J. Coat. Technol. Res., 5 (1) 93–98, 2008 DOI 10.1007/s11998-007-9057-5

Effect of vitreous enamel coating on the oxidation behavior of Ti6Al4V and TiAl alloys at high temperatures Yuming Xiong, Shenglong Zhu, Fuhui Wang, Changhee Lee


FSCT and OCCA 2007 Abstract Vitreous enamel coating is a promising candidate as a high temperature protective coating for titanium (Ti)-based alloys due to its high thermochemical stability, compatibility, and matching thermal expansion coefficient to the substrates. Vitreous enamel coating is economically attractive because of its low cost and easy handling. The oxidation behavior of Ti6Al4V (at 700C) and Ti–48Al (at 800–900C), with and without the vitreous enamel coating exposed to air, are investigated in this article. The results show that the vitreous enamel coating could markedly protect the substrate (Ti6Al4V and Ti48Al) from oxidation at elevated temperatures. In comparison, the TiAlCr coating might not provide long-term protection for the Ti6Al4V alloys due to the heavy interfacial interdiffusion at high temperatures, although a protective Al2O3 scale could form at the initial oxidation stage. The vitreous enamel coating remains intact, uniform, compact, and adhesive to the substrate, however, with undetectable interfacial reaction after oxidation. It is also worth noting that some new phases form in the coating during oxidation at 900C, although the protectiveness of the coating seems to be unaffected. Keywords Ti6Al4V, TiAl, Enamel coating, Oxidation, Interfacial reaction Y. Xiong, S. Zhu, F. Wang (&) State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, Shengyang 110016, China e-mail: [email protected] Y. Xiong (&), C. Lee Kinetic Spray Coatings Laboratory, Division of Materials Science and Engineering, College of Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133791, South Korea e-mail: [email protected]

Introduction Since the introduction of titanium (Ti) and titanium alloys in the early 1950s, these materials have become the backbone materials for the aerospace, energy, and chemical industries due to their low density, high specific yield strength and stiffness, good oxidation resistance, and good creep properties in high temperatures. Among them, a + b Ti–6Al–4V alloy is known as the ‘‘workhorse’’ and has the most common applications by far. Some of its applications include aircraft turbine engine components, aircraft structural components, aerospace fasteners, high performance automotive parts, marine applications, medical devices, and sports equipment at low to moderate temperatures. As for TiAl intermetallics, they possess good high-temperature (700C) oxidation resistance and low cost (with the addition of aluminum [Al]) apart from the low density and high specific strength. In addition, their oxidation resistance and strength could be further improved by adding proper alloying elements. For all the Ti-based alloys and intermetallics, however, high oxygen sensitivity is a fatal shortcoming when they are used in an oxygen-containing environment at high temperatures. It is difficult for Ti-based alloys to form protective Al2O3 scale due to the similar oxygen affinity between Ti and Al when exposed to oxidizing environments. Meanwhile, oxygen dissolves into the alloys to form an oxygen-rich layer (oxygen embrittlement), which degrades the mechanical properties of the alloys.1 One promising way to overcome the above drawbacks of Ti-based alloys would be to deposit stable coatings to protect the substrate from long-term oxidation and oxygen embrittlement. There is an ongoing interest in the development of oxygen-barrier coatings, such as ceramic oxides (Al2O32,3, SiO24), MCrAlY5,6, aluminide7,8, and TiAlCr-based9–14 coatings. In recent years, however, a promising enamel

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coating was developed to protect Ti-based alloys from oxidation and corrosion due to its high thermochemical stability and matched thermal expansion coefficient (8.8–11 · 10–6/C) with the substrate.15–19 It is especially economically attractive for its applications due to its low cost and easy operation.20 In this article, the effect of enamel coating on the oxidation behavior of Ti6Al4V (at 700C) and TiAl (at 800–900C) alloys is investigated. For comparison purposes, the TiAlCr coating is also deposited on Ti6Al4V alloys by magnetron sputtering.

specimens were cooled in air to room temperature after continuous oxidation at experimental temperatures for every 10 or 20 h in a muffle furnace, and then weighed (the spalling scales included) using a balance (0.1 mg resolution) to characterize the oxidation kinetics. The surface morphologies and the oxide phases of the specimens after oxidation were characterized using scanning electron microscopy (SEM)— equipped by Energy Dispersive X-ray Spectroscopy (EDS)—and X-ray diffraction, and then mounted in epoxy resins, cross sectioned, polished, and coated with a thin layer of conductive carbon to reveal the crosssectional microstructures by SEM.

