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A study of the corrosion behavior of TiN films
Materials Science and Engineering, 69 (1985) 89-93
89
A Study of the Corrosion Behavior of TiN Films* A. ERDEMIR, W. B. CARTER and R. F. HOCHMAN School of Chemical Engineering, Metallurgy Program, Georgia Institute of Technology, Atlanta, GA 30332-0100 (U.S.A.) E. I. MELETIS Metal Science and Technology, Illinois Institute of Technology, Chicago, IL 60616 (U.S.A.) (Received September 17, 1984)
ABSTRACT General corrosion characteristics of TiN films produced by three different techniques were compared. One technique was the sputter deposition of a titanium film followed by implantation with N + ions to a fluence of 4 X 1017 ions cm -2 at an acceleration voltage o f 50 keV. A post-implantation vacuum anneal was performed to yield stable TiN. Sputter deposition and ion plating were the alternative techniques used to produce TiN films. The substrates were fiat discs of M50 steel. The chemical and structural composition o f the films were characterized by X-ray diffraction, electron spectroscopy for chemical analysis and scanning electron microscopy. Corrosion testing was carried out potentiodynamically in deaerated 1 N H2S04 aqueous solution at 25 °C. Test results indicate that dense ion-plated TiN films have a passive corrosion current density as much as five orders o f magnitude less than other coatings.
1. INTRODUCTION The production of thin layers of TiN on the surfaces of various engineering components by a variety of techniques has received considerable attention in the past few years [13 ]. This interest in TiN m a y be attributed to a number of exceptional properties that this material possesses (i.e. high hardness, good wear and corrosion resistance, chemical *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30
stability and an attractive gold-like color [4, 5]). Metal cutting and forming tools in particular are being coated routinely with TiN. Its production by a variety of deposition techniques is well documented in the literature [6-8]. The test results of a majority of these studies differ. This can be attributed to the wide variations in structure and composition achieved because of the different deposition techniques used. It is well known that thin films produced by physical vapor deposition fall into one of three distinct zone morphologies depending on the deposition temperature [9], the surrounding gas pressure [10] and the voltage applied to the substrate [11]. If the deposition is carried out in a reactive atmosphere to produce c o m p o u n d films, non-stoichiometric coatings generally result. For example, although TiN films generally have a gold-like appearance, the morphology and chemistry may differ substantially from film to film. The present investigation was undertaken to examine the general corrosion behavior of thin TiN films produced by a variety of techniques and in particular to evaluate the effects of various film morphologies on the corrosion behavior.
2. EXPERIMENTAL DETAILS TiN films (designated as film 1, film 2 and film 3) were produced on finely polished surfaces of M50 bearing steel by the following methods. (1) In the first method, a titanium film (about 1 pm thick) was sputter deposited onto the substrate at 260 °C by utilizing a d.c. © Elsevier Sequoia/Printed in The Netherlands
90 magnetron source in an argon atmosphere. It was then implanted with N + ions to a dose of 4 × 1017 ions cm -2 at an acceleration voltage of 50 keV. In the final step, it was vacuum annealed in a vacuum of 5 × 10 -6 Torr at 600 °C for 30 min. In similar research, Armigliato et al. [12] were able to produce a TiN layer as thick as 810 A. In their research the titanium film was implanted with N ÷ ions to a fluence of 3.4 × 1017 ions cm -2 at an acceleration voltage of 50 keV. Post-implantation vacuum annealing was performed at 600 °C for 15 min. The implantation and vacuum-annealing conditions used in the present research were expected to result in a thicker layer of dense TiN (approximately 0.1 pm). X-ray diffraction of the film revealed a stable TiN phase with a strong peak from (111) planes. Electron spectroscopy for chemical analysis of the film before and after sputter removal of a 300 A layer verified t h a t titanium was present as TiN (verified by the Ti 2p3/2 peak at approximately 455.7 eV [13]). (2) Film 2 was produced by sputter deposition of TiN in the same system as film 1. In addition to argon, nitrogen gas was also bled into the deposition chamber during coating. The film thickness was about 1 pm, and the deposition was performed at 260 °C. (3) Film 3 was produced by ion plating in an activated plasma consisting of argon and nitrogen gases at a substrate temperature of 500 °C. The film thickness was of the order of 1 pm. The chemical and morphological states of the films were characterized by X-ray diffraction, electron spectroscopy for chemical analysis and scanning electron microscopy. Corrosion testing was potentiodynamically carried out in deaerated 1 N H2SO4 aqueous solution at 25 °C.
