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Improved oxidation resistance of Ti with a thermal sprayed Ti3Al(O)–Al2O3 composite coating
Scripta Materialia 48 (2003) 1649–1653 www.actamat-journals.com
Improved oxidation resistance of Ti with a thermal sprayed Ti3Al(O)–Al2O3 composite coating Z.-W. Li a, W. Gao a
a,*
, D.-Y. Ying b, D.-L. Zhang
b
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Department of Materials and Processes Engineering, The University of Waikato, Private Bag 3105, Hamilton, New Zealand
b
Received 14 January 2003; received in revised form 24 February 2003; accepted 6 March 2003
Abstract A Ti3 Al(O)–Al2 O3 in situ composite was explored as a coating system for Ti using thermal spray. Oxidation tests at 700–800 °C showed that this coating remarkably decreased the oxidation rate and increased the scale spallation resistance compared with Ti. The mechanisms for these improvements were then briefly discussed. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Intermetallic; Composite; Coating; Oxidation
1. Introduction Titanium has been reputed as the third element due to its importance in industry which is the second only to iron and aluminium. In the aerospace industry, many Ti-based alloys have been developed based on their lightweight, high strength and excellent corrosion resistance. Moreover, during the past half-century, a number of Ti-based alloys, as structural materials, have been spreading to applications in other areas including energy production, petrochemical, automobile, metallurgical, papermaking, medical, and food industries [1]. As the structural materials used at moderately high temperatures, however, the commercial Ti-
* Corresponding author. Tel.: +64-9-373-7599x8175; fax: +64-9-373-7463. E-mail address: [email protected] (W. Gao).
based alloys have useful strength and oxidation resistance only up to 600 °C, far below 1100 °C of the limited service temperature for nickel-based superalloys. Ti-based intermetallics, such as a2 Ti3 Al, c-TiAl, and TiAl3 , thereby, are being developed actively due to their attractive elevated temperature properties (strength, ductility, environment resistance, etc.) [2,3]. Recently, intermetallic matrix composites (IMCs) are becoming popular. They possess balanced mechanical, physical, and chemical properties for their wider applications, as additions of the second phase often leads to improved properties [4,5]. Recently, an in situ Ti-based composite system, Ti3 Al(O)–Al2 O3 , has been developed by University of Waikato and Titanox Development Ltd. with cheap raw materials and easy fabrication [6,7]. It showed good oxidation resistance and excellent scale spallation resistance at the temperatures ranging from 700 to 900 °C [8]. In this study, we further studied the performance of this composite
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00133-7
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Z.-W. Li et al. / Scripta Materialia 48 (2003) 1649–1653
as a coating system and applied it onto Ti sheet. Tests also showed great improvement in oxidation resistance at 700–800 °C.
2. Experimental The Ti3 Al(O)–Al2 O3 composite powder was produced using a combination of high-energy mechanical milling of a mixture of Al and TiO2 powder to make an Al/TiO2 composite powder and then thermal treatment [7]. The powder was thermally sprayed onto Ti stripes (>99.9%Ti) to form a coating layer. The samples were cut from the stripe to the size of 10 10 mm. The surfaces of pure Ti samples were ground to 1200 grit paper, ultrasonically cleaned in acetone, distilled water, and then dried with blowing hot air. Isothermal oxidation tests were carried out discontinuously at 700 and 800 °C in a tube-furnace up to the period of 400 h. All the samples were put into quartz crucibles separately, which were previously heated at 900 °C to have constant masses. After oxidation, the surfaces and cross-sections of the specimens were studied with a HRSEM (Philips XL-30S) with an energy dispersive spectrometer (EDS). Oxidation products were also characterised using an X-ray diffractometer with Cu-Ka radiation (Bruker, D8).
3. Results Fig. 1 represents the metallographically polished cross-section of Ti with the Ti3 Al(O)–Al2 O3 composite coating. The coating has a thickness of 50 lm in average, and covers the whole surface with reasonably good uniformity. Two phases can be seen directly; the dark areas were detected as Al and O, while the light is characterised as Ti–Al–O with the Ti:Al ratio about 3:1 in average and 10–15 at.%O. At some locations, the content of Al might be low and Ti was high. XRD showed the peaks for a-Al2 O3 (corresponding to the dark area), Ti3 Al, TiO, and Al. The total oxidation mass gains and the amount of scale spallation of the uncoated and coated Ti samples at 700 and 800 °C in air are shown in
Fig. 1. A typical cross-section of pure Ti with the thermal sprayed Ti3 Al(O)–Al2 O3 in situ composite coating.
