PERGAMON
Micron 32 (2001) 807±815
www.elsevier.com/locate/micron
Study of the interfacial structure and chemistry of CVD k-Al2O3/TiC
multilayer coatings
M. Halvarsson a,*, A. Larsson a, S. Ruppi b
a
Department of Experimental Physics, Chalmers University of Technology and GoÈteborg University, SE-412 96 GoÈteborg, Sweden
b
Research and Development, Seco Tools AB, SE-737 82 Fagersta, Sweden
Abstract
This article describes the interfacial regions in CVD grown TiC/k-Al2O3 multilayers. A number of microanalytical techniques were used
including HREM, EDX and EELS. Occasionally, the ®rst 50 nm of the alumina layers deposited on the intermediate TiC layers grew as a
cubic alumina, heavily faulted, containing small amounts of sulphur (S), maybe as a stabiliser. The presence of slightly rounded TiC (111)
facets may act as preferred nucleation sites for the cubic Al2O3 phase, with a `cube on cube' orientation relationship. In this way the
nucleation of k-Al2O3 is less favourable. After some tens of nanometres the cubic phase cannot be stabilised any longer and the layer
continues to grow as k-Al2O3. A number of observations point towards the reaction zone (RZ) being h- and/or g-Al2O3. The diffraction work
Ê , which matches with h- and g-Al2O3. The EELS
and the FFT analysis of the HREM images show that the RZ is an fcc phase with a ˆ 7.9 A
Al ®ne structure indicate more tetrahedral Al ions than in k-Al2O3, as in h- and g-Al2O3. The RZ contains small amounts of S, as has been
reported for g-Al2O3. Due to the structural similarities between h- and g-Al2O3 it was not possible to determine which of these cubic phases is
present in the RZ. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: HREM; CVD; Interface; k-Al2O3; g-Al2O3; h-Al2O3; TiC; Multilayer; Reaction zone
1. Introduction
Chemical vapour deposition (CVD) is widely used to
produce wear-resistant coatings on cemented carbide
cutting tools (Lux et al., 1986; Chat®eld et al., 1989;
Ruppi and Halvarsson, 1999). Typical coating materials
are TiC, TiN, Ti(C,N), k-Al2O3 and a-Al2O3. They are
often used in combination to produce multilayer coatings (Vuorinen and Skogsmo, 1990; Halvarsson et al.,
1993).
A recent study (Berne et al., 1999) focused on the thermal
stability of TiC/k-Al2O3 and TiN/k-Al2O3 multilayer coatings. k-Al2O3 is a metastable phase and will transform to aAl2O3 at about 10008C. This transformation is associated
with a volume decrease and may cause the coating to
crack. It was found that the two multilayers behaved quite
differently despite TiC and TiN having the same type of
crystal structure (sodium chloride structure) and approximately the same lattice parameters. The TiN/k-Al2O3 multilayer transformed about twice as fast as TiC/k-Al2O3 in the
temperature range 1030±10908C. It was believed that the
difference in transformation rate could be explained by a
* Corresponding author.
difference in microstructure. Therefore, a detailed transmission electron microscopy (TEM) investigation was carried
out (Trancik et al., to be published).
It was found that the microstructure of the two different
multilayers was quite similar; columnar, twinned grains of
k-Al2O3 with the c-axis parallel to the growth direction and
orientation relationships between TiX (X ˆ C or N) and kAl2O3 were often found, re¯ecting growth of close-packed
planes of one phase on close-packed planes of the other. The
difference in microstructure was found in the interfacial
regions. Porosity was often found in TiN/k-Al2O3 interfaces,
especially where k-Al2O3 had grown on TiN. So the difference in transformation rate could be explained by a difference in free surface (pores) along the TiX/k-Al2O3
interfaces.
However, it was also found that occasionally a `reaction zone' (RZ) had formed on top of the TiC layers.
Thus, the difference in transformation rate could also
depend on the nature of this RZ. The aim of this article
is to describe in detail the interfacial region, including
the reaction zone, in TiC//k-Al2O3 multilayers. The
compositional variations, crystal structures and growth
behaviour will be treated by a number of microanalytical
techniques.
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M. Halvarsson et al. / Micron 32 (2001) 807±815
Fig. 1. TEM micrograph of the multilayer coating. The k-Al2O3 layers are approximately 1 mm thick separated by thin intermediate TiC layers. The growth
direction is to the left in this picture.
