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Characterization of (Ti,Al)N films deposited by pulsed d.c. plasma-enhanced chemical vapor deposition
Thin Solid Films 386 Ž2001. 271᎐275
Characterization of ŽTi,Al. N films deposited by pulsed d.c. plasma-enhanced chemical vapor deposition Kazuki Kawataa,b,U , Hiroyuki Sugimuraa , Osamu Takai a a
Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya Uni¨ ersity, Furo-chou, Chikusa-ku, Nagoya 464-8603, Japan b Oriental Engineering Company Ltd., Research De¨ elopment Di¨ ision, 2-8-49, Yoshinodai Kawagoe-shi, Saitama 350-0833, Japan Received 17 June 1999; received in revised form 1 May 2000; accepted 1 May 2000
Abstract Ti 0.58 Al 0.42 NŽupper.rTiNŽlower. double-layered films were prepared on steel substrates by pulsed d.c. plasma-enhanced chemical vapor deposition ŽPECVD. at 823 K using gas mixtures of TiCl 4 , AlCl 3 , N2 , H 2 and Ar. We evaluated structural, compositional, mechanical and chemical properties of the films. Glow discharge optical emission spectroscopy ŽGDOS. analysis revealed that the chlorine concentration was much lower in the upper-layered Ti 0.58 Al 0.42 N film than in the lower-layered TiN film. X-Ray diffraction ŽXRD. analysis showed that the upper-layered Ti 0.58 Al 0.42 N film had the same NaCl structure as TiN. The double-layered films had high oxidation resistance, a low friction coefficient and high wear resistance. Furthermore, the double-layered films demonstrated the superior soldering and corrosion resistance in a molten aluminum alloy at 953 K. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ŽTi,Al.N; PECVD; Characterization
1. Introduction Many papers have reported the preparation of TiN, TiC and TiCN films by physical vapor deposition ŽPVD., chemical vapor deposition ŽCVD., and plasma-enhanced chemical vapor deposition ŽPECVD., and their applications to cutting tools and metal moulds w1᎐6x. These films, however, have poor oxidation resistance. Recently, ŽTi,Al.N films with superior oxidation resistance have been developed and investigated w7᎐11x. At present, ŽTi,Al.N films deposited by PVD such as sputtering w7,8x and cathodic arc ion plating w9᎐11x are applied to cutting tools because they have high wear and oxidation resistance. However, the application of the PVD processes to metal moulds of complicated
U
Corresponding author Tel.: q81-492255811; fax: q81-492252325. E-mail address: [email protected] ŽK. Kawata..
geometry is difficult because the throwing power Ži.e. the ability to coat irregularly shaped objects. of PVD is less than that of PECVD. On the other hand, ŽTi,Al.N films deposited by PECVD are applicable to metal moulds of complicated geometry such as aluminum die casting dies and aluminum extrusion dies because the PECVD process has good throwing power at low temperatures w12᎐15x. We can supply a Ti-source gas and an Al-source gas individually in the PECVD process, and easily produce multi-layer films or compositionally graded films. Lee et al. w16,17x have reported the influence of deposition conditions on the composition, structure, surface morphology, hardness and adhesion of ŽTi 1y X Al X .N coatings deposited by r.f. PECVD. However, they have not investigated oxidation and wear resistance and corrosion resistance in a molten aluminum alloy. The study on oxidation and wear resistance, and corrosion resistance in a molten aluminum
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 7 2 - 2
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alloy of ŽTi 1y X Al X .N coatings was necessary from an industrial application’s point of view. This paper reports on the preparation of ŽTi,Al.NŽupper.rTiNŽlower. double-layered films by pulsed d.c. PECVD, and their tribological behavior. This includes their oxidation resistance in air and corrosion resistance in a molten aluminum alloy. In this case, the lower TiN layer was used to improve the adhesion between a substrate and the upper ŽTi, Al.N layer. 2. Experimental Fig. 1 shows a schematic diagram of the pulsed d.c. PECVD apparatus used. The pulsed d.c. PECVD apparatus consisted of a reaction chamber, an external heater, a vacuum system, a rotation system of a working table, a pulsed d.c. power supply, a gas supply system and a computer control system. The effective dimensions of workpieces treated in this apparatus are 460 mm in diameter and 800 mm in height. The deposition conditions of TiN Žlower layer. and ŽTi,Al.N Župper layer. films are shown in Table 1. The specimens Žsubstrates . used were JIS ŽJapanese Industrial Standards. SKD61 Žhot working tool steel. and JIS SKH51 Žhigh speed steel.. The SKD61 specimens were used for hardness measurements, compositional analysis, oxidation tests and corrosion tests, and the SKH51 specimens were used for friction and wear tests. They were heat-treated in a vacuum furnace before the deposition. Their Vickers hardness ŽHv. was 450 for SKD61 and 750 for SKH51. The size of the SKD61 specimens was 20 mm in diameter and 5 mm in thickness for hardness measurement, compositional analysis and oxidation tests, and was 10 mm in diameter and 100 mm in length for corrosion tests. The size of the SKH51 specimens was 20 mm in diameter and 5 mm in thickness. Nitrocarburized specimens Žcompound layer thickness, 9 m; diffusion layer thickness, 0.15 mm., were
Fig. 1. Schematic diagram of the pulsed d.c. PECVD apparatus.
