- Email: [email protected]
Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance
Current Applied Physics 10 (2010) 698–702
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance Yongjun Xu, Zhongping Yao *, Fangzhou Jia, Yunlong Wang, Zhaohua Jiang, Haitao Bu School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 14 May 2009 Received in revised form 25 July 2009 Accepted 8 September 2009 Available online 10 September 2009 PACS: 64.70.kd Keywords: Plasma electrolytic oxidation Ceramic coatings Thermal shock resistance High temperature oxidation resistance Ti–6Al–4V alloy
a b s t r a c t Ceramic coatings were prepared in Na2SiO3–Na2CO3–NaOH system by pulsed bi-polar plasma electrolytic oxidation on Ti–6Al–4V alloy. The phase composition, structure and the elemental distribution of the coatings were studied by XRD, SEM and energy dispersive spectroscopy, respectively. The thermal shock resistance of the coated samples at 850 °C was evaluated by the thermal shock tests. The high temperature oxidation resistance of the coating samples at 500 °C was investigated. The results showed that the coating was mainly composed of rutile- and anatase TiO2, Increasing the concentration of Na2SiO3, TiO2 content decreased gradually while the thickness of the coating increased. There were a large amount of micro pores and sintered particles on the surface of the coatings. Increasing concentration of Na2SiO3, the sintered particles on the surface turned large, and the Si content increased while the Ti content decreased gradually. When the concentration of Na2SiO3 was 15 g/L, the thermal shock resistance of the coatings was better than that of the coatings that prepared under other Na2SiO3 concentrations. The coating samples prepared under the optimized technique process based on the thermal shock tests improved the high temperature oxidation resistance at 500 °C greatly, whether considering the isothermal oxidation or the cyclic oxidation. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Ti–6Al–4V alloy has been applied in many fields, due to their high strength, non-magnetism, corrosion resistance and other characters. However, its safety working temperature is generally less than 400–500 °C; therefore, Ti–6Al–4V alloy is usually considered as a sort of heat-unstable material, which greatly limited their further application. Presently, many kinds of surface modification techniques such as pre-oxidation and coating technologies were developed to improve the high temperature oxidation resistance of Ti alloys [1–3]. At present, plasma electrolytic oxidation (PEO) technique can also be used for the surface treatment of Ti alloys. Using this technique, a dense ceramic coating can be grown in situ on the surface of Ti alloy [4–6]. Now, some researches have focused on the structure of the coatings on Ti alloy prepared by PEO and their properties such as corrosion resistance, anti-abrasion and so on [7–9]. However, there are only a few instances of coatings on Ti alloys with the high temperature oxidation resistance. Hao et al. and Tang et al. investigated the effects of PEO technique on the high temperature oxidation resistance of TiAl alloy under 1000 and 850 °C,
* Corresponding author. Tel.: +86 0451 86413710; fax: +86 0451 86415647. E-mail address: [email protected] (Z. Yao). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.09.003
respectively, and found the high temperature oxidation resistance of TiAl alloy was improved in a different degree [10,11]. Zhou et al. and our research group preliminarily studied the high temperature oxidation resistance of PEO coatings on Ti–6Al–4V alloy at 700– 1000 °C [12,13]. In this paper, we prepared compound ceramic coatings in Na2SiO3–Na2CO3–NaOH system by pulsed bi-polar plasma electrolytic oxidation on Ti–6Al–4V alloy. Meanwhile, the structure, thermal shock resistance and high temperature oxidation resistance at 500 °C of such coatings were investigated. 2. Experimental details 2.1. Preparation of the ceramic coatings by plasma electrolytic oxidation Plate samples of Ti–6Al–4V alloy with a reaction dimension of 20 mm 10 mm 6 mm were used as working electrode and the electrolyser made of stainless steel served as the counter electrode. The electrolyte used in the experiments was Na2SiO3 solution with different concentrations, Na2CO3 (4 g/L) and NaOH (0.5 g/L). A home-made high power pulsed bi-polar electrical source with power of 10 kW was used for plasma electrolytic oxidation under the current densities of 1.2 A/dm2 for anode pulse and 0.4 A/dm2 for cathode pulse, and the working frequency was
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
699
the PEO treatment, the coated samples were rinsed with water and dried in air. 2.2. Analysis of phase composition and structure of the coatings
Fig. 1. The thickness of the coatings prepared under different concentrations of Na2SiO3.
