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Cyclic oxidation behavior of Al–Cu–Fe–Cr quasicrystalline coating on titanium alloy
Materials Science and Engineering A 386 (2004) 362–366
Cyclic oxidation behavior of Al–Cu–Fe–Cr quasicrystalline coating on titanium alloy Chungen Zhou∗ , Fei Cai, Huibin Xu, Shengkai Gong Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China Received 19 November 2003; received in revised form 15 July 2004
Abstract The Al–Cu–Fe–Cr quasicrystalline coating was deposited by low pressure plasma spraying (LPPS). Formation of Al–Cu–Fe–Cr quasicrystalline phase is closely related to annealing temperature. As-deposited Al–Cu–Fe–Cr coating is a kind of mixtures of quasicrystalline and crystalline phases. With increasing heat treatment temperature, icosahedral phase initially is increased and then decreased, the amount of crystalline phases is decreased and the amount of decagonal quasicrystalline phase is increased. Al–Cu–Fe–Cr quasicrystalline coating improved the cyclic oxidation resistance of titanium-based alloys. The weight gains at 650 and 800 ◦ C for Al–Cu–Fe–Cr quasicrystalline coating are very low. During the oxidation period there is no evident spallation of the coating from the substrate. Oxide formed on the surface of Al–Cu–Fe–Cr quasicrystalline coating after oxidation consisted of Al2 O3 . © 2004 Elsevier B.V. All rights reserved. Keywords: Quasicrystal; Low pressure plasma spraying; Cyclic oxidation; Titanium alloy
1. Introduction Quasicrystal is a new class of materials characterized with quasi-period order. The phases with five-fold rotational symmetry was first reported in Al–Mn alloy by Shechtman and Chan in 1982 [1], which arouses great interest among materials scientists. Due to its special structure, quasicrystals exhibit a series of unique properties such as low conductivity with positive thermal coefficient [2], low friction coefficient [3], excellent oxidation and corrosion resistance [4,5]. The shortcoming of quasicrystals generally manifests itself as extreme brittleness in bulk form. They cannot be applied as structural materials due to their brittle nature at ambient temperature. However, the combination of excellent physical and mechanical properties makes them the potential materials for surface application [3,6–9]. Titanium alloys are widely used as structural components in aerospace, chemical, petrochemical and marine industries owing to their low density, high ∗
Corresponding author. Tel.: +86 108 231 6000; fax: +86 108 233 8200. E-mail addresses: [email protected] (C. Zhou), [email protected] (H. Xu). 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.08.026
specific strength and high temperature mechanical properties. However, the maximum service temperature of titanium alloys is limited to around 550 ◦ C due to strong oxidation and descending of mechanical properties under high temperature [10]. The present investigation deals with the development of low pressure plasma sprayed [11] Al–Cu–Fe–Cr quasicrystalline coating, and investigates the influence of the coating on the cyclic oxidation behavior of titanium-based alloys.
2. Experimental procedures The substrates were cut into coupons with a dimension of 13 mm × 10 mm × 2.5 mm from a wrought sheet of titanium-base alloy with nominal composition (wt.%) of Ti–2.5Al–5V–5Mo. In order to improve the adherence of the coating, these coupons were then grit-blasted, using 250 m alumina grit, to obtain a sharp-peaked surface contour with a roughness average of 4–5 m. A quasicrystalline coating with nominal composition (at.%) of Al71 –Cu10 –Fe8.5 –Cr10.5 powder was vacuum plasma-sprayed by Sultzer Metco onto all surfaces of the specimens. The particle size of the powder
C. Zhou et al. / Materials Science and Engineering A 386 (2004) 362–366 Table 1 Conditions for the LPPS deposition of the coatings Volts (V) Current (A) Argon gas (L/min) Hydrogen gas (L/min) Powder feed rate (g/min) Substrate temperature (◦ C) Chamber pressure (Pa)
50–55 520–550 60 20 18 ∼400 5 × 103
ranged from 25 to 50 m. The deposition condition for the coating in this study is given in Table 1. As-sprayed Al–Cu–Fe–Cr coating sealed in quartz tube was annealed at different temperatures for 5 h and subsequently furnace-cooled to room temperature by turning off the furnace in order to obtain the effect of heat treatment temperature on the formation of quasicrystalline phase. Cyclic oxidation tests were conducted for evaluation of oxidation resistance of the coating. The cyclic oxidation tests were performed under atmospheric pressure at 650 and 800 ◦ C in static air. Each cycle consisted of an hour of holding the test coupon at predefined temperature in the furnace followed by a 20 min cool down to room temperature. The sample weights were measured to 0.0001 g at each cycle until the end of their respective tests. Before and after oxidation, the samples were analyzed using X-ray diffraction (D/max 2200pc, Rigaku) using a Cu K␣ source, and scanning electron microscopy (JSM-5800, JEOL) with energy dispersive X-ray analysis (EDAX).