Table 1 Deposition conditions of ŽTi,Al.NrTiN double-layered films TiN Žlower layer. Thickness Žm. Deposition temperature ŽK. Deposition time Žmin. Pressure ŽPa. Discharge voltage ŽV. Frequency ŽkHz. Duty cycle Ž%. Flow rate of gas Žcm3 rmin. N2 Ar H2 TiCl4 AlCl3
ŽTi,Al.N Župper layer.
2.5 823
1.0 823
150 270 700 20 10᎐50
120 270 700 20 10᎐50
500 100 2000 60 0
500 100 2000 20 80
prepared by gas nitrocarburizing process. TiN singlelayered films Ž3 m in thickness . were prepared by hollow cathode discharge ion plating ŽPVD process.. TiN single-layered films Ž4 m in thickness . were also prepared by the pulsed d.c. PECVD. These were used to compare the ŽTi,Al.NrTiN double-layered films prepared by the pulsed d.c. PECVD. The material of these specimens was SKD61. The ŽTi,Al.NrTiN double-layered films were analyzed by scanning electron microscopy ŽSEM., energydispersive X-ray spectroscopy ŽEDX., glow discharge optical emission spectroscopy ŽGDOS. and X-ray diffraction ŽXRD.. Hardness, oxidation resistance in air for 1 h and corrosion resistance at 953 K in a molten JIS AD.C.12 aluminum die casting alloy were also investigated. Friction coefficients and wear volumes were measured with a ball-on-disk type tribometer. In this case, surface roughness of all disk samples was unified to be lower than R a s 0.01 m, R y s 0.1 m by lapping with diamond paste. The testing conditions were as follows: ball, JIS SUJ2, 6 mm in diameter, 815 Hv; load, 10 N; sliding speed, 400 mmrs; sliding distance, 500 m; relative humidity, 60%; temperature, 298 K; unlubricated conditions. In the corrosion test, after the specimens were immersed into the molten aluminum alloy for 2 h, they were taken out from it. Subsequently, they were cleaned in a NaOH alkaline solution until most of the soldered aluminum alloy was leached out from the specimen surface. The mass gain of the specimen was measured with an electric balance and a microstructure analysis was also performed. The procedure of dipping into the molten aluminum alloy and removing soldered aluminum alloy was repeated three times. The immersion was carried out intermittently and the specimens were immersed for 6 h in total. For the specimens
K. Kawata et al. r Thin Solid Films 386 (2001) 271᎐275
Fig. 2. Cross-sectional SEM image of a ŽTi,Al.NrTiN double-layered film deposited by the pulsed d.c. PECVD on a SKD61 substrate.
soldered with the aluminum alloy and the specimens from which the aluminum alloy was removed, surfaces and cross-sections of the specimens were analyzed by optical microscopy ŽOMS., SEM and EDX. 3. Results and discussion 3.1. Surface and cross-sectional morphology Fig. 2 shows a cross-sectional SEM image of a ŽTi,Al.NrTiN double-layered film deposited by the pulsed d.c. PECVD on a SKD61 substrate. The film had a columnar structure and was composed of two layers of a lower TiN film, and an upper ŽTi,Al.N film. The thickness of the lower TiN film was 2.5 m and that of the upper ŽTi,Al.N film was 1.0 m, and the total thickness of the double-layered film was 3.5 m. The surface observation by SEM demonstrated that the double-layered film contained no defects such as pinholes and droplets. Therefore, we can prepare defectfree ŽTi,Al.N films by the present pulsed d.c. PECVD process. This is the advantage of pulsed d.c. PECVD compared to PVD processes. 3.2. Hardness
273
Fig. 3. GDOS concentration depth profile of the Ti 0.58 Al 0.42 NrTiN double-layered film deposited by the pulsed d.c. PECVD on a SKD61 substrate.
