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Oxidation behavior of thermal barrier coatings obtained by detonation spraying
Surface and Coatings Technology 166 (2003) 189–194
Oxidation behavior of thermal barrier coatings obtained by detonation spraying Y.N. Wu*, F.H. Wang, W.G. Hua, J. Gong, C. Sun, L.S. Wen Institute of Metal Research, Chinese Academy of Sciences, State Key Laboratory for Corrosion and Protection, 72 Wenhua Road, Shenyang, 110016, PR China Received 18 June 2002; accepted 25 October 2002
Abstract Thermal barrier coatings (TBCs) have been successfully obtained by detonation spraying, through optimizing the spray parameters (especially the ratio of C2 H2 to O2 ). The oxidation behaviors of detonation sprayed TBCs at 1000 and 1100 8C were studied. The results indicated that the detonation sprayed TBCs were uniform and dense, with a few microcracks in the ceramic coats and a rough surface of bond coats. At the high temperature, the dense detonation sprayed ceramic coats with low porosity could obviously decrease the diffusive channels for oxygen and reduce the oxygen pressure (PO2 ) at the ceramicybond coat interface. Under the lower oxygen pressure at the interface, it was advantageous to the formation of a continuous protective Al2O3 layer and the growth of a thermally grown oxide layer (TGO) obeyed the fourth power law. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal barrier coatings; Detonation spraying; Oxidation; Thermally grown oxide
1. Introduction In order to increase the gas turbine inlet temperature, thermal barrier coatings (TBCs) coated on the gas turbine components by plasma spraying (PS) w1,2x and electron beam physical vapor deposition (EB-PVD) w3,4x have been investigated and developed for over 40 years. In spite of significant advancements in the plasma spraying process, plasma sprayed TBCs exhibit nonuniform lamellar structure, including voids, unmelted particles and oxide inclusions w5x. These defects may be disadvantageous to the high temperature properties of TBCs. Although the high temperature performances of TBCs obtained by EB-PVD are better than those of the plasma sprayed TBCs, the low deposition rate and complex and expensive equipments are unsatisfactory. Therefore, some new preparation techniques for TBCs should be tried and discussed. Gas detonation equipment was originally developed and patented by Union Carbide (now Praxair) in 1955 w6x. Up to now, the detonation spraying technique has *Corresponding author. Tel.: q86-24-83978235; fax: q86-2423843436. E-mail address: [email protected] (Y.N. Wu).
been widely applied, especially to the aircraft industries in the United States, Japan, and the former Soviet Union w6–9x. Compared with other thermal spray techniques, the detonation spraying technique is characterized by the detonation wave and impact on the substrate surface at the highest velocity of 800–1200 m sy1. During detonation spraying, the sufficiently high velocity of the particles can produce a uniform and dense coating with high hardness and strong adhesion to the substrate. This method permits the deposition of quality coatings for various purposes from various powder materials. Due to some technical difficulties, such as low torch temperature and very short heating time for ZrO2 – Y2O3 (YPSZ) ceramic particles during the spraying process, the detonation spraying technique has scarcely been used to obtain TBCs. In this paper, TBCs were successfully obtained by detonation spraying, through optimizing the spray parameters, especially the ratio of C2H2 to O2. The oxidation kinetics of the detonation sprayed TBCs at 1000 and 1100 8C were studied. The growth mechanism of the thermally grown oxide layer (TGO) at the interface between ceramic and bond coats was also discussed.