Phase composition of the coatings was examined with a RICOH D/max-rB automatic X-ray diffractometer (XRD) using a Cu Ka source. Surface morphologies of the produced coatings were studied by scanning electron microscopy (SEM; Hitachi S-570). The element distribution on the surface of the coating was investigated by energy dispersive spectroscopy (EDS; US PN5502). The thickness of the coating was measured, using an eddy current-based thickness gauge (CTG-10, Time Company, China). The average thickness of each sample was obtained from 10 measurements at different positions. 2.3. Thermal shock tests of the coatings
100 Hz, the duty ratio for both pulse was 40% and the reaction time was for 25 min. The output mode of both pulses for the pulsed source is shown in Ref. [14]. The reaction temperature was controlled to below 30 °C by adjusting the cooling water flow. After
Thermal shock tests of the coatings were carried out in the muffle. The coatings were kept for 2 min under 850 °C and then put into the cool water of 20 °C. Repeating the tests and observing the experimental phenomena until the spallation of the coating oc-
Fig. 2. The surface SEM of the coatings prepared under different concentrations of Na2SiO3 (a): 5 g/L; (b): 10 g/L; (c): 15 g/L; and (d): 20 g/L.
700
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
curred. The times of the thermal shock tests was recorded. Three parallel samples were used in the thermal shock tests to ensure the reliability of the experiments. 2.4. High temperature oxidation tests of the coating samples The coating samples and the Ti–6Al–4V substrates, which have already been weighed, were oxidized in the kryptol heater for different time at the temperature of 500 °C, respectively. And then, the samples were taken out and cooled naturally in air to the room temperature and weighed again. The weight of the samples was measured by CP 225D Sartorius electron analytical balance with the accuracy of 10 5 g. Three parallel samples with the same surface area of 7.5 cm2 were used in each measurement to ensure the reliability of the experiments. The duration of the isothermal oxidation and cyclic oxidation process was 200 h. 3. Results and discussion
Fig. 3. The XRD patterns of the coatings prepared under different concentrations of Na2SiO3.
the swelling parts at the low diffraction angles in the XRD patterns [15]. Furthermore, the characteristic diffraction peaks corresponding to the titanium substrate also appeared in the patterns.
3.1. The thickness of the coatings The prepared coatings are all smooth and gray. The color of the coatings turned deeply with the increase of the concentration of Na2SiO3. Fig. 1 is the mean thickness of the coatings prepared under different concentration of Na2SiO3. It can be noted that the thickness of the coating increased with the increase of the concentration of Na2SiO3. But, at low concentrations, the thickness of the coating changed not apparently, while that of at high concentrations increasing greatly. 3.2. Surface SEM of the coatings and EDS analyses Fig. 2 is the surface SEM of the coatings prepared under different concentrations of Na2SiO3. There were a large amount of micro pores and sintered particles on the surface of the coatings. The amount of micro pores decreased with the increasing concentration of Na2SiO3, whereas the sintered particles increased and became larger and larger. And when the concentration of Na2SiO3 was 20 g/L, the large particles were connected each other and formed a comparatively continuous bump. Table 1 is the relative contents of Si and Ti main elements on the surface of the coatings by EDS analysis. Increasing the concentration of Na2SiO3, the Si content on the surface of the coating increased while the Ti content decreased gradually. 3.3. Phase composition of the coatings Fig. 3 is the XRD patterns of the coatings prepared under different concentrations of Na2SiO3. The coatings were composed of rutile and anatase TiO2, and the content of rutile TiO2 was more than that of anatase TiO2. Increasing the concentration of Na2SiO3, TiO2 content decreased gradually, which is consistent with the results of EDS analysis. When the concentration of Na2SiO3 was 20 g/L, anatase TiO2 was hardly detected by XRD. According to the EDS analysis, a large amount of Si from the electrolyte joined the PEO process, but the relative crystallized substance were not detected by XRD, which meant that element Si existed in the form of amorphous state. This amorphous Si–O compound was corresponding to
3.4. Thermal shock resistance of the coating samples The thermal shock resistance of the coating is a key factor for the application of such materials. Fig. 4 is the numbers of thermal shock tests of the coatings prepared under different concentration of Na2SiO3 at 800 °C. Increasing the concentration of Na2SiO3, the number of the thermal shock resistance of the coatings firstly increased, and then decreased. When the concentration of Na2SiO3 was 15 g/L, the thermal shock resistance of the coatings was the best among the samples prepared under other experimental conditions. The thermal shock resistance of the coatings was related to the structure and composition. Since the coatings had the similar composition, the thermal shock resistance of the coatings was mainly attributable to the structure of the coatings. The PEO ceramic coatings had the heat resistant property, which was improved with increase of the thickness of the coating. Increasing the concentration of Na3SiO3, the thickness of the coating increased which was helpful to improve the heat resistant property. Besides, the micro pores and micro cracks on the surface of the coating were liable for the release of the residual stress during the thermal shock tests. Both of them were useful to increase the thermal shock resistance of the coatings. On the other side, the increase of the thickness accompanied the increase of the growth stress of the coating during PEO process, and the micro pores and micro cracks would strengthen the oxidation of the substrate due to the diffusion of O2 into the interface between the coating and the substrate. Both
Table 1 The relative contents of Si and Ti on the surface of the coatings (atom%).
