- Email: [email protected]
Aluminide diffusion coatings on Inconel 738 using a pre-heated AlCl3 + H2 gas mixture
NATERIMS SCIENCE & EMOlWEERlNG ELSEVIER
B
Materials Science and Engineering B39 (1996) Ll-L4
Letter
Aluminide
diffusion coatings on Inconel 738 using a pre-heated AlCl, + H, gas mixture T. Araki”,
S. Motojimab3*
aBusiness Detielopment Division, Iwatani Int. Co. Ltd., 3-4-8 bDepartment of Applied Chemistry, Faculty of Engineeriilg,
Homrnachi, Chuo-ku, Osaka G$i University, G& 501-l
541, Japan I, Japan
Received 22 May 1995; in revised form 17 November 1995
Abstract
Aluminide diffusion coatings were obtained on an Inconel 738 specimen at 1000 “C using a pre-heated AlCl, + H, gas mixture. It was found that the pre-heating treatment was effective for the formation of AlCl, and the weight gain and thickness of the aluminide diffusion layers were 50%-65% higher than those obtained without pre-heating. Keywords:
Aluminide coatings; NiAl; Chemical vapour deposition
Alluminide diffusion coatings on refractory metals or alloys are very interesting due to their good high-temperature oxidation and hot corrosion resistances. The pack coating process is the most widely used technique for the deposition of aluminide coatings [l-4]. However, it is difficult for this process to apply such coatings on substrates such as turbine blades which contain narrow passages for cooling air (below 0.5 mm in diameter), because it is difficult to feed the pack powder uniformly into these narrow passages and subsequently remove the powder. The metal-organic chemical vapour deposition (CVD) process using trialkyl aluminium, such as Al(CH,),, Al(&H&, etc., has been used to deposit aluminium, or aluminide coatings [5,6]. However, the aluminium or aluminide coatings obtained by this process are generally not adherent to the substrate and the growth rate of aluminide or its diffusion layers is very low. Furthermore, the coating operation is not easy, because the alkyl aluminium source is very unstable to moisture and oxygen and is flammable in air. The conventional thermal CVD process using AlC13, AlBr, or AlCl as the aluminium source has frequently been used for the deposition of aluminide coatings. The formation of aluminide coatings using the direct hydrogen reduction of AlCl, or AIBr, requires a temperature * Corresponding
author.
0921-5107/96/$15.00 0 1996 - Elsevier Science S.A. All rights reserved
above 1000 “C, or about 1200 “C if thick coating layers are needed. Accordingly, AlCl, which has a high reactivity and is obtained by reaction of AlCl, with molten’ aluminium at 900-1000 “C, is generally used [7-g]. However, this molten aluminium process has the disadvantage of difficulties in handling molten aluminium. A small amount of AlCl is also formed by the direct hydrogen reduction of AlCl, above 1000 “C. If a sufficient amount of AlCl gas is formed from the AlCl, + H, system at a temperature of about 1000 “C, this process may be more simple, flexible and productive than the use of molten aluminium, and may be very useful for obtaining aluminide coatings. In this work, we obtained aluminide diffusion coatings on an Inconel 738 specimen at 1000 “C from an AlCl, + H, gas mixture which was pre-heated at 1000 “C for 0.3 s using a pre-heating chamber. The effect of pre-heating of the AlCl, + H, gas mixture on the aluminide coatings was examined. A schematic diagram of the apparatus, including the pre-heating chamber, is shown in Fig. 1. The AlCl, gas was prepared by the chlorination of aluminium metal by HCl gas at 330 “C. The gas mixture of AlCl, + H, was introduced into the pre-heating chamber prior to introduction into the reaction zone. The pre-heating chamber was made of graphite, and had 3280 cm3 of total path volume as shown in Fig. 2. The experiment
L2
T. Araki, S. Motojitnn 1 Materials Science atld Engitzeeritlg B39 (1996) Ll -L4
was also carried out without the pre-heating chamber for reference. The coating chamber (Inconel 601; inside diameter, 350 mm; height 1430 mm) was heated from the outside by a Bell-type furnace having five separate heating zones whose temperatures could be separately controlled. An Inconel 738 disc (diameter, 25 mm; thickness, 5 mm) was used as a specimen for the coatings. The reaction conditions were fixed to the values shown in Table 1, irrespective of the pre-heating conditions.