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 8 3 - 1
Y.N. Wu et al. / Surface and Coatings Technology 166 (2003) 189–194
190
2. Experimental details The substrates were superalloy M38G (C: 0.13–0.20; Cr: 15.3–16.3; Co: 8.0–9.0; W: 2.3–2.9; Mo: 1.4–2.0; Al: 3.5–4.5; Ti: 3.2–4.0; Nb: 0.4–1.0; Ta: 1.4–2.0; Zr: 0.05–0.15; B: 0.005–0.015; Ni: Balance, wt.%) with a size of 25=15=2 mm, which were degreased and sandblasted with Al2O3 (40 mesh) before spraying. The powder materials for spraying were Ni–25Cr–5Al–0.5Y used for the bond coats and ZrO2 –8Y2O3 (YPSZ) for the ceramic coats, with the size distribution of 50–75 mm. A Russian detonation spraying system (‘ob’-type) with two powder feeders was employed. The ranges of thickness of bond coats and ceramic coats were 80–100 mm and 250–350 mm, respectively. Isothermal oxidation tests were carried out at 1000 8C in static air. The specimens were placed in alumina crucibles, oxidized at 1000 8C and then cooled to room temperature at regular intervals for mass measurements. Cyclic oxidation tests were performed in a furnace controlled automatically. The cycles consisted of a 60min isothermal hold at 1100 8C in static ambient air, and cooling for 10 min. The total time in the furnace at 1100 8C was 200 h. At regular times, the mass of samples was measured. The sensitivity of the balance used in this paper was 0.01 mg. The oxidation behavior was evaluated by the mass gain of the samples. After the tests, the cross-sectional morphology of TBCs and the chemical composition of TGO were analyzed by a S-360 scanning electron microscope (Cambridge Instruments, UK), equipped with an X-ray energy-dispersive spectrum (EDS). 3. Results 3.1. Detonation sprayed TBCs First, the maximum temperature of detonation torch is typically 3500–4500 8C, much lower than that of the plasma spray flame. Secondly, the heating time for particles in the detonation torch is very short and the heat transfer between the detonation wave and the particles is very limited, because of high velocity flow. Therefore, it is difficult to melt the ZrO2 –Y2O3 (YPSZ) ceramic particles (Tms2600 8C) during detonation spraying. The melting behavior of sprayed particles is sensitive to the spraying parameters, especially the ratio
Fig. 1. Cross-sectional morphology of detonation sprayed TBCs (a) and the ZrO2 –8Y2O3 ceramic coats after etching (b).
of C2H2 to O2. In Gavrilenko’s studies w10x, the simulated temperature of the detonation wave is increased with increasing the composition of C2H2 in the mixed fuel gas. In our experiments, the results indicated that the uniform YPSZ ceramic coats without the unmelted particles were successfully obtained on the bond coats, by increasing the ratio of C2H2 to O2 up to 1:1.08. The optimum parameters (such as the ratio of C2H2 to O2, spray distance, powder flow rate, diameter of spot) are shown in Table 1. As shown in Fig. 1a, a dense ceramic coat was obtained by detonation spraying, with a few micro-
Table 1 Parameters of detonation spraying Parameters
C2H2:O2
Frequency (shot sy1)
Diameter of spot (mm)
Distance (mm)
Powder flow rate (g sy1)
Bond coats (Ni25Cr5Al0.5Y) Ceramic coats (ZrO2 –8Y2O3)
1:(1.20–1.25)
4–6
25
100–140
0.3–0.6
1:(1.03–1.08)
4–6
25
80–120
0.3–0.9
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191
Fig. 3. Oxidation kinetics plotted as DW 2 –t and DW 4 –t for alloy M38G and detonation sprayed TBCs at 1000 8C. Fig. 2. Isothermal oxidation kinetics of alloy M38G and the detonation sprayed TBCs at 1000 8C.
cracks. The principal cracks were vertical, perpendicular to the substrate. There were also some horizontal branch cracks. The detonation sprayed bond coat had a very rough surface. A few microcracks in the ceramic coat and a rough bond coat surface can enhance the thermal shock resistance and increase the lifetime of TBCs during thermal cycling. Fig. 1b represents the crosssectional morphology of the detonation sprayed ceramic coat after etching. Due to the intermittent process during detonation spraying, the dense ceramic coat was the regular and thin lamellar structure, with few pores and voids. The lamellar interface oriented perpendicular to the direction of heat flow is beneficial to decreasing the thermal conductivity of ceramic coats.