Experimental procedure Ti–48Al (at.%) intermetallic was produced by the fusion of high purity metals in an induction furnace with protective Ar2 atmosphere, following molding in a cylindrical mold. After the cast alloys were homogenized at 1000C for 1 h, they were subsequently water quenched to room temperature. The as-received Ti– 6Al–4V alloy is the commercial one. The alloy ingots were cut into 15 x 10 x 2.5 mm specimens by spark cutting and ground down to 600#-SiC sandpaper. The nominal composition of enamel frit was SiO2 58.2, Al2O3 6.3, ZrO2 5.3, ZnO 9.0, CaO 4.1, and others 17.0 (wt%). In proportion, the mixture of raw mineral materials was melted at 1450C for about 10 h, and then the molten enamel fusion was quenched in water to get frit. The enamel frit in acetone was ultrafine after being milled for about 300 h, with some additives to lower its vitrifying temperature. The ultrafine enamel frit layer formed on the round angle substrates of Ti-based alloys, which were first grit-blasted, using an airspraying technology at room temperature. The enamel coating was formed after vitrification for 30 min at 900C in air. The detailed preparation process of the enamel coating has been shown elsewhere.20 In comparison, the Ti–35.45Al–20.05Cr (at.%) coating was prepared with magnetron-sputtering technique using direct current power supplies at ambient temperatures with argon plasma.11 The thickness of the two coatings was 20–40 lm. The Ti–6Al–4V (at 700C) and TiAl (at 800–900C) alloys, with and without enamel coating were oxidized discontinuously in open air for up to 100 h. The

Results and discussion Microstructures of as-vitrified enamel coatings The enamel frit size ranges from several microns to 300 nm after milling for 300 h. Its vitrifying temperature (900C) is about 150C lower than that of the conventional frit (1050C, 20 lm), as shown elsewhere.16 Figure 1 shows the cross-sectional microstructures of as-vitrified enamel coatings on Ti6Al4V and Ti48Al alloys. In comparison with conventional coatings,15 the compactness and uniformity of enamel coatings are improved by using the ultrafine instead of the conventional frit. The coating shows good bonding to the substrates. It consists of gray vitreous matrix and white particles, which are evenly precipitated in the matrix and enriched in ZrO2 and Al2O3. Oxidation of Ti6Al4V at 700C Figure 2 shows the oxidation kinetics of Ti6Al4V alloys with and without coatings (TiAlCr and enamel) at 700C exposed to air. The oxidation kinetics of bare Ti6Al4V alloy shows the approximately linear law. Both TiAlCr and enamel coatings could protect the alloys from oxidation at 700C with slight oxidation mass gain. The alloy with TiAlCr coating shows undetectable mass gain after initially slight weight increase, while the enamel-coated alloy shows stable

Fig. 1: The cross-sectional microstructures of as-vitrified enamel coatings on (a) Ti6Al4V alloy and (b) TiAl intermetallics

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Ti6Al4V+TiAlCr Ti6Al4V Ti6Al4V+Enamel

8

2

Mass Gain (mg/cm )

