alumina composite fibers and YAG fiber by sol–gel method

Composites: Part A 32 (2001) 1127±1131

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Preparation of polycrystalline YAG/alumina composite ®bers and YAG ®ber by sol±gel method A. Towata a,*, H.J. Hwang a, M. Yasuoka a, M. Sando a, K. Niihara b a

b

National Industrial Research Institute of Nagoya, 462-8510, 1-1 Hirate-cho, Kita-ku, Nagoya, Japan The Institute of Scienti®c and Industrial Research, Osaka University, 567-0047, 8-1 Mihogaoka, Ibaraki, Osaka, Japan Received 26 May 2000; revised 28 November 2000; accepted 28 November 2000

Abstract Polycrystalline yttrium±aluminum garnet, Y3Al5O12 (YAG) ®ber and a-alumina and YAG matrix composite ®ber were prepared by the sol±gel method. a-Alumina and YAG matrix composite ®ber with ®ne and homogeneous microstructure could be successfully fabricated by interpenetrating YAG in alumina matrix and adding a-alumina of seed particles to ®bers. Effect of a-alumina seed particles and YAG on crystallization and microstructure of composite ®ber were discussed. The size of alumina matrix of the composite ®bers heated at 16008C for 4 h was below 2 mm. The tensile of strength alumina ®ber heat-treated at 15008C was 0.2 GPa, while that of the composite ®ber was 1.1 GPa. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic ®bre; Alumina; E. Powder processing; D. Thermal analysis

1. Introduction The ceramic matrix composite has an intended use at high temperatures in several applications including advanced aircraft and gas turbine engine. Fibers used in these continuous ®ber-reinforced ceramic composites are required to exhibit good thermal and mechanical properties in high temperature oxidizing environments. Oxide ®bers have the potential to maintain durability and mechanical properties in oxidizing environments. However, the applications of commercially available oxide ®bers are limited by the degradation of fracture strength and creep properties above 12008C. For example, Al2O3 ®bers derived by sol± gel process, which have been commercially used for a number of years, contained a mixture of alumina spinel and amorphous silica, resulting in poor creep resistance [1]. Thus, recent research work has been focused on developing ®ber materials with inherently better hightemperature properties. The creep resistance of polycrystalline ceramics can be improved by the addition of hard second-phase particles [2±3]. The second-phase particles can affect the deformation rate of a polycrystal in two ways [4]. One is by the macroscopic modi®cation of the continuum deformation mechanics, and the other is the microscopic change of the * Corresponding author. Tel.: 181-52-911-2111; fax: 181-52-916-2802. E-mail address: [email protected] (A. Towata).

interface-related deformation characteristics such as suppressing interface reactions (vacancy or interstitial creation/annihilation) or hindering grain boundary sliding. In addition, the second-phase particles may affect ®ber grain growth, microstructure, and therefore creep rate by grain boundary pinning. YAG (yttrium±aluminum garnet, Y3Al5O12) is a prime candidate for hardening Al2O3 because of its excellent creep resistance [5,6]. Parthasarathy et al. [7] investigated the creep behavior of hot-pressed polycrystalline YAG with the grain size of 3 mm and found that it indicated a lower creep rate than that calculated for polycrystalline Al2O3 with a similar grain size. Directionally solidi®ed Al2O3 ± Y3Al5O12 eutectic composites have also been shown to exhibit better high-temperature properties. This paper describes new fabrication process of Al2O3 and YAG composite ®ber and YAG ®ber. Transformation behavior and microstructure development of Al2O3 based composite ®ber were studied. The effect of a-Al2O3 and YAG seed particles and mechanical properties of these ®bers were also examined. 2. Experimental procedure YAG and alumina composite ®ber and YAG ®ber were prepared. The composite ®bers had 0.5 and 40 vol% yttrium content. The aluminum isopropoxide (Nakarai Chemical

1359-835X/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 1359-835 X(01)00 014-8

