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κ-Al2O3 multilayer coatings on cemented carbides
15 (1997) 169-178 © 1997ElsevierScienceLimited Printed in Great Britain. All rights reserved (1263-4368/97/$17.00
Int. J. of Refractory Metals & Hard Materials
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ELSEVIER
S0263-4368(96)00028-5
Microstructural Investigations of CVD TiN/ K-ALO3 Multilayer Coatings on Cemented Carbides M. Halvarsson Department of Physics, Chalmers University of Technology, S-412 96 G6teborg, Sweden
& S. Vuorinen Research and Development, Seco Tools AB, S-737 82 Fagersta, Sweden (Received 24 October 1995; accepted 24 April 1996) Multilayer coatings of TiN and K-AI~O, have been examined by x-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy. The general microstructure of the coatings, such as grain size, grain shape, twinning, dislocations, pores and interfaces was investigated and, based on these findings, conclusions about the detailed structure of K-AI203 were made. A model is presented for the domain formation in CVD K-AI20~. The height of the columnar, twinned K-grains was of the order of the K-layer thickness. The preferred growth direction for K-AI203 was along the c-axis. The grain morphology is very different for TiN deposited on to cemented carbide as compared to TiN deposited on to K-AI203; columnar grains with an aspect ratio of about 5 and equiaxed grains, respectively. No pores or dislocations could be found within the TiN and K-AI203layers. However, tc-AI203/TiN interfaces below the TiN-layers exhibit a large number of pores. The deposition rate of TiN was two-three times higher for TiN deposited on cemented carbide than for TiN deposited on to K-AI203. Epitaxy was frequently found both for TiN on K-AI20~ and for K-AI20~ on TiN. Three twin-related K-domains grew epitaxially on two twin-related TiN domains which grew epitaxially on three twin-related K-domains. The orientation relationships could be described as:
Abstract:
(001),, //
(Ill)TiN //
(001),,
1100],.,
[lI21,.,N,
[100l,.,
[1 101,,2 //
[lT2]-nNe//
[1 10],,2
[ho],,.,
0-1o1,,.~
The observed twinning of TiN on h--A120~ indicates that the (001)-planes in h--AI20~ are close-packed in an .. ABAC.. stacking sequence. There are two types of geometrical relationships of neighbouring domains in CVD K-A1203: (i) twinrelated domains due to different positions of the aluminium ions within the oxygen-ion sub lattice and; (ii) translation-related domains due to different stacking sequences of the oxygen-ion planes. © 1997 Elsevier Science Limited
1 INTRODUCTION
coatings are often p r o d u c e d by Chemical Vapour Deposition (CVD). A l u m i n a exhibits a n u m b e r of crystallographic modifications. H o w e v e r , only two are of interest for cutting tool applications, namely the
Alumina (A1203)1-7 and titanium nitride (TIN) 8-'3 are widely used as wear-resistant coatings on c e m e n t e d carbide cutting tools. T h e 169
170
M. Halvarsson, S. Vuorinen
stable c~ and the metastable x polymorph. At elevated temperatures, coatings of K-A1203 will transform to the stable ~ phase and this has been reported earlier. 14-'7 ~-AI:O, has a well established crystal structure, '"-2'' but the complete structure of t~-AI:O~ has not been determined. It has been shown 21"22with selected area electron diffraction (SAED) in transmission electron microscopy (TEM) that the Bravais lattice of t c - A l 2 0 3 is primitive orthorhombic and that CVD grown h'-grains consist of a large number of domains, related by a 120 ° rotation along the c-axis. This domain structure of K - A I 2 0 3 is probably the reason for the previous confusion in the literature about the crystal structure of K - A I 2 0 3 . The lattice parameters of tc-AI203 have been determined to be a=4.84 A, b=8.31 A and c=8.94~. 23 Based on the symmetry determination of K - A I e O 3 and the dimensions of the unit cell, it was proposed that h:-AI:O3 may consist of close-packed oxygen-ion planes with the aluminium ions in interstitial positions. 22 A close packed structure was also suggested by Okimiya et al. 24 based on density measurements. An atomic model for x-AI:O, has been suggested by Liu and Skogsmo :2 with an .. ABAC.. stacking of oxygen-ion planes with aluminium ions occupying both octahedral (~) and tetrahedral (fl') positions, giving an ..A~B~Afl'Cfl'.. stacking sequence along the c-axis. However, many atomic models are possible and the suggested model has not been verified by diffraction experiments. Titanium nitride has the NaCl-structure (FCC) and has the lattice parameter a=4.24 A, which, however, varies with deviations from stoichiometry. Previous investigations ~-'3 on CVD TiN have ranged from the relations between process parameters and the microstructure, and kinetics and thermodynamics, to different wear characteristics during machining. An icosahedral surface morphology of TiN has been reported,'" which suggests that the TiN crystals are highly twinned. Transmission electron microscopy studies have also revealed planar faults in CVD TiN, which were interpreted as twins?'~5 In this work, multilayer coatings of TiN and x-A1203 have been examined by analytical scanning and transmission electron microscopy. The aim was to describe the general microstructure of the coatings, such as grain size, grain shape,
twinning, dislocations, pores and interfacial structures. Based on these findings, conclusions about the detailed structure of ~c-AI20~ are made and a model is presented for two different types of domain in CVD ~:-A1203.
