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SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior
Corrosion Science 74 (2013) 367–378
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior Wenbo Li, Shenglong Zhu ⇑, Cheng Wang, Minghui Chen, Mingli Shen, Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 9 March 2013 Accepted 14 May 2013 Available online 22 May 2013 Keywords: A: Titanium B: EPMA B: TEM C: Oxidation
a b s t r a c t A SiO2–Al2O3–glass composite coating was prepared on Ti–6Al–4V alloy by air spraying and subsequent firing. The oxidation behavior of the specimens at 800 °C and 900 °C for 100 h was studied. The thermal shock resistance of the coating was tested by heating up to 900 °C and then quenching in water. The composite coating acted as an oxygen migration barrier and exhibited good resistance against high temperature oxidation, thermal shock, and oxygen permeation on the Ti–6Al–4V alloy. Coating/alloy interfacial reaction occurred, forming a Ti5Si3/Ti3Al bilayer structure. A thin Al2O3 rich layer formed beneath the composite coating during oxidation at 900 °C. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Titanium alloys were widely used as structural materials in aero-industries over the past sixty years because of their high specific strength and excellent corrosion resistance [1]. However, they form a less protective scale of rutile TiO2 or TiO2 + Al2O3 mixture, which lead to inefficient oxidation resistance and limit their applications at high temperatures [2–4]. In addition, high oxygen solubility of titanium alloys results in a brittle oxygen-rich sublayer beneath the oxide scale during high temperature exposure and therefore decreases the plasticity significantly [5–8]. Oxygen incorporates interstitially into the alloy, either at random or ordered sites of alternate basal plane of the hexagonal a-Ti structure, and thus forces anisotropic lattice deformation [5]. Xiong et al. [6] reported that the elongation of fresh Ti60 alloy at room temperature was about 12.7% and decreased down to 6.1% after exposure in air at 600 °C for 1000 h. Therefore, titanium alloys must be protected by coatings when used at elevated temperatures. The glass–ceramic coatings are extensively studied as oxidation resistant coatings for titanium alloy [6,9–11] and TiAl intermetallic compounds [12–15] due to their good chemical stability and thermal durability at high temperatures as well as better process capability and more economic than vacuum vapor deposited coatings. Moskalewicz [10] reported a double layered glass–ceramic coating with higher hardness and better oxidation resistance than the substrate on near-a titanium alloy. Glass–ceramic coating can protect TiAl compounds not only from high temperature oxidation, but
⇑ Corresponding author. Tel.: +86 24 23904856; fax: +86 24 23893624. E-mail address: [email protected] (S. Zhu). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.05.010
also from the hot corrosion attack at high temperatures [14]. Another important advantage of the glass–ceramic coatings is their composition diversity; therefore, it is possible to produce the coatings that possess proper thermo-mechanical properties compatible with the substrates by carefully choosing the glass composition [16]. The glass–ceramic coatings are conventionally prepared by a multi-step melt-quenched method [6,10–12]. Firstly, raw mineral materials were mixed according to the glass formula; secondly, the mixture was heated and formed a molten glass, usually at temperatures above 1500 °C, and then the molten glass was quenched in water to get frit; thirdly, the glass frits were milled into glass powders; fourthly, a green glass coating was prepared by spraying a slurry of glass powders; finally, the coated component was fired or heat treated at elevated temperature. Well-designed glass composition and heat treatments for nucleation and growth of crystalline phases in the glass are essential for good thermal and mechanical properties [17]. Therefore, it is more convenient to tailor the thermal and mechanical properties by adding ceramic inclusions into the glass matrix and controlling the species and amount of the inclusions. The aqueous solution of binary alkali silicates has been known for a long time and widely used as inorganic binder in high temperature paint and lightweight refractory material. When losing water in the solution, the small colloids aggregate and gel with amorphous structure forms [18]. The alkali silicate gel forms molten glass when being heating to temperatures high enough. So it is probable to shorten the preparation processing of the glass–ceramic composite coating by using a slurry made from the aqueous solution of potassium silicate and the ceramic powders.
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In this study, a SiO2–Al2O3–glass composite coating was prepared on the Ti–6Al–4V alloy by air spraying such a slurry and subsequent firing. The powders of a-Al2O3 and quartz were selected as the inclusions due to their good compatibility with the alkali silicates glass and the low oxygen permeability. Ti–6Al–4V alloy was chosen as the substrate because it is one of the most widely used titanium alloys and is expected to be utilized at temperature up to 900 °C if reinforced by SiC fibers [2,19]. The effect of SiO2– Al2O3–glass composite coating on the oxidation behavior and oxygen dissolution behavior of Ti–6Al–4V alloy at 800 °C and 900 °C was evaluated. The interfacial reactions between the Ti–6Al–4V alloy and composite coating were characterized. 2. Experimental procedures 2.1. Coating preparation The Ti–6Al–4V (wt.%) alloy with two phases (a + b) was used as the substrate material. Specimens with nominal dimensions of 15 10 2.5 mm were cut from a Ti–6Al–4V alloy ingot by spark line saw and ground down to 600-grit SiC paper, then ultrasonically cleaned in ethanol, followed by sand blasting with alumina and 5 min rinsing in ethanol. The concentration of the aqueous solution of potassium silicate (ASPS) in this study was 40 wt.%. The ratio of SiO2 to K2O was about 3:1 (Dayang Chemical Plant). Both the sizes of the a-Al2O3 (Shenyang Laisheng Plant) and quartz (Sinopharm Chemical Reagent Co., Ltd) powders were about 1–10 lm. The processes to prepare SiO2–Al2O3–glass composite coating are as follows: first, the a-Al2O3 and quartz particles were dispersed into ASPS to form a slurry with the ratio of 15 g quartz : 5 g a-Al2O3: 50 g ASPS. Then, a green coating with thickness about 35 lm was formed on the surface of Ti–6Al–4V alloy by air spraying using a manual painting gun. The air pressure of spraying is about 0.4Mpa. Finally, the green coating was solidified in air at room temperature for 12 h, followed by serial baking in the oven for 12 h at 70 °C, 10 h at 120 °C and then 5 h at 260 °C, and finally firing at 900 °C for 60 min to form a dense SiO2–Al2O3–glass composite coating. 2.2. Oxidation tests The oxidation behavior of the Ti–6Al–4V alloy with and without the composite coating was examined by the methods of interrupted oxidation and cyclic oxidation. The interrupted oxidation tests were conducted in a muffle furnace at 800 °C and 900 °C up to 100 h, respectively. The specimens were put into a furnace where the temperature has already been elevated to the test temperatures. After certain oxidation time, they were removed quickly from the furnace and cooled in the air down to the room temperature. The mass changes of the specimens were measured using an electronic balance with a sensitivity of 105 g. Three samples of each type were used under the same test conditions, and the oxidation kinetics curves were plotted using the average mass change. Cyclic oxidation tests were conducted at 900 °C in static air. The specimens were kept at 900 °C for 1 h and cooled down to room temperature for 10 min as a cycle. Specimens were weighed with an electronic balance after 20 cycles to measure the mass changes. The cyclic oxidation was conducted for 100 cycles. Thermal shock tests were conducted in muffle furnace where the temperature had been elevated at 900 °C. Each cycle contains inserting the specimens into the furnace, followed by 10 min holding in the furnace and then plunging the specimen into water at room temperature (about 25 °C). The mass changes of the specimens were measured using balance after each 5 cycles.
2.3. Analytical characterization The microstructures of the specimens before and after oxidation were characterized by SEM (FEI INSPECT F 50, FEI, Hillsboro, OR) with an EDS (OXFORD X-Max, Oxford Instruments, Oxford, UK). X-ray diffraction (Panalytical X’ Pert PRO, Cu Ka radiation at 40 kV, PA Analytical, Almelo, Holland) was used to analyze the phases of the oxides. Electron probe microanalysis (EPMA-1610, Shimadzu, Kyoto, Japan) and transmission electron microscope (JEM 2100F transmission electron microscope, JEOL) were used to study the details of the interfacial reactions between composite coating and substrate. Because the hardness of Ti-based alloys is very sensitive to the content of the dissolved oxygen in the alloy, the microhardness depth profile was used to study the dissolution profile of oxygen in the alloy beneath the oxide scale or coating. The Vickers microhardness measurements (Micromet 5114, Buehler, Mitutoyo Co., Kanagawa, Japan) were carried out on the cross-section of the specimens by indentation method with the load of 100 g and duration of 10 s.