Si Ti
5 g/L
10 g/L
15 g/L
20 g/L
34.12 62.44
43.53 53.06
54.53 43.82
60.06 38.45
Fig. 4. The numbers of thermal shock tests of the coatings prepared under different concentration of Na2SiO3.
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
701
Fig. 5. The SEM of the coatings after high temperature oxidation tests. (a) Isothermal oxidation, and (b) cyclic oxidation.
of them would result in the decrease of thermal shock resistance reversely. Consequently, such conflicts determined that coating prepared under the 15 g/L presented the best thermal shock resistance.
3.5. High temperature oxidation tests of the coating samples at 500 °C According to the results of thermal shock tests, the ceramic coatings prepared at 15 g/L of Na2SiO3 were used to investigate the high temperature oxidation resistance at 500 °C through isothermal oxidation and cyclic oxidation tests. Accompany to those
Fig. 6. The XRD patterns of the coatings after high temperature oxidation tests. (a) Isothermal oxidation, and (b) cyclic oxidation.
high temperature oxidation tests, the color of the substrate and coatings changed greatly: the substrate turned light blue and deep purple gradually, while the coating turned from the gray to light yellow and deep brown. And the coating did not spall off the substrate. Figs. 5 and 6 are the surface morphologies and phase compositions of the coatings correspondingly after high temperature oxidation tests. It can be noted that the surface images of the coating changed less, but the composition changed greatly. The amount of TiO2 was increased and the diffraction peaks corresponding to the substrate was also intensified. This meant that substrate was oxidized in a certain degree and the density of the coating was reduced after 200 h of high temperature oxidation. Fig. 7 is the changing curves of the weight for the coating samples and substrate versus time under isothermal oxidation and cyclic oxidation tests of 500 °C, respectively. Both tests present the similar changing regularity. The weight of the substrate samples increased quickly at the beginning and then comparatively slowly with increasing oxidation time, whereas interestingly the weight of the coating samples decreased quickly and then increased slowly. For the isothermal oxidation, the mean weight gain of the substrate samples after 200 h was 0.1053 mg/cm2. Because of the weigh loss of the coating samples at the beginning, the weigh gain of the coating sample was calculated approximately through DW1 as shown in the Fig. 5, with the results of 0.0373 mg/cm2, which is nearly 1/3 that of the substrate sample. This meant that PEO technique can effectively decrease the weight gain of the isothermal oxidation of the Ti–6Al–4V at 500 °C. For the cyclic oxidation, the mean weight gain of the substrate samples after 200 h was 0.1004 mg/cm2, while the weigh gain of the coating samples after the same approximate calculation through DW2 was 0.0124 mg/
Fig. 7. Changing curves of the weight for the coating samples and the substrate versus time at 500 °C for 200 h. (a) Isothermal oxidation, and (b) cyclic oxidation.