Consequently, the chemical formula of the ŽTi,Al.N films prepared in this experiment can be written as Ti 0.58 Al 0.42 N. The composition of the lower TiN film was Tis 49.23 at.%, Ns 49.23 at.% and Cl s 1.54 at.%. Fig. 3 shows GDOS concentration depth profiles for each element for the Ti 0.58 Al 0.42 NrTiN double-layered film. It is confirmed from the concentration distributions of Ti and Al that the film is composed of two layers. The chlorine concentration was much lower in the Ti 0.58 Al 0.42 N film than in the TiN film. There was no concentration of chlorine at the interface of the film and the substrate. Chlorine was distributed uniformly in the respective films. Fig. 4 shows the XRD pattern of the Ti 0.58 Al 0.42 NrTiN double-layered film. Because an incident angle was set at 0.5⬚ in this XRD analysis, this diffraction pattern corresponds to the diffraction at the upper Ti 0.58 Al 0.42 N film. This Ti 0.58 Al 0.42 N film has the same NaCl structure as TiN. The lattice parameter for the upper-layered Ti 0.58 Al 0.42 N film was 0.420 nm, which was smaller than that of TiN. The Ti 0.58 Al 0.42 N film showed no preferred orientation. The TiN films prepared by the pulsed d.c. PECVD method had a preferred orientation of 200 and the lattice parameter was 0.427 nm. Whereas the TiN films
The hardness of the ŽTi,Al.NrTiN double-layered films, the TiN films prepared by PECVD and the TiN films prepared by PVD was 2400, 2300 and 1920 Hv, respectively. The load was 0.1 N. The hardness of the ŽTi,Al.NrTiN double-layered films is within the range of the reported hardness values of 1750᎐2900 Hv, of the ŽTi 1y X Al X .N films obtained by PVD w7,9,10x or PECVD w16x. 3.3. Compositional and structural analysis The composition of the upper ŽTi,Al.N film determined by the EDX analysis was Tis 28.65 at.%, Al s 21.10 at.%, Ns 49.75 at.% and Cl s 0.50 at.%.
Fig. 4. XRD pattern of the Ti 0.58 Al 0.42 NrTiN double-layered film.
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of the Ti 0.58 Al 0.42 NrTiN film is due to the formation of a stable amorphous aluminum oxide layer on top of the film, which prevents the film from further oxidation w7x. 3.6. Corrosion properties
Fig. 5. The relationship between friction coefficient and sliding distance for the Ti 0.58 Al 0.42 NrTiN double-layered film, the TiN film and the uncoated SKH51 specimen Žball, SUJ2; load, 10 N; sliding speed, 400 mmrs; relative humidity, 60%; temperature, 298 K; unlubricated condition..
prepared by PVD had a preferred orientation of 111, and the lattice parameter was 0.427 nm. 3.4. Tribological properties Fig. 5 shows the relationship between the friction coefficient and the sliding distance for the Ti 0.58 Al 0.42 NrTiN double-layered film, the TiN film prepared by the pulsed d.c. PECVD, and uncoated SKH51 specimen, respectively. The value of the friction coefficient for the Ti 0.58 Al 0.42 NrTiN double-layered film is 0.47᎐0.55, and this value is the lowest of all the specimens tested in this investigation. The wear volumes for a disk coated by a Ti 0.58 Al 0.42 NrTiN film and a ball had the lowest value for all the specimens. Thus, the Ti 0.58 Al 0.42 NrTiN double-layered film exhibited better tribological properties under frictional conditions such as high load and high sliding speed for a steel ball. It is considered that the low friction coefficient and high wear resistance of the Ti 0.58 Al 0.42 NrTiN double-layered film is due to the suppression of the attachment of the film to the ball because the film is exceptionally hard, and resistant to oxidation which will be described in the next section. In this case, it is considered that high oxidation resistance of the film improves wear resistance by restraining formation of a porous and soft titanium oxide layer on top of the film due to frictional heat.
