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Influence of columnar microstructure of a sputtered NiAl coating on its oxidation behavior at 1000 °C
Intermetallics 10 (2002) 467–471 www.elsevier.com/locate/intermet
Influence of columnar microstructure of a sputtered NiAl coating on its oxidation behavior at 1000 C Songlan Yanga,b,*, Fuhui Wanga, Zhengming Sunb, Shenglong Zhua a
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China b AIST Tohoku, National Institute of Advanced Industrial Science and Technology 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan Received 14 November 2001; received in revised form 25 January 2002; accepted 30 January 2002
Abstract The influence of columnar microstructure of a sputtered nanocrystalline NiAl coating on its oxidation behavior at 1000 C in air was investigated. Results showed that the existence of columnar boundaries significantly affected its initial oxidation kinetics. However, no such obvious effect on the scale thickness was observed after long term oxidation. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Nickel aluminides, based on NiAl; B. Oxidation; C. Coatings, intermetallic and otherwise; D. Grain boundary, structure
1. Introduction NiAl intermetallic compound is being considered as a material for high temperature applications, due to the combination of its low density, high melting point, and high thermal conductivity, and in particular to its excellent oxidation resistance. Based on the high aluminum activity, NiAl forms only Al2O3 upon high temperature oxidation. However, the Al2O3 scale formed on cast b-NiAl alloys suffers from thermal spallation due to the poor adhesion between the scale and the substrate [1]. Former studies [2,3] concerning the oxidation behavior of cast NiAl alloys with and without trace amount yttrium addition and sputtered NiAl coatings at 1000 C in air showed that the nanocrystallization as well as the addition of Y obviously restricted the void formation at the scale/substrate interface, and significantly improved the scale adhesion. Furthermore, the nanocrystallization accelerated the phase transformation from the metastable y-Al2O3 to the stable a-Al2O3 in the scale, while the addition of yttrium retarded the phase transformation [2], and it also gave an ideal protection perfor-
* Corresponding author at second address. Tel.: +81-22-2375211; fax: +81-22-2375216. E-mail address: [email protected] (S. Yang).
mance for an NiAl–TiC composite [4]. Investigations on the cross section of cast NiAl, NiAlY alloys and NiAl nanocrystalline coating after 100 h isothermal exposure indicated that all the scales were about 3 mm in thickness [2]. However, the corresponding isothermal oxidation kinetics for the nanocrystallized NiAl coating was different from those of other alloys. The mass gain was almost two times higher than that of cast NiAl and NiAlY alloys [2]. In fact, from the kinetics data, the difference in oxidation kinetics primarily occurred at the initial oxidation stage, which was less than 1.5 h for the NiAl coating. Previous study on K38G nanocrystalline coating [5] showed that the mass gain decreased about 20% when the coating was treated by removing the surface scale oxidized at 1000 C and then re-oxidized, which was attributed to the rougher surface of the coating. However, this surface effect can not account for such significant difference in oxidation behavior observed in the present work. There may be some other factors playing an important role during the oxidation of the nanocrystalline NiAl coating. It is true that, columnar structures commonly exist in the SIP (sputter iron plating) coatings, resulting in a great number of columnar boundaries, which may remarkably affect the oxidation kinetics [2]. In the present study, our attention is focused on such columnar
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00020-1
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S. Yang et al. / Intermetallics 10 (2002) 467–471
boundaries to evaluate their effect on the oxidation behavior of the nanocrystalline NiAl coating.
2. Experimental procedure NiAl coating was produced by the magnetron sputtering. Sputtering parameters were: argon pressure 0.25 Pa; power 600 V4 A; substrate temperature 230 C; sputtering time 7 h [2,3,4]. The grain size and the thickness of the coating are about 300 nm and 30 mm, respectively. The surface morphology and the fractured cross section of the NiAl coating are shown in Fig. 1. Nodules with a size about 2 mm distribute uniformly on the whole surface. Some micro-voids exist between the adjacent nodules [Fig. 1(a)]. The columnar grains, with a width corresponding to the size of the surface nodules, extend from the coating/substrate interface to the surface. And boundaries between columnar grains are visible [Fig. 1(b)]. In fact, these columnar boundaries are the internal development of micro-voids existing between nodules.
To examine the effect of nodules and particularly columnar boundaries, on the oxidation behavior of sputtered NiAl coating, different treatments were carried out on the surface of NiAl coating. We prepared three different kinds of coating samples in the present study, including a sample in the as-sputtered state [sample (1)], a sample whose surface has been polished to exclude the surface roughness due to the existence of irregular nodules [sample (2)], and a sample which had been pre-oxidized at 1000 C in air for 1.5 h and then the formed scale was removed by polishing for the following oxidation test [sample (3)]. On the basis of the isothermal oxidation kinetics of NiAl coating, the most obvious mass gain occurred at the initial few seconds. Therefore, if columnar boundaries played an important role in the initial oxidation of NiAl coating, those sites must have been filled up with Al2O3 after 1.5 h exposure, consequently there will be no or negligible influence due to the presence of columnar boundaries in the following oxidation test. The mass gain of the three different surface-state coatings during isothermal tests were measured using a microbalance. Samples were put into a vertical furnace when it achieved the desired temperature, and the mass change was started to be recorded automatically. Cyclic oxidation of samples (1) and (3) were performed in a vertical furnace. Specimens were kept in the furnace at 1000 C for 1 h and cooled in air for 10 min as one cycle. The cross section of sample (3) after 100 cycles oxidation was examined by SEM to evaluate the effect of such treatment on the scale adhesion.
3. Results and discussions
Fig. 1. SEM surface morphology (a) and the fractured cross section (b) of the sputtered NiAl coating.
Fig. 2 is the isothermal oxidation kinetics for the three NiAl coatings at 1000 C in air. From Fig. 2(a) it can be seen that, compared with the as-sputtered sample, the long-term kinetics is decreased at about 10% when the coating surface was polished. However, for the sample whose pre-oxidized scale was removed, the mass gain during the following oxidation is reduced to about half that of the as-sputtered sample. Moreover, Fig. 2(b) revealed that, for the as-sputtered and surface polished sample, the intensive mass-gain, especially during the initial oxidation stage, is explicit. However, when the coating was pre-oxidized and the formed scale was removed, such severe mass gain can be avoided. Therefore, the removal of the pre-oxidized scale is more effective in reducing the coating oxidation kinetics. Fig. 3 shows a cross section of a NiAl as sputtered coating after 100 h oxidation at 1000 C in air. A continuous scale about 3 mm in thickness was formed on the coating. Internal oxidation along columnar boundaries happened, forming a great number of fine tree-root shape protrusions or micropegs across the coating. And
S. Yang et al. / Intermetallics 10 (2002) 467–471
the distance between such micropegs is about 2 mm, which is consistent with the size of surface nodules. As mentioned above, most of visible columnar boundaries are the internal development of micro-voids existing between nodules, and the surface of such micro-voids/ boundaries can be treated as the additional free surface, especially at the initial oxidation stage. To quantitatively analyse the effect of the additional free surface due to the existence of columnar boundaries on the initial oxidation of NiAl coatings, a columnar grain is treated as a single prism with a L L square top surface, whose height H is equal to the average depth of the internal oxidation along columnar boundaries. So the surface area of the top surface of the columnar grain A is: A¼LL
ð1Þ
469
And the total lateral surface area S is: S¼4LH
ð2Þ
If we define B to be the ratio of S to A, and then, B ¼ S=A ¼ 4 H=L
ð3Þ
Therefore, in the case of the present sputtered NiAl coating, the existence of boundaries between columnar structures will increase the free surface area as much as 4H/L times, and the finer the columnar grains, the larger the additional free surface will be. For the present study, if H is equal to 30 mm and L is equal to 2 mm, the corresponding additional free surface is 60 times as that of the coating surface, and in this situation the effect of the size of surface nodules may be too little. During the
Fig. 2. (a) Thirty hour and (b) the magnification of transient isothermal oxidation kinetics for the three NiAl coatings at 1000 C in air.
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S. Yang et al. / Intermetallics 10 (2002) 467–471
Fig. 3. Cross section of the NiAl coating after 100 h isothermal oxidation at 1000 C in air.
initial oxidation stage, columnar boundaries act as ‘‘easy passages’’ for oxygen inwards and aluminum outwards diffusion. Al2O3 micropegs can immediately form at these sites, when oxygen and aluminum meet together. However, this process can not continue for too long due to the limitation of the size of the columnar boundary, it is only several seconds according to the kinetics data [2], which is agreed with results in Fig. 2(b). If the columnar boundaries have been filled up with Al2O3, it is impossible or difficult for oxygen to transport across this long alumina micropegs in columnar boundaries of the coating, and then the following oxidation will predominantly occur on the surface of the coating. The formation of such great number of micropegs or protrusions in columnar boundaries increases the initial oxidation kinetics obviously, and increases the contact
area between the scale and the substrate significantly, enhancing the bonding strength of the scale and the scale adhesion by mechanical keying greatly [6,7]. Furthermore, if the interface between scale and substrate is flat, it will be subjected to catastrophic failure because a crack can propagate across the entire substrate surface quickly if the stress is of sufficient magnitude to overcome the metal-oxide bond, to result in an almost total loss of the external scale. However, the existence of the micropegs can retard the propagation of an interfacial crack. For cyclic oxidation only the effect of the removal of pre-oxidized scale [sample (3)] was took into account on the basis of the above discussion. The mass change is also decreased significantly after the treatment, because those columnar boundaries have been filled up with Al2O3, and the oxidation predominantly occurred on the surface of the coating. For the sample whose preoxidized scale was removed, no abrupt mass loss was observed for the whole test period (Fig. 4), implying that no scale spallation occurred, and the scale adhesion is as strong as the as coated sample (Fig. 5). The grains of the sputtered coating are nanocrystalline and metastable, and the grain growth may take place during the exposure in air at high temperatures, while, as reported, this factor is too small to affect the following oxidation process [5]. Actually, during the magnetron sputterring, it is unavoidable to produce some oxides at grain boundaries even if such oxidation is too little to be detected, especially, in the case of NiAl alloy with high aluminium activity to ensure the easy formation of Al2O3. The mechanical pinning of these oxides limited the further growth of grains, resulting in too little change in the grain size. Nanocrystalline
Fig. 4. Cyclic oxidation kinetics of the coating.
S. Yang et al. / Intermetallics 10 (2002) 467–471
471
4. Conclusions The existence of boundaries between columnar structures significantly affects the initial oxidation kinetics of the presently investigated sputtered nanocrystalline NiAl coating. However, there is no such obvious effect on the thickness of the scale formed after long term oxidation. The removal of the pre-oxidized scale does not affect the scale adhesion in the following cyclic process.
Acknowledgements
Fig. 5. Cross section of sample (3) after 100 cycles oxidation at 1000 C in air.
microstructures ensures that the fast short-circuit diffusion of Al is available which is resulted from the presence of a great number of grain boundaries/or dislocations, hence no Al-depletion layer is formed adjacent to the pre-oxidized scale [3,7]. Therefore, the scale formed on the coating whose pre-oxidized scale has been removed is almost the same as that formed on the coating in the as-sputtered state with good continuity and also intimate contact with their substrates as well as the excellent adhesion.
This project was financially supported by the National Nature Science Foundation of China (NSFC) for Outstanding Youth Foundation under the grant No. 59625103. Dr. Yang also wishes to acknowledge the Japan Society for the Promotion of Science (JSPS) for a postdoctoral fellowship.
References [1] Smialek JL. Metall Trans 1978;A9:309. [2] Yang S, Wang F, Wu W, Niu Y. Materials Science Forum 2001; 361:369–72. [3] Yang S, Wang F, Wu W. Intermetallics 2001;9:741. [4] Yang S, Wang F, Wu W, Niu Y. Oxid Met 2001;56:33. [5] Lou H, Zhu S, Wang F. Oxid Met 1995;43:317. [6] Wang F, Lou H, Zhu S, Wu W. Oxid Met 1996;45:39. [7] Wang F. Oxid Met 1997;48:215.
Influence of columnar microstructure of a sputtered NiAl coating on its oxidation behavior at 1000 C Songlan Yanga,b,*, Fuhui Wanga, Zhengming Sunb, Shenglong Zhua a
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China b AIST Tohoku, National Institute of Advanced Industrial Science and Technology 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan Received 14 November 2001; received in revised form 25 January 2002; accepted 30 January 2002
Abstract The influence of columnar microstructure of a sputtered nanocrystalline NiAl coating on its oxidation behavior at 1000 C in air was investigated. Results showed that the existence of columnar boundaries significantly affected its initial oxidation kinetics. However, no such obvious effect on the scale thickness was observed after long term oxidation. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Nickel aluminides, based on NiAl; B. Oxidation; C. Coatings, intermetallic and otherwise; D. Grain boundary, structure
1. Introduction NiAl intermetallic compound is being considered as a material for high temperature applications, due to the combination of its low density, high melting point, and high thermal conductivity, and in particular to its excellent oxidation resistance. Based on the high aluminum activity, NiAl forms only Al2O3 upon high temperature oxidation. However, the Al2O3 scale formed on cast b-NiAl alloys suffers from thermal spallation due to the poor adhesion between the scale and the substrate [1]. Former studies [2,3] concerning the oxidation behavior of cast NiAl alloys with and without trace amount yttrium addition and sputtered NiAl coatings at 1000 C in air showed that the nanocrystallization as well as the addition of Y obviously restricted the void formation at the scale/substrate interface, and significantly improved the scale adhesion. Furthermore, the nanocrystallization accelerated the phase transformation from the metastable y-Al2O3 to the stable a-Al2O3 in the scale, while the addition of yttrium retarded the phase transformation [2], and it also gave an ideal protection perfor-
* Corresponding author at second address. Tel.: +81-22-2375211; fax: +81-22-2375216. E-mail address: [email protected] (S. Yang).
mance for an NiAl–TiC composite [4]. Investigations on the cross section of cast NiAl, NiAlY alloys and NiAl nanocrystalline coating after 100 h isothermal exposure indicated that all the scales were about 3 mm in thickness [2]. However, the corresponding isothermal oxidation kinetics for the nanocrystallized NiAl coating was different from those of other alloys. The mass gain was almost two times higher than that of cast NiAl and NiAlY alloys [2]. In fact, from the kinetics data, the difference in oxidation kinetics primarily occurred at the initial oxidation stage, which was less than 1.5 h for the NiAl coating. Previous study on K38G nanocrystalline coating [5] showed that the mass gain decreased about 20% when the coating was treated by removing the surface scale oxidized at 1000 C and then re-oxidized, which was attributed to the rougher surface of the coating. However, this surface effect can not account for such significant difference in oxidation behavior observed in the present work. There may be some other factors playing an important role during the oxidation of the nanocrystalline NiAl coating. It is true that, columnar structures commonly exist in the SIP (sputter iron plating) coatings, resulting in a great number of columnar boundaries, which may remarkably affect the oxidation kinetics [2]. In the present study, our attention is focused on such columnar
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00020-1
468
S. Yang et al. / Intermetallics 10 (2002) 467–471
boundaries to evaluate their effect on the oxidation behavior of the nanocrystalline NiAl coating.
2. Experimental procedure NiAl coating was produced by the magnetron sputtering. Sputtering parameters were: argon pressure 0.25 Pa; power 600 V4 A; substrate temperature 230 C; sputtering time 7 h [2,3,4]. The grain size and the thickness of the coating are about 300 nm and 30 mm, respectively. The surface morphology and the fractured cross section of the NiAl coating are shown in Fig. 1. Nodules with a size about 2 mm distribute uniformly on the whole surface. Some micro-voids exist between the adjacent nodules [Fig. 1(a)]. The columnar grains, with a width corresponding to the size of the surface nodules, extend from the coating/substrate interface to the surface. And boundaries between columnar grains are visible [Fig. 1(b)]. In fact, these columnar boundaries are the internal development of micro-voids existing between nodules.
To examine the effect of nodules and particularly columnar boundaries, on the oxidation behavior of sputtered NiAl coating, different treatments were carried out on the surface of NiAl coating. We prepared three different kinds of coating samples in the present study, including a sample in the as-sputtered state [sample (1)], a sample whose surface has been polished to exclude the surface roughness due to the existence of irregular nodules [sample (2)], and a sample which had been pre-oxidized at 1000 C in air for 1.5 h and then the formed scale was removed by polishing for the following oxidation test [sample (3)]. On the basis of the isothermal oxidation kinetics of NiAl coating, the most obvious mass gain occurred at the initial few seconds. Therefore, if columnar boundaries played an important role in the initial oxidation of NiAl coating, those sites must have been filled up with Al2O3 after 1.5 h exposure, consequently there will be no or negligible influence due to the presence of columnar boundaries in the following oxidation test. The mass gain of the three different surface-state coatings during isothermal tests were measured using a microbalance. Samples were put into a vertical furnace when it achieved the desired temperature, and the mass change was started to be recorded automatically. Cyclic oxidation of samples (1) and (3) were performed in a vertical furnace. Specimens were kept in the furnace at 1000 C for 1 h and cooled in air for 10 min as one cycle. The cross section of sample (3) after 100 cycles oxidation was examined by SEM to evaluate the effect of such treatment on the scale adhesion.
3. Results and discussions
Fig. 1. SEM surface morphology (a) and the fractured cross section (b) of the sputtered NiAl coating.
Fig. 2 is the isothermal oxidation kinetics for the three NiAl coatings at 1000 C in air. From Fig. 2(a) it can be seen that, compared with the as-sputtered sample, the long-term kinetics is decreased at about 10% when the coating surface was polished. However, for the sample whose pre-oxidized scale was removed, the mass gain during the following oxidation is reduced to about half that of the as-sputtered sample. Moreover, Fig. 2(b) revealed that, for the as-sputtered and surface polished sample, the intensive mass-gain, especially during the initial oxidation stage, is explicit. However, when the coating was pre-oxidized and the formed scale was removed, such severe mass gain can be avoided. Therefore, the removal of the pre-oxidized scale is more effective in reducing the coating oxidation kinetics. Fig. 3 shows a cross section of a NiAl as sputtered coating after 100 h oxidation at 1000 C in air. A continuous scale about 3 mm in thickness was formed on the coating. Internal oxidation along columnar boundaries happened, forming a great number of fine tree-root shape protrusions or micropegs across the coating. And
S. Yang et al. / Intermetallics 10 (2002) 467–471
the distance between such micropegs is about 2 mm, which is consistent with the size of surface nodules. As mentioned above, most of visible columnar boundaries are the internal development of micro-voids existing between nodules, and the surface of such micro-voids/ boundaries can be treated as the additional free surface, especially at the initial oxidation stage. To quantitatively analyse the effect of the additional free surface due to the existence of columnar boundaries on the initial oxidation of NiAl coatings, a columnar grain is treated as a single prism with a L L square top surface, whose height H is equal to the average depth of the internal oxidation along columnar boundaries. So the surface area of the top surface of the columnar grain A is: A¼LL
ð1Þ
469
And the total lateral surface area S is: S¼4LH
ð2Þ
If we define B to be the ratio of S to A, and then, B ¼ S=A ¼ 4 H=L
ð3Þ
Therefore, in the case of the present sputtered NiAl coating, the existence of boundaries between columnar structures will increase the free surface area as much as 4H/L times, and the finer the columnar grains, the larger the additional free surface will be. For the present study, if H is equal to 30 mm and L is equal to 2 mm, the corresponding additional free surface is 60 times as that of the coating surface, and in this situation the effect of the size of surface nodules may be too little. During the
Fig. 2. (a) Thirty hour and (b) the magnification of transient isothermal oxidation kinetics for the three NiAl coatings at 1000 C in air.