3. Results and discussion 3.1. Annealing effect on formation of Al–Cu–Fe–Cr quasicrystalline coating It was found that during the spraying loss of Al occurs due to the poor thermal conductivity of quasicrystals. The loss of Al shifts the coating composition away from the quasicrystalline phase region since the quasicrystalline phase region in the phase diagram is very small [12–14]. Therefore, as-deposited Al–Cu–Fe–Cr coating is a kind of mixtures of quasicrystalline and crystalline phases. In order to decrease the crystalline phases in the coating, the heat treatment should be performed. Fig. 1 shows X-ray diffraction pattern of as-sprayed Al–Cu–Fe–Cr coating and as-sprayed Al–Cu–Fe–Cr coating annealed in vacuum at different temperatures for 5 h. As-sprayed Al–Cu–Fe–Cr coating is a kind of mixtures of icosahedral (Al65 Cu20 Fe15 ) and decagonal quasicrystalline phases (Al65 Cr5 Cu20 Fe10 ), and crystalline phases (Fig. 1(a)). The result is in accordance with the results of Dubois et al. [15] and Dong and Dubois [16]. With increasing heat treatment temperature, As shown in Fig. 1, icosahedral phase at around 43◦ in 2θ initially is increased and then decreased. The amount of crystalline phases is decreased and the amount of decagonal quasicrys-
363
talline phase is increased. As shown in Fig. 1(e), as sprayed Al–Cu–Fe–Cr coating after heat treatment at 950 ◦ C for 5 h is consisted of decagonal phase and very small amount of cubic phase peaks, and icosahedral quasicrystalline phase is disappeared. Huttunen-Saarivirta et al. investigated influence of Cr alloying on the microstructure of thermally sprayed quasicrystalline Al–Cu–Fe coating [17]. Huttunen-Saarivirta et al.’s work shows that Cr addition in Al–Cu–Fe alloy introduces two different icosahedral phases (Al80 Cr13.5 Fe6.5 and Al13 Cr3 Cu4 ) in the thermal sprayed coating. In the present investigation icosahedral Al80 Cr13.5 Fe6.5 or Al13 Cr3 Cu4 was not found in the thermal sprayed coating. The difference in phase composition may result from different spraying parameters and powder compositions. 3.2. Microstructures of the LPPS Al–Cu–Fe–Cr quasicrystalline coating Fig. 2 shows a typical lamellar splat formation of the low pressure plasma sprayed Al–Cu–Fe–Cr quasicrystalline coating. The coating was found to be relatively dense, and some cracks were observed. The thickness of the coating was about 100 m. 3.3. Cyclic oxidation kinetics Cyclic oxidation test was performed to investigate the influence of Al–Cu–Fe–Cr quasicrystalline coating on the oxidation behavior of the titanium alloy. Fig. 3 shows a plot of the weight change per unit area versus number of cycles for the test performed at 650 and 800 ◦ C in static air. The titanium alloy oxidized at a high rate. It maintains a good appearance only up to two cycles, but yellow nodules appear on the surface after three cycles. Spallation occurred during the course of cyclic oxidation. For Al–Cu–Fe–Cr quasicrystalline coat-
Fig. 1. X-ray diffraction pattern of as-sprayed Al–Cu–Fe–Cr coating and assprayed Al–Cu–Fe–Cr coating annealed in vacuum at different temperatures for 5 h.