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Co., Japan) and yttrium isopropoxide (High Purity Chemical Co., Japan) were dissolved in isopropyl alcohol solution with ethyl 3-oxobutanate by re¯uxing under air atmosphere at 608C for 20 h according to the procedure described by Yogo et al. [8]. In the case of ®ber with seed particles, a-alumina (average particle size: 0.2 m, Taimei Chemical Co., Japan) was added to the solution. The solution was hydrolyzed by adding hydrochloric acid and water, and was condensed by re¯uxing at 608C for one day. In order to control the viscosity of the precursor, the isopropyl alcohol involved in the solution was concentrated by evaporation. Viscosity of the solution increased gradually. The solution was then spun into the precursor ®ber by inserting a wire into the viscous solution and pulling it upward quickly by hand under 45% relative humidity. The resulting precursor ®bers were aged and dried at room temperature for one day and with 30±60% relative humidity. Then they were put in an alumina boat and heat-treated in air at various temperatures. Heat treatment was carried out at temperatures ranging from 1200 to 15008C with a heating rate of 108C min 21. The holding time at the desired temperature was ®xed at 4 h. After designed heat treatment the ®bers were cooled in the furnace. The crystalline phase of the composite ®ber was identi®ed by X-ray powder diffraction. The viscosity was measured by a cone viscometer. The crystalline phase of the composite ®ber was identi®ed by X-ray powder diffraction (RU-200B, Rigaku Co. Ltd, Japan). The crystallization temperature of the ®ber was investigated by DSC (Shikuu-rikou, Japan). The ®ber morphology and microstructure were observed by a scanning electron microscope (SEM, JSM-6320FK, JEOL, Japan). The ratio of Y/Al of the ®ber was measured by inductively coupled plasma-emission spectrometry (ICPES, ICAP-1000S, NIPPON Jarrell-Ash, Japan). The tensile strength of single ®bers was measured using an universal mechanical testing machine (RTC-1210A, A &D Co.). Fibers were mounted with acrylic adhesive on cardboard tabs for aligning and gripping. A 5-mm gage length and a crosshead speed of 2 mm min 21 were used in all tests. Owing to the non-circular cross-section of the ®bers, the fracture load was converted to tensile strength by measuring the cross-sectional area of the ®ber with an optical microscope.

Fig. 1. Change of viscosity for precursor of solution as a function of rotating speed.

transformation temperature shifted to a higher temperature. In the case of 9.7 vol% YAG content, the transformation temperature was 11708C. These results indicate that yttrium addition plays a important role on the crystallization behavior of Al2O3 ®bers. The transformation to a-phase of the composite ®ber shifted to higher temperature than for Al2O3 ®ber. This tendency was also found in the unseeded ®bers heat-treated at the same temperature. It can, therefore, be inferred that yttrium inhibits the a-Al2O3 crystallization. It was probable that another peak of in this ®gure showed some yttrium oxide formation. Crystallization products with a-Al2O3, YAP (yttrium± aluminum perovskite, YalO3) and YAG composition were observed to follow multi-step transformations to the fully crystalline state. Crystallization of YAG appears to involve the formation of hexagonal and orthorhombic YAP as an intermediate as shown in the temperature vs. time diagram determined using X-ray diffraction analysis (Fig. 3). Crystallization of YAG depends on the yttrium content and

3. Results and discussion Fig. 1 shows the viscosity of polymeric precursor solution as a function of rotating speed. The precursor solution is suf®ciently spinnable. The viscosity was 40 Pa s 21 at 2 rpm and decreased with increasing rotating speed. Fig. 2 shows DSC curves of the composite ®bers. The transformation temperature from u- to a-phase alumina in the case of the composite ®ber with seed particles of 1.5 wt% was 10408C. As the YAG content increased, the

Fig. 2. DSC of the seeded (1.5 wt%) composite ®bers with 0±9.7 vol% YAG. Samples were heated at the speed of 108C min 21 in air.

