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The control of the composition and structure of aluminide layers formed by vapour aluminising
Surface and Coatings Technology 120–121 (1999) 118–123 www.elsevier.nl/locate/surfcoat
The control of the composition and structure of aluminide layers formed by vapour aluminising A. Squillace a, R. Bonetti b, N.J. Archer c, *, J.A. Yeatman c a Fiat Avio SpA, Pomigliano D’Arco, Italy b Berna AG, Olten, Switzerland c Archer Technicoat Ltd., High Wycombe, UK
Abstract Vapour aluminising or out-of-pack aluminising is a major production technique for producing aluminide coatings on gas turbine parts. The aluminide layers enhance the oxidation resistance of hot-section components. The layers are formed by the inter-diffusion of aluminium from the vapour phase and elements from the base alloy. The final composition of the layer is dependent on the method of its formation and the substrate alloy. This paper presents a comparison between two vapour aluminising processes widely used in production. One produces an initially high concentration (high activity) of aluminium at the surface of the part with predominantly inward diffusion of aluminium, and needs subsequent heat treatment. The other produces a lower concentration ( low activity) of aluminium at the surface of the part with substantial outward diffusion of nickel from the substrate alloy. Both processes achieve an aluminide layer with acceptable oxidation resistance for specific applications. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Aluminide layers; Oxidation resistance; Vapour aluminising
1. Introduction The nickel alloys used in the hot sections of gas turbines require protection from oxidation. Their excellent creep properties are not matched by their oxidation properties. The diffusion of aluminium into the alloy surface substantially enhances their oxidation resistance. The aluminium reacts to form very stable nickel aluminide compounds. The presence of the aluminium within the surface region generates a thin surface layer of aluminium oxide, which provides the oxidation resistance. This protects the surface so long as it is replenished by aluminium from the underlying diffusion layer. Balanced against the gain in oxidation resistance, there is a loss of strength in the components corresponding to the cross-sectional area of the coating. A thin, effective and uniform coating is always the objective. Aluminising has been successfully used for the protection of gas turbine parts for some 20 years [1]. It has been used mostly in aero gas turbines, and is being used increasingly on land-based turbines because their operat* Corresponding author. Tel.: +44 1494 462101; fax: +44 1494 463049.
ing temperatures are increasing in the interest of greater efficiency. In that time various methods have been evolved to create the aluminide diffusion layer [2–6 ]. The control of the structure and composition of these layers has gradually become more important. This paper compares the results achieved by two different production processes operated in the same equipment under different conditions. Aluminising has been industrially successful because it is a very friendly production process [7]. The raw materials are low cost; large batches of parts can be coated at one time; out-of-tolerance parts can be stripped and re-coated, as can used parts; failed batches are rare. It can also be used in combination with other coatings to produce Cr/Al, Si/Al, and Pt/Al layers. The established processes for aluminising gas turbine parts are, in order of antiquity, pack aluminising, outof-pack aluminising, and CVD aluminising. The pack process involves submersing the parts to be processed in a powder pack at 900–1100°C. The large amount of powder used makes this procedure environmentally unfriendly (and expensive), and it is being replaced by out-of-pack processing. Out-of-pack processing is itself being replaced by CVD aluminising, which has the
0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 34 7 - 3
A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
ability to coat internal passages, as well as external surfaces. CVD aluminising requires no powder, and is the cleanest process of the three. The chemical reactions in all three aluminising processes are similar [8–10]. All processes operate through the gas phase although the range of transport in the pack process is rather small. The key step is the formation of a volatile aluminium subhalide, AlX ( X=F, Cl, n Br; n<3), which occurs only at temperatures >800°C. This species reacts with the surface of a nickel alloy to deposit aluminium by the following reaction: AlX +NiAlNi +AlX . (1) n y 3 The difference between in the processes lies in the way that the AlX is generated and transported. The intern metallic layer created at the surface has the composition AlNi (3≥y≥1/3). Whilst the aluminium is brought to y the surface by a vapour phase process, its diffusion into the surface is a solid state process. The two transport processes control the growth rate and composition of the layer. In the out-of-pack process, the AlX is generated by n the reaction between Al metal and AlX , known as the 3 activator: Al+AlX AlX . (2) 3 n AlX is more stable thermodynamically than AlX as n 3 the temperature increases. A layer of Al O , Al, and 2 3 AlX powder is placed close to the parts to be processed 3 at a temperature in the range 900–1150°C. The activator reacts with the aluminium to form the subhalide, or more accurately, a mixture of subhalides. These diffuse in the vapour phase to the surface of the part where reaction (1) occurs. The activator, AlX , is one of the 3 products from reaction (1), and it can diffuse back to the aluminium source to continue the process. The process continues in this way:
