- Email: [email protected]
Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6– 8 0 1
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation Chong-gui Li a,b,⁎, You Wang a , Wei Tian a , Yong Yang a a b
Laboratory of Nano Surface Engineering, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
Plasma-sprayed nanostructured Al2O3–13 wt.%TiO2 coatings were successfully fabricated on
Received 21 January 2010
titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the
Received in revised form
titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma
30 April 2010
gun. The microstructure, phase constituents and mechanical properties of the titanium
Accepted 4 May 2010
alloys before and after ablation were investigated by scanning electron microscope (SEM), Xray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-
Keywords:
sectional microstructure and hardness of titanium alloys with coatings are similar before
Nanostructured coating
and after ablation. In contrast, the microstructure and mechanical properties of the
Plasma spraying
titanium alloy without coating are significantly changed after ablation. The surface coating
Titanium alloy
is found to serve as a protective coating during ablation.
Ablation
1.
© 2010 Elsevier Inc. All rights reserved.
Introduction
Titanium and titanium alloys have attracted much interest and have been used in a number of applications in industry ranging from aircraft components, gas turbine engines to chemical processing facilities owing to their high strength to weight ratio and excellent high corrosion resistance [1,2]. However, in general, titanium and titanium alloys readily absorb oxygen when used at elevated temperatures in air. Titanium alloys exposed to high-temperature environment are liable to surface failure, including hot corrosion, oxidation, microcracking as well as surface degradation. Therefore, the thermal stability of titanium and titanium alloys needs to be improved. A large number of attempts have been made to improve the thermal stability of titanium alloys, including physical vapor deposition [3], hot-dip aluminizing [4], and mechanical alloying [5]. Among various methods used for this purpose, plasma spraying has been a versatile and widely used method [6].
Plasma-sprayed Al2O3 and Al2O3–TiO2 ceramic coatings have been extensively used in many applications as surface coating to protect components against wear, corrosion and oxidation owing to their thermal, chemical and mechanical stability. Recently, plasma-sprayed nanostructured Al2O3–TiO2 coatings have been successfully developed, which possess more attractive bond strength, toughness, abrasive wear and thermal shock resistance, compared with their conventional counterparts [7–9]. In the present study, nanostructured Al2O3–13 wt.%TiO2 coatings were deposited by plasma spraying on titanium alloys (Ti–6Al–4V). Ablation experiments were carried out on titanium alloys with or without nanostructured Al2O3–13 wt.% TiO2 coatings. Ti–6Al–4V (TC4) is one of the most widely used titanium alloys and has got extensive applications as a space age alloy, which demands high reliability during service. The aim of this study is to propose a new method for protecting titanium alloys against thermal instability.
⁎ Corresponding author. Laboratory of Nano Surface Engineering, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 86402752; fax: +86 451 86413922. E-mail address: [email protected] (C.G. Li). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.05.002
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M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6 –8 0 1
2.
Experimental
2.1.
Material
The material used in this study was a Ti–6Al–4V alloy. The chemical composition (wt.%) of the as-received alloy is listed as follows: 6.00 Al, 4.30 V, 0.30 Fe, 0.10 Si, 0.10 C, 0.15 O, and the balance is Ti. The samples were cut into small discs with a diameter of 15 mm and a thickness of 3 mm by using an electric spark wire-cutting machine.
2.2.
Table 1 – Parameters of the ablation experiments.
A B
Current/ (A)
Voltage/ (V)
Distance/ (mm)
Ablation time/ (minute)
500 600
60 60
100 100
1 1
gas flow rate is 120 standard cubic feet per hour (SCFH), (f) powder feed rate is 1000–1500 g/h, and (g) spray distance is about 100 mm. The thickness of the coatings ranges from 300 to 400 μm.
Feedstock Reconstitution and Plasma Spraying 2.3.