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Fig. 5. SEM photograph of alumina/YAG composite ®ber (YAG: 40 vol%) heated at 16008C.

heat-treatment temperature. In the case of 1.0 and 2.1 vol% of YAG, orthorhombic YAP appeared at 13008C. With the increasing temperature YAP completely transformed to YAG at 15008C, and the composite ®ber consisted of a-Al2O3 and YAG at 15008C. When the YAG content was increased up to 2.4 vol%, the crystallization of YAG shifted to a lower temperature (13008C). However, the same

temperature (15008C) was required to obtain the composite ®ber with a-Al2O3 and YAG. The composite ®ber with yttrium more than 7 vol% showed a different transformation behavior. Hexagonal YAP, which was not observed in 1.0, 2.1 and 3.5 vol% of YAG, formed at 11008C and it disappeared above 12008C. Crystallization of YAG was signi®cantly accelerated with an increasing yttrium content. For example, heat treatment at 13008C of the precursor ®ber with 9.7 vol% of YAG resulted in a-Al2O3 composite ®ber which dispersed only with YAG. In the case of 40 vol% of YAG, YAP appeared at 10008C. Fig. 4 shows the X-ray diffraction patterns of alumina/ 40 vol% YAG composite ®ber heat-treated at 1000, 1100, 1200, 1300, 1400 and 15008C. It seemed that yttrium reacted with alumina as the heat-treatment temperature

Fig. 4. X-ray diffraction pattern of Al2O3/YAG composite ®bers (YAG: 40 vol%) heat-treated at various temperature for 4 h.

Fig. 6. X-ray diffraction pattern of YAG ®bers heat-treated at various temperature for 4 h.

Fig. 3. Crystallization behavior of the composite ®bers with different YAG content.

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Fig. 7. SEM photograph of YAG ®ber heated at 16008C.

increased, to produce an hexagonal YAP above 10008C. At the same time, YAG appeared at 11008C. With increasing heat-treatment temperature, the relative intensity of YAG increased, and YAP completely disappeared at 13008C. The composite ®ber consisted of a-alumina and YAG at 13008C. Fig. 5 shows the SEM photograph for thermally etched surface of the alumina/40 vol% YAG composite ®ber heat-treated at 16008C for 4 h. The composite ®ber consisted with the 1 mm, alumina and YAG grains were intertwined. Fig. 6 shows the X-ray diffraction patterns of YAG ®ber heat-treated at 700, 800 and 9008C. An amorphous YAG ®ber was still observed at 7008C. The crystallization of YAG started at 8008C, and well-crystallized YAG ®ber was obtained at as low as 9008C. A SEM micrograph of the seeded YAG ®ber heated at 16008C is shown in Fig. 7. The grain size was 1±3 mm.

Fig. 9. Tensile strength of ®bers measured at room temperature.

Fig. 8 shows DSC curves of the YAG ®bers including with and without YAG seed particles. The seeded ®ber had two peaks between 700 and 9008C, whereas the unseeded ®ber had only one. The ®rst peak of the seeded ®ber at 7408C seems to result from the crystallization of hexagonal YAP. The second peak at 8508C is due to the transformation from hexagonal YAP phase to YAG phase [9]. The peak of the seeded ®ber appeared at lower temperature than the unseeded YAG ®ber. Fig. 9 shows the tensile strength measured at room temperature of the alumina ®ber and composite ®ber ®red at 1400, 1500 and 16008C for 4 h. The tensile strength of alumina ®ber is 0.2 GPa, while that of composite ®ber is 1.1 GPa. It can, therefore, be concluded that composite ®ber is stronger than alumina ®ber. This result may be due to the difference of alumina size. The grain sizes of alumina and the composite ®ber are 3±5, and 1 mm, respectively. The tensile strength of the YAG-rich composite ®ber and YAG are also shown in Fig. 9. YAG ®ber was 0.1 GPa at 16008C. The strength of composite ®ber decreased with increasing YAG content, and may also be because of the change in the alumina grain size. 4. Conclusions

Fig. 8. DSC curve of YAG ®bers heated at the speed of 108C min 21 in air.

Polycrystalline alumina, alumina/YAG composite ®bers and YAG ®ber were successfully fabricated by sol±gel process containing a-alumina or YAG seed particles. The a-alumina seed particle accelerated the phase transformation from u- to a-phase. The YAG seed particle affected multi-step of the yttrium and aluminum oxide. The composite ®bers with YAG (YAG ˆ 40 vol%) consisted of alumina and YAG grains with size of 1±2 mm at 16008C. The tensile strength of alumina ®ber heat-treated at 15008C was 0.2 GPa, and the strength was signi®cantly improved in the composite ®ber with 10 vol% of YAG.

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