JAm. Ceru. Soc,871259-126402004) urna High-Temperature Fatigue Strength of Crack-Healed Al2O3 Toughened by Sic Whiskers Sang-Kee Lee, Koji Takahashi, *,T,s Masahiro Yokouchi,*, f Hiroyoshi Suenaga, and Kotoji Ando Department of Energy and Safety Engineering, Yokohama National University, 79-5, Hodogaya, Yokohama, 240-8501, Japan Kokan Keisoku KK, Kawasaki-ku, Kawasaki, 210-0855, Japan Al,O3 reinforced by SiC whiskers(Al O,SiC-w)was hot- size that can be completely healed, the high-temperature pressed to investigate the fatigue strength of crack-healed strength of crack-healed zones, o,>- assessment of the cyclic specimens at high temperature Semielliptical surface cracks of and static fatigue strengths of crack-healed ceramic compo- 100 um surface length were introduced on each specimen nents,- and crack-healing behavior under static or cyclic urface. These specimens were crack-healed at 1300C for 1 h stress.2 It has been shown that Si, N, /SiC and mullite/SiC in air, and static and cyclie fatigue strengths were systemati- have excellent crack-healing abilities. It was also reported that cally investigated at room temperature, 900 and 1100oc by three-point bending. The static and cyclic fatigue limits of the ability similar to Si,N /SiC8,22 and mullite/SiC.23,2 crack-healed specimens were more than 70% of the average However, higher fracture toughness is desirable for the struc- ending strength at each testing temperature. Crack-heales tural integrity of Al2O, ceramics. It is well known that whisker specimens of Al2O/SiC-w were not sensitive to statie and reinforcement is very effective for increasing the fracture tough cyclic fatigue at room temperature and high temperatures. ness of structural ceramics. -If whisker-reinforced Al,, could Therefore the combination of crack-healing and whisker express excellent crack-healing ability, it would be desirable for reinforcement can play an important role in increasing static the structural integrity of Al2O, components. In a previous and cyclic fatigue strengths at high temperature. tudy, we sintered Al,O, reinforced by SiC whiskers(AlOy/ Sic-W) and studied basic crack-healing behaviors, We found that the combination of crack-healing and whisker reinforcement is . Introduction very useful for inducing self-crack-healing ability and increasing A LUMINA(Al2,)is a very popular ceramic used in various monotonic strength and fracture toughness fields. However, it has three weak points: low bendin It is essential to determine fatigue behavior under the appropri- strength(400 MPa), low fracture toughness(3 MPam),and ate loading, such as static, dynamic, or cyclic loadings indispens- a low heat-resistance limit(-900oC)for bending strength. These weaknesses limit the application of Al,O, for important compo- tructural purposes. A considerable number of studies have been nents. These weaknesses can be overcome in the following two conducted with regard to the fatigue behavior of engineering ways:(1)by toughening Al, O, with fibers or whiskers, and (2) by ramics 30-35 Few critical studies have been conducted regarding inducing a self-crack-healing ability static and cyclic fatigue strength of alumina ceramics at high Niihara proposed a new concept related to"nanocomposite temperatures, although activity in this field has recently been ceramics.Al,O3/SiC nanocomposite sintered by Niihara et al. ncreasing. In a previous study, we examined the high- temperature strength of Al,O /SiC-W and found that the lim showed excellent bending strength, fracture toughness, and heat- temperature for the bending strength of crack-healed specimen resistance limit for bending strength. Many studies have reinforcement and many useful results have been reported. -ger conducted with the aim of toughening Al,O, by fiber or whisker was 1100C. However, the fatigue strength of the crack-healed specimen was not studied. This paper focuses on the fatigue However, the fracture toughness of Al,O3 is not high, thus behaviors of crack-healed Al,O SiC-w composite ceramics at Al2O3 is very sensitive to cracking and is less reliable than metal. room temperature and high temperatures It