7 6 5 4 3 2 1 0 0

20

40

60

80

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Oxidation Time (h) Fig. 2: The oxidation kinetics of Ti6Al4V, with and without coatings at 700C exposed to air

weight increase with extremely gentle slope. Figure 3 shows the cross-sectional microstructures of Ti6Al4V alloys with or without coatings after oxidation 100 h at 700C in air. After oxidation for 100 h at 700C, the bare Ti6Al4V alloy suffered from heavy attack with thick and spalling oxide scales that are primarily composed of TiO2 by XRD and EDS analysis. The bare alloys show breakaway oxidation features. Both TiAlCr and enamel coating, however, could significantly protect Ti6Al4V alloy from oxidation at 700C exposed to air, acting as oxygen barriers on the substrate. XRD and EDS results show Al2O3 scale forms on TiAlCr coating during oxidation at 700C. Furthermore, the formation of pinning oxides along the grain boundary of typical TiAlCr columnar crys-

talline improves the adhesion of the external oxide scales (as shown in Fig. 3b). The compact and continuous Al2O3 scale could protect the substrate from oxidation due to the extremely low oxygen diffusion rate in it (hardly detectable mass gain in oxidation kinetics as shown in Fig. 2). Nevertheless, the severe interdiffusion at the interface of TiAlCr/Ti6Al4V (see Fig. 3b, d) would be substantial because the coating would degrade rapidly due to the formation of an Al depletion sublayer under the protective Al2O3 scale. Meanwhile, it also has a detrimental effect on the mechanical properties of Ti6Al4V alloy substrate due to the inward diffusion of alloying elements in the coating. The composition of A and B regions in Fig. 3d is 27Al–73Ti and 11Al–83Ti–6V (at.%), respectively, in terms of EDS analysis. It could therefore be assumed that TiAlCr coating would fail during long-term oxidation due to the heavy interfacial interdiffusion. In contrast to TiAlCr coating, vitreous enamel coating seems to be a good candidate for the protection against long-term oxidation of Ti6Al4V alloys. There is no detectable interfacial reaction and interdiffusion of components at the interface of enamel/Ti6Al4V after oxidation for 100 h at 700C. The oxidation mass gain is primarily dependent on the inward diffusion of oxygen through the compact enamel coating with constant thickness. Accordingly, the oxidation kinetics approximately follows linear law with extremely gentle slope as mentioned above (see Fig. 2). In addition, the interface between enamel coating and the Ti6Al4V substrate stays intact after oxidation, benefiting from the matched thermal expansion coefficient and high compactness and uniformity of the enamel coating. No variation occurs for the enamel coating after oxidation in comparison with the as-vitrified one (see Fig. 1a).

Fig. 3: The cross-sectional microstructures of Ti6Al4V alloys (a) without and (b) with TiAlCr, and (c) enamel coatings after oxidation for 100 h at 700C, as well as the interfacial close-up (d, schematically marked in b)

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Oxidation of TiAl at 800–900C Figure 4 shows the oxidation kinetics of TiAl with or without enamel coating at 800 and 900C in air, respectively. The oxidation kinetics of bare Ti–48Al intermetallics follows linear law at both temperatures. The enamel-coated alloys, however, show much lower mass gain than the correlative bare alloys. Since oxidation behaviors of TiAl with TiAlCr coatings were conducted by many authors9–14 they will be not repeated in this article. Figure 5 shows the cross-sectional microstructures of Ti–48Al alloys with or without enamel coating after oxidation for 100 h at 800 and 900C, respectively. TiAl intermetallics suffer from heavy oxidation attack at both temperatures. The porous and spalling mixture oxides of TiO2 and Al2O3 form on the substrate during oxidation. At the initial oxidation stage, a porous TiO2 scale forms because TiAl

a

5

TiAl TiAl+Enamel

2

Mass gain ( mg/cm )

4

3

2

1

0

0

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40

60

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100

80

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Oxidation time (h)

b

12

TiAl TiAl+Enamel

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Mass gain ( mg/cm )

10

8

6

4

2

0 0

20

40

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Oxidation time (h) Fig. 4: The oxidation kinetics of TiAl intermetallics with or without enamel coating at (a) 800C and (b) 900C exposed to air, respectively

