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Oxidation behavior of sputtered Ni–Cr–Al–Ti nanocrystalline coating
Surface and Coatings Technology 123 (2000) 92–96 www.elsevier.nl/locate/surfcoat
Oxidation behavior of sputtered Ni–Cr–Al–Ti nanocrystalline coating G. Chen *, H. Lou State Key Lab for Corrosion and Protection, Institute of Corrosion and Protection of Metals, Academia Sinica, Shenyang 110015, People’s Republic of China Received 22 June 1999; accepted in revised form 6 August 1999
Abstract Magnetron sputter deposition was used to prepare Ni–9.7Cr–5.5Al–7Ti (wt%) nanocrystalline coatings on the substrates of the same material. The oxidation behavior of the cast Ni–Cr–Al–Ti alloy and its sputtered coating at 1000°C in air was investigated. An a-Al O scale with TiO dispersions was formed on the sputtered coating after 200 h of oxidation; even so good 2 3 2 oxidation resistance was exhibited. The formation of TiO was attributed to the high ratio of Ti/Al in the coating. The high 2 content of Cr and the precipitation of Ni (AlTi) in the sputtered coating during oxidation could restrict the formation and growth 3 of TiO . Therefore, good oxidation resistance was shown by the sputtered Ni–Cr–Al–Ti coating despite the formation of TiO . 2 2 © 2000 Elsevier Science S.A. All rights reserved. Keywords: Magnetron sputtering; Nanocrystalline coating; Ni–Cr–Al–Ti; Oxidation
1. Introduction Ni-base superalloys are widely used as turbine blade material for advanced engines that are required to operate at elevated temperatures and greatly depend on the presence of Al or Cr to form a protective oxide scale of alumina or chromina. However, a certain amount of Ti is contained in superalloys. The existence of Ti in superalloys can impair the oxidation resistance by forming TiO , which can destroy the continuity of the 2 protective oxide scale [1,2]. Recently, nanocrystalline coatings prepared by the magnetron sputtering technique have attracted much attention. The nanocrystalline coatings on the substrates of the same materials such as Ni-based superalloys K38G, K17F and LDZ125 showed excellent oxidation resistance by developing a unitary layer of Al O [3–5]. TiO was not formed on 2 3 2 these coated superalloys; therefore, no evidence can be used to evaluate the effect of TiO on the oxidation 2 resistance of these sputtered coatings. Nanocrystallization can promote the selective oxidation of Al due to the low free energy of formation of Al O [6 ]. It 2 3 has been suggested that the free energies of formation
of TiO and Al O are almost the same [7]. Therefore, 2 2 3 TiO and Al O would have almost the same propensity 2 2 3 to form. The results that no TiO was formed may be 2 due to the low ratio of Ti/Al in the sputtered superalloys coating. Because TiO could be formed when the ratio 2 of Ti/Al was increased slightly, this can be evidenced by the studies of the rehealing ability of sputtered K17F and K38G coatings [8,9]. However, TiO could not be 2 found on the oxide scale. The results suggested that TiO was well dispersed in the oxide scale. It is important 2 to study the oxidation behavior of the sputtered nanocrystalline coating when TiO can be formed and 2 observed on the coating so as to evaluate the effect of TiO on the oxidation resistance of the sputtered coating. 2 In this paper, based upon the contents of Cr, Al and Ti in the superalloy LDZ125, by increasing the content of Ti in order to promote the formation of TiO , a simple 2 Ni–Cr–Al–Ti alloy was developed. The oxidation resistance and oxide formation of this alloy and its sputtered Ni–Cr–Al–Ti nanocrystalline coating have been investigated.
2. Experimental * Corresponding author. Tel.: +86-24-23915908; fax: +86-24-238941149. E-mail address: [email protected] (G. Chen)
The Ni–Cr–Al–Ti alloy used in this study was prepared by vacuum induction melting and casting using
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 47 0 - 3
G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
93
Table 1 Chemical compositions of the Ni–Cr–Al–Ti and LDZ 125 alloys (wt%)
NiCrAlTi LDZ 125
Ni
Cr
Al
Ti
Co
W
Ta
Mo
Bal. Bal.