(h 2s2Kpt; h, the thickness of TGO; Kp, the parabolic rate constant) w12x, the growth of TGO at the interface of detonation sprayed TBCs accorded with the fourth power law (as shown in Fig. 3): DW4sKpt
(1)
3.2. Oxidation tests
where Kp is the constant. The value of Kp at 1000 8C was 1.38=10y2 mg4 cmy8 hy1. In Miller’s studies w11x, the different laws of oxidation kinetics are related to the oxygen partial pressures at the ceramicybond coat interface. After oxidation at 1000 8C for 100 h, a continuous TGO was formed at the ceramicybond coat interface, as shown in Fig. 4. From the result of EDS (Fig. 5), the main composition of TGO was Al2O3, which could hinder the diffusion of oxygen into the bond coat and
3.2.1. Isothermal oxidation Fig. 2 represents the mass gains as a function of time for TBCs and alloy M38G at 1000 8C. Compared with the kinetic curve of alloy M38G, the oxidation kinetic curve of TBCs showed slower mass gain (indicating growth of a continuous, protective scale) after the initial oxidation stage defined by sharp mass gain. In the early stage of oxidation (-20 h), the volatilization of MoO3 and CrO3 from the surface possibly induced to the slower mass gain of M38G than that of TBCs. With the depletion of Cr and Mo, M38G showed higher oxidation mass gain than TBCs. The oxidation velocity of TBCs was 1.06=10y2 mg cmy2 hy1, lower than that of alloy M38G (1.70=10y2 mg cmy2 hy1). Different from the growth law of TGO of plasma sprayed TBCs (lnWsb0qb1lnt, where b0 and b1 change with the increasing of oxidation time and temperature) w11x and that of TBCs obtained by EB-PVD
Fig. 4. Cross-sectional morphology of detonation sprayed TBCs after oxidation at 1000 8C for 100 h.
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detonation sprayed TBCs became gentle. The growth of TGO also obeyed the fourth power law at 1100 8C (in Fig. 6b). As shown in Fig. 7a, the thickness of TGO was also approximately 1 mm at 1100 8C for 200 cyclic times. With the increasing of oxidation temperature and time, the thickness increase of TGO was slow. The main oxides of TGO were Cr2O3, Al2O3 and NiO (in Fig. 7b). With the increasing of oxidation temperature, the diffusion rate of oxygen through the ceramic coat was increased, which induced to the increase of oxygen pressure at the ceramicybond coat interface. The change of oxygen pressure would lead to the variation of composition of TGO at the interface. Therefore, the compositions of TGO which were Cr2O3, Al2O3 and NiO after oxidation at 1100 8C were different from the continuous Al2O3 at 1000 8C. 4. Discussion The formation of a continuous Al2O3 layer at the ceramicybond coat interface is beneficial to improving
Fig. 5. Cross-sectional morphology of TGO (a) and the point scanned EDS result of TGO (b) after oxidation at 1000 8C for 100 h.
reduce the oxidation rate of TBCs. The average thickness of the Al2O3 layer was approximately 1 mm. 3.2.2. Cyclic oxidation After cyclic oxidation at 1100 8C for 200 cyclic times, no spallation occurred on the detonation sprayed TBCs. The roughness of the bond coat surface can provide mechanical interlocking at the ceramicybond coat interface and prevent or postpone TBCs spallation w13x. The thermal expansion mismatch stress can be uncoupled from the metallic substrate by the vertical cracks in the ceramic coat w14x. Therefore, a few microcracks in the ceramic coat and a rough bond coat surface can be advantageous to increasing the lifetime of TBCs during thermal cycling. The cyclic oxidation dynamic curves of detonation sprayed TBCs and alloy M38G are shown in Fig. 6. It could be seen that the mass gain of alloy M38G began to decrease after oxidation for 40 h, induced by the vaporization of CrO3 and spallation of oxide scales from the bare alloy M38G specimens. After sharp mass gain in the initial oxidation stage, the oxidation curve of
Fig. 6. Cyclic oxidation kinetics (a) and oxidation kinetics plotted as DW 4 –t (b) for alloy M38G and detonation sprayed TBCs at 1100 8C.