702
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
cm2, which is only 12.4% that of the substrates. Therefore, the weight gains of the cyclic oxidation were reduced greatly, compared with the substrate. In terms of the above analyses, the ceramic coatings reduced the weight gains of Ti alloy at 500 °C, whether considering the isothermal oxidation or the cyclic oxidation, that is to say, the ceramic coatings greatly improved the high temperature oxidation resistance of Ti alloy and may be used as a kind of oxidation resistance coating of Ti alloy at 500 °C in practice. The decrease of the weight of the coating samples at the initial stage in our experiments may be related to the structure of the coating. In the coating there may be some unstable phases such as amorphous hydrated Si–O compounds or carbonates. Under the high temperature oxidation of 500 °C, the dehydration or decomposition reactions occurred, which may lead to the weight loss of the coating samples decreased at the beginning of the high temperature oxidation tests. After the weight loss reactions, the weight gains of the coating samples can be apparently observed because of the oxidation of the substrate in the experiments. 4. Conclusions Ceramic coatings on Ti alloy were prepared in silicate solution by plasma electrolytic oxidation. The structure, thermal shock resistance and high temperature oxidation resistance of the coatings were investigated and the following conclusions can be drawn: (1) The ceramic coatings were composed of rutile and anatase TiO2. Increasing the concentration of Na2SiO3, TiO2 content decreased gradually while the thickness of the coating increased. There were a large amount of micro pores and sintered particles on the surface of the coatings. Increasing concentration of Na2SiO3, the amount of micro pores decreased whereas the sintered particles increased and became larger and larger, and meantime, the Si content on the surface of the coating increased while the Ti content decreased gradually. (2) When the concentration of Na2SiO3 was 15 g/L, the thermal shock resistance of the coatings was the best among the samples prepared under other experimental conditions. The high temperature oxidation tests shows the surface morphologies of the coatings changed less while the phase composition changed greatly due to the oxidation of the substrate. The ceramic coatings reduced the weight gains of Ti alloy at 500 °C greatly, whether considering the isother-
mal oxidation or the cyclic oxidation and therefore, improved the high temperature oxidation resistance of Ti alloy at 500 °C.
Acknowledgement This work was financially supported by Special Foundation for New Teachers of Doctor course in Chinese Education Ministry (Grant No. 200802131065). References [1] J. Sun, J.S. Wu, B. Zhao, F. Wang, Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys, Mater. Sci. Eng. A329–A331 (2002) 713–717. [2] Y.G. Zhao, W. Zhou, Q.D. Qin, Y.H. Liang, Q.C. Jiang, Effects of preoxidation on high temperature oxidation behavior of Ti alloys, Spec. Cast. Nonferrous Alloys 24 (2004) 34–36 (in Chinese). [3] J.S. Xiao, G.D. Xu, Several ways to improve mechanical properties of hightemperature Ti-based alloys, Chin. J. Nonferrous Met. 7 (1997) 97–105. [4] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [5] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Characterization of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy, Surf. Coat. Technol. 130 (2000) 195–206. [6] Y.J. Guan, Y. Xia, Review on plasma electrolytic deposition, Adv. Mech. 34 (2004) 237 (in Chinese). [7] Z.P. Yao, Z.H. Jiang, X.H. Wu, X.T. Sun, Z.D. Wu, Effects of ceramic coating by micro-plasma oxidation on the corrosion resistance of Ti–6Al–4V alloy, Surf. Coat. Technol. 200 (2005) 2445–2450. [8] W.B. Xue, C. Wang, R.Y. Chen, Structure and properties characterization of ceramic coatings produced on Ti–6Al–4V alloy by microarc oxidation in aluminate solution, Mater. Lett. 52 (2002) 435–441. [9] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Microarc oxidation and spraying graphite duplex coating formed on titanium alloy for antifriction purpose, Appl. Surf. Sci. 246 (2005) 214–221. [10] Z.L. Tang, F.H. Wang, W.T. Wu, Effect of micro-arc oxidation treatment on oxidation resistance of TiAl alloy, Chin. J. Nonferrous Met. 9 (S1) (1999) 63–68 (in Chinese). [11] J.M. Hao, W.Q. Jie, H. Chen, Y.D. Ye, The effect of SiO32- on micro-arc oxidization ceramic layer of TiAl alloy, Rare Met. Mater. Eng. 34 (9) (2005) 1455–1459 (in Chinese). [12] H. Zhou, Z.T. Liu, Z.X. Li, J.H. Du, Microarc oxidation coating and hightemperature oxidation resistant property on Ti alloy, Rare Met. Mater. Eng. 11 (34) (2005) 1835–1837 (in Chinese). [13] Z.P. Yao, Z.H. Jiang, F.P. Wang, G.D. Hao, Oxidation behavior of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation, J. Mater. Process. Technol. 190 (2007) 117–122. [14] Z.P. Yao, Z.H. Jiang, X.T. Sun, S.G. Xin, Z.D. Wu, Influence of the frequency on the structure and corrosion resistance of ceramic coatings on Ti–6Al–4V alloy produced by micro-plasma oxidation, Mater. Chem. Phys. 92 (2005) 408–412. [15] X.T. Sun, Z.H. Jiang, Y.P. Li, F.P. Wang, Y.D. Lv, Effect of the oxidation time on properties of ceramic coatings produced on Ti–6Al–4V by micro-arc oxidation, J. Mater. Technol. Sci. 21 (2005) 281–285.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance Yongjun Xu, Zhongping Yao *, Fangzhou Jia, Yunlong Wang, Zhaohua Jiang, Haitao Bu School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 14 May 2009 Received in revised form 25 July 2009 Accepted 8 September 2009 Available online 10 September 2009 PACS: 64.70.kd Keywords: Plasma electrolytic oxidation Ceramic coatings Thermal shock resistance High temperature oxidation resistance Ti–6Al–4V alloy