119
diffuses outwards, both driven by the energy of formation of the intermetallics and the concentration gradients. The rate of diffusion obeys the Arrhenius equation in the form [11]: D=A exp(−Q/RT ) where D is the diffusion coefficient, A is the frequency factor and Q is the activation energy. The diffusion coefficient for Ni has a higher temperature dependence than that of Al. At 900°C the diffusion rate of the aluminium is much greater than that of the nickel, but at 1150°C the diffusion rates are comparable. Consequently, at the low temperature the surface diffusion layers are created by the inward diffusion of aluminium, whereas at the higher temperature, the nickel diffuses equally rapidly. Exactly the same chemical processes occur in pack and CVD aluminising. In the case of the pack aluminising, all of the processes occur within the pack, and the vapor transport is very short range from the powder to the parts. In out-of-pack alumising, the transport is of a longer range from powder to parts over distances up to 300 mm. In CVD aluminising, the chemical steps are separated. Reaction (2) occurs in a generator placed remotely from the deposition zone. The temperature in the generator must be above 800°C. The reactive species are now being transported by gas flow as well as diffusion, which increases the range over which coatings can be formed up to 1–2000 mm. The CVD process gives greater control over the concentration of the AlX , which in turn gives greater control over composin tion and growth rate. Finally, the CVD process is the only one capable of coating long internal passages because the vapour can easily be conducted from the generator.
2. Experimental
At the surface of the alloy, the aluminium arrives and diffuses into the surface, driven by the formation of the NiAl intermetallics. All of the chemical reactions y shown are equilibria, and are thus reversible. It is the thermodynamic stability of these intermetallics which drives the whole process. Within the alloy surface, two main diffusion processes occur. The aluminium diffuses inwards, and the nickel
In this paper we have the opportunity to compare the structures produced by two commercial out-of-pack processes, one operating at 960 and the other at 1150°C. Both are operated by Alfa Romeo Avio (now Fiat Avio), and both use the same equipment under similar conditions except for the temperature. The equipment, an Archer Technicoat VA2436 reactor, uses a fixed retort with an inverted bell furnace which moves between two retorts ( Fig. 1). A two-base system is operated so that the furnace can be moved to the second base while the first cools and is unloaded. Both processes operate at reduced pressure (<100 mbar). A vacuum furnace is used, working at the same pressure as the process in order to minimise the pressure differential across the inner retort wall, ensuring that it does not distort at the operating temperature. Effectively both processes use AlF as the activator. In 3
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Fig. 1. Overall view of the reactor.
one case NH F is added to the powder which dissociates 4 into HF and NH at about 4–500°C. The HF then 3 reacts immediately with the Al metal in the pack to form AlF . The vapour pressure of the AlF is substan3 3 tial (<1 mbar) above 900°C so that it evaporates and reacts with the Al metal to form AlF which is also volatile. It evaporates slowly and diffuses to parts where it reacts to form the diffusion layer and AlF . This 3 diffuses back to the powder and repeats the procedure. Slowly all of the AlF diffuses to the cold parts of 3 the retort. In both processes the parts are distributed on grids
above the powder ( Fig. 2). The separation between the grid and the powder is approximately 100 mm. The conditions used for the two processes are given in Table 1. The high temperature process is used for the Rene´ 77 alloys , and the low temperature process is used for Rene´ 80, INCO 100, and CMFX. We also have the possibility to compare these structures with that obtained by CVD aluminising on internal surfaces. These coatings were obtained under the conditions given in Table 1. The equipment used was similar to that shown in Fig. 1. The activator was chloride, and there were external and internal generators to form AlCl and the subchloride. 3
3. Observations
Fig. 2. Layout of the parts within the retort.