In this study, the feedstock used for plasma spraying was reconstituted agglomerates derived from nanoparticles. The raw material powders consisted of Al2O3 (δ and γ phases, 99.9% purity, Degussa Co., Ltd., Germany) with particle size of 20–45 nm, TiO2 (anatase phase, 99.9% purity, Nanjing High Technology Nano Materials Co., Ltd., China) with particle size of 20–50 nm, ZrO2 (tetragonal phase, 99.9% purity, Cug-Nano Materials Manufacturing Co., Ltd., China) with particle size of 20–50 nm and CeO2 (cubic phase, 99.9% purity, Rare Chem. Co., Ltd., China) with particle size of 20–40 nm. ZrO2 and CeO2 were used as additives and their weight content was about 5%, respectively. All the nanoparticles were mixed by wet ball milling. An emulsion of polyvinyl alcohol (PVA) was added into the slurry as a binder addition. The reconstitution process of the feedstock also included spray drying, agglomerates sintering and plasma treatment. The SEM micrographs of the as-prepared Al2O3–13 wt.%TiO2 feedstock are shown in Fig. 1. The spherical powders possess excellent flowability owing to its roundness and smoothness. The sizes of the powders vary from 20 to 50 μm. Atmospheric plasma spraying was used to deposit the nanostructured coatings on freshly grit-blasted substrates, which were degreased by ultrasonic cleaning in acetone prior to spraying. A Metco plasma spray control system equipped with a Metco 9MB gun (Sulzer Metco, USA) was selected to deposit the coatings. A mixture of Ar and H2 was used as plasma gas and Ar was used as powder carrier gas. The parameters of plasma spraying are as follows: (a) current is 600 A, (b) voltage is 65 V, (c) primary Ar gas pressure is about 690 kPa, (d) secondary H2 gas pressure is about 380 kPa, (e) Ar
Fig. 1 – SEM micrographs of the as-prepared Al2O3–13 wt.% TiO2 feedstock.
Ablation Experiments
The ablation experiments for the titanium alloy samples with or without the nanostructured Al2O3–13 wt.%TiO2 coating were performed using a Metco 9MB plasma gun (Sulzer Metco, USA). The working principle of the gun is similar to an electric arc welding machine widely used in industry. The plasma gun was controlled by a Metco plasma spray control system. The Ar and H2 gas can be heated up and ionized to form a high-temperature plasma flame. The high-pressure cool air could form a plasma between the two electrodes of the plasma gun by applying high voltage or a strong current. The air plasma was accelerated to high speed. The temperature of the plasma flame can be adjusted by controlling the electric current and voltage. The temperature of the flame during ablation was estimated to be over 2000 °C. The samples were fixed using a holding fixture and placed at a distance of 100 mm from the nozzle. The experimental parameters for the ablation experiments are listed in Table 1. The details of the ablation facilities are illustrated in Fig. 2.
2.4.
Characterization
The phase constituents of the titanium alloys before and after ablation experiments were examined by an X-ray diffractometer (D/max-γB, Japan) using CuKα radiation (λ = 0.15418 nm) operated at 45 kV and 40 mA. The data was collected in the 2θ range of 30–90°. A field-emission gun scanning electron microscope (S-570, Hitachi, Japan) was employed to investigate the surface morphologies and cross-sectional microstructures of the titanium alloys before and after ablation experiments. In order to examine the cross-sectional microstructures of the alloys, the samples were cut perpendicular to the surface using an electric spark wire-cutting machine. Metallographic samples were prepared by standard polishing
Fig. 2 – Diagram illustration of the ablation experiment.
798
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6– 8 0 1
techniques. The cross-sectional areas of the samples were etched in a solution of HF–HNO3–H2O (2:3:95 in volume ratio; 25 °C; 1 min). Hardness of the titanium alloys was measured on a Vickers microhardness tester (HV-1000) under 300 g load and 15 s dwell time. The as-received titanium alloy samples with and without the nanostructured Al2O3–13 wt.%TiO2 coatings are referred to as R2 and R1, respectively. The samples ablated with the ablation parameters of group A are referred to as A1 (without coating) and A2 (with coating), respectively. The samples ablated with the ablation parameters of group B are referred to as B1 (without coating) and B2 (with coating), respectively.
Fig. 3 – Macroscopic features of the samples, (a) R1 and R2, before the ablation experiments, (b) A1 and A2, after ablation experiments, using the parameters of group A, and (c) B1 and B2, after ablation experiments, using the parameters of group B.
3.
Results and Discussion
3.1.