2 EXPERIMENTAL Chemical vapour deposition
Deposition of the experimental coatings was carried out in a computer-controlled hot-wall CVD reactor. The TiN coatings were deposited from the TiC14-N2-H2 system, the overall reaction being thus TiCI4 (g) +0.5 N 2 (g) +2H2 (g)--,TiN (s) + 4 HC1 (g) Deposition of TiN was carried out at a pressure of 600 mbar in order to suppress the formation of Ti(C,N) in the first TiN layer which was deposited directly on a cemented carbide substrate. The K-AI203 coatings were deposited using the gases AIC13, C O 2 and H2. ~° AIC13 was generated within the deposition system through the chlorination of AI with HCI. The A1203 coatings were deposited at a pressure of 50 mbar. The inlet gas compositions which were used to deposit TiN and K - A I 2 0 3 a r e given in Table 1. The experimental multilayer coating was composed of seven individual layers of either K-AI203 or TiN. The coating is shown in Fig. 1 and is described in detail below. Material characterisation
The coatings were examined by scanning electron microscopy (SEM) using a Camscan S48DV and by analytical transmission electron microscopy (TEM/EDX) using a JEOL 2000FX equipped with a LINK AN10.000 EDX system. Cross-section thin foils were prepared by a
Table 1. The inlet gas compositions (%) 142 TiN A1203
Balance Balance
TiCL
N2
C02
HCl
AICl,
8 --
55 --
-6
2 1
-2
Multilayer coatings of TiN and tc-A1203
171
Table 2. The deposition time, thickness and growth rate for the CVD layers
Layer
TiN on WC/Co TiN on K-AI20~ h-A1203 on TiN
Fig. 1. A cross-section SEM micrograph of the TiN/ K-A1203 multilayer coating and the cemented carbide substrate. The bright layers are TiN and the dark layers are /':-A1203.
method described elsewhere. 2"5 This method produces specimens with electron transparency in all layers and interfaces in the multilayer coating simultaneously. In addition, standard techniques for plane-view specimens were used. X-ray diffractometry (XRD) was carried out using a Philips PW1700 system.
3 RESULTS AND DISCUSSION General microstructure
The general mlcrostructure of the multilayer coating, which is composed of seven individual layers, is shown in Fig. 1. A relatively thick intermediate layer of TiN was deposited directly on the cemented carbide substrate followed by three x-Ale03 layers which were separated by intermediate layers of TiN. A thin layer of TiN, hardly visible in Fig. 1, was applied on top of the uppermost ~c-Al203 layer. The innermost TiN layer is about 3/~m thick. The K-A1203 layers are approximately 0.8/~m thick, and the outer TiN layers (except the outermost one) are 0.4 #m thick. The total thickness of the coating is consequently about 6-5/~m. As the deposition times are known, the approximate growth rates for the layers can be calculated and are presented in Table 2. All K-layers had the same growth rate, about 0.8 pm/h, while the growth rate for TiN on
Deposition time (min )
Thickness ( l~m)
Growth rate ( l~m/h)
315 100 60
3 0.4 0.8
0.57 0.24 0.80
cemented carbide was much higher than for TiN on x-Al203, ~0.6/~m/h as compared with ,~0.2/~m/h. Thus, the deposition rate was twothree times higher for TiN deposited on cemented carbide. As shown in Fig. 2(a), the innermost TiN layer is often composed of columnar crystals, especially above binder phase regions. Using XRD, the preferred growth direction was found to be (100). The grain size is about 0.2 × 1 #m and thus the aspect ratio is about 5. No dislocations or pores could be observed. Epitaxial growth of TiN on WC was frequently observed. These orientation relationships will be dealt with in a separate paper, but were of the same type as reported earlier by Vuorinen and Hoel for TiC on WC. 27 The ~c-A1203 layers consisted of columnar twinned grains, the grain height often being equal to the layer thickness. It was shown, using selected area electron diffraction (SAED) in TEM, that the preferred growth direction for K-AI203 is along the c-axis, (001). This is in accordance with earlier observations on multilayer coatings of K-A1203.3"28 Figure 2(b) shows a cross-section of a TiNlayer between two ~:-A1203 layers. The TiN grains are equiaxed with a size of about 100 nm. Even though the process parameters were identical for all TiN layers, the grain morphology and growth rate are very different for TiN on cemented carbide, as compared to TiN on K-AI203. No pores and very few dislocations could be found within the TiN-layer. However, in the x-AI203/TiN interface there are a large number of pores, with a size in the range of 50-200 nm. All pores are located below the TiN-layer, no pores can be found in the TiN/ K-A1203 interface. Although the process parameters had been adjusted to favour deposition of TiN without
172
M. Halvarsson, S. Vuorinen
incorporation of carbon, the higher growth rate of TiN on cemented carbide indicates that some carbon may have been supplied by the substrate (binder phase) to the coating and hence increased the TiN growth rate. As alumina is an effective diffusion barrier, no carbon could have been supplied from the substrate to the TiN layer grown on K-AI203, which resulted in a lower growth rate and more equiaxed grains. However, it is difficult to quantify and compare the C/N ratio in the 'TIN' layers by EDX, due to the poor yield of x-rays by light elements and the contamination of the specimen with hydrocarbons in the microscope vacuum.
Fig. 2.
Orientation relationships for TiN on K-AI20~ and for K'AI203 on TiN
Epitaxial growth of TiN on K-AI203 and K-A1203 on TiN were frequently observed [Fig. 3(a) and (b)]. In order to determine the orientation relationships for TiN o n K-A1203, three selected area electron diffraction patterns (DPs) were obtained from the region marked A in Fig. 3(a). The first DP was taken from TiN and is shown in Fig. 4(a). Then the selected area aperture was moved slightly within the TiN layer and a new DP was obtained, shown in Fig. 4(b). Finally, the selected area aperture was moved
(a) Cross-section TEM micrograph of TiN deposited on to the substrate. (b) Pores (arrowed) are only localised in the h'-AI20~/TiN interface below the TiN layer. TEM.
Multilayer coatings of TiN and K-AI20 ,
down towards the substrate, to include spots from the underlying K-AI203 grain [Fig. 4(c)]. The first DP can be indexed as a (single crystal) [1-23] zone, as shown in Fig. 4(d). The second DP, however, does not originate from a single crystal, but from two twin related domains. The indexed pattern in Fig. 4(e) shows spots from two TiN domains and extra spots arising from double diffraction between the two domains. The twins are related to each other by a 180° rotation along the [lll]-axis. Finally, the DP in Fig. 4(c) is a superposition of spots from TiN, tc-A1203, and double diffraction, and the correct indices are given in Fig. 4(f), where TiN and
Fig. 3.
173
double diffracted spots have been omitted for clarity. The DP is a superposition of the [123]T~N, [12-3]TIN and [810],< zones. The orientation relationships can therefore be expressed as follows:
(O01),
(1)
where TiN1 and TiN2 stand for the two TiN domains. When the same type of DP analysis was carried out for x-AI203 deposited on TiN, i.e. from region B in Fig. 3(a), the same type of
Orientation relationships were frequently found, both for K-AI203 on TiN and for TiN on K-AI203 as shown here in cross-section TEM: (a) bright field and; (b) dark field mode.