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contains lower Al content than the critical value of 67 (at.%) for the formation of continuously protective Al2O3 scale.21 Subsequently, the Al oxides begin to form due to the oxidation consumption of Ti with the rapid diffusion of oxygen across the porous TiO2 scale. Of course, it is impossible to form a continuous Al2O3 scale under the TiO2 scale due to the similar oxygen affinity between Al and Ti, and the high oxygen sensitivity of Ti. Accordingly, the Ti and Al oxides form by turns or/and synchronously. As shown in Fig. 5a and b, the I region (2 in Fig. 5b) is the TiO2 enriched oxides, while the II region (1 in Fig. 5b) is the Al2O3-enriched oxides. The brittle oxides scales would therefore be broken from the substrate under thermal stress. It is worth noting that relatively thick oxygen-enriched layers (2 and 3 in Fig. 5b) form during oxidation. Enamel coating could markedly protect TiAl intermetallics from oxidation attack at 800C and even 900C, which is close to the vitrifying temperature (950C). Other than the cases of niobium (Nb)containing TiAl intermetallics in our previous work,17 the enamel coating remains intact and adhesive to Ti48Al and Ti6Al4V substrates after oxidation, although there exists slight interfacial reaction or interdiffusion at the interface of the coating/substrate. At the interface of enamel/TiAl (see Fig. 5a, b), interfacial Ti-enriched (at the substrate side) and Alenriched (at the coating side) layers are formed. It indicates that the selective oxidation of Al tends to occur at the interfacial low-oxygen partial environment. Its oxidation rate, however, is extremely slow. The interfacial reaction becomes severe with the increase of the oxidation temperature. EDS results show that the ratio of Ti, Al, and silicon (Si) at A, B, and C is 2:3:26, 51:7:26, and 2:1:0, respectively. In spite of the formation of interfacial mixture oxides of Ti–Al–Si, there is not a continuous Al2O3 scale. Previous work17 has shown that the doping of additional Nb could improve the formation of protective Al2O3 scale on TiAl alloys at high temperatures. The formation of interfacial Al2O3 scale by the active effect of a small Nb addition is therefore detrimental to the adhesion of enamel coating to the substrate of TiAl–5Nb. The enamel-coated TiAlNb alloys showed inferior oxidation resistance than the bare alloys due to the spallation of coating during discontinuous oxidation. It is worth noting, however, that some new phases in the enamel coating form during oxidation at elevated temperatures. Comparing with the as-vitrified coating (see Fig. 1b), the new arisen white phases seem to result from the phase transformation of the vitreous matrix of enamel coating at high temperatures. The effect of the occurrence of new phases in enamel coating during oxidation needs further investigation, even though the enamel coating remained compact and adhesive after oxidation. The protectiveness of enamel coating seems to be unaffected by the formation of new phases.

J. Coat. Technol. Res., 5 (1) 93–98, 2008

Fig. 5: The cross-sectional microstructures of Ti48Al alloys without or with enamel coating after oxidation for 100 h at (a, b) 800C and (c, d) 900C, respectively

Conclusions In this article, the results showed that the enamel coating could markedly protect Ti6Al4V alloys and Ti48Al intermetallics from oxidation at high temperatures. The enamel coating remains intact and adhesive to the substrate after oxidation, although there exists a slight interfacial reaction at the interface of TiAl/ enamel. In comparison with the heavy interfacial interdiffusion at the interface of Ti6Al4V/TiAlCrcoating, the enamel coating might be a good candidate for protecting Ti6Al4V alloy from long-term oxidation due to undetectable interfacial reaction during oxidation. In addition, the protectiveness of the enamel coating to TiAl intermetallics has been unaffected in spite of the formation of some new phases in the coating during oxidation at 900C. Acknowledgments This project was supported by the NSFC for Outstanding Young Scientists, the National High-Tech Research and Development Program of China, and a Korean Science & Engineering Foundation (KOSEF) grant funded by the Korean Government (MOST) (No. 2006-02289).

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