Macroscopic Morphologies
A digital camera was used to record the macroscopic morphologies of the samples before and after ablation, as shown in Fig. 3. The as-received TC4 alloy sample (R1) exhibits a bright surface. The titanium alloy sample R2 is covered with a smooth coating. After ablation experiment using the parameters of group A, the surface of the sample without coating (A1) is covered with a gray dark oxidation layer. In contrast, the coating remains almost undestroyed, though the surface color turned gray yellow, as shown in Fig. 3b. It can be suggested that the titanium alloy substrate is well protected by the surface coating. After ablation experiment using the parameters of group B, ablation pits and burning tracks are
Fig. 4 – Surface morphologies of the samples, (a) R2, (b) A2, (c) B2, and (d) B1.
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6 –8 0 1
found at the surface of the sample without coating (B1) owing to the ablation under high-temperature plasma flame. Some minor ablation pits are found at the surface of the sample with coating (B2). However, the surface coating is not integrally peeled off from the substrate.
3.2.
Microstructure
Fig. 4 shows the surface morphologies of samples R2, A2, B2 and B1. After ablation using the parameters of group A, no considerable change in the surface morphology of the sample A2 is observed, compared with that of the sample R2 (Fig. 4a and b). After ablation using the parameters of group B, some cracks are observed at the coating surface, as shown in Fig. 4c. A rough morphology and some ablation pits are observed at the surface of the sample without coating (after ablation using parameters of group B), as shown in Fig. 4d. The typical cross-sectional microstructure of the asreceived titanium alloy is shown in Fig. 5a and b. The SEM observations reveal that the as-received alloy possesses a near equiaxed microstructure. After ablation using the parameters of group A, the microstructural features of sample A2 (with coatings) are similar to those of the as-received alloys, as shown in Fig. 5c and d. It shows that the titanium alloy is well
799
protected by the surface coating. Fig. 5e and f presents a significantly different microstructural features of the sample A1 (without coating), compared with those of the as-received alloys. The SEM observation of the sample A1 proves the presence of lath martensitic microstructure, and needle like microstructure is also visible. Fine grained microstructure is also observed, compared to the as-received alloys. It is noted that the plasma gun used in the ablation experiment was operated at hot-working conditions with an ultra-high temperature, and the samples were thus subjected to rapid heating. In addition, the samples had small sizes, which facilitated cooling from the high temperature. Under this process of a rapid heating and subsequent cooling cycle, the new nucleated grains were unable to grow large during the renucleation process. Consequently, the fine grained microstructure was formed. Moreover, the titanium alloy tended to react with O2 in the atmosphere under high temperature owing to its reactive nature [10]. Thus, an oxidation layer might be formed at the ablated surface, as shown in Fig. 5e.
3.3.
Phase Constituents
Fig. 6 shows the X-ray diffraction patterns of samples R1, A1 and A2 (the surface coating of the sample A2 was removed before
Fig. 5 – Cross-sectional micrographs of the samples before and after ablation, (a) and (b) as-received titanium alloy, (c) and (d) A2, (e) and (f) A1, and (b), (d) and (f) are the high magnification SEM images of (a), (c) and (e).
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MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6– 8 0 1
SEM observations, under such nonequilibrium conditions, the martensitic transformation would occur during the subsequent rapid cooling cycle, and thus the β-Ti phase might be quenched to α′ upon cooling. As an unstable phase, α′ is further transformed to stable α-Ti phase at room temperature. In addition, when exposed to high temperature, Ti and Al in the titanium alloy sample tend to react with each other owing to their increased reactive behavior under high temperature. Intermetallics of aluminum and titanium like Ti3Al, Al2Ti and Al3Ti phases are found in the sample A1, as a result of the reaction and possible diffusion processes during the heating and cooling cycle. The rapid cooling of the materials restricts the transformation of the β-Ti phase into the phases compatible with the phase diagram. It is worth noting that the peaks of the sample A1 are much broader than those of R1. The broadening of the peaks found in the XRD patterns of the sample A1 may also indicate a fine grained microstructure. On the contrary, for the sample A2 (with coating), α-Ti, β-Ti and Ti3Al peaks are found, which are close to those of R1.
3.4.
Hardness
The hardness profiles of the titanium alloys before and after ablation experiments are presented in Fig. 7. It can be seen that the average hardness of the as-received titanium alloy is about 350 HV0.3. The average hardness of the sample A2 (with coating) after the ablation experiment is close to that of the asreceived alloy. However, the average hardness of the sample A1 (without coating) is increased to about 420 HV0.3 after the ablation experiment. It could be found that the hardness of the sample A1 is considerably changed after ablated by the hightemperature plasma flame, which is supposed to have a negative impact on the performances of the titanium alloy under circumstances requiring high reliability, e.g., serving as a component of aerospace vehicle. The significant increase of hardness for the sample A1 may be attributed to the formation of new intermetallic phases as well as fine grained
Fig. 6 – XRD patterns of the titanium alloys, (a) as-received titanium alloy R1, (b) A1, and (c) A2.
