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Plasma-enhanced CVD of (Ti,Al)N films from chloridic precursors in a DC glow discharge
Surface and Coatings Technology 133᎐134 Ž2000. 208᎐214
Plasma-enhanced CVD of ž Ti,Al/ N films from chloridic precursors in a DC glow discharge R. Prange, R. Cremer U , D. Neuschutz ¨ Lehrstuhl fur Rheinisch-Westfalische Technische Hochschule Aachen, D-52056 Aachen, Germany ¨ Theoretische Huttenkunde, ¨ ¨
Abstract Metastable Ti 1yx Al x N films have been deposited from gaseous mixtures of TiCl 4 ᎐AlCl 3 ᎐N2 ᎐H 2 ᎐Ar in a pulsed DC glow discharge at 510⬚C. When the discharge voltage was kept constant, the Al content x of the films increased linearly with the AlCl 3rTiCl 4 ratio in the feed gas. Increasing the discharge voltage also increased the Al content. Up to compositions of Ti 0.09 Al 0.91 N the layers remained single-phase cubic with a strong 1004 texture. Films with a higher Al content consisted of two phases and their cubic phase showed a weak 1114 texture. The lattice parameter of the homogeneous cubic films decreased with increasing Al content in accordance with Vegard’s law. Films with a low Al content exhibited a columnar morphology, while the films with high Al contents had a fine-grained structure. Increasing the discharge voltage also caused the grain size to decrease. The microhardness of the single-phase coatings increased with increasing Al content up to 3947 HV 0.05 for xs 0.83, while the two-phase layers showed hardness values of approximately 5000 HV 0.05. The metastable films began to decompose at temperatures between 750 and 800⬚C, depending on the Al content. The decomposition of the films with an AlrTi ratio below 1 caused the lattice parameter of the cubic phase to increase and the microhardness to decrease. Films with high Al contents did not show any increase in the lattice parameter after annealing and their microhardness strongly increased. Investigation of the oxide layer formed on a Ti 0.21 Al 0.79 N film after annealing in air at 800⬚C showed that an amorphous alumina layer with a thickness of approximately 100 nm was formed on the surface, preventing further oxidation. The films with high Al content exhibited advantageous tribological properties with friction coefficients of 0.5. Thus, they seem to be especially well suited for an application on cutting and metal working tools. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: X-Ray diffraction; PACVD; Aluminium nitride; Titanium nitride
1. Introduction Metastable ŽTi,Al.N films offer superior oxidation and wear resistance in combination with advantageous tribological properties. They have become a standard coating for cutting tools and have been studied for more than 10 years w1᎐20x. They are usually deposited by PVD techniques, namely reactive magnetron sputtering ŽMSIP. and reactive arc ion plating w1᎐11x, but U
Corresponding author. Tel.: q49-241-805995; fax: q49-2418888295. E-mail addresses: [email protected] ŽR. Cremer., n e u s c h u e t z @ l t h .r w t h - a a c h e n .d e Ž D . N e u s c h u ¨tz. [email protected] ŽR. Prange..
attempts to deposit ŽTi,Al.N films by plasma-enhanced CVD ŽPECVD. can also be found in the literature w12᎐19x. The metastable phase diagram for the pseudo-binary system TiN᎐AlN was theoretically investigated by Spencer and co-workers w20x and experimentally confirmed for PVD by Cremer et al. w4x. At deposition temperatures of 500⬚C, it was possible to substitute up to 63 at.% titanium by aluminium in the cubic unit cell of TiN. Films with a higher Al content were two-phase. Above an AlrTi ratio of 3:1 the layers exhibited the hexagonal structure of AlN. With higher deposition temperatures the two-phase region became broader. Although cubic ŽTi,Al.N is a metastable phase, the films did not show any signs of phase separation when
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 4 1 - 5
R. Prange et al. r Surface and Coatings Technology 133᎐134 (2000) 208᎐214
annealed at temperatures below 800⬚C in inert atmospheres w6x. Apart from better wear resistance in comparison with TiN, ŽTi,Al.N coatings are of particular interest because of their superior oxidation resistance. They do not show any visible signs of oxidation when annealed in air at temperatures of 800⬚C for 1 h w6,7x. Detailed analysis of the oxide layer with a thickness of only 200᎐400 nm revealed that a thin amorphous alumina layer was formed on the surface of the coatings preventing their further oxidation w6᎐9x. Although most investigations of ŽTi,Al.N have been carried out on films deposited by PVD, several studies of the deposition by PECVD have appeared recently. Lee et al. w12,13x deposited ŽTi,Al.N films from mixtures of TiCl 4 ᎐AlCl 3 ᎐H 2 ᎐NH 3 ᎐Ar in an r.f. plasma. Their films showed the cubic TiN structure with a 1004 texture up to compositions of Ti 0.2 Al 0.8 N. Films with higher Al contents consisted of two phases. Kim et al. w14,15x reported the deposition of ŽTi,Al.N from TiCl 4 ᎐AlCl 3 ᎐H 2 ᎐N2 ᎐Ar mixtures also using an r.f. plasma as excitation method. The films were singlephase cubic with a 1004 texture and had compositions from pure TiN to Ti 0.3 Al 0.7 N. Increasing the AlrTi ratio, however, leads to an increase of the chlorine content of the coatings, deteriorating their properties at AlN contents of 12% and higher. Other groups also used N2 as nitrogen source, but a pulsed DC plasma as excitation method w16᎐19x. They observed an increase of the Al-content of the films when increasing the AlCl 3 content in the feed gas and the plasma power.