a b s t r a c t Ceramic coatings were prepared in Na2SiO3–Na2CO3–NaOH system by pulsed bi-polar plasma electrolytic oxidation on Ti–6Al–4V alloy. The phase composition, structure and the elemental distribution of the coatings were studied by XRD, SEM and energy dispersive spectroscopy, respectively. The thermal shock resistance of the coated samples at 850 °C was evaluated by the thermal shock tests. The high temperature oxidation resistance of the coating samples at 500 °C was investigated. The results showed that the coating was mainly composed of rutile- and anatase TiO2, Increasing the concentration of Na2SiO3, TiO2 content decreased gradually while the thickness of the coating increased. There were a large amount of micro pores and sintered particles on the surface of the coatings. Increasing concentration of Na2SiO3, the sintered particles on the surface turned large, and the Si content increased while the Ti content decreased gradually. When the concentration of Na2SiO3 was 15 g/L, the thermal shock resistance of the coatings was better than that of the coatings that prepared under other Na2SiO3 concentrations. The coating samples prepared under the optimized technique process based on the thermal shock tests improved the high temperature oxidation resistance at 500 °C greatly, whether considering the isothermal oxidation or the cyclic oxidation. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Ti–6Al–4V alloy has been applied in many fields, due to their high strength, non-magnetism, corrosion resistance and other characters. However, its safety working temperature is generally less than 400–500 °C; therefore, Ti–6Al–4V alloy is usually considered as a sort of heat-unstable material, which greatly limited their further application. Presently, many kinds of surface modification techniques such as pre-oxidation and coating technologies were developed to improve the high temperature oxidation resistance of Ti alloys [1–3]. At present, plasma electrolytic oxidation (PEO) technique can also be used for the surface treatment of Ti alloys. Using this technique, a dense ceramic coating can be grown in situ on the surface of Ti alloy [4–6]. Now, some researches have focused on the structure of the coatings on Ti alloy prepared by PEO and their properties such as corrosion resistance, anti-abrasion and so on [7–9]. However, there are only a few instances of coatings on Ti alloys with the high temperature oxidation resistance. Hao et al. and Tang et al. investigated the effects of PEO technique on the high temperature oxidation resistance of TiAl alloy under 1000 and 850 °C,
* Corresponding author. Tel.: +86 0451 86413710; fax: +86 0451 86415647. E-mail address: [email protected] (Z. Yao). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.09.003
respectively, and found the high temperature oxidation resistance of TiAl alloy was improved in a different degree [10,11]. Zhou et al. and our research group preliminarily studied the high temperature oxidation resistance of PEO coatings on Ti–6Al–4V alloy at 700– 1000 °C [12,13]. In this paper, we prepared compound ceramic coatings in Na2SiO3–Na2CO3–NaOH system by pulsed bi-polar plasma electrolytic oxidation on Ti–6Al–4V alloy. Meanwhile, the structure, thermal shock resistance and high temperature oxidation resistance at 500 °C of such coatings were investigated. 2. Experimental details 2.1. Preparation of the ceramic coatings by plasma electrolytic oxidation Plate samples of Ti–6Al–4V alloy with a reaction dimension of 20 mm 10 mm 6 mm were used as working electrode and the electrolyser made of stainless steel served as the counter electrode. The electrolyte used in the experiments was Na2SiO3 solution with different concentrations, Na2CO3 (4 g/L) and NaOH (0.5 g/L). A home-made high power pulsed bi-polar electrical source with power of 10 kW was used for plasma electrolytic oxidation under the current densities of 1.2 A/dm2 for anode pulse and 0.4 A/dm2 for cathode pulse, and the working frequency was
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
699
the PEO treatment, the coated samples were rinsed with water and dried in air. 2.2. Analysis of phase composition and structure of the coatings
Fig. 1. The thickness of the coatings prepared under different concentrations of Na2SiO3.