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Fig. 7. Cross-sectional morphology of TGO (a) and the point scanned EDS result of TGO (b) after oxidation at 1100 8C for 200 h.
the life of TBCs, due to the slow growth rate of Al2O3 and low diffusion of oxygen in Al2O3. It is advantageous to the formation of a protective Al2O3 layer through reducing the PO2 at the ceramicybond coat interface, because the decomposed pressure of Al2O3 is lower than that of Cr2O3 and NiO (Al2O3: 10y14 Pa, Cr2O3: 10y8 Pa, NiO: 10y4 Pa, at 1000 8C w15x). The zirconia helps to transport oxygen from the gas stream to the bond coat by two mechanisms w16x: ionic transportation through the lattice by reverse movement of oxygen ion vacancies, and gaseous diffusion along the networks of interconnected microcracks and pores. Detonation spray is characterized by higher velocity of particles, which produce a dense ceramic coat. Compared with the plasma sprayed ceramic coat (porosity: 5–15%), the dense ceramic coats with low porosity (0.5–2%) can decrease the diffusive channels for oxygen and reduce the value of oxygen pressure (PO2) at the ceramicybond coat interface. At the lower oxygen pressure, the growth of Cr2O3 and NiO is restrained and a continuous single Al2O3 layer is prone to form at the ceramicybond coat interface. The lower the oxygen pressure at the interface, the more advantageous to the formation of a protective Al2O3 layer. In our experiments, the results indicated that a continuous Al2O3 layer formed at the ceramicybond coat interface of
detonation sprayed TBCs after oxidation at 1000 8C for 100 h. Despite the low porosity of detonation sprayed YPSZ, a few microcracks in YPSZ and a rough bond coat surface could also accommodate the thermal stress during thermal cycling. The results indicated that no spallation occurred on the detonation sprayed TBCs after cycling oxidation at 1100 8C for 200 cycling times. At 1000 and 1100 8C, the growth of TGO obeyed the fourth power law, which possessed the better oxidation resistance. The decreasing of oxygen pressure at the interface of detonation sprayed TBCs could slow down the growth rate of TGO and change the growth kinetics of TGO, because the lower the oxygen pressure at the interface, the more advantageous to the formation of a protective Al2O3 layer. 5. Conclusions The detonation sprayed TBCs displayed dense structures, with a few microcracks in the ceramic coats and a rough surface of the bond coats which could enhance the thermal shock resistance and increase the lifetime of TBCs during thermal cycling. The dense ceramic coat with low porosity decreased the diffusive channels for oxygen through the ceramic
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coat and reduced the oxygen pressure at the ceramicy bond coat interface, which induced the formation of continuous Al2O3 and a different growth law of TGO from the plasma sprayed TBCs and the TBCs obtained by EB-PVD. References w1x M.P. Borom, C.A. Johnson, Surf. Coat. Technol. 54y55 (1992) 45. w2x R. Taylor, J.R. Brandon, Surf. Coat. Technol. 50 (1992) 141. w3x A.A. Tchizhik, A.I. Rybnikov, I.S. Malashenko, S.A. Leontiev, A.S. Osyka, Surf. Coat. Technol. 78 (1996) 113. w4x J. Cheng, E.H. Jordan, B. Barber, M. Gell, Acta Mater. 46 (1998) 5839. w5x T.A. Mahank, J. Singh, A.K. Kulkarni, Mater. Manu. Proc. 13 (1998) 829.