3. Results 3.1. Microstructure of the as-prepared composite coating The XRD patterns of the as-prepared SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy are shown in the Fig. 1. Quartz, aAl2O3, and weak peaks of Ti5Si3 and Ti3Al were detected. Quartz and a-Al2O3 were the inclusions in the composite coating. Ti5Si3 and Ti3Al may be the new phases formed during preparation of the composite coating. Fig. 2 shows the cross-sectional images and elemental line-scan profile of the as-prepared SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy. The composite coating was well adherent to the substrate, and no pore or crack was observed in the coating. The quartz and alumina particles are well dispersed in the coating. High-magnification cross-section image (Fig. 2b) shows that there was an interfacial reaction zone beneath the coating. Besides a layer rich in Ti and Al, another layer composed mainly of Ti and Si may be observed in the interfacial reaction zone (Fig. 2c). One can also observe some oxide dispersion in the Ti-Si layer. Combining the EDS results and XRD patterns, it may be concluded that Ti5Si3 and Ti3Al formed beneath the composite coating during the preparation of the coating.
Fig. 1. XRD patterns of the as-prepared SiO2–Al2O3–glass composite coating on Ti– 6Al–4V alloy.
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Fig. 2. Low-magnification cross-sectional image (a), high-magnification cross-sectional image (b), and elemental line-scan profile (analyzed by EDS) (c) of the as-prepared SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy.
3.2. Interrupted oxidation 3.2.1. Oxidation kinetics The oxidation kinetics curves of Ti–6Al–4V alloy with and without the SiO2–Al2O3–glass composite coating at 800 °C and 900 °C are shown in the Fig. 3. At the two temperatures, the oxidation kinetics of bare Ti–6Al–4V alloy followed an approximately parabolic law. The total mass gains of the bare Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C were about 30 and 70 mg/ cm2, respectively. However, the mass gains of coated ones were much smaller, only about 0.8 and 2.5 mg/cm2, respectively, indicating that the composite coating decreases the oxidation rate of the Ti–6Al–4V alloy significantly. 3.2.2. Microstructure of the oxides on uncoated Ti–6Al–4V alloy The XRD patterns for the bare Ti–6Al–4V alloy after 100 h oxidation tests at 800 °C and 900 °C are shown in the Fig. 4. RutileTiO2 and weak diffraction peaks of a-Al2O3 were detected. The cross-sectional microstructure of the alloy after 100 h oxidation at 800 °C and 900 °C is shown in the Fig. 6a and b. The scale structure of the bare Ti–6Al–4V alloy after 100 h oxidation at 800 °C was similar to that formed at 900 °C except that the thickness of the former was about half of the latter, corresponding to the oxidation kinetics. The oxide scale is of layered structure. Thick rutile-TiO2 was frequently interrupted by thin discontinuous layers of Al2O3. Cracks paralleled to the surface of the specimen were observed in the oxide scale. 3.2.3. Microstructure of the oxides on coated Ti–6Al–4V alloy The XRD patterns of the coated Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C are shown in the Fig. 5. Quartz, cristobalite, a-Al2O3, Ti5Si3, and Ti3Al were detected. Cristobalite, which was detected only for the specimen after oxidation at 900 °C, may be transformed from the quartz particles in the composite coating.
The cross-sectional microstructure of the coated specimens after 100 h oxidation at 800 °C and 900 °C are shown in the Fig. 6c and d. The composite coating maintained intact after oxidation tests, implying that the composite coating acted as an oxygen migration barrier and improved the oxidation resistance of the Ti– 6Al–4V alloy. EDS analysis shows that the composition of the glass in the composite coating (region A in Fig. 6d) after 100 h oxidation at 900 °C is about 63.8% O, 28.3% Si, 4.9% K, and 3% Al. No oxide of Ti was detected in the composite coating. Blurry quartz/glass interface and circumferential cracks surrounding big SiO2 inclusions were observed. Interfacial reaction zone with layered structure was observed between the coating and the alloy. The interfacial reaction between composite coating and substrate during oxidation at 800 °C is similar to that at 900 °C, though the latter was stronger than the former. The chemical composition of region B (Fig. 6f) is 61.6 Ti, 35.5Si, 1.9V and 0.9Al (by EDS), suggesting that region B was mainly composed of Ti5Si3. The region C (Fig. 6f) is composed of 75.4Ti, 20.8Al, 2.2V and 1.5Si, so it is probably Ti3Al. Both dispersed oxides and voids were observed in the Ti5Si3 layer. The EPMA mapping technique was used to analyze the interfacial reaction between the composite coating and the Ti–6Al–4V alloy at 800 °C and 900 °C, as shown in the Figs. 7 and 8. For the specimen after 100 h oxidation at 800 °C, from the substrate to the coating, a Ti–Al alloy layer was found beneath the Ti-Si alloy layer, corresponding to Ti3Al and Ti5Si3, respectively. Small amount of Al was detected in the glass surrounding the Al2O3 particles, suggesting that the Al2O3 particles may partially dissolve into glass matrix. For the specimen after 100 h oxidation at 900 °C, besides the Ti3Al and Ti5Si3 layer, it is worth noting that a thin Al rich layer was observed beneath the composite coating, indicating selective oxidation of Al may occur and lead to the formation of thin Al2O3 layer beneath the coating (Fig. 8). Al was detected almost in the whole glass matrix, even though the content was low.
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Fig. 5. XRD patterns of Ti–6Al–4V alloy with the composite coating after 100 h oxidation at 800 °C and 900 °C.
Fig. 3. Interrupted oxidation kinetics of Ti–6Al–4V alloy with and without the composite coating at 800 °C (a) and 900 °C (b). Each data point represents the average value of three specimens. The scatter band is the mean deviation.
Ti3Al/Ti5Si3 interface. One may remember that some voids much larger were also observed at the Ti3Al/Ti5Si3 interface and in the Ti5Si3 layer (Fig. 6f). The thin Al2O3 layer formed beneath the composite coating was also observed by TEM. It is interesting that two different interfaces were observed. One is the Ti5Si3/Al2O3/glass interface, as shown in the Fig. 10. The element mapping technique shows that the SiO2 and K2O are detected in the Al2O3 layer, indicating that the reaction between the newly formed Al2O3 and the glass matrix occurred. Oxide of Ti was also observed in the glass, and it may exist just in the coating closing to the Ti5Si3/coating interface. The other is Ti5Si3/Al2O3/quartz interface, as shown in the Fig. 11. Even though the Al2O3/quartz interface is not obvious, the reaction between the quartz and Al2O3 is not as strong as that between glass matrix and Al2O3. Selective area electron diffraction indicated that a-Al2O3 was formed at the interface. The thickness of Al2O3 layer is less than 0.5 lm. Almost no oxide of Ti was observed at the interface. Al2O3 and TiO2 were observed in the Ti5Si3 layer, as shown in the Fig. 11. Combining the above results, it could be concluded that a Ti5Si3/ Ti3Al bilayer structure formed at the coating/alloy interface after 100 h oxidation due to the interfacial reaction between coating and Ti–6Al–4V alloy. And it is interesting that a thin Al2O3 rich layer formed at the Ti5Si3/coating interface after 100 h oxidation at 900 °C. 3.3. Cyclic oxidation and thermal shock
Fig. 4. XRD patterns of bare Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C.
TEM was also used to analyze the interfacial reaction between coating and alloy after 100 h oxidation at 900 °C. The interface between the Ti3Al and Ti5Si3 is shown in the Fig. 9; the grain size of the Ti3Al is larger than 2 lm, while the grain size of the Ti5Si3 is in the range of 200–500 nm. Some voids were observed at the
Optical images of the Ti–6Al–4V alloy with and without composite coating after cyclic oxidation at 900 °C for 40 cycles and 100 cycles were shown in the Fig. 12. Severe spallation of the oxide scale on the bare Ti–6Al–4V alloy was observed, as shown in the Fig. 12a and c. However, no spallation but glassy blaze was observed on the surface of the coated Ti–6Al–4V alloy (Fig. 12b and d). Oxide of the substrate was observed at the surrounding of the processing hole because the coating at these surfaces was imperfect. The cyclic oxidation kinetics of Ti–6Al–4V alloy with and without the composite coating at 900 °C is shown in the Fig. 13. Even though severe spallation occurred, the mass gain of the bare Ti–6Al–4V alloy kept monotonously increasing, suggesting that the mass gain due to oxidation is larger than the mass loss due to spallation. For the coated specimen, the cyclic oxidation kinetics is almost the same as the interrupted oxidation kinetics, proving good cyclic oxidation resistance of the composite coating on Ti– 6Al–4V alloy.
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Fig. 6. Cross-sectional image (a and b) of bare Ti–6Al–4V alloy, low-magnification cross-sectional image (c and d), and high-magnification cross-sectional image (e and f) of the coated Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C.