174
M. Halvarsson, S. Vuorinen
B = [i231
d
,;9
,%
"
,#
8'
• TiN-domain 1 TiN-domain 2 x double diffraction v I¢-A1203 o
• B = [i23] .. ° B = [t23]
e
d
".,,,
® O
s¢,~, i
o
c~o qY ,O, '~ ~ ~
o
.
.B = [i231 oB = [ t ~ l VB=[810]
®
"4~ " ~ 1 , ~ ,~
Fig. 4. (a) Diffraction pattern from one TiN domain. (b) Diffraction pattern from two neighbouring TiN domains. (c) Diffraction pattern as in (b), but also including spots from the ~c-grain on to which the two TiN domains nucleated. Indexing of the diffraction patterns in (a)-(c). The legend is shown_i_n (d). (d) The DP in (a) could be indexed as the [123] zone. (e) The DP in (b) consisted of two TiN twins with [123]//[123]. (f) The indices for the [810] zone of ~-AI20~. The double diffracted spots and the indices for TiN have been omitted for clarity.
Multilayer coatings of TiN and x-Ale03
DPs were obtained as from region A. Thus, x-AI203 grows epitaxially on TiN with the same relation (1) as above. It has previously been shown that CVD x-grains consist of three domains rotated by 120° around the [001] direction. ~' If the relationship (1) is rotated approximately 11° around (001),. and all three x-domains are included, the full relationships are written as: (001),¢ // [100],., [110],.2 //
[i10],,3
( l l l ) T i N //
[il2]TiN, __ [ll2]wiNz //
(001),,. [100],¢, [110],.2
[i101,,3
(2)
where xl, ~:2 and K3 stand for three twinrelated domains in x-A1203. Thus, three twinrelated ~:-domains grow epitaxially on two twin-related TiN domains which grow epitaxially on three twin-related ~c-domains. Twinning mechanism of TiN and structure of i¢-A]203 The formation of the TiN twins on (001) planes of/(-m1203 indicates that these planes are closepacked• This is described in Fig. 5(a) and (b) where the large open circles are oxygen ions in rc-AIa03 and the small filled circles are atoms in close-packed planes of TiN, e.g. Ti. Three neighbouring unit cells of tc-AI203 are outlined• In close packed structures it is common to describe atom positions with three letters, A, B and C. If the oxygen ions are in A-positions, the Ti-atoms can be placed in B or C positions on the oxygen plane. In the left-hand cell of Fig. 5(a), Ti-atoms occupy B-positions, while in the right hand cell they occupy C-positions. Therefore, two TiN crystals with different stacking sequences can be formed, one with an •.ABCABC.. stacking sequence and one with an ..ACBACB.. sequence, as is shown in sideview in Fig. 5(b). This corresponds to two TiN twins related by a 180° rotation around (111). This twinning mechanism, which is identical to that found experimentally in this work, only occurs on close-packed surfaces. According to the DP analyses above, the two TiN twins grew on (001) planes of x-A1203. Thus, the conclusion is that (001),, is close-packed.
175
As has been discussed by Liu and Skogsmo, 22 only two stacking sequences of close packed oxygen-ion planes in x-Alz03 are possible if four planes should complete one unit cell in the c-direction, and these are ..ABAB.. and .. A B A C . . . However, only.. ABAC.. is allowed by the symmetry determined for K-Al~03. The positions of the AI ions in the x-AI203 unit cell cannot be determined from the observed twinning mechanism, but it can be concluded that K-AI203 consists of close-packed oxygen ion planes with aluminium ions in interstitial positions. This is shown schematically in Fig. 6(a). The orientation relationships (2) are similar to those reported earlier for multi-layers of K-AI203.TM In the former work the x-layers were separated by thin x-modification layers. The x-modification layers are used to renucleate K-A1203 in order to obtain grain refinement of the K-AI203 coating. 2"3 It has previously been reported that the x-modification layer consists of (AI,Ti) (C,O) 2"2" and exhibits an FCC structure. TM Thus, that multilayer system is quite similar, from a crystal structure point of view, to the multilayer examined in this work. It is then possible that the grains in the ~c-modification layer are twinned in the same way as reported here for TiN. However, more work is needed on the modification/A1203 layer interfaces. Orientation relationships for x-A1203 on TiN have been investigated earlier by other workers, 2~ and relationships of the type (O001),J/(lll)T,N were reported• However, that diffraction work was performed on plane-view specimens and therefore the DPs from K-A1203 were a superposition of the three rotated ~c-domains, which produced a false hexagonal superlattice. However, no twinning was reported for TiN. Domain formation in K-AI203 For /~-Al203 deposited on TiN, different stacking sequences may occur in a similary way to that described above for the TiN domains. If the top surface of TiN is a close packed (111) layer in e.g. a C-position, x-Al20.~ can grow in three different stacking sequences, as shown in Fig. 6(b). However, the three stacking sequences for ~:-AI20~ do not represent twinning. All three domains have the same sequence ..ABAC.. and are not rotated, but instead translated relative to each other, and will therefore produce identical DPs. By TEM analysis
176
M. Halvarsson, S. Vuorinen
within one x-grain, it was possible to confirm the existence of neighbouring x-domains with identical DPs, which then supports the model of translated ~:-domains. As the space group of K-A1203 is Pna2,, and the oxygen sub-lattice with a stacking sequence
(a)
TiN domain 1
of ..ABAC.. has the space group P63/mmc, the positions of the AI ions reduce the symmetry to Pna2,. In the oxygen-ion sub-lattice, the AI ions can be placed in the oxygen ion sub-lattice in three ways, related by a 120° rotation with respect to each other. This leads to the forma-
TiN domain 2
K-AI203
[010]
[110l ~-~[il2l
[1101
~100l
C
A
l (b)
TiN domain 2 ~ (] 1])
TiNdomain 1 T(]I]) A
A
A
A
C
C
B
B
B
B
C
C AOc)
A(~:) ~-A120 3 ~ (001) surface
Fig. 5. The drawings show how the TiN twins are formed on h--A1203. (a) Top view. Large open circles represent oxygen ions in an (001) K-plane and the small filled circles are atoms in a close-packed (lll)-plane in TiN, e.g. Ti atoms. Three unit cells of h'-AI20~ are outlined. The oxygen ions are in A-positions, while TiN has nucleated on B- and C-positions in the left and right unit cell, respectively. (b) Edge-on view. The figure shows the difference in stacking sequence for the two TiN domains in (a).