A dipping test was carried out to evaluate the corrosion resistance of variously surface-treated and uncoated SKD61 specimens. In this test, all specimens were dipped into a molten ADC12 aluminum alloy. Fig. 6 shows the relationship between the immersion time and the corrosion loss for variously surface-treated specimens. The corrosion loss increases in the following order: Ti 0.58 Al 0.42 NrTiN-coated specimen; TiN-coated ŽPECVD. specimen; TiN-coated ŽPVD. specimen; gas nitrocarburized specimen and uncoated specimen. The corrosion loss for the uncoated specimen increases linearly with immersion time. The loss for the gas nitrocarburized specimen is small up to 4 h. However, the loss increases after 4 h at the same corrosion speed as the uncoated specimen. The TiN film prepared by the PECVD has better corrosion resistance than that prepared by PVD. The Ti 0.58 Al 0.42 NrTiN-coated specimen shows almost no corrosion loss and the best corrosion resistance. The surfaces of the specimens from which the soldered aluminum alloy was removed after the 6-h dipping test were investigated with OMS and SEM. For the uncoated and gas nitrocarburized specimens, all the dipped parts of the specimens were corroded severely. For TiN-coated ŽPVD. specimen, the film remained on the substrate, although many corrosion pits were observed. For the TiN-coated ŽPECVD. specimen, a similar corrosion pits were observed, although the
3.5. Oxidation properties W e m e a su re d th e m a ss ga in s of th e Ti 0.58 Al 0.42 NrTiN film, and the TiN film in the oxidation test. Oxidation of the TiN film commenced at 873 K, and the mass gain increased steeply with oxidation temperature reaching 3.2 mgrcm2 at 1073 K. On the other hand, the mass gain of the Ti 0.58 Al 0.42 NrTiN film was zero at 1073 K! The high oxidation resistance
Fig. 6. The relationship between immersion time and corrosion loss for various surface-treated specimens in a molten ADC12 aluminum alloy Žtemperature of molten aluminum alloy, 953 K..
K. Kawata et al. r Thin Solid Films 386 (2001) 271᎐275
number of the pits was small. On the contrary, for the Ti 0.58 Al 0.42 NrTiN-coated specimen, the pitting corrosion was scarcely observed. Next, the surfaces of the specimens from which soldered aluminum alloy was not removed after the 6-h dipping test were investigated. For the uncoated and gas nitrocarburized specimens, a large amount of soldered aluminum alloy was observed on the surfaces. On the other hand, the other coated specimens had much less soldered aluminum alloy on the surface. In particular, the soldered aluminum alloy remained least on the Ti 0.58 Al 0.42 NrTiN-coated specimen. Consequently the Ti 0.58 Al 0.42 NrTiN-coated specimen demonstrated the best soldering resistance. The cross-sectional structure of the specimens from which soldered aluminum alloy was not removed after the 6-h dipping test was analyzed using SEM and EDX. For the uncoated specimen, three layers and a soldered aluminum alloy layer were observed throughout the cross-sectional area. EDX analysis indicated that these three layers consisted of intermetallic compounds ŽAlSi-Fe. with varying composition. For the gas nitrocarburized specimen, the same intermetallic compound layers as the uncoated specimen were observed. For the TiN-coated ŽPVD. specimen, the TiN film and pitting corrosion were observed. At the corroded part, the same intermetallic compound layers as the uncoated specimen were observed. For the TiN-coated ŽPECVD. specimen, the same result as the TiN-coated ŽPVD. specimen was obtained. On the other hand, for the Ti 0.58 Al 0.42 NrTiN-coated specimen, no corrosion area was detected in its cross-section. Therefore, the Ti 0.58 Al 0.42 NrTiN-coated specimen kept its initial appearance and demonstrated the best corrosion resistance amongst all the specimens. Its superior corrosion resistance is attributed to few pinholes on the films, low reactivity with a molten
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aluminum alloy, and exceptional resistance to oxidation at high temperatures. 4. Conclusions The Ti 0.58 Al 0.42 NrTiN double-layered films prepared by pulsed d.c. PECVD show exceptional hardness Ž2400 Hv., a low friction coefficient Ž0.47᎐0.55., resistance to oxidation in air Žno oxidation at 1073 K., and excellent soldering and corrosion resistance in a molten aluminum alloy at 953 K. Therefore, these films are widely applicable to various moulds, mechanical parts, cutting tools, etc. The PECVD method has the potential to coat any type of specimens of complicated geometry. References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x
M. Podob, Met. Progr., 121 Ž1982. 50. P.A. Dearnley, Surf. Eng. 1 Ž1985. 43. J.C. Knight, T.F. Page, I.M. Hutchings, Surf. Eng. 5 Ž1989. 213. F. Hohl, ¨ H.-R. Stock, P. Mayr, Surf. Coat. Technol. 54r55 Ž1992. 160. K. Kawata, Surf. Coat. Technol. 54r55 Ž1992. 604. K.-T. Rie, A. Gebauer, J. Wohl, C. Blawit, Surf. ¨ H.K. Tonshoff, ¨ Coat. Technol. 74-75 Ž1995. 375. W.-D. Munz, ¨ J. Vac. Sci. Technol. A4 Ž1986. 2717. I. Efeoglu, R.D. Arnell, S.F. Tinston, D.G. Teer, Surf. Coat. Technol. 57 Ž1993. 117. T. Ikeda, H. Satoh, Thin Solid Films 195 Ž1991. 99. O. Knotec, H.-G. Prengel, Surf. Modif. Technol. 4 Ž1991. 507. B.F. Coll, P. Sathrum, R. Fontana, J.P. Peyre, D. Duchateau, M. Benmalek, Surf. Coat. Technol. 52 Ž1992. 57. K. Kawata, Met. Press 28 Ž4. Ž1996. 24 ŽŽin Japanese... K. Kawata, Special Steel 45 Ž9. Ž1996. 26 ŽŽin Japanese... K. Kawata, Sokeizai 38 Ž7. Ž1997. 15 ŽŽin Japanese... K. Kawata, Die Mould Technol. 14 Ž5. Ž1999. 65 Ž) Žin Japanese... S.-H. Lee, H.-J. Ryoo, J.-J. Lee, J. Vac. Sci. Technol. A12 Ž1994. 1602. S.-H. Lee, J.-J. Lee, J. Vac. Sci. Technol. A13 Ž1995. 2030.