A similar metallic grey coating is produced in both cases. The overall coating thickness was 60–70 mm for both processes. However, the coatings have substantially different structures. In the low temperature process, there are three layers visible in the coating ( Figs. 3–5). The inner zone has a striated, columnar appearance. The outer two layers are equi-axed. The middle layer is featureless, whereas the outer layer shows many inclusions. As deposited, the layer contains 36–40 wt.% alu-
Table 1 Conditions used in the two aluminising processes Low temperature out-of-pack process
High temperature out-of-pack process
CVD aluminising process
Powder Temperature (°C ) Time (h) Pressure (mbar|) Cooling
Al/AlF /Al O 3 2 3 900–1000 6–7 10–30 Rapid
Al 900–1000 2–3 <1 Rapid
Subsequent heat treatment Substrate alloys
1 h at 1150°C in vacuum Rene´ 80, INCO 100, CMFX
Cr/Al/NH F 4 1160 3–5 <15 from 1160 to 1080°C in 70 min; from 1080 to 700°C in 10 min No Rene´ 77
1 h at 1250°C in vacuum IN738LC
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Fig. 3. Cross-section of the aluminide layer after heat treatment formed by the low temperature process on Rene´ 80.
Fig. 5. Cross-section of the aluminide layer after heat treatment formed by the low temperature process on CMFX.
Table 2
Aluminium concentration in weight per cent measured by quantitative EDX
Fig. 4. Cross-section of the aluminide layer after heat treatment formed by the low temperature process on INCO 100.
minium. After the diffusion heat treatment, the inner columnar zone increases in thickness and the aluminium content of the layer decreases to about 29–30 wt.%. Traces of oxide can be seen at about one-third of the coating thickness indicating the original position of the surface. This shows that material has diffused inwards rather than outwards. Closer examination of the inner region reveals the presence of alloying elements from the base alloy. In the high temperature process, the coating has fewer features than the low temperature coating ( Fig. 6). It does not show the inner striated layer. The main feature is a line of different composition about twothirds from the surface. This represents the original surface showing the majority of the coating has been
Internal positiona
Before diffusion heat treatment
After diffusion heat treatment
1O 1M 1I 2O 2M 2I 3O 3M 3I 4O 4M 4I 5O 5M 5I 6O 6M 6I 7O 7M 7I
35.1 33.8 12.4 36.5 33.5 12.6 37.7 33.2 13.0 37.4 33.6 12.5 36.1 31.2 12.3 38.3 34.5 13.1 36.9 30.3 11.7
28.5 20.4 9.7 27.7 20.7 10.5 27.9 21.0 8.9 29.2 22.0 9.8 28.6 21.1 9.6 28.1 19.7 9.5 30.1 20.1 8.6
a O=outer layer, close to the surface; M=middle layer; I=inner layer, the inter-diffusion zone.
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A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
Table 3
Internal thickness measurements before diffusion heat treatment No.
Thickness (mm)
No.
Thickness (mm)
No.
Thickness (mm)
No.
Thickness (mm)
1 2 3 4 5 6
50 46 46 44 46 46
7 8 9 10 11 12
44 40 42 43 40 42
13 14 15 16 17 18
40 42 40 42 40 40
19 20 21 22 23
42 44 46 46 50
Fig. 6. Cross section of the aluminide layer without heat treatment formed by the high temperature process on Rene´ 77.
Fig. 7. Cross-section of an oxidised aluminide layer formed by the low temperature process on Rene´ 80.
formed by outward diffusion. The aluminium content of this layer is 28–34 wt.% aluminium. This layer does not require a secondary diffusion heat treatment. The two processes result in layers with similar aluminium content and similar thickness, but a very different structure. Although we have oxidation data for both coatings, we do not have strictly comparable data. Fig. 7 shows the degradation of the aluminide layer by oxidation. Most important is the uniform behaviour of the layer so no part fails prematurely. The CVD aluminising process produced a three-layer structure identical to that shown in Fig. 4, corresponding to high activity, inward diffusion. The uniformity of both thickness and composition is shown in Tables 2 and 3, despite the variation in the cross-section of the passage. Comparable results have been obtained at
temperatures in the range 850–1000°C, the rate of growth being slower at the lower temperature. We have coated passages with a diameter 0.7 mm and length up to 700 mm using this method.