Multilayer coatings of TiN and x-Al20,
tion of three twin related x-A1203 domains, in a similar way as described above for TiN grown on x-A1203. The three possibilities are shown schematically in Fig. 6(c). Consequently, there are two types of domains in CVD x-A1203: (i) twin-related domains due to the positions of the AI ions within the oxygen-ion sub-lattice and; (ii) translation-related
(a)
177
domains due to different stacking sequences of the oxygen-ion planes.
4 CONCLUSIONS
(a) The
height of the columnar, twinned tc-A1203 grains was approximately that of
A
O 2"
? . . . . . . . . . . . . . . . . . . . A13÷ C
O 2"
? . . . . . . . . . . . . . . . . . . . A13+ c
A
02-
? . . . . . . . . . . . . . . . . . . . AI 3+ 02-
B
? . . . . . . . . . . . . . . . . . . . A13÷ A
(b)
O 2-
Domain I
Domain II
Domain III
I¢-A1203 ~ (001)
r-A1203 ~ ( 0 0 1 )
K:-AI203 ~ (001)
C
C B
B A
A
A
A A
A C
C
B
B C
CA
A
A
A A
A B
B C (TIN)
C (TIN)
t
TiN t[ (] 1i) surface
(c) o
o o
0
o
0 0
0 0
0
0
0
0 0
t oi o
0 0
o
o
0
o a
o
0
0
°
~ v
o
o
(1)
o
(2)
(3)
Fig. 6. (a) A schematic drawing of the stacking sequence of the x-Ai203 u_nitcell. (b) Three different stacking sequences are possible for the oxygen-ion planes of the tc-AlzO 3 deposited on a (111) TiN plane in C-position. They represent domains displaced fractions of the height of the tc-A1203 unit cell. (c) The 6, symmetry of the ABAC-stacked oxygen sublattice of the tc-mlzo3 is broken down by the positions of the aluminium ions to a remaining 2, symmetry for x-A120~. This can be obtained in three different ways, leading to three domains, rotated 120° relative to each other. The circles represent oxygen ions.
178
M. Halvarsson, S. Vuorinen
the K-AI203 layer thickness. The preferred growth direction for ~:-AI20~ was (001). (b) The grain morphology is very different for TiN deposited on the cemented carbide as compared to TiN deposited on to sc-A1203; the former consists of columnar grains with an aspect ratio of about 5 and the latter consists of equiaxed grains. These observations may be attributed to carbon supply from the substrate. The deposition rate of TiN was two-three times higher for TiN deposited on the cemented carbide as compared to TiN on ~:-A1203. The preferred growth direction for TiN was (100). (c) No pores or dislocations could be found within the TiN and K-AI203 layers. However, in the K-AIRO3/TiN interface below the TiN-layer there is a large number of pores. (d) Orientation relationships were frequently found both for TiN on K-AI203 and for K-A1203 on TiN. Three twin-related K-domains grow epitaxially on two twinrelated TiN domains which grow epitaxially on three twin-related K-domains and can be described as: (001),, //
( l l l ) T i N //
(001),~
[100l,,,
[]-12lwiN,
[100],~,
[ii0]h-2 //
[1121TIN2 //
[']-i0]h. 2
[i101,,3
[i101,,3
(e) It was concluded from the observed twinning mechanism of TiN on ~:-A1203 that K-AI203 consists of close packed oxygenion planes in an ..ABAC.. stacking sequence with aluminium ions in interstitial positions. (f) There are two types of domain in CVD x-A1203: (i) twin-related domains due to the positions of the A1 ions within the oxygen-ion sub-lattice and; (ii) translationrelated domains due to different stacking sequences of the oxygen-ion planes. ACKNOWLEDGEMENTS Financial support was provided by the Swedish National Board for Technical Development (STU). The authors wish to thank Dr H. Nord6n (Chalmers University of Technology) for
helpful discussions and Dr P. O. Snell (Seco Tools AB) for permission to publish this work.
