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Sensors and Materials, Vol. 36, No. 8 (2024) 3407–3418
MYU Tokyo
S & M 3740

Mechanical Characteristics of Al2O3, TiCN, and TiAlN Hard
Coating Films Measured by Microcantilever Bending Test
Takahiro Namazu,1* Tomohide Ide,2 Tsuyoshi Yamasaki,3 and Takahito Tanibuchi4
1Kyoto University of Advanced Science, 18 Yamanouchigotanda-cho, Ukyo, Kyoto 615-8577, Japan
2Aichi Institute of Technology, 1247 Yachigusa, Yakusa, Toyota, Aichi 470-0392, Japan
3Kyocera Corporation, 45 Wadai, Tsukuba, Ibaraki 300-4247, Japan

4Kyocera Corporation, 1810 Taki-cho, Satsumasendai, Kagoshima 895-0292, Japan(Received May 23, 2024; accepted July 2, 2024)

Keywords: hard coating film, Al2O3, TiCN, TiAlN, PVD, CVD, bending test, strength, fracture toughness,internal stress

In this paper, we describe the mechanical characteristics of Al2O3 and TiCN thin films
formed by chemical vapor deposition (CVD) and a TiAlN thin film formed by physical vapor
deposition (PVD), which are used as hard coating films. A focused ion beam (FIB) was used tofabricate a microcantilever beam for strength tests and fracture toughness tests under bending. A
nanoindentation system was used to apply normal force to cantilever beams. All the films
showed a linear force–deflection relationship, indicating brittle fracture during elastic
deformation. The Young’s modulus ranged from 340 to 379 GPa, with no significant difference
among the films, whereas the fracture strength of the Al2O3 films was 5.5 GPa, which was
almost twice those of the other two films. For the fracture toughness test, a precrack withvarious depths was made using FIB at 2 μm from the fixed end of the cantilever beam. The
Al2O3 film toughness values ranged from 4.1 to 5.5 MPa√m, which were almost twice that of the
TiCN film at each precrack depth. Both CVD films showed a similar trend, i.e., an increase in
fracture toughness with decreasing precrack depth, However, the fracture toughness of the
TiAlN film showed no precrack depth dependence. The difference in fracture toughness
between the CVD and PVD films is discussed on the basis of fracture surface observation
results and information on the crystal grain size and orientation.

1.

Introduction

Hard coating films such as Al2O3, TiCN, and TiAlN films play an important role inpreventing the chipping and wearing of cutting tools, leading to the improvement of their longterm durability and reliability.(1–5) Improving the adhesion of a coating film to a tool is one of thesignificant concerns to overcome delamination. Also, investigating the mechanical properties
such as Young’s modulus, fracture strength, and fracture toughness is significant for realizing
the high performance and reliability of a film-coated cutting tool. For thin films, many
*Corresponding author: e-mail: namazu.takahiro@kuas.ac.jp

https://doi.org/10.18494/SAM5152

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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researchers have developed their original tensile mechanical testing technologies.(6–15) Shaping
and chucking a film sample are considered the technical challenges for thin-film tensile testing.
Tsuchiya et al. proposed an electrostatic chucking technique, which is effective mainly forsemiconductor solid films because the sample is produced using microfabrication
technologies.(6) Sato et al. developed an on-chip tensile test chip, which includes a thin-film
sample, torsion beams, and loading levers.(7) The technique is advantageous because nochucking process is necessary, and it can be applied to single-crystal Si and Si-related materials
because those mechanical components are batch-processed. We developed a mechanicalchucking technique with a pin-hole combination, which can be applied to various types of thinfilm material if a sample, consisting of a thin-film section, chucking holes, Si springs forsupporting a movable part, and frame, can be prepared using microfabrication technologies.(8–15)
Although those techniques are sophisticated, it is difficult to use them for evaluating hardcoating films because sample preparation is technically difficult owing to their hardness andchemical inertness.
The focused ion beam (FIB) is a powerful tool for directly fabricating materials to a 3D shapewithout photolithography and etching. It works well for making a mechanical test piece with awire shape even if the material is difficult to process.(16–20) The purpose of this study is toestablish a strength and fracture toughness evaluation method for hard-coating thin-film
materials and to compare the mechanical characteristics among Al2O3, TiCN, and TiAlN films.
FIB was utilized to prepare a microscale cantilever beam made of those film materials. A
microcantilever bending test was performed using a nanoindentation system.