Characterization of ŽTi,Al. N films deposited by pulsed d.c. plasma-enhanced chemical vapor deposition Kazuki Kawataa,b,U , Hiroyuki Sugimuraa , Osamu Takai a a
Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya Uni¨ ersity, Furo-chou, Chikusa-ku, Nagoya 464-8603, Japan b Oriental Engineering Company Ltd., Research De¨ elopment Di¨ ision, 2-8-49, Yoshinodai Kawagoe-shi, Saitama 350-0833, Japan Received 17 June 1999; received in revised form 1 May 2000; accepted 1 May 2000
Abstract Ti 0.58 Al 0.42 NŽupper.rTiNŽlower. double-layered films were prepared on steel substrates by pulsed d.c. plasma-enhanced chemical vapor deposition ŽPECVD. at 823 K using gas mixtures of TiCl 4 , AlCl 3 , N2 , H 2 and Ar. We evaluated structural, compositional, mechanical and chemical properties of the films. Glow discharge optical emission spectroscopy ŽGDOS. analysis revealed that the chlorine concentration was much lower in the upper-layered Ti 0.58 Al 0.42 N film than in the lower-layered TiN film. X-Ray diffraction ŽXRD. analysis showed that the upper-layered Ti 0.58 Al 0.42 N film had the same NaCl structure as TiN. The double-layered films had high oxidation resistance, a low friction coefficient and high wear resistance. Furthermore, the double-layered films demonstrated the superior soldering and corrosion resistance in a molten aluminum alloy at 953 K. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ŽTi,Al.N; PECVD; Characterization
1. Introduction Many papers have reported the preparation of TiN, TiC and TiCN films by physical vapor deposition ŽPVD., chemical vapor deposition ŽCVD., and plasma-enhanced chemical vapor deposition ŽPECVD., and their applications to cutting tools and metal moulds w1᎐6x. These films, however, have poor oxidation resistance. Recently, ŽTi,Al.N films with superior oxidation resistance have been developed and investigated w7᎐11x. At present, ŽTi,Al.N films deposited by PVD such as sputtering w7,8x and cathodic arc ion plating w9᎐11x are applied to cutting tools because they have high wear and oxidation resistance. However, the application of the PVD processes to metal moulds of complicated
U
Corresponding author Tel.: q81-492255811; fax: q81-492252325. E-mail address: [email protected] ŽK. Kawata..