9.7 9.47
5.5 5.2
7 3.8
– 10.38
– 7.42
– 4
– 2
pure Ni, Cr, Al and Ti (99.9%) as raw metals. The nominal chemical compositions of Ni–Cr–Al–Ti and LDZ 125 superalloy are listed in Table 1. The ingot was cut into 20×10×3 mm specimens. The specimens were ground to 600-SiC paper, peened and ultrasonically cleaned in ethanol. The magnetron sputter technique was used to prepare nanocrystalline coating of Ni–Cr– Al–Ti on the substrates of the same alloy under the following sputtering parameters: Ar working pressure 0.2 Pa, power 2 kW, substrate temperature 250°C. The compositions of the sputtered coating were the same as the substrates, and the thickness of the coating was about 50 mm. Oxidation tests on the specimens were conducted at 1000°C in air for up to 200 h. The specimens were placed in alumina crucibles, oxidized at 1000°C and then cooled to room temperature at regular intervals of 20 h for mass measurements. The sensitivity of the balance used in the study was 10−4 g. The as-sputtered Ni–Cr–Al–Ti coating was examined by atomic force microscopy (AFM ). After the oxidation tests, specimens were examined by X-ray diffraction with CuKa ( XRD), scanning electron microscopy (SEM ) and energy dispersive analysis by X-ray ( EDAX ).
3. Results 3.1. Microstructure An AFM image of the as-sputtered Ni–Cr–Al–Ti coating is shown in Fig. 1. Based on the AFM morphology, it can be seen that the average grain size of the
Fig. 1. AFM image of the as-sputtered Ni–Cr–Al–Ti coating.
Fig. 2. Micrograph of cast Ni–Cr–Al–Ti alloy.
sputtered Ni–Cr–Al–Ti coating was several tens of nanometres. Fig. 2 shows a SEM micrograph of the cast Ni– Cr–Al–Ti alloy. The average grain size of the cast alloy was found to be several micrometres, which was larger than that of the sputtered coating by two orders of magnitude. 3.2. Oxidation kinetics The oxidation kinetics of the cast Ni–Cr–Al–Ti and its sputtered coating are illustrated in Fig. 3. For comparison, the oxidation kinetics of the cast LDZ125 alloy and its sputtered coating from Ref. [5] are also given in Fig. 3. After the sharp increase in mass gains at the initial oxidation stage, the kinetic curve of the sputtered Ni–Cr–Al–Ti coating showed an extensive period of very slow mass gain, which can be attributed to the rapid formation of a protective oxide scale. However, the kinetic curve of the cast Ni–Cr–Al–Ti alloy showed this propensity after 40 h of oxidation, which is later than the sputtered coating. Although the protective oxide scale of the cast alloy formed later than the sputtered coating, both showed good oxidation resistance. As compared with the sputtered LDZ125 coating, the sputtered Ni–Cr–Al–Ti coating shows a slightly
Fig. 3. Oxidation kinetics of cast Ni–Cr–Al–Ti, LDZ 125 alloys and their sputtered coatings.
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Fig. 4. XRD patterns of cast Ni–Cr–Al–Ti alloy and the sputtered coating after 200 h of oxidation.
higher mass gain, which is due to the much higher mass gain of the sputtered Ni–Cr–Al–Ti coating in the initial oxidation period.
scale can be seen on the sputtered coating. Besides the a-Al O scale, a particle of TiO is also observed. It can 2 3 2 also be seen that TiO did not directly connect with the 2 coating below. Fig. 6 shows the surface morphology and the crosssectional microstructure of the cast Ni–Cr–Al–Ti alloy after 200 h of oxidation. A duplex oxide scale was formed on the cast alloy. The outer layer was composed of Cr O , NiCr O and TiO , and the inner layer was 2 3 2 4 2 mainly a-Al O . 2 3 Fig. 7 shows micrograghs of TiO after 1, 20 and 2 200 h of oxidation. It can be observed that the growth of TiO was very fast during the early stage of oxidation. 2 With increasing oxidation time, TiO grew slowly. This 2 inferred that the formation of TiO only contributed to 2 the rapid mass gain in the initial oxidation period and showed little influence on the kinetics of the sputtered coating in the following oxidation period.