The surface and cross-sectional morphologies of coated Ti–6Al– 4V alloy after cyclic oxidation at 900 °C for 100 cycles are shown in Fig. 14. Neither crack nor spallation was observed; the protrusions at the surface of the coating are either Al2O3 or SiO2 particles. The composite coating was still well adherent to the substrate. The cross-sectional microstructure of the coated Ti–6Al–4V alloy after cyclic oxidation is similar to that after interrupted oxidation, as shown in the Fig. 14b and c. Therefore, the composite coating significantly improved the cyclic oxidation resistance of Ti–6Al–4V alloy. Fig. 15 shows the weight gain of Ti–6Al–4V alloy with composite coating after 50 cycles of thermal shock. One can see that the coated Ti–6Al–4V alloy exhibited normal positive weight gains similar to the oxidized specimens, and the weight gains after 50 cycles of thermal shock are about 0.33 mg/cm2. The surface morphology of the Ti–6Al– 4V alloy with composite coating after 50 cycles of thermal shock was shown in the Fig. 16. Neither crack nor spallation was observed, indicating good thermal shock resistance of the composite coating. 3.4. Hardness tests Mircohardness depth profiles of Ti–6Al–4V alloy with and without the composite coating after 100 h interrupted oxidation at 800 °C and 900 °C are shown in Fig. 17. The zero points in Fig. 17
correspond to either the oxide scale/alloy interface for the bare Ti–6Al–4V alloy or the coating/alloy interface for the coated Ti– 6Al–4V alloy, respectively. For the bare Ti–6Al–4V alloy, the microhardness of the alloy close to the oxide scale/alloy interface were as high as 9 GPa after oxidation tests, almost 3 times of that before oxidation (3.5 GPa). The microhardness decreased exponentially from the interface to the center of the sample. The thickness of the hardened layer on the alloy after oxidation at 800 °C is about 130 lm, much thinner than that after oxidation at 900 °C (>700 lm). In contrast, the microhardness beneath the coating/alloy interface was about 4.5 GPa for both coated specimens after oxidation, which is only a little higher than that of the fresh Ti–6Al–4V alloy, indicating only slight oxygen contamination during oxidation. The microhardness tests on the bare samples also resulted in the cracks at the surrounding of the indentations, as shown in the Fig. 18. Especially for the sample after 100 h oxidation at 900 °C, the location of the third indentation (Fig. 18c) is about 75 lm away from the oxide scale/alloy interface, while a crack around this indentation was also observed, which means that dissolution of oxygen decreased the plasticity of the alloy greatly. For the coated samples, no such cracks were observed around the indentations.
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Fig. 7. Element mapping (Al, O, Ti, Si, and K) for the Ti–6Al–4V alloy with composite coating after 100 h oxidation at 800 °C.
Fig. 8. Element mapping (Al, O, Ti, Si, and K) for the Ti–6Al–4V alloy with composite coating after 100 h oxidation at 900 °C.
4. Discussion 4.1. Oxidation behavior of the bare Ti–6Al–4V alloy Because Ti-based alloys have very high solid solubility of oxygen and form less protective rutile TiO2 scale on their surfaces, severe oxidation and oxygen dissolution occurred for the Ti-based alloys at high temperatures [4,17,20].
The stress formed during the oxidation tests could be divided into two parts. One is the growth stress due to the growth of the oxide, the other one is the thermal stress formed during the cooling due to the mismatch of the thermal expansion coefficient (TEC) between the oxide scale and the alloy. Du et al. [2] reported that it is possible to maintain contact between the oxide scale and substrate by plastic flow of the oxide scale at the initial stages of oxidation, but cracks will form between the oxide scale and substrate once a
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Fig. 9. TEM image, electron diffraction patterns, and element mapping (Ti, Al, and Si) of the Ti3Al/Ti5Si3 interface of the coated Ti–6Al–4V alloy after 100 h oxidation at 900 °C.
Fig. 10. TEM image, electron diffraction patterns, and element mapping (Al, Ti, Si, O, and K) of the Ti5Si3/Al2O3/glass interfaces of the coated Ti–6Al–4V alloy after 100 h oxidation at 900 °C.
critical thickness of the oxide scale is exceeded. In present study, cracks paralleled to the surface of the substrate were presented in the oxide scale. Because the Pilling-Bed-Worth ratio (PBR) of TiO2/ Ti is as high as 1.73 [21], the growth of the oxide may result in high compressive stress in the oxide scale. One may remember that a hardened sublayer formed beneath the oxide scale due to the oxygen dissolution (Fig. 12). The plasticity of the hardened sublayer is much lower than that of the substrate at high temperature, so the high growth stresses of the oxide scale may not be able to be released by the plastic flow of the substrate and probably result in the formation of the cracks. Another reason for the spallation of the oxide scale
is the mismatch of the TEC between the oxide scale and the substrate. The TEC of the TiO2 is about 8.2 106 (K1) [22], while that of the Ti–6Al–4V alloy is about 1012 106 (K1) [23]. Mismatch of the TEC between the scale and the alloy resulted in the formation of large compressive stresses in the oxide scale and then the spallation of the oxide scale during cooling, as shown in the Fig. 12a and c. Vanadium was detected by EDS in the oxide scale, while no oxide of V was shown in the corresponding XRD patterns. Considering that the ionic radii of the V4+ (0.63) is similar to that of Ti4+ (0.68), it is probable that the V4+ incorporate into the rutile lattice by replacing the Ti4+.
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Fig. 11. TEM image, electron diffraction patterns, and element mapping (Al, Ti, Si, O, K, and V) of the Ti5Si3/Al2O3/quartz interfaces of the coated Ti–6Al–4V alloy after 100 h oxidation at 900 °C.
Fig. 12. Optical images of the bare and coated Ti–6Al–4V alloy after 40 h (a and b) and 100 h (c and d) cyclic oxidation at 900 °C.
Fig. 13. cyclic oxidation kinetics of Ti–6Al–4V alloy with and without the composite coating at 900 °C. Each data point represents the average value of three specimens. The scatter band is the mean deviation.
Many thin discontinuous Al2O3 layers were observed in the oxide scale (Fig. 8a and b). Formation of TiO2 layer results in local enrichment of Al beneath the TiO2 and then leads to the formation of Al2O3 due to selective oxidation. However, the content of the Al is so low (6wt.%) that the growth of the Al2O3 layer is not able to be maintained. Therefore, alternant growth of TiO2 and Al2O3 will occur, and the alternant multilayer of thick TiO2 and discontinuous Al2O3 formed on the bare Ti–6Al–4V alloy. Beneath the oxide scale, a hardened layer in the alloy was detected by the microhardness tests (Fig. 17). The hardness of the oxygen containing titanium alloy depends upon the content of oxygen in the alloy. The higher the oxygen content is, the higher hardness the alloy possesses. In this study, the oxygen dissolution in the bare Ti–6Al–4V alloy was obvious. The oxygen content in the alloy decreased exponentially from the oxide/alloy interface to the center of the alloy, suggesting the oxygen dissolution is dominated by inward oxygen diffusion. The thickness of the oxygen dissolution layer increased with the increase in the temperature. The oxygen dissolution effect is detrimental to the mechanical performance of Ti-based alloys, especially to the alloy plasticity
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Fig. 14. Surface morphology (a), cross-sectional image (b), and elemental line-scan profile (analyzed by EDS) (c) of the SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy after 100 h cyclic oxidation at 900 °C.
Fig. 15. Thermal shock kinetic of the coated Ti–6Al–4V alloy at 900 °C.
[4]. The cracks due to the indentations shown in the Fig. 18 proved the oxygen dissolution lead to the formation of the brittle layer beneath the oxide scale.
4.2. Interfacial reaction between the composite coating and the Ti– 6Al–4V alloy Compared with the poor oxidation resistance of bare Ti–6Al–4V alloy, the Ti–6Al–4V alloy with SiO2–Al2O3–glass composite coating shows much better oxidation resistance. The composite coating acted as an oxygen migration barrier, not only reduced the high temperature oxidation rate of the alloy, but also decreased the oxygen dissolution in the alloy. Similar results have been reported [14,24– 26]. The short-term interfacial reaction between the glass and titanium alloys was extensively researched [27–32]. Because TiO2 is
Fig. 16. Surface morphology of the coated Ti–6Al–4V alloy after 50 cycles of thermal shock.