w6x Y.A. Kharlamov, Mater. Sci. Eng. 93 (1987) 1. w7x E. Kadyrov, V. Kadyrov, Adv. Mater. Process 8 (1995) 21. w8x X.Q. Ma, Y.M. Rhyim, H.W. Jin, C.G. Park, Mater. Manu. Proc. 14 (1999) 195. w9x R.C. Tucker, J. Vac. Sci. Technol. 11 (1974) 725. w10x T.P. Gavrilenko, Y.A. Nikolaev, V.Y. Ulianitsky, Proceedings of the 8th National Thermal Spray Conference, ASM International, Materials Park, Ohio, Houston, Texas, 1995, p. 51. w11x B.A. Miller, J. Am. Ceram. Soc. 67 (1984) 517. w12x A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505. w13x T.A. Taylor, D.L. Appleby, A.E. Weatherill, J. Griffiths, Surf. Coat. Technol. 43y44 (1990) 470. w14x C.H. Hsuech, J.A. Haynes, M.J. Lance, et al., J. Am. Ceram. Soc. 82 (1999) 1073. w15x N. Birks, G.H. Meier, Introduction to High Temperature Oxidation of Metals, Edward Arnold, London, 1983. w16x A. Bennett, Mater. Sci. Technol. 2 (1986) 257.
Oxidation behavior of thermal barrier coatings obtained by detonation spraying Y.N. Wu*, F.H. Wang, W.G. Hua, J. Gong, C. Sun, L.S. Wen Institute of Metal Research, Chinese Academy of Sciences, State Key Laboratory for Corrosion and Protection, 72 Wenhua Road, Shenyang, 110016, PR China Received 18 June 2002; accepted 25 October 2002
Abstract Thermal barrier coatings (TBCs) have been successfully obtained by detonation spraying, through optimizing the spray parameters (especially the ratio of C2 H2 to O2 ). The oxidation behaviors of detonation sprayed TBCs at 1000 and 1100 8C were studied. The results indicated that the detonation sprayed TBCs were uniform and dense, with a few microcracks in the ceramic coats and a rough surface of bond coats. At the high temperature, the dense detonation sprayed ceramic coats with low porosity could obviously decrease the diffusive channels for oxygen and reduce the oxygen pressure (PO2 ) at the ceramicybond coat interface. Under the lower oxygen pressure at the interface, it was advantageous to the formation of a continuous protective Al2O3 layer and the growth of a thermally grown oxide layer (TGO) obeyed the fourth power law. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal barrier coatings; Detonation spraying; Oxidation; Thermally grown oxide
1. Introduction In order to increase the gas turbine inlet temperature, thermal barrier coatings (TBCs) coated on the gas turbine components by plasma spraying (PS) w1,2x and electron beam physical vapor deposition (EB-PVD) w3,4x have been investigated and developed for over 40 years. In spite of significant advancements in the plasma spraying process, plasma sprayed TBCs exhibit nonuniform lamellar structure, including voids, unmelted particles and oxide inclusions w5x. These defects may be disadvantageous to the high temperature properties of TBCs. Although the high temperature performances of TBCs obtained by EB-PVD are better than those of the plasma sprayed TBCs, the low deposition rate and complex and expensive equipments are unsatisfactory. Therefore, some new preparation techniques for TBCs should be tried and discussed. Gas detonation equipment was originally developed and patented by Union Carbide (now Praxair) in 1955 w6x. Up to now, the detonation spraying technique has *Corresponding author. Tel.: q86-24-83978235; fax: q86-2423843436. E-mail address: [email protected] (Y.N. Wu).
been widely applied, especially to the aircraft industries in the United States, Japan, and the former Soviet Union w6–9x. Compared with other thermal spray techniques, the detonation spraying technique is characterized by the detonation wave and impact on the substrate surface at the highest velocity of 800–1200 m sy1. During detonation spraying, the sufficiently high velocity of the particles can produce a uniform and dense coating with high hardness and strong adhesion to the substrate. This method permits the deposition of quality coatings for various purposes from various powder materials. Due to some technical difficulties, such as low torch temperature and very short heating time for ZrO2 – Y2O3 (YPSZ) ceramic particles during the spraying process, the detonation spraying technique has scarcely been used to obtain TBCs. In this paper, TBCs were successfully obtained by detonation spraying, through optimizing the spray parameters, especially the ratio of C2H2 to O2. The oxidation kinetics of the detonation sprayed TBCs at 1000 and 1100 8C were studied. The growth mechanism of the thermally grown oxide layer (TGO) at the interface between ceramic and bond coats was also discussed.