XRD examination). It reveals that the as-received titanium alloy (R1) consists of α-Ti and β-Ti. After ablation experiments using the parameters of group A, α-Ti, Ti3Al, Al2Ti and Al3Ti peaks are present in the sample A1 (without coating). As stated above, the temperature of the flame during ablation is estimated to be over 2000 °C. The α-Ti phase (closepacked hexagonal structure) in the sample A1 is transformed to β-Ti phase (body centered cubic structure) upon heating, according to a Ti–6Al–4V phase diagram. As indicated by the
Fig. 7 – Hardness profiles of the titanium alloys before and after ablation experiment, R1: as-received titanium alloy, A1: ablated under the parameters of group A, without coating, and A2: ablated under the parameters of group A, with coating.
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6 –8 0 1
microstructure. In contrast, the hardness of the sample A2 is close to that of the as-received alloy, which indicates the protection function of the plasma-sprayed coating.
4.
Conclusions
Nanostructured Al2O3–13 wt.%TiO2 coatings were successfully fabricated on the titanium alloy by plasma spraying using nanostructured feedstock. The surface coating is found to serve as a protective coating to protect the titanium alloy from thermal instability during ablation. For the titanium alloy with coating, the surface morphologies, cross-sectional microstructure and hardness are similar before and after the ablation experiment under a high-temperature plasma flame produced at voltage of 60 V and current of 500 A. However, in the case of the titanium alloy without coating, the microstructure is changed from near equiaxed microstructure to lath microstructure owing to the ablation of the high-temperature plasma flame. In addition, the hardness of the titanium alloy without coating is changed significantly after ablation. Therefore, the nanostructured Al2O3–13 wt.%TiO2 coating prepared by plasma spraying is suggested to be an effective protective coating for the titanium alloy, which can significantly restrain the thermal instability of titanium alloy.
Acknowledgments The authors would like to thank the Natural Science Foundation of Heilongjiang Province, China for providing financial support. We are also grateful to Prof. M.J. Sun, F.J. Wang, X.D. Zhang, L. Wang and B. Zhong from Harbin Institute of Technology for helpful discussions.
801
REFERENCES [1] Boyer RR. An overview on the use of titanium in the aerospace industry. Mater Sci Eng A 1996;213:103–14. [2] Gurrappa I. Characterization of titanium alloy Ti–6Al–4V for chemical, marine and industrial applications. Mater Charact 2003;51:131–9. [3] James AS, Matthews A. Thermal stability of partially-yttria-stabilized zirconia thermal barrier coatings deposited by r.f. plasma-assisted physical vapour deposition. Surf Coat Tech 1990;41:305–13. [4] Cammarota GP, Casagrande A, Sambogna G. Effect of Ni, Si and Cr in the structural formation of diffusion aluminide coatings on commercial-purity titanium. Surf Coat Tech 2006;201:230–42. [5] Romankov S, Sha W, Kaloshkin SD, Kaevitser K. Fabrication of Ti–Al coatings by mechanical alloying method. Surf Coat Tech 2006;201:3235–45. [6] McKee DW, Luthra KL. Plasma-sprayed coatings for titanium alloy oxidation protection. Surf Coat Tech 1993;56:109–17. [7] Wang Y, Jiang S, Wang MD, Wang SH, Xiao TD, Strutt PR. Abrasive wear characteristics of plasma sprayed nanostructured alumina/titania coatings. Wear 2000;237: 176–85. [8] Jordan EH, Gell M, Sohn YH, Goberman D, Shaw L, Jiang S, et al. Fabrication and evaluation of plasma sprayed nanostructured alumina–titania coatings with superior properties. Mater Sci Eng A 2001;301:80–9. [9] Wang Y, Tian W, Yang Y. Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying. Surf Coat Tech 2007;201: 7746–54. [10] Yue X, He P, Feng JC, Zhang JH, Zhu FQ. Microstructure and interfacial reactions of vacuum brazing titanium alloy to stainless steel using an AgCuTi filler metal. Mater Charact 2008;59:1721–7.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation Chong-gui Li a,b,⁎, You Wang a , Wei Tian a , Yong Yang a a b
Laboratory of Nano Surface Engineering, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
Plasma-sprayed nanostructured Al2O3–13 wt.%TiO2 coatings were successfully fabricated on
Received 21 January 2010
titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the
Received in revised form
titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma
30 April 2010
gun. The microstructure, phase constituents and mechanical properties of the titanium
Accepted 4 May 2010
alloys before and after ablation were investigated by scanning electron microscope (SEM), Xray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-
Keywords:
sectional microstructure and hardness of titanium alloys with coatings are similar before
Nanostructured coating
and after ablation. In contrast, the microstructure and mechanical properties of the
Plasma spraying
titanium alloy without coating are significantly changed after ablation. The surface coating
Titanium alloy
is found to serve as a protective coating during ablation.