more thermodynamically stable than SiO2, the SiO2 in the glass is reduced by the Ti in the alloy during the firing process or oxidation. Formation of the Ti5Si3 at the glass/alloy interface was observed by Moskalewicz [10] and Oku [31]. Several titanium/glass coating interfacial reaction mechanisms have been proposed. Pazo [28] and Oku [31] proposed an interfacial reaction as follows:
5Ti þ 3SiO2 ðglassÞ ¼ Ti5 Si3 þ 3O2
ð1Þ
They believed that the released oxygen is the reason for the voids formed at the interface in their study, but the calculated standard free energy of the reaction (1) at 800 °C and 900 °C is about 1555 kJ/mol and 1501 kJ/mol (The standard free energy of this reaction and the following two reactions is calculated by for-
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Fig. 17. Microhardness depth profiles of Ti–6Al–4V alloy with and without composite coating after 100 h oxidation at 800 °C (a) and 900 °C (b). H H mula:DGH r ðTÞ ¼ DHr ð298 KÞ T DSr ð298 KÞ, the data are from the Table 1 [33]). So, this reaction could not spontaneously take place at test temperatures. In present study, the Ti5Si3 was observed in the as-prepared specimen and in the specimens after 100 h oxidation at 800 °C and 900 °C. The dissolution of the TiO2 into the glass was also observed. The probable reaction is as follows:
The formation of Ti3Al may have close relationship with the formation of Ti5Si3. Because of the low solubility of Al in Ti5Si3 phase (Fig. 8), Al is driven away from the Ti5Si3 layer, either formed Al2O3 at the coating/alloy interface or diffused into the deeper alloy. Once the content of Al in the alloy exceeded the solubility of Al in the Ti, according to the binary Ti–Al phase diagrams, the probable phase formed beneath the interfacial reaction is Ti3Al. The most interesting part of the interfacial reaction may be the formation of the thin Al2O3 rich layer at the coating/alloy interface after 100 h oxidation at 900 °C, which has not been reported in the previous literature. There may be two reasons for the formation of the thin Al2O3 layer: one is the local enrichment of Al due to the formation of the Ti5Si3 layer, and the other is the low oxygen partial pressure at the coating/Ti5Si3 interface; both promotes the selective oxidation of Al by Eq. (3). Li and Taniguchi [34] prepared a Ti5Si3/Al-rich bilayer on the Ti–48Al–1.3Fe–1.1V–0.3B (at.%) alloy and found that the Al-rich zone affected the oxidation of the Ti5Si3 layer. They reported that the Al-rich zone beneath the Ti5Si3 modified layer resulted in large amounts of Al2O3 after long-term oxidation and the final oxides of the sample were composed of TiO2, Al2O3 and amorphous SiO2, which is beneficial to the oxidation resistance of the alloy. In present study, the composite coating acts as an oxygen migration barrier, decreasing the oxygen partial pressure at the Ti5Si3/composite coating interface. Compared with the oxidation of the Ti5Si3/Al-rich bilayer in air, the low oxygen partial pressure at the interface promotes the selective oxidation of the Al and the formation of Al2O3. Large amounts of grain boundaries in the Ti5Si3 layer may benefit the outward diffusion of Al which is essential for the alumina growth. Litter [35] reported that a silica layer, which formed at the MoxRuySiz compounds/glass interface due to the selective oxidation of the Si in the compounds, dissolved into the alkali borosilicate glass quickly at 1350 °C. In this study, the dissolution of the newly formed Al2O3 layer was also observed at the Al2O3 layer/glass interface (Fig. 10). The growth of the Al2O3 layer competed with the dissolution of Al2O3 layer during the oxidation tests. Al was detected in the glass by EDS, but the main source of the Al in the glass is not the newly formed Al2O3 layer, but the Al2O3 inclusions in the composite coating, which we were discussed in the Section 4.3. The dissolution of the alumina inclusions in the glass may decrease the dissolution of the newly formed alumina layer. Only dispersed alumina particles were observed at the interface after oxidation at 800 °C (Fig. 6), while a continuous alumina layer formed at 900 °C (Fig. 8), may be attributed to the lower growth rate of alumina at 800 °C. 4.3. Microstructure change of the composite coating
8Ti þ 3SiO2 ¼ 3TiO2 þ Ti5 Si3
ð2Þ
The standard free energy of reaction (2) at 800 °C and 900 °C is 682 kJ/mol and 680 kJ/mol, respectively. This reaction was also proposed by Gomez-Vega [30], even the TiO2 was not observed in their study. Ti in the alloy diffused outward and reacted with the SiO2–K2O glass, leading to not only the dissolution of TiO2 in the glass but also the formation of the Ti5Si3 layer with some TiO2 dispersion at the coating/alloy interface, as shown in the Figs. 10 and 11. It is interesting that some Al2O3 dispersion was also observed in the Ti5Si3 layer, indicating that the following reaction may also occurred:
5Ti þ 4Al þ 3SiO2 ¼ Ti5 Si3 þ 2Al2 O3
ð3Þ
The standard free energy of reaction (3) at 800 °C and 900 °C is 1123 kJ/mol and 1115 kJ/mol, respectively. The grain size of the Ti5Si3 was in the range of 200–500 nm, probably because the dispersed TiO2 and Al2O3 in it restricted the growth of the grain.
Because the glass transition temperature of the glass in the composite coating is as low as about 495 °C [36], the glass softens and acts as a viscous liquid at 900 °C; therefore, it is not surprising that the reactions between glass and inclusions occurred during oxidation tests. Both EDS and EPMA analysis detected Al in the glass matrix. Compared with the as-prepared coating (Fig. 2), less Al2O3 inclusions were observed in the coating after 100 h oxidation at 900 °C (Fig 6d). All these results indicated that the Al2O3 inclusions partially dissolved into the glass matrix during oxidation tests. The Al2O3 inclusions, but not the newly formed Al2O3 layer, are the main source of the Al in the glass matrix because the Al2O3 inclusions/ glass interfaces are much larger than the Al2O3 layer/glass interface. The dissolution of the Al2O3 inclusions turned the binary silicate glass (K2O–SiO2) into a ternary glass (K2O–Al2O3–SiO2), which may improve the chemical stability of the glass [37]. Quartz particles were also instable in the glass. Phase transition of quartz to cristobalite was observed at 900 °C (Fig. 5). According to Fenner [38], with the existence of mineralizer, such as Na2O, K2O
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Fig. 18. Indentations on the cross-section of the bare Ti–6Al–4V alloy (a and c) and of the coated Ti–6Al–4V alloy (b and d) after 100 h oxidation at 800 °C and 900 °C.
Table 1 Standard enthalpies of formation and standard entropies for the relevant substances at 298 K. The data are from [33]. Compound
1 DHH i ð298 KÞ (kJ mol )
1 mol1) SH i ð298 KÞ (J K
Ti Al SiO2 Ti5Si3 TiO2 (rutile) a-Al2O3 O2
0 0 905.20 579.07 944.75 1675.27 0
30.65 28.32 46.28 217.99 50.33 50.94 205.04
and CaO, the quartz would transform into cristobalite or tridymite when the temperature is higher than 870 °C. The dissolution of the quartz particles may also occur during oxidation tests. Because the profile of the original quartz particles is angular, however, the profile of the quartz particles in the composite coating was smooth, and the quartz/glass interface was blurry. Similar results have been reported [39]. Circumferential cracks surrounding big remaining quartz particles were observed in the Fig. 6f. Gilabert [40] studied the fracture patterns of quartz particles in glass feldspar matrix and reported that rounded particles tend to generate circumferential flaws close to the matrix/quartz interface. In this study, the formation of the circumferential cracks may be attributed to the phase transition of b-quartz to a-quartz at around 573 °C during cooling. This phase transition is accompanied by relatively large volume shrinkage and completes in a short time, resulting in tensile stress in the radical direction in the glass phase surrounding the quartz inclusions. Once the tensile stress surpasses the tensile strength of the glass phase, circumferential cracks would form in the glass phase surrounding the quartz inclusions.
5. Conclusions From the above study, the following conclusions can be drawn:
(1) An oxidation resistant SiO2–Al2O3–glass composite coating was successfully prepared on the Ti–6Al–4V alloy by air spraying a slurry which was made from the aqueous solution of potassium silicate and the quartz and alumina powders, and subsequent firing. It remarkably improved the oxidation resistance of the Ti–6Al–4V alloy at 800 °C and 900 °C. The composite coating exhibited good thermal cycle resistance and thermal shock resistance on Ti–6Al–4V alloy. (2) A hardened layer formed beneath the oxide scale due to the oxygen dissolution in the bare Ti–6Al–4V alloy. However, the SiO2–Al2O3–glass composite coating remarkably decreased the oxygen dissolution in the Ti–6Al–4V alloy at 800 °C and 900 °C. (3) Interfacial reactions between composite coating and alloy occurred, forming a layered structure (Ti5Si3/Ti3Al) beneath the composite coating. A thin Al2O3 rich layer formed at the coating/Ti5Si3 interface at 900 °C. (4) There was dissolution of Al2O3 and quartz inclusions into the glass matrix, as well as the phase transformation of quartz into cristobalite during oxidation tests.
Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 51201171 and the National High Technology Research and Development Program of China (863 Program) under Grant No. 2012AA03A512. References [1] M. Peters, J. Kumpfert, C.H. Ward, C. Leyens, Titanium alloy for aerospace applications, Adv. Eng. Mater. 5 (2003) 419–427. [2] H.L. Du, P.K. Datta, D.B. Lewis, J.S. Burnell-Gray, Air oxidation behavior of Ti– 6Al–4V alloy between 650 and 850 °C, Corros. Sci. 36 (1994) 631–642. [3] Z.W. Li, W. Gao, D.L. Zhang, Z.H. Cai, High temperature oxidation behaviour of a TiAl–Al2O3 intermetallic matrix composite, Corros. Sci. 46 (2004) 1997–2007.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior Wenbo Li, Shenglong Zhu ⇑, Cheng Wang, Minghui Chen, Mingli Shen, Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 9 March 2013 Accepted 14 May 2013 Available online 22 May 2013 Keywords: A: Titanium B: EPMA B: TEM C: Oxidation
a b s t r a c t A SiO2–Al2O3–glass composite coating was prepared on Ti–6Al–4V alloy by air spraying and subsequent firing. The oxidation behavior of the specimens at 800 °C and 900 °C for 100 h was studied. The thermal shock resistance of the coating was tested by heating up to 900 °C and then quenching in water. The composite coating acted as an oxygen migration barrier and exhibited good resistance against high temperature oxidation, thermal shock, and oxygen permeation on the Ti–6Al–4V alloy. Coating/alloy interfacial reaction occurred, forming a Ti5Si3/Ti3Al bilayer structure. A thin Al2O3 rich layer formed beneath the composite coating during oxidation at 900 °C. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Titanium alloys were widely used as structural materials in aero-industries over the past sixty years because of their high specific strength and excellent corrosion resistance [1]. However, they form a less protective scale of rutile TiO2 or TiO2 + Al2O3 mixture, which lead to inefficient oxidation resistance and limit their applications at high temperatures [2–4]. In addition, high oxygen solubility of titanium alloys results in a brittle oxygen-rich sublayer beneath the oxide scale during high temperature exposure and therefore decreases the plasticity significantly [5–8]. Oxygen incorporates interstitially into the alloy, either at random or ordered sites of alternate basal plane of the hexagonal a-Ti structure, and thus forces anisotropic lattice deformation [5]. Xiong et al. [6] reported that the elongation of fresh Ti60 alloy at room temperature was about 12.7% and decreased down to 6.1% after exposure in air at 600 °C for 1000 h. Therefore, titanium alloys must be protected by coatings when used at elevated temperatures. The glass–ceramic coatings are extensively studied as oxidation resistant coatings for titanium alloy [6,9–11] and TiAl intermetallic compounds [12–15] due to their good chemical stability and thermal durability at high temperatures as well as better process capability and more economic than vacuum vapor deposited coatings. Moskalewicz [10] reported a double layered glass–ceramic coating with higher hardness and better oxidation resistance than the substrate on near-a titanium alloy. Glass–ceramic coating can protect TiAl compounds not only from high temperature oxidation, but
⇑ Corresponding author. Tel.: +86 24 23904856; fax: +86 24 23893624. E-mail address: [email protected] (S. Zhu). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.05.010
also from the hot corrosion attack at high temperatures [14]. Another important advantage of the glass–ceramic coatings is their composition diversity; therefore, it is possible to produce the coatings that possess proper thermo-mechanical properties compatible with the substrates by carefully choosing the glass composition [16]. The glass–ceramic coatings are conventionally prepared by a multi-step melt-quenched method [6,10–12]. Firstly, raw mineral materials were mixed according to the glass formula; secondly, the mixture was heated and formed a molten glass, usually at temperatures above 1500 °C, and then the molten glass was quenched in water to get frit; thirdly, the glass frits were milled into glass powders; fourthly, a green glass coating was prepared by spraying a slurry of glass powders; finally, the coated component was fired or heat treated at elevated temperature. Well-designed glass composition and heat treatments for nucleation and growth of crystalline phases in the glass are essential for good thermal and mechanical properties [17]. Therefore, it is more convenient to tailor the thermal and mechanical properties by adding ceramic inclusions into the glass matrix and controlling the species and amount of the inclusions. The aqueous solution of binary alkali silicates has been known for a long time and widely used as inorganic binder in high temperature paint and lightweight refractory material. When losing water in the solution, the small colloids aggregate and gel with amorphous structure forms [18]. The alkali silicate gel forms molten glass when being heating to temperatures high enough. So it is probable to shorten the preparation processing of the glass–ceramic composite coating by using a slurry made from the aqueous solution of potassium silicate and the ceramic powders.
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In this study, a SiO2–Al2O3–glass composite coating was prepared on the Ti–6Al–4V alloy by air spraying such a slurry and subsequent firing. The powders of a-Al2O3 and quartz were selected as the inclusions due to their good compatibility with the alkali silicates glass and the low oxygen permeability. Ti–6Al–4V alloy was chosen as the substrate because it is one of the most widely used titanium alloys and is expected to be utilized at temperature up to 900 °C if reinforced by SiC fibers [2,19]. The effect of SiO2– Al2O3–glass composite coating on the oxidation behavior and oxygen dissolution behavior of Ti–6Al–4V alloy at 800 °C and 900 °C was evaluated. The interfacial reactions between the Ti–6Al–4V alloy and composite coating were characterized. 2. Experimental procedures 2.1. Coating preparation The Ti–6Al–4V (wt.%) alloy with two phases (a + b) was used as the substrate material. Specimens with nominal dimensions of 15 10 2.5 mm were cut from a Ti–6Al–4V alloy ingot by spark line saw and ground down to 600-grit SiC paper, then ultrasonically cleaned in ethanol, followed by sand blasting with alumina and 5 min rinsing in ethanol. The concentration of the aqueous solution of potassium silicate (ASPS) in this study was 40 wt.%. The ratio of SiO2 to K2O was about 3:1 (Dayang Chemical Plant). Both the sizes of the a-Al2O3 (Shenyang Laisheng Plant) and quartz (Sinopharm Chemical Reagent Co., Ltd) powders were about 1–10 lm. The processes to prepare SiO2–Al2O3–glass composite coating are as follows: first, the a-Al2O3 and quartz particles were dispersed into ASPS to form a slurry with the ratio of 15 g quartz : 5 g a-Al2O3: 50 g ASPS. Then, a green coating with thickness about 35 lm was formed on the surface of Ti–6Al–4V alloy by air spraying using a manual painting gun. The air pressure of spraying is about 0.4Mpa. Finally, the green coating was solidified in air at room temperature for 12 h, followed by serial baking in the oven for 12 h at 70 °C, 10 h at 120 °C and then 5 h at 260 °C, and finally firing at 900 °C for 60 min to form a dense SiO2–Al2O3–glass composite coating. 2.2. Oxidation tests The oxidation behavior of the Ti–6Al–4V alloy with and without the composite coating was examined by the methods of interrupted oxidation and cyclic oxidation. The interrupted oxidation tests were conducted in a muffle furnace at 800 °C and 900 °C up to 100 h, respectively. The specimens were put into a furnace where the temperature has already been elevated to the test temperatures. After certain oxidation time, they were removed quickly from the furnace and cooled in the air down to the room temperature. The mass changes of the specimens were measured using an electronic balance with a sensitivity of 105 g. Three samples of each type were used under the same test conditions, and the oxidation kinetics curves were plotted using the average mass change. Cyclic oxidation tests were conducted at 900 °C in static air. The specimens were kept at 900 °C for 1 h and cooled down to room temperature for 10 min as a cycle. Specimens were weighed with an electronic balance after 20 cycles to measure the mass changes. The cyclic oxidation was conducted for 100 cycles. Thermal shock tests were conducted in muffle furnace where the temperature had been elevated at 900 °C. Each cycle contains inserting the specimens into the furnace, followed by 10 min holding in the furnace and then plunging the specimen into water at room temperature (about 25 °C). The mass changes of the specimens were measured using balance after each 5 cycles.
2.3. Analytical characterization The microstructures of the specimens before and after oxidation were characterized by SEM (FEI INSPECT F 50, FEI, Hillsboro, OR) with an EDS (OXFORD X-Max, Oxford Instruments, Oxford, UK). X-ray diffraction (Panalytical X’ Pert PRO, Cu Ka radiation at 40 kV, PA Analytical, Almelo, Holland) was used to analyze the phases of the oxides. Electron probe microanalysis (EPMA-1610, Shimadzu, Kyoto, Japan) and transmission electron microscope (JEM 2100F transmission electron microscope, JEOL) were used to study the details of the interfacial reactions between composite coating and substrate. Because the hardness of Ti-based alloys is very sensitive to the content of the dissolved oxygen in the alloy, the microhardness depth profile was used to study the dissolution profile of oxygen in the alloy beneath the oxide scale or coating. The Vickers microhardness measurements (Micromet 5114, Buehler, Mitutoyo Co., Kanagawa, Japan) were carried out on the cross-section of the specimens by indentation method with the load of 100 g and duration of 10 s.
3. Results 3.1. Microstructure of the as-prepared composite coating The XRD patterns of the as-prepared SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy are shown in the Fig. 1. Quartz, aAl2O3, and weak peaks of Ti5Si3 and Ti3Al were detected. Quartz and a-Al2O3 were the inclusions in the composite coating. Ti5Si3 and Ti3Al may be the new phases formed during preparation of the composite coating. Fig. 2 shows the cross-sectional images and elemental line-scan profile of the as-prepared SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy. The composite coating was well adherent to the substrate, and no pore or crack was observed in the coating. The quartz and alumina particles are well dispersed in the coating. High-magnification cross-section image (Fig. 2b) shows that there was an interfacial reaction zone beneath the coating. Besides a layer rich in Ti and Al, another layer composed mainly of Ti and Si may be observed in the interfacial reaction zone (Fig. 2c). One can also observe some oxide dispersion in the Ti-Si layer. Combining the EDS results and XRD patterns, it may be concluded that Ti5Si3 and Ti3Al formed beneath the composite coating during the preparation of the coating.
Fig. 1. XRD patterns of the as-prepared SiO2–Al2O3–glass composite coating on Ti– 6Al–4V alloy.
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Fig. 2. Low-magnification cross-sectional image (a), high-magnification cross-sectional image (b), and elemental line-scan profile (analyzed by EDS) (c) of the as-prepared SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy.
3.2. Interrupted oxidation 3.2.1. Oxidation kinetics The oxidation kinetics curves of Ti–6Al–4V alloy with and without the SiO2–Al2O3–glass composite coating at 800 °C and 900 °C are shown in the Fig. 3. At the two temperatures, the oxidation kinetics of bare Ti–6Al–4V alloy followed an approximately parabolic law. The total mass gains of the bare Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C were about 30 and 70 mg/ cm2, respectively. However, the mass gains of coated ones were much smaller, only about 0.8 and 2.5 mg/cm2, respectively, indicating that the composite coating decreases the oxidation rate of the Ti–6Al–4V alloy significantly. 3.2.2. Microstructure of the oxides on uncoated Ti–6Al–4V alloy The XRD patterns for the bare Ti–6Al–4V alloy after 100 h oxidation tests at 800 °C and 900 °C are shown in the Fig. 4. RutileTiO2 and weak diffraction peaks of a-Al2O3 were detected. The cross-sectional microstructure of the alloy after 100 h oxidation at 800 °C and 900 °C is shown in the Fig. 6a and b. The scale structure of the bare Ti–6Al–4V alloy after 100 h oxidation at 800 °C was similar to that formed at 900 °C except that the thickness of the former was about half of the latter, corresponding to the oxidation kinetics. The oxide scale is of layered structure. Thick rutile-TiO2 was frequently interrupted by thin discontinuous layers of Al2O3. Cracks paralleled to the surface of the specimen were observed in the oxide scale. 3.2.3. Microstructure of the oxides on coated Ti–6Al–4V alloy The XRD patterns of the coated Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C are shown in the Fig. 5. Quartz, cristobalite, a-Al2O3, Ti5Si3, and Ti3Al were detected. Cristobalite, which was detected only for the specimen after oxidation at 900 °C, may be transformed from the quartz particles in the composite coating.
The cross-sectional microstructure of the coated specimens after 100 h oxidation at 800 °C and 900 °C are shown in the Fig. 6c and d. The composite coating maintained intact after oxidation tests, implying that the composite coating acted as an oxygen migration barrier and improved the oxidation resistance of the Ti– 6Al–4V alloy. EDS analysis shows that the composition of the glass in the composite coating (region A in Fig. 6d) after 100 h oxidation at 900 °C is about 63.8% O, 28.3% Si, 4.9% K, and 3% Al. No oxide of Ti was detected in the composite coating. Blurry quartz/glass interface and circumferential cracks surrounding big SiO2 inclusions were observed. Interfacial reaction zone with layered structure was observed between the coating and the alloy. The interfacial reaction between composite coating and substrate during oxidation at 800 °C is similar to that at 900 °C, though the latter was stronger than the former. The chemical composition of region B (Fig. 6f) is 61.6 Ti, 35.5Si, 1.9V and 0.9Al (by EDS), suggesting that region B was mainly composed of Ti5Si3. The region C (Fig. 6f) is composed of 75.4Ti, 20.8Al, 2.2V and 1.5Si, so it is probably Ti3Al. Both dispersed oxides and voids were observed in the Ti5Si3 layer. The EPMA mapping technique was used to analyze the interfacial reaction between the composite coating and the Ti–6Al–4V alloy at 800 °C and 900 °C, as shown in the Figs. 7 and 8. For the specimen after 100 h oxidation at 800 °C, from the substrate to the coating, a Ti–Al alloy layer was found beneath the Ti-Si alloy layer, corresponding to Ti3Al and Ti5Si3, respectively. Small amount of Al was detected in the glass surrounding the Al2O3 particles, suggesting that the Al2O3 particles may partially dissolve into glass matrix. For the specimen after 100 h oxidation at 900 °C, besides the Ti3Al and Ti5Si3 layer, it is worth noting that a thin Al rich layer was observed beneath the composite coating, indicating selective oxidation of Al may occur and lead to the formation of thin Al2O3 layer beneath the coating (Fig. 8). Al was detected almost in the whole glass matrix, even though the content was low.
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Fig. 5. XRD patterns of Ti–6Al–4V alloy with the composite coating after 100 h oxidation at 800 °C and 900 °C.
Fig. 3. Interrupted oxidation kinetics of Ti–6Al–4V alloy with and without the composite coating at 800 °C (a) and 900 °C (b). Each data point represents the average value of three specimens. The scatter band is the mean deviation.
Ti3Al/Ti5Si3 interface. One may remember that some voids much larger were also observed at the Ti3Al/Ti5Si3 interface and in the Ti5Si3 layer (Fig. 6f). The thin Al2O3 layer formed beneath the composite coating was also observed by TEM. It is interesting that two different interfaces were observed. One is the Ti5Si3/Al2O3/glass interface, as shown in the Fig. 10. The element mapping technique shows that the SiO2 and K2O are detected in the Al2O3 layer, indicating that the reaction between the newly formed Al2O3 and the glass matrix occurred. Oxide of Ti was also observed in the glass, and it may exist just in the coating closing to the Ti5Si3/coating interface. The other is Ti5Si3/Al2O3/quartz interface, as shown in the Fig. 11. Even though the Al2O3/quartz interface is not obvious, the reaction between the quartz and Al2O3 is not as strong as that between glass matrix and Al2O3. Selective area electron diffraction indicated that a-Al2O3 was formed at the interface. The thickness of Al2O3 layer is less than 0.5 lm. Almost no oxide of Ti was observed at the interface. Al2O3 and TiO2 were observed in the Ti5Si3 layer, as shown in the Fig. 11. Combining the above results, it could be concluded that a Ti5Si3/ Ti3Al bilayer structure formed at the coating/alloy interface after 100 h oxidation due to the interfacial reaction between coating and Ti–6Al–4V alloy. And it is interesting that a thin Al2O3 rich layer formed at the Ti5Si3/coating interface after 100 h oxidation at 900 °C. 3.3. Cyclic oxidation and thermal shock
Fig. 4. XRD patterns of bare Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C.
TEM was also used to analyze the interfacial reaction between coating and alloy after 100 h oxidation at 900 °C. The interface between the Ti3Al and Ti5Si3 is shown in the Fig. 9; the grain size of the Ti3Al is larger than 2 lm, while the grain size of the Ti5Si3 is in the range of 200–500 nm. Some voids were observed at the
Optical images of the Ti–6Al–4V alloy with and without composite coating after cyclic oxidation at 900 °C for 40 cycles and 100 cycles were shown in the Fig. 12. Severe spallation of the oxide scale on the bare Ti–6Al–4V alloy was observed, as shown in the Fig. 12a and c. However, no spallation but glassy blaze was observed on the surface of the coated Ti–6Al–4V alloy (Fig. 12b and d). Oxide of the substrate was observed at the surrounding of the processing hole because the coating at these surfaces was imperfect. The cyclic oxidation kinetics of Ti–6Al–4V alloy with and without the composite coating at 900 °C is shown in the Fig. 13. Even though severe spallation occurred, the mass gain of the bare Ti–6Al–4V alloy kept monotonously increasing, suggesting that the mass gain due to oxidation is larger than the mass loss due to spallation. For the coated specimen, the cyclic oxidation kinetics is almost the same as the interrupted oxidation kinetics, proving good cyclic oxidation resistance of the composite coating on Ti– 6Al–4V alloy.
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Fig. 6. Cross-sectional image (a and b) of bare Ti–6Al–4V alloy, low-magnification cross-sectional image (c and d), and high-magnification cross-sectional image (e and f) of the coated Ti–6Al–4V alloy after 100 h oxidation at 800 °C and 900 °C.
The surface and cross-sectional morphologies of coated Ti–6Al– 4V alloy after cyclic oxidation at 900 °C for 100 cycles are shown in Fig. 14. Neither crack nor spallation was observed; the protrusions at the surface of the coating are either Al2O3 or SiO2 particles. The composite coating was still well adherent to the substrate. The cross-sectional microstructure of the coated Ti–6Al–4V alloy after cyclic oxidation is similar to that after interrupted oxidation, as shown in the Fig. 14b and c. Therefore, the composite coating significantly improved the cyclic oxidation resistance of Ti–6Al–4V alloy. Fig. 15 shows the weight gain of Ti–6Al–4V alloy with composite coating after 50 cycles of thermal shock. One can see that the coated Ti–6Al–4V alloy exhibited normal positive weight gains similar to the oxidized specimens, and the weight gains after 50 cycles of thermal shock are about 0.33 mg/cm2. The surface morphology of the Ti–6Al– 4V alloy with composite coating after 50 cycles of thermal shock was shown in the Fig. 16. Neither crack nor spallation was observed, indicating good thermal shock resistance of the composite coating. 3.4. Hardness tests Mircohardness depth profiles of Ti–6Al–4V alloy with and without the composite coating after 100 h interrupted oxidation at 800 °C and 900 °C are shown in Fig. 17. The zero points in Fig. 17
correspond to either the oxide scale/alloy interface for the bare Ti–6Al–4V alloy or the coating/alloy interface for the coated Ti– 6Al–4V alloy, respectively. For the bare Ti–6Al–4V alloy, the microhardness of the alloy close to the oxide scale/alloy interface were as high as 9 GPa after oxidation tests, almost 3 times of that before oxidation (3.5 GPa). The microhardness decreased exponentially from the interface to the center of the sample. The thickness of the hardened layer on the alloy after oxidation at 800 °C is about 130 lm, much thinner than that after oxidation at 900 °C (>700 lm). In contrast, the microhardness beneath the coating/alloy interface was about 4.5 GPa for both coated specimens after oxidation, which is only a little higher than that of the fresh Ti–6Al–4V alloy, indicating only slight oxygen contamination during oxidation. The microhardness tests on the bare samples also resulted in the cracks at the surrounding of the indentations, as shown in the Fig. 18. Especially for the sample after 100 h oxidation at 900 °C, the location of the third indentation (Fig. 18c) is about 75 lm away from the oxide scale/alloy interface, while a crack around this indentation was also observed, which means that dissolution of oxygen decreased the plasticity of the alloy greatly. For the coated samples, no such cracks were observed around the indentations.
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Fig. 7. Element mapping (Al, O, Ti, Si, and K) for the Ti–6Al–4V alloy with composite coating after 100 h oxidation at 800 °C.
Fig. 8. Element mapping (Al, O, Ti, Si, and K) for the Ti–6Al–4V alloy with composite coating after 100 h oxidation at 900 °C.
4. Discussion 4.1. Oxidation behavior of the bare Ti–6Al–4V alloy Because Ti-based alloys have very high solid solubility of oxygen and form less protective rutile TiO2 scale on their surfaces, severe oxidation and oxygen dissolution occurred for the Ti-based alloys at high temperatures [4,17,20].
The stress formed during the oxidation tests could be divided into two parts. One is the growth stress due to the growth of the oxide, the other one is the thermal stress formed during the cooling due to the mismatch of the thermal expansion coefficient (TEC) between the oxide scale and the alloy. Du et al. [2] reported that it is possible to maintain contact between the oxide scale and substrate by plastic flow of the oxide scale at the initial stages of oxidation, but cracks will form between the oxide scale and substrate once a
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Fig. 9. TEM image, electron diffraction patterns, and element mapping (Ti, Al, and Si) of the Ti3Al/Ti5Si3 interface of the coated Ti–6Al–4V alloy after 100 h oxidation at 900 °C.
Fig. 10. TEM image, electron diffraction patterns, and element mapping (Al, Ti, Si, O, and K) of the Ti5Si3/Al2O3/glass interfaces of the coated Ti–6Al–4V alloy after 100 h oxidation at 900 °C.
critical thickness of the oxide scale is exceeded. In present study, cracks paralleled to the surface of the substrate were presented in the oxide scale. Because the Pilling-Bed-Worth ratio (PBR) of TiO2/ Ti is as high as 1.73 [21], the growth of the oxide may result in high compressive stress in the oxide scale. One may remember that a hardened sublayer formed beneath the oxide scale due to the oxygen dissolution (Fig. 12). The plasticity of the hardened sublayer is much lower than that of the substrate at high temperature, so the high growth stresses of the oxide scale may not be able to be released by the plastic flow of the substrate and probably result in the formation of the cracks. Another reason for the spallation of the oxide scale
is the mismatch of the TEC between the oxide scale and the substrate. The TEC of the TiO2 is about 8.2 106 (K1) [22], while that of the Ti–6Al–4V alloy is about 1012 106 (K1) [23]. Mismatch of the TEC between the scale and the alloy resulted in the formation of large compressive stresses in the oxide scale and then the spallation of the oxide scale during cooling, as shown in the Fig. 12a and c. Vanadium was detected by EDS in the oxide scale, while no oxide of V was shown in the corresponding XRD patterns. Considering that the ionic radii of the V4+ (0.63) is similar to that of Ti4+ (0.68), it is probable that the V4+ incorporate into the rutile lattice by replacing the Ti4+.
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Fig. 11. TEM image, electron diffraction patterns, and element mapping (Al, Ti, Si, O, K, and V) of the Ti5Si3/Al2O3/quartz interfaces of the coated Ti–6Al–4V alloy after 100 h oxidation at 900 °C.
Fig. 12. Optical images of the bare and coated Ti–6Al–4V alloy after 40 h (a and b) and 100 h (c and d) cyclic oxidation at 900 °C.
Fig. 13. cyclic oxidation kinetics of Ti–6Al–4V alloy with and without the composite coating at 900 °C. Each data point represents the average value of three specimens. The scatter band is the mean deviation.
Many thin discontinuous Al2O3 layers were observed in the oxide scale (Fig. 8a and b). Formation of TiO2 layer results in local enrichment of Al beneath the TiO2 and then leads to the formation of Al2O3 due to selective oxidation. However, the content of the Al is so low (6wt.%) that the growth of the Al2O3 layer is not able to be maintained. Therefore, alternant growth of TiO2 and Al2O3 will occur, and the alternant multilayer of thick TiO2 and discontinuous Al2O3 formed on the bare Ti–6Al–4V alloy. Beneath the oxide scale, a hardened layer in the alloy was detected by the microhardness tests (Fig. 17). The hardness of the oxygen containing titanium alloy depends upon the content of oxygen in the alloy. The higher the oxygen content is, the higher hardness the alloy possesses. In this study, the oxygen dissolution in the bare Ti–6Al–4V alloy was obvious. The oxygen content in the alloy decreased exponentially from the oxide/alloy interface to the center of the alloy, suggesting the oxygen dissolution is dominated by inward oxygen diffusion. The thickness of the oxygen dissolution layer increased with the increase in the temperature. The oxygen dissolution effect is detrimental to the mechanical performance of Ti-based alloys, especially to the alloy plasticity
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Fig. 14. Surface morphology (a), cross-sectional image (b), and elemental line-scan profile (analyzed by EDS) (c) of the SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy after 100 h cyclic oxidation at 900 °C.
Fig. 15. Thermal shock kinetic of the coated Ti–6Al–4V alloy at 900 °C.
[4]. The cracks due to the indentations shown in the Fig. 18 proved the oxygen dissolution lead to the formation of the brittle layer beneath the oxide scale.
4.2. Interfacial reaction between the composite coating and the Ti– 6Al–4V alloy Compared with the poor oxidation resistance of bare Ti–6Al–4V alloy, the Ti–6Al–4V alloy with SiO2–Al2O3–glass composite coating shows much better oxidation resistance. The composite coating acted as an oxygen migration barrier, not only reduced the high temperature oxidation rate of the alloy, but also decreased the oxygen dissolution in the alloy. Similar results have been reported [14,24– 26]. The short-term interfacial reaction between the glass and titanium alloys was extensively researched [27–32]. Because TiO2 is
Fig. 16. Surface morphology of the coated Ti–6Al–4V alloy after 50 cycles of thermal shock.
more thermodynamically stable than SiO2, the SiO2 in the glass is reduced by the Ti in the alloy during the firing process or oxidation. Formation of the Ti5Si3 at the glass/alloy interface was observed by Moskalewicz [10] and Oku [31]. Several titanium/glass coating interfacial reaction mechanisms have been proposed. Pazo [28] and Oku [31] proposed an interfacial reaction as follows:
5Ti þ 3SiO2 ðglassÞ ¼ Ti5 Si3 þ 3O2
ð1Þ
They believed that the released oxygen is the reason for the voids formed at the interface in their study, but the calculated standard free energy of the reaction (1) at 800 °C and 900 °C is about 1555 kJ/mol and 1501 kJ/mol (The standard free energy of this reaction and the following two reactions is calculated by for-
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Fig. 17. Microhardness depth profiles of Ti–6Al–4V alloy with and without composite coating after 100 h oxidation at 800 °C (a) and 900 °C (b). H H mula:DGH r ðTÞ ¼ DHr ð298 KÞ T DSr ð298 KÞ, the data are from the Table 1 [33]). So, this reaction could not spontaneously take place at test temperatures. In present study, the Ti5Si3 was observed in the as-prepared specimen and in the specimens after 100 h oxidation at 800 °C and 900 °C. The dissolution of the TiO2 into the glass was also observed. The probable reaction is as follows:
The formation of Ti3Al may have close relationship with the formation of Ti5Si3. Because of the low solubility of Al in Ti5Si3 phase (Fig. 8), Al is driven away from the Ti5Si3 layer, either formed Al2O3 at the coating/alloy interface or diffused into the deeper alloy. Once the content of Al in the alloy exceeded the solubility of Al in the Ti, according to the binary Ti–Al phase diagrams, the probable phase formed beneath the interfacial reaction is Ti3Al. The most interesting part of the interfacial reaction may be the formation of the thin Al2O3 rich layer at the coating/alloy interface after 100 h oxidation at 900 °C, which has not been reported in the previous literature. There may be two reasons for the formation of the thin Al2O3 layer: one is the local enrichment of Al due to the formation of the Ti5Si3 layer, and the other is the low oxygen partial pressure at the coating/Ti5Si3 interface; both promotes the selective oxidation of Al by Eq. (3). Li and Taniguchi [34] prepared a Ti5Si3/Al-rich bilayer on the Ti–48Al–1.3Fe–1.1V–0.3B (at.%) alloy and found that the Al-rich zone affected the oxidation of the Ti5Si3 layer. They reported that the Al-rich zone beneath the Ti5Si3 modified layer resulted in large amounts of Al2O3 after long-term oxidation and the final oxides of the sample were composed of TiO2, Al2O3 and amorphous SiO2, which is beneficial to the oxidation resistance of the alloy. In present study, the composite coating acts as an oxygen migration barrier, decreasing the oxygen partial pressure at the Ti5Si3/composite coating interface. Compared with the oxidation of the Ti5Si3/Al-rich bilayer in air, the low oxygen partial pressure at the interface promotes the selective oxidation of the Al and the formation of Al2O3. Large amounts of grain boundaries in the Ti5Si3 layer may benefit the outward diffusion of Al which is essential for the alumina growth. Litter [35] reported that a silica layer, which formed at the MoxRuySiz compounds/glass interface due to the selective oxidation of the Si in the compounds, dissolved into the alkali borosilicate glass quickly at 1350 °C. In this study, the dissolution of the newly formed Al2O3 layer was also observed at the Al2O3 layer/glass interface (Fig. 10). The growth of the Al2O3 layer competed with the dissolution of Al2O3 layer during the oxidation tests. Al was detected in the glass by EDS, but the main source of the Al in the glass is not the newly formed Al2O3 layer, but the Al2O3 inclusions in the composite coating, which we were discussed in the Section 4.3. The dissolution of the alumina inclusions in the glass may decrease the dissolution of the newly formed alumina layer. Only dispersed alumina particles were observed at the interface after oxidation at 800 °C (Fig. 6), while a continuous alumina layer formed at 900 °C (Fig. 8), may be attributed to the lower growth rate of alumina at 800 °C. 4.3. Microstructure change of the composite coating
8Ti þ 3SiO2 ¼ 3TiO2 þ Ti5 Si3
ð2Þ
The standard free energy of reaction (2) at 800 °C and 900 °C is 682 kJ/mol and 680 kJ/mol, respectively. This reaction was also proposed by Gomez-Vega [30], even the TiO2 was not observed in their study. Ti in the alloy diffused outward and reacted with the SiO2–K2O glass, leading to not only the dissolution of TiO2 in the glass but also the formation of the Ti5Si3 layer with some TiO2 dispersion at the coating/alloy interface, as shown in the Figs. 10 and 11. It is interesting that some Al2O3 dispersion was also observed in the Ti5Si3 layer, indicating that the following reaction may also occurred:
5Ti þ 4Al þ 3SiO2 ¼ Ti5 Si3 þ 2Al2 O3
ð3Þ
The standard free energy of reaction (3) at 800 °C and 900 °C is 1123 kJ/mol and 1115 kJ/mol, respectively. The grain size of the Ti5Si3 was in the range of 200–500 nm, probably because the dispersed TiO2 and Al2O3 in it restricted the growth of the grain.
Because the glass transition temperature of the glass in the composite coating is as low as about 495 °C [36], the glass softens and acts as a viscous liquid at 900 °C; therefore, it is not surprising that the reactions between glass and inclusions occurred during oxidation tests. Both EDS and EPMA analysis detected Al in the glass matrix. Compared with the as-prepared coating (Fig. 2), less Al2O3 inclusions were observed in the coating after 100 h oxidation at 900 °C (Fig 6d). All these results indicated that the Al2O3 inclusions partially dissolved into the glass matrix during oxidation tests. The Al2O3 inclusions, but not the newly formed Al2O3 layer, are the main source of the Al in the glass matrix because the Al2O3 inclusions/ glass interfaces are much larger than the Al2O3 layer/glass interface. The dissolution of the Al2O3 inclusions turned the binary silicate glass (K2O–SiO2) into a ternary glass (K2O–Al2O3–SiO2), which may improve the chemical stability of the glass [37]. Quartz particles were also instable in the glass. Phase transition of quartz to cristobalite was observed at 900 °C (Fig. 5). According to Fenner [38], with the existence of mineralizer, such as Na2O, K2O
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Fig. 18. Indentations on the cross-section of the bare Ti–6Al–4V alloy (a and c) and of the coated Ti–6Al–4V alloy (b and d) after 100 h oxidation at 800 °C and 900 °C.
Table 1 Standard enthalpies of formation and standard entropies for the relevant substances at 298 K. The data are from [33]. Compound
1 DHH i ð298 KÞ (kJ mol )
1 mol1) SH i ð298 KÞ (J K
Ti Al SiO2 Ti5Si3 TiO2 (rutile) a-Al2O3 O2
0 0 905.20 579.07 944.75 1675.27 0
30.65 28.32 46.28 217.99 50.33 50.94 205.04
and CaO, the quartz would transform into cristobalite or tridymite when the temperature is higher than 870 °C. The dissolution of the quartz particles may also occur during oxidation tests. Because the profile of the original quartz particles is angular, however, the profile of the quartz particles in the composite coating was smooth, and the quartz/glass interface was blurry. Similar results have been reported [39]. Circumferential cracks surrounding big remaining quartz particles were observed in the Fig. 6f. Gilabert [40] studied the fracture patterns of quartz particles in glass feldspar matrix and reported that rounded particles tend to generate circumferential flaws close to the matrix/quartz interface. In this study, the formation of the circumferential cracks may be attributed to the phase transition of b-quartz to a-quartz at around 573 °C during cooling. This phase transition is accompanied by relatively large volume shrinkage and completes in a short time, resulting in tensile stress in the radical direction in the glass phase surrounding the quartz inclusions. Once the tensile stress surpasses the tensile strength of the glass phase, circumferential cracks would form in the glass phase surrounding the quartz inclusions.
5. Conclusions From the above study, the following conclusions can be drawn:
(1) An oxidation resistant SiO2–Al2O3–glass composite coating was successfully prepared on the Ti–6Al–4V alloy by air spraying a slurry which was made from the aqueous solution of potassium silicate and the quartz and alumina powders, and subsequent firing. It remarkably improved the oxidation resistance of the Ti–6Al–4V alloy at 800 °C and 900 °C. The composite coating exhibited good thermal cycle resistance and thermal shock resistance on Ti–6Al–4V alloy. (2) A hardened layer formed beneath the oxide scale due to the oxygen dissolution in the bare Ti–6Al–4V alloy. However, the SiO2–Al2O3–glass composite coating remarkably decreased the oxygen dissolution in the Ti–6Al–4V alloy at 800 °C and 900 °C. (3) Interfacial reactions between composite coating and alloy occurred, forming a layered structure (Ti5Si3/Ti3Al) beneath the composite coating. A thin Al2O3 rich layer formed at the coating/Ti5Si3 interface at 900 °C. (4) There was dissolution of Al2O3 and quartz inclusions into the glass matrix, as well as the phase transformation of quartz into cristobalite during oxidation tests.
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