Fig. 2. At 700 °C, oxidation kinetics of Ti followed an approximately parabolic law, though breakdown behaviour commenced after a long time exposure. While, the coated Ti exhibited a different behaviour, in the first 10 h, 1.16 mg/cm2 mass gain was achieved, about 50% of the total mass gain after 400 h exposure; after that a much slower oxidation followed. The oxide scale formed on the Ti samples started spallation at about 300 h exposure. Large pieces of scale spalled off, resulting in a subsequent high oxidation rate. A cross-section was presented in Fig. 3a, showing that a rutile (TiO2 ) layer was formed with a thickness of about 30 lm, and a very long crack was developed in its lower part. This crack was believed to harm the integrity of the scale. Actually, separation of the upper part scale with this crack could be clearly seen. In contrast, no spallation could be observed and measured for the coated Ti sample. The crosssection after oxidation showed little difference to the original coated cross-section, except for the more obvious outer rutile layer and the increased thickness in total (Fig. 3b). Therefore, it is quite difficult to determine the thickness of coating layer oxidised. In general, the scale, consisted of rutile and a-Al2 O3 as characterized by XRD, was dense and adherent to the substrate. Additionally, with EDS line scanning, it was found that after 400 h exposure, the Ti sample had an oxygen-dissolved zone of about 15 lm, while this zone could not be clearly found on the coated counterpart. At 800 °C, catastrophic oxidation occurred on Ti as believed. The oxidation kinetics showed a
Z.-W. Li et al. / Scripta Materialia 48 (2003) 1649–1653
1651
4.8
50 Mass Gain (mg/cm2)
Mass Gain (mg/cm2)
60
Ti
(a)
4.0 3.2 2.4 1.6
Ti, coated
0.8
Ti
(b)
40 30
3.31mg/cm2
20
Ti, coated
10 0
0.0 0
40
80
120
160
200
240
280
320
360
400
0
40
80
Time (hrs)
120
160
200
240
Time (hrs)
Spallation (mg/cm2)
6
(c)
5
Ti, 800°C
4
Ti, 700 °C
3 2
Ti, coated No spallation
1 0 0
40
80
120
160 200 240 Time (hrs)
280
320
360
400
Fig. 2. Total oxidation mass gains and amount of scale spallation of Ti with and without composite coating: (a) 700 °C, total oxidation mass gain, (b) 800 °C, total oxidation mass gain, and (c) 700–800 °C, amount of scale spallation.
Fig. 3. Cross-sections of Ti with and without composite coating: (a) 700 °C, 400 h, Ti, (b) 700 °C, 400 h, Ti coated, (c) 800 °C, 240 h, Ti, and (d) 800 °C, 240 h, Ti coated.
fast, linear reaction behaviour and severe scale spallation (Fig. 2b and c). As shown in Fig. 3c, a very thick, about 400 lm, rutile layer formed, showing a layered structure with long cracks par-
allel to the interface. Meantime, the oxidation mass gain of the coated sample showed a quick increase in the first 10 h, similar to the one at 700 °C, and got 48% of the total mass gain. After
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that, protective oxidation behaviour exhibited with a much lower oxidation rate. Scale spallation could never be found in the whole testing time period. Examination of the cross-section showed that the scale had a thickness of 70 lm, with randomly distributed TiO2 and a-Al2 O3 particles (Fig. 3d). Elemental line scanning performed across the interface showed that an oxygen-dissolved zone about 40 lm was formed on Ti metal, while this zone could not be revealed on the coated samples. Instead, an Al diffusion region of 10 lm thick in Ti close to the interface can be detected.
4. Discussion It is known that pure Ti has poor high temperature oxidation resistance. At 400–600 °C, its oxidation kinetics follows a parabolic or approximately cubic law. Above 600–700 °C, its oxidation rate in air and oxygen containing media is typically parabolic, and changes into approximately linear after an extended reaction period. While at 900– 1000 °C the linear oxidation is followed by a decreasing rate of oxidation with time, which has been attributed to the formation of a compact diffusion barrier layer within the scale caused by the sintering and grain growth of the oxide [9]. In the present study, the transition from parabolic to linear commenced at 140 h at 700 °C, while the linear behaviour with extremely high rate constant occurred at 800 °C. Therefore, the oxidation kinetics basically revealed that pure Ti had limited or no protectiveness at temperatures above 650 °C. From the microstructures of the cross-sections, it is clearly seen that the Ti oxide scale contains a considerable amount of voids, porosity, and cracks. And it is apparent that the substrate-scale interface is weak. Since the mobility of Ti and O through the scale is high, and TiO2 has a relatively high Pilling–Bedworth ratio (PBR) (1.70–1.78) [10], it is understandable that large compressive stresses will be generated in the scale, developing tensile stresses at the scale-substrate interface. Thermal stresses also developed in the scale due to the different coefficients of thermal expansion (CTE). When the oxide scale reaches a critical thickness, the stresses in the scale will also reach a critical
value, starting cracks in the scale. The cracks tend to propagate laterally along or parallel to the interface, severely damaging the integrity of the scale. These cracks also provide additional paths for the inward transportation of oxygen, resulting in breakdown oxidation behaviour all the time. Furthermore, a notable process accompanied with the oxidation of Ti is the dissolution of large quantity of oxygen in the substrate during the oxidation process. This is often accompanied by severe embrittlement of the substrate material. Below the a–b transition of Ti at 882.5 °C, oxygen dissolves in the a-phase, up to about 30 at.% and shows a small variation with temperature. Studies on the reacted specimens showed that 25–30% or more of the reacted oxygen dissolved in the metal at temperatures of 700–750 °C and more than 50% at 900–950 °C [9]. In the present study, we also detected a relative thick oxygen-dissolution zone along the interface in Ti. It is believed that the dissolution of oxygen contributes to the total mass gain, and also has close relation with the formation of the layered structure of oxide scale, and therefore the poor interfacial bonding strength. Ti with thermal sprayed Ti3 Al(O)–Al2 O3 in situ composite coating, on the other hand, exhibited high oxidation and spallation resistance under the same testing conditions. In comparison with the previous results with bulk Ti3 Al(O)–Al2 O3 composite, the coatings have the similar oxidation mass gains. Additionally, it appeared that there was little change in the thickness of the coatings after oxidation. Hence, the oxidation process took place mainly in the coating layer. The difference between the coating and bulk material is that the coatings have a relatively high initial oxidation rate in the first 10 h (Fig. 2a and b). This can be understood since the coating was produced by thermal spraying of Ti3 Al–Al2 O3 composite powder. There is a certain degree of porosity in the microstructure, which increases the actual reaction area at the beginning. The microstructure of the coatings is also not as uniform as the bulk material due to their processing. Some metallic Al and Ti– Al regions in the sprayed coatings might oxidise fast when exposed to high temperature. The following mechanisms are suggested as the reasons that the Ti3 Al(O)–Al2 O3 composite coat-
Z.-W. Li et al. / Scripta Materialia 48 (2003) 1649–1653
ings showed superior oxidation and scale spallation resistance. 1. As oxidation was mainly conducted within the coating, the overall behaviour thereby, should be similar to Ti3 Al. Obviously, Ti3 Al has better oxidation resistance than Ti due to its high Al content, which could partially promote the formation of Al2 O3 . As discussed previously, incorporation of alumina particles could further decrease the oxidation mass gain, since they could act as diffusion barrier to O and Ti with reduced cross-section area. Pre-dissolution of oxygen in Ti3 Al could also contribute to the lower mass gain as inward diffusion and dissolution of oxygen in the substrate were reduced significantly [8]. 2. Excellent scale spallation resistance is also closely related with the alumina particles. It can be seen that these particles have good interfacial bonding with the newly formed oxide. The alumina particles have formed a 3-D network, holding the oxide scale strongly. Besides this, the presence of alumina particles could inhibit the lateral growth of cracks in the oxide scale. This is in contrast to the cracks observed in the scales on pure Ti, where the crack can propagate laterally without interruption. Additionally, the fine oxide grains originally formed by a combination of mechanical milling and thermal treatment of the powder could promote the plastic deformation of the scale, reducing the risk of cracking [11]. It is also suggested that the incorporation of Al2 O3 could decrease the CTE of the composite (7.0–8.7 10 6 , 8.0– 8.3 10 6 , 12 10 6 , and 11 10 6 /K for TiO2 , a-Al2 O3 , Ti3 Al, and Ti respectively). Thus the outer oxide scale and the coating might have a better adhesion to each other.
5. Conclusion A Ti3 Al(O)–Al2 O3 in situ composite coating was applied on pure Ti substrate through thermal
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spraying. Oxidation tests at 700 and 800 °C up to 400 h showed that this coating decreased the oxidation rate and improved the scale spallation resistance of Ti remarkably. The enhanced resistance is attributed to the incorporation of alumina into the coating, which serves as a diffusion barrier, an inhibitor to crack propagation, and connectors between the coating and oxide scale. The coating can increase the application temperature of Ti and Ti alloys from 650 °C to at least 800 °C.
Acknowledgements This project is partially funded by the Foundation for Research, Science and Technology, New Zealand through a New Economy Research Fund (NERF) grant. Titanox Development Ltd., Auckland, New Zealand, is the licenser of the powder making technology. ZWL thanks the postdoctoral fellowship from the University. The authors would also like to thank the technical staff at the department and the Research Centre for Surface and Materials Science for various assistance.
References [1] Zhu ZF. Corrosion resistance and applications of nonferrous metals. Beijing: Chemical Industry Press; 1995. [2] Froes FH, Suryanarayana C, Elieze D. J Mater Sci 1992;27:5113. [3] Sauthoff G. Intermetallics. Weinheim: VCH; 1995. [4] Ward-Close CM, Minor R, Doorbar PJ. Intermetallics 1996;4:217. [5] Miracle DA. Metal matrix composites. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 3. Amsterdam: Elsevier; 2000. p. 741. [6] Zhang DL, Newby MR. US patent, No.US6 264 719 B1. [7] Zhang DL, Ying DY, Adam G. Mater Sci Forum 2002;386–388:287. [8] Gao W, Li Z, Zhang D. Oxid Met 2002;57:99. [9] Kofstad P. High temperature corrosion. London: Elsevier; 1988. [10] Zhu RZ, He YD, Qi HB. High temperature corrosion and its resistant materials. Shanghai: Shanghai Science and Technology Press; 1995. [11] Coble RJ. J Appl Phys 1963;34:1679.
Improved oxidation resistance of Ti with a thermal sprayed Ti3Al(O)–Al2O3 composite coating Z.-W. Li a, W. Gao a
a,*
, D.-Y. Ying b, D.-L. Zhang
b
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Department of Materials and Processes Engineering, The University of Waikato, Private Bag 3105, Hamilton, New Zealand
b
Received 14 January 2003; received in revised form 24 February 2003; accepted 6 March 2003
Abstract A Ti3 Al(O)–Al2 O3 in situ composite was explored as a coating system for Ti using thermal spray. Oxidation tests at 700–800 °C showed that this coating remarkably decreased the oxidation rate and increased the scale spallation resistance compared with Ti. The mechanisms for these improvements were then briefly discussed. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Intermetallic; Composite; Coating; Oxidation
1. Introduction Titanium has been reputed as the third element due to its importance in industry which is the second only to iron and aluminium. In the aerospace industry, many Ti-based alloys have been developed based on their lightweight, high strength and excellent corrosion resistance. Moreover, during the past half-century, a number of Ti-based alloys, as structural materials, have been spreading to applications in other areas including energy production, petrochemical, automobile, metallurgical, papermaking, medical, and food industries [1]. As the structural materials used at moderately high temperatures, however, the commercial Ti-
* Corresponding author. Tel.: +64-9-373-7599x8175; fax: +64-9-373-7463. E-mail address: [email protected] (W. Gao).
based alloys have useful strength and oxidation resistance only up to 600 °C, far below 1100 °C of the limited service temperature for nickel-based superalloys. Ti-based intermetallics, such as a2 Ti3 Al, c-TiAl, and TiAl3 , thereby, are being developed actively due to their attractive elevated temperature properties (strength, ductility, environment resistance, etc.) [2,3]. Recently, intermetallic matrix composites (IMCs) are becoming popular. They possess balanced mechanical, physical, and chemical properties for their wider applications, as additions of the second phase often leads to improved properties [4,5]. Recently, an in situ Ti-based composite system, Ti3 Al(O)–Al2 O3 , has been developed by University of Waikato and Titanox Development Ltd. with cheap raw materials and easy fabrication [6,7]. It showed good oxidation resistance and excellent scale spallation resistance at the temperatures ranging from 700 to 900 °C [8]. In this study, we further studied the performance of this composite
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00133-7
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Z.-W. Li et al. / Scripta Materialia 48 (2003) 1649–1653
as a coating system and applied it onto Ti sheet. Tests also showed great improvement in oxidation resistance at 700–800 °C.
2. Experimental The Ti3 Al(O)–Al2 O3 composite powder was produced using a combination of high-energy mechanical milling of a mixture of Al and TiO2 powder to make an Al/TiO2 composite powder and then thermal treatment [7]. The powder was thermally sprayed onto Ti stripes (>99.9%Ti) to form a coating layer. The samples were cut from the stripe to the size of 10 10 mm. The surfaces of pure Ti samples were ground to 1200 grit paper, ultrasonically cleaned in acetone, distilled water, and then dried with blowing hot air. Isothermal oxidation tests were carried out discontinuously at 700 and 800 °C in a tube-furnace up to the period of 400 h. All the samples were put into quartz crucibles separately, which were previously heated at 900 °C to have constant masses. After oxidation, the surfaces and cross-sections of the specimens were studied with a HRSEM (Philips XL-30S) with an energy dispersive spectrometer (EDS). Oxidation products were also characterised using an X-ray diffractometer with Cu-Ka radiation (Bruker, D8).
3. Results Fig. 1 represents the metallographically polished cross-section of Ti with the Ti3 Al(O)–Al2 O3 composite coating. The coating has a thickness of 50 lm in average, and covers the whole surface with reasonably good uniformity. Two phases can be seen directly; the dark areas were detected as Al and O, while the light is characterised as Ti–Al–O with the Ti:Al ratio about 3:1 in average and 10–15 at.%O. At some locations, the content of Al might be low and Ti was high. XRD showed the peaks for a-Al2 O3 (corresponding to the dark area), Ti3 Al, TiO, and Al. The total oxidation mass gains and the amount of scale spallation of the uncoated and coated Ti samples at 700 and 800 °C in air are shown in
Fig. 1. A typical cross-section of pure Ti with the thermal sprayed Ti3 Al(O)–Al2 O3 in situ composite coating.
Fig. 2. At 700 °C, oxidation kinetics of Ti followed an approximately parabolic law, though breakdown behaviour commenced after a long time exposure. While, the coated Ti exhibited a different behaviour, in the first 10 h, 1.16 mg/cm2 mass gain was achieved, about 50% of the total mass gain after 400 h exposure; after that a much slower oxidation followed. The oxide scale formed on the Ti samples started spallation at about 300 h exposure. Large pieces of scale spalled off, resulting in a subsequent high oxidation rate. A cross-section was presented in Fig. 3a, showing that a rutile (TiO2 ) layer was formed with a thickness of about 30 lm, and a very long crack was developed in its lower part. This crack was believed to harm the integrity of the scale. Actually, separation of the upper part scale with this crack could be clearly seen. In contrast, no spallation could be observed and measured for the coated Ti sample. The crosssection after oxidation showed little difference to the original coated cross-section, except for the more obvious outer rutile layer and the increased thickness in total (Fig. 3b). Therefore, it is quite difficult to determine the thickness of coating layer oxidised. In general, the scale, consisted of rutile and a-Al2 O3 as characterized by XRD, was dense and adherent to the substrate. Additionally, with EDS line scanning, it was found that after 400 h exposure, the Ti sample had an oxygen-dissolved zone of about 15 lm, while this zone could not be clearly found on the coated counterpart. At 800 °C, catastrophic oxidation occurred on Ti as believed. The oxidation kinetics showed a
Z.-W. Li et al. / Scripta Materialia 48 (2003) 1649–1653
1651
4.8
50 Mass Gain (mg/cm2)
Mass Gain (mg/cm2)
60
Ti
(a)
4.0 3.2 2.4 1.6
Ti, coated
0.8
Ti
(b)
40 30
3.31mg/cm2
20
Ti, coated
10 0
0.0 0
40
80
120
160
200
240
280
320
360
400
0
40
80
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160
200
240
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Spallation (mg/cm2)
6
(c)
5
Ti, 800°C
4
Ti, 700 °C
3 2
Ti, coated No spallation
1 0 0
40
80
120
160 200 240 Time (hrs)
280
320
360
400
Fig. 2. Total oxidation mass gains and amount of scale spallation of Ti with and without composite coating: (a) 700 °C, total oxidation mass gain, (b) 800 °C, total oxidation mass gain, and (c) 700–800 °C, amount of scale spallation.
Fig. 3. Cross-sections of Ti with and without composite coating: (a) 700 °C, 400 h, Ti, (b) 700 °C, 400 h, Ti coated, (c) 800 °C, 240 h, Ti, and (d) 800 °C, 240 h, Ti coated.
fast, linear reaction behaviour and severe scale spallation (Fig. 2b and c). As shown in Fig. 3c, a very thick, about 400 lm, rutile layer formed, showing a layered structure with long cracks par-
allel to the interface. Meantime, the oxidation mass gain of the coated sample showed a quick increase in the first 10 h, similar to the one at 700 °C, and got 48% of the total mass gain. After
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that, protective oxidation behaviour exhibited with a much lower oxidation rate. Scale spallation could never be found in the whole testing time period. Examination of the cross-section showed that the scale had a thickness of 70 lm, with randomly distributed TiO2 and a-Al2 O3 particles (Fig. 3d). Elemental line scanning performed across the interface showed that an oxygen-dissolved zone about 40 lm was formed on Ti metal, while this zone could not be revealed on the coated samples. Instead, an Al diffusion region of 10 lm thick in Ti close to the interface can be detected.
4. Discussion It is known that pure Ti has poor high temperature oxidation resistance. At 400–600 °C, its oxidation kinetics follows a parabolic or approximately cubic law. Above 600–700 °C, its oxidation rate in air and oxygen containing media is typically parabolic, and changes into approximately linear after an extended reaction period. While at 900– 1000 °C the linear oxidation is followed by a decreasing rate of oxidation with time, which has been attributed to the formation of a compact diffusion barrier layer within the scale caused by the sintering and grain growth of the oxide [9]. In the present study, the transition from parabolic to linear commenced at 140 h at 700 °C, while the linear behaviour with extremely high rate constant occurred at 800 °C. Therefore, the oxidation kinetics basically revealed that pure Ti had limited or no protectiveness at temperatures above 650 °C. From the microstructures of the cross-sections, it is clearly seen that the Ti oxide scale contains a considerable amount of voids, porosity, and cracks. And it is apparent that the substrate-scale interface is weak. Since the mobility of Ti and O through the scale is high, and TiO2 has a relatively high Pilling–Bedworth ratio (PBR) (1.70–1.78) [10], it is understandable that large compressive stresses will be generated in the scale, developing tensile stresses at the scale-substrate interface. Thermal stresses also developed in the scale due to the different coefficients of thermal expansion (CTE). When the oxide scale reaches a critical thickness, the stresses in the scale will also reach a critical
value, starting cracks in the scale. The cracks tend to propagate laterally along or parallel to the interface, severely damaging the integrity of the scale. These cracks also provide additional paths for the inward transportation of oxygen, resulting in breakdown oxidation behaviour all the time. Furthermore, a notable process accompanied with the oxidation of Ti is the dissolution of large quantity of oxygen in the substrate during the oxidation process. This is often accompanied by severe embrittlement of the substrate material. Below the a–b transition of Ti at 882.5 °C, oxygen dissolves in the a-phase, up to about 30 at.% and shows a small variation with temperature. Studies on the reacted specimens showed that 25–30% or more of the reacted oxygen dissolved in the metal at temperatures of 700–750 °C and more than 50% at 900–950 °C [9]. In the present study, we also detected a relative thick oxygen-dissolution zone along the interface in Ti. It is believed that the dissolution of oxygen contributes to the total mass gain, and also has close relation with the formation of the layered structure of oxide scale, and therefore the poor interfacial bonding strength. Ti with thermal sprayed Ti3 Al(O)–Al2 O3 in situ composite coating, on the other hand, exhibited high oxidation and spallation resistance under the same testing conditions. In comparison with the previous results with bulk Ti3 Al(O)–Al2 O3 composite, the coatings have the similar oxidation mass gains. Additionally, it appeared that there was little change in the thickness of the coatings after oxidation. Hence, the oxidation process took