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Fig. 2. Morphologies of LPPS Al–Cu–Fe–Cr quasicrystalline coating: (a) surface and (b) cross-section.
ing, at the initial stage it shows some weight change. After 25 cycles only a little change in weight is observed. During the course of oxidation no evident spallation occurs. After 624 cycles at 650 ◦ C and 290 cycles at 800 ◦ C, the weight changes per unit area were about 0.7 and 1.5 mg/cm2 , respectively. Gurrappa [18] investigated the effect of aluminizing on the oxidation rate of titanium alloy at 800 ◦ C. It was found that the weight gain was about 2.0 mg/cm2 after cyclic-isothermal
oxidation at 800 ◦ C for 300 h. Compared with the result given by Gurrappa, the oxidation resistance of Al–Cu–Fe–Cr quasicrystalline coating is same as aluminide coating. It can be drawn from the above result that the Al–Cu–Fe–Cr quasicrystalline coatings provide excellent protection of the titanium alloys against cyclic oxidation at both 650 and 800 ◦ C. 3.4. Scale structure To identify the phases developed on the surface of LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation in air at 800 ◦ C for 290 cycles, the specimen surfaces were analyzed by X-ray diffraction. The result is shown in Fig. 4. XRD shows peaks for ␥-Al2 O3 , showing the oxide formed on the LPPS Al–Cu–Fe–Cr quasicrystalline coating is composed of ␥-Al2 O3 and the thickness of the oxide is very thin since the peak intensities for the coating are very strong. 3.5. Morphologies of the oxide Fig. 5 shows the surface morphologies of the LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation
Fig. 3. Plot of weight change per unit area vs. number of cycles for cyclic oxidation test at (a) 650 ◦ C and (b) 800 ◦ C.
Fig. 4. XRD spectrum from the Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation at 800 ◦ C for 290 cycles.
C. Zhou et al. / Materials Science and Engineering A 386 (2004) 362–366
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Fig. 5. Surface morphologies of LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation at (a) 650 ◦ C for 624 cycles and (b) 800 ◦ C for 290 cycles.
in air at 650 ◦ C for 624 cycles and at 800 ◦ C for 290 cycles. SEM image of the surface for the LPPS Al–Cu–Fe–Cr quasicrystalline coating oxidized at 650 ◦ C for 624 cycles (Fig. 5(a)) is different from that of 800 ◦ C (Fig. 5(b)). The coating oxidized at 800 ◦ C for 290 cycles has smaller grain (2–3 m) than that of at 650 ◦ C (∼6 m) for 624 cycles. The surfaces of the coatings are very dense and the oxides formed are composed of small round grains. No evident cracks on the surfaces are observed after cyclic oxidation test. Fig. 6 shows the cross-sectional image of the coatings after cyclic oxidation at 800 ◦ C for 290 cycles. Very thin oxide layers of about 2–5 m can be seen on the surface. EDS analysis indicates that the oxide layer formed on Al–Cu–Fe–Cr quasicrystalline coating is composed of Al and O elements. The result is in accordance with XRD result. For the oxidation of Al–Cu–Fe–Cr quasicrystalline coating, during the very initial stage of oxidation there is a rapid uptake of oxygen by the coating and various oxides such as CuO, Al2 O3 , FeO as well as Cr2 O3 are formed on the surface of the coating [19]. Because Al2 O3 is the most stable thermodynamically, replacement reaction of Al occurs, resulting in the formation of Al2 O3 on the coating. The thermal stress developed in the oxide layer during cyclic oxidation can be estimated by the following equation [19]: σox = −