3. RESULTS AND DISCUSSION
All films had the characteristic gold-like color of TiN prior to corrosion testing. Film 1 exhibited a densely packed columnar morphology. At a high magnification (Fig. l(a)) it was observed that this film contained a number of small pores. The origin of these pores is under investigation and at this time no speculation will be made. Figure l(b) is a scanning electron micrograph of the same film after
Fig. 1. Scanning electron micrographs of film 1 (a) before and (b) after testing in an aqueous 1 N H2SO 4 solution.
corrosion testing. It is evident that the TiN film was selectively dissolved and that some film detachment occurred. Film 2 displayed a columnar morphology with a smooth surface as shown in Fig. 2(a). The scanning electron micrograph in Fig. 2(b) revealed that the film was selectively removed during corrosion testing. Film 3 had a dense nearly equiaxed morphology as evident in Fig. 3(a). After corrosion testing, no indication of selective attack or film dissolution was detected in this film as shown in Fig. 3(b).
91
Fig. 2. Scanning electron micrographs of film 2 (a) before and (b) after testing in an aqueous 1 N H2SO4 solution.
Figure 4 shows the p o t e n t i o d y n a m i c polarization curves for each TiN film in aqueous 1 N H2SO4 solution. F r o m these observations, it is clear t h a t thin TiN layers with different morphologies and thicknesses may exhibit marked variation in their response to corrosive environments. F o r example, the dissolution o f film 1 and film 2 was most pr obabl y due to their co lu m na r m o r p h o l o g y . Bunshah [14] has r ecen tly concluded that, in pure metal deposits with c ol um nar morphologies, grain boundaries are easily dissolved in a
Fig. 3. Scanning electron micrographs of film 3 (a) before and (b) after testing in an aqueous 1 N H2SO4 solution.
n u m b e r of corrosive environments. T he corrosion behavior of film 1 is partly attributed to the surface defects shown in Fig. l ( a ) and insufficient u n i f o r m i t y of the TiN. Poor adhesion of b o t h films appears to facilitate the dissolution process during corrosion testing. An exceptionally low passivation current density for film 3 correlates well with the equiaxed and dense nature of this film. No sign of selective dissolution of film 3 was observed (Fig. 3{b)). Only the scratch marks
92
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Fig. 4. Potentiodynamic polarization curves for specimens tested in an aqueous 1 N H 2 SO 4 solution (the voltage was measured with respect to a saturated calomel electrode (SCE)): , film 3 ; - - , film 2 ; - - - , film 1; . . . . . M50 steel.
in the underlying substrate surface were visible.
4. CONCLUSIONS
From the results of this study the following conclusions can be drawn. (1) Titanium films implanted with N + ions can be heat treated to produce essentially pure TiN. (2) Film morphology is important to the subsequent properties of the TiN film. Columnar films in general, including those which had been N + ion implanted, have poor corrosion resistance. (3) Well-formed TiN films (in this case, those produced by reactive ion plating) have excellent corrosion resistance in the solution studied. (4) This work shows how the film characteristics control the nature of the TiN resistance to corrosion. Thus, researchers must develop controlled surface modification processes and determine the factors to be controlled.
(5) It should be possible to use ion implantation as well as ion beam mixing to obtain TiN with corrosion resistance as good as that formed by the ion-plating process used in this study. Control and understanding of the process variables is all important.