place mainly in the coating layer. The difference between the coating and bulk material is that the coatings have a relatively high initial oxidation rate in the first 10 h (Fig. 2a and b). This can be understood since the coating was produced by thermal spraying of Ti3 Al–Al2 O3 composite powder. There is a certain degree of porosity in the microstructure, which increases the actual reaction area at the beginning. The microstructure of the coatings is also not as uniform as the bulk material due to their processing. Some metallic Al and Ti– Al regions in the sprayed coatings might oxidise fast when exposed to high temperature. The following mechanisms are suggested as the reasons that the Ti3 Al(O)–Al2 O3 composite coat-
Z.-W. Li et al. / Scripta Materialia 48 (2003) 1649–1653
ings showed superior oxidation and scale spallation resistance. 1. As oxidation was mainly conducted within the coating, the overall behaviour thereby, should be similar to Ti3 Al. Obviously, Ti3 Al has better oxidation resistance than Ti due to its high Al content, which could partially promote the formation of Al2 O3 . As discussed previously, incorporation of alumina particles could further decrease the oxidation mass gain, since they could act as diffusion barrier to O and Ti with reduced cross-section area. Pre-dissolution of oxygen in Ti3 Al could also contribute to the lower mass gain as inward diffusion and dissolution of oxygen in the substrate were reduced significantly [8]. 2. Excellent scale spallation resistance is also closely related with the alumina particles. It can be seen that these particles have good interfacial bonding with the newly formed oxide. The alumina particles have formed a 3-D network, holding the oxide scale strongly. Besides this, the presence of alumina particles could inhibit the lateral growth of cracks in the oxide scale. This is in contrast to the cracks observed in the scales on pure Ti, where the crack can propagate laterally without interruption. Additionally, the fine oxide grains originally formed by a combination of mechanical milling and thermal treatment of the powder could promote the plastic deformation of the scale, reducing the risk of cracking [11]. It is also suggested that the incorporation of Al2 O3 could decrease the CTE of the composite (7.0–8.7 10 6 , 8.0– 8.3 10 6 , 12 10 6 , and 11 10 6 /K for TiO2 , a-Al2 O3 , Ti3 Al, and Ti respectively). Thus the outer oxide scale and the coating might have a better adhesion to each other.
5. Conclusion A Ti3 Al(O)–Al2 O3 in situ composite coating was applied on pure Ti substrate through thermal
1653
spraying. Oxidation tests at 700 and 800 °C up to 400 h showed that this coating decreased the oxidation rate and improved the scale spallation resistance of Ti remarkably. The enhanced resistance is attributed to the incorporation of alumina into the coating, which serves as a diffusion barrier, an inhibitor to crack propagation, and connectors between the coating and oxide scale. The coating can increase the application temperature of Ti and Ti alloys from 650 °C to at least 800 °C.
Acknowledgements This project is partially funded by the Foundation for Research, Science and Technology, New Zealand through a New Economy Research Fund (NERF) grant. Titanox Development Ltd., Auckland, New Zealand, is the licenser of the powder making technology. ZWL thanks the postdoctoral fellowship from the University. The authors would also like to thank the technical staff at the department and the Research Centre for Surface and Materials Science for various assistance.
References [1] Zhu ZF. Corrosion resistance and applications of nonferrous metals. Beijing: Chemical Industry Press; 1995. [2] Froes FH, Suryanarayana C, Elieze D. J Mater Sci 1992;27:5113. [3] Sauthoff G. Intermetallics. Weinheim: VCH; 1995. [4] Ward-Close CM, Minor R, Doorbar PJ. Intermetallics 1996;4:217. [5] Miracle DA. Metal matrix composites. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 3. Amsterdam: Elsevier; 2000. p. 741. [6] Zhang DL, Newby MR. US patent, No.US6 264 719 B1. [7] Zhang DL, Ying DY, Adam G. Mater Sci Forum 2002;386–388:287. [8] Gao W, Li Z, Zhang D. Oxid Met 2002;57:99. [9] Kofstad P. High temperature corrosion. London: Elsevier; 1988. [10] Zhu RZ, He YD, Qi HB. High temperature corrosion and its resistant materials. Shanghai: Shanghai Science and Technology Press; 1995. [11] Coble RJ. J Appl Phys 1963;34:1679.