Eox T (αM − αOX ) 1 + 2(Eox /EM ξ/ h)
(1)
Table 2 Constants used in Eq. (1) System
Thermal-expansion coefficient (K−1 )
Young’s modulus (GPa)
Temperature change (K)
Al2 O3 Al–Cu–Fe–Cr
9.0 × 10−6 [21] (14–17) × 10−6 [20]
400 [21] 68 [22]
775 775
where EM and Eox are Young’s moduli of the coating and the oxide, respectively; αM and αox the linear thermal expansion coefficient of the coating and oxide, respectively; ξ the oxide thickness (∼4 m); and h the half thickness of the coating (∼50 m). For Al–Cu–Fe–Cr quasicrystal, its linear thermal expansion coefficient has not been reported. However, Dubois [20] reported values between 14 × 10−6 and 17 × 10−6 K−1 for Al–Cu–Fe quasicrystal. The linear thermal expansion coefficient of Al–Cu–Fe–Cr quasicrystal should be very similar. Using the relevant constants listed in Table 2, the thermal stress in Al2 O3 without considering creep release is calculated as about −1100 MPa. On the other hand, the maximum compressive strength of Al2 O3 is given as 3000 MPa [23]. This means that the oxide scales of Al2 O3 formed on the coating would be able to withstand the thermal contraction during the cooling and would not be spalled, which protects the Cu and Fe components from oxidizing. Therefore, Al–Cu–Fe–Cr quasicrystalline coating improved the cyclic oxidation resistance of titanium-based alloys.
4. Conclusion
Fig. 6. Cross-sectional morphologies of LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation at 800 ◦ C for 290 cycles.
Al–Cu–Fe–Cr quasicrystalline coating has been made by low pressure plasma spraying (LPPS) method and characterized. Formation of Al–Cu–Fe–Cr quasicrystalline phase is closely related to annealing temperature. As-deposited Al–Cu–Fe–Cr coating is a kind of mixtures of quasicrystalline and crystalline phases. With increasing heat treatment temperature, icosahedral phase at around 43◦ in 2θ initially is increased and then decreased. The amount of crystalline phases is decreased and the amount of decagonal quasicrystalline phase is increased. Under atmospheric pressure in static air at 650 and 800 ◦ C, the coating exhibits excellent
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cyclic oxidation resistance. The oxide formed on the surface of LPPS Al–Cu–Fe quasicrystalline coating after cyclic oxidation consisted of Al2 O3 .
Acknowledgement The Aviation Science Foundation of China is acknowledged for financial support under Grant No. 00H51006.
References [1] D. Shechtman, J.W. Chan, Phys. Rev. Lett. 53 (1984) 1951. [2] T. Klein, A. Gozlan, C. Berger, F. Cyrot-Lackmann, Y. Calvayrac, A. Quivy, Euro-Phys. Lett. 13 (2) (1990) 129. [3] S.S. Kang, J.M. Dubois, J. von Stebut, J. Mater. Res. 8 (10) (1993) 2471. [4] S-L. Chang, W.B. Chin, C-M. Zhang, C.J. Jenks, P.A. Thiel, Surf. Sci. 337 (1995) 135. [5] D.J. Sordelet, M.J. Kramer, I.E. Anderson, M.F. Besser, in: C. Janot, R. Mosseri (Eds.), Proceedings of the ICQ5, Avignon, France, 1995, p. 778. [6] S.D. Palos, S. Usmani, K. Kishi, S. Sampath, D.J. Sordelet, M.F. Besser, Proceedings of the 15th International Thermal Spray Conference, Nice, France, 25–29 May, 1998.