Fig. 1. Schematic drawing of the apparatus: 1, AK.& generator (SUS 304); 2, Bell-type furnace; 3, coating chamber (Inconel 601; Inside diameter 350 mm; height 1430 mm; 4, specimen (Inconel 738; diameter, thickness 25 mm; 5 mm;5, pre-heating chamber: 6, PR thermocouple; 7, cyclone; 8, liquid ring pump; 9, butterfly valve; 10, pressure gauge; 11, coating chamber without pre-heating chamber.
Fig. 2. Detailed pre-heating chamber: material, graphite; inside diameter , 180 mm; height, 200 mm, total path volume, 3280 cm3; A, source gas (AlCl, + HJ inlet; B, pre-heated gas outlet.
Table 1 Reaction conditions Reaction temperature (“C) Pre-heating temperature (“C) Reaction pressure (Pa) Reaction time (h) Pre-heating time (s) AlCl, Flow rate (seem) H2 Flow rate (seem)
1000 1000 GO0 4 0.3 200 3000
The weight gain of the aluminized specimenusing the pre-heated AlCl, + H2 gas mixture was 2.3 mg cm-“, whereas a value of 1.5 mg cm-’ was obtained without pre-heating. Therefore the weight gain of the specimen increased with pre-heating by about 1.65 times relative to that obtained without preheating. Fig. 3 shows the SEM and EPMA images of the polished cross-section of the aluminized specimensobtained with and without pre-heating. The deposited layers were composed of two parts. The thickness and weight gain are shown in Table 2. The total thickness of the deposited layers obtained with pre-heating was 19 jrm; this value was 1.5 times greater than that obtained without pre-heating. The thickness of the inner diffusion layer with pre-heating was twice that observed without pre-heating, but the thickness of the outer layer was the same irrespective of the pre-heating conditions. The inner diffusion layer obtained with preheating was very dense,while that without pre-heating was rough. Therefore pre-heating leads to an increasein the thickness of the inner dense diffusion layer, but the reason for this phenomenon is not yet known. The adhesion of the deposited layers to the substrate due to pre-heating was comparable with that obtained during the molten aluminium process. There were no differences observed in the thermal or chemical stability
Fig. 3. SEM and EPMA images of the polished cross-section of the aluminized specimens: (a), (b) with pre-heating; (c), (d) without pre-heating; A, outer aluminized layer; B, inner diffusion layer; C, substrate layer (Inconel 738).
T. Amki,
S. Motojima
/ Materials
Science
aud Engineering
B39 (1996)
D
Table 2 Weight gain and thickness of the aluminized layers Condition
Weight gain (mg cm-‘)
With pre-heating Without pre-heating
2.3 1.5
L3
Ll -L4 l
NiAl
Thickness (pm) Outer layer
Inner layer
Total layers
8 8
11 5
19 13
between the pre-heating and molten aluminium processes. The deposition rate of aluminide layers increases with increasing AlCl gas flow rate [9]. The main reaction product in the pre-heating chamber is AICI. Accordingly, the high deposition rate observed with pre-heating is caused by the higher generation rate of AlCl gas from the AlCl, + H, gas mixture than that obtained without pre-heating. However, the formation rate of AlCl in the pre-heating chamber is not yet known. Furthermore, the possibility of the formation of reactive precursors other than AlCl in the pre-heating chamber cannot be excluded. The holding time of the gas mixture in the preheating chamber was only 0.3~. Accordingly, it may be l-
20
40
60 CuKa,
80
100
(2 e)
Fig. 5. XRD spectrum of the surface of the as-grown aluminized layer.
postulated that the graphite surface of the pre-heating chamber may have a catalytic effect for the hydrogen reduction of AlC& to AlCl. Aluminium was contained in both the outer and inner layers as shown in Fig. 3(b) and Fig. 3(d). Fig. 4 shows the line profiles of Al Ka and Ni Ka in the cross-sections shown in Fig. 3(a) and Fig. 3(c). The aluminium content in the outer layer decreased from the outer part (surface) to the inner part and then significantly increased at the layer boundary. The aluminium content in the inner layer was nearly constant at about 10 wt.%. However, without pre-heating, the aluminium content in the inner layer adjacent to the specimen layer was about 4 wt.%. These differences in aluminium content may partly be caused by the different chemical species and amounts present in the vapour phase. The XRD profile of the in situ surface of the aluminized layer with pre-heating is shown in Fig. 5. The sharp peaks of NiAl can be seen, i.e. the surface of the aluminized layer is composed of NiAl phase. In conclusion, we have successfully obtained aluminide coatings on an Inconel 738 substrate at 1000 “C using AlCl, + H2 gas mixtures pre-heated at 1000 “C for 0.3 s in a graphite pre-heating chamber. This preheating process using AlCl, $ Hz is useful as it is simple, flexible and productive.