Ablation
1.
© 2010 Elsevier Inc. All rights reserved.
Introduction
Titanium and titanium alloys have attracted much interest and have been used in a number of applications in industry ranging from aircraft components, gas turbine engines to chemical processing facilities owing to their high strength to weight ratio and excellent high corrosion resistance [1,2]. However, in general, titanium and titanium alloys readily absorb oxygen when used at elevated temperatures in air. Titanium alloys exposed to high-temperature environment are liable to surface failure, including hot corrosion, oxidation, microcracking as well as surface degradation. Therefore, the thermal stability of titanium and titanium alloys needs to be improved. A large number of attempts have been made to improve the thermal stability of titanium alloys, including physical vapor deposition [3], hot-dip aluminizing [4], and mechanical alloying [5]. Among various methods used for this purpose, plasma spraying has been a versatile and widely used method [6].
Plasma-sprayed Al2O3 and Al2O3–TiO2 ceramic coatings have been extensively used in many applications as surface coating to protect components against wear, corrosion and oxidation owing to their thermal, chemical and mechanical stability. Recently, plasma-sprayed nanostructured Al2O3–TiO2 coatings have been successfully developed, which possess more attractive bond strength, toughness, abrasive wear and thermal shock resistance, compared with their conventional counterparts [7–9]. In the present study, nanostructured Al2O3–13 wt.%TiO2 coatings were deposited by plasma spraying on titanium alloys (Ti–6Al–4V). Ablation experiments were carried out on titanium alloys with or without nanostructured Al2O3–13 wt.% TiO2 coatings. Ti–6Al–4V (TC4) is one of the most widely used titanium alloys and has got extensive applications as a space age alloy, which demands high reliability during service. The aim of this study is to propose a new method for protecting titanium alloys against thermal instability.
⁎ Corresponding author. Laboratory of Nano Surface Engineering, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 86402752; fax: +86 451 86413922. E-mail address: [email protected] (C.G. Li). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.05.002
797
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6 –8 0 1
2.
Experimental
2.1.
Material
The material used in this study was a Ti–6Al–4V alloy. The chemical composition (wt.%) of the as-received alloy is listed as follows: 6.00 Al, 4.30 V, 0.30 Fe, 0.10 Si, 0.10 C, 0.15 O, and the balance is Ti. The samples were cut into small discs with a diameter of 15 mm and a thickness of 3 mm by using an electric spark wire-cutting machine.
2.2.
Table 1 – Parameters of the ablation experiments.
A B
Current/ (A)
Voltage/ (V)
Distance/ (mm)
Ablation time/ (minute)
500 600
60 60
100 100
1 1
gas flow rate is 120 standard cubic feet per hour (SCFH), (f) powder feed rate is 1000–1500 g/h, and (g) spray distance is about 100 mm. The thickness of the coatings ranges from 300 to 400 μm.
Feedstock Reconstitution and Plasma Spraying 2.3.