4. Conclusions This work has demonstrated that in one reactor it is possible to control consistently the structure and composition of the aluminide layer by changing only the operating conditions. CVD aluminising achieves exactly comparable results in the internal passages. In the future it may be possible to get even closer control over the layer composition using CVD aluminising where much more control is available over the vapour phase species.
A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
Engine designs should be able to take advantage of these improvements. More consistent coatings can lead to a lower specified thickness with a closer tolerance range. This reduces the degradation of mechanical performance, which must be accommodated in exchange for oxidation resistance.
References [1] J. Stringer, R. Viswanathan, Proceedings ASM Materials Congress Materials Week ’93, Pittsburgh, PA, (1993).
123
[2] J.E. Restall, C. Hayman, Br. Patent 1 549 845, 1979. [3] J.E. Restall, C. Hayman, US Patent 4 156 042, 1979. [4] R.A. Levy, M.L. Green, P.K. Gallagher, J.Electrochem. Soc. 131 (9) (1984) 2175. [5] R.A. Levy, P.K. Gallagher, R. Contolini, F. Schrey, J. Electrochem. Soc. 132 (2) (1985) 457. [6 ] A. Reich, L. Singheiser, G. Wahl, in: Proceedings 4th European Conference on CVD, Eindhoven (1983) 122–129. [7] J.E. Restall, B.J. Gill, C. Hayman, N.J. Archer, in: Proceedings ASME Conference on Superalloys (1980) 405–411. [8] L. Vandebulcke, B. Neiri, in: Proceedings 5th European Conference on CVD, Uppsala (1985) 283–292. [9] E. Genieys, A. Saida, US Patent 5 648 120, 1997. [10] S. Shankar, G.W. Goward, US Patent 4 526 814, 1985. [11] E.A. Brandes, in: Smithells Metals Reference Book, 6th ed., Butterworths, London, 1983., p. 13-1.
The control of the composition and structure of aluminide layers formed by vapour aluminising A. Squillace a, R. Bonetti b, N.J. Archer c, *, J.A. Yeatman c a Fiat Avio SpA, Pomigliano D’Arco, Italy b Berna AG, Olten, Switzerland c Archer Technicoat Ltd., High Wycombe, UK
Abstract Vapour aluminising or out-of-pack aluminising is a major production technique for producing aluminide coatings on gas turbine parts. The aluminide layers enhance the oxidation resistance of hot-section components. The layers are formed by the inter-diffusion of aluminium from the vapour phase and elements from the base alloy. The final composition of the layer is dependent on the method of its formation and the substrate alloy. This paper presents a comparison between two vapour aluminising processes widely used in production. One produces an initially high concentration (high activity) of aluminium at the surface of the part with predominantly inward diffusion of aluminium, and needs subsequent heat treatment. The other produces a lower concentration ( low activity) of aluminium at the surface of the part with substantial outward diffusion of nickel from the substrate alloy. Both processes achieve an aluminide layer with acceptable oxidation resistance for specific applications. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Aluminide layers; Oxidation resistance; Vapour aluminising
1. Introduction The nickel alloys used in the hot sections of gas turbines require protection from oxidation. Their excellent creep properties are not matched by their oxidation properties. The diffusion of aluminium into the alloy surface substantially enhances their oxidation resistance. The aluminium reacts to form very stable nickel aluminide compounds. The presence of the aluminium within the surface region generates a thin surface layer of aluminium oxide, which provides the oxidation resistance. This protects the surface so long as it is replenished by aluminium from the underlying diffusion layer. Balanced against the gain in oxidation resistance, there is a loss of strength in the components corresponding to the cross-sectional area of the coating. A thin, effective and uniform coating is always the objective. Aluminising has been successfully used for the protection of gas turbine parts for some 20 years [1]. It has been used mostly in aero gas turbines, and is being used increasingly on land-based turbines because their operat* Corresponding author. Tel.: +44 1494 462101; fax: +44 1494 463049.
