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JAm. Ceram soc,86[12]2143-4702003) ournal Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers Koji Takahashi, * Masahiro Yokouchi, Sang-Kee Lee, t and Kotoji Ando*t Department of Energy and Safety Engineering, Yokohama National University, 79-5, Hodogaya, Yokohama 240-8501, Japan Al,O3 reinforced by Sic whiskers (Al,O3/SiC-w) was hot- crack-healin Thus, higher fracture toughness is desirable for pressed to investigate the crack-healing behavior. Semiellipti the structural integrity of ceramics. It is well-known that whisker cal surface cracks of 100 um in surface length were introduced reinforcement is very effective for increasing the fracture tough using a Vickers indenter. The specimens containing precracks ness of structural ceramics. 4-6 Thus, if whisker-reinforced Al-O were crack-healed at temperatures between 1000 and 1300oC possesses excellent crack-healing ability, it would be desirable for for 1 h in air, and their strengths were measured by three the structural integrity of Al2O3 components. However, the cracl oint bending tests at room temperature and elevated temper healing behavior of whisker-reinforced Al,O, has not yet been atures between 400 and 1300%C. The results show that tudied. Thus, Al,O3 reinforced by SiC whiskers(Al,O, SiC-W) Al2OSiC-w possesses considerable crack-healing ability. The has been hot-pressed, and its crack-healing behavior has been surface cracks with length of 2c 100 um could be healed by investigated systematically in this stud crack-healing at 1200 or 1300C for 1 h in air. Fracture The following research objectives were chosen for the oughness of the material was also determined. As expected, study: (1)to hot-press Al,O3 reinforced by SiC whiskers the SiC whiskers made their AlO, tougher. Sic-W);(2)to investigate the crack-healing behavior of Al, O, Sic-W as a function of crack-healing temperature; (3)to determine . Introduction the effect of the test temperature on gn of a crack-healed member The fracture toughness of the material was also determined LUMINA(Al2O,)is a very popular ceramic that is used in various fields. However, it has three weak points: low bending strength -400 MPa), low fracture toughness (3-4 MPam), and low I. Experimental Procedure heat-resistance limit(-900C). These weaknesses restrict the appli The alumina powder used in this investigation is AKP-20(mean cation of Al,O, for important components. Niharaproposed a new particle size=0.4-0.6 Hm; purity =99.99%)from Sumitomo concept called"nanocomposite" ceramics Using this concept, two of Chemical (Tokyo, Japan). The SiC whiskers used were SCW the above weaknesses, low n and low heat-resistance limit, have 1-0.8(length 30-100 um; diameter =0.8-1.0 um) from been overcome. However, since the fracture toughness of Al,O3is Tateho Chemical Industries(Ako, Japan). The quantity of SiC not high, Al,O, is very sensitive to cracks and has lower reliability whiskers added was 20 vol% relative to Al,O, powder. The To overcome these weaknesses, there are two methods:(1)toughen- ing AlO, with fibers or whiskers and (2) inducing a self-crack- mixture was blended in isopropyl alcohol for 12 h using alumina balls and a mill pot. Thereafter, the mixture was dried. Rectangular healing ability. Many studies have been conducted on toughening plates of 9 mm X 50 mm X 50 mm were hot-pressed under the Al,O, by fiber or whisker reinforcement, and many useful results following conditions: temperature= 1850C, time I h, pres- have been reported. -It is well-known that monolithic Al_O3 and sure =40 MPa, and in an argon environment. The density of the Al2O3 reinforced by Sic particles have an interesting crack-healing test material measured by the Archimedes technique was 3.83 ability.- Applying ceramics with high crack-healing ability to g/cm, which was 99.9% of the theoretical density of the material structural components for engineering use could result in great The average grain size of Al, O, was 1-2 um. The SiC whiskers components and reduced inspection, machining, and polishing costs was shortened to 5-20 um during the ball milling process. of ceramic components. To maximize the benefits, however, more The fracture toughness was evaluated by the single-edge pre systematic studies have to be conducted, including crack-healing cracked beam (SEPB) method and indentation-fracture (IF) behavior as a function of healing temperature and time, the effects method according to Japanese Industrial Standard (JIS). Five of environment on crack-healing, 2, 13, 16, 7 the maximum crack size pecimens were used in both SEPB and IF methods. In the SEPB that can be completely healed, -the high-temperature strength of method, precracked specimens having dimensions of 3 mm X 4 crack-healed zones, assessment of the cyclic and static fatigue mm X 23 mm were subjected to three-point bending with a span strengths of crack-healed ceramic components, ,22-3 and crack of 16 mm using a crosshead speed of 0.5 mm/min at room healing behavior under static or cyclic stress.2729-31 temperature. As the starter of precrack, three Vickers indentations It has been shown that Si3 N /SiC, 9,2 and mullite/SiC were introduced on the specimen surface, using a indent load of have excellent crack-healing abilities. However, embedded flaws 196 N. Then, a straight-through crack was introduced by the ure toughness can be calculated by J. J. Petrovic--contributing editor where F is the load, S is the span, B is the width, W is the height of the specimen, and a is the crack length. y is a geometry factor lember,Amey (B)No. 14750056 of the term from 2002 to 2oooung Scientists 1.99-A(1-A)(2.15-3.93A+2.7A2 aUthor to whom correspondence