2.

Experimental Procedure

Figure 1 shows photographs of Al2O3, TiCN, and TiAlN films deposited on a machine tool
along with their surface photographs. WC-Co was used as the deposition substrate, which istypically used for a machine tool. The one-side length in-plane and thickness of the substrate

Fig. 1.

(Color online) Surface observation photographs of Al2O3, TiCN, and TiAlN films.

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were around 13 mm and 4.5 mm, respectively. The reason why the shapes of the substrates
differed from each other is that the machines used for cutting were different. For the Al2O3 film
sample, first, a TiCN film was deposited onto the WC-Co substrate by chemical vapor deposition(CVD). The TiCN film worked as an adhesive layer for depositing the Al2O3 film and its
thickness was around 10 μm. Then, an Al2O3 film of 4 μm thickness was deposited using 0.35
vol.% H2S and the remaining H2. The substrate temperature and pressure during the CVD were
set to be 1020 °C and 9.0 kPa, respectively, with mixed gases of 2.0 vol.% AlCl3, 4.5 vol.% CO2,
and 2.2 vol.% HCl. The film deposition rate was 1.0 μm/h. For the TiCN film sample, a 5-μmthick TiCN film was deposited onto the WC-Co substrate by CVD using mixed gases of 23
vol.% N2, 1.3 vol.% TiCl4, and 0.8 vol.% CH3CN, and the remaining H2. The substratetemperature and pressure were set to 820 °C and 7.5 kPa, respectively, and the film deposition
rate was 0.9 μm/h. For the TiAlN film samples, the arc-ion plating technique, which is a type ofphysical vapor deposition (PVD), was employed to deposit the TiAlN film directly onto the WCCo substrate. The deposition was conducted using a TiAl alloy target in N2 gas at a pressure of
3–5 Pa. The bias voltage and arc current were set to be 50 V and 150 A, respectively. Under these
conditions, the film deposition rate was maintained at 1.0 μm/h. The film thickness was around
3 μm. In the magnified photographs, many dark dots can be seen on the surfaces of all the films.
Those are considered as some type of defect (e.g., droplets) formed during the deposition
process.
Figure 2 shows a cross-sectional view of the Al2O3 sample as a process chart for a
microcantilever beam. A commercial FIB system (SII Nano Technology, SMI3050) was used.
First, the Al2O3 film surface was polished mechanically because the as-deposited film surface
was slightly rough for fabricating a microcantilever beam using FIB. Second, the cantilever
beam was processed using FIB. Third, the in-plane cantilever beam shape was roughly fabricatedat the acceleration voltage of 30 kV. The beam current and diameter were set to be 13 nA and2000 nm, respectively. Then, the vacancy beneath the cantilever beam was made under the same
conditions as the last process, so that the thickness of the cantilever beam was 2.5 μm from the
top surface of the evaluating film. After that, the sidewall and bottom surfaces were finely
formed and made as smooth as possible using FIB under the conditions of 30 kV, 700 pA, and 60
nm for acceleration voltage, beam current, and beam diameter, respectively. In the case of thesamples for the fracture toughness test, a precrack was introduced on the cantilever beam
surface under the same above-mentioned conditions. In the scanning electron microscopy (SEM)
images shown in Fig. 3(a), it was found that all the cantilever beams were finely formed using
FIB as bending test samples. The width, length, and thickness were 10, 20, and 2.5 μm,
respectively, which were precisely controlled using FIB. In the fracture toughness test samples
shown in Fig. 3(b), it was found that a precrack was finely made using FIB in the vicinity of the
fixed end of each cantilever beam. The precrack of 2 μm was processed from the fixed end. The
depth varied from 0.15 to 1.0 μm to investigate the fracture toughness values place by place inthe thickness direction. Concave portions, which were defects, were found on the top surface ofeach film. All the cantilever beam structures were processed to avoid those defects as much as
possible because they were considered to be stress concentration spots during bending.
For strength and fracture toughness tests under bending, a commercial nanoindentation

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Fig. 2.

Fig. 3.

(Color online) Schematic of process flow for fabricating a microcantilever sample for bending test.