References
A-
B+C
Fig. 4. Line profiles of the cross-sections shown in Figs. 3(a) and 3(c): (a) with pre-heating; (b) without pre-heating; A, outer aluminized layer; B, inner diffusion layer; C, substrate layer (Inconel 738).
[l] J.R. Nicholls and D.J. Stephenson, in Metals and Materials, 1991, p. 156. [2] R.A. Papp, D. Wang and T. Weisert, in High Temperature Coatiizg, 1987, p. 131. [3] J.L. Cocking, P.G. Richards and G.R. Johnson, Surf. Coat. Technol., 36 (1988) 37. [4] M.A. Harper, D.M. Miller and R.A. Rapp, Oxid. Met., 42 (1994) 3.
L4
T. Araki,
S. Motojima
/ Materials
Science
[5] J.O. Carisson, S. Gorbatkin, D. Lubben and J.E. Greene, J. Vat. Sci. Technol., B, 9 (1991) 2759. [6] A. L. Cabrera, J.E. Zehner and J.N. Armor, Oxid. Met., 36 (1991) 3. [7] L. Singheiser, G. Wahl and W. Thiele, Thin Solid Films, 107 (1982) 35.
and Engineering
B39 (1996)
Ll-L4
[S] L. Singheiser, G. Wahl and W. Thiele, ?%/I Solid Filuu, 107 (1983) 443. [9] K. Brennfleck, E. Figzer and D. Kehr, Proc. 7th bu. Cor15 CVD, The Electrochemical Society, Princeton, NJ, 2979, p, 578. [lo] T.W. Hashman and SE. Pratsinis, J. Am. C~IYM. Sot, 75(1992) 920.
B
Materials Science and Engineering B39 (1996) Ll-L4
Letter
Aluminide
diffusion coatings on Inconel 738 using a pre-heated AlCl, + H, gas mixture T. Araki”,
S. Motojimab3*
aBusiness Detielopment Division, Iwatani Int. Co. Ltd., 3-4-8 bDepartment of Applied Chemistry, Faculty of Engineeriilg,
Homrnachi, Chuo-ku, Osaka G$i University, G& 501-l
541, Japan I, Japan
Received 22 May 1995; in revised form 17 November 1995
Abstract
Aluminide diffusion coatings were obtained on an Inconel 738 specimen at 1000 “C using a pre-heated AlCl, + H, gas mixture. It was found that the pre-heating treatment was effective for the formation of AlCl, and the weight gain and thickness of the aluminide diffusion layers were 50%-65% higher than those obtained without pre-heating. Keywords:
Aluminide coatings; NiAl; Chemical vapour deposition
Alluminide diffusion coatings on refractory metals or alloys are very interesting due to their good high-temperature oxidation and hot corrosion resistances. The pack coating process is the most widely used technique for the deposition of aluminide coatings [l-4]. However, it is difficult for this process to apply such coatings on substrates such as turbine blades which contain narrow passages for cooling air (below 0.5 mm in diameter), because it is difficult to feed the pack powder uniformly into these narrow passages and subsequently remove the powder. The metal-organic chemical vapour deposition (CVD) process using trialkyl aluminium, such as Al(CH,),, Al(&H&, etc., has been used to deposit aluminium, or aluminide coatings [5,6]. However, the aluminium or aluminide coatings obtained by this process are generally not adherent to the substrate and the growth rate of aluminide or its diffusion layers is very low. Furthermore, the coating operation is not easy, because the alkyl aluminium source is very unstable to moisture and oxygen and is flammable in air. The conventional thermal CVD process using AlC13, AlBr, or AlCl as the aluminium source has frequently been used for the deposition of aluminide coatings. The formation of aluminide coatings using the direct hydrogen reduction of AlCl, or AIBr, requires a temperature * Corresponding
author.