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 8 3 - 1
Y.N. Wu et al. / Surface and Coatings Technology 166 (2003) 189–194
190
2. Experimental details The substrates were superalloy M38G (C: 0.13–0.20; Cr: 15.3–16.3; Co: 8.0–9.0; W: 2.3–2.9; Mo: 1.4–2.0; Al: 3.5–4.5; Ti: 3.2–4.0; Nb: 0.4–1.0; Ta: 1.4–2.0; Zr: 0.05–0.15; B: 0.005–0.015; Ni: Balance, wt.%) with a size of 25=15=2 mm, which were degreased and sandblasted with Al2O3 (40 mesh) before spraying. The powder materials for spraying were Ni–25Cr–5Al–0.5Y used for the bond coats and ZrO2 –8Y2O3 (YPSZ) for the ceramic coats, with the size distribution of 50–75 mm. A Russian detonation spraying system (‘ob’-type) with two powder feeders was employed. The ranges of thickness of bond coats and ceramic coats were 80–100 mm and 250–350 mm, respectively. Isothermal oxidation tests were carried out at 1000 8C in static air. The specimens were placed in alumina crucibles, oxidized at 1000 8C and then cooled to room temperature at regular intervals for mass measurements. Cyclic oxidation tests were performed in a furnace controlled automatically. The cycles consisted of a 60min isothermal hold at 1100 8C in static ambient air, and cooling for 10 min. The total time in the furnace at 1100 8C was 200 h. At regular times, the mass of samples was measured. The sensitivity of the balance used in this paper was 0.01 mg. The oxidation behavior was evaluated by the mass gain of the samples. After the tests, the cross-sectional morphology of TBCs and the chemical composition of TGO were analyzed by a S-360 scanning electron microscope (Cambridge Instruments, UK), equipped with an X-ray energy-dispersive spectrum (EDS). 3. Results 3.1. Detonation sprayed TBCs First, the maximum temperature of detonation torch is typically 3500–4500 8C, much lower than that of the plasma spray flame. Secondly, the heating time for particles in the detonation torch is very short and the heat transfer between the detonation wave and the particles is very limited, because of high velocity flow. Therefore, it is difficult to melt the ZrO2 –Y2O3 (YPSZ) ceramic particles (Tms2600 8C) during detonation spraying. The melting behavior of sprayed particles is sensitive to the spraying parameters, especially the ratio
Fig. 1. Cross-sectional morphology of detonation sprayed TBCs (a) and the ZrO2 –8Y2O3 ceramic coats after etching (b).
of C2H2 to O2. In Gavrilenko’s studies w10x, the simulated temperature of the detonation wave is increased with increasing the composition of C2H2 in the mixed fuel gas. In our experiments, the results indicated that the uniform YPSZ ceramic coats without the unmelted particles were successfully obtained on the bond coats, by increasing the ratio of C2H2 to O2 up to 1:1.08. The optimum parameters (such as the ratio of C2H2 to O2, spray distance, powder flow rate, diameter of spot) are shown in Table 1. As shown in Fig. 1a, a dense ceramic coat was obtained by detonation spraying, with a few micro-
Table 1 Parameters of detonation spraying Parameters
C2H2:O2
Frequency (shot sy1)
Diameter of spot (mm)
Distance (mm)
Powder flow rate (g sy1)
Bond coats (Ni25Cr5Al0.5Y) Ceramic coats (ZrO2 –8Y2O3)
1:(1.20–1.25)
4–6
25
100–140
0.3–0.6
1:(1.03–1.08)
4–6
25
80–120
0.3–0.9
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Fig. 3. Oxidation kinetics plotted as DW 2 –t and DW 4 –t for alloy M38G and detonation sprayed TBCs at 1000 8C. Fig. 2. Isothermal oxidation kinetics of alloy M38G and the detonation sprayed TBCs at 1000 8C.