4. Discussion 3.3. XRD analysis Fig. 4 shows the XRD patterns of oxidation products for the cast Ni–Cr–Al–Ti alloy and sputtered coating after 200 h of oxidation. It can be seen that both a-Al O and TiO were formed on the sputtered coating; 2 3 2 the latter resulted from the high content of Ti that promoted the formation of TiO . Compared with the 2 oxide compositions on the sputtered coating, more complex oxides such as a-Al O , TiO , Cr O and NiCr O 2 3 2 2 3 2 4 were formed on the cast alloy. 3.4. Morphologies Fig. 5 illustrates the surface morphology and the cross-sectional microstructure of the sputtered Ni–Cr– Al–Ti coating after 200 h of oxidation. Some TiO 2 particles dispersed on the oxide scale formed on the sputtered coating. From the cross-sectional microstructure, a curved, dense, complete and adherent a-Al O 2 3
The formation of TiO can impair the oxidation 2 resistance of the cast alloy in that TiO can destroy the 2 continuity of the formed oxide scale [1]. It has been indicated that the formation and growth of TiO were 2 directly related to the presence of TiC in the superalloys [2]. The lack of formation of TiO on the coated K38G, 2 K17F, LDZ125 superalloys was probably attributed to the absence of TiC in the sputtered superalloys coatings. The present results indicated that TiO was formed on 2 the cast Ni–Cr–Al–Ti alloy and its sputtered coating. No TiC existed in the Ni–Cr–Al–Ti alloy; therefore, the harmful effect of TiO was not apparent for the cast 2 Ni–Cr–Al–Ti alloy. This was due to the formation of a complete Al O subscale preceded by the formation of 2 3 transient oxides such as Cr O , TiO and NiCr O . The 2 3 2 2 4 high ratio of Ti/Al contributed to the formation of TiO on the sputtered Ni–Cr–Al–Ti coating. The appear2 ance of TiO on the protective coating may contribute 2 to the acceleration of mass gain [10]. However, in
Fig. 5. (a) Surface morphology and (b) the cross-sectional microstructure of the sputtered Ni–Cr–Al–Ti coating after 200 h of oxidation.
G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
95
Fig. 6. Surface morphology (a) and the cross-sectional microstructure (b) of the cast Ni–Cr–Al–Ti alloy after 200 h of oxidation.
Fig. 7. Micrographs of TiO after (a) 1 h, (b) 20 h, (c) 200 h of oxidation. 2
comparison with the sputtered LDZ125 superalloy coating on which a unitary Al O scale was formed, the 2 3 mass gain of sputtered Ni–Cr–Al–Ti coating on which TiO and Al O both formed was not seriously influ2 2 3 enced, indicating good oxidation resistance. The ratio of Ti/Al in the sputtered Ni–Cr–Al–Ti coating was larger than 1, and the free energies of formation of TiO and Al O were similar. However, a 2 2 3 layer of Al O in which small TiO particles were 2 3 2 dispersed was formed on the sputtered Ni–Cr–Al–Ti coating. It was suggested that the high content of Cr could restrict the formation of TiO [11]. In addition, 2 Cr could promote the formation of Al O [12]. 2 3
Therefore, the selective oxidation of Al was more predominant than Ti, and an Al O scale with dispersed 2 3 TiO particles was formed on the sputtered coating. 2 Besides the effect of Cr, the precipitation of Ni (AlTi) also restricted the formation of TiO . Fig. 8 3 2 shows the XRD spectra for the sputtered Ni–Cr–Al–Ti coating after different periods of oxidation, in which the peaks of the c phase were emphasized, whereas the peaks of any oxides were not obvious. Only the c phase was detected in the as-sputtered Ni–Cr–Al–Ti coating. Ni (AlTi) was precipitated during the oxidation process 3 due to the supersaturation state of Al and Ti in the c phase. According to the phase diagram for Ni–Cr–Ti
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G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
generated, and then these stresses may be relieved by plastic deformation of the oxide scale [6 ]. The curved oxide scale shown in Fig. 5(b) provides evidence for such deformation.