geometry is difficult because the throwing power Ži.e. the ability to coat irregularly shaped objects. of PVD is less than that of PECVD. On the other hand, ŽTi,Al.N films deposited by PECVD are applicable to metal moulds of complicated geometry such as aluminum die casting dies and aluminum extrusion dies because the PECVD process has good throwing power at low temperatures w12᎐15x. We can supply a Ti-source gas and an Al-source gas individually in the PECVD process, and easily produce multi-layer films or compositionally graded films. Lee et al. w16,17x have reported the influence of deposition conditions on the composition, structure, surface morphology, hardness and adhesion of ŽTi 1y X Al X .N coatings deposited by r.f. PECVD. However, they have not investigated oxidation and wear resistance and corrosion resistance in a molten aluminum alloy. The study on oxidation and wear resistance, and corrosion resistance in a molten aluminum
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 7 2 - 2
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alloy of ŽTi 1y X Al X .N coatings was necessary from an industrial application’s point of view. This paper reports on the preparation of ŽTi,Al.NŽupper.rTiNŽlower. double-layered films by pulsed d.c. PECVD, and their tribological behavior. This includes their oxidation resistance in air and corrosion resistance in a molten aluminum alloy. In this case, the lower TiN layer was used to improve the adhesion between a substrate and the upper ŽTi, Al.N layer. 2. Experimental Fig. 1 shows a schematic diagram of the pulsed d.c. PECVD apparatus used. The pulsed d.c. PECVD apparatus consisted of a reaction chamber, an external heater, a vacuum system, a rotation system of a working table, a pulsed d.c. power supply, a gas supply system and a computer control system. The effective dimensions of workpieces treated in this apparatus are 460 mm in diameter and 800 mm in height. The deposition conditions of TiN Žlower layer. and ŽTi,Al.N Župper layer. films are shown in Table 1. The specimens Žsubstrates . used were JIS ŽJapanese Industrial Standards. SKD61 Žhot working tool steel. and JIS SKH51 Žhigh speed steel.. The SKD61 specimens were used for hardness measurements, compositional analysis, oxidation tests and corrosion tests, and the SKH51 specimens were used for friction and wear tests. They were heat-treated in a vacuum furnace before the deposition. Their Vickers hardness ŽHv. was 450 for SKD61 and 750 for SKH51. The size of the SKD61 specimens was 20 mm in diameter and 5 mm in thickness for hardness measurement, compositional analysis and oxidation tests, and was 10 mm in diameter and 100 mm in length for corrosion tests. The size of the SKH51 specimens was 20 mm in diameter and 5 mm in thickness. Nitrocarburized specimens Žcompound layer thickness, 9 m; diffusion layer thickness, 0.15 mm., were
Fig. 1. Schematic diagram of the pulsed d.c. PECVD apparatus.
Table 1 Deposition conditions of ŽTi,Al.NrTiN double-layered films TiN Žlower layer. Thickness Žm. Deposition temperature ŽK. Deposition time Žmin. Pressure ŽPa. Discharge voltage ŽV. Frequency ŽkHz. Duty cycle Ž%. Flow rate of gas Žcm3 rmin. N2 Ar H2 TiCl4 AlCl3
ŽTi,Al.N Župper layer.
2.5 823
1.0 823
150 270 700 20 10᎐50
120 270 700 20 10᎐50
500 100 2000 60 0
500 100 2000 20 80
prepared by gas nitrocarburizing process. TiN singlelayered films Ž3 m in thickness . were prepared by hollow cathode discharge ion plating ŽPVD process.. TiN single-layered films Ž4 m in thickness . were also prepared by the pulsed d.c. PECVD. These were used to compare the ŽTi,Al.NrTiN double-layered films prepared by the pulsed d.c. PECVD. The material of these specimens was SKD61. The ŽTi,Al.NrTiN double-layered films were analyzed by scanning electron microscopy ŽSEM., energydispersive X-ray spectroscopy ŽEDX., glow discharge optical emission spectroscopy ŽGDOS. and X-ray diffraction ŽXRD.. Hardness, oxidation resistance in air for 1 h and corrosion resistance at 953 K in a molten JIS AD.C.12 aluminum die casting alloy were also investigated. Friction coefficients and wear volumes were measured with a ball-on-disk type tribometer. In this case, surface roughness of all disk samples was unified to be lower than R a s 0.01 m, R y s 0.1 m by lapping with diamond paste. The testing conditions were as follows: ball, JIS SUJ2, 6 mm in diameter, 815 Hv; load, 10 N; sliding speed, 400 mmrs; sliding distance, 500 m; relative humidity, 60%; temperature, 298 K; unlubricated conditions. In the corrosion test, after the specimens were immersed into the molten aluminum alloy for 2 h, they were taken out from it. Subsequently, they were cleaned in a NaOH alkaline solution until most of the soldered aluminum alloy was leached out from the specimen surface. The mass gain of the specimen was measured with an electric balance and a microstructure analysis was also performed. The procedure of dipping into the molten aluminum alloy and removing soldered aluminum alloy was repeated three times. The immersion was carried out intermittently and the specimens were immersed for 6 h in total. For the specimens
K. Kawata et al. r Thin Solid Films 386 (2001) 271᎐275
Fig. 2. Cross-sectional SEM image of a ŽTi,Al.NrTiN double-layered film deposited by the pulsed d.c. PECVD on a SKD61 substrate.