REFERENCES 1. Chatfield, C., Lindstr6m, J. N. & Sj6strand, M. E., J. Physique (Paris), Colloq. C5, 50 (1989) 377. 2. Vuorinen, S. & Skogsmo, J., Thin Solid Films, 193/194 (1990) 536. 3. Halvarsson, M., Vuorinen, S. & Nord6n, H., Surface and Coatings Technol., 61 (1993) 177. 4. Lux, B., Colombier, C. & Altena, H., Thin Solid Films, 138 (1985) 49. 5. Vuorinen, S. & Skogsmo, J. In Surface Modification Technologies, eds T. S. Sudarshan & D. G. Bhat. TMS, Phoenix, AZ, 1988, p. 143. 6. Cho, J. S., Nam, S. W. & Chun, J. S., J. Mater Sci., 17 (1982) 2495. 7. Halvarsson, M., Nord6n, H. & Vuorinen, S., Surface and Coatings Technology, 68/69 (1994) 266. 8. Shah, D. C. & Bhat, D. G. In Proc. 4th Int. Conf. on Surface Modification Technologies, Paris, France, 1990, p. 74. 9. Kim, M. S. & Chun, J. S., Thin Solid Films, 107 (1983) 129. 10. Bhat, D. G. In Proc. llth Int. Conf. on CVD, Seattle, WA, 1990, p. 648. 11. Roman, O. V., Kirilyuk, L. M., Dubrovskaya, G. N., Anikin, V. N. & Anikiev, A. I., Powder Met. Int., 13 (1981) 192. 12. Wokulksi, Z., Cryst. Res. Technol., 26 (1991) 8. 13. Sj6strand, M. E. In Proc. 7th Int. Conf. on CVD, eds T. O. Sedgwich & H. Lytdin. The Electrochemical Society, Pennington, NJ, 1979, p. 452. 14. Vuorinen, S. & Karlsson, L., Thin Solid Films, 214 (1992) 132. 15. Skogsmo, J., Halvarsson, M. & Vuorinen, S., Surface and Coatings Technol., 54/55 (1992) 186. 16. Lindulf, N., Halvarsson, M., Nord6n, H. & Vuorinen, S., Thin Solid Films, 253 (1994) 311. 17. Hansson, P., Halvarsson, M. & Vuorinen, S., Surface and Coatings Technology, 78-77 (1995) 256. 18. Kronberg, M. L.,Acta Met., 5 (1957) 508. 19. Pauling, L. & Hendricks, S. B., J. Am. Chem. Soc., 47 (1925) 781. 20. Newnham, R. E. & de Hahn, Y. M., Zeitschrifi fiir Kristallographie, 117 (1962) 235. 21. Skogsmo, J., Liu, P., Chatfield, C. & Nord6n, H. In Proc. 12th Int. Plansee Seminar, eds H. Bilstein & H. M. Ortner. Vol 3, Metallwerk Plansee, Reutte, Austria, 1989, p. 129. 22. Liu, P. & Skogsmo, J.,Acta Cryst., B47 (1991) 425. 23. Halvarsson, M., Langer, V. & Vuorinen, S., Surface and Coatings Technology, 78-77 (1995) 358. 24. Okimiya, M., Yamaguchi, G., Yamada, O. & Ono, S., Bull. Chem. Soc. Jap., 44 (1971) 418. 25. Vuorinen, S. In Proc. 11th Int. Congr. on Electron Micr., Kyoto, Japan, 1986, p. 1401. 26. Ruppi, S., US Patent 5137774 (1992). 27. Vuorinen, S. & Hoel, R. H., Thin Solid Films, 232 (1993) 73. 28. Halvarsson, M., Vuorinen, S. & Nord6n, H., Mat. Res. Symp. Proc., 314 (1993) 83. 29. Chatfield, C., Int. J. Refractory Metals and Hard Materials, September (1990) 132.
Int. J. of Refractory Metals & Hard Materials
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S0263-4368(96)00028-5
Microstructural Investigations of CVD TiN/ K-ALO3 Multilayer Coatings on Cemented Carbides M. Halvarsson Department of Physics, Chalmers University of Technology, S-412 96 G6teborg, Sweden
& S. Vuorinen Research and Development, Seco Tools AB, S-737 82 Fagersta, Sweden (Received 24 October 1995; accepted 24 April 1996) Multilayer coatings of TiN and K-AI~O, have been examined by x-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy. The general microstructure of the coatings, such as grain size, grain shape, twinning, dislocations, pores and interfaces was investigated and, based on these findings, conclusions about the detailed structure of K-AI203 were made. A model is presented for the domain formation in CVD K-AI20~. The height of the columnar, twinned K-grains was of the order of the K-layer thickness. The preferred growth direction for K-AI203 was along the c-axis. The grain morphology is very different for TiN deposited on to cemented carbide as compared to TiN deposited on to K-AI203; columnar grains with an aspect ratio of about 5 and equiaxed grains, respectively. No pores or dislocations could be found within the TiN and K-AI203layers. However, tc-AI203/TiN interfaces below the TiN-layers exhibit a large number of pores. The deposition rate of TiN was two-three times higher for TiN deposited on cemented carbide than for TiN deposited on to K-AI203. Epitaxy was frequently found both for TiN on K-AI20~ and for K-AI20~ on TiN. Three twin-related K-domains grew epitaxially on two twin-related TiN domains which grew epitaxially on three twin-related K-domains. The orientation relationships could be described as:
Abstract:
(001),, //
(Ill)TiN //
(001),,
1100],.,
[lI21,.,N,
[100l,.,
[1 101,,2 //
[lT2]-nNe//
[1 10],,2
[ho],,.,
0-1o1,,.~
The observed twinning of TiN on h--A120~ indicates that the (001)-planes in h--AI20~ are close-packed in an .. ABAC.. stacking sequence. There are two types of geometrical relationships of neighbouring domains in CVD K-A1203: (i) twinrelated domains due to different positions of the aluminium ions within the oxygen-ion sub lattice and; (ii) translation-related domains due to different stacking sequences of the oxygen-ion planes. © 1997 Elsevier Science Limited
1 INTRODUCTION
coatings are often p r o d u c e d by Chemical Vapour Deposition (CVD). A l u m i n a exhibits a n u m b e r of crystallographic modifications. H o w e v e r , only two are of interest for cutting tool applications, namely the
Alumina (A1203)1-7 and titanium nitride (TIN) 8-'3 are widely used as wear-resistant coatings on c e m e n t e d carbide cutting tools. T h e 169
170
M. Halvarsson, S. Vuorinen
stable c~ and the metastable x polymorph. At elevated temperatures, coatings of K-A1203 will transform to the stable ~ phase and this has been reported earlier. 14-'7 ~-AI:O, has a well established crystal structure, '"-2'' but the complete structure of t~-AI:O~ has not been determined. It has been shown 21"22with selected area electron diffraction (SAED) in transmission electron microscopy (TEM) that the Bravais lattice of t c - A l 2 0 3 is primitive orthorhombic and that CVD grown h'-grains consist of a large number of domains, related by a 120 ° rotation along the c-axis. This domain structure of K - A I 2 0 3 is probably the reason for the previous confusion in the literature about the crystal structure of K - A I 2 0 3 . The lattice parameters of tc-AI203 have been determined to be a=4.84 A, b=8.31 A and c=8.94~. 23 Based on the symmetry determination of K - A I e O 3 and the dimensions of the unit cell, it was proposed that h:-AI:O3 may consist of close-packed oxygen-ion planes with the aluminium ions in interstitial positions. 22 A close packed structure was also suggested by Okimiya et al. 24 based on density measurements. An atomic model for x-AI:O, has been suggested by Liu and Skogsmo :2 with an .. ABAC.. stacking of oxygen-ion planes with aluminium ions occupying both octahedral (~) and tetrahedral (fl') positions, giving an ..A~B~Afl'Cfl'.. stacking sequence along the c-axis. However, many atomic models are possible and the suggested model has not been verified by diffraction experiments. Titanium nitride has the NaCl-structure (FCC) and has the lattice parameter a=4.24 A, which, however, varies with deviations from stoichiometry. Previous investigations ~-'3 on CVD TiN have ranged from the relations between process parameters and the microstructure, and kinetics and thermodynamics, to different wear characteristics during machining. An icosahedral surface morphology of TiN has been reported,'" which suggests that the TiN crystals are highly twinned. Transmission electron microscopy studies have also revealed planar faults in CVD TiN, which were interpreted as twins?'~5 In this work, multilayer coatings of TiN and x-A1203 have been examined by analytical scanning and transmission electron microscopy. The aim was to describe the general microstructure of the coatings, such as grain size, grain shape,
twinning, dislocations, pores and interfacial structures. Based on these findings, conclusions about the detailed structure of ~c-AI20~ are made and a model is presented for two different types of domain in CVD ~:-A1203.