2. Materials and methods
2.1. Chemical vapour deposition
In order to investigate the TiC/k-Al2O3 interfaces a multilayer was deposited. The multilayer was several layers
thick, up to eight k-Al2O3 and seven TiC layers. The thicknesses of the individual k-Al2O3 and TiC layers were about
1 mm and 20 nm, respectively.
The experimental coatings of k-Al2O3 were deposted
from the AlCl3-CO2 ±H2 system in a computer-controlled
hot-wall CVD reactor (Ruppi, 1992). The intermediate
TiC layers were deposited from the system TiCl4 ±CH4 ±
H2. AlCl3 was generated within the deposition system
through the chlorination of aluminium with HCl. This was
carried out in a special AlCl3 generator where HCl was
allowed to pass over aluminium chips. The inlet gas composition was AlCl3 (3%), HCl (1%), CO2 (6%), H2S (,1%)
and H2 (balance).
The TiC/k±Al2O3 multilayers were deposited on Ti(C,N)
coated cemented carbide inserts (SNUN 120412). The
cemented carbide substrate was composed of 85.5 wt%
WC and 5.5 wt% Co, the balance being cubic carbides.
2.2. Electron microscopy
Cross-section thin foils for the TEM investigation were
prepared by a technique described elsewhere (Vuorinen and
Skogsmo, 1988, 1990). It involves cutting, grinding and
polishing, followed by dimpling a hole very close to the
coating substrate interface. Ion beam milling slowly
expands the hole towards the coating. The process is
stopped when the hole is next to the coating. The coating
and the upper part of the substrate will then be electron
transparent.
A Philips CM200 FEG transmission electron microscope
was used for the analyses. Energy dispersive X-ray analysis
(EDX) was carried out using a Link Isis system equipped
with special software for controlling the beam during line
scan acquisition. A Gatan image ®lter (GIF) was used for
the electron energy loss spectroscopy (EELS) investigation.
The spectra were acquired in the diffraction mode.
3. Results and discussion
An overview of the multilayer coating can be seen in the
TEM micrograph in Fig. 1. The coating growth direction is
to the left in the ®gure. The 1 mm thick k-Al2O3 layers are
separated by the thin imtermediate TiC layers. Columns of
twinned k-Al2O3 grains often extend from one TiC layer to
the next. Pores can occasionally be seen in the TiC/k-Al2O3
interfaces, below the TiC layer, never above. Generally the
k-Al2O3 grows directly on the underlying TiC layer, in some
regions it grows on a thin `reaction zone' (RZ), which has
formed on the TiC layer. The RZ is typically 10±30 nm high
and some hundred nanometres wide. The RZ is always
formed in regions where epitaxial columns of grains (´ ´´kAl2O3/TiC/k-Al2O3/TiC´ ´ ´) have grown, see below for
details about the orientation. However, in some epitaxial
M. Halvarsson et al. / Micron 32 (2001) 807±815
809
Fig. 2. HREM image of the k-Al2O3/TiC/RZ/k-Al2O3 layers.
regions no RZ has formed. The nature of the RZ will now be
studied in more detail.
Figs. 2±4 are high-resolution electron micrographs from
an area consisting of two k-Al2O3 layers separated by one
TiC layer and an RZ. Fig. 2 is an overview, while Fig. 3 is
taken from the TiC layer and Fig. 4 from the RZ. In order to
Fig. 3. HREM image of two twin related TiC domains. The twin boundary
can be seen from the bottom left corner to the top right corner.
investigate the crystal structure of the layers, a sequence of
fast Fourier transforms (FFTs) were acquired (Figs. 5±10)
along a line across the interfaces in Fig. 2. The FFT from the
k-Al2O3 layer is shown in Fig. 5 and re¯ects the orthorhombic structure (Liu and Skogsmo, 1991; Ollivier et al., 1997;
Yourdshahyan et al., 1999) of k-Al2O3. In this part of the
coating the k-Al2O3 layer consists of one crystalline grain
which has grown along the c-axis. The crystal is viewed
along a close-packed direction of oxygen ions in the cplane. No twinned domains can be seen in this ®gure.