soldered with the aluminum alloy and the specimens from which the aluminum alloy was removed, surfaces and cross-sections of the specimens were analyzed by optical microscopy ŽOMS., SEM and EDX. 3. Results and discussion 3.1. Surface and cross-sectional morphology Fig. 2 shows a cross-sectional SEM image of a ŽTi,Al.NrTiN double-layered film deposited by the pulsed d.c. PECVD on a SKD61 substrate. The film had a columnar structure and was composed of two layers of a lower TiN film, and an upper ŽTi,Al.N film. The thickness of the lower TiN film was 2.5 m and that of the upper ŽTi,Al.N film was 1.0 m, and the total thickness of the double-layered film was 3.5 m. The surface observation by SEM demonstrated that the double-layered film contained no defects such as pinholes and droplets. Therefore, we can prepare defectfree ŽTi,Al.N films by the present pulsed d.c. PECVD process. This is the advantage of pulsed d.c. PECVD compared to PVD processes. 3.2. Hardness
273
Fig. 3. GDOS concentration depth profile of the Ti 0.58 Al 0.42 NrTiN double-layered film deposited by the pulsed d.c. PECVD on a SKD61 substrate.
Consequently, the chemical formula of the ŽTi,Al.N films prepared in this experiment can be written as Ti 0.58 Al 0.42 N. The composition of the lower TiN film was Tis 49.23 at.%, Ns 49.23 at.% and Cl s 1.54 at.%. Fig. 3 shows GDOS concentration depth profiles for each element for the Ti 0.58 Al 0.42 NrTiN double-layered film. It is confirmed from the concentration distributions of Ti and Al that the film is composed of two layers. The chlorine concentration was much lower in the Ti 0.58 Al 0.42 N film than in the TiN film. There was no concentration of chlorine at the interface of the film and the substrate. Chlorine was distributed uniformly in the respective films. Fig. 4 shows the XRD pattern of the Ti 0.58 Al 0.42 NrTiN double-layered film. Because an incident angle was set at 0.5⬚ in this XRD analysis, this diffraction pattern corresponds to the diffraction at the upper Ti 0.58 Al 0.42 N film. This Ti 0.58 Al 0.42 N film has the same NaCl structure as TiN. The lattice parameter for the upper-layered Ti 0.58 Al 0.42 N film was 0.420 nm, which was smaller than that of TiN. The Ti 0.58 Al 0.42 N film showed no preferred orientation. The TiN films prepared by the pulsed d.c. PECVD method had a preferred orientation of 200 and the lattice parameter was 0.427 nm. Whereas the TiN films
The hardness of the ŽTi,Al.NrTiN double-layered films, the TiN films prepared by PECVD and the TiN films prepared by PVD was 2400, 2300 and 1920 Hv, respectively. The load was 0.1 N. The hardness of the ŽTi,Al.NrTiN double-layered films is within the range of the reported hardness values of 1750᎐2900 Hv, of the ŽTi 1y X Al X .N films obtained by PVD w7,9,10x or PECVD w16x. 3.3. Compositional and structural analysis The composition of the upper ŽTi,Al.N film determined by the EDX analysis was Tis 28.65 at.%, Al s 21.10 at.%, Ns 49.75 at.% and Cl s 0.50 at.%.
Fig. 4. XRD pattern of the Ti 0.58 Al 0.42 NrTiN double-layered film.
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of the Ti 0.58 Al 0.42 NrTiN film is due to the formation of a stable amorphous aluminum oxide layer on top of the film, which prevents the film from further oxidation w7x. 3.6. Corrosion properties
Fig. 5. The relationship between friction coefficient and sliding distance for the Ti 0.58 Al 0.42 NrTiN double-layered film, the TiN film and the uncoated SKH51 specimen Žball, SUJ2; load, 10 N; sliding speed, 400 mmrs; relative humidity, 60%; temperature, 298 K; unlubricated condition..
prepared by PVD had a preferred orientation of 111, and the lattice parameter was 0.427 nm. 3.4. Tribological properties Fig. 5 shows the relationship between the friction coefficient and the sliding distance for the Ti 0.58 Al 0.42 NrTiN double-layered film, the TiN film prepared by the pulsed d.c. PECVD, and uncoated SKH51 specimen, respectively. The value of the friction coefficient for the Ti 0.58 Al 0.42 NrTiN double-layered film is 0.47᎐0.55, and this value is the lowest of all the specimens tested in this investigation. The wear volumes for a disk coated by a Ti 0.58 Al 0.42 NrTiN film and a ball had the lowest value for all the specimens. Thus, the Ti 0.58 Al 0.42 NrTiN double-layered film exhibited better tribological properties under frictional conditions such as high load and high sliding speed for a steel ball. It is considered that the low friction coefficient and high wear resistance of the Ti 0.58 Al 0.42 NrTiN double-layered film is due to the suppression of the attachment of the film to the ball because the film is exceptionally hard, and resistant to oxidation which will be described in the next section. In this case, it is considered that high oxidation resistance of the film improves wear resistance by restraining formation of a porous and soft titanium oxide layer on top of the film due to frictional heat.