In this study, the feedstock used for plasma spraying was reconstituted agglomerates derived from nanoparticles. The raw material powders consisted of Al2O3 (δ and γ phases, 99.9% purity, Degussa Co., Ltd., Germany) with particle size of 20–45 nm, TiO2 (anatase phase, 99.9% purity, Nanjing High Technology Nano Materials Co., Ltd., China) with particle size of 20–50 nm, ZrO2 (tetragonal phase, 99.9% purity, Cug-Nano Materials Manufacturing Co., Ltd., China) with particle size of 20–50 nm and CeO2 (cubic phase, 99.9% purity, Rare Chem. Co., Ltd., China) with particle size of 20–40 nm. ZrO2 and CeO2 were used as additives and their weight content was about 5%, respectively. All the nanoparticles were mixed by wet ball milling. An emulsion of polyvinyl alcohol (PVA) was added into the slurry as a binder addition. The reconstitution process of the feedstock also included spray drying, agglomerates sintering and plasma treatment. The SEM micrographs of the as-prepared Al2O3–13 wt.%TiO2 feedstock are shown in Fig. 1. The spherical powders possess excellent flowability owing to its roundness and smoothness. The sizes of the powders vary from 20 to 50 μm. Atmospheric plasma spraying was used to deposit the nanostructured coatings on freshly grit-blasted substrates, which were degreased by ultrasonic cleaning in acetone prior to spraying. A Metco plasma spray control system equipped with a Metco 9MB gun (Sulzer Metco, USA) was selected to deposit the coatings. A mixture of Ar and H2 was used as plasma gas and Ar was used as powder carrier gas. The parameters of plasma spraying are as follows: (a) current is 600 A, (b) voltage is 65 V, (c) primary Ar gas pressure is about 690 kPa, (d) secondary H2 gas pressure is about 380 kPa, (e) Ar
Fig. 1 – SEM micrographs of the as-prepared Al2O3–13 wt.% TiO2 feedstock.
Ablation Experiments
The ablation experiments for the titanium alloy samples with or without the nanostructured Al2O3–13 wt.%TiO2 coating were performed using a Metco 9MB plasma gun (Sulzer Metco, USA). The working principle of the gun is similar to an electric arc welding machine widely used in industry. The plasma gun was controlled by a Metco plasma spray control system. The Ar and H2 gas can be heated up and ionized to form a high-temperature plasma flame. The high-pressure cool air could form a plasma between the two electrodes of the plasma gun by applying high voltage or a strong current. The air plasma was accelerated to high speed. The temperature of the plasma flame can be adjusted by controlling the electric current and voltage. The temperature of the flame during ablation was estimated to be over 2000 °C. The samples were fixed using a holding fixture and placed at a distance of 100 mm from the nozzle. The experimental parameters for the ablation experiments are listed in Table 1. The details of the ablation facilities are illustrated in Fig. 2.
2.4.
Characterization
The phase constituents of the titanium alloys before and after ablation experiments were examined by an X-ray diffractometer (D/max-γB, Japan) using CuKα radiation (λ = 0.15418 nm) operated at 45 kV and 40 mA. The data was collected in the 2θ range of 30–90°. A field-emission gun scanning electron microscope (S-570, Hitachi, Japan) was employed to investigate the surface morphologies and cross-sectional microstructures of the titanium alloys before and after ablation experiments. In order to examine the cross-sectional microstructures of the alloys, the samples were cut perpendicular to the surface using an electric spark wire-cutting machine. Metallographic samples were prepared by standard polishing
Fig. 2 – Diagram illustration of the ablation experiment.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6– 8 0 1
techniques. The cross-sectional areas of the samples were etched in a solution of HF–HNO3–H2O (2:3:95 in volume ratio; 25 °C; 1 min). Hardness of the titanium alloys was measured on a Vickers microhardness tester (HV-1000) under 300 g load and 15 s dwell time. The as-received titanium alloy samples with and without the nanostructured Al2O3–13 wt.%TiO2 coatings are referred to as R2 and R1, respectively. The samples ablated with the ablation parameters of group A are referred to as A1 (without coating) and A2 (with coating), respectively. The samples ablated with the ablation parameters of group B are referred to as B1 (without coating) and B2 (with coating), respectively.
Fig. 3 – Macroscopic features of the samples, (a) R1 and R2, before the ablation experiments, (b) A1 and A2, after ablation experiments, using the parameters of group A, and (c) B1 and B2, after ablation experiments, using the parameters of group B.
3.
Results and Discussion
3.1.
Macroscopic Morphologies
A digital camera was used to record the macroscopic morphologies of the samples before and after ablation, as shown in Fig. 3. The as-received TC4 alloy sample (R1) exhibits a bright surface. The titanium alloy sample R2 is covered with a smooth coating. After ablation experiment using the parameters of group A, the surface of the sample without coating (A1) is covered with a gray dark oxidation layer. In contrast, the coating remains almost undestroyed, though the surface color turned gray yellow, as shown in Fig. 3b. It can be suggested that the titanium alloy substrate is well protected by the surface coating. After ablation experiment using the parameters of group B, ablation pits and burning tracks are
Fig. 4 – Surface morphologies of the samples, (a) R2, (b) A2, (c) B2, and (d) B1.
