《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_j2916.2003.tb03622

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(@ 2143

Crack-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 hot￾pressed to investigate the crack-healing behavior. Semiellipti￾cal 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 three￾point bending tests at room temperature and elevated temper￾atures 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 MPa
m1/2), and low heat-resistance limit (900°C). These weaknesses restrict the appli￾cation 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) toughen￾ing Al2O3 with fibers or whiskers and (2) inducing a self-crack￾healing 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 crack￾healing 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 tough￾ness 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 crack￾healing 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, pres￾sure 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 pre￾cracked 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 frac￾ture toughness can be calculated by32,33 KIC

3FS 2BW2a
Y 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

2144 Journal of the American Ceramic Sociery-Takahashi et al. VoL 86. No. 12 In the IF method, fracture toughness can be estimated by the following equation in accordance with JIS 32 o Smooth specimens heat-treated Kic=0.018(E/HV).(P/c) at 1300C for lh in As cracked(2c=100um) where E is Young's modulus, P is the indent load, c is half the surface crack length, and HV is Vickers hardness. Vickers inden- a Crack-healed for lh in air tations were introduced on the polished surface by applying an 1400 load of 98N for 20 s. The material has the following ties:Young' s modulus, E=380 GPa, and Vickers hardness, 1200 19.6 GPa The effect of crack-healing was investigated using specimens 5 three specimens were used in crack-healing and subsequent bend- F 1000 having dimensions of 3 mm X 4 mm X 23 mm. Normally, two or ensile surface (4 mm wide side) of each test specimen was 800 mirror-finished. Semielliptical surface cracks were made at the s center of the tensile surface of each test specimen with a Vickers an indenter, using a load of 19.6 N. By this method, semielliptic 600 cracks of about 100 um in surface length were made. The ratio of E depth(a) to half the surface length(c)of the crack(aspect ratio was a/c=0.9 84/4 treatment at temperatures between 1000 and 1300C for I h in air E to investigate the effect of healing temperature on the bending 200 strength at room temperature. The bending strength of crack healed specimens was also measured at elevated temperatures ranging from 400%to 1300.C in air For elevated temperature tests, R.T100011001200130014001500 precracked specimens were subjected to heat treatment at 1300C for 1 h The specimens were heated at a rate of 10.C/min, Healing Temperature(C) and cooling was spontaneous in the furnace. The bending strength of smooth specimens(without indent Fig. 1. Relationship between the bending strength of Al,O,/SiC-W at cracks) was also investigated. It has been reported that small room temperature and the crack-healing tempera surface flaws caused by polishing decreased the bending strength of ceramics. 7 Thus, all smooth specimens were heat-treated at 1300 C for I h in air before the bending test to heal preexisting surface flaws. The specimens which were in- almost the same as that of the heat-treated smooth specimens 1000 MPa). Thus, the surface cracks with 2c= 100 um can be dented have not received this heat treatment before indentation healed completely at 1200 or 1300@C for I h in air. However, the because the polishing flaws were much smaller than the indenta- value of on for the specimens crack-healed below 1 100.C is lower tion cracks Bending tests were conducted on a three-point loading system below 1 100oC the degree of crack-healing decreased with decreas- with a span of 16 mm at both room and elevated temperatures. The crosshead speed in the bending tests was 0.5 mm/min. The Ing tem specimen surfaces and fracture surfaces were analyzed using a scanning electron microscope (SEM). The X-ray diffraction ethod was used to investigate the surface oxidized layer. The Fracture Surfaces radiation used in X-ray diffraction was CuKo at 50-k voltage and 60-mA electric current. Figure 2 shows the SEM micrographs of the specimen surfaces ter heat treatment at temperatures between 1 100 and 1300C for I h in air. In Fig. 2, the light regions are alumina and the dark II. Results and Diseussion regions are SiC. When the specimens were heat-treated at 1300C oxidation products like water droplets originating from SiC whis (I Fracture Toughness kers (gray phase) were clearly observed, as shown in Fig. 2(c) The average values of kic for five data evaluated by the SEPB However, when the samples were heat-treated below 1200oC, the method and IF method are Kic = 5.7 + 0.2 and 5.6+ 0.2 oxidation products were much smaller. On the basis of their morphology, the oxidation products are considered to be a glassy small. The values of Kic for AL, O,/SiC-W are larger than that of phase. The formation mechanism of the oxidation products will be monolithic alumina(KIC=3-4 MPa'm2). Therefore, SiC whis- kers apparently contribute to increasing the fracture toughness of Figure 3 shows a SEM micrograph of the fracture surface of a Al,O/SiC-W crack-healed specimen, which fractured from the crack-healed zone. The precrack front is indicated by the white dotted line Figure 3(b) shows the details of the crack-healed zone. The dark 2) Efect of Crack-Healing Temperature on the Bending egions in Fig 3(b)are oxidized; the light ones, unoxidized. Figure Strength of Crack-Healed specimens 3(c)shows the details of outside the crack-healed zone. Interac- circles)is about 1000 MPa. The op of the precracked specimens iC-H ute to a>ker ck and the matrix, such as bridging and/or Figure I shows the effect of healing temperature on the bend strength of the crack-healed specimens at room temperature. The pullouts by whiskers, are clearly observed. Thus, the Sic whiskers average value of oB for the heat-treated smooth specimens(solid increase in the fracture toughness of Al,O3/ with 2c= 100 um(open triangles) is 440 MPa, which is a id algoman mool investigated the oxidation behavior of reduction from uB of the heat-treated smooth specimens. The sol c in argon at various levels of oxidation triangles indicate the gn of crack-healed specimens. The averag partial pressure. 7 d out that in a strongly oxidizing values of oB for the specimen crack-healed at 1200 and 1300C tmosphere, such as air, SiO, was first formed by oxidation of Sic. are 970 and 980 MPa, respectively. These bending strengths are after which a reaction between this SiO2 and the matrix Al,O

In the IF method, fracture toughness can be estimated by the following equation in accordance with JIS.32 KIC
0.018E/HV 0.5P/c1.5 where E is Young’s modulus, P is the indent load, c is half the surface crack length, and HV is Vickers hardness. Vickