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High temperature oxidation behavior of TiO2 + ZrO2 composite ceramic coatings prepared by microarc oxidation on Ti6Al4V alloy
SCT-19892; No of Pages 7 Surface & Coatings Technology xxx (2014) xxx–xxx
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High temperature oxidation behavior of TiO2 + ZrO2 composite ceramic coatings prepared by microarc oxidation on Ti6Al4V alloy Chao Wang, Jianmin Hao ⁎, Yazhe Xing, Chaofei Guo, Hong Chen School of Materials Science and Engineering, Chang'an University, Xi'an 710061, China
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Article history: Received 1 August 2014 Accepted in revised form 12 November 2014 Available online xxxx Keywords: Microarc oxidation Ti6Al4V TiO2 + ZrO2 High temperature oxidation behavior
a b s t r a c t Microarc oxidation (MAO) was used to prepare TiO2 + ZrO2 composite coatings on Ti6Al4V alloy in Zr-containing electrolytes. The high temperature oxidation resistance and oxidation mechanism of MAO coatings were investigated and discussed. The composite ceramic coatings exhibit a porous structure with small pores (0.4–1 μm in diameter) on the inner wall of larger pores, and consist of m-ZrO2 and ZrTiO4. In the case of static oxidation at different temperatures, the high temperature oxidation resistance of Ti6Al4V is improved 2–10 times after MAO treatment. The coated samples oxidized at different temperatures present various oxidation behaviors. A porous TiO2 layer forms after exposing oxidation at 500 °C for 300 h, while a continuous TiO2 layer forms after oxidation at 600 °C for 75 h. However, when the coated samples oxidized at 700 °C and 800 °C, a multilayer structure of MAO coating/transitional layer/TiO2 layer/TiN layer forms on Ti6Al4V with the combined effect of interface migration of MAO coating and air diffusion inward, and the diffusion of substrate elements outward. Especially, it is found that a TiN layer with preferential growth towards (111) lattice planes forms in the multilayer structure in coated samples, which is much different from the oxidation behavior of Ti6Al4V at 700 °C and 800 °C. The presence of multilayer structure can provide an efficient diffusion barrier, which restrains the growth of TiO2 and TiN. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys have been widely used in aerospace and civil fields, due to their excellent comprehensive properties such as high specific strength, good corrosion resistance, non-magnetism and so on. However, titanium alloys are prone to be oxidized when serving at high temperature conditions. At present, many surface modification techniques, such as thermal spray [1], laser cladding [2], ion implantation [3], and double glow plasma surface alloying [4], were adopted to enhance the high temperature oxidation resistance. These methods can improve the thermal oxidation resistance of substrate to some extent. However, there are still some problems, such as poor adhesion, big thermal affected zone, complex processing and high cost, that cannot be ignored, which limit the application of above methods. Microarc oxidation (MAO) is an in-situ growth technique for preparing ceramic coatings on valve metals. MAO coatings own the advantages of high adhesion property, simple process, high thermal shock resistance, and environmental protection. The composition and microstructure can be controlled through designing the electrolytes and adjusting the electrical parameters. It is well known that the coatings used for high temperature protection should have a relatively high thermal ⁎ Corresponding author at: School of Materials Science and Engineering, Chang'an University, Chang'an road, Xi'an 710061, China. Tel./fax: +86 029 82334590. E-mail address: [email protected] (J. Hao).
stability to resist thermal shock and thermal oxidation. Hence, it is necessary to introduce some substances with high thermal stability into MAO coating. The MAO coatings prepared on titanium alloys in aluminate electrolytes mainly were composed of Al2TiO5 [5,6] and the investigation of MAO coatings on high temperature properties turned out to be thermal-protective. ZrO2 coatings have been applied for thermal barrier coatings in aerospace fields for its low thermal conductivity and high thermal stability. Therefore, it is of great importance to introduce ZrO2 to MAO coatings. Currently, there are usually two ways applied to obtaining these coatings. One is the addition of disperse ZrO2 particles with a size of nano to microlevel to the electrolyte. The other way is the use of soluble zirconate in the electrolyte. In the past few years, many works have been made to produce Al2O3 + ZrO2/Al [7–10], and MgO + ZrO2/Mg [11–13] composite coatings. The results showed that the introduction of ZrO2 improved the thermal shock resistance and thermal oxidation resistance [12]. Presently, an increasing works [14–21] of preparing TiO2 + ZrO2 composite coating on titanium alloys have been reported, some [14–17] of which concern the thermal oxidation behavior. Vasilyeva [14] studied the influence of annealing at different temperatures on the thermal stability of TiO2 + ZrO2 composite coating. The results showed that the composite coating exhibits relatively high thermal stability from 500 °C to 800 °C. Rudnev studied [17] the specific surface structure of ZrO2 + TiO2 and ZrO2 + CeOx + TiO2 coatings on titanium and whiskers were found on the surface of ZrO2 + TiO2 coatings before annealing, and perfectly edged crystals composed of TiO2 and
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ZrO2 oxides were found after annealing at 850 °C for 24 h in air. However, few works were conducted concerning the high temperature oxidation resistance and oxidation mechanism of TiO2 + ZrO2 composite coating. In this paper, Ti6Al4V was MAO-treated in zirconate electrolyte to obtain TiO2 + ZrO2 composite ceramic coating. The main work was focused on studying the oxidation behavior and mechanism of TiO2 + ZrO2 composite coatings at different temperatures. 2. Experiment 2.1. Coating preparation Ti6Al4V alloy was machined into the block samples with dimension of 15 mm × 15 mm × 4 mm. The surface of samples was grounded with 240# and 600#SiC abrasive papers, and then ultrasonically cleaned in acetone and distilled water, respectively. 20 g/L K2ZrF6 and 15 g/L (NaPO3)6 were selected as basic electrolytes. NH4HF2 (2–4 g/L) and sodium citrate (2 g/L) were used as additives. A home-made MAO operating unit (MAO-100D) was used to prepare ceramic coating. The samples were treated in a constant voltage control mode with 500 Hz pulse frequency, 10% duty ratio, 30 min treatment time and 425 V working voltage. The temperature of the electrolyte solution was maintained between 25 and 40 °C with the circulation cooling system. NH4HF2 with concentration varied in the range of 2 g/L–5 g/L was used to keep the pH of the electrolyte at 4–6. 2.2. High temperature oxidation tests High temperature oxidation tests were performed at 500 °C, 600 °C, 700 °C and 800 °C in a muffle furnace. The weight of the samples was measured by Sartorius electronic analytical balance with the accuracy of 10−4 g. The samples pre-dried at 50 °C were weighed together with alumina crucibles, which were pre-dried to a constant mass at 850 °C. The samples were heated at the pre-set temperature for certain hours and then cooled to room temperature, which was defined as a cycle. The samples were weighed after each cycle was finished. The duration of oxidation is 300 h at 500 °C and 75 h at the rest temperatures. 2.3. Coating characterization The phase composition of the coating was identified by an X-ray diffraction (XRD, Bruker D8 ADVANCE). The surface and cross-section morphologies of the coating were characterized by scanning electron microscopy (SEM, Hitachi S4800). The elemental distribution and content were analyzed by energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Surface and cross-sectional microstructures of the MAO coatings The surface morphology with different magnifications of MAO coating is shown in Fig. 1(a). The composite ceramic coatings exhibit a porous structure with small pores (0.4–1 μm in diameter) on the inner wall of larger pores. Several ceramic particles are distributed on the skeleton of MAO coating, as the arrows pointed. The formation of coating skeleton and ceramic particles is due to different discharge types, according to Hussein's research on MAO discharge behaviors [22]. As is shown in Fig. 1(b), the cross-section morphology of MAO coating exhibits a double-layer structure with an inner compact layer and an outer loose layer. The EDS analysis indicates that the loose layer (point A: at.%, Ti 9.13, Zr 21.22, O 68.84, Al 0.81) has more Zr from the electrolyte, while the compact layer (point B: at.%, Ti 19.13%, Zr 14.01, O 66.35, Al 0.51) has relatively equivalent amount of Ti and Zr both from the substrate and electrolyte. According to the study [21] on growth characteristics of MAO coating on Ti6Al4V in K2ZrF6–H3PO4 and NaAlO2–Na3PO4 solutions, the loose coatings were mainly structured by elements from
Fig. 1. SEM images of surface morphology (a) and cross-sectional structure (b) of MAO coating.
the electrolytes, while the coatings exhibiting dense inner layer and loose outer layer were structured by elements from both the substrate and electrolyte. In our present work, EDS analysis and phase composition (mainly composed of ZrTiO4) indicate that the compact layer is mainly structured by both Ti from the substrate and Zr from the electrolyte, which is consistent with the structure exhibited in Fig. 1b. Besides, the high pulse voltage produces high intensity of spark discharging and large amount of products accumulate rapidly to form coarse structure at the initial stage. With the increase of the coating thickness, the transportation resistance of the dissolved Ti through discharging channels increased [21]. Thus, the growth of coating towards the substrate predominates the growing process, which contributes to the compact structure in the late stage. After growing for 60 min, the coating obtains a final thickness of about 30 μm and bonds well with the substrate. 3.2. Phase composition Fig. 2 shows the XRD pattern of MAO coating formed on Ti6Al4V. The MAO coating mainly consists of monoclinic ZrO2(m-ZrO2) and ZrTiO4. ZrTiO4 is a composite oxide of TiO2 and ZrO2. The formation of ZrTiO4 can be presented as follows. First, ZrF26 − absorbs on the surface of anode under electric field and forms colloidal Zr(OH)4 with the reaction of OH−: ZrF62− + 4OH− → Zr(OH)4 + F− [8]. When the discharge sparks appear on the surface of anode, the absorbed Zr(OH)4 at the interface was drawn into the discharge channel.
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detected by XRD analysis, which indicated that more ZrO2 forms than TiO2 during the MAO reaction and extra ZrO2 was reserved to be monoclinic ZrO2 after forming ZrTiO4.
3.3. Oxidation kinetic
Fig. 2. XRD pattern of the MAO ceramic coating formed on Ti6Al4V.
Then a series of plasma chemistry, hydrothermal synthesis, thermolysis and solid state interactions took place in the channel filled with the components from both the substrate and electrolyte [23]. A decomposition reaction of the Zr(OH)4 occurs under the high pressure and high temperature conditions: Zr(OH)4 → ZrO2 + 2H2O. ZrO2 reacts with oxidized titanium to produce ZrTiO4: ZrO2 + TiO2 → ZrTiO4. In addition, TiO2 was not
Fig. 3 shows the oxidation kinetic curves of coated and uncoated samples after cyclic oxidation at different temperatures. From Fig. 3(a) and (b), the weight of coated sample losses at the beginning of cycles. According to Xu's work [24], the weight loss is due to the decomposition or dehydration of some unstable hydrated compounds. When the weight gain exceeds the loss factors, the weight change turns into a positive value. After several cycles, the oxidation of coated samples tends to be stable and the weight gain barely changes, while the weight gain of uncoated sample shows an increasing tendency. From Fig. 3(c) and (d), the oxidation kinetic curves present a similar changing regularity. After oxidation at 700 °C for 2 h, the weight gains of the coated and uncoated samples are 0.25 mg/cm2 and 0.26 mg/cm2 respectively. However, the weight gain of coated sample increases slowly in the next four cycles, whereas the weight gain of uncoated sample increases in a linear fashion. In the following cycles, the uncoated sample increases comparatively quickly and the coated sample increases at a very low speed. After oxidation at 700 °C for 75 h, the weight change of uncoated sample is 3.29 mg/cm2, which is 2.6 times as many as that of coated sample. After oxidation at 800 °C for 75 h, the weight change of uncoated sample is 17.2 mg/cm2, which is 3.5 times as many as that of coated sample.
Fig. 3. The oxidation kinetic curves of coated and uncoated samples oxidized at different temperatures: (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C.
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In order to study the kinetic law of oxidation, the weight gain data in Fig. 3 can be fitted by the following equation [25]: n
ΔM ¼ kn t;
ð1Þ
where ΔM is the weight gain per unit area (mg/cm2), n is the reaction index, kn is the oxidation rate constant (mgn/(cm2nh)) and t is the oxidation time(h). The deformation of Eq. (1) is established: nlnΔM ¼ lnkn þ lnt:
ð2Þ
The values of n and kn are automatically fitted by Origin software based on Eq. (2). Table 1 displays the values of n and kn. At a certain temperature, the kinetic model is parabolic if n = 2 and linear if n = 1. The values in Table 1 indicates that Ti6Al4V with MAO coatings oxidized at 500 °C, 600 °C and 700 °C presents a pseudo-parabolic law (n ≈ 2), while that at 800 °C is in accordance with the parabolic law (n = 2.01). The kinetic model of Ti6Al4V at 500 °C and 600 °C is pseudo-parabolic law (n ≈ 2). With the increase of temperature, the oxidized products turn to be brittle and tend to peel off from the substrate. Therefore, the kinetic model of Ti6Al4V at 700 °C and 800 °C complies with linear law, which is consistent with the previous oxidation kinetic study of Ti6Al4V [26]. Besides, kn of Ti6Al4V in Table 1 at 700 °C and 800 °C is very close to the reported values showed in lgk ~ 1/T × 104 [26]. The value of the oxidation rate constant kn would reflect the oxidation rate. Compared with oxidation reaction rate constant of Ti6Al4V with MAO coatings, kn of Ti6Al4V is 2–10 times higher, which indicates that ZrO 2 + TiO 2 coating formed on Ti6Al4V can greatly improve the high temperature oxidation resistance. The excellent performance at high temperatures is attributed to the microstructure and phase composition of MAO coating. First, the phase composition determines the MAO coating to be a protective coating. As discussed before, MAO coating consists of m-ZrO 2 and ZrTiO 4 . Both phases have good thermal stability and low coefficient of thermal expansion at present operating temperature, which avoids decomposing or peeling off at high temperature. Moreover, the interface between the MAO coating and substrate is a thin layer of TiO2 . Ti and TiO 2 have a similar coefficient of thermal expansion (8.6 × 10 − 6 K − 1 and 8.3 × 10 − 6 K − 1 , respectively), which avoids the interfacial debonding or delamination under thermal shock condition. As a result, the coating can efficiently protect Ti6Al4V substrate from high temperature oxidation. Second, the microstructure of MAO coating reduces the oxidation rate. As shown in Fig. 1(a) and (b), the micropore structure and compact layer can be an efficient barrier for air diffusion and slow down the speed of the air diffusion inward. Besides, the protection of thermal barrier results from the low thermal conductivity of ZrO2 contributes to the high temperature oxidation resistance. To conclude, ZrO2 + TiO2 composite ceramic coating can be used for thermal protective coating of titanium alloys. 3.4. High temperature oxidation behavior and mechanism Fig. 4 exhibits the surface and cross-section morphologies of the coated samples oxidized at different temperatures. After exposing oxidation Table 1 Values of n and kn of Ti6Al4V with and without MAO coatings at different temperatures. Temperature/°C 500 °C 600 °C 700 °C 800 °C
Sample
kn/mgn/cm2n·h2
MAO Ti6Al4V MAO Ti6Al4V MAO Ti6Al4V MAO Ti6Al4V
0.98 3.11 12.9 130 300 1000 3280 6700
× × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4
n
t/h
1.64 1.81 1.83 1.53 2.25 1.41 2.01 1.37
300 300 75 75 75 75 75 75
h h h h h h h h
at 500 °C for 300 h, only a few microcracks occur on the surface of MAO coating and the coating/substrate interface becomes a little bit porous compared with that before oxidation, as shown in Fig. 4(a) and (b). The EDS analysis of point A (at.%, Ti 20.42, O 75.17, Al 3.2, V 1.21) marked in Fig. 5(b) indicates an oxide layer (1 μm thick) formed at the interface during oxidation at 500 °C. The thickness of the oxide layer is much thinner than that reported in the study [27] on thermal oxidation of the Ti6Al4V oxidized at 500 °C. The MAO sample oxidized at 600 °C for 75 h is shown in Fig. 4(c) and (d). Several microcracks occur on the coating surface. The EDS analysis of point B (at.%,Ti 25.44, O 70.66, Al 3.06, V 0.84) marked in Fig. 4(d) indicates that an oxide layer (2.5 μm thick) forms at the MAO coating/substrate interface. According to the study [28] on Ti6Al4V oxidized at 600 °C for 4 h, the thickness of oxide scale is 1.2 μm. This indicates that MAO coating efficiently prevents the substrate from oxidation. From Fig. 4(e), more microcracks occur on the surface of MAO coating than the sample oxidized at 600 °C. Fig. 4(f) exhibits a multilayer structure oxidized at 700 °C, which is similar with that oxidized at 800 °C, as shown in Fig. 4(h). From Fig. 4(g), the surface morphology of the MAO coating oxidized at 800 °C for 75 h undergoes obvious change. The surface structure becomes more compact and the width of cracks increased markedly. The coated sample oxidized at 500 °C shows a simple oxidation mechanism. The air diffuses inward through the residual discharge channel to react with titanium. Because of the compact MAO coating and relatively low temperature, oxygen is not easy to pass through the MAO coating and react with titanium. Only a discontinuous oxide layer (1 μm thick) is formed after oxidation at 500 °C for 300 h. Compared with oxidation at 500 °C, more oxygen can reach the surface of substrate at 600 °C. TiO2 forms prior to Al2O3, which leads to an Alrich zone between TiO2 and substrate. Then Al atoms diffuse outward and form Al2O3. As is shown in Fig. 5(a), the content of aluminum at MAO coating/TiO2 interface is higher, which indicates that the interface between TiO2 and MAO coating is an Al-rich zone. However, due to the limited content of Al, a continuous layer to protect oxygen from invasion cannot be formed. Thus, oxygen diffuses inward through the mixture oxides and forms an oxygen-dissolved layer. As a result, the oxide formed after oxidation shows excess oxygen. The marked point F (at.%, Ti 21.67, O 65.68, Al 0.98, Zr 11.66) in Fig. 4(f) is MAO coating. Point E in Fig. 4(f) lies in the interface between MAO coating and TiO2. The content (at.%, Ti 25.41, O 65.94, Al 3.71, Zr 4.00, V 0.95) of point E shows that it is a transitional layer (which is probably composed of oxides of aluminum, titanium and zirconium) from TiO2 to MAO coating. The EDS analysis of the marked point D (at.%, Ti 31.06, O 68.94) in Fig. 4(f) indicates that the layer is mainly composed of TiO2. The EDS analysis of point C (at.%, Ti 57.16, N 41.90, Al 0.93) in Fig. 4(f) indicates that a titanium nitride layer forms in the coated sample oxidized at 700 °C, which is much different from Ti6Al4V oxidized at the same temperature. According to the study [29] on high temperature oxidation of TiAl alloy, there is a “nitrogen effect” during oxidation. The research of Rakowski and Dettenwanger [29] suggested that the reason why the continuous layer of Al2O3 could not be formed on Ti–Al two binary alloy at 800 °C–900 °C in air was due to the formation of TiN at the beginning of the oxidation. Fig. 6 shows the schematic diagram by Rakowski and Dettenwanger showing the development of the oxidation morphology on TiAl in air at the temperature range of 800 °C– 900 °C. Therefore, in the present work, TiN distributes in the outer layer of the oxide scale in the early stage of the oxidation and is then oxidized into TiO2 and N atoms. N diffuses inward under a high concentration difference condition. This indicates that TiN can be formed in the inner layer during high temperature oxidation. However, there is no literature reported regarding the formation of TiN on MAO-treated Ti6Al4V. In order to verify the presence of TiN, MAO coating and TiO2 layer were stripped from coated sample oxidized at 700 °C and the reserved was identified by XRD. Due to the nonuniformity of the coating thickness and very limited thickness of TiN layer, it is hard to strip the precise
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Fig. 4. Surface and cross-sectional images of coated samples oxidized at different temperatures: (a)(b) 500 °C, (c)(d) 600 °C, (e)(f) 700 °C, (g)(h) 800 °C.
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Fig. 6. Rakowski et al., schematic mechanism of nitrogening effect for binary TiAl alloys at 800 °C–900 °C in air.
Fig. 5. EDS spectra of the coated samples oxidized at (a) 600 °C (b) 800 °C.
depth. After removing the surface, parts of the samples are golden yellow, which is similar with the color of TiN. The XRD pattern in Fig. 7 indicates that TiN forms in the inner layer of the coating. The growth of TiN presents a preferential orientation towards (111) lattice planes. According to the study [30] on preferential growth of TiN, the texture evolved from random orientation to a pure (111) lattice planes with increasing the thickness of TiN. Similar study [31] indicated that preferential TiN (111) films were deposited on the substrate of Si at low temperature. Thus, in our present work, TiN of 3 μm in thickness forms after oxidation at 700 °C for 75 h. However, titanium oxides and Ti substrate are inevitably detected as well. Al2O3 is not found since the Al-rich zone (as Fig. 5(b) shows) lies in the outer layer of TiO2, which has been stripped. The cross-section morphology oxidized at 700 °C and 800 °C exhibits a similar structure. Fig. 5(b) shows the line scanning spectra of coated sample oxidized at 800 °C. The content of titanium decreases sharply from TiN layer to TiO2 layer and then reduces gradually outward while the content of aluminum is very low in TiN layer and TiO2 layer and increases markedly in the transitional layer. The formation of Al-rich zone is similar with that at 600 °C, as discussed above. The content of zirconium decreases inward in the MAO coating and then increases in the transitional zone. The change of Zr content indicates that Zr in MAO coating diffuses to this transitional zone, which causes the interface migration of MAO coating inward during oxidation at 700 °C and 800 °C. From Fig. 4(f) and(h), the thicknesses of TiN layer formed at 700 °C and 800 °C are similar, while the thickness of TiO2 layer formed at 800 °C is much thicker than that at 700 °C. The diffusion of oxygen at 800 °C
is much faster than that at 700 °C. However, limited amounts of TiN formed at the early stage of oxidation, which is discontinuous in the inner layer. Thus, thicker TiO2 forms at 800 °C before the formation of the multilayer structure. When the continuous TiN forms, the multilayer structure acts as an efficient diffusion barrier. As a result, the substrate is well protected from O2 and N2 and the interface of TiO2 and TiN did not migrate apparently inward. Afterwards, the growth of TiN and TiO2 decreased dramatically and the oxidation tends to be stable, which is in good agreement with the oxidation kinetic curves. Therefore, the final thicknesses of TiN at 700 °C and 800 °C are approximately the same, and TiO2 layer at 800 °C is much thicker than that at 700 °C. To summarize the discussions above, the coated samples present various oxidation mechanisms at different temperatures. A doublelayer structure forms due to the oxidation of substrate at 500 °C and 600 °C, while a multilayer structure formed with the combined
Fig. 7. XRD pattern of the inner layer on coated sample oxidized at 700 °C.
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effect of interface migration of MAO coating and air diffusion inward, and the diffusion of the substrate elements outward at 700 °C and 800 °C. 4. Conclusion TiO 2 + ZrO2 composite ceramic coatings were prepared in Zrcontaining electrolytes by microarc oxidation on Ti6Al4V alloy. The microstructure, phase composition, oxidation behavior and mechanism at 500 °C, 600 °C, 700 °C and 800 °C were investigated and discussed. And following conclusions can be drawn: 1 The oxidation kinetic curves at different temperature indicate that the high temperature oxidation resistance of Ti6Al4V is improved 2–10 times after MAO treatment. 2 The coated samples present various oxidation behaviors oxidized at different temperatures. A porous TiO2 layer of 1 μm in thickness forms after oxidation at 500 °C for 300 h, while a continuous TiO2 layer of 2.5 μm in thickness forms after oxidation at 600 °C for 75 h. However, after oxidation at 700 °C and 800 °C oxidized for 75 h, a multilayer structure of MAO coating/transitional layer (a mixture oxides of aluminum, titanium and zirconium)/TiO2 layer/TiN layer forms with the combined effect of interface migration of MAO coating and air diffusion inward, and the diffusion of substrate elements outward. 3 It is found that a TiN layer with preferential growth towards (111) lattice planes formed in the multilayer structure in the coated samples, which is much different from the oxidation behavior of Ti6Al4V at 700 °C and 800 °C. The presence of multilayer structure can provide an efficient diffusion barrier. As a result, the growth of TiO2 and TiN dramatically decreases and the oxidation tends to be stable, which is in good agreement with the oxidation kinetic curves.
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Acknowledgments This work was supported by the National Natural Science Foundation of China under grant no. 51301022.
Please cite this article as: C. Wang, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.11.031
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High temperature oxidation behavior of TiO2 + ZrO2 composite ceramic coatings prepared by microarc oxidation on Ti6Al4V alloy Chao Wang, Jianmin Hao ⁎, Yazhe Xing, Chaofei Guo, Hong Chen School of Materials Science and Engineering, Chang'an University, Xi'an 710061, China
a r t i c l e
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Article history: Received 1 August 2014 Accepted in revised form 12 November 2014 Available online xxxx Keywords: Microarc oxidation Ti6Al4V TiO2 + ZrO2 High temperature oxidation behavior
a b s t r a c t Microarc oxidation (MAO) was used to prepare TiO2 + ZrO2 composite coatings on Ti6Al4V alloy in Zr-containing electrolytes. The high temperature oxidation resistance and oxidation mechanism of MAO coatings were investigated and discussed. The composite ceramic coatings exhibit a porous structure with small pores (0.4–1 μm in diameter) on the inner wall of larger pores, and consist of m-ZrO2 and ZrTiO4. In the case of static oxidation at different temperatures, the high temperature oxidation resistance of Ti6Al4V is improved 2–10 times after MAO treatment. The coated samples oxidized at different temperatures present various oxidation behaviors. A porous TiO2 layer forms after exposing oxidation at 500 °C for 300 h, while a continuous TiO2 layer forms after oxidation at 600 °C for 75 h. However, when the coated samples oxidized at 700 °C and 800 °C, a multilayer structure of MAO coating/transitional layer/TiO2 layer/TiN layer forms on Ti6Al4V with the combined effect of interface migration of MAO coating and air diffusion inward, and the diffusion of substrate elements outward. Especially, it is found that a TiN layer with preferential growth towards (111) lattice planes forms in the multilayer structure in coated samples, which is much different from the oxidation behavior of Ti6Al4V at 700 °C and 800 °C. The presence of multilayer structure can provide an efficient diffusion barrier, which restrains the growth of TiO2 and TiN. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys have been widely used in aerospace and civil fields, due to their excellent comprehensive properties such as high specific strength, good corrosion resistance, non-magnetism and so on. However, titanium alloys are prone to be oxidized when serving at high temperature conditions. At present, many surface modification techniques, such as thermal spray [1], laser cladding [2], ion implantation [3], and double glow plasma surface alloying [4], were adopted to enhance the high temperature oxidation resistance. These methods can improve the thermal oxidation resistance of substrate to some extent. However, there are still some problems, such as poor adhesion, big thermal affected zone, complex processing and high cost, that cannot be ignored, which limit the application of above methods. Microarc oxidation (MAO) is an in-situ growth technique for preparing ceramic coatings on valve metals. MAO coatings own the advantages of high adhesion property, simple process, high thermal shock resistance, and environmental protection. The composition and microstructure can be controlled through designing the electrolytes and adjusting the electrical parameters. It is well known that the coatings used for high temperature protection should have a relatively high thermal ⁎ Corresponding author at: School of Materials Science and Engineering, Chang'an University, Chang'an road, Xi'an 710061, China. Tel./fax: +86 029 82334590. E-mail address: [email protected] (J. Hao).
stability to resist thermal shock and thermal oxidation. Hence, it is necessary to introduce some substances with high thermal stability into MAO coating. The MAO coatings prepared on titanium alloys in aluminate electrolytes mainly were composed of Al2TiO5 [5,6] and the investigation of MAO coatings on high temperature properties turned out to be thermal-protective. ZrO2 coatings have been applied for thermal barrier coatings in aerospace fields for its low thermal conductivity and high thermal stability. Therefore, it is of great importance to introduce ZrO2 to MAO coatings. Currently, there are usually two ways applied to obtaining these coatings. One is the addition of disperse ZrO2 particles with a size of nano to microlevel to the electrolyte. The other way is the use of soluble zirconate in the electrolyte. In the past few years, many works have been made to produce Al2O3 + ZrO2/Al [7–10], and MgO + ZrO2/Mg [11–13] composite coatings. The results showed that the introduction of ZrO2 improved the thermal shock resistance and thermal oxidation resistance [12]. Presently, an increasing works [14–21] of preparing TiO2 + ZrO2 composite coating on titanium alloys have been reported, some [14–17] of which concern the thermal oxidation behavior. Vasilyeva [14] studied the influence of annealing at different temperatures on the thermal stability of TiO2 + ZrO2 composite coating. The results showed that the composite coating exhibits relatively high thermal stability from 500 °C to 800 °C. Rudnev studied [17] the specific surface structure of ZrO2 + TiO2 and ZrO2 + CeOx + TiO2 coatings on titanium and whiskers were found on the surface of ZrO2 + TiO2 coatings before annealing, and perfectly edged crystals composed of TiO2 and