cracks. The principal cracks were vertical, perpendicular to the substrate. There were also some horizontal branch cracks. The detonation sprayed bond coat had a very rough surface. A few microcracks in the ceramic coat and a rough bond coat surface can enhance the thermal shock resistance and increase the lifetime of TBCs during thermal cycling. Fig. 1b represents the crosssectional morphology of the detonation sprayed ceramic coat after etching. Due to the intermittent process during detonation spraying, the dense ceramic coat was the regular and thin lamellar structure, with few pores and voids. The lamellar interface oriented perpendicular to the direction of heat flow is beneficial to decreasing the thermal conductivity of ceramic coats.
(h 2s2Kpt; h, the thickness of TGO; Kp, the parabolic rate constant) w12x, the growth of TGO at the interface of detonation sprayed TBCs accorded with the fourth power law (as shown in Fig. 3): DW4sKpt
(1)
3.2. Oxidation tests
where Kp is the constant. The value of Kp at 1000 8C was 1.38=10y2 mg4 cmy8 hy1. In Miller’s studies w11x, the different laws of oxidation kinetics are related to the oxygen partial pressures at the ceramicybond coat interface. After oxidation at 1000 8C for 100 h, a continuous TGO was formed at the ceramicybond coat interface, as shown in Fig. 4. From the result of EDS (Fig. 5), the main composition of TGO was Al2O3, which could hinder the diffusion of oxygen into the bond coat and
3.2.1. Isothermal oxidation Fig. 2 represents the mass gains as a function of time for TBCs and alloy M38G at 1000 8C. Compared with the kinetic curve of alloy M38G, the oxidation kinetic curve of TBCs showed slower mass gain (indicating growth of a continuous, protective scale) after the initial oxidation stage defined by sharp mass gain. In the early stage of oxidation (-20 h), the volatilization of MoO3 and CrO3 from the surface possibly induced to the slower mass gain of M38G than that of TBCs. With the depletion of Cr and Mo, M38G showed higher oxidation mass gain than TBCs. The oxidation velocity of TBCs was 1.06=10y2 mg cmy2 hy1, lower than that of alloy M38G (1.70=10y2 mg cmy2 hy1). Different from the growth law of TGO of plasma sprayed TBCs (lnWsb0qb1lnt, where b0 and b1 change with the increasing of oxidation time and temperature) w11x and that of TBCs obtained by EB-PVD
Fig. 4. Cross-sectional morphology of detonation sprayed TBCs after oxidation at 1000 8C for 100 h.
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detonation sprayed TBCs became gentle. The growth of TGO also obeyed the fourth power law at 1100 8C (in Fig. 6b). As shown in Fig. 7a, the thickness of TGO was also approximately 1 mm at 1100 8C for 200 cyclic times. With the increasing of oxidation temperature and time, the thickness increase of TGO was slow. The main oxides of TGO were Cr2O3, Al2O3 and NiO (in Fig. 7b). With the increasing of oxidation temperature, the diffusion rate of oxygen through the ceramic coat was increased, which induced to the increase of oxygen pressure at the ceramicybond coat interface. The change of oxygen pressure would lead to the variation of composition of TGO at the interface. Therefore, the compositions of TGO which were Cr2O3, Al2O3 and NiO after oxidation at 1100 8C were different from the continuous Al2O3 at 1000 8C. 4. Discussion The formation of a continuous Al2O3 layer at the ceramicybond coat interface is beneficial to improving
Fig. 5. Cross-sectional morphology of TGO (a) and the point scanned EDS result of TGO (b) after oxidation at 1000 8C for 100 h.