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6 –8 0 1
found at the surface of the sample without coating (B1) owing to the ablation under high-temperature plasma flame. Some minor ablation pits are found at the surface of the sample with coating (B2). However, the surface coating is not integrally peeled off from the substrate.
3.2.
Microstructure
Fig. 4 shows the surface morphologies of samples R2, A2, B2 and B1. After ablation using the parameters of group A, no considerable change in the surface morphology of the sample A2 is observed, compared with that of the sample R2 (Fig. 4a and b). After ablation using the parameters of group B, some cracks are observed at the coating surface, as shown in Fig. 4c. A rough morphology and some ablation pits are observed at the surface of the sample without coating (after ablation using parameters of group B), as shown in Fig. 4d. The typical cross-sectional microstructure of the asreceived titanium alloy is shown in Fig. 5a and b. The SEM observations reveal that the as-received alloy possesses a near equiaxed microstructure. After ablation using the parameters of group A, the microstructural features of sample A2 (with coatings) are similar to those of the as-received alloys, as shown in Fig. 5c and d. It shows that the titanium alloy is well
799
protected by the surface coating. Fig. 5e and f presents a significantly different microstructural features of the sample A1 (without coating), compared with those of the as-received alloys. The SEM observation of the sample A1 proves the presence of lath martensitic microstructure, and needle like microstructure is also visible. Fine grained microstructure is also observed, compared to the as-received alloys. It is noted that the plasma gun used in the ablation experiment was operated at hot-working conditions with an ultra-high temperature, and the samples were thus subjected to rapid heating. In addition, the samples had small sizes, which facilitated cooling from the high temperature. Under this process of a rapid heating and subsequent cooling cycle, the new nucleated grains were unable to grow large during the renucleation process. Consequently, the fine grained microstructure was formed. Moreover, the titanium alloy tended to react with O2 in the atmosphere under high temperature owing to its reactive nature [10]. Thus, an oxidation layer might be formed at the ablated surface, as shown in Fig. 5e.
3.3.
Phase Constituents
Fig. 6 shows the X-ray diffraction patterns of samples R1, A1 and A2 (the surface coating of the sample A2 was removed before
Fig. 5 – Cross-sectional micrographs of the samples before and after ablation, (a) and (b) as-received titanium alloy, (c) and (d) A2, (e) and (f) A1, and (b), (d) and (f) are the high magnification SEM images of (a), (c) and (e).
800
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6– 8 0 1
SEM observations, under such nonequilibrium conditions, the martensitic transformation would occur during the subsequent rapid cooling cycle, and thus the β-Ti phase might be quenched to α′ upon cooling. As an unstable phase, α′ is further transformed to stable α-Ti phase at room temperature. In addition, when exposed to high temperature, Ti and Al in the titanium alloy sample tend to react with each other owing to their increased reactive behavior under high temperature. Intermetallics of aluminum and titanium like Ti3Al, Al2Ti and Al3Ti phases are found in the sample A1, as a result of the reaction and possible diffusion processes during the heating and cooling cycle. The rapid cooling of the materials restricts the transformation of the β-Ti phase into the phases compatible with the phase diagram. It is worth noting that the peaks of the sample A1 are much broader than those of R1. The broadening of the peaks found in the XRD patterns of the sample A1 may also indicate a fine grained microstructure. On the contrary, for the sample A2 (with coating), α-Ti, β-Ti and Ti3Al peaks are found, which are close to those of R1.
3.4.
Hardness
The hardness profiles of the titanium alloys before and after ablation experiments are presented in Fig. 7. It can be seen that the average hardness of the as-received titanium alloy is about 350 HV0.3. The average hardness of the sample A2 (with coating) after the ablation experiment is close to that of the asreceived alloy. However, the average hardness of the sample A1 (without coating) is increased to about 420 HV0.3 after the ablation experiment. It could be found that the hardness of the sample A1 is considerably changed after ablated by the hightemperature plasma flame, which is supposed to have a negative impact on the performances of the titanium alloy under circumstances requiring high reliability, e.g., serving as a component of aerospace vehicle. The significant increase of hardness for the sample A1 may be attributed to the formation of new intermetallic phases as well as fine grained
Fig. 6 – XRD patterns of the titanium alloys, (a) as-received titanium alloy R1, (b) A1, and (c) A2.