[7] C. Zhou, H. Xu, S. Gong, G. Kang, J. Mater. Sci. Technol. 18 (2002) 251. [8] E. Fleury, Y.C. Kim, J.S. Kim, D.H. Kim, W.T. Kim, H.S. Ahn, S.M. Lee, J. Alloys Comp. 342 (2002) 321. [9] E. Fleury, S.M. Lee, W.T. Kim, D.H. Kim, J. Non-Cryst. Solids 278 (2000) 194. [10] M.W. Kearns, J.E. Restall, Proceedings of the Sixth World Conference on Titanium, Cannes, 1988, Paper SU8, p. 396, Les Editions de Physique, Les Ulis, 1988. [11] H. Guruer, Thin Solid Films 118 (1984) 409. [12] D.J. Sordelet, S.D. Widener, Y. Tang, M.F. Besser, Mater. Sci. Eng. A 294–296 (2000) 834. [13] D.J. Sordelet, M.F. Besser, J.L. Logsdon, Mater. Sci. Eng. A 255 (1998) 54. [14] E. Huttunen-Saarivirta, E. Turunen, M. Kallio, J. Alloys Comp. 354 (2003) 269. [15] J.M. Dubois, S.S. Kang, J. von Stebut, J. Mater. Sci. Lett. 10 (1991) 537. [16] C. Dong, J.M. Dubois, J. Mater. Sci. 26 (1991) 1647–1654. [17] E. Huttunen-Saarivirta, E. Turunen, M. Kallio, Intermetallics 11 (2003) 879. [18] I. Gurrappa, Oxid. Met. 56 (2001) 73. [19] N. Birks, G.H. Meier, Introduction to High Temperature Oxidation of Metals, Edward Arnold Ltd., 1983. [20] D.J. Dubois, MRS Bulletin, November 1997. [21] Z. Liu, W. Gao, K.L. Dahm, F. Wang, Oxid. Met. 50 (1998) 51. [22] C. Dong, Quasicrystalline Materials, National Defense Industry Press of China, 1994. [23] P. Kofstad, Oxid. Met. 24 (1985) 265.
Cyclic oxidation behavior of Al–Cu–Fe–Cr quasicrystalline coating on titanium alloy Chungen Zhou∗ , Fei Cai, Huibin Xu, Shengkai Gong Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China Received 19 November 2003; received in revised form 15 July 2004
Abstract The Al–Cu–Fe–Cr quasicrystalline coating was deposited by low pressure plasma spraying (LPPS). Formation of Al–Cu–Fe–Cr quasicrystalline phase is closely related to annealing temperature. As-deposited Al–Cu–Fe–Cr coating is a kind of mixtures of quasicrystalline and crystalline phases. With increasing heat treatment temperature, icosahedral phase initially is increased and then decreased, the amount of crystalline phases is decreased and the amount of decagonal quasicrystalline phase is increased. Al–Cu–Fe–Cr quasicrystalline coating improved the cyclic oxidation resistance of titanium-based alloys. The weight gains at 650 and 800 ◦ C for Al–Cu–Fe–Cr quasicrystalline coating are very low. During the oxidation period there is no evident spallation of the coating from the substrate. Oxide formed on the surface of Al–Cu–Fe–Cr quasicrystalline coating after oxidation consisted of Al2 O3 . © 2004 Elsevier B.V. All rights reserved. Keywords: Quasicrystal; Low pressure plasma spraying; Cyclic oxidation; Titanium alloy
1. Introduction Quasicrystal is a new class of materials characterized with quasi-period order. The phases with five-fold rotational symmetry was first reported in Al–Mn alloy by Shechtman and Chan in 1982 [1], which arouses great interest among materials scientists. Due to its special structure, quasicrystals exhibit a series of unique properties such as low conductivity with positive thermal coefficient [2], low friction coefficient [3], excellent oxidation and corrosion resistance [4,5]. The shortcoming of quasicrystals generally manifests itself as extreme brittleness in bulk form. They cannot be applied as structural materials due to their brittle nature at ambient temperature. However, the combination of excellent physical and mechanical properties makes them the potential materials for surface application [3,6–9]. Titanium alloys are widely used as structural components in aerospace, chemical, petrochemical and marine industries owing to their low density, high ∗
Corresponding author. Tel.: +86 108 231 6000; fax: +86 108 233 8200. E-mail addresses: [email protected] (C. Zhou), [email protected] (H. Xu). 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.08.026
specific strength and high temperature mechanical properties. However, the maximum service temperature of titanium alloys is limited to around 550 ◦ C due to strong oxidation and descending of mechanical properties under high temperature [10]. The present investigation deals with the development of low pressure plasma sprayed [11] Al–Cu–Fe–Cr quasicrystalline coating, and investigates the influence of the coating on the cyclic oxidation behavior of titanium-based alloys.