ing temperatures are increasing in the interest of greater efficiency. In that time various methods have been evolved to create the aluminide diffusion layer [2–6 ]. The control of the structure and composition of these layers has gradually become more important. This paper compares the results achieved by two different production processes operated in the same equipment under different conditions. Aluminising has been industrially successful because it is a very friendly production process [7]. The raw materials are low cost; large batches of parts can be coated at one time; out-of-tolerance parts can be stripped and re-coated, as can used parts; failed batches are rare. It can also be used in combination with other coatings to produce Cr/Al, Si/Al, and Pt/Al layers. The established processes for aluminising gas turbine parts are, in order of antiquity, pack aluminising, outof-pack aluminising, and CVD aluminising. The pack process involves submersing the parts to be processed in a powder pack at 900–1100°C. The large amount of powder used makes this procedure environmentally unfriendly (and expensive), and it is being replaced by out-of-pack processing. Out-of-pack processing is itself being replaced by CVD aluminising, which has the
0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 34 7 - 3
A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
ability to coat internal passages, as well as external surfaces. CVD aluminising requires no powder, and is the cleanest process of the three. The chemical reactions in all three aluminising processes are similar [8–10]. All processes operate through the gas phase although the range of transport in the pack process is rather small. The key step is the formation of a volatile aluminium subhalide, AlX ( X=F, Cl, n Br; n<3), which occurs only at temperatures >800°C. This species reacts with the surface of a nickel alloy to deposit aluminium by the following reaction: AlX +NiAlNi +AlX . (1) n y 3 The difference between in the processes lies in the way that the AlX is generated and transported. The intern metallic layer created at the surface has the composition AlNi (3≥y≥1/3). Whilst the aluminium is brought to y the surface by a vapour phase process, its diffusion into the surface is a solid state process. The two transport processes control the growth rate and composition of the layer. In the out-of-pack process, the AlX is generated by n the reaction between Al metal and AlX , known as the 3 activator: Al+AlX AlX . (2) 3 n AlX is more stable thermodynamically than AlX as n 3 the temperature increases. A layer of Al O , Al, and 2 3 AlX powder is placed close to the parts to be processed 3 at a temperature in the range 900–1150°C. The activator reacts with the aluminium to form the subhalide, or more accurately, a mixture of subhalides. These diffuse in the vapour phase to the surface of the part where reaction (1) occurs. The activator, AlX , is one of the 3 products from reaction (1), and it can diffuse back to the aluminium source to continue the process. The process continues in this way:
119
diffuses outwards, both driven by the energy of formation of the intermetallics and the concentration gradients. The rate of diffusion obeys the Arrhenius equation in the form [11]: D=A exp(−Q/RT ) where D is the diffusion coefficient, A is the frequency factor and Q is the activation energy. The diffusion coefficient for Ni has a higher temperature dependence than that of Al. At 900°C the diffusion rate of the aluminium is much greater than that of the nickel, but at 1150°C the diffusion rates are comparable. Consequently, at the low temperature the surface diffusion layers are created by the inward diffusion of aluminium, whereas at the higher temperature, the nickel diffuses equally rapidly. Exactly the same chemical processes occur in pack and CVD aluminising. In the case of the pack aluminising, all of the processes occur within the pack, and the vapor transport is very short range from the powder to the parts. In out-of-pack alumising, the transport is of a longer range from powder to parts over distances up to 300 mm. In CVD aluminising, the chemical steps are separated. Reaction (2) occurs in a generator placed remotely from the deposition zone. The temperature in the generator must be above 800°C. The reactive species are now being transported by gas flow as well as diffusion, which increases the range over which coatings can be formed up to 1–2000 mm. The CVD process gives greater control over the concentration of the AlX , which in turn gives greater control over composin tion and growth rate. Finally, the CVD process is the only one capable of coating long internal passages because the vapour can easily be conducted from the generator.