On top of the k-Al2O3 grain a TiC layer has been deposited. The TiC layer consists of many small domains, but they
are all of two types. This can be seen in the FFTs in Figs. 6±
8. The FFT from one of the domain types is shown in Fig. 6
and the FFT from the other in Fig. 7. The indexing of the
FFT in Fig. 7 is shown in Fig. 11. It was thus found that the
growth direction of the TiC grain is [1Å1Å1]. The FFT of many
domains is shown in Fig. 8, which is a superposition of fcc
crystals with two orientations, both having [1Å1Å1] as the
growth direction. Thus, the (1Å1Å1) planes of TiC have
grown on the (001) planes of k-Al2O3. It can be clearly
seen that the two TiC domains are related to each other by
a 1808 rotation along [1Å1Å1]. Thus, they correspond to
´´´ABCABC´´´ and ´´´ACBACB´´´ stacking of the closepacked {111} planes, respectively. This is the same situation
as that previously reported for CVD TiN on k-Al2O3
(Halvarsson and Vuorinen, 1997). It can also be seen in
Fig. 2 that although there exists an orientation relationship
between TiC and k-Al2O3, the interface between them is not
¯at, but slightly rounded.
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M. Halvarsson et al. / Micron 32 (2001) 807±815
Fig. 6. FFT from TiC domain 1. The upper (and lower) edges of the image
are parallel with the layer interfaces and the left (and right) edges are
parallel with the deposition direction.
Fig. 4. HREM image of several twin related RZ domains. The twin boundaries are parallel to the diagonal from the bottom left corner to the top right
corner.
In Fig. 2 it can be seen that the RZ has grown onto the TiC
layer. The RZ is heavily faulted and each single crystal
domain is only a few nanometres high. Laterally the
domains can extend more than 100 nm. An FFT from the
RZ is shown in Fig. 9, which closely resembles the FFT
from the TiC layer, indicating that the RZ also consists of
two fcc domains. Indeed, it was possible to obtain an FFT
(Fig. 10) from only one domain in the RZ and it closely
resembles the FFT from one of the TiC domains (Fig. 7).
The distance between the spots in the FFT from the RZ is
about half the distance for TiC. Therefore, the lattice parameter of the RZ should be approximately double that of TiC.
Fig. 5. FFT from k-Al2O3 layer. The upper (and lower) edges of the image
are parallel with the layer interfaces and the left (and right) edges are
parallel with the deposition direction.
Fig. 7. FFT from TiC domain 2. The upper (and lower) edges of the image
are parallel with the layer interfaces and the left (and right) edges are
parallel with the deposition direction.
Fig. 8. FFT from two TiC domains. The upper (and lower) edges of the
images are parallel with the layer interfaces and the left (and right) edges
are parallel with the deposition direction.
M. Halvarsson et al. / Micron 32 (2001) 807±815
811
Fig. 11. Indexes for one fcc domain viewed along [011], as in Figs. 7
and 10.
Fig. 9. FFT from two RZ domains. The upper (and lower) edges of the
images are parallel with the layer interfaces and the left (and right) edges
are parallel with the deposition direction.
TiC and the RZ is shown in Fig. 13. In addition, by using a
re¯ection from the RZ the dark ®eld image in Fig. 14 was
formed. Only the RZ is bright in the image as the other
phases did not contribute to the diffraction spot used. It
was thus possible to determine which spots belonged to
the RZ, although they were weak due to the relatively
small area of the RZ.
In order to try to determine which alumina phase the RZ
consists of, selected area electron diffraction (SAED) was
used. We know from the FFT analysis above that the RZ is
an fcc phase with about twice the lattice parameter of TiC,
but by using SAED, the lattice parameter can be determined
more accurately. Diffraction patterns were obtained from
the interfacial region and by measuring the distance
between the diffraction spots a relationship between
k-Al2O3 and the RZ was determined:
6 z …112†RZ ˆ …100†k
Fig. 10. FFT from RZ domain 1. The upper (and lower) edges of the image
are parallel with the layer interfaces and the left (and right) edges are
parallel with the deposition direction.
Thus, the RZ is an fcc phase consisting of two twins,
twinned on the [1Å1Å1] plane by 1808. Approximately 30 nm
from the TiC interface, the RZ starts to disappear, and the
structure grown changes to k-Al2O3. The rest of the layer
grows as k-Al2O3.
In order to determine the composition of the RZ, EDX
analyses were carried out. A typical line scan across the kAl2O3/TiC/RZ/k-Al2O3 layers is shown in Fig. 12. To the
left, the composition in the lower k-Al2O3 layer is shown. In
the middle there is a sharp peak for Ti, corresponding to the
TiC layer. To the right, in the RZ, we ®nd Al and O, but the
oxygen signal is slightly lower than in k-Al2O3. Note that
the minimum in the O signal is shifted about 5 nm to the
right of the minimum in the Al signal. Thus, the RZ is an
alumina phase, somewhat depleted in oxygen. On top of the
RZ (to the right in the ®gure), the k-Al2O3 layer exhibits no
depletion in oxygen.