should be addressed. e-mail: andokoto(@ 2143Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers Koji Takahashi,* Masahiro Yokouchi,† Sang-Kee Lee,† and Kotoji Ando* ,‡ Department of Energy and Safety Engineering, Yokohama National University, 79-5, Hodogaya, Yokohama 240-8501, Japan Al2O3 reinforced by SiC whiskers (Al2O3/SiC-W) was hotpressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 m in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by threepoint bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al2O3/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2c 100 m could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al2O3 tougher. I. Introduction ALUMINA (Al2O3) is a very popular ceramic that is used in various fields. However, it has three weak points: low bending strength (B, 400 MPa), low fracture toughness (3–4 MPam1/2), and low heat-resistance limit (900°C). These weaknesses restrict the application of Al2O3 for important components. Niihara1,2 proposed a new concept called “nanocomposite” ceramics. Using this concept, two of the above weaknesses, low B and low heat-resistance limit, have been overcome.1–3 However, since the fracture toughness of Al2O3 is not high, Al2O3 is very sensitive to cracks and has lower reliability. To overcome these weaknesses, there are two methods: (1) toughening Al2O3 with fibers or whiskers and (2) inducing a self-crackhealing ability. Many studies have been conducted on toughening Al2O3 by fiber or whisker reinforcement, and many useful results have been reported.4–7 It is well-known that monolithic Al2O3 and Al2O3 reinforced by SiC particles have an interesting crack-healing ability.8–13 Applying ceramics with high crack-healing ability to structural components for engineering use could result in great benefits,8–31 such as increased reliability of the structural ceramic components and reduced inspection, machining, and polishing costs of ceramic components. To maximize the benefits, however, more systematic studies have to be conducted, including crack-healing behavior as a function of healing temperature and time,20,21 the effects of environment on crack-healing,12,13,16,17 the maximum crack size that can be completely healed,19,22,27 the high-temperature strength of crack-healed zones,19–24 assessment of the cyclic and static fatigue strengths of crack-healed ceramic components,19,22–31 and crackhealing behavior under static or cyclic stress.27,29–31 It has been shown that Si3N4/SiC17,19,21 and mullite/SiC22,25,27 have excellent crack-healing abilities. However, embedded flaws cannot be healed in these ceramics because oxygen is necessary for crack-healing.16,17 Thus, higher fracture toughness is desirable for the structural integrity of ceramics. It is well-known that whisker reinforcement is very effective for increasing the fracture toughness of structural ceramics.4–6 Thus, if whisker-reinforced Al2O3 possesses excellent crack-healing ability, it would be desirable for the structural integrity of Al2O3 components. However, the crackhealing behavior of whisker-reinforced Al2O3 has not yet been studied. Thus, Al2O3 reinforced by SiC whiskers (Al2O3/SiC-W) has been hot-pressed, and its crack-healing behavior has been investigated systematically in this study. The following research objectives were chosen for the present study: (1) to hot-press Al2O3 reinforced by SiC whiskers (Al2O3/ SiC-W); (2) to investigate the crack-healing behavior of Al2O3/ SiC-W as a function of crack-healing temperature; (3) to determine the effect of the test temperature on B of a crack-healed member. The fracture toughness of the material was also determined. II. Experimental Procedure The alumina powder used in this investigation is 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. The mixture was blended in isopropyl alcohol for 12 h using alumina balls and a mill pot. Thereafter, the mixture was dried. Rectangular plates of 9 mm 50 mm 50 mm were hot-pressed under the following conditions: temperature 1850°C, time 1 h, pressure 40 MPa, and in an argon environment. 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. The SiC whiskers were found in grain boundaries. The length of the SiC whiskers was shortened to 5–20 m during the ball milling process. The fracture toughness was evaluated by the single-edge precracked beam (SEPB) method and indentation-fracture (IF) method according to Japanese Industrial Standard (JIS).32 Five specimens were used in both SEPB and IF methods. In the SEPB method, precracked specimens having dimensions of 3 mm 4 mm 23 mm were subjected to three-point bending with a span of 16 mm using a crosshead speed of 0.5 mm/min at room temperature. As the starter of precrack, three Vickers indentations were introduced on the specimen surface, using a indent load of 196 N. Then, a straight-through crack was introduced by the bridge-indentation method according to the standard.32 The fracture toughness can be calculated by32,33 KIC 3FS 2BW2aY where F is the load, S is the span, B is the width, W is the height of the specimen, and a is the crack length. Y is a geometry factor calculated by32,33 Y 1.99 1 2.15 3.93 2.72 1 21 3/ 2 where a/W. J. J. Petrovic—contributing editor Manuscript No. 186914. Received June 10, 2002; approved August 4, 2003. A part of this study was supported by a Grant-in-Aid for scientific research allocated to the Japan Society for the Promotion Science (JSPS), Young Scientists Research Category (B) No. 14750056 of the term from 2002 to 2003. *Member, American Ceramic Society. † Postgraduate student. ‡ Author to whom correspondence should be addressed. e-mail: andokoto@ ynu.ac.jp. J. Am. Ceram. Soc., 86 [12] 2143–47 (2003) 2143 journal