2. Experimental procedures The substrates were cut into coupons with a dimension of 13 mm × 10 mm × 2.5 mm from a wrought sheet of titanium-base alloy with nominal composition (wt.%) of Ti–2.5Al–5V–5Mo. In order to improve the adherence of the coating, these coupons were then grit-blasted, using 250 m alumina grit, to obtain a sharp-peaked surface contour with a roughness average of 4–5 m. A quasicrystalline coating with nominal composition (at.%) of Al71 –Cu10 –Fe8.5 –Cr10.5 powder was vacuum plasma-sprayed by Sultzer Metco onto all surfaces of the specimens. The particle size of the powder
C. Zhou et al. / Materials Science and Engineering A 386 (2004) 362–366 Table 1 Conditions for the LPPS deposition of the coatings Volts (V) Current (A) Argon gas (L/min) Hydrogen gas (L/min) Powder feed rate (g/min) Substrate temperature (◦ C) Chamber pressure (Pa)
50–55 520–550 60 20 18 ∼400 5 × 103
ranged from 25 to 50 m. The deposition condition for the coating in this study is given in Table 1. As-sprayed Al–Cu–Fe–Cr coating sealed in quartz tube was annealed at different temperatures for 5 h and subsequently furnace-cooled to room temperature by turning off the furnace in order to obtain the effect of heat treatment temperature on the formation of quasicrystalline phase. Cyclic oxidation tests were conducted for evaluation of oxidation resistance of the coating. The cyclic oxidation tests were performed under atmospheric pressure at 650 and 800 ◦ C in static air. Each cycle consisted of an hour of holding the test coupon at predefined temperature in the furnace followed by a 20 min cool down to room temperature. The sample weights were measured to 0.0001 g at each cycle until the end of their respective tests. Before and after oxidation, the samples were analyzed using X-ray diffraction (D/max 2200pc, Rigaku) using a Cu K␣ source, and scanning electron microscopy (JSM-5800, JEOL) with energy dispersive X-ray analysis (EDAX).
3. Results and discussion 3.1. Annealing effect on formation of Al–Cu–Fe–Cr quasicrystalline coating It was found that during the spraying loss of Al occurs due to the poor thermal conductivity of quasicrystals. The loss of Al shifts the coating composition away from the quasicrystalline phase region since the quasicrystalline phase region in the phase diagram is very small [12–14]. Therefore, as-deposited Al–Cu–Fe–Cr coating is a kind of mixtures of quasicrystalline and crystalline phases. In order to decrease the crystalline phases in the coating, the heat treatment should be performed. Fig. 1 shows X-ray diffraction pattern of as-sprayed Al–Cu–Fe–Cr coating and as-sprayed Al–Cu–Fe–Cr coating annealed in vacuum at different temperatures for 5 h. As-sprayed Al–Cu–Fe–Cr coating is a kind of mixtures of icosahedral (Al65 Cu20 Fe15 ) and decagonal quasicrystalline phases (Al65 Cr5 Cu20 Fe10 ), and crystalline phases (Fig. 1(a)). The result is in accordance with the results of Dubois et al. [15] and Dong and Dubois [16]. With increasing heat treatment temperature, As shown in Fig. 1, icosahedral phase at around 43◦ in 2θ initially is increased and then decreased. The amount of crystalline phases is decreased and the amount of decagonal quasicrys-
363