5. Conclusion
Fig. 8. XRD spectra of the sputtered Ni–Cr–Al–Ti coating after different periods of oxidation.
a-Al O and TiO were formed on the sputtered Ni– 2 3 2 Cr–Al–Ti nanocrystalline coating after 200 h of oxidation at 1000°C in air. Good oxidation resistance was shown by the sputtered Ni–Cr–Al–Ti coating. The presence of TiO did not impair the continuity of the 2 Al O scale, which resulted from the effect of Cr and 2 3 the precipitation of Ni (AlTi). 3 Acknowledgement
and Ni–Al–Ti, the presence of Cr and Al could greatly decrease the solid solution of Ti in Ni-base c phase [13]. However, the solid solution of Al in Ni-base c phase could not be influenced. Therefore, it is very possible that there was much less Al than Ti in Ni (AlTi). With 3 the precipitation of Ni (AlTi), more Ti could be tied 3 up in Ni (AlTi) and the system would become more 3 ordered. In a Monte Carlo simulation study [14], the diffusion of Ti in Ni (AlTi) was found to be reduced 3 markedly as the system became more ordered compared with Ti in the c phase. Thus, the diffusivity of Ti decreased, and thus the growth rate of TiO could be 2 decreased. The stagnancy of TiO growth shown in 2 Fig. 7 was attributed to the reduced availability of Ti tied up in the Ni (AlTi) phase and the coalescence into 3 a complete scale of the Al O formed. 2 3 The oxide scale will be subjected to thermal and growth stresses generated during oxidation process. Spalling may be induced by these stresses, resulting in accelerated oxidation. No spallation was observed in the oxide scale formed on the sputtered coating. Therefore, the stresses must somehow have been relieved. The plastic deformation of the oxide scale formed on the nanocrystalline coating could be easily
The work was sponsored by the National Natural Science Foundation of China under grant 59671060.
References [1] S.W. Yang, Oxid. Met. 15 (1981) 375. [2] J. Litz, A. Rahmel, M. Schorr, Oxid. Met. 30 (1988) 95. [3] H. Lou, F. Wang, S. Zhu, W. Wu, Surf. Coat. Technol. 63 (1994) 105. [4] H. Lou, Y. Tang, X. Sun, H. Guan, Mater. Sci. Eng. A 207 (1996) 121. [5] J. Zhang, H. Lou, Acta Metall. Sinica 34 (1998) 627 in Chinese. [6 ] F. Wang, Oxid. Met. 48 (1997) 215. [7] A. Rahmel, P.S. Spencer, Oxid. Met. 35 (1991) 53. [8] H. Lou, S. Zhu, F. Wang, Oxid. Met. 43 (1995) 317. [9] J. Zhang, H. Lou, Chi. J. Nonferrous Met. 8 (Suppl. 1) (1998) 30 in Chinese. [10] Z. Tang, F. Wang, W. Wu, Surf. Coat. Technol. 99 (1998) 248. [11] D.W. McKee, S.C. Huang, Corros. Sci. 33 (1992) 1899. [12] F.H. Stott, G.C. Wood, J. Stringer, Oxid. Met. 44 (1995) 113. [13] R.Z. Zhu, Y.D. He, H.B. Qi, in: High-temperature Corrosion and High-temperature Corrosion Resistance Materials, Shanghai Science and Technology Publishing House, Shanghai, 1995, p. 298. in Chinese. [14] W.M. Young, E.W. Elcock, Proc. Phys. Soc. 89 (1966) 735.