XRD examination). It reveals that the as-received titanium alloy (R1) consists of α-Ti and β-Ti. After ablation experiments using the parameters of group A, α-Ti, Ti3Al, Al2Ti and Al3Ti peaks are present in the sample A1 (without coating). As stated above, the temperature of the flame during ablation is estimated to be over 2000 °C. The α-Ti phase (closepacked hexagonal structure) in the sample A1 is transformed to β-Ti phase (body centered cubic structure) upon heating, according to a Ti–6Al–4V phase diagram. As indicated by the
Fig. 7 – Hardness profiles of the titanium alloys before and after ablation experiment, R1: as-received titanium alloy, A1: ablated under the parameters of group A, without coating, and A2: ablated under the parameters of group A, with coating.
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 7 9 6 –8 0 1
microstructure. In contrast, the hardness of the sample A2 is close to that of the as-received alloy, which indicates the protection function of the plasma-sprayed coating.
4.
Conclusions
Nanostructured Al2O3–13 wt.%TiO2 coatings were successfully fabricated on the titanium alloy by plasma spraying using nanostructured feedstock. The surface coating is found to serve as a protective coating to protect the titanium alloy from thermal instability during ablation. For the titanium alloy with coating, the surface morphologies, cross-sectional microstructure and hardness are similar before and after the ablation experiment under a high-temperature plasma flame produced at voltage of 60 V and current of 500 A. However, in the case of the titanium alloy without coating, the microstructure is changed from near equiaxed microstructure to lath microstructure owing to the ablation of the high-temperature plasma flame. In addition, the hardness of the titanium alloy without coating is changed significantly after ablation. Therefore, the nanostructured Al2O3–13 wt.%TiO2 coating prepared by plasma spraying is suggested to be an effective protective coating for the titanium alloy, which can significantly restrain the thermal instability of titanium alloy.
Acknowledgments The authors would like to thank the Natural Science Foundation of Heilongjiang Province, China for providing financial support. We are also grateful to Prof. M.J. Sun, F.J. Wang, X.D. Zhang, L. Wang and B. Zhong from Harbin Institute of Technology for helpful discussions.
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REFERENCES [1] Boyer RR. An overview on the use of titanium in the aerospace industry. Mater Sci Eng A 1996;213:103–14. [2] Gurrappa I. Characterization of titanium alloy Ti–6Al–4V for chemical, marine and industrial applications. Mater Charact 2003;51:131–9. [3] James AS, Matthews A. Thermal stability of partially-yttria-stabilized zirconia thermal barrier coatings deposited by r.f. plasma-assisted physical vapour deposition. Surf Coat Tech 1990;41:305–13. [4] Cammarota GP, Casagrande A, Sambogna G. Effect of Ni, Si and Cr in the structural formation of diffusion aluminide coatings on commercial-purity titanium. Surf Coat Tech 2006;201:230–42. [5] Romankov S, Sha W, Kaloshkin SD, Kaevitser K. Fabrication of Ti–Al coatings by mechanical alloying method. Surf Coat Tech 2006;201:3235–45. [6] McKee DW, Luthra KL. Plasma-sprayed coatings for titanium alloy oxidation protection. Surf Coat Tech 1993;56:109–17. [7] Wang Y, Jiang S, Wang MD, Wang SH, Xiao TD, Strutt PR. Abrasive wear characteristics of plasma sprayed nanostructured alumina/titania coatings. Wear 2000;237: 176–85. [8] Jordan EH, Gell M, Sohn YH, Goberman D, Shaw L, Jiang S, et al. Fabrication and evaluation of plasma sprayed nanostructured alumina–titania coatings with superior properties. Mater Sci Eng A 2001;301:80–9. [9] Wang Y, Tian W, Yang Y. Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying. Surf Coat Tech 2007;201: 7746–54. [10] Yue X, He P, Feng JC, Zhang JH, Zhu FQ. Microstructure and interfacial reactions of vacuum brazing titanium alloy to stainless steel using an AgCuTi filler metal. Mater Charact 2008;59:1721–7.