Phase composition of the coatings was examined with a RICOH D/max-rB automatic X-ray diffractometer (XRD) using a Cu Ka source. Surface morphologies of the produced coatings were studied by scanning electron microscopy (SEM; Hitachi S-570). The element distribution on the surface of the coating was investigated by energy dispersive spectroscopy (EDS; US PN5502). The thickness of the coating was measured, using an eddy current-based thickness gauge (CTG-10, Time Company, China). The average thickness of each sample was obtained from 10 measurements at different positions. 2.3. Thermal shock tests of the coatings
100 Hz, the duty ratio for both pulse was 40% and the reaction time was for 25 min. The output mode of both pulses for the pulsed source is shown in Ref. [14]. The reaction temperature was controlled to below 30 °C by adjusting the cooling water flow. After
Thermal shock tests of the coatings were carried out in the muffle. The coatings were kept for 2 min under 850 °C and then put into the cool water of 20 °C. Repeating the tests and observing the experimental phenomena until the spallation of the coating oc-
Fig. 2. The surface SEM of the coatings prepared under different concentrations of Na2SiO3 (a): 5 g/L; (b): 10 g/L; (c): 15 g/L; and (d): 20 g/L.
700
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
curred. The times of the thermal shock tests was recorded. Three parallel samples were used in the thermal shock tests to ensure the reliability of the experiments. 2.4. High temperature oxidation tests of the coating samples The coating samples and the Ti–6Al–4V substrates, which have already been weighed, were oxidized in the kryptol heater for different time at the temperature of 500 °C, respectively. And then, the samples were taken out and cooled naturally in air to the room temperature and weighed again. The weight of the samples was measured by CP 225D Sartorius electron analytical balance with the accuracy of 10 5 g. Three parallel samples with the same surface area of 7.5 cm2 were used in each measurement to ensure the reliability of the experiments. The duration of the isothermal oxidation and cyclic oxidation process was 200 h. 3. Results and discussion
Fig. 3. The XRD patterns of the coatings prepared under different concentrations of Na2SiO3.
the swelling parts at the low diffraction angles in the XRD patterns [15]. Furthermore, the characteristic diffraction peaks corresponding to the titanium substrate also appeared in the patterns.
3.1. The thickness of the coatings The prepared coatings are all smooth and gray. The color of the coatings turned deeply with the increase of the concentration of Na2SiO3. Fig. 1 is the mean thickness of the coatings prepared under different concentration of Na2SiO3. It can be noted that the thickness of the coating increased with the increase of the concentration of Na2SiO3. But, at low concentrations, the thickness of the coating changed not apparently, while that of at high concentrations increasing greatly. 3.2. Surface SEM of the coatings and EDS analyses Fig. 2 is the surface SEM of the coatings prepared under different concentrations of Na2SiO3. There were a large amount of micro pores and sintered particles on the surface of the coatings. The amount of micro pores decreased with the increasing concentration of Na2SiO3, whereas the sintered particles increased and became larger and larger. And when the concentration of Na2SiO3 was 20 g/L, the large particles were connected each other and formed a comparatively continuous bump. Table 1 is the relative contents of Si and Ti main elements on the surface of the coatings by EDS analysis. Increasing the concentration of Na2SiO3, the Si content on the surface of the coating increased while the Ti content decreased gradually. 3.3. Phase composition of the coatings Fig. 3 is the XRD patterns of the coatings prepared under different concentrations of Na2SiO3. The coatings were composed of rutile and anatase TiO2, and the content of rutile TiO2 was more than that of anatase TiO2. Increasing the concentration of Na2SiO3, TiO2 content decreased gradually, which is consistent with the results of EDS analysis. When the concentration of Na2SiO3 was 20 g/L, anatase TiO2 was hardly detected by XRD. According to the EDS analysis, a large amount of Si from the electrolyte joined the PEO process, but the relative crystallized substance were not detected by XRD, which meant that element Si existed in the form of amorphous state. This amorphous Si–O compound was corresponding to
3.4. Thermal shock resistance of the coating samples The thermal shock resistance of the coating is a key factor for the application of such materials. Fig. 4 is the numbers of thermal shock tests of the coatings prepared under different concentration of Na2SiO3 at 800 °C. Increasing the concentration of Na2SiO3, the number of the thermal shock resistance of the coatings firstly increased, and then decreased. When the concentration of Na2SiO3 was 15 g/L, the thermal shock resistance of the coatings was the best among the samples prepared under other experimental conditions. The thermal shock resistance of the coatings was related to the structure and composition. Since the coatings had the similar composition, the thermal shock resistance of the coatings was mainly attributable to the structure of the coatings. The PEO ceramic coatings had the heat resistant property, which was improved with increase of the thickness of the coating. Increasing the concentration of Na3SiO3, the thickness of the coating increased which was helpful to improve the heat resistant property. Besides, the micro pores and micro cracks on the surface of the coating were liable for the release of the residual stress during the thermal shock tests. Both of them were useful to increase the thermal shock resistance of the coatings. On the other side, the increase of the thickness accompanied the increase of the growth stress of the coating during PEO process, and the micro pores and micro cracks would strengthen the oxidation of the substrate due to the diffusion of O2 into the interface between the coating and the substrate. Both
Table 1 The relative contents of Si and Ti on the surface of the coatings (atom%).