SEM images of processed microcantilever samples for bending and fracture toughness tests.

system (Elionix, ENT-1100a) with a Berkovich indenter tip was used. The indenter tip was
penetrated at 15 μm from the fixed end of each cantilever beam. Bending loading was performed
at a constant loading rate of 10 nm/s. In the cantilever bending test, the relationship between
bending force (F) and deflection (δ) is given by
F = (wt3E)δ/4l3,(1)where w, t, and l are the width, thickness, and length of the cantilever beam, respectively, and E
is Young’s modulus. Here, l indicates the distance between the fixed end and the loading point (=15 μm). The maximum tensile stress (σmax) is generated on the top surface of the fixed end,
which can be expressed as
σmax = M/Z = 6Fl/wt2,(2)

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where M and Z are the bending moment and section modulus, respectively. The fracture
toughness (K IC) in the cantilever beam with a notch can be expressed as an empirical equation:
K IC = Fmaxlα/wt3/2,(3)

α = 1.46 + 24.36d/t – 47.21(d/t)2 + 75.18(d/t)3.(21)(4)

All the tests were conducted at 25 ℃ in ambient air.

3.

Results and Discussion

Figure 4 shows a representative load–displacement relationship of microcantilever beams
made of Al2O3, TiCN, and TiAlN films evaluated by the bending test. The displacement isindicative of the deflection at the loading point. It was found that the three microcantilever
beams show a linear load–displacement relationship, indicating that all the materials fractured
in a brittle manner during elastic deformation. In the case that the cantilever beam is thin andlong, an indenter tip is known to slide before fracture owing to a large deflection. It provides anonlinear load–displacement relationship. In this study, only linear relationships were obtained,
which is evidence that the bending tests were correctly carried out. The microcantilever beams
made of TiCN and TiAlN fractured at around 2 mN, whereas that made of Al2O3 fractured atover 5 mN, twice those of the other two materials. Five cantilever beams for each film were
subjected to the bending test to evaluate their mechanical properties. The Young’s moduli
obtained in this study were 342 ± 51, 377 ± 42, and 367 ± 57 GPa for the Al2O3, TiCN, andFig. 4.

(Color online) Representative load–displacement relationship obtained in microcantilever bending test.

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TiAlN microcantilevers, respectively, which are almost comparable to their bulk values. The
fracture strength of Al2O3 was 5.4 ± 0.8 GPa on average, which was almost twice those of TiCN
and TiAlN (2.6 ± 0.5 and 2.9 ± 0.3 GPa, respectively). The reason why only Al2O3 showed highstrength is that Al2O3 has higher purity and fewer defects than the others. TiCN is known as ahigh-purity hard coating film as well, but some defects are possibly included in the film. The
TiAlN film, which is the sole PVD film in this study, is considered to possess droplets of arounda few μm diameter, which would have affected the stress concentration site during bending.
Figure 5 shows representative fracture surfaces observed by SEM. The Al2O3 sample showed
an irregular fracture surface. It was found that a river-like pattern from the top surface to thebottom can be seen, indicating that a crack initiated on the top surface and propagated to thebottom, which is typically observed in a brittle fracture. No intrinsic defects such as voids orsurface notches were detected. The fracture surfaces of the other two samples showed a different
trend from that of the Al2O3 sample. The fracture surface of the TiCN sample was found to
consist of columnar structures. The diameter of each column was roughly estimated to be 100
nm, which was consistent throughout the film. Although the fracture origin and propagation
direction were unclear because of its strong columnar structure, it was found that the cantilever
beam showed brittle fracture. The TiAlN cantilever beam showed a columnar fracture surface
similar to that of the TiCN beam, but the shape of each column was not as sharp as that of the
TiCN film. This implies that the TiAlN film was denser than the TiCN film. Typically, theinterface between columns is known to work as a stress concentration site, so thin films withclearer columns possess lower fracture strength. From the fracture surface comparison, it was
found that the Al2O3 film was the densest among the three films, which would have provided
high fracture strength that is almost twice those of the TiCN and TiAlN films.
Figure 6 shows a representative load–displacement relationship of microcantilever beams
with a precrack. In those cases, the ratios of precrack depth to cantilever thickness were 0.4,
0.44, and 0.4 for the Al2O3, TiCN, and TiAlN cantilever beams, respectively. All the load–
displacement relationships were found to be linear until fracture. All the samples fractured at the
precrack portion. The slopes of the TiCN and TiAlN samples looked almost the same, whereasthat of the Al2O3 sample was found to be smaller than those of the other two samples. This might
be caused by the slightly smaller thickness of the Al2O3 sample than of the other two, leading to

Fig. 5.