2. Experimental
At the surface of the alloy, the aluminium arrives and diffuses into the surface, driven by the formation of the NiAl intermetallics. All of the chemical reactions y shown are equilibria, and are thus reversible. It is the thermodynamic stability of these intermetallics which drives the whole process. Within the alloy surface, two main diffusion processes occur. The aluminium diffuses inwards, and the nickel
In this paper we have the opportunity to compare the structures produced by two commercial out-of-pack processes, one operating at 960 and the other at 1150°C. Both are operated by Alfa Romeo Avio (now Fiat Avio), and both use the same equipment under similar conditions except for the temperature. The equipment, an Archer Technicoat VA2436 reactor, uses a fixed retort with an inverted bell furnace which moves between two retorts ( Fig. 1). A two-base system is operated so that the furnace can be moved to the second base while the first cools and is unloaded. Both processes operate at reduced pressure (<100 mbar). A vacuum furnace is used, working at the same pressure as the process in order to minimise the pressure differential across the inner retort wall, ensuring that it does not distort at the operating temperature. Effectively both processes use AlF as the activator. In 3
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A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
Fig. 1. Overall view of the reactor.
one case NH F is added to the powder which dissociates 4 into HF and NH at about 4–500°C. The HF then 3 reacts immediately with the Al metal in the pack to form AlF . The vapour pressure of the AlF is substan3 3 tial (<1 mbar) above 900°C so that it evaporates and reacts with the Al metal to form AlF which is also volatile. It evaporates slowly and diffuses to parts where it reacts to form the diffusion layer and AlF . This 3 diffuses back to the powder and repeats the procedure. Slowly all of the AlF diffuses to the cold parts of 3 the retort. In both processes the parts are distributed on grids
above the powder ( Fig. 2). The separation between the grid and the powder is approximately 100 mm. The conditions used for the two processes are given in Table 1. The high temperature process is used for the Rene´ 77 alloys , and the low temperature process is used for Rene´ 80, INCO 100, and CMFX. We also have the possibility to compare these structures with that obtained by CVD aluminising on internal surfaces. These coatings were obtained under the conditions given in Table 1. The equipment used was similar to that shown in Fig. 1. The activator was chloride, and there were external and internal generators to form AlCl and the subchloride. 3
3. Observations
Fig. 2. Layout of the parts within the retort.
A similar metallic grey coating is produced in both cases. The overall coating thickness was 60–70 mm for both processes. However, the coatings have substantially different structures. In the low temperature process, there are three layers visible in the coating ( Figs. 3–5). The inner zone has a striated, columnar appearance. The outer two layers are equi-axed. The middle layer is featureless, whereas the outer layer shows many inclusions. As deposited, the layer contains 36–40 wt.% alu-
Table 1 Conditions used in the two aluminising processes Low temperature out-of-pack process
High temperature out-of-pack process
CVD aluminising process
Powder Temperature (°C ) Time (h) Pressure (mbar|) Cooling
Al/AlF /Al O 3 2 3 900–1000 6–7 10–30 Rapid
Al 900–1000 2–3 <1 Rapid
Subsequent heat treatment Substrate alloys
1 h at 1150°C in vacuum Rene´ 80, INCO 100, CMFX
Cr/Al/NH F 4 1160 3–5 <15 from 1160 to 1080°C in 70 min; from 1080 to 700°C in 10 min No Rene´ 77
1 h at 1250°C in vacuum IN738LC
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A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
Fig. 3. Cross-section of the aluminide layer after heat treatment formed by the low temperature process on Rene´ 80.
Fig. 5. Cross-section of the aluminide layer after heat treatment formed by the low temperature process on CMFX.
Table 2
Aluminium concentration in weight per cent measured by quantitative EDX
Fig. 4. Cross-section of the aluminide layer after heat treatment formed by the low temperature process on INCO 100.