A bright ®eld image of the k-Al2O3 layers separated by
6aRZ
p ˆ ak ˆ 4:84 A
6
aRZ ˆ 7:9 A
Thus, the lattice parameter for the fcc alumina RZ was
Ê . Two fcc alumina phases exist, gfound to be about 7.9 A
Al2O3 and h-Al2O3 (Lippens and Boer, 1964; Zhou and
Snyder, 1991), space group Fd3m, with lattice parameters
Ê . The two phases are very similar and using
a ˆ 7.91 A
SAED it is not possible to distinguish between the two
phases. g-Al2O3 is more frequently reported in the literature
as a coating material, indicating that this may be the correct
phase for the RZ. However, it cannot be ruled out that the
RZ is h-Al2O3. Therefore, we will write h/g-Al2O3 as the
RZ phase, indicating that both phases are possible.
It has been shown that g-Al2O3 can be produced by
CVD (Larsson and Ruppi, to be published), especially
when high doping levels of H2S are used. In these cases,
thick ( < 1 mm) layers of pure g-Al2O3 can be deposited. Small amounts of sulphur were found within the gAl2O3 layers, which may have a stabilising effect on gAl2O3. Therefore EDX spectra were acquired, both in
the RZ and in the k-Al2O3 only 10 nm from the RZ.
The results are presented in Figs. 15 and 16 for the RZ
and k-Al2O3, respectively. Even after long acquisition
times, no S can be found in k-Al2O3. For the RZ, on
the other hand, a clear sulphur peak can be seen. The
sulphur level was determined to be 0.2 ^ 0.1 at.%. Thus,
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M. Halvarsson et al. / Micron 32 (2001) 807±815
Fig. 12. EDX line scan across the k-Al2O3/TiC/RZ/k-Al2O3 layers. The lower k-Al2O3 layer is the left in the ®gure.
Fig. 13. Bright ®eld image of two k-Al2O3 layers separated by TiC and
the RZ.
Fig. 14. Dark ®eld TEM image using a re¯ection from the RZ.
M. Halvarsson et al. / Micron 32 (2001) 807±815
813
Fig. 15. EDX spectrum from the RZ.
at the onset of the alumina deposition onto the TiC
layer, the H2S level may possibly be high enough to
stabilise g-Al2O3.
In order to further investigate the possibility that the RZ
may be g-Al2O3, EELS analysis was carried out. Fig. 17
shows an EEL spectrum of the Al L edge, taken from the
RZ. There is a distinct difference in appearance of the onset
of the edge for the RZ as compared to that for k-Al2O3
(Larsson et al., to be published). For k-Al2O3 there are
two peaks with approximately the same height. For the
RZ, however, the ®rst (indicated (1) in Fig. 17) is much
smaller than the second peak (labelled (2)). This indicates
a higher occupation of Al ions in tetrahedral positions than
in k-Al2O3, which is the case for h- and g-Al2O3. Furthermore, the spectrum is quite similar to the spectrum from gAl2O3 (Hansen et al., 1994; Levin et al., 1998).
4. Concluding discussion
There are a number of observations pointing towards the
RZ being h- and/or g-Al2O3. The diffraction work and the
FFT analyses of the HREM images show that the RZ is an
Ê , which matches with h- and gfcc phase with a ˆ 7.9 A
Al2O3. The EDX analyses give the composition Al2O3,
slightly depleted in oxygen. The EELS Al ®ne structure
indicate more tetrahedral Al ions than in k-Al2O3, as in h-
and g-Al2O3. The RZ contains small amounts of S, as has
been reported for g-Al2O3.
The most important observation is, however, that the
whole alumina layers do not always grow as k-Al2O3.
Occasionally, the ®rst 50 nm regions grow as a cubic
alumina, heavily faulted, containing small amounts of
S, maybe as a stabiliser. The presence of slightly rounded
TiC (111) facets may act as preferred nucleation sites for
h- or g-Al2O3. k-Al2O3 grows epitaxially on (111) facets
of TiC, while g-Al2O3 has been reported to grow on all
TiC facets, `cube-on-cube'. Thus, the steps of the
rounded TiC layer should act as fast nucleation sites,
which in turn favour h- and possibly g-Al2O3. After
some tens of nanometres the cubic phase cannot be stabilised any longer and the layer continues to grow as kAl2O3. The presence of thin RZs in the multilayer could
affect the mechanical properties as well as the thermal
properties of the coating.