Oxidation behavior of sputtered Ni–Cr–Al–Ti nanocrystalline coating G. Chen *, H. Lou State Key Lab for Corrosion and Protection, Institute of Corrosion and Protection of Metals, Academia Sinica, Shenyang 110015, People’s Republic of China Received 22 June 1999; accepted in revised form 6 August 1999
Abstract Magnetron sputter deposition was used to prepare Ni–9.7Cr–5.5Al–7Ti (wt%) nanocrystalline coatings on the substrates of the same material. The oxidation behavior of the cast Ni–Cr–Al–Ti alloy and its sputtered coating at 1000°C in air was investigated. An a-Al O scale with TiO dispersions was formed on the sputtered coating after 200 h of oxidation; even so good 2 3 2 oxidation resistance was exhibited. The formation of TiO was attributed to the high ratio of Ti/Al in the coating. The high 2 content of Cr and the precipitation of Ni (AlTi) in the sputtered coating during oxidation could restrict the formation and growth 3 of TiO . Therefore, good oxidation resistance was shown by the sputtered Ni–Cr–Al–Ti coating despite the formation of TiO . 2 2 © 2000 Elsevier Science S.A. All rights reserved. Keywords: Magnetron sputtering; Nanocrystalline coating; Ni–Cr–Al–Ti; Oxidation
1. Introduction Ni-base superalloys are widely used as turbine blade material for advanced engines that are required to operate at elevated temperatures and greatly depend on the presence of Al or Cr to form a protective oxide scale of alumina or chromina. However, a certain amount of Ti is contained in superalloys. The existence of Ti in superalloys can impair the oxidation resistance by forming TiO , which can destroy the continuity of the 2 protective oxide scale [1,2]. Recently, nanocrystalline coatings prepared by the magnetron sputtering technique have attracted much attention. The nanocrystalline coatings on the substrates of the same materials such as Ni-based superalloys K38G, K17F and LDZ125 showed excellent oxidation resistance by developing a unitary layer of Al O [3–5]. TiO was not formed on 2 3 2 these coated superalloys; therefore, no evidence can be used to evaluate the effect of TiO on the oxidation 2 resistance of these sputtered coatings. Nanocrystallization can promote the selective oxidation of Al due to the low free energy of formation of Al O [6 ]. It 2 3 has been suggested that the free energies of formation
of TiO and Al O are almost the same [7]. Therefore, 2 2 3 TiO and Al O would have almost the same propensity 2 2 3 to form. The results that no TiO was formed may be 2 due to the low ratio of Ti/Al in the sputtered superalloys coating. Because TiO could be formed when the ratio 2 of Ti/Al was increased slightly, this can be evidenced by the studies of the rehealing ability of sputtered K17F and K38G coatings [8,9]. However, TiO could not be 2 found on the oxide scale. The results suggested that TiO was well dispersed in the oxide scale. It is important 2 to study the oxidation behavior of the sputtered nanocrystalline coating when TiO can be formed and 2 observed on the coating so as to evaluate the effect of TiO on the oxidation resistance of the sputtered coating. 2 In this paper, based upon the contents of Cr, Al and Ti in the superalloy LDZ125, by increasing the content of Ti in order to promote the formation of TiO , a simple 2 Ni–Cr–Al–Ti alloy was developed. The oxidation resistance and oxide formation of this alloy and its sputtered Ni–Cr–Al–Ti nanocrystalline coating have been investigated.
2. Experimental * Corresponding author. Tel.: +86-24-23915908; fax: +86-24-238941149. E-mail address: [email protected] (G. Chen)
The Ni–Cr–Al–Ti alloy used in this study was prepared by vacuum induction melting and casting using
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 47 0 - 3
G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
93
Table 1 Chemical compositions of the Ni–Cr–Al–Ti and LDZ 125 alloys (wt%)
NiCrAlTi LDZ 125
Ni
Cr
Al
Ti
Co
W
Ta
Mo
Bal. Bal.