ACKNOWLEDGMENT
This work was supported by National Aeronautics and Space Administration Contract NAS 8-35048 and NIDR Training Grant DE07054. REFERENCES 1 A. K. Suri, R, Nimmagadda and R. F. Bunshah, Thin Solid Films, 64 (1979) 191. 2 A. Matthews and D. G. Teer, Thin Solid Films, 72 (1980) 541. 3 W. D. Mfinz, D. Hofmann and K. Hartig, Thin Solid Films, 96 (1982} 79. 4 P. Agarwal, P. Nath, H. J. Doerr, R. F. Bunshah, G. Kuhlman and A. J. Koury, Thin Solid Films, 83 (1981) 37. 5 S0 Bair, S. Ramalingam and W. O. Winer, Wear, 60 (1980) 413.
93 6 W. Schintlmeister, W. Wallgram and J. Kanz, Thin Solid Films, 107 (1983) 117. 7 K. Nakamura, K. Inagawa, K. Tsuruoka and S. Komiya, Thin Solid Films, 40 (1977) 155. 8 T. Sato, M. Tada, Y. C. Huang and H. Takei, Thin Solid Films, 54 (1978) 61. 9 B. A. Movchan and A. V. Demchishin, Phys. Met. Metallogr. (Engl. Transl.), 28 (1969) 83. 10 J. A. Thornton, J. Vac. Sci. Technol., 12 (1975) 830.
11 M. Lardon, R. Buhl, H. Signer, H. K. Pulker and E. Moll, Thin Solid Films, 54 (1978) 317. 12 A. Armigliato, G. Celotti, A. Garulli, S. Guerri, P. Ostoja and C. Summonte, Le Vide, 218 (1983) 401. 13 D. Briggs and M. P. Seah (eds.), Practial Surface Analysis, Wiley, New York, 1983. 14 R. F. Bunshah, in R. F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Noyes, Park Ridge, NJ, 1982, p. 88.
89
A Study of the Corrosion Behavior of TiN Films* A. ERDEMIR, W. B. CARTER and R. F. HOCHMAN School of Chemical Engineering, Metallurgy Program, Georgia Institute of Technology, Atlanta, GA 30332-0100 (U.S.A.) E. I. MELETIS Metal Science and Technology, Illinois Institute of Technology, Chicago, IL 60616 (U.S.A.) (Received September 17, 1984)
ABSTRACT General corrosion characteristics of TiN films produced by three different techniques were compared. One technique was the sputter deposition of a titanium film followed by implantation with N + ions to a fluence of 4 X 1017 ions cm -2 at an acceleration voltage o f 50 keV. A post-implantation vacuum anneal was performed to yield stable TiN. Sputter deposition and ion plating were the alternative techniques used to produce TiN films. The substrates were fiat discs of M50 steel. The chemical and structural composition o f the films were characterized by X-ray diffraction, electron spectroscopy for chemical analysis and scanning electron microscopy. Corrosion testing was carried out potentiodynamically in deaerated 1 N H2S04 aqueous solution at 25 °C. Test results indicate that dense ion-plated TiN films have a passive corrosion current density as much as five orders o f magnitude less than other coatings.
1. INTRODUCTION The production of thin layers of TiN on the surfaces of various engineering components by a variety of techniques has received considerable attention in the past few years [13 ]. This interest in TiN m a y be attributed to a number of exceptional properties that this material possesses (i.e. high hardness, good wear and corrosion resistance, chemical *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30
stability and an attractive gold-like color [4, 5]). Metal cutting and forming tools in particular are being coated routinely with TiN. Its production by a variety of deposition techniques is well documented in the literature [6-8]. The test results of a majority of these studies differ. This can be attributed to the wide variations in structure and composition achieved because of the different deposition techniques used. It is well known that thin films produced by physical vapor deposition fall into one of three distinct zone morphologies depending on the deposition temperature [9], the surrounding gas pressure [10] and the voltage applied to the substrate [11]. If the deposition is carried out in a reactive atmosphere to produce c o m p o u n d films, non-stoichiometric coatings generally result. For example, although TiN films generally have a gold-like appearance, the morphology and chemistry may differ substantially from film to film. The present investigation was undertaken to examine the general corrosion behavior of thin TiN films produced by a variety of techniques and in particular to evaluate the effects of various film morphologies on the corrosion behavior.
2. EXPERIMENTAL DETAILS TiN films (designated as film 1, film 2 and film 3) were produced on finely polished surfaces of M50 bearing steel by the following methods. (1) In the first method, a titanium film (about 1 pm thick) was sputter deposited onto the substrate at 260 °C by utilizing a d.c. © Elsevier Sequoia/Printed in The Netherlands
90 magnetron source in an argon atmosphere. It was then implanted with N + ions to a dose of 4 × 1017 ions cm -2 at an acceleration voltage of 50 keV. In the final step, it was vacuum annealed in a vacuum of 5 × 10 -6 Torr at 600 °C for 30 min. In similar research, Armigliato et al. [12] were able to produce a TiN layer as thick as 810 A. In their research the titanium film was implanted with N ÷ ions to a fluence of 3.4 × 1017 ions cm -2 at an acceleration voltage of 50 keV. Post-implantation vacuum annealing was performed at 600 °C for 15 min. The implantation and vacuum-annealing conditions used in the present research were expected to result in a thicker layer of dense TiN (approximately 0.1 pm). X-ray diffraction of the film revealed a stable TiN phase with a strong peak from (111) planes. Electron spectroscopy for chemical analysis of the film before and after sputter removal of a 300 A layer verified t h a t titanium was present as TiN (verified by the Ti 2p3/2 peak at approximately 455.7 eV [13]). (2) Film 2 was produced by sputter deposition of TiN in the same system as film 1. In addition to argon, nitrogen gas was also bled into the deposition chamber during coating. The film thickness was about 1 pm, and the deposition was performed at 260 °C. (3) Film 3 was produced by ion plating in an activated plasma consisting of argon and nitrogen gases at a substrate temperature of 500 °C. The film thickness was of the order of 1 pm. The chemical and morphological states of the films were characterized by X-ray diffraction, electron spectroscopy for chemical analysis and scanning electron microscopy. Corrosion testing was potentiodynamically carried out in deaerated 1 N H2SO4 aqueous solution at 25 °C.
3. RESULTS AND DISCUSSION
All films had the characteristic gold-like color of TiN prior to corrosion testing. Film 1 exhibited a densely packed columnar morphology. At a high magnification (Fig. l(a)) it was observed that this film contained a number of small pores. The origin of these pores is under investigation and at this time no speculation will be made. Figure l(b) is a scanning electron micrograph of the same film after
Fig. 1. Scanning electron micrographs of film 1 (a) before and (b) after testing in an aqueous 1 N H2SO 4 solution.
corrosion testing. It is evident that the TiN film was selectively dissolved and that some film detachment occurred. Film 2 displayed a columnar morphology with a smooth surface as shown in Fig. 2(a). The scanning electron micrograph in Fig. 2(b) revealed that the film was selectively removed during corrosion testing. Film 3 had a dense nearly equiaxed morphology as evident in Fig. 3(a). After corrosion testing, no indication of selective attack or film dissolution was detected in this film as shown in Fig. 3(b).
91
Fig. 2. Scanning electron micrographs of film 2 (a) before and (b) after testing in an aqueous 1 N H2SO4 solution.