Si Ti
5 g/L
10 g/L
15 g/L
20 g/L
34.12 62.44
43.53 53.06
54.53 43.82
60.06 38.45
Fig. 4. The numbers of thermal shock tests of the coatings prepared under different concentration of Na2SiO3.
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
701
Fig. 5. The SEM of the coatings after high temperature oxidation tests. (a) Isothermal oxidation, and (b) cyclic oxidation.
of them would result in the decrease of thermal shock resistance reversely. Consequently, such conflicts determined that coating prepared under the 15 g/L presented the best thermal shock resistance.
3.5. High temperature oxidation tests of the coating samples at 500 °C According to the results of thermal shock tests, the ceramic coatings prepared at 15 g/L of Na2SiO3 were used to investigate the high temperature oxidation resistance at 500 °C through isothermal oxidation and cyclic oxidation tests. Accompany to those
Fig. 6. The XRD patterns of the coatings after high temperature oxidation tests. (a) Isothermal oxidation, and (b) cyclic oxidation.
high temperature oxidation tests, the color of the substrate and coatings changed greatly: the substrate turned light blue and deep purple gradually, while the coating turned from the gray to light yellow and deep brown. And the coating did not spall off the substrate. Figs. 5 and 6 are the surface morphologies and phase compositions of the coatings correspondingly after high temperature oxidation tests. It can be noted that the surface images of the coating changed less, but the composition changed greatly. The amount of TiO2 was increased and the diffraction peaks corresponding to the substrate was also intensified. This meant that substrate was oxidized in a certain degree and the density of the coating was reduced after 200 h of high temperature oxidation. Fig. 7 is the changing curves of the weight for the coating samples and substrate versus time under isothermal oxidation and cyclic oxidation tests of 500 °C, respectively. Both tests present the similar changing regularity. The weight of the substrate samples increased quickly at the beginning and then comparatively slowly with increasing oxidation time, whereas interestingly the weight of the coating samples decreased quickly and then increased slowly. For the isothermal oxidation, the mean weight gain of the substrate samples after 200 h was 0.1053 mg/cm2. Because of the weigh loss of the coating samples at the beginning, the weigh gain of the coating sample was calculated approximately through DW1 as shown in the Fig. 5, with the results of 0.0373 mg/cm2, which is nearly 1/3 that of the substrate sample. This meant that PEO technique can effectively decrease the weight gain of the isothermal oxidation of the Ti–6Al–4V at 500 °C. For the cyclic oxidation, the mean weight gain of the substrate samples after 200 h was 0.1004 mg/cm2, while the weigh gain of the coating samples after the same approximate calculation through DW2 was 0.0124 mg/
Fig. 7. Changing curves of the weight for the coating samples and the substrate versus time at 500 °C for 200 h. (a) Isothermal oxidation, and (b) cyclic oxidation.