2. Experimental ŽTi,Al.N films have been deposited on nitrided hot work tool steel AISI H11 Ž0.38% C, 5.0% Cr, 1.2% Mo, 0.4% V. and ISO K 15 cemented carbide insert tips by means of PECVD in a pulsed DC glow discharge from TiCl 4 ᎐AlCl 3 ᎐H 2 ᎐N2 ᎐Ar gaseous mixtures. The experimental setup is given elsewhere w19x. All gases used were of 99.999% purity except HCl which was of 99.995% purity. The gases were metered and controlled by mass flow controllers with an accuracy of 2%. The TiCl 4 of a purity higher than 99% was evaporated, metered by a flow meter and controlled by a needle valve. The AlCl 3 was generated in situ by flowing HCl over Al chips of a purity higher than 99.99% at a temperature of 500⬚C. After grinding and polishing with 1-m diamond suspension, the specimens were cleaned ultrasonically, rinsed in methanol, dried in a nitrogen jet and placed on the cathode of the pulsed DC plasma source with the bell of the vacuum chamber acting as the anode. The deposition chamber had an auxiliary resistance
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Table 1 Deposition parameters Specimens Temperature Pressure Coating time Discharge voltage Pulse time Pulserpause ratio HrCl input ratio NrŽAl q Ti. input ratio AlrTi input ratio
Hot work tool steel AISI H11 Cemented carbide ISO K 15 510⬚C 1.5 mbar 4h 430᎐530 V 50 s 0.5᎐2.0 10 40 0.4᎐4.0
heater to allow the independent adjustment of temperature and plasma parameters. The deposition parameters are given in Table 1. The composition of the films was examined by electron probe microanalysis ŽEPMA. in a Camebax SX 50 using a 7-keV electron beam, while their morphology was investigated by scanning electron microscopy ŽSEM.. The crystallographic structure was determined by grazing incidence ŽGD. X-ray diffraction ŽXRD. in a Siemens D500 goniometer with GD attachment using the Cu K ␣-line at an incidence angle of 5⬚. Additional texture analyses were carried out in BraggrBrentano geometry. In order to study the thermal stability of the films the goniometer was equipped with a high temperature chamber. The specimens were annealed in an Ar atmosphere with less than 0.1 ppm O 2 for 30 min. After annealing, the diffraction pattern was recorded and the specimen was annealed again at a 50⬚C higher temperature. This procedure was continued until the peaks of the hexagonal phase could be clearly identified. Microhardness measurements were carried out with a Leitz Durimet microhardness tester and the tribological properties were measured with a pin on disc tribometer with a Ck 45 carbon steel pin and the coated specimen forming the disc.
3. Results and discussion 3.1. Chemical composition The layers contained between 0.5 and 2.0 at.% oxygen. Apart from the films deposited at 430 V, the amount of Cl incorporated from the incomplete dissociation of the chloridic precursors was below 5.0 at.%, decreasing strongly with the discharge voltage and the Al content of the films. The deposited films were overstochiometric, with an NrŽTiq Al. ratio between 1.02 and 1.25. An increase of the N content with the discharge voltage was observed, which is attributed to the higher amount and higher energy of nitrogen ions in the plasma at higher discharge voltages.
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Fig. 1. x in Ti 1y x Al x N as a function of Al content of the feed gas and discharge voltage.
In all layers Al substituted Ti leading to films of the composition Ti 1y x Al x N. The value x, which describes the mole fraction of AlN in the film, ranged between 0.02 and 0.96, depending on the AlCl 3rTiCl 4 ratio of the feed gas and the discharge voltage, Fig. 1. At a given discharge voltage, x was a linear function of the AlCl 3rTiCl 4 ratio. Increasing the voltage of the glow discharge ᎏ and hence the power in the plasma ᎏ also led to a substantial increase of the Al content of the coatings: While x was 0.07 in layers deposited at an AlCl 3rTiCl 4 feed gas ratio of 0.4 and a discharge voltage of 480 V, Ti 0.49 Al 0.51 N was deposited at the same AlCl 3rTiCl 4 ratio and a discharge voltage of 530 V. At 430 V, an AlCl 3rTiCl 4 ratio of 1.5 resulted in films of the composition Ti 0.95 Al 0.05 N. 3.2. Morphology Fig. 2 shows the SEM micrographs of the cross-sec-
tions of ŽTi,Al.N films with AlN mole fractions between 2 and 96%. All films had a crack-free and smooth surface and no indications of intergranular porosity or voids were detected. With the exception of the Ti 0.04 Al 0.96 N film, deposited at 530 V and an AlCl 3rTiCl 4 ratio of 3.1, all films showed a growth direction perpendicular to the interface. While the layers with a low Al content had a columnar structure with grains extending from the interface to the surface, coatings with a higher Al content had a rather fine grained morphology. The growth of the crystallites was continuously interrupted by the nucleation of new grains. The transition between columnar and finegrained structure took place between Ti 0.61 Al 0.39 N and Ti 0.48 Al 0.52 N. In addition to that, an increase of the discharge voltage also led to a finer grained structure. The Ti 0.04 Al 0.96 N film, formed at AlCl 3rTiCl 4 s 3.1 and Us 530 V ŽFig. 2, lower right. had a completely different appearance. It exhibited a globular morphology and no clear boundaries between the grains could be observed. Furthermore, no growth direction could be detected. 3.3. Crystallographic structure With the exception of the films deposited at AlCl 3rTiCl 4 feed gas ratios higher than 3.0 and discharge voltages of 530 V, all layers exhibited the cubic structure of TiN and no hexagonal phase could be detected. Fig. 3 shows four spectra of these films. The position of the diffraction peaks shifted continuously towards higher diffraction angles with increasing Al
Fig. 2. SEM micrographs of the cross-sections of several Ti 1y x Al x N films.
R. Prange et al. r Surface and Coatings Technology 133᎐134 (2000) 208᎐214
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Fig. 3. XRD spectra of four homogeneous cubic Ti 1y x Al x N films.
content. This corresponds to a decrease of the lattice parameter a of the cubic unit cell when Ti atoms are replaced by the smaller Al atoms ŽFig. 4.. The lattice parameter was a function of the AlrTi ratio in the coatings and it decreased according to Vegard’s law. The films deposited at AlCl 3rTiCl 4 ratios of approximately 3 and a discharge voltage of 530 V consisted of two phases, one cubic and the other hexagonal. With ˚ the lattice parameter of the cubic 4.0878" 0.01 A, phase was even lower than the value of cubic AlN tabulated in the JCPDS data w21x, indicating high compressive stresses in this phases. The lattice parameters a and c of the hexagonal phase were 3.1192 and 4.9665 ˚ , respectively. These values are in good agreement A ˚ and c0 s 4.9792 with the JCPDS data Ž a0 s 3.1114 A ˚ .. A In order to investigate the texture of the films, additional XRD spectra were recorded in the BraggrBrentano geometry ŽFig. 5.. They revealed a strong 1004 texture for the single phase cubic films. The intensity of the 2004 peak is much higher than any of the other peaks in the spectra. According to the data for randomly oriented powders recorded in the JCPDS-files w21x, the 1114 and the 2204 peaks should have 72 and 45% of the intensity of the 2004 peak, respectively. The cubic phase in the layers consisting of two phases exhibited only a very weak 1114 orienta-
Fig. 4. Decrease of the lattice parameter a of the homogeneous cubic Ti 1y x Al x N films with the Al content; values for pure TiN and AlN from JCPDS w21x.