SEM images of fracture surface after microcantilever bending test.

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Fig. 6.

3413(Color online) Representative load–displacement relationship obtained in fracture toughness test.

its higher flexibility. The TiAlN sample was found to show better mechanical resistance than the
TiCN sample under the same precrack depth-to-thickness ratio. This indicates that the TiAlN
film was mechanically tougher than the TiCN film.
Figure 7 shows fracture toughness as a function of the precrack depth-to-thickness ratio (d/t)
of microcantilever beam samples. In the Al2O3 sample, the fracture toughness was measured to
be 4.1 MPa√m at the d/t of 0.4. With a decreasing d/t, the fracture toughness was found toincrease exponentially. At the d/t of 0.14, the fracture toughness took the maximum value of 5.5
MPa√m, which is 2.5–3.3 times higher than the literature value.(22) The difference between the
maximum and minimum fracture toughness values was 2.2 MPa√m, indicating that the fracture
toughness value differed from place to place in the thickness direction. The TiCN sample,
deposited by CVD as well, was found to show a trend similar to the Al2O3 sample, although themagnitude of the fracture toughness was low by around 2.5 MPa√m at each d/t. On the other
hand, the TiAlN sample, deposited by PVD, showed a different trend where the fracture
toughness value stayed constant at around 3.0 MPa√m throughout the d/t range tested here. Thatis, it can be said that the PVD film showed no thickness dependence on fracture toughness.
On the basis of the experimental results, the following two things should be discussed. The
first is about the reason why the Al2O3 film showed a higher fracture toughness than the TiAlN
film. Considering the difference in thermal expansion coefficient between the Al2O3 film andthe WC-Co substrate, tensile residual stress is probably included in the CVD film,(1) which istypically seen in hard coating films formed by CVD. In the case of TiCN, another CVD film
evaluated in this study, Fig. 8 clearly shows that multiple cracks were introduced in the TiCN
film in the bending fracture, although single cracking happened in the TiAlN film formed by
PVD. This is evidence that the CVD film having tensile residual stress was weak against
externally applied tensile stress, and brittle fracture happened easily. Actually, the TiCN film
showed lower fracture toughness than the TiAlN film, which is considered to possess

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Fig. 7.

(Color online) Fracture toughness as a function of notch depth-to-sample thickness ratio.

Fig. 8.

SEM images of cracking introduced after microcantilever bending test.

compressive residual stress after PVD. Note, however, that the Al2O3 film showed higher
fracture toughness than the other two films, although it had tensile residual stress. It might be
attributed to grain size. As shown in Fig. 9, cross-sectional electron backscatter diffraction(EBSD) analyses suggest that the crystal grains in the Al2O3 film were definitely larger thanthose in the others. That is, the in-plane diameter ranged from 1 to 2 μm, roughly twice those of
the others. In addition, the TiCN and TiAlN films possessed a columnar structure, whereas the
Al2O3 film possessed a grained and noncolumnar structure. This structural difference might
have provided high fracture toughness in the Al2O3 film. Moreover, the crystal orientation might
have influenced the fracture toughness, although it was not precisely controlled during thedeposition in this study.
The second one is about the thickness effect on fracture toughness seen in the two CVD
films. Although the thickness effect was not obtained in the PVD film, it was clearly seen in the
two CVD films. The thickness effect happened possibly because of the crystal grain size. In the
EBSD results shown in Fig. 9, it was found that the grain size in the CVD films was different

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Fig. 9.

