by the working
gas during deposition and this forms some ions of argon which can incorporate the crystalline
structure to improve residual stresses. ZrO2Al2O3Y2O3 coatings present also an increase in
compressive stresses for higher applied negative bias.
7
3.4. Hardness and Elastic Modulus Evaluated by Nanoindentation Measurements.
The indentation hardness is not a fundamental material property. Rather it depends on the way it
is obtained (testing technique, evaluation,…). Despite of this, the indentation hardness is widely
used as a parameter for characterizing the mechanical properties of materials, in particular of
metals, for several reasons: indentation hardness tests are relatively easy to perform and provide
a value that gives a direct measure for loading – bearing capacity of the investigated material.
Moreover, for completely plastic contacts the hardness of a material also gives an indirect
measure of its uniaxial tensile yield stress [36].
H  c y
(2)
where c, the constraint factor, depends on the ratio E  y of the material. For most metals, for
which E  y  100 , the constraint factor has a value of c  3 [36]. Another reason for the
widespread application of hardness tests is closely related with the definition of indentation
hardness as a local measure of the mechanical properties.
In this work the hardness and elastic modulus were calculated using Berkovich nanoidentation
measurements. The physical principles and models used to determine H and E from indentation
load – displacement data uses the Oliver – Pharr theory [22].
A schematic representation of indentation load (P) versus displacement (h) data obtained during
a one full cycle of loading and unloading is presented in figure 3. The important quantities are
the peak load (Pmax), the maximum depth (hmax), the final or residual depth after unloading (hf),
and the slope of the upper portion of the unloading curve ( S  dP dh ). The parameter S is known
as the elastic contact stiffness. The hardness and elastic modulus are derived from these
quantities. The continuous stiffness measurement technique is used to monitor the average
contact pressure (ACP) for loading segment. To monitor the average contact pressure in the
indent during reloading of the indenter, the procedure suggested in [23] was used.
The fundamental relations from wich H and E are determined are:
H
Pmax
A
(3)
where A is the projected contact area at that load, and:
8
Er 
 S
2
(4)
A
where Er is the reduced elastic modulus and account for the fact that elastic displacements occur
in both indenter and the sample. The elastic modulus of the test material, E, is calculated from Er
using:
1 1  2 1  i


Er
E
Ei
2
(5)
where  is the Poisson’s ratio for the test material, and Ei and  i are the elastic modulus and
Poisson’s ratio, respectively, of the indenter. For diamond, the elastic constants Ei = 1141 GPa
and  i  0.07 are often used [37]. Equation 2 is a working definition for the hardness. By this
definition, the hardness is a measure of the load – bearing capacity of the contact computed by
dividing the applied load by the projected area of contact under load. The most widely used
method for establish the contact area was developed by Oliver – Pharr theory [36]
In figure 4 are plotted values of hardness (H) and elastic modulus (E), extracted from the
loading-unloading curves of tests performed at 5 mN high load peak to assure no substrate
influence in the retrieved data .
These results show an increase in hardness values with deposition temperature and higher
applied bias. The highest value reaches to 15.3 GPa for the sample prepared at 350ºC and -70V
bias. Similar results were obtained for the elastic modulus. The value for elastic modulus is
almost constant for the samples prepared at 250ºC and 350ºC as well as sample prepared at -70V
bias. The sample doped with Y2O3 presents lower values for hardness and elastic modulus.
In table 3 are listed the mean values of H and E for tree different sets of tests performed at
different maximum loads (5, 10 and 30 mN). All samples have a similar behaviour, that is
characterized by the decrease in H and E values with increase in maximum peak load. This effect
9
is explained by the contribution of substrate because the measurements at higher loads have
promoted indentation deeps superior than 10-15 % of coatings thickness, in the case of 30 mN
these values reaches about 1/3 of the thickness. The increase in hardness and residual stresses
values with temperature can be due the higher densification of nanocoatings structure a, that is
evident for the sample ZrAl7. Hardness was also related with grain sise and surface roughness.
The hardness increases for lower grain size, it is according with other studies and is due the
increase in number of grain boundaries that increase the surface energy and reduce dislocations.
In the case of surface roughness it is well observed that hardness increases as roughness
decreases, and can be related because higher hardness implies worth surface compactness and the
top of columnar growth is freer and permit higher dislocations during indentation. The 15.3 GPa
as maximum hardness is in agreement with the relations presented before, because this sample
has the higher compressive residual stresses, lower grain size and lower roughness.
The adhesion was evaluated qualitatively performing nanoindentation measurements at higher
loads (120mN) and no delamination of the coatings from the substrate was observed.
4. CONCLUSION
With the aim to stabilize the tetragonal phase of zirconia we grew nanocomposite sputtered
coatings of zirconia and alumina, zirconia, yttria and alumina using metallic targets. For the
deposition of stabilized zirconia it is possible to control at an atomic level the addition of these
elements to the zirconia matrix, and thus providing an improved uniformization of the crystalline
phases presented in the coating.