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ZrO2 oxides were found after annealing at 850 °C for 24 h in air. However, few works were conducted concerning the high temperature oxidation resistance and oxidation mechanism of TiO2 + ZrO2 composite coating. In this paper, Ti6Al4V was MAO-treated in zirconate electrolyte to obtain TiO2 + ZrO2 composite ceramic coating. The main work was focused on studying the oxidation behavior and mechanism of TiO2 + ZrO2 composite coatings at different temperatures. 2. Experiment 2.1. Coating preparation Ti6Al4V alloy was machined into the block samples with dimension of 15 mm × 15 mm × 4 mm. The surface of samples was grounded with 240# and 600#SiC abrasive papers, and then ultrasonically cleaned in acetone and distilled water, respectively. 20 g/L K2ZrF6 and 15 g/L (NaPO3)6 were selected as basic electrolytes. NH4HF2 (2–4 g/L) and sodium citrate (2 g/L) were used as additives. A home-made MAO operating unit (MAO-100D) was used to prepare ceramic coating. The samples were treated in a constant voltage control mode with 500 Hz pulse frequency, 10% duty ratio, 30 min treatment time and 425 V working voltage. The temperature of the electrolyte solution was maintained between 25 and 40 °C with the circulation cooling system. NH4HF2 with concentration varied in the range of 2 g/L–5 g/L was used to keep the pH of the electrolyte at 4–6. 2.2. High temperature oxidation tests High temperature oxidation tests were performed at 500 °C, 600 °C, 700 °C and 800 °C in a muffle furnace. The weight of the samples was measured by Sartorius electronic analytical balance with the accuracy of 10−4 g. The samples pre-dried at 50 °C were weighed together with alumina crucibles, which were pre-dried to a constant mass at 850 °C. The samples were heated at the pre-set temperature for certain hours and then cooled to room temperature, which was defined as a cycle. The samples were weighed after each cycle was finished. The duration of oxidation is 300 h at 500 °C and 75 h at the rest temperatures. 2.3. Coating characterization The phase composition of the coating was identified by an X-ray diffraction (XRD, Bruker D8 ADVANCE). The surface and cross-section morphologies of the coating were characterized by scanning electron microscopy (SEM, Hitachi S4800). The elemental distribution and content were analyzed by energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Surface and cross-sectional microstructures of the MAO coatings The surface morphology with different magnifications of MAO coating is shown in Fig. 1(a). The composite ceramic coatings exhibit a porous structure with small pores (0.4–1 μm in diameter) on the inner wall of larger pores. Several ceramic particles are distributed on the skeleton of MAO coating, as the arrows pointed. The formation of coating skeleton and ceramic particles is due to different discharge types, according to Hussein's research on MAO discharge behaviors [22]. As is shown in Fig. 1(b), the cross-section morphology of MAO coating exhibits a double-layer structure with an inner compact layer and an outer loose layer. The EDS analysis indicates that the loose layer (point A: at.%, Ti 9.13, Zr 21.22, O 68.84, Al 0.81) has more Zr from the electrolyte, while the compact layer (point B: at.%, Ti 19.13%, Zr 14.01, O 66.35, Al 0.51) has relatively equivalent amount of Ti and Zr both from the substrate and electrolyte. According to the study [21] on growth characteristics of MAO coating on Ti6Al4V in K2ZrF6–H3PO4 and NaAlO2–Na3PO4 solutions, the loose coatings were mainly structured by elements from
Fig. 1. SEM images of surface morphology (a) and cross-sectional structure (b) of MAO coating.
the electrolytes, while the coatings exhibiting dense inner layer and loose outer layer were structured by elements from both the substrate and electrolyte. In our present work, EDS analysis and phase composition (mainly composed of ZrTiO4) indicate that the compact layer is mainly structured by both Ti from the substrate and Zr from the electrolyte, which is consistent with the structure exhibited in Fig. 1b. Besides, the high pulse voltage produces high intensity of spark discharging and large amount of products accumulate rapidly to form coarse structure at the initial stage. With the increase of the coating thickness, the transportation resistance of the dissolved Ti through discharging channels increased [21]. Thus, the growth of coating towards the substrate predominates the growing process, which contributes to the compact structure in the late stage. After growing for 60 min, the coating obtains a final thickness of about 30 μm and bonds well with the substrate. 3.2. Phase composition Fig. 2 shows the XRD pattern of MAO coating formed on Ti6Al4V. The MAO coating mainly consists of monoclinic ZrO2(m-ZrO2) and ZrTiO4. ZrTiO4 is a composite oxide of TiO2 and ZrO2. The formation of ZrTiO4 can be presented as follows. First, ZrF26 − absorbs on the surface of anode under electric field and forms colloidal Zr(OH)4 with the reaction of OH−: ZrF62− + 4OH− → Zr(OH)4 + F− [8]. When the discharge sparks appear on the surface of anode, the absorbed Zr(OH)4 at the interface was drawn into the discharge channel.
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detected by XRD analysis, which indicated that more ZrO2 forms than TiO2 during the MAO reaction and extra ZrO2 was reserved to be monoclinic ZrO2 after forming ZrTiO4.
3.3. Oxidation kinetic
Fig. 2. XRD pattern of the MAO ceramic coating formed on Ti6Al4V.
Then a series of plasma chemistry, hydrothermal synthesis, thermolysis and solid state interactions took place in the channel filled with the components from both the substrate and electrolyte [23]. A decomposition reaction of the Zr(OH)4 occurs under the high pressure and high temperature conditions: Zr(OH)4 → ZrO2 + 2H2O. ZrO2 reacts with oxidized titanium to produce ZrTiO4: ZrO2 + TiO2 → ZrTiO4. In addition, TiO2 was not
Fig. 3 shows the oxidation kinetic curves of coated and uncoated samples after cyclic oxidation at different temperatures. From Fig. 3(a) and (b), the weight of coated sample losses at the beginning of cycles. According to Xu's work [24], the weight loss is due to the decomposition or dehydration of some unstable hydrated compounds. When the weight gain exceeds the loss factors, the weight change turns into a positive value. After several cycles, the oxidation of coated samples tends to be stable and the weight gain barely changes, while the weight gain of uncoated sample shows an increasing tendency. From Fig. 3(c) and (d), the oxidation kinetic curves present a similar changing regularity. After oxidation at 700 °C for 2 h, the weight gains of the coated and uncoated samples are 0.25 mg/cm2 and 0.26 mg/cm2 respectively. However, the weight gain of coated sample increases slowly in the next four cycles, whereas the weight gain of uncoated sample increases in a linear fashion. In the following cycles, the uncoated sample increases comparatively quickly and the coated sample increases at a very low speed. After oxidation at 700 °C for 75 h, the weight change of uncoated sample is 3.29 mg/cm2, which is 2.6 times as many as that of coated sample. After oxidation at 800 °C for 75 h, the weight change of uncoated sample is 17.2 mg/cm2, which is 3.5 times as many as that of coated sample.
Fig. 3. The oxidation kinetic curves of coated and uncoated samples oxidized at different temperatures: (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C.