9.7 9.47
5.5 5.2
7 3.8
– 10.38
– 7.42
– 4
– 2
pure Ni, Cr, Al and Ti (99.9%) as raw metals. The nominal chemical compositions of Ni–Cr–Al–Ti and LDZ 125 superalloy are listed in Table 1. The ingot was cut into 20×10×3 mm specimens. The specimens were ground to 600-SiC paper, peened and ultrasonically cleaned in ethanol. The magnetron sputter technique was used to prepare nanocrystalline coating of Ni–Cr– Al–Ti on the substrates of the same alloy under the following sputtering parameters: Ar working pressure 0.2 Pa, power 2 kW, substrate temperature 250°C. The compositions of the sputtered coating were the same as the substrates, and the thickness of the coating was about 50 mm. Oxidation tests on the specimens were conducted at 1000°C in air for up to 200 h. The specimens were placed in alumina crucibles, oxidized at 1000°C and then cooled to room temperature at regular intervals of 20 h for mass measurements. The sensitivity of the balance used in the study was 10−4 g. The as-sputtered Ni–Cr–Al–Ti coating was examined by atomic force microscopy (AFM ). After the oxidation tests, specimens were examined by X-ray diffraction with CuKa ( XRD), scanning electron microscopy (SEM ) and energy dispersive analysis by X-ray ( EDAX ).
3. Results 3.1. Microstructure An AFM image of the as-sputtered Ni–Cr–Al–Ti coating is shown in Fig. 1. Based on the AFM morphology, it can be seen that the average grain size of the
Fig. 1. AFM image of the as-sputtered Ni–Cr–Al–Ti coating.
Fig. 2. Micrograph of cast Ni–Cr–Al–Ti alloy.
sputtered Ni–Cr–Al–Ti coating was several tens of nanometres. Fig. 2 shows a SEM micrograph of the cast Ni– Cr–Al–Ti alloy. The average grain size of the cast alloy was found to be several micrometres, which was larger than that of the sputtered coating by two orders of magnitude. 3.2. Oxidation kinetics The oxidation kinetics of the cast Ni–Cr–Al–Ti and its sputtered coating are illustrated in Fig. 3. For comparison, the oxidation kinetics of the cast LDZ125 alloy and its sputtered coating from Ref. [5] are also given in Fig. 3. After the sharp increase in mass gains at the initial oxidation stage, the kinetic curve of the sputtered Ni–Cr–Al–Ti coating showed an extensive period of very slow mass gain, which can be attributed to the rapid formation of a protective oxide scale. However, the kinetic curve of the cast Ni–Cr–Al–Ti alloy showed this propensity after 40 h of oxidation, which is later than the sputtered coating. Although the protective oxide scale of the cast alloy formed later than the sputtered coating, both showed good oxidation resistance. As compared with the sputtered LDZ125 coating, the sputtered Ni–Cr–Al–Ti coating shows a slightly
Fig. 3. Oxidation kinetics of cast Ni–Cr–Al–Ti, LDZ 125 alloys and their sputtered coatings.
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G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
Fig. 4. XRD patterns of cast Ni–Cr–Al–Ti alloy and the sputtered coating after 200 h of oxidation.
higher mass gain, which is due to the much higher mass gain of the sputtered Ni–Cr–Al–Ti coating in the initial oxidation period.
scale can be seen on the sputtered coating. Besides the a-Al O scale, a particle of TiO is also observed. It can 2 3 2 also be seen that TiO did not directly connect with the 2 coating below. Fig. 6 shows the surface morphology and the crosssectional microstructure of the cast Ni–Cr–Al–Ti alloy after 200 h of oxidation. A duplex oxide scale was formed on the cast alloy. The outer layer was composed of Cr O , NiCr O and TiO , and the inner layer was 2 3 2 4 2 mainly a-Al O . 2 3 Fig. 7 shows micrograghs of TiO after 1, 20 and 2 200 h of oxidation. It can be observed that the growth of TiO was very fast during the early stage of oxidation. 2 With increasing oxidation time, TiO grew slowly. This 2 inferred that the formation of TiO only contributed to 2 the rapid mass gain in the initial oxidation period and showed little influence on the kinetics of the sputtered coating in the following oxidation period.