Acknowledgements
Financial support from the Swedish National Board for
Industrial and Technical Development (Nutek) and the
Swedish Research Council for Engineering Sciences
(TFR) is gratefully acknowledged.
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M. Halvarsson et al. / Micron 32 (2001) 807±815
Fig. 16. EDX spectrum from the k-Al2O3 region next to the RZ.
Fig. 17. EEL spectrum (Al L edge) from the RZ. The labels (1) and (2) refer to peaks at the onset of the edge. The spectrum was acquired in the diffraction
mode.
M. Halvarsson et al. / Micron 32 (2001) 807±815
References
Berne, C., Halvarsson, M., Larsson, A., Ruppi, S., 1999. The phase transformation of CVD kappa alumina multilayers separated by TiC or TiN.
Solid±Solid Phase Transformations '99, Kyoto, Japan. The Japan Institute of Metals, pp. 1321±1324.
Chat®eld, C., LindstroÈm, J.N., SjoÈstrand, M.E., 1989. Microstructure of
CVD Al2O3. J. Phys. C5, 377±387.
Halvarsson, M., Vuorinen, S., 1997. Microstructural investigation of CVD
TiN/k-Al2O3 coatings on cemented carbides. International Journal of
Refractory Metals and Hard Materials 15, 169±178.
Halvarsson, M., Vuorinen, S., NordeÂn, H., 1993. Microstructural investigation of CVD a-Al2O3/k-Al2O3 multilayer coatings . Surf. Coat. Technol.
61, 177±181.
Hansen, P., Brydson, R., McComb, D., Richardson, I., 1994. EELS ®ngerprint of Al-coordination in silicates. Microsc. Microanal. Microstruct.
5, 173±182.
Larsson, A., Ruppi, S. (to be published).
Larsson, A., Zackrisson, J., Ruberto, C., Halvarsson, M., Ruppi, S. (to be
published).
Levin, I., Berner, A., Scheu, C., Muelljans, H., Brandon, D., 1998. Electron
energy-loss near-edge structure of alumina polymorphs. Microchim.
Acta 15, 93±96.
Lippens, B., Boer, J.D., 1964. Study of the phase transformation during
calcination of aluminium hydroxides by selected area diffraction. Acta
Cryst. 17, 1312.
Liu, P., Skogsmo, J., 1991. Space-group determination and structure model
815
for k-Al2O3 by convergent-beam electron diffraction. Acta Cryst. B47,
425±433.
Lux, B., Colombier, C., Altena, H., Stjernberg, K., 1986. Preparation of
alumina coatings by chemical vapour deposition. Thin Solid Films 138,
49±64.
Ollivier, B., Retoux, R., Lacorre, P., Massiot, D., FeÂrey, G., 1997. Crystal
structure of k-alumina: an X-ray powder diffraction TEM and NMR
study. J. Mater. Chem. 7 (6), 1049±1056.
Ruppi, S., 1992. Multi-oxide coated carbide body and the method of producing the same. US Patent 5,137,774.
Ruppi, S., Halvarsson, M., 1999. TEM investigation of wear mechanisms
during metal machining. Thin Solid Films 353, 182±188.
Trancik, J., Halvarsson, M., Vuorinen, S. The microstructure of CVD kAl2O3 multilayers separated by thin intermediate TiN or TiC layers (to
be published).
Vuorinen, S., Skogsmo, J., 1988. Microstructural characterization
of wear resistant coatings. In: Sudarshan, T., Bhat, D. (Eds.).
Surface Modi®cation Technologies. TMS, Phoenix, AZ, pp.
143±168.
Vuorinen, S., Skogsmo, J., 1990. Characterization of a-Al2O3, k-Al2O3 and
a±k multioxide coatings on cemented carbide. Thin Solid Films 193±
194, 536±546.
Yourdshahyan, Y., Ruberto, C., et al., 1999. Theoretical structure determination of a complex material: k-Al2O3. J. Am. Ceram. Soc. 82 (6),
1365±1380.
Zhou, R., Snyder, R., 1991. Structures and transformation of the h, g, and u
transition aluminas. Acta Cryst. B47, 617±630.