talline phase is increased. As shown in Fig. 1(e), as sprayed Al–Cu–Fe–Cr coating after heat treatment at 950 ◦ C for 5 h is consisted of decagonal phase and very small amount of cubic phase peaks, and icosahedral quasicrystalline phase is disappeared. Huttunen-Saarivirta et al. investigated influence of Cr alloying on the microstructure of thermally sprayed quasicrystalline Al–Cu–Fe coating [17]. Huttunen-Saarivirta et al.’s work shows that Cr addition in Al–Cu–Fe alloy introduces two different icosahedral phases (Al80 Cr13.5 Fe6.5 and Al13 Cr3 Cu4 ) in the thermal sprayed coating. In the present investigation icosahedral Al80 Cr13.5 Fe6.5 or Al13 Cr3 Cu4 was not found in the thermal sprayed coating. The difference in phase composition may result from different spraying parameters and powder compositions. 3.2. Microstructures of the LPPS Al–Cu–Fe–Cr quasicrystalline coating Fig. 2 shows a typical lamellar splat formation of the low pressure plasma sprayed Al–Cu–Fe–Cr quasicrystalline coating. The coating was found to be relatively dense, and some cracks were observed. The thickness of the coating was about 100 m. 3.3. Cyclic oxidation kinetics Cyclic oxidation test was performed to investigate the influence of Al–Cu–Fe–Cr quasicrystalline coating on the oxidation behavior of the titanium alloy. Fig. 3 shows a plot of the weight change per unit area versus number of cycles for the test performed at 650 and 800 ◦ C in static air. The titanium alloy oxidized at a high rate. It maintains a good appearance only up to two cycles, but yellow nodules appear on the surface after three cycles. Spallation occurred during the course of cyclic oxidation. For Al–Cu–Fe–Cr quasicrystalline coat-
Fig. 1. X-ray diffraction pattern of as-sprayed Al–Cu–Fe–Cr coating and assprayed Al–Cu–Fe–Cr coating annealed in vacuum at different temperatures for 5 h.
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Fig. 2. Morphologies of LPPS Al–Cu–Fe–Cr quasicrystalline coating: (a) surface and (b) cross-section.
ing, at the initial stage it shows some weight change. After 25 cycles only a little change in weight is observed. During the course of oxidation no evident spallation occurs. After 624 cycles at 650 ◦ C and 290 cycles at 800 ◦ C, the weight changes per unit area were about 0.7 and 1.5 mg/cm2 , respectively. Gurrappa [18] investigated the effect of aluminizing on the oxidation rate of titanium alloy at 800 ◦ C. It was found that the weight gain was about 2.0 mg/cm2 after cyclic-isothermal
oxidation at 800 ◦ C for 300 h. Compared with the result given by Gurrappa, the oxidation resistance of Al–Cu–Fe–Cr quasicrystalline coating is same as aluminide coating. It can be drawn from the above result that the Al–Cu–Fe–Cr quasicrystalline coatings provide excellent protection of the titanium alloys against cyclic oxidation at both 650 and 800 ◦ C. 3.4. Scale structure To identify the phases developed on the surface of LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation in air at 800 ◦ C for 290 cycles, the specimen surfaces were analyzed by X-ray diffraction. The result is shown in Fig. 4. XRD shows peaks for ␥-Al2 O3 , showing the oxide formed on the LPPS Al–Cu–Fe–Cr quasicrystalline coating is composed of ␥-Al2 O3 and the thickness of the oxide is very thin since the peak intensities for the coating are very strong. 3.5. Morphologies of the oxide Fig. 5 shows the surface morphologies of the LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation
Fig. 3. Plot of weight change per unit area vs. number of cycles for cyclic oxidation test at (a) 650 ◦ C and (b) 800 ◦ C.
Fig. 4. XRD spectrum from the Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation at 800 ◦ C for 290 cycles.