is well known that monolithic Al,O3 and Al,O3 reinforced by SiC particles have an interesting crack-healing ability. -4If this ability is used in structural components for engineering uses, great Il. Materials, Specimens, and Test Method benefits can be anticipated, such as increased reliability of the The alumina powder used in this investigation was AKP-20 structural ceramic components and reduced inspection, machining, mean particle size 0.4-0.6 um, purity 99.99%)from Sumitomo nd polishing costs of such compor lowever, before this Chemical (Tokyo, Japan). The SiC whiskers used were SCW No healing ability is realized in structural ceramics, many problem 1-0.8 (length 30-100 um, diameter 0.8-1.0 Hm)from Tateho must be resolved. These include the crack-healing behavior as a Chemical Industries(Ako, Japan). The quantity of Sic whiskers function of healing temperature and time, the effects of ior. 3-I8 the maximum crack added was 20 vol% relative to AL, O, powder. Isopropyl alcohol nt on crack-healing behavi was added to the mixture and the mixture was blended completely for 12 h using alumina balls and a mill pot. Thereafter, the mixture was placed in an evaporator to extract the solvent, and then in a T: Rouxel-contributing editor vacuum desiccator to produce a dry powder mixture. Rectangular plates with dimensions of 9 mm X 50 mm X 50 mm were hot-pressed at 1850.C and 40 MPa for I h in an argon environ ment. The value of fracture toughness(Kic) of the test material ript No. 10275. Received June 25, 2003, approved March 16, 2004. was Kic =5.7 MPam". The density of the test material measured by the Archimedes technique was 3.83 g/cm, which was 99.9% of the theoretical density of the material. The average grain size of to whom correspondence should be addressed. e-mail: kaka @ynu ac jp Al,, was 1-2 um, and most of the Sic whiskers were located in 59High-Temperature Fatigue Strength of Crack-Healed Al2O3 Toughened by SiC Whiskers Sang-Kee Lee,† Koji Takahashi,* ,†,§ Masahiro Yokouchi,* ,† Hiroyoshi Suenaga,‡ and Kotoji Ando* ,† Department of Energy and Safety Engineering, Yokohama National University, 79-5, Hodogaya, Yokohama, 240-8501, Japan Kokan Keisoku K. K., Kawasaki-ku, Kawasaki, 210-0855, Japan Al2O3 reinforced by SiC whiskers (Al2O3/SiC-W) was hot￾pressed to investigate the fatigue strength of crack-healed specimens at high temperature. Semielliptical surface cracks of 100
m surface length were introduced on each specimen surface. These specimens were crack-healed at 1300°C for 1 h in air, and static and cyclic fatigue strengths were systemati￾cally investigated at room temperature, 900° and 1100°C by three-point bending. The static and cyclic fatigue limits of the crack-healed specimens were more than 70% of the average bending strength at each testing temperature. Crack-healed specimens of Al2O3/SiC-W were not sensitive to static and cyclic fatigue at room temperature and high temperatures. Therefore, the combination of crack-healing and whisker reinforcement can play an important role in increasing static and cyclic fatigue strengths at high temperature. I. Introduction ALUMINA (Al2O3) is a very popular ceramic used in various fields. However, it has three weak points: low bending strength (
400 MPa), low fracture toughness (
3 MPa
m1/2), and a low heat-resistance limit (
900°C) for bending strength. These weaknesses limit the application of Al2O3 for important compo￾nents. These weaknesses can be overcome in the following two ways: (1) by toughening Al2O3 with fibers or whiskers, and (2) by inducing a self-crack-healing ability. Niihara proposed a new concept related to “nanocomposite” ceramics.1,2 Al2O3/SiC nanocomposite sintered by Niihara et al. showed excellent bending strength, fracture toughness, and heat￾resistance limit for bending strength. Many studies have been conducted with the aim of toughening Al2O3 by fiber or whisker reinforcement and many useful results have been reported.3–8 However, the fracture toughness of Al2O3 is not high; thus Al2O3 is very sensitive to cracking and is less reliable than metal. It is well known that monolithic Al2O3 and Al2O3 reinforced