Figure 4 shows the p o t e n t i o d y n a m i c polarization curves for each TiN film in aqueous 1 N H2SO4 solution. F r o m these observations, it is clear t h a t thin TiN layers with different morphologies and thicknesses may exhibit marked variation in their response to corrosive environments. F o r example, the dissolution o f film 1 and film 2 was most pr obabl y due to their co lu m na r m o r p h o l o g y . Bunshah [14] has r ecen tly concluded that, in pure metal deposits with c ol um nar morphologies, grain boundaries are easily dissolved in a
Fig. 3. Scanning electron micrographs of film 3 (a) before and (b) after testing in an aqueous 1 N H2SO4 solution.
n u m b e r of corrosive environments. T he corrosion behavior of film 1 is partly attributed to the surface defects shown in Fig. l ( a ) and insufficient u n i f o r m i t y of the TiN. Poor adhesion of b o t h films appears to facilitate the dissolution process during corrosion testing. An exceptionally low passivation current density for film 3 correlates well with the equiaxed and dense nature of this film. No sign of selective dissolution of film 3 was observed (Fig. 3{b)). Only the scratch marks
92
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164
CURRENT
DENSITY
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163
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Fig. 4. Potentiodynamic polarization curves for specimens tested in an aqueous 1 N H 2 SO 4 solution (the voltage was measured with respect to a saturated calomel electrode (SCE)): , film 3 ; - - , film 2 ; - - - , film 1; . . . . . M50 steel.
in the underlying substrate surface were visible.
4. CONCLUSIONS
From the results of this study the following conclusions can be drawn. (1) Titanium films implanted with N + ions can be heat treated to produce essentially pure TiN. (2) Film morphology is important to the subsequent properties of the TiN film. Columnar films in general, including those which had been N + ion implanted, have poor corrosion resistance. (3) Well-formed TiN films (in this case, those produced by reactive ion plating) have excellent corrosion resistance in the solution studied. (4) This work shows how the film characteristics control the nature of the TiN resistance to corrosion. Thus, researchers must develop controlled surface modification processes and determine the factors to be controlled.
(5) It should be possible to use ion implantation as well as ion beam mixing to obtain TiN with corrosion resistance as good as that formed by the ion-plating process used in this study. Control and understanding of the process variables is all important.
ACKNOWLEDGMENT
This work was supported by National Aeronautics and Space Administration Contract NAS 8-35048 and NIDR Training Grant DE07054. REFERENCES 1 A. K. Suri, R, Nimmagadda and R. F. Bunshah, Thin Solid Films, 64 (1979) 191. 2 A. Matthews and D. G. Teer, Thin Solid Films, 72 (1980) 541. 3 W. D. Mfinz, D. Hofmann and K. Hartig, Thin Solid Films, 96 (1982} 79. 4 P. Agarwal, P. Nath, H. J. Doerr, R. F. Bunshah, G. Kuhlman and A. J. Koury, Thin Solid Films, 83 (1981) 37. 5 S0 Bair, S. Ramalingam and W. O. Winer, Wear, 60 (1980) 413.
93 6 W. Schintlmeister, W. Wallgram and J. Kanz, Thin Solid Films, 107 (1983) 117. 7 K. Nakamura, K. Inagawa, K. Tsuruoka and S. Komiya, Thin Solid Films, 40 (1977) 155. 8 T. Sato, M. Tada, Y. C. Huang and H. Takei, Thin Solid Films, 54 (1978) 61. 9 B. A. Movchan and A. V. Demchishin, Phys. Met. Metallogr. (Engl. Transl.), 28 (1969) 83. 10 J. A. Thornton, J. Vac. Sci. Technol., 12 (1975) 830.
11 M. Lardon, R. Buhl, H. Signer, H. K. Pulker and E. Moll, Thin Solid Films, 54 (1978) 317. 12 A. Armigliato, G. Celotti, A. Garulli, S. Guerri, P. Ostoja and C. Summonte, Le Vide, 218 (1983) 401. 13 D. Briggs and M. P. Seah (eds.), Practial Surface Analysis, Wiley, New York, 1983. 14 R. F. Bunshah, in R. F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Noyes, Park Ridge, NJ, 1982, p. 88.