0921-5107/96/$15.00 0 1996 - Elsevier Science S.A. All rights reserved
above 1000 “C, or about 1200 “C if thick coating layers are needed. Accordingly, AlCl, which has a high reactivity and is obtained by reaction of AlCl, with molten’ aluminium at 900-1000 “C, is generally used [7-g]. However, this molten aluminium process has the disadvantage of difficulties in handling molten aluminium. A small amount of AlCl is also formed by the direct hydrogen reduction of AlCl, above 1000 “C. If a sufficient amount of AlCl gas is formed from the AlCl, + H, system at a temperature of about 1000 “C, this process may be more simple, flexible and productive than the use of molten aluminium, and may be very useful for obtaining aluminide coatings. In this work, we obtained aluminide diffusion coatings on an Inconel 738 specimen at 1000 “C from an AlCl, + H, gas mixture which was pre-heated at 1000 “C for 0.3 s using a pre-heating chamber. The effect of pre-heating of the AlCl, + H, gas mixture on the aluminide coatings was examined. A schematic diagram of the apparatus, including the pre-heating chamber, is shown in Fig. 1. The AlCl, gas was prepared by the chlorination of aluminium metal by HCl gas at 330 “C. The gas mixture of AlCl, + H, was introduced into the pre-heating chamber prior to introduction into the reaction zone. The pre-heating chamber was made of graphite, and had 3280 cm3 of total path volume as shown in Fig. 2. The experiment
L2
T. Araki, S. Motojitnn 1 Materials Science atld Engitzeeritlg B39 (1996) Ll -L4
was also carried out without the pre-heating chamber for reference. The coating chamber (Inconel 601; inside diameter, 350 mm; height 1430 mm) was heated from the outside by a Bell-type furnace having five separate heating zones whose temperatures could be separately controlled. An Inconel 738 disc (diameter, 25 mm; thickness, 5 mm) was used as a specimen for the coatings. The reaction conditions were fixed to the values shown in Table 1, irrespective of the pre-heating conditions.
Fig. 1. Schematic drawing of the apparatus: 1, AK.& generator (SUS 304); 2, Bell-type furnace; 3, coating chamber (Inconel 601; Inside diameter 350 mm; height 1430 mm; 4, specimen (Inconel 738; diameter, thickness 25 mm; 5 mm;5, pre-heating chamber: 6, PR thermocouple; 7, cyclone; 8, liquid ring pump; 9, butterfly valve; 10, pressure gauge; 11, coating chamber without pre-heating chamber.
Fig. 2. Detailed pre-heating chamber: material, graphite; inside diameter , 180 mm; height, 200 mm, total path volume, 3280 cm3; A, source gas (AlCl, + HJ inlet; B, pre-heated gas outlet.
Table 1 Reaction conditions Reaction temperature (“C) Pre-heating temperature (“C) Reaction pressure (Pa) Reaction time (h) Pre-heating time (s) AlCl, Flow rate (seem) H2 Flow rate (seem)
1000 1000 GO0 4 0.3 200 3000
The weight gain of the aluminized specimenusing the pre-heated AlCl, + H2 gas mixture was 2.3 mg cm-“, whereas a value of 1.5 mg cm-’ was obtained without pre-heating. Therefore the weight gain of the specimen increased with pre-heating by about 1.65 times relative to that obtained without preheating. Fig. 3 shows the SEM and EPMA images of the polished cross-section of the aluminized specimensobtained with and without pre-heating. The deposited layers were composed of two parts. The thickness and weight gain are shown in Table 2. The total thickness of the deposited layers obtained with pre-heating was 19 jrm; this value was 1.5 times greater than that obtained without pre-heating. The thickness of the inner diffusion layer with pre-heating was twice that observed without pre-heating, but the thickness of the outer layer was the same irrespective of the pre-heating conditions. The inner diffusion layer obtained with preheating was very dense,while that without pre-heating was rough. Therefore pre-heating leads to an increasein the thickness of the inner dense diffusion layer, but the reason for this phenomenon is not yet known. The adhesion of the deposited layers to the substrate due to pre-heating was comparable with that obtained during the molten aluminium process. There were no differences observed in the thermal or chemical stability
Fig. 3. SEM and EPMA images of the polished cross-section of the aluminized specimens: (a), (b) with pre-heating; (c), (d) without pre-heating; A, outer aluminized layer; B, inner diffusion layer; C, substrate layer (Inconel 738).