2 EXPERIMENTAL Chemical vapour deposition
Deposition of the experimental coatings was carried out in a computer-controlled hot-wall CVD reactor. The TiN coatings were deposited from the TiC14-N2-H2 system, the overall reaction being thus TiCI4 (g) +0.5 N 2 (g) +2H2 (g)--,TiN (s) + 4 HC1 (g) Deposition of TiN was carried out at a pressure of 600 mbar in order to suppress the formation of Ti(C,N) in the first TiN layer which was deposited directly on a cemented carbide substrate. The K-AI203 coatings were deposited using the gases AIC13, C O 2 and H2. ~° AIC13 was generated within the deposition system through the chlorination of AI with HCI. The A1203 coatings were deposited at a pressure of 50 mbar. The inlet gas compositions which were used to deposit TiN and K - A I 2 0 3 a r e given in Table 1. The experimental multilayer coating was composed of seven individual layers of either K-AI203 or TiN. The coating is shown in Fig. 1 and is described in detail below. Material characterisation
The coatings were examined by scanning electron microscopy (SEM) using a Camscan S48DV and by analytical transmission electron microscopy (TEM/EDX) using a JEOL 2000FX equipped with a LINK AN10.000 EDX system. Cross-section thin foils were prepared by a
Table 1. The inlet gas compositions (%) 142 TiN A1203
Balance Balance
TiCL
N2
C02
HCl
AICl,
8 --
55 --
-6
2 1
-2
Multilayer coatings of TiN and tc-A1203
171
Table 2. The deposition time, thickness and growth rate for the CVD layers
Layer
TiN on WC/Co TiN on K-AI20~ h-A1203 on TiN
Fig. 1. A cross-section SEM micrograph of the TiN/ K-A1203 multilayer coating and the cemented carbide substrate. The bright layers are TiN and the dark layers are /':-A1203.
method described elsewhere. 2"5 This method produces specimens with electron transparency in all layers and interfaces in the multilayer coating simultaneously. In addition, standard techniques for plane-view specimens were used. X-ray diffractometry (XRD) was carried out using a Philips PW1700 system.
3 RESULTS AND DISCUSSION General microstructure
The general mlcrostructure of the multilayer coating, which is composed of seven individual layers, is shown in Fig. 1. A relatively thick intermediate layer of TiN was deposited directly on the cemented carbide substrate followed by three x-Ale03 layers which were separated by intermediate layers of TiN. A thin layer of TiN, hardly visible in Fig. 1, was applied on top of the uppermost ~c-Al203 layer. The innermost TiN layer is about 3/~m thick. The K-A1203 layers are approximately 0.8/~m thick, and the outer TiN layers (except the outermost one) are 0.4 #m thick. The total thickness of the coating is consequently about 6-5/~m. As the deposition times are known, the approximate growth rates for the layers can be calculated and are presented in Table 2. All K-layers had the same growth rate, about 0.8 pm/h, while the growth rate for TiN on
Deposition time (min )
Thickness ( l~m)
Growth rate ( l~m/h)
315 100 60
3 0.4 0.8
0.57 0.24 0.80
cemented carbide was much higher than for TiN on x-Al203, ~0.6/~m/h as compared with ,~0.2/~m/h. Thus, the deposition rate was twothree times higher for TiN deposited on cemented carbide. As shown in Fig. 2(a), the innermost TiN layer is often composed of columnar crystals, especially above binder phase regions. Using XRD, the preferred growth direction was found to be (100). The grain size is about 0.2 × 1 #m and thus the aspect ratio is about 5. No dislocations or pores could be observed. Epitaxial growth of TiN on WC was frequently observed. These orientation relationships will be dealt with in a separate paper, but were of the same type as reported earlier by Vuorinen and Hoel for TiC on WC. 27 The ~c-A1203 layers consisted of columnar twinned grains, the grain height often being equal to the layer thickness. It was shown, using selected area electron diffraction (SAED) in TEM, that the preferred growth direction for K-AI203 is along the c-axis, (001). This is in accordance with earlier observations on multilayer coatings of K-A1203.3"28 Figure 2(b) shows a cross-section of a TiNlayer between two ~:-A1203 layers. The TiN grains are equiaxed with a size of about 100 nm. Even though the process parameters were identical for all TiN layers, the grain morphology and growth rate are very different for TiN on cemented carbide, as compared to TiN on K-AI203. No pores and very few dislocations could be found within the TiN-layer. However, in the x-AI203/TiN interface there are a large number of pores, with a size in the range of 50-200 nm. All pores are located below the TiN-layer, no pores can be found in the TiN/ K-A1203 interface. Although the process parameters had been adjusted to favour deposition of TiN without
172
M. Halvarsson, S. Vuorinen
incorporation of carbon, the higher growth rate of TiN on cemented carbide indicates that some carbon may have been supplied by the substrate (binder phase) to the coating and hence increased the TiN growth rate. As alumina is an effective diffusion barrier, no carbon could have been supplied from the substrate to the TiN layer grown on K-AI203, which resulted in a lower growth rate and more equiaxed grains. However, it is difficult to quantify and compare the C/N ratio in the 'TIN' layers by EDX, due to the poor yield of x-rays by light elements and the contamination of the specimen with hydrocarbons in the microscope vacuum.
Fig. 2.