10
All coatings have a dense columnar structure and are transparent. For all coatings the alumina
stay in amorphous phase and only the zirconia presents a crystalline phases with predominance
for the (200) tetragonal direction. The ZrO2Al2O3 coatings with a content of 2wt% of Al2O3
present as predominant phase the monoclinic.
Increasing the deposition temperature it’s observed an increase in the grain size and a decrease
for higher negative bias.
The hardness and Elastic Modulus were determined by nanoindentation and it is observed an
increase in their values for higher deposition temperatures and higher negative bias. The
hardness of the films reaches 15.3 GPa for a ZrO2Al2O3 nanostructured coating deposited at bias
-70V and 350ºC. No delamination of the films from substrate was observed for 120 mN peak
load.
The compressive residual stresses of coatings increase with deposition temperature and with
negative bias.
The hardness increase with the compressive residual stresses, decrease in roughness values and
with decrease of grain size.
Acknowledgments
This work was partially financially supported by FCT- Fundação para a Ciência e TecnologiaFCT under the project POCTI/EME/39316/2001: “PVDCOAT-Composite and multilayered
protective coatings for efficient energy systems”. A. Portinha is gratefully for the Research Grant
supported by F.C.T.- Portuguese Foundation for Science and Technology.
11
List Of Tables
Table 1: Sputtering deposition parameters for ZrO2Al2O3 and ZrO2Y2O3Al2O3 coatings.
Thickness and %Wt of Al2O3 and Y2O3.
PARAMETERS
Current (A)
RF Power for
aluminium (w)
Base pressure (Pa)
Substrate
temperature (ºC)
% of O2 Flux
Work Pressure (Pa)
Bias voltage (V)
Deposition time (s)
Thickness (nm)
%Wt of Al2O3
%Wt of Y2O3
Sample Codes
ZrAl7 ZrAl8 Zr8YAl1
2.86
ZrAl2
ZrAl4
ZrAl6
Zr8YAl2
Zr8YAl3
600
250
600
600
600
600
600
600
>10-4
10-4
6*10-5
>10-4
7*10-5
2*10-4
2*10-4
2*10
250
250
350
350
100
250
250
250
-70
36000
1200
14.6
4.1
-100
36000
1420
14.6
4.1
-4
20
6*10-1
28800
1200
9.1
-
-50
34200
1540
1.7
-
28800
1350
5
-
-70
14400
760
9.1
-
21600
1280
9.1
-
-50
36000
1350
14.6
4.1
12
Table 2: Surface roughness, grain size, I(200)/I(111) ratio and residual stresses.
Sample Codes
Ra (nm)
Rms (nm)
Grain size (nm)
I(200)/I(111)
Residual Stresses σ (GPa)
ZrAl2
5.81
7.4
24.7
3.4
-0.89
ZrAl6
7.56
9.46
28.1
8.0
0.639
ZrAl7
4.51
5.69
24.6
0.2
-3.401
ZrAl8
7.52
9.32
23.9
0.9
0.584
Zr8YAl1
13.69
17.22
35.8
7.5
-0.149
Zr8YAl2
25.2
9.0
-0.254
Zr8YAl3
25.2
7.4
-0.5
13
Table 3: Hardness and elastic modulus for various indentation loads.
Test Load (mN)
ZrAl2
ZrAl6
ZrAl7
ZrAl8
Zr8YAl1
Properties (GPa)
Hardness (H)
Elastic modulus (E)
5
10
30
5
10
30
11.7 9.65 6.83
196
182
162
12.1 11.8
8.6
191
190
179
15.3 12.6
6.4
194
185
157
10.6 10.7
6.0
174
178
147
9.19 8.15 7.16
173
168
165
14
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15
List of Figures
Ra = 4.51 nm
Rms = 5.69 nm
Small hillock
Figure 1: Tree-dimensional AFM micrograph of a ZrAl7 sample (sputtered at 350ºC and bias
-70V) illustrating the surface morphology.
16
60000
DT = 350 ºC (Bias = -70V)
DT = 100 ºC
DT = 250 ºC
DT = 350 ºC
Substrate
% Al2O3 = 2 wt%
Intensity [a.u.]
50000
Substrate
t(220)
40000
t(111)
Substrate
t(400)
t(131)
t(200)
30000
20000
10000
m(200)
t(200)
m(110)
m(220) t(220)
m(310)
0
20
25
30
35
40
45
50
55
60
65
70
75
80
2 [º]
Figure 2: XRD patterns for the ZrO2Al2O3 nanostructured coatings.
17
P10max
Y Axis Title
8
Loading
6
Unloading
4
S
2
0
0
hf
2
X Axis Title
hmax
4
Displacement, h
Figure 3: Schematic representation of indentation load-displacement data during one complete
cycle of loading and unloading
18
200
195
16
H
E
Bias: -50V
Bias -70V
15
14
13
185
12
H [GPa]
E [GPa]
190
180
11
ZrO2Al2O3 doped
with Y2O3
(Sample Zr8YAl1)
175
170
10
9
100
150
200
250
300
350
Temperature [ºC]
Figure 4: Hardness and Elastic Modulus values function of deposition parameters for tests
performed at 5mN maximum load.
19