4. Discussion 3.3. XRD analysis Fig. 4 shows the XRD patterns of oxidation products for the cast Ni–Cr–Al–Ti alloy and sputtered coating after 200 h of oxidation. It can be seen that both a-Al O and TiO were formed on the sputtered coating; 2 3 2 the latter resulted from the high content of Ti that promoted the formation of TiO . Compared with the 2 oxide compositions on the sputtered coating, more complex oxides such as a-Al O , TiO , Cr O and NiCr O 2 3 2 2 3 2 4 were formed on the cast alloy. 3.4. Morphologies Fig. 5 illustrates the surface morphology and the cross-sectional microstructure of the sputtered Ni–Cr– Al–Ti coating after 200 h of oxidation. Some TiO 2 particles dispersed on the oxide scale formed on the sputtered coating. From the cross-sectional microstructure, a curved, dense, complete and adherent a-Al O 2 3
The formation of TiO can impair the oxidation 2 resistance of the cast alloy in that TiO can destroy the 2 continuity of the formed oxide scale [1]. It has been indicated that the formation and growth of TiO were 2 directly related to the presence of TiC in the superalloys [2]. The lack of formation of TiO on the coated K38G, 2 K17F, LDZ125 superalloys was probably attributed to the absence of TiC in the sputtered superalloys coatings. The present results indicated that TiO was formed on 2 the cast Ni–Cr–Al–Ti alloy and its sputtered coating. No TiC existed in the Ni–Cr–Al–Ti alloy; therefore, the harmful effect of TiO was not apparent for the cast 2 Ni–Cr–Al–Ti alloy. This was due to the formation of a complete Al O subscale preceded by the formation of 2 3 transient oxides such as Cr O , TiO and NiCr O . The 2 3 2 2 4 high ratio of Ti/Al contributed to the formation of TiO on the sputtered Ni–Cr–Al–Ti coating. The appear2 ance of TiO on the protective coating may contribute 2 to the acceleration of mass gain [10]. However, in
Fig. 5. (a) Surface morphology and (b) the cross-sectional microstructure of the sputtered Ni–Cr–Al–Ti coating after 200 h of oxidation.
G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
95
Fig. 6. Surface morphology (a) and the cross-sectional microstructure (b) of the cast Ni–Cr–Al–Ti alloy after 200 h of oxidation.
Fig. 7. Micrographs of TiO after (a) 1 h, (b) 20 h, (c) 200 h of oxidation. 2
comparison with the sputtered LDZ125 superalloy coating on which a unitary Al O scale was formed, the 2 3 mass gain of sputtered Ni–Cr–Al–Ti coating on which TiO and Al O both formed was not seriously influ2 2 3 enced, indicating good oxidation resistance. The ratio of Ti/Al in the sputtered Ni–Cr–Al–Ti coating was larger than 1, and the free energies of formation of TiO and Al O were similar. However, a 2 2 3 layer of Al O in which small TiO particles were 2 3 2 dispersed was formed on the sputtered Ni–Cr–Al–Ti coating. It was suggested that the high content of Cr could restrict the formation of TiO [11]. In addition, 2 Cr could promote the formation of Al O [12]. 2 3
Therefore, the selective oxidation of Al was more predominant than Ti, and an Al O scale with dispersed 2 3 TiO particles was formed on the sputtered coating. 2 Besides the effect of Cr, the precipitation of Ni (AlTi) also restricted the formation of TiO . Fig. 8 3 2 shows the XRD spectra for the sputtered Ni–Cr–Al–Ti coating after different periods of oxidation, in which the peaks of the c phase were emphasized, whereas the peaks of any oxides were not obvious. Only the c phase was detected in the as-sputtered Ni–Cr–Al–Ti coating. Ni (AlTi) was precipitated during the oxidation process 3 due to the supersaturation state of Al and Ti in the c phase. According to the phase diagram for Ni–Cr–Ti
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G. Chen, H. Lou / Surface and Coatings Technology 123 (2000) 92–96
generated, and then these stresses may be relieved by plastic deformation of the oxide scale [6 ]. The curved oxide scale shown in Fig. 5(b) provides evidence for such deformation.