3415(Color online) Cross-sectional EBSD image of Al2O3, TiCN, and TiAlN films.

from place to place in the thickness direction. Although the crystal grain in the vicinity of thesubstrate was smaller than 1 μm at least, it grew during the deposition process until it finallybecame 1 or 2 μm on the top surface. On the other hand, in the PVD film, the diameter of eachcrystal column looks almost constant throughout its thickness direction. In the CVD process, afilm is deposited in an epitaxial-like manner more than in the PVD process.(23) In epitaxial
growth, since the crystallinity of the deposited film follows that of a substrate, the crystal grain
size increases as the deposition proceeds. It means that the crystal grain in the vicinity of the top
surface of a film is definitely larger than that at the bottom.(1) This “grain size nonuniformity”
might have provided the “fracture toughness nonuniformity” in the out-of-plane direction.
However, this should not be concluded until the relationship between grain size and fracture
toughness has been experimentally found and analyzed.

4.

Conclusions

In this study, the mechanical properties of hard coating films were evaluated by amicrocantilever bending test. CVD was employed for forming Al2O3 and TiCN films, and PVD
was employed for forming a TiAlN film. The Young’s modulus ranged from 340 to 379 GPa,with no significant difference among the films, whereas the fracture strength of the Al2O3 film
was 5.5 GPa, almost twice those of the other two films. In the fracture toughness test, the two
CVD films showed thickness dependence on fracture toughness, whereas the PVD film showeda constant value irrespective of precrack depth. The Al2O3 film having tensile residual stresspossessed a maximum fracture toughness of 5.5 MPa√m, which was higher than that of the
TiAlN film having compressive residual stress. The reason was discussed considering the
crystal grain size and orientation.
Mechanical characterization of hard coating films is known to be difficult technically. In thisstudy, the microcantilevers made of Al2O3, TiCN, and TiAlN films were fabricated using FIB,

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and the bending test was conducted using a nanoindentation tester. The combination of the FIB
process and nanoindentation demonstrated that the mechanical characterization of hard coating
films could be carried out well, and we were able to discuss the differences in strength andfracture toughness among the three films.

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About the Authors
Takahiro Namazu received his B.S., M.S., and Ph.D. degrees in mechanicalengineering from Ritsumeikan University, Kusatsu, Japan, in 1997, 1999, and2002, respectively. From 2002 to 2006, he was an assistant professor at the
Department of Mechanical and Systems Engineering, Graduate School ofEngineering, University of Hyogo, Himeji, Japan. In 2007, he became an
associate professor at the university. In 2010, he joined the Precursory
Research for Embryonic Science and Technology (PRESTO) program,“Nanosystems and Emergent Functions” of the Japan Science and Technology
Agency (JST), as a researcher. In 2016, he became a professor at the
Department of Mechanical Engineering, Aichi Institute of Technology,
Toyota, Aichi, Japan. In 2020, he became a professor at the Faculty ofEngineering, Kyoto University of Advanced Science (KUAS). He is currently
engaged in studies on functional film materials, such as self-propagating
exothermic materials, and their applications to micro/nano-electro-mechanicalsystems (NMEMS). His research interests also include the development ofmaterial testing techniques for measuring the mechanical properties of micro/
nanoscale materials, such as carbon nanotubes and silicon nanowires, with afocus on clarifying the nanomaterials’ size effect phenomena. The evaluationof the reliability of MEMS and semiconductor devices is among his interests
towards realizing the design of ultralong-life microdevices. (namazu.
takahiro@kuas.ac.jp)
Tomohide Ide received his B.E. and M.E. degrees from Aichi Institute ofTechnology, Japan, in 2018 and 2020, respectively. His research interests are
the development of mechanical characterization techniques for hard-coating
thin-film materials and the measurement of the mechanical properties ofmaterials used for MEMS and semiconductor devices. He is also interested indeveloping functional materials, such as shape memory alloy films and selfpropagating exothermic films, by sputtering.
Tsuyoshi Yamasaki received his B.E. degree from Gifu University in 2009. In
2009, he joined Kyocera Corporation. From 2009 to 2021, he was mainly
engaged in the development of hard thin films at the Cutting Tool Materials
R&D Division of Kyocera Corporation. Since 2021, he has been working on
the crystal growth of nitride semiconductors in the R&D Division. His
research field is power devices.

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Takahito Tanibuchi received his B.S. and M.S. degrees from Ritsumeikan
University in 1999 and 2001, respectively. He joined Kyocera Corporation in
2001. He has been engaged in the development of materials for metal cutting
tools at the Cutting Tool Materials R&D Division of Kyocera Corporation
since 2001. His research interests are in hard coatings and ceramics withexcellent mechanical properties and heat resistance.