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In order to study the kinetic law of oxidation, the weight gain data in Fig. 3 can be fitted by the following equation [25]: n
ΔM ¼ kn t;
ð1Þ
where ΔM is the weight gain per unit area (mg/cm2), n is the reaction index, kn is the oxidation rate constant (mgn/(cm2nh)) and t is the oxidation time(h). The deformation of Eq. (1) is established: nlnΔM ¼ lnkn þ lnt:
ð2Þ
The values of n and kn are automatically fitted by Origin software based on Eq. (2). Table 1 displays the values of n and kn. At a certain temperature, the kinetic model is parabolic if n = 2 and linear if n = 1. The values in Table 1 indicates that Ti6Al4V with MAO coatings oxidized at 500 °C, 600 °C and 700 °C presents a pseudo-parabolic law (n ≈ 2), while that at 800 °C is in accordance with the parabolic law (n = 2.01). The kinetic model of Ti6Al4V at 500 °C and 600 °C is pseudo-parabolic law (n ≈ 2). With the increase of temperature, the oxidized products turn to be brittle and tend to peel off from the substrate. Therefore, the kinetic model of Ti6Al4V at 700 °C and 800 °C complies with linear law, which is consistent with the previous oxidation kinetic study of Ti6Al4V [26]. Besides, kn of Ti6Al4V in Table 1 at 700 °C and 800 °C is very close to the reported values showed in lgk ~ 1/T × 104 [26]. The value of the oxidation rate constant kn would reflect the oxidation rate. Compared with oxidation reaction rate constant of Ti6Al4V with MAO coatings, kn of Ti6Al4V is 2–10 times higher, which indicates that ZrO 2 + TiO 2 coating formed on Ti6Al4V can greatly improve the high temperature oxidation resistance. The excellent performance at high temperatures is attributed to the microstructure and phase composition of MAO coating. First, the phase composition determines the MAO coating to be a protective coating. As discussed before, MAO coating consists of m-ZrO 2 and ZrTiO 4 . Both phases have good thermal stability and low coefficient of thermal expansion at present operating temperature, which avoids decomposing or peeling off at high temperature. Moreover, the interface between the MAO coating and substrate is a thin layer of TiO2 . Ti and TiO 2 have a similar coefficient of thermal expansion (8.6 × 10 − 6 K − 1 and 8.3 × 10 − 6 K − 1 , respectively), which avoids the interfacial debonding or delamination under thermal shock condition. As a result, the coating can efficiently protect Ti6Al4V substrate from high temperature oxidation. Second, the microstructure of MAO coating reduces the oxidation rate. As shown in Fig. 1(a) and (b), the micropore structure and compact layer can be an efficient barrier for air diffusion and slow down the speed of the air diffusion inward. Besides, the protection of thermal barrier results from the low thermal conductivity of ZrO2 contributes to the high temperature oxidation resistance. To conclude, ZrO2 + TiO2 composite ceramic coating can be used for thermal protective coating of titanium alloys. 3.4. High temperature oxidation behavior and mechanism Fig. 4 exhibits the surface and cross-section morphologies of the coated samples oxidized at different temperatures. After exposing oxidation Table 1 Values of n and kn of Ti6Al4V with and without MAO coatings at different temperatures. Temperature/°C 500 °C 600 °C 700 °C 800 °C
Sample
kn/mgn/cm2n·h2
MAO Ti6Al4V MAO Ti6Al4V MAO Ti6Al4V MAO Ti6Al4V
0.98 3.11 12.9 130 300 1000 3280 6700
× × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4
n
t/h
1.64 1.81 1.83 1.53 2.25 1.41 2.01 1.37
300 300 75 75 75 75 75 75
h h h h h h h h
at 500 °C for 300 h, only a few microcracks occur on the surface of MAO coating and the coating/substrate interface becomes a little bit porous compared with that before oxidation, as shown in Fig. 4(a) and (b). The EDS analysis of point A (at.%, Ti 20.42, O 75.17, Al 3.2, V 1.21) marked in Fig. 5(b) indicates an oxide layer (1 μm thick) formed at the interface during oxidation at 500 °C. The thickness of the oxide layer is much thinner than that reported in the study [27] on thermal oxidation of the Ti6Al4V oxidized at 500 °C. The MAO sample oxidized at 600 °C for 75 h is shown in Fig. 4(c) and (d). Several microcracks occur on the coating surface. The EDS analysis of point B (at.%,Ti 25.44, O 70.66, Al 3.06, V 0.84) marked in Fig. 4(d) indicates that an oxide layer (2.5 μm thick) forms at the MAO coating/substrate interface. According to the study [28] on Ti6Al4V oxidized at 600 °C for 4 h, the thickness of oxide scale is 1.2 μm. This indicates that MAO coating efficiently prevents the substrate from oxidation. From Fig. 4(e), more microcracks occur on the surface of MAO coating than the sample oxidized at 600 °C. Fig. 4(f) exhibits a multilayer structure oxidized at 700 °C, which is similar with that oxidized at 800 °C, as shown in Fig. 4(h). From Fig. 4(g), the surface morphology of the MAO coating oxidized at 800 °C for 75 h undergoes obvious change. The surface structure becomes more compact and the width of cracks increased markedly. The coated sample oxidized at 500 °C shows a simple oxidation mechanism. The air diffuses inward through the residual discharge channel to react with titanium. Because of the compact MAO coating and relatively low temperature, oxygen is not easy to pass through the MAO coating and react with titanium. Only a discontinuous oxide layer (1 μm thick) is formed after oxidation at 500 °C for 300 h. Compared with oxidation at 500 °C, more oxygen can reach the surface of substrate at 600 °C. TiO2 forms prior to Al2O3, which leads to an Alrich zone between TiO2 and substrate. Then Al atoms diffuse outward and form Al2O3. As is shown in Fig. 5(a), the content of aluminum at MAO coating/TiO2 interface is higher, which indicates that the interface between TiO2 and MAO coating is an Al-rich zone. However, due to the limited content of Al, a continuous layer to protect oxygen from invasion cannot be formed. Thus, oxygen diffuses inward through the mixture oxides and forms an oxygen-dissolved layer. As a result, the oxide formed after oxidation shows excess oxygen. The marked point F (at.%, Ti 21.67, O 65.68, Al 0.98, Zr 11.66) in Fig. 4(f) is MAO coating. Point E in Fig. 4(f) lies in the interface between MAO coating and TiO2. The content (at.%, Ti 25.41, O 65.94, Al 3.71, Zr 4.00, V 0.95) of point E shows that it is a transitional layer (which is probably composed of oxides of aluminum, titanium and zirconium) from TiO2 to MAO coating. The EDS analysis of the marked point D (at.%, Ti 31.06, O 68.94) in Fig. 4(f) indicates that the layer is mainly composed of TiO2. The EDS analysis of point C (at.%, Ti 57.16, N 41.90, Al 0.93) in Fig. 4(f) indicates that a titanium nitride layer forms in the coated sample oxidized at 700 °C, which is much different from Ti6Al4V oxidized at the same temperature. According to the study [29] on high temperature oxidation of TiAl alloy, there is a “nitrogen effect” during oxidation. The research of Rakowski and Dettenwanger [29] suggested that the reason why the continuous layer of Al2O3 could not be formed on Ti–Al two binary alloy at 800 °C–900 °C in air was due to the formation of TiN at the beginning of the oxidation. Fig. 6 shows the schematic diagram by Rakowski and Dettenwanger showing the development of the oxidation morphology on TiAl in air at the temperature range of 800 °C– 900 °C. Therefore, in the present work, TiN distributes in the outer layer of the oxide scale in the early stage of the oxidation and is then oxidized into TiO2 and N atoms. N diffuses inward under a high concentration difference condition. This indicates that TiN can be formed in the inner layer during high temperature oxidation. However, there is no literature reported regarding the formation of TiN on MAO-treated Ti6Al4V. In order to verify the presence of TiN, MAO coating and TiO2 layer were stripped from coated sample oxidized at 700 °C and the reserved was identified by XRD. Due to the nonuniformity of the coating thickness and very limited thickness of TiN layer, it is hard to strip the precise
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Fig. 4. Surface and cross-sectional images of coated samples oxidized at different temperatures: (a)(b) 500 °C, (c)(d) 600 °C, (e)(f) 700 °C, (g)(h) 800 °C.