Fig. 5. XRD spectra of a single-phase cubic film Ža. and a layer consisting of two phases Žb. recorded in BraggrBrentano geometry.
tion. These findings correspond very well with the observed morphology. While the single phase coatings showed a very ordered structure with grains growing perpendicular to the interface, no growth direction whatsoever could be detected in the films composed of two phases. 3.4. Microhardness The microhardness of the ŽTi,Al.N films is shown in Fig. 6. It increases with the Al content of the coatings. Introducing Al into the cubic structure of TiN causes a distortion of the unit cell leading to higher intrinsic stresses and hence a higher resistivity against plastic deformation. The dependency of the microhardness on the Al content is not linear. While the increase is small up to films of the composition Ti 0.75 Al 0.25 N, the slope of the curve gets steeper with increasing Al content. This behaviour is attributed to the change in the microstructure of the films with increasing Al content. While layers with a low Al content still exhibit the columnar structure of pure TiN, coatings with higher Al contents are fine grained. Studies of the evolution of the morphology and the microhardness of ŽTi,Al.N coatings deposited by PVD revealed a strong influence of the microstructure on the hardness w10x: The de-
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R. Prange et al. r Surface and Coatings Technology 133᎐134 (2000) 208᎐214
Fig. 6. Microhardness of the Ti 1y x Al x N films as a function of their Al content.
crease in grain size and the change from columnar to fine grained morphology led to a sharp increase in the microhardness, reaching values similar to those measured here. The maximum hardness of the single phase coatings was 3947 HV 0.05 for the Ti 0.17 Al 0.83 N film, while layers composed of two phases exhibited had a much higher hardness at approximately 5000 HV 0.05. A different mechanism than the one described above for the microhardness increase of the single-phase cubic films seems to be responsible for this difference. As is known from the literature that two phase nanocrystalline ŽTi,Al.N coatings can exhibit such high hardness values w11x, a similar explanation is suggested here. The morphology of the two-phase layers was completely different from the single phase ones ŽFig. 2.. The grain size was dramatically decreased and no clear boundary between the crystallites could be identified. Thus, the increase in the microhardness with the change from single-phase to two-phase films is attributed to the different morphology.
that of pure TiN Ž2200 HV 0.05.. This behaviour is well known for ŽTi,Al.N films deposited by PVD. The Ti 0.21 Al 0.79 N film exhibited a completely different behaviour during the experiments. The lattice parameter of the cubic phase did not increase when the hexagonal phase was formed and the microhardness increased from 3842 to 5235 HV 0.05, which is similar to the hardness of the coatings consisting of two phases already after deposition. The reason for this needs further investigation, but it seems that the segregation of the second phase prohibits the relaxation of the lattice of the cubic phase, causing additional intrinsic stresses and leading to a state similar to that of the layers originally consisting of two phases. 3.6. Oxidation beha¨ iour The oxidation behaviour was investigated by annealing of the coatings in air at 800⬚C for 1 h. Fig. 8 shows two SEM micrographs of the cross-section of an oxidised Ti 0.21 Al 0.79 N film. A thin oxide layer of 100 nm thickness was formed on the surface of the film. Although the oxide was able to penetrate the film along the grain boundaries ŽFig. 8, right., no cracks were formed in the film. Fig. 9 gives the results of an EPMA linescan along the flank of a dimple ground into the
3.5. Thermal stability Depending on their Al content, the ŽTi,Al.N films showed a different behaviour during annealing in inert atmosphere ŽFig. 7.. While the Ti 0.61 Al 0.39 N coating started to decompose at 750⬚C, the Ti 0.21 Al 0.79 N film showed the first peaks of the hexagonal phase at 700⬚C. The mechanism of the decomposition also seems to be different in both cases. The segregation of a hexagonal AlN phase was accompanied by a shift of the diffraction peaks of the cubic phase towards lower diffraction angles in case of the Ti 0.61 Al 0.39 N layer. This corresponds to an increase of the lattice parameter of the cubic phase. While the Al content of the cubic phase decreases due to the appearance of the hexagonal phase, the unit cell expands because of the larger amount of Ti atoms in it. The separation of the two phases also causes the microhardness of the coating to decrease from 2926 to 2350 HV 0.05, which is close to
Fig. 7. Evolution of the hexagonal phase in two Ti 1y x Al x N films during annealing in Ar atmosphere.
R. Prange et al. r Surface and Coatings Technology 133᎐134 (2000) 208᎐214
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Ti content showed a contrary behaviour. At the surface the Al concentration is increased and the Ti content is decreased in comparison to the bulk of the film. Underneath the surface layer an area with increased Ti and decreased Al concentration was found. Therefore it is concluded that the oxide layer is formed by the outward diffusion of Al which causes a drop in the Al content of the underlying zone. As the oxygen concentration of the bulk of the film was not changed after the experiment, it is followed that the oxide layer was able to prevent further oxidation of the film. Furthermore, an XRD analysis of the oxidised coating did not show any sign of TiO 2 phases, which is an additional confirmation of this conclusion. 3.7. Tribological properties
Fig. 8. SEM micrographs of the cross-section of the oxidised Ti 0.21 Al 0.29 N film.
oxidised film. The measured N content of the surface layer is due to the information depth of the EPMA technique. Under the conditions of the measurement, the recorded N signal is generated in a volume of 150 nm thickness w22x. As the thickness of oxide layer is only 100 nm, it can be assumed that the origin of the N signal lies below the oxidised surface of the film. Below the surface oxide layer the oxygen content decreased sharply within the first 400 nm and reached the concentration of the as-deposited film after 1 m. This effect is attributed to the oxide that was formed along the grain boundaries in the film. Within the effected region of the film, the Al and the
Fig. 9. Depth profile of the oxidised Ti 0.21 Al 0.79 N film, measured by EPMA with an information depth of 150 nm for O and N, 130 nm for Al and 100 nm for Ti w22x.
The friction coefficient of the ŽTi,Al.N coatings was determined by Pin on Disk measurements. After a short period at the beginning of the experiments, during which was high due to the surface roughness of the as-deposited coatings, the friction coefficient had values of approximately 0.5. The curve of showed spikes throughout the measurement in case of the single phase Ti 0.21 Al 0.79 N film. They are attributed to local sticking of the pin to the coating during the experiment. With the Ti 0.04 Al 0.96 N layer which consisted of two phases, no sticking occurred and remained steady during the time of the experiment.
4. Conclusions Metastable Ti 1y x Al x N films have been deposited by PECVD form chloridic precursors. Depending on the Al content of the feed gas and the voltage of the pulsed glow discharge, x varied from 0.02 to 0.96. Up to compositions of Ti 0.09 Al 0.91 N the films exhibited the cubic structure of TiN with a strong 1004 texture and no sign of a second, hexagonal phase was found. The lattice parameter decreased with increasing Al content according to Vegard’s law. Films with an AlN content higher than 90% content consisted of two phases. With increasing Al content the morphology of the films changed from a columnar to a fine grained microstructure with a growth direction perpendicular to the interface. No growth direction was detectable in the twophase layers. The microhardness of the homogeneous cubic films increased with increasing Al content, which is attributed to intrinsic stresses and the change in their microstructure. A nanocrystalline state of the two-phase coatings is proposed as the reason for their much higher hardness values of approximately 5000 HV 0.05. The coatings started to decompose between 700 and 750⬚C. While the properties of coatings with low Al
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content deteriorated during the segregation of the second, hexagonal phase, layers with high Al content showed a strong increase in their microhardness, reaching values of the films originally consisting of two phases. The coatings exhibited a good oxidation resistance due to the formation of a 100-nm thick Al-rich surface layer, which prevented a further oxidation of the film. This, and their good tribological behaviour with a friction coefficient of approximately 0.5, makes ŽTi,Al.N coatings deposited by PECVD a promising coating for cutting tools as well as metal forming tools. Further investigations have to be carried out in order to understand why the PECVD technique is capable of depositing single phase ŽTi,Al.N coatings with a higher Al content than in the case of PVD techniques. In addition to that, the decomposition mechanism of the coatings needs further investigation with respect to the different behaviour of coatings with low and high Al content.
Acknowledgements The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft ŽDFG. within the Collaborative Research Centre ŽSFB. 289 ‘Forming of metals in the semi-solid state and their properties’.
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References w1x O. Knotek, M. Bohmer, T. Leyendecker, J. Vac. Sci. Technol. ¨ A 4 Ž1986. 2695᎐2700. w2x O. Knotek, T. Leyendecker, J. Solid State Chem. 70 Ž1987. 318᎐322. w3x A. von Richthofen, R. Cremer, D. Neuschutz, ¨ Mikrochim. Acta 125 Ž1997. 143᎐148. w4x R. Cremer, M. Witthaut, D. Neuschutz, ¨ Experimental determi-
w19x w20x w21x
w22x
nation of the metastable ŽTi,Al.N phase diagram up to 700⬚C, in: W.D. Cho, H.Y. Sohn ŽEds.., Value-Addition Metallurgy, TMS Publications, Warrendale, 1998, pp. 249᎐258. R. Cremer, M. Witthaut, A. von Richthofen, D. Neuschutz, ¨ Fresenius J. Anal. Chem. 361 Ž1998. 642᎐645. D. McIntyre, J.E. Greene, G. Hakansson, J.-E. Sundgren, W.-D. ˚ Munz, ¨ J. Appl. Phys. 67 Ž1990. 1542᎐1553. R. Cremer, M. Witthaut, A. von Richthofen, D. Neuschutz, ¨ Comparison of the oxidation behaviour of cubic and hexagonal metastable Ti 1y x Al x N films, in: I. Olefjord, L. Nyborg, D. Briggs ŽEds.., ECASIA 97, 7 th European Conf. on App. of Surf. and Interface Anal., Goteborg 16᎐20 June 1997, Wiley, Chich¨ ester, 1997, pp. 963᎐966. A. von Richthofen, R. Cremer, M. Witthaut, R. Domnick, D. Neuschutz, ¨ Thin Solid Films 312 Ž1998. 190᎐194. M. Witthaut, R. Cremer, A. von Richthofen, D. Neuschutz, ¨ Fresenius J. Anal. Chem. 361 Ž1998. 639᎐641. G. Hakansson, J.-E. Sundgren, D. McIntyre, J.E. Greene, W.-D. ˚ Munz, ¨ Thin Solid Films 153 Ž1987. 55. J. Musil, H. Hruby, ´ Superhard nanocomposite Ti 1y x Al x N films prepared by magnetron sputtering, in: M. Hrabovsky, ´ M. Konrad, ´ V. Kopecky´ ŽEds.., Proc. of the 14th Int. Symp. on Plasma Chemistry, Institute for Plasma Physics, Prague, Czech Republic, 2᎐6 August 1999, p. 1605. S.H. Lee, B.J. Kim, H.H. Kim, J.J. Lee, J. Appl. Phys. 80 Ž1996. 1469᎐1473. S.H. Lee, J.J. Lee, J. Vac. Sci. Technol. A 13 Ž1995. 2030᎐2034. K.H. Kim, S.H. Lee, Thin Solid Films 283 Ž1996. 165᎐170. K.H. Kim, S.H. Lee, J. Mater. Sci. Lett. 14 Ž1995. 1531᎐1533. A. Leonhardt, K. Bartsch, I. Endler, Surf. Coat. Technol. 76᎐77 Ž1995. 225᎐230. K. Bartsch, A. Leonhardt, U. Langer, K. Kunanz, Surf. Coat. ¨ Technol. 94-94 Ž1997. 168᎐173. C. Jarms., H.-R. Stock, P. Mayr, Surf. Coat. Technol. 108-109 Ž1998. 206᎐210. R. Prange, D. Neuschutz, ¨ J. Phys. IV France 9 Ž1999. Pr8811᎐818. H. Stolten, P. Spencer, D. Neuschutz, ¨ J. Chim. Phys. 90 Ž1993. 209᎐219. JCPDS-Powder Diffraction File, International Center for Diffraction Data, 1601 Park Lane, Swarthmore, Pennsylvania 19081-2389, USA. P. Karduck, Mikrochim. Acta 15 Ž1998. 109.
Plasma-enhanced CVD of ž Ti,Al/ N films from chloridic precursors in a DC glow discharge R. Prange, R. Cremer U , D. Neuschutz ¨ Lehrstuhl fur Rheinisch-Westfalische Technische Hochschule Aachen, D-52056 Aachen, Germany ¨ Theoretische Huttenkunde, ¨ ¨
Abstract Metastable Ti 1yx Al x N films have been deposited from gaseous mixtures of TiCl 4 ᎐AlCl 3 ᎐N2 ᎐H 2 ᎐Ar in a pulsed DC glow discharge at 510⬚C. When the discharge voltage was kept constant, the Al content x of the films increased linearly with the AlCl 3rTiCl 4 ratio in the feed gas. Increasing the discharge voltage also increased the Al content. Up to compositions of Ti 0.09 Al 0.91 N the layers remained single-phase cubic with a strong 1004 texture. Films with a higher Al content consisted of two phases and their cubic phase showed a weak 1114 texture. The lattice parameter of the homogeneous cubic films decreased with increasing Al content in accordance with Vegard’s law. Films with a low Al content exhibited a columnar morphology, while the films with high Al contents had a fine-grained structure. Increasing the discharge voltage also caused the grain size to decrease. The microhardness of the single-phase coatings increased with increasing Al content up to 3947 HV 0.05 for xs 0.83, while the two-phase layers showed hardness values of approximately 5000 HV 0.05. The metastable films began to decompose at temperatures between 750 and 800⬚C, depending on the Al content. The decomposition of the films with an AlrTi ratio below 1 caused the lattice parameter of the cubic phase to increase and the microhardness to decrease. Films with high Al contents did not show any increase in the lattice parameter after annealing and their microhardness strongly increased. Investigation of the oxide layer formed on a Ti 0.21 Al 0.79 N film after annealing in air at 800⬚C showed that an amorphous alumina layer with a thickness of approximately 100 nm was formed on the surface, preventing further oxidation. The films with high Al content exhibited advantageous tribological properties with friction coefficients of 0.5. Thus, they seem to be especially well suited for an application on cutting and metal working tools. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: X-Ray diffraction; PACVD; Aluminium nitride; Titanium nitride
1. Introduction Metastable ŽTi,Al.N films offer superior oxidation and wear resistance in combination with advantageous tribological properties. They have become a standard coating for cutting tools and have been studied for more than 10 years w1᎐20x. They are usually deposited by PVD techniques, namely reactive magnetron sputtering ŽMSIP. and reactive arc ion plating w1᎐11x, but U
Corresponding author. Tel.: q49-241-805995; fax: q49-2418888295. E-mail addresses: [email protected] ŽR. Cremer., n e u s c h u e t z @ l t h .r w t h - a a c h e n .d e Ž D . N e u s c h u ¨tz. [email protected] ŽR. Prange..
attempts to deposit ŽTi,Al.N films by plasma-enhanced CVD ŽPECVD. can also be found in the literature w12᎐19x. The metastable phase diagram for the pseudo-binary system TiN᎐AlN was theoretically investigated by Spencer and co-workers w20x and experimentally confirmed for PVD by Cremer et al. w4x. At deposition temperatures of 500⬚C, it was possible to substitute up to 63 at.% titanium by aluminium in the cubic unit cell of TiN. Films with a higher Al content were two-phase. Above an AlrTi ratio of 3:1 the layers exhibited the hexagonal structure of AlN. With higher deposition temperatures the two-phase region became broader. Although cubic ŽTi,Al.N is a metastable phase, the films did not show any signs of phase separation when
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 4 1 - 5
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annealed at temperatures below 800⬚C in inert atmospheres w6x. Apart from better wear resistance in comparison with TiN, ŽTi,Al.N coatings are of particular interest because of their superior oxidation resistance. They do not show any visible signs of oxidation when annealed in air at temperatures of 800⬚C for 1 h w6,7x. Detailed analysis of the oxide layer with a thickness of only 200᎐400 nm revealed that a thin amorphous alumina layer was formed on the surface of the coatings preventing their further oxidation w6᎐9x. Although most investigations of ŽTi,Al.N have been carried out on films deposited by PVD, several studies of the deposition by PECVD have appeared recently. Lee et al. w12,13x deposited ŽTi,Al.N films from mixtures of TiCl 4 ᎐AlCl 3 ᎐H 2 ᎐NH 3 ᎐Ar in an r.f. plasma. Their films showed the cubic TiN structure with a 1004 texture up to compositions of Ti 0.2 Al 0.8 N. Films with higher Al contents consisted of two phases. Kim et al. w14,15x reported the deposition of ŽTi,Al.N from TiCl 4 ᎐AlCl 3 ᎐H 2 ᎐N2 ᎐Ar mixtures also using an r.f. plasma as excitation method. The films were singlephase cubic with a 1004 texture and had compositions from pure TiN to Ti 0.3 Al 0.7 N. Increasing the AlrTi ratio, however, leads to an increase of the chlorine content of the coatings, deteriorating their properties at AlN contents of 12% and higher. Other groups also used N2 as nitrogen source, but a pulsed DC plasma as excitation method w16᎐19x. They observed an increase of the Al-content of the films when increasing the AlCl 3 content in the feed gas and the plasma power.
2. Experimental ŽTi,Al.N films have been deposited on nitrided hot work tool steel AISI H11 Ž0.38% C, 5.0% Cr, 1.2% Mo, 0.4% V. and ISO K 15 cemented carbide insert tips by means of PECVD in a pulsed DC glow discharge from TiCl 4 ᎐AlCl 3 ᎐H 2 ᎐N2 ᎐Ar gaseous mixtures. The experimental setup is given elsewhere w19x. All gases used were of 99.999% purity except HCl which was of 99.995% purity. The gases were metered and controlled by mass flow controllers with an accuracy of 2%. The TiCl 4 of a purity higher than 99% was evaporated, metered by a flow meter and controlled by a needle valve. The AlCl 3 was generated in situ by flowing HCl over Al chips of a purity higher than 99.99% at a temperature of 500⬚C. After grinding and polishing with 1-m diamond suspension, the specimens were cleaned ultrasonically, rinsed in methanol, dried in a nitrogen jet and placed on the cathode of the pulsed DC plasma source with the bell of the vacuum chamber acting as the anode. The deposition chamber had an auxiliary resistance
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Table 1 Deposition parameters Specimens Temperature Pressure Coating time Discharge voltage Pulse time Pulserpause ratio HrCl input ratio NrŽAl q Ti. input ratio AlrTi input ratio
Hot work tool steel AISI H11 Cemented carbide ISO K 15 510⬚C 1.5 mbar 4h 430᎐530 V 50 s 0.5᎐2.0 10 40 0.4᎐4.0
heater to allow the independent adjustment of temperature and plasma parameters. The deposition parameters are given in Table 1. The composition of the films was examined by electron probe microanalysis ŽEPMA. in a Camebax SX 50 using a 7-keV electron beam, while their morphology was investigated by scanning electron microscopy ŽSEM.. The crystallographic structure was determined by grazing incidence ŽGD. X-ray diffraction ŽXRD. in a Siemens D500 goniometer with GD attachment using the Cu K ␣-line at an incidence angle of 5⬚. Additional texture analyses were carried out in BraggrBrentano geometry. In order to study the thermal stability of the films the goniometer was equipped with a high temperature chamber. The specimens were annealed in an Ar atmosphere with less than 0.1 ppm O 2 for 30 min. After annealing, the diffraction pattern was recorded and the specimen was annealed again at a 50⬚C higher temperature. This procedure was continued until the peaks of the hexagonal phase could be clearly identified. Microhardness measurements were carried out with a Leitz Durimet microhardness tester and the tribological properties were measured with a pin on disc tribometer with a Ck 45 carbon steel pin and the coated specimen forming the disc.
3. Results and discussion 3.1. Chemical composition The layers contained between 0.5 and 2.0 at.% oxygen. Apart from the films deposited at 430 V, the amount of Cl incorporated from the incomplete dissociation of the chloridic precursors was below 5.0 at.%, decreasing strongly with the discharge voltage and the Al content of the films. The deposited films were overstochiometric, with an NrŽTiq Al. ratio between 1.02 and 1.25. An increase of the N content with the discharge voltage was observed, which is attributed to the higher amount and higher energy of nitrogen ions in the plasma at higher discharge voltages.
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Fig. 1. x in Ti 1y x Al x N as a function of Al content of the feed gas and discharge voltage.
In all layers Al substituted Ti leading to films of the composition Ti 1y x Al x N. The value x, which describes the mole fraction of AlN in the film, ranged between 0.02 and 0.96, depending on the AlCl 3rTiCl 4 ratio of the feed gas and the discharge voltage, Fig. 1. At a given discharge voltage, x was a linear function of the AlCl 3rTiCl 4 ratio. Increasing the voltage of the glow discharge ᎏ and hence the power in the plasma ᎏ also led to a substantial increase of the Al content of the coatings: While x was 0.07 in layers deposited at an AlCl 3rTiCl 4 feed gas ratio of 0.4 and a discharge voltage of 480 V, Ti 0.49 Al 0.51 N was deposited at the same AlCl 3rTiCl 4 ratio and a discharge voltage of 530 V. At 430 V, an AlCl 3rTiCl 4 ratio of 1.5 resulted in films of the composition Ti 0.95 Al 0.05 N. 3.2. Morphology Fig. 2 shows the SEM micrographs of the cross-sec-
tions of ŽTi,Al.N films with AlN mole fractions between 2 and 96%. All films had a crack-free and smooth surface and no indications of intergranular porosity or voids were detected. With the exception of the Ti 0.04 Al 0.96 N film, deposited at 530 V and an AlCl 3rTiCl 4 ratio of 3.1, all films showed a growth direction perpendicular to the interface. While the layers with a low Al content had a columnar structure with grains extending from the interface to the surface, coatings with a higher Al content had a rather fine grained morphology. The growth of the crystallites was continuously interrupted by the nucleation of new grains. The transition between columnar and finegrained structure took place between Ti 0.61 Al 0.39 N and Ti 0.48 Al 0.52 N. In addition to that, an increase of the discharge voltage also led to a finer grained structure. The Ti 0.04 Al 0.96 N film, formed at AlCl 3rTiCl 4 s 3.1 and Us 530 V ŽFig. 2, lower right. had a completely different appearance. It exhibited a globular morphology and no clear boundaries between the grains could be observed. Furthermore, no growth direction could be detected. 3.3. Crystallographic structure With the exception of the films deposited at AlCl 3rTiCl 4 feed gas ratios higher than 3.0 and discharge voltages of 530 V, all layers exhibited the cubic structure of TiN and no hexagonal phase could be detected. Fig. 3 shows four spectra of these films. The position of the diffraction peaks shifted continuously towards higher diffraction angles with increasing Al
Fig. 2. SEM micrographs of the cross-sections of several Ti 1y x Al x N films.
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Fig. 3. XRD spectra of four homogeneous cubic Ti 1y x Al x N films.
content. This corresponds to a decrease of the lattice parameter a of the cubic unit cell when Ti atoms are replaced by the smaller Al atoms ŽFig. 4.. The lattice parameter was a function of the AlrTi ratio in the coatings and it decreased according to Vegard’s law. The films deposited at AlCl 3rTiCl 4 ratios of approximately 3 and a discharge voltage of 530 V consisted of two phases, one cubic and the other hexagonal. With ˚ the lattice parameter of the cubic 4.0878" 0.01 A, phase was even lower than the value of cubic AlN tabulated in the JCPDS data w21x, indicating high compressive stresses in this phases. The lattice parameters a and c of the hexagonal phase were 3.1192 and 4.9665 ˚ , respectively. These values are in good agreement A ˚ and c0 s 4.9792 with the JCPDS data Ž a0 s 3.1114 A ˚ .. A In order to investigate the texture of the films, additional XRD spectra were recorded in the BraggrBrentano geometry ŽFig. 5.. They revealed a strong 1004 texture for the single phase cubic films. The intensity of the 2004 peak is much higher than any of the other peaks in the spectra. According to the data for randomly oriented powders recorded in the JCPDS-files w21x, the 1114 and the 2204 peaks should have 72 and 45% of the intensity of the 2004 peak, respectively. The cubic phase in the layers consisting of two phases exhibited only a very weak 1114 orienta-
Fig. 4. Decrease of the lattice parameter a of the homogeneous cubic Ti 1y x Al x N films with the Al content; values for pure TiN and AlN from JCPDS w21x.
Fig. 5. XRD spectra of a single-phase cubic film Ža. and a layer consisting of two phases Žb. recorded in BraggrBrentano geometry.
tion. These findings correspond very well with the observed morphology. While the single phase coatings showed a very ordered structure with grains growing perpendicular to the interface, no growth direction whatsoever could be detected in the films composed of two phases. 3.4. Microhardness The microhardness of the ŽTi,Al.N films is shown in Fig. 6. It increases with the Al content of the coatings. Introducing Al into the cubic structure of TiN causes a distortion of the unit cell leading to higher intrinsic stresses and hence a higher resistivity against plastic deformation. The dependency of the microhardness on the Al content is not linear. While the increase is small up to films of the composition Ti 0.75 Al 0.25 N, the slope of the curve gets steeper with increasing Al content. This behaviour is attributed to the change in the microstructure of the films with increasing Al content. While layers with a low Al content still exhibit the columnar structure of pure TiN, coatings with higher Al contents are fine grained. Studies of the evolution of the morphology and the microhardness of ŽTi,Al.N coatings deposited by PVD revealed a strong influence of the microstructure on the hardness w10x: The de-
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Fig. 6. Microhardness of the Ti 1y x Al x N films as a function of their Al content.
crease in grain size and the change from columnar to fine grained morphology led to a sharp increase in the microhardness, reaching values similar to those measured here. The maximum hardness of the single phase coatings was 3947 HV 0.05 for the Ti 0.17 Al 0.83 N film, while layers composed of two phases exhibited had a much higher hardness at approximately 5000 HV 0.05. A different mechanism than the one described above for the microhardness increase of the single-phase cubic films seems to be responsible for this difference. As is known from the literature that two phase nanocrystalline ŽTi,Al.N coatings can exhibit such high hardness values w11x, a similar explanation is suggested here. The morphology of the two-phase layers was completely different from the single phase ones ŽFig. 2.. The grain size was dramatically decreased and no clear boundary between the crystallites could be identified. Thus, the increase in the microhardness with the change from single-phase to two-phase films is attributed to the different morphology.
that of pure TiN Ž2200 HV 0.05.. This behaviour is well known for ŽTi,Al.N films deposited by PVD. The Ti 0.21 Al 0.79 N film exhibited a completely different behaviour during the experiments. The lattice parameter of the cubic phase did not increase when the hexagonal phase was formed and the microhardness increased from 3842 to 5235 HV 0.05, which is similar to the hardness of the coatings consisting of two phases already after deposition. The reason for this needs further investigation, but it seems that the segregation of the second phase prohibits the relaxation of the lattice of the cubic phase, causing additional intrinsic stresses and leading to a state similar to that of the layers originally consisting of two phases. 3.6. Oxidation beha¨ iour The oxidation behaviour was investigated by annealing of the coatings in air at 800⬚C for 1 h. Fig. 8 shows two SEM micrographs of the cross-section of an oxidised Ti 0.21 Al 0.79 N film. A thin oxide layer of 100 nm thickness was formed on the surface of the film. Although the oxide was able to penetrate the film along the grain boundaries ŽFig. 8, right., no cracks were formed in the film. Fig. 9 gives the results of an EPMA linescan along the flank of a dimple ground into the
3.5. Thermal stability Depending on their Al content, the ŽTi,Al.N films showed a different behaviour during annealing in inert atmosphere ŽFig. 7.. While the Ti 0.61 Al 0.39 N coating started to decompose at 750⬚C, the Ti 0.21 Al 0.79 N film showed the first peaks of the hexagonal phase at 700⬚C. The mechanism of the decomposition also seems to be different in both cases. The segregation of a hexagonal AlN phase was accompanied by a shift of the diffraction peaks of the cubic phase towards lower diffraction angles in case of the Ti 0.61 Al 0.39 N layer. This corresponds to an increase of the lattice parameter of the cubic phase. While the Al content of the cubic phase decreases due to the appearance of the hexagonal phase, the unit cell expands because of the larger amount of Ti atoms in it. The separation of the two phases also causes the microhardness of the coating to decrease from 2926 to 2350 HV 0.05, which is close to
Fig. 7. Evolution of the hexagonal phase in two Ti 1y x Al x N films during annealing in Ar atmosphere.
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Ti content showed a contrary behaviour. At the surface the Al concentration is increased and the Ti content is decreased in comparison to the bulk of the film. Underneath the surface layer an area with increased Ti and decreased Al concentration was found. Therefore it is concluded that the oxide layer is formed by the outward diffusion of Al which causes a drop in the Al content of the underlying zone. As the oxygen concentration of the bulk of the film was not changed after the experiment, it is followed that the oxide layer was able to prevent further oxidation of the film. Furthermore, an XRD analysis of the oxidised coating did not show any sign of TiO 2 phases, which is an additional confirmation of this conclusion. 3.7. Tribological properties
Fig. 8. SEM micrographs of the cross-section of the oxidised Ti 0.21 Al 0.29 N film.
oxidised film. The measured N content of the surface layer is due to the information depth of the EPMA technique. Under the conditions of the measurement, the recorded N signal is generated in a volume of 150 nm thickness w22x. As the thickness of oxide layer is only 100 nm, it can be assumed that the origin of the N signal lies below the oxidised surface of the film. Below the surface oxide layer the oxygen content decreased sharply within the first 400 nm and reached the concentration of the as-deposited film after 1 m. This effect is attributed to the oxide that was formed along the grain boundaries in the film. Within the effected region of the film, the Al and the
Fig. 9. Depth profile of the oxidised Ti 0.21 Al 0.79 N film, measured by EPMA with an information depth of 150 nm for O and N, 130 nm for Al and 100 nm for Ti w22x.
The friction coefficient of the ŽTi,Al.N coatings was determined by Pin on Disk measurements. After a short period at the beginning of the experiments, during which was high due to the surface roughness of the as-deposited coatings, the friction coefficient had values of approximately 0.5. The curve of showed spikes throughout the measurement in case of the single phase Ti 0.21 Al 0.79 N film. They are attributed to local sticking of the pin to the coating during the experiment. With the Ti 0.04 Al 0.96 N layer which consisted of two phases, no sticking occurred and remained steady during the time of the experiment.
4. Conclusions Metastable Ti 1y x Al x N films have been deposited by PECVD form chloridic precursors. Depending on the Al content of the feed gas and the voltage of the pulsed glow discharge, x varied from 0.02 to 0.96. Up to compositions of Ti 0.09 Al 0.91 N the films exhibited the cubic structure of TiN with a strong 1004 texture and no sign of a second, hexagonal phase was found. The lattice parameter decreased with increasing Al content according to Vegard’s law. Films with an AlN content higher than 90% content consisted of two phases. With increasing Al content the morphology of the films changed from a columnar to a fine grained microstructure with a growth direction perpendicular to the interface. No growth direction was detectable in the twophase layers. The microhardness of the homogeneous cubic films increased with increasing Al content, which is attributed to intrinsic stresses and the change in their microstructure. A nanocrystalline state of the two-phase coatings is proposed as the reason for their much higher hardness values of approximately 5000 HV 0.05. The coatings started to decompose between 700 and 750⬚C. While the properties of coatings with low Al
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content deteriorated during the segregation of the second, hexagonal phase, layers with high Al content showed a strong increase in their microhardness, reaching values of the films originally consisting of two phases. The coatings exhibited a good oxidation resistance due to the formation of a 100-nm thick Al-rich surface layer, which prevented a further oxidation of the film. This, and their good tribological behaviour with a friction coefficient of approximately 0.5, makes ŽTi,Al.N coatings deposited by PECVD a promising coating for cutting tools as well as metal forming tools. Further investigations have to be carried out in order to understand why the PECVD technique is capable of depositing single phase ŽTi,Al.N coatings with a higher Al content than in the case of PVD techniques. In addition to that, the decomposition mechanism of the coatings needs further investigation with respect to the different behaviour of coatings with low and high Al content.
Acknowledgements The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft ŽDFG. within the Collaborative Research Centre ŽSFB. 289 ‘Forming of metals in the semi-solid state and their properties’.
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References w1x O. Knotek, M. Bohmer, T. Leyendecker, J. Vac. Sci. Technol. ¨ A 4 Ž1986. 2695᎐2700. w2x O. Knotek, T. Leyendecker, J. Solid State Chem. 70 Ž1987. 318᎐322. w3x A. von Richthofen, R. Cremer, D. Neuschutz, ¨ Mikrochim. Acta 125 Ž1997. 143᎐148. w4x R. Cremer, M. Witthaut, D. Neuschutz, ¨ Experimental determi-
w19x w20x w21x
w22x
nation of the metastable ŽTi,Al.N phase diagram up to 700⬚C, in: W.D. Cho, H.Y. Sohn ŽEds.., Value-Addition Metallurgy, TMS Publications, Warrendale, 1998, pp. 249᎐258. R. Cremer, M. Witthaut, A. von Richthofen, D. Neuschutz, ¨ Fresenius J. Anal. Chem. 361 Ž1998. 642᎐645. D. McIntyre, J.E. Greene, G. Hakansson, J.-E. Sundgren, W.-D. ˚ Munz, ¨ J. Appl. Phys. 67 Ž1990. 1542᎐1553. R. Cremer, M. Witthaut, A. von Richthofen, D. Neuschutz, ¨ Comparison of the oxidation behaviour of cubic and hexagonal metastable Ti 1y x Al x N films, in: I. Olefjord, L. Nyborg, D. Briggs ŽEds.., ECASIA 97, 7 th European Conf. on App. of Surf. and Interface Anal., Goteborg 16᎐20 June 1997, Wiley, Chich¨ ester, 1997, pp. 963᎐966. A. von Richthofen, R. Cremer, M. Witthaut, R. Domnick, D. Neuschutz, ¨ Thin Solid Films 312 Ž1998. 190᎐194. M. Witthaut, R. Cremer, A. von Richthofen, D. Neuschutz, ¨ Fresenius J. Anal. Chem. 361 Ž1998. 639᎐641. G. Hakansson, J.-E. Sundgren, D. McIntyre, J.E. Greene, W.-D. ˚ Munz, ¨ Thin Solid Films 153 Ž1987. 55. J. Musil, H. Hruby, ´ Superhard nanocomposite Ti 1y x Al x N films prepared by magnetron sputtering, in: M. Hrabovsky, ´ M. Konrad, ´ V. Kopecky´ ŽEds.., Proc. of the 14th Int. Symp. on Plasma Chemistry, Institute for Plasma Physics, Prague, Czech Republic, 2᎐6 August 1999, p. 1605. S.H. Lee, B.J. Kim, H.H. Kim, J.J. Lee, J. Appl. Phys. 80 Ž1996. 1469᎐1473. S.H. Lee, J.J. Lee, J. Vac. Sci. Technol. A 13 Ž1995. 2030᎐2034. K.H. Kim, S.H. Lee, Thin Solid Films 283 Ž1996. 165᎐170. K.H. Kim, S.H. Lee, J. Mater. Sci. Lett. 14 Ž1995. 1531᎐1533. A. Leonhardt, K. Bartsch, I. Endler, Surf. Coat. Technol. 76᎐77 Ž1995. 225᎐230. K. Bartsch, A. Leonhardt, U. Langer, K. Kunanz, Surf. Coat. ¨ Technol. 94-94 Ž1997. 168᎐173. C. Jarms., H.-R. Stock, P. Mayr, Surf. Coat. Technol. 108-109 Ž1998. 206᎐210. R. Prange, D. Neuschutz, ¨ J. Phys. IV France 9 Ž1999. Pr8811᎐818. H. Stolten, P. Spencer, D. Neuschutz, ¨ J. Chim. Phys. 90 Ž1993. 209᎐219. JCPDS-Powder Diffraction File, International Center for Diffraction Data, 1601 Park Lane, Swarthmore, Pennsylvania 19081-2389, USA. P. Karduck, Mikrochim. Acta 15 Ž1998. 109.