by SiC particles have an interesting crack-healing ability.9–14 If this ability is used in structural components for engineering uses, great benefits can be anticipated, such as increased reliability of the structural ceramic components and reduced inspection, machining, and polishing costs of such components. However, before this healing ability is realized in structural ceramics, many problems must be resolved. These include the crack-healing behavior as a function of healing temperature and time,15,16 the effects of environment on crack-healing behavior,13–18 the maximum crack size that can be completely healed,19–21 the high-temperature strength of crack-healed zones,15,16,19–23 assessment of the cyclic and static fatigue strengths of crack-healed ceramic compo￾nents,21–28 and crack-healing behavior under static or cyclic stress.21,26–28 It has been shown that Si3N4/SiC and mullite/SiC have excellent crack-healing abilities. It was also reported that Al2O3 14 reinforced by SiC particles had excellent crack-healing ability similar to Si3N4/SiC18,22 and mullite/SiC.23,25–27 However, higher fracture toughness is desirable for the struc￾tural integrity of Al2O3 ceramics. It is well known that whisker reinforcement is very effective for increasing the fracture tough￾ness of structural ceramics.3–7 If whisker-reinforced Al2O3 could express excellent crack-healing ability, it would be desirable for the structural integrity of Al2O3 components. In a previous study,29 we sintered Al2O3 reinforced by SiC whiskers (Al2O3/ SiC-W) and studied basic crack-healing behaviors. We found that the combination of crack-healing and whisker reinforcement is very useful for inducing self-crack-healing ability and increasing monotonic strength and fracture toughness. It is essential to determine fatigue behavior under the appropri￾ate loading, such as static, dynamic, or cyclic loadings indispens￾able in the engineering application of ceramic materials for structural purposes. A considerable number of studies have been conducted with regard to the fatigue behavior of engineering ceramics.30–35 Few critical studies have been conducted regarding static and cyclic fatigue strength of alumina ceramics at high temperatures, although activity in this field has recently been increasing.30,34 In a previous study,29 we examined the high￾temperature strength of Al2O3/SiC-W and found that the limit temperature for the bending strength of crack-healed specimens was 1100°C. However, the fatigue strength of the crack-healed specimen was not studied. This paper focuses on the fatigue behaviors of crack-healed Al2O3/SiC-W composite ceramics at room temperature and high temperatures. II. Materials, Specimens, and Test Method The alumina powder used in this investigation was AKP-20 (mean particle size 0.4–0.6 m, purity 99.99%) from Sumitomo Chemical (Tokyo, Japan). The SiC whiskers used were SCW No. 1-0.8 (length 30–100 m, diameter 0.8–1.0 m) from Tateho Chemical Industries (Ako, Japan). The quantity of SiC whiskers added was 20 vol% relative to Al2O3 powder. Isopropyl alcohol was added to the mixture and the mixture was blended completely for 12 h using alumina balls and a mill pot. Thereafter, the mixture was placed in an evaporator to extract the solvent, and then in a vacuum desiccator to produce a dry powder mixture. Rectangular plates with dimensions of 9 mm 50 mm 50 mm were hot-pressed at 1850°C and 40 MPa for 1 h in an argon environ￾ment. The value of fracture toughness (KIC) of the test material was KIC  5.7 MPa
m1/2. The density of the test material measured by the Archimedes technique was 3.83 g/cm3 , which was 99.9% of the theoretical density of the material. The average grain size of Al2O3 was 1–2 m, and most of the SiC whiskers were located in T. Rouxel—contributing editor Manuscript No. 10275. Received June 25, 2003; approved March 16, 2004. *Member, American Ceramic Society. † Department of Energy and Safety Engineering, Yokohama National University. ‡ Kokan Keisoku K. K. § Author to whom correspondence should be addressed. e-mail: ktaka@ynu.ac.jp. J. Am. Ceram. Soc., 87 [7] 1259–1264 (2004) 1259 journal