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S. Yang et al. / Intermetallics 10 (2002) 467–471
Fig. 3. Cross section of the NiAl coating after 100 h isothermal oxidation at 1000 C in air.
initial oxidation stage, columnar boundaries act as ‘‘easy passages’’ for oxygen inwards and aluminum outwards diffusion. Al2O3 micropegs can immediately form at these sites, when oxygen and aluminum meet together. However, this process can not continue for too long due to the limitation of the size of the columnar boundary, it is only several seconds according to the kinetics data [2], which is agreed with results in Fig. 2(b). If the columnar boundaries have been filled up with Al2O3, it is impossible or difficult for oxygen to transport across this long alumina micropegs in columnar boundaries of the coating, and then the following oxidation will predominantly occur on the surface of the coating. The formation of such great number of micropegs or protrusions in columnar boundaries increases the initial oxidation kinetics obviously, and increases the contact
area between the scale and the substrate significantly, enhancing the bonding strength of the scale and the scale adhesion by mechanical keying greatly [6,7]. Furthermore, if the interface between scale and substrate is flat, it will be subjected to catastrophic failure because a crack can propagate across the entire substrate surface quickly if the stress is of sufficient magnitude to overcome the metal-oxide bond, to result in an almost total loss of the external scale. However, the existence of the micropegs can retard the propagation of an interfacial crack. For cyclic oxidation only the effect of the removal of pre-oxidized scale [sample (3)] was took into account on the basis of the above discussion. The mass change is also decreased significantly after the treatment, because those columnar boundaries have been filled up with Al2O3, and the oxidation predominantly occurred on the surface of the coating. For the sample whose preoxidized scale was removed, no abrupt mass loss was observed for the whole test period (Fig. 4), implying that no scale spallation occurred, and the scale adhesion is as strong as the as coated sample (Fig. 5). The grains of the sputtered coating are nanocrystalline and metastable, and the grain growth may take place during the exposure in air at high temperatures, while, as reported, this factor is too small to affect the following oxidation process [5]. Actually, during the magnetron sputterring, it is unavoidable to produce some oxides at grain boundaries even if such oxidation is too little to be detected, especially, in the case of NiAl alloy with high aluminium activity to ensure the easy formation of Al2O3. The mechanical pinning of these oxides limited the further growth of grains, resulting in too little change in the grain size. Nanocrystalline
Fig. 4. Cyclic oxidation kinetics of the coating.
S. Yang et al. / Intermetallics 10 (2002) 467–471
471
4. Conclusions The existence of boundaries between columnar structures significantly affects the initial oxidation kinetics of the presently investigated sputtered nanocrystalline NiAl coating. However, there is no such obvious effect on the thickness of the scale formed after long term oxidation. The removal of the pre-oxidized scale does not affect the scale adhesion in the following cyclic process.
Acknowledgements
Fig. 5. Cross section of sample (3) after 100 cycles oxidation at 1000 C in air.
microstructures ensures that the fast short-circuit diffusion of Al is available which is resulted from the presence of a great number of grain boundaries/or dislocations, hence no Al-depletion layer is formed adjacent to the pre-oxidized scale [3,7]. Therefore, the scale formed on the coating whose pre-oxidized scale has been removed is almost the same as that formed on the coating in the as-sputtered state with good continuity and also intimate contact with their substrates as well as the excellent adhesion.
This project was financially supported by the National Nature Science Foundation of China (NSFC) for Outstanding Youth Foundation under the grant No. 59625103. Dr. Yang also wishes to acknowledge the Japan Society for the Promotion of Science (JSPS) for a postdoctoral fellowship.
References [1] Smialek JL. Metall Trans 1978;A9:309. [2] Yang S, Wang F, Wu W, Niu Y. Materials Science Forum 2001; 361:369–72. [3] Yang S, Wang F, Wu W. Intermetallics 2001;9:741. [4] Yang S, Wang F, Wu W, Niu Y. Oxid Met 2001;56:33. [5] Lou H, Zhu S, Wang F. Oxid Met 1995;43:317. [6] Wang F, Lou H, Zhu S, Wu W. Oxid Met 1996;45:39. [7] Wang F. Oxid Met 1997;48:215.