minium. After the diffusion heat treatment, the inner columnar zone increases in thickness and the aluminium content of the layer decreases to about 29–30 wt.%. Traces of oxide can be seen at about one-third of the coating thickness indicating the original position of the surface. This shows that material has diffused inwards rather than outwards. Closer examination of the inner region reveals the presence of alloying elements from the base alloy. In the high temperature process, the coating has fewer features than the low temperature coating ( Fig. 6). It does not show the inner striated layer. The main feature is a line of different composition about twothirds from the surface. This represents the original surface showing the majority of the coating has been
Internal positiona
Before diffusion heat treatment
After diffusion heat treatment
1O 1M 1I 2O 2M 2I 3O 3M 3I 4O 4M 4I 5O 5M 5I 6O 6M 6I 7O 7M 7I
35.1 33.8 12.4 36.5 33.5 12.6 37.7 33.2 13.0 37.4 33.6 12.5 36.1 31.2 12.3 38.3 34.5 13.1 36.9 30.3 11.7
28.5 20.4 9.7 27.7 20.7 10.5 27.9 21.0 8.9 29.2 22.0 9.8 28.6 21.1 9.6 28.1 19.7 9.5 30.1 20.1 8.6
a O=outer layer, close to the surface; M=middle layer; I=inner layer, the inter-diffusion zone.
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Table 3
Internal thickness measurements before diffusion heat treatment No.
Thickness (mm)
No.
Thickness (mm)
No.
Thickness (mm)
No.
Thickness (mm)
1 2 3 4 5 6
50 46 46 44 46 46
7 8 9 10 11 12
44 40 42 43 40 42
13 14 15 16 17 18
40 42 40 42 40 40
19 20 21 22 23
42 44 46 46 50
Fig. 6. Cross section of the aluminide layer without heat treatment formed by the high temperature process on Rene´ 77.
Fig. 7. Cross-section of an oxidised aluminide layer formed by the low temperature process on Rene´ 80.
formed by outward diffusion. The aluminium content of this layer is 28–34 wt.% aluminium. This layer does not require a secondary diffusion heat treatment. The two processes result in layers with similar aluminium content and similar thickness, but a very different structure. Although we have oxidation data for both coatings, we do not have strictly comparable data. Fig. 7 shows the degradation of the aluminide layer by oxidation. Most important is the uniform behaviour of the layer so no part fails prematurely. The CVD aluminising process produced a three-layer structure identical to that shown in Fig. 4, corresponding to high activity, inward diffusion. The uniformity of both thickness and composition is shown in Tables 2 and 3, despite the variation in the cross-section of the passage. Comparable results have been obtained at
temperatures in the range 850–1000°C, the rate of growth being slower at the lower temperature. We have coated passages with a diameter 0.7 mm and length up to 700 mm using this method.
4. Conclusions This work has demonstrated that in one reactor it is possible to control consistently the structure and composition of the aluminide layer by changing only the operating conditions. CVD aluminising achieves exactly comparable results in the internal passages. In the future it may be possible to get even closer control over the layer composition using CVD aluminising where much more control is available over the vapour phase species.
A. Squillace et al. / Surface and Coatings Technology 120–121 (1999) 118–123
Engine designs should be able to take advantage of these improvements. More consistent coatings can lead to a lower specified thickness with a closer tolerance range. This reduces the degradation of mechanical performance, which must be accommodated in exchange for oxidation resistance.
References [1] J. Stringer, R. Viswanathan, Proceedings ASM Materials Congress Materials Week ’93, Pittsburgh, PA, (1993).
123
[2] J.E. Restall, C. Hayman, Br. Patent 1 549 845, 1979. [3] J.E. Restall, C. Hayman, US Patent 4 156 042, 1979. [4] R.A. Levy, M.L. Green, P.K. Gallagher, J.Electrochem. Soc. 131 (9) (1984) 2175. [5] R.A. Levy, P.K. Gallagher, R. Contolini, F. Schrey, J. Electrochem. Soc. 132 (2) (1985) 457. [6 ] A. Reich, L. Singheiser, G. Wahl, in: Proceedings 4th European Conference on CVD, Eindhoven (1983) 122–129. [7] J.E. Restall, B.J. Gill, C. Hayman, N.J. Archer, in: Proceedings ASME Conference on Superalloys (1980) 405–411. [8] L. Vandebulcke, B. Neiri, in: Proceedings 5th European Conference on CVD, Uppsala (1985) 283–292. [9] E. Genieys, A. Saida, US Patent 5 648 120, 1997. [10] S. Shankar, G.W. Goward, US Patent 4 526 814, 1985. [11] E.A. Brandes, in: Smithells Metals Reference Book, 6th ed., Butterworths, London, 1983., p. 13-1.