5. Conclusion
Fig. 8. XRD spectra of the sputtered Ni–Cr–Al–Ti coating after different periods of oxidation.
a-Al O and TiO were formed on the sputtered Ni– 2 3 2 Cr–Al–Ti nanocrystalline coating after 200 h of oxidation at 1000°C in air. Good oxidation resistance was shown by the sputtered Ni–Cr–Al–Ti coating. The presence of TiO did not impair the continuity of the 2 Al O scale, which resulted from the effect of Cr and 2 3 the precipitation of Ni (AlTi). 3 Acknowledgement
and Ni–Al–Ti, the presence of Cr and Al could greatly decrease the solid solution of Ti in Ni-base c phase [13]. However, the solid solution of Al in Ni-base c phase could not be influenced. Therefore, it is very possible that there was much less Al than Ti in Ni (AlTi). With 3 the precipitation of Ni (AlTi), more Ti could be tied 3 up in Ni (AlTi) and the system would become more 3 ordered. In a Monte Carlo simulation study [14], the diffusion of Ti in Ni (AlTi) was found to be reduced 3 markedly as the system became more ordered compared with Ti in the c phase. Thus, the diffusivity of Ti decreased, and thus the growth rate of TiO could be 2 decreased. The stagnancy of TiO growth shown in 2 Fig. 7 was attributed to the reduced availability of Ti tied up in the Ni (AlTi) phase and the coalescence into 3 a complete scale of the Al O formed. 2 3 The oxide scale will be subjected to thermal and growth stresses generated during oxidation process. Spalling may be induced by these stresses, resulting in accelerated oxidation. No spallation was observed in the oxide scale formed on the sputtered coating. Therefore, the stresses must somehow have been relieved. The plastic deformation of the oxide scale formed on the nanocrystalline coating could be easily
The work was sponsored by the National Natural Science Foundation of China under grant 59671060.
References [1] S.W. Yang, Oxid. Met. 15 (1981) 375. [2] J. Litz, A. Rahmel, M. Schorr, Oxid. Met. 30 (1988) 95. [3] H. Lou, F. Wang, S. Zhu, W. Wu, Surf. Coat. Technol. 63 (1994) 105. [4] H. Lou, Y. Tang, X. Sun, H. Guan, Mater. Sci. Eng. A 207 (1996) 121. [5] J. Zhang, H. Lou, Acta Metall. Sinica 34 (1998) 627 in Chinese. [6 ] F. Wang, Oxid. Met. 48 (1997) 215. [7] A. Rahmel, P.S. Spencer, Oxid. Met. 35 (1991) 53. [8] H. Lou, S. Zhu, F. Wang, Oxid. Met. 43 (1995) 317. [9] J. Zhang, H. Lou, Chi. J. Nonferrous Met. 8 (Suppl. 1) (1998) 30 in Chinese. [10] Z. Tang, F. Wang, W. Wu, Surf. Coat. Technol. 99 (1998) 248. [11] D.W. McKee, S.C. Huang, Corros. Sci. 33 (1992) 1899. [12] F.H. Stott, G.C. Wood, J. Stringer, Oxid. Met. 44 (1995) 113. [13] R.Z. Zhu, Y.D. He, H.B. Qi, in: High-temperature Corrosion and High-temperature Corrosion Resistance Materials, Shanghai Science and Technology Publishing House, Shanghai, 1995, p. 298. in Chinese. [14] W.M. Young, E.W. Elcock, Proc. Phys. Soc. 89 (1966) 735.