A dipping test was carried out to evaluate the corrosion resistance of variously surface-treated and uncoated SKD61 specimens. In this test, all specimens were dipped into a molten ADC12 aluminum alloy. Fig. 6 shows the relationship between the immersion time and the corrosion loss for variously surface-treated specimens. The corrosion loss increases in the following order: Ti 0.58 Al 0.42 NrTiN-coated specimen; TiN-coated ŽPECVD. specimen; TiN-coated ŽPVD. specimen; gas nitrocarburized specimen and uncoated specimen. The corrosion loss for the uncoated specimen increases linearly with immersion time. The loss for the gas nitrocarburized specimen is small up to 4 h. However, the loss increases after 4 h at the same corrosion speed as the uncoated specimen. The TiN film prepared by the PECVD has better corrosion resistance than that prepared by PVD. The Ti 0.58 Al 0.42 NrTiN-coated specimen shows almost no corrosion loss and the best corrosion resistance. The surfaces of the specimens from which the soldered aluminum alloy was removed after the 6-h dipping test were investigated with OMS and SEM. For the uncoated and gas nitrocarburized specimens, all the dipped parts of the specimens were corroded severely. For TiN-coated ŽPVD. specimen, the film remained on the substrate, although many corrosion pits were observed. For the TiN-coated ŽPECVD. specimen, a similar corrosion pits were observed, although the
3.5. Oxidation properties W e m e a su re d th e m a ss ga in s of th e Ti 0.58 Al 0.42 NrTiN film, and the TiN film in the oxidation test. Oxidation of the TiN film commenced at 873 K, and the mass gain increased steeply with oxidation temperature reaching 3.2 mgrcm2 at 1073 K. On the other hand, the mass gain of the Ti 0.58 Al 0.42 NrTiN film was zero at 1073 K! The high oxidation resistance
Fig. 6. The relationship between immersion time and corrosion loss for various surface-treated specimens in a molten ADC12 aluminum alloy Žtemperature of molten aluminum alloy, 953 K..
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number of the pits was small. On the contrary, for the Ti 0.58 Al 0.42 NrTiN-coated specimen, the pitting corrosion was scarcely observed. Next, the surfaces of the specimens from which soldered aluminum alloy was not removed after the 6-h dipping test were investigated. For the uncoated and gas nitrocarburized specimens, a large amount of soldered aluminum alloy was observed on the surfaces. On the other hand, the other coated specimens had much less soldered aluminum alloy on the surface. In particular, the soldered aluminum alloy remained least on the Ti 0.58 Al 0.42 NrTiN-coated specimen. Consequently the Ti 0.58 Al 0.42 NrTiN-coated specimen demonstrated the best soldering resistance. The cross-sectional structure of the specimens from which soldered aluminum alloy was not removed after the 6-h dipping test was analyzed using SEM and EDX. For the uncoated specimen, three layers and a soldered aluminum alloy layer were observed throughout the cross-sectional area. EDX analysis indicated that these three layers consisted of intermetallic compounds ŽAlSi-Fe. with varying composition. For the gas nitrocarburized specimen, the same intermetallic compound layers as the uncoated specimen were observed. For the TiN-coated ŽPVD. specimen, the TiN film and pitting corrosion were observed. At the corroded part, the same intermetallic compound layers as the uncoated specimen were observed. For the TiN-coated ŽPECVD. specimen, the same result as the TiN-coated ŽPVD. specimen was obtained. On the other hand, for the Ti 0.58 Al 0.42 NrTiN-coated specimen, no corrosion area was detected in its cross-section. Therefore, the Ti 0.58 Al 0.42 NrTiN-coated specimen kept its initial appearance and demonstrated the best corrosion resistance amongst all the specimens. Its superior corrosion resistance is attributed to few pinholes on the films, low reactivity with a molten
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aluminum alloy, and exceptional resistance to oxidation at high temperatures. 4. Conclusions The Ti 0.58 Al 0.42 NrTiN double-layered films prepared by pulsed d.c. PECVD show exceptional hardness Ž2400 Hv., a low friction coefficient Ž0.47᎐0.55., resistance to oxidation in air Žno oxidation at 1073 K., and excellent soldering and corrosion resistance in a molten aluminum alloy at 953 K. Therefore, these films are widely applicable to various moulds, mechanical parts, cutting tools, etc. The PECVD method has the potential to coat any type of specimens of complicated geometry. References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x
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