702
Y. Xu et al. / Current Applied Physics 10 (2010) 698–702
cm2, which is only 12.4% that of the substrates. Therefore, the weight gains of the cyclic oxidation were reduced greatly, compared with the substrate. In terms of the above analyses, the ceramic coatings reduced the weight gains of Ti alloy at 500 °C, whether considering the isothermal oxidation or the cyclic oxidation, that is to say, the ceramic coatings greatly improved the high temperature oxidation resistance of Ti alloy and may be used as a kind of oxidation resistance coating of Ti alloy at 500 °C in practice. The decrease of the weight of the coating samples at the initial stage in our experiments may be related to the structure of the coating. In the coating there may be some unstable phases such as amorphous hydrated Si–O compounds or carbonates. Under the high temperature oxidation of 500 °C, the dehydration or decomposition reactions occurred, which may lead to the weight loss of the coating samples decreased at the beginning of the high temperature oxidation tests. After the weight loss reactions, the weight gains of the coating samples can be apparently observed because of the oxidation of the substrate in the experiments. 4. Conclusions Ceramic coatings on Ti alloy were prepared in silicate solution by plasma electrolytic oxidation. The structure, thermal shock resistance and high temperature oxidation resistance of the coatings were investigated and the following conclusions can be drawn: (1) The ceramic coatings were composed of rutile and anatase TiO2. Increasing the concentration of Na2SiO3, TiO2 content decreased gradually while the thickness of the coating increased. There were a large amount of micro pores and sintered particles on the surface of the coatings. Increasing concentration of Na2SiO3, the amount of micro pores decreased whereas the sintered particles increased and became larger and larger, and meantime, the Si content on the surface of the coating increased while the Ti content decreased gradually. (2) When the concentration of Na2SiO3 was 15 g/L, the thermal shock resistance of the coatings was the best among the samples prepared under other experimental conditions. The high temperature oxidation tests shows the surface morphologies of the coatings changed less while the phase composition changed greatly due to the oxidation of the substrate. The ceramic coatings reduced the weight gains of Ti alloy at 500 °C greatly, whether considering the isother-
mal oxidation or the cyclic oxidation and therefore, improved the high temperature oxidation resistance of Ti alloy at 500 °C.
Acknowledgement This work was financially supported by Special Foundation for New Teachers of Doctor course in Chinese Education Ministry (Grant No. 200802131065). References [1] J. Sun, J.S. Wu, B. Zhao, F. Wang, Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys, Mater. Sci. Eng. A329–A331 (2002) 713–717. [2] Y.G. Zhao, W. Zhou, Q.D. Qin, Y.H. Liang, Q.C. Jiang, Effects of preoxidation on high temperature oxidation behavior of Ti alloys, Spec. Cast. Nonferrous Alloys 24 (2004) 34–36 (in Chinese). [3] J.S. Xiao, G.D. Xu, Several ways to improve mechanical properties of hightemperature Ti-based alloys, Chin. J. Nonferrous Met. 7 (1997) 97–105. [4] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [5] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Characterization of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy, Surf. Coat. Technol. 130 (2000) 195–206. [6] Y.J. Guan, Y. Xia, Review on plasma electrolytic deposition, Adv. Mech. 34 (2004) 237 (in Chinese). [7] Z.P. Yao, Z.H. Jiang, X.H. Wu, X.T. Sun, Z.D. Wu, Effects of ceramic coating by micro-plasma oxidation on the corrosion resistance of Ti–6Al–4V alloy, Surf. Coat. Technol. 200 (2005) 2445–2450. [8] W.B. Xue, C. Wang, R.Y. Chen, Structure and properties characterization of ceramic coatings produced on Ti–6Al–4V alloy by microarc oxidation in aluminate solution, Mater. Lett. 52 (2002) 435–441. [9] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Microarc oxidation and spraying graphite duplex coating formed on titanium alloy for antifriction purpose, Appl. Surf. Sci. 246 (2005) 214–221. [10] Z.L. Tang, F.H. Wang, W.T. Wu, Effect of micro-arc oxidation treatment on oxidation resistance of TiAl alloy, Chin. J. Nonferrous Met. 9 (S1) (1999) 63–68 (in Chinese). [11] J.M. Hao, W.Q. Jie, H. Chen, Y.D. Ye, The effect of SiO32- on micro-arc oxidization ceramic layer of TiAl alloy, Rare Met. Mater. Eng. 34 (9) (2005) 1455–1459 (in Chinese). [12] H. Zhou, Z.T. Liu, Z.X. Li, J.H. Du, Microarc oxidation coating and hightemperature oxidation resistant property on Ti alloy, Rare Met. Mater. Eng. 11 (34) (2005) 1835–1837 (in Chinese). [13] Z.P. Yao, Z.H. Jiang, F.P. Wang, G.D. Hao, Oxidation behavior of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation, J. Mater. Process. Technol. 190 (2007) 117–122. [14] Z.P. Yao, Z.H. Jiang, X.T. Sun, S.G. Xin, Z.D. Wu, Influence of the frequency on the structure and corrosion resistance of ceramic coatings on Ti–6Al–4V alloy produced by micro-plasma oxidation, Mater. Chem. Phys. 92 (2005) 408–412. [15] X.T. Sun, Z.H. Jiang, Y.P. Li, F.P. Wang, Y.D. Lv, Effect of the oxidation time on properties of ceramic coatings produced on Ti–6Al–4V by micro-arc oxidation, J. Mater. Technol. Sci. 21 (2005) 281–285.