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Fig. 6. Rakowski et al., schematic mechanism of nitrogening effect for binary TiAl alloys at 800 °C–900 °C in air.
Fig. 5. EDS spectra of the coated samples oxidized at (a) 600 °C (b) 800 °C.
depth. After removing the surface, parts of the samples are golden yellow, which is similar with the color of TiN. The XRD pattern in Fig. 7 indicates that TiN forms in the inner layer of the coating. The growth of TiN presents a preferential orientation towards (111) lattice planes. According to the study [30] on preferential growth of TiN, the texture evolved from random orientation to a pure (111) lattice planes with increasing the thickness of TiN. Similar study [31] indicated that preferential TiN (111) films were deposited on the substrate of Si at low temperature. Thus, in our present work, TiN of 3 μm in thickness forms after oxidation at 700 °C for 75 h. However, titanium oxides and Ti substrate are inevitably detected as well. Al2O3 is not found since the Al-rich zone (as Fig. 5(b) shows) lies in the outer layer of TiO2, which has been stripped. The cross-section morphology oxidized at 700 °C and 800 °C exhibits a similar structure. Fig. 5(b) shows the line scanning spectra of coated sample oxidized at 800 °C. The content of titanium decreases sharply from TiN layer to TiO2 layer and then reduces gradually outward while the content of aluminum is very low in TiN layer and TiO2 layer and increases markedly in the transitional layer. The formation of Al-rich zone is similar with that at 600 °C, as discussed above. The content of zirconium decreases inward in the MAO coating and then increases in the transitional zone. The change of Zr content indicates that Zr in MAO coating diffuses to this transitional zone, which causes the interface migration of MAO coating inward during oxidation at 700 °C and 800 °C. From Fig. 4(f) and(h), the thicknesses of TiN layer formed at 700 °C and 800 °C are similar, while the thickness of TiO2 layer formed at 800 °C is much thicker than that at 700 °C. The diffusion of oxygen at 800 °C
is much faster than that at 700 °C. However, limited amounts of TiN formed at the early stage of oxidation, which is discontinuous in the inner layer. Thus, thicker TiO2 forms at 800 °C before the formation of the multilayer structure. When the continuous TiN forms, the multilayer structure acts as an efficient diffusion barrier. As a result, the substrate is well protected from O2 and N2 and the interface of TiO2 and TiN did not migrate apparently inward. Afterwards, the growth of TiN and TiO2 decreased dramatically and the oxidation tends to be stable, which is in good agreement with the oxidation kinetic curves. Therefore, the final thicknesses of TiN at 700 °C and 800 °C are approximately the same, and TiO2 layer at 800 °C is much thicker than that at 700 °C. To summarize the discussions above, the coated samples present various oxidation mechanisms at different temperatures. A doublelayer structure forms due to the oxidation of substrate at 500 °C and 600 °C, while a multilayer structure formed with the combined
Fig. 7. XRD pattern of the inner layer on coated sample oxidized at 700 °C.
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effect of interface migration of MAO coating and air diffusion inward, and the diffusion of the substrate elements outward at 700 °C and 800 °C. 4. Conclusion TiO 2 + ZrO2 composite ceramic coatings were prepared in Zrcontaining electrolytes by microarc oxidation on Ti6Al4V alloy. The microstructure, phase composition, oxidation behavior and mechanism at 500 °C, 600 °C, 700 °C and 800 °C were investigated and discussed. And following conclusions can be drawn: 1 The oxidation kinetic curves at different temperature indicate that the high temperature oxidation resistance of Ti6Al4V is improved 2–10 times after MAO treatment. 2 The coated samples present various oxidation behaviors oxidized at different temperatures. A porous TiO2 layer of 1 μm in thickness forms after oxidation at 500 °C for 300 h, while a continuous TiO2 layer of 2.5 μm in thickness forms after oxidation at 600 °C for 75 h. However, after oxidation at 700 °C and 800 °C oxidized for 75 h, a multilayer structure of MAO coating/transitional layer (a mixture oxides of aluminum, titanium and zirconium)/TiO2 layer/TiN layer forms with the combined effect of interface migration of MAO coating and air diffusion inward, and the diffusion of substrate elements outward. 3 It is found that a TiN layer with preferential growth towards (111) lattice planes formed in the multilayer structure in the coated samples, which is much different from the oxidation behavior of Ti6Al4V at 700 °C and 800 °C. The presence of multilayer structure can provide an efficient diffusion barrier. As a result, the growth of TiO2 and TiN dramatically decreases and the oxidation tends to be stable, which is in good agreement with the oxidation kinetic curves.
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Acknowledgments This work was supported by the National Natural Science Foundation of China under grant no. 51301022.
Please cite this article as: C. Wang, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.11.031