Orientation relationships for TiN on K-AI20~ and for K'AI203 on TiN
Epitaxial growth of TiN on K-AI203 and K-A1203 on TiN were frequently observed [Fig. 3(a) and (b)]. In order to determine the orientation relationships for TiN o n K-A1203, three selected area electron diffraction patterns (DPs) were obtained from the region marked A in Fig. 3(a). The first DP was taken from TiN and is shown in Fig. 4(a). Then the selected area aperture was moved slightly within the TiN layer and a new DP was obtained, shown in Fig. 4(b). Finally, the selected area aperture was moved
(a) Cross-section TEM micrograph of TiN deposited on to the substrate. (b) Pores (arrowed) are only localised in the h'-AI20~/TiN interface below the TiN layer. TEM.
Multilayer coatings of TiN and K-AI20 ,
down towards the substrate, to include spots from the underlying K-AI203 grain [Fig. 4(c)]. The first DP can be indexed as a (single crystal) [1-23] zone, as shown in Fig. 4(d). The second DP, however, does not originate from a single crystal, but from two twin related domains. The indexed pattern in Fig. 4(e) shows spots from two TiN domains and extra spots arising from double diffraction between the two domains. The twins are related to each other by a 180° rotation along the [lll]-axis. Finally, the DP in Fig. 4(c) is a superposition of spots from TiN, tc-A1203, and double diffraction, and the correct indices are given in Fig. 4(f), where TiN and
Fig. 3.
173
double diffracted spots have been omitted for clarity. The DP is a superposition of the [123]T~N, [12-3]TIN and [810],< zones. The orientation relationships can therefore be expressed as follows:
(O01),
(1)
where TiN1 and TiN2 stand for the two TiN domains. When the same type of DP analysis was carried out for x-AI203 deposited on TiN, i.e. from region B in Fig. 3(a), the same type of
Orientation relationships were frequently found, both for K-AI203 on TiN and for TiN on K-AI203 as shown here in cross-section TEM: (a) bright field and; (b) dark field mode.
174
M. Halvarsson, S. Vuorinen
B = [i231
d
,;9
,%
"
,#
8'
• TiN-domain 1 TiN-domain 2 x double diffraction v I¢-A1203 o
• B = [i23] .. ° B = [t23]
e
d
".,,,
® O
s¢,~, i
o
c~o qY ,O, '~ ~ ~
o
.
.B = [i231 oB = [ t ~ l VB=[810]
®
"4~ " ~ 1 , ~ ,~
Fig. 4. (a) Diffraction pattern from one TiN domain. (b) Diffraction pattern from two neighbouring TiN domains. (c) Diffraction pattern as in (b), but also including spots from the ~c-grain on to which the two TiN domains nucleated. Indexing of the diffraction patterns in (a)-(c). The legend is shown_i_n (d). (d) The DP in (a) could be indexed as the [123] zone. (e) The DP in (b) consisted of two TiN twins with [123]//[123]. (f) The indices for the [810] zone of ~-AI20~. The double diffracted spots and the indices for TiN have been omitted for clarity.
Multilayer coatings of TiN and x-Ale03
DPs were obtained as from region A. Thus, x-AI203 grows epitaxially on TiN with the same relation (1) as above. It has previously been shown that CVD x-grains consist of three domains rotated by 120° around the [001] direction. ~' If the relationship (1) is rotated approximately 11° around (001),. and all three x-domains are included, the full relationships are written as: (001),¢ // [100],., [110],.2 //
[i10],,3
( l l l ) T i N //
[il2]TiN, __ [ll2]wiNz //
(001),,. [100],¢, [110],.2
[i101,,3
(2)
where xl, ~:2 and K3 stand for three twinrelated domains in x-A1203. Thus, three twinrelated ~:-domains grow epitaxially on two twin-related TiN domains which grow epitaxially on three twin-related ~c-domains. Twinning mechanism of TiN and structure of i¢-A]203 The formation of the TiN twins on (001) planes of/(-m1203 indicates that these planes are closepacked• This is described in Fig. 5(a) and (b) where the large open circles are oxygen ions in rc-AIa03 and the small filled circles are atoms in close-packed planes of TiN, e.g. Ti. Three neighbouring unit cells of tc-AI203 are outlined• In close packed structures it is common to describe atom positions with three letters, A, B and C. If the oxygen ions are in A-positions, the Ti-atoms can be placed in B or C positions on the oxygen plane. In the left-hand cell of Fig. 5(a), Ti-atoms occupy B-positions, while in the right hand cell they occupy C-positions. Therefore, two TiN crystals with different stacking sequences can be formed, one with an •.ABCABC.. stacking sequence and one with an ..ACBACB.. sequence, as is shown in sideview in Fig. 5(b). This corresponds to two TiN twins related by a 180° rotation around (111). This twinning mechanism, which is identical to that found experimentally in this work, only occurs on close-packed surfaces. According to the DP analyses above, the two TiN twins grew on (001) planes of x-A1203. Thus, the conclusion is that (001),, is close-packed.
175
As has been discussed by Liu and Skogsmo, 22 only two stacking sequences of close packed oxygen-ion planes in x-Alz03 are possible if four planes should complete one unit cell in the c-direction, and these are ..ABAB.. and .. A B A C . . . However, only.. ABAC.. is allowed by the symmetry determined for K-Al~03. The positions of the AI ions in the x-AI203 unit cell cannot be determined from the observed twinning mechanism, but it can be concluded that K-AI203 consists of close-packed oxygen ion planes with aluminium ions in interstitial positions. This is shown schematically in Fig. 6(a). The orientation relationships (2) are similar to those reported earlier for multi-layers of K-AI203.TM In the former work the x-layers were separated by thin x-modification layers. The x-modification layers are used to renucleate K-A1203 in order to obtain grain refinement of the K-AI203 coating. 2"3 It has previously been reported that the x-modification layer consists of (AI,Ti) (C,O) 2"2" and exhibits an FCC structure. TM Thus, that multilayer system is quite similar, from a crystal structure point of view, to the multilayer examined in this work. It is then possible that the grains in the ~c-modification layer are twinned in the same way as reported here for TiN. However, more work is needed on the modification/A1203 layer interfaces. Orientation relationships for x-A1203 on TiN have been investigated earlier by other workers, 2~ and relationships of the type (O001),J/(lll)T,N were reported• However, that diffraction work was performed on plane-view specimens and therefore the DPs from K-A1203 were a superposition of the three rotated ~c-domains, which produced a false hexagonal superlattice. However, no twinning was reported for TiN. Domain formation in K-AI203 For /~-Al203 deposited on TiN, different stacking sequences may occur in a similary way to that described above for the TiN domains. If the top surface of TiN is a close packed (111) layer in e.g. a C-position, x-Al20.~ can grow in three different stacking sequences, as shown in Fig. 6(b). However, the three stacking sequences for ~:-AI20~ do not represent twinning. All three domains have the same sequence ..ABAC.. and are not rotated, but instead translated relative to each other, and will therefore produce identical DPs. By TEM analysis
176
M. Halvarsson, S. Vuorinen
within one x-grain, it was possible to confirm the existence of neighbouring x-domains with identical DPs, which then supports the model of translated ~:-domains. As the space group of K-A1203 is Pna2,, and the oxygen sub-lattice with a stacking sequence
(a)
TiN domain 1
of ..ABAC.. has the space group P63/mmc, the positions of the AI ions reduce the symmetry to Pna2,. In the oxygen-ion sub-lattice, the AI ions can be placed in the oxygen ion sub-lattice in three ways, related by a 120° rotation with respect to each other. This leads to the forma-
TiN domain 2
K-AI203
[010]
[110l ~-~[il2l
[1101
~100l
C
A
l (b)
TiN domain 2 ~ (] 1])
TiNdomain 1 T(]I]) A
A
A
A
C
C
B
B
B
B
C
C AOc)
A(~:) ~-A120 3 ~ (001) surface
Fig. 5. The drawings show how the TiN twins are formed on h--A1203. (a) Top view. Large open circles represent oxygen ions in an (001) K-plane and the small filled circles are atoms in a close-packed (lll)-plane in TiN, e.g. Ti atoms. Three unit cells of h'-AI20~ are outlined. The oxygen ions are in A-positions, while TiN has nucleated on B- and C-positions in the left and right unit cell, respectively. (b) Edge-on view. The figure shows the difference in stacking sequence for the two TiN domains in (a).
Multilayer coatings of TiN and x-Al20,
tion of three twin related x-A1203 domains, in a similar way as described above for TiN grown on x-A1203. The three possibilities are shown schematically in Fig. 6(c). Consequently, there are two types of domains in CVD x-A1203: (i) twin-related domains due to the positions of the AI ions within the oxygen-ion sub-lattice and; (ii) translation-related
(a)
177
domains due to different stacking sequences of the oxygen-ion planes.
4 CONCLUSIONS
(a) The
height of the columnar, twinned tc-A1203 grains was approximately that of
A
O 2"
? . . . . . . . . . . . . . . . . . . . A13÷ C
O 2"
? . . . . . . . . . . . . . . . . . . . A13+ c
A
02-
? . . . . . . . . . . . . . . . . . . . AI 3+ 02-
B
? . . . . . . . . . . . . . . . . . . . A13÷ A
(b)
O 2-
Domain I
Domain II
Domain III
I¢-A1203 ~ (001)
r-A1203 ~ ( 0 0 1 )
K:-AI203 ~ (001)
C
C B
B A
A
A
A A
A C
C
B
B C
CA
A
A
A A
A B
B C (TIN)
C (TIN)
t
TiN t[ (] 1i) surface
(c) o
o o
0
o
0 0
0 0
0
0
0
0 0
t oi o
0 0
o
o
0
o a
o
0
0
°
~ v
o
o
(1)
o
(2)
(3)
Fig. 6. (a) A schematic drawing of the stacking sequence of the x-Ai203 u_nitcell. (b) Three different stacking sequences are possible for the oxygen-ion planes of the tc-AlzO 3 deposited on a (111) TiN plane in C-position. They represent domains displaced fractions of the height of the tc-A1203 unit cell. (c) The 6, symmetry of the ABAC-stacked oxygen sublattice of the tc-mlzo3 is broken down by the positions of the aluminium ions to a remaining 2, symmetry for x-A120~. This can be obtained in three different ways, leading to three domains, rotated 120° relative to each other. The circles represent oxygen ions.
178
M. Halvarsson, S. Vuorinen
the K-AI203 layer thickness. The preferred growth direction for ~:-AI20~ was (001). (b) The grain morphology is very different for TiN deposited on the cemented carbide as compared to TiN deposited on to sc-A1203; the former consists of columnar grains with an aspect ratio of about 5 and the latter consists of equiaxed grains. These observations may be attributed to carbon supply from the substrate. The deposition rate of TiN was two-three times higher for TiN deposited on the cemented carbide as compared to TiN on ~:-A1203. The preferred growth direction for TiN was (100). (c) No pores or dislocations could be found within the TiN and K-AI203 layers. However, in the K-AIRO3/TiN interface below the TiN-layer there is a large number of pores. (d) Orientation relationships were frequently found both for TiN on K-AI203 and for K-A1203 on TiN. Three twin-related K-domains grow epitaxially on two twinrelated TiN domains which grow epitaxially on three twin-related K-domains and can be described as: (001),, //
( l l l ) T i N //
(001),~
[100l,,,
[]-12lwiN,
[100],~,
[ii0]h-2 //
[1121TIN2 //
[']-i0]h. 2
[i101,,3
[i101,,3
(e) It was concluded from the observed twinning mechanism of TiN on ~:-A1203 that K-AI203 consists of close packed oxygenion planes in an ..ABAC.. stacking sequence with aluminium ions in interstitial positions. (f) There are two types of domain in CVD x-A1203: (i) twin-related domains due to the positions of the A1 ions within the oxygen-ion sub-lattice and; (ii) translationrelated domains due to different stacking sequences of the oxygen-ion planes. ACKNOWLEDGEMENTS Financial support was provided by the Swedish National Board for Technical Development (STU). The authors wish to thank Dr H. Nord6n (Chalmers University of Technology) for
helpful discussions and Dr P. O. Snell (Seco Tools AB) for permission to publish this work.
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