reduce the oxidation rate of TBCs. The average thickness of the Al2O3 layer was approximately 1 mm. 3.2.2. Cyclic oxidation After cyclic oxidation at 1100 8C for 200 cyclic times, no spallation occurred on the detonation sprayed TBCs. The roughness of the bond coat surface can provide mechanical interlocking at the ceramicybond coat interface and prevent or postpone TBCs spallation w13x. The thermal expansion mismatch stress can be uncoupled from the metallic substrate by the vertical cracks in the ceramic coat w14x. Therefore, a few microcracks in the ceramic coat and a rough bond coat surface can be advantageous to increasing the lifetime of TBCs during thermal cycling. The cyclic oxidation dynamic curves of detonation sprayed TBCs and alloy M38G are shown in Fig. 6. It could be seen that the mass gain of alloy M38G began to decrease after oxidation for 40 h, induced by the vaporization of CrO3 and spallation of oxide scales from the bare alloy M38G specimens. After sharp mass gain in the initial oxidation stage, the oxidation curve of
Fig. 6. Cyclic oxidation kinetics (a) and oxidation kinetics plotted as DW 4 –t (b) for alloy M38G and detonation sprayed TBCs at 1100 8C.
Y.N. Wu et al. / Surface and Coatings Technology 166 (2003) 189–194
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Fig. 7. Cross-sectional morphology of TGO (a) and the point scanned EDS result of TGO (b) after oxidation at 1100 8C for 200 h.
the life of TBCs, due to the slow growth rate of Al2O3 and low diffusion of oxygen in Al2O3. It is advantageous to the formation of a protective Al2O3 layer through reducing the PO2 at the ceramicybond coat interface, because the decomposed pressure of Al2O3 is lower than that of Cr2O3 and NiO (Al2O3: 10y14 Pa, Cr2O3: 10y8 Pa, NiO: 10y4 Pa, at 1000 8C w15x). The zirconia helps to transport oxygen from the gas stream to the bond coat by two mechanisms w16x: ionic transportation through the lattice by reverse movement of oxygen ion vacancies, and gaseous diffusion along the networks of interconnected microcracks and pores. Detonation spray is characterized by higher velocity of particles, which produce a dense ceramic coat. Compared with the plasma sprayed ceramic coat (porosity: 5–15%), the dense ceramic coats with low porosity (0.5–2%) can decrease the diffusive channels for oxygen and reduce the value of oxygen pressure (PO2) at the ceramicybond coat interface. At the lower oxygen pressure, the growth of Cr2O3 and NiO is restrained and a continuous single Al2O3 layer is prone to form at the ceramicybond coat interface. The lower the oxygen pressure at the interface, the more advantageous to the formation of a protective Al2O3 layer. In our experiments, the results indicated that a continuous Al2O3 layer formed at the ceramicybond coat interface of
detonation sprayed TBCs after oxidation at 1000 8C for 100 h. Despite the low porosity of detonation sprayed YPSZ, a few microcracks in YPSZ and a rough bond coat surface could also accommodate the thermal stress during thermal cycling. The results indicated that no spallation occurred on the detonation sprayed TBCs after cycling oxidation at 1100 8C for 200 cycling times. At 1000 and 1100 8C, the growth of TGO obeyed the fourth power law, which possessed the better oxidation resistance. The decreasing of oxygen pressure at the interface of detonation sprayed TBCs could slow down the growth rate of TGO and change the growth kinetics of TGO, because the lower the oxygen pressure at the interface, the more advantageous to the formation of a protective Al2O3 layer. 5. Conclusions The detonation sprayed TBCs displayed dense structures, with a few microcracks in the ceramic coats and a rough surface of the bond coats which could enhance the thermal shock resistance and increase the lifetime of TBCs during thermal cycling. The dense ceramic coat with low porosity decreased the diffusive channels for oxygen through the ceramic
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