T. Amki,
S. Motojima
/ Materials
Science
aud Engineering
B39 (1996)
D
Table 2 Weight gain and thickness of the aluminized layers Condition
Weight gain (mg cm-‘)
With pre-heating Without pre-heating
2.3 1.5
L3
Ll -L4 l
NiAl
Thickness (pm) Outer layer
Inner layer
Total layers
8 8
11 5
19 13
between the pre-heating and molten aluminium processes. The deposition rate of aluminide layers increases with increasing AlCl gas flow rate [9]. The main reaction product in the pre-heating chamber is AICI. Accordingly, the high deposition rate observed with pre-heating is caused by the higher generation rate of AlCl gas from the AlCl, + H, gas mixture than that obtained without pre-heating. However, the formation rate of AlCl in the pre-heating chamber is not yet known. Furthermore, the possibility of the formation of reactive precursors other than AlCl in the pre-heating chamber cannot be excluded. The holding time of the gas mixture in the preheating chamber was only 0.3~. Accordingly, it may be l-
20
40
60 CuKa,
80
100
(2 e)
Fig. 5. XRD spectrum of the surface of the as-grown aluminized layer.
postulated that the graphite surface of the pre-heating chamber may have a catalytic effect for the hydrogen reduction of AlC& to AlCl. Aluminium was contained in both the outer and inner layers as shown in Fig. 3(b) and Fig. 3(d). Fig. 4 shows the line profiles of Al Ka and Ni Ka in the cross-sections shown in Fig. 3(a) and Fig. 3(c). The aluminium content in the outer layer decreased from the outer part (surface) to the inner part and then significantly increased at the layer boundary. The aluminium content in the inner layer was nearly constant at about 10 wt.%. However, without pre-heating, the aluminium content in the inner layer adjacent to the specimen layer was about 4 wt.%. These differences in aluminium content may partly be caused by the different chemical species and amounts present in the vapour phase. The XRD profile of the in situ surface of the aluminized layer with pre-heating is shown in Fig. 5. The sharp peaks of NiAl can be seen, i.e. the surface of the aluminized layer is composed of NiAl phase. In conclusion, we have successfully obtained aluminide coatings on an Inconel 738 substrate at 1000 “C using AlCl, + H2 gas mixtures pre-heated at 1000 “C for 0.3 s in a graphite pre-heating chamber. This preheating process using AlCl, $ Hz is useful as it is simple, flexible and productive.
References
A-
B+C
Fig. 4. Line profiles of the cross-sections shown in Figs. 3(a) and 3(c): (a) with pre-heating; (b) without pre-heating; A, outer aluminized layer; B, inner diffusion layer; C, substrate layer (Inconel 738).
[l] J.R. Nicholls and D.J. Stephenson, in Metals and Materials, 1991, p. 156. [2] R.A. Papp, D. Wang and T. Weisert, in High Temperature Coatiizg, 1987, p. 131. [3] J.L. Cocking, P.G. Richards and G.R. Johnson, Surf. Coat. Technol., 36 (1988) 37. [4] M.A. Harper, D.M. Miller and R.A. Rapp, Oxid. Met., 42 (1994) 3.
L4
T. Araki,
S. Motojima
/ Materials
Science
[5] J.O. Carisson, S. Gorbatkin, D. Lubben and J.E. Greene, J. Vat. Sci. Technol., B, 9 (1991) 2759. [6] A. L. Cabrera, J.E. Zehner and J.N. Armor, Oxid. Met., 36 (1991) 3. [7] L. Singheiser, G. Wahl and W. Thiele, Thin Solid Films, 107 (1982) 35.
and Engineering
B39 (1996)
Ll-L4
[S] L. Singheiser, G. Wahl and W. Thiele, ?%/I Solid Filuu, 107 (1983) 443. [9] K. Brennfleck, E. Figzer and D. Kehr, Proc. 7th bu. Cor15 CVD, The Electrochemical Society, Princeton, NJ, 2979, p, 578. [lo] T.W. Hashman and SE. Pratsinis, J. Am. C~IYM. Sot, 75(1992) 920.