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Fig. 5. Surface morphologies of LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation at (a) 650 ◦ C for 624 cycles and (b) 800 ◦ C for 290 cycles.
in air at 650 ◦ C for 624 cycles and at 800 ◦ C for 290 cycles. SEM image of the surface for the LPPS Al–Cu–Fe–Cr quasicrystalline coating oxidized at 650 ◦ C for 624 cycles (Fig. 5(a)) is different from that of 800 ◦ C (Fig. 5(b)). The coating oxidized at 800 ◦ C for 290 cycles has smaller grain (2–3 m) than that of at 650 ◦ C (∼6 m) for 624 cycles. The surfaces of the coatings are very dense and the oxides formed are composed of small round grains. No evident cracks on the surfaces are observed after cyclic oxidation test. Fig. 6 shows the cross-sectional image of the coatings after cyclic oxidation at 800 ◦ C for 290 cycles. Very thin oxide layers of about 2–5 m can be seen on the surface. EDS analysis indicates that the oxide layer formed on Al–Cu–Fe–Cr quasicrystalline coating is composed of Al and O elements. The result is in accordance with XRD result. For the oxidation of Al–Cu–Fe–Cr quasicrystalline coating, during the very initial stage of oxidation there is a rapid uptake of oxygen by the coating and various oxides such as CuO, Al2 O3 , FeO as well as Cr2 O3 are formed on the surface of the coating [19]. Because Al2 O3 is the most stable thermodynamically, replacement reaction of Al occurs, resulting in the formation of Al2 O3 on the coating. The thermal stress developed in the oxide layer during cyclic oxidation can be estimated by the following equation [19]: σox = −
Eox T (αM − αOX ) 1 + 2(Eox /EM ξ/ h)
(1)
Table 2 Constants used in Eq. (1) System
Thermal-expansion coefficient (K−1 )
Young’s modulus (GPa)
Temperature change (K)
Al2 O3 Al–Cu–Fe–Cr
9.0 × 10−6 [21] (14–17) × 10−6 [20]
400 [21] 68 [22]
775 775
where EM and Eox are Young’s moduli of the coating and the oxide, respectively; αM and αox the linear thermal expansion coefficient of the coating and oxide, respectively; ξ the oxide thickness (∼4 m); and h the half thickness of the coating (∼50 m). For Al–Cu–Fe–Cr quasicrystal, its linear thermal expansion coefficient has not been reported. However, Dubois [20] reported values between 14 × 10−6 and 17 × 10−6 K−1 for Al–Cu–Fe quasicrystal. The linear thermal expansion coefficient of Al–Cu–Fe–Cr quasicrystal should be very similar. Using the relevant constants listed in Table 2, the thermal stress in Al2 O3 without considering creep release is calculated as about −1100 MPa. On the other hand, the maximum compressive strength of Al2 O3 is given as 3000 MPa [23]. This means that the oxide scales of Al2 O3 formed on the coating would be able to withstand the thermal contraction during the cooling and would not be spalled, which protects the Cu and Fe components from oxidizing. Therefore, Al–Cu–Fe–Cr quasicrystalline coating improved the cyclic oxidation resistance of titanium-based alloys.
4. Conclusion
Fig. 6. Cross-sectional morphologies of LPPS Al–Cu–Fe–Cr quasicrystalline coating after cyclic oxidation at 800 ◦ C for 290 cycles.
Al–Cu–Fe–Cr quasicrystalline coating has been made by low pressure plasma spraying (LPPS) method and characterized. Formation of Al–Cu–Fe–Cr quasicrystalline phase is closely related to annealing temperature. As-deposited Al–Cu–Fe–Cr coating is a kind of mixtures of quasicrystalline and crystalline phases. With increasing heat treatment temperature, icosahedral phase at around 43◦ in 2θ initially is increased and then decreased. The amount of crystalline phases is decreased and the amount of decagonal quasicrystalline phase is increased. Under atmospheric pressure in static air at 650 and 800 ◦ C, the coating exhibits excellent
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cyclic oxidation resistance. The oxide formed on the surface of LPPS Al–Cu–Fe quasicrystalline coating after cyclic oxidation consisted of Al2 O3 .
Acknowledgement The Aviation Science Foundation of China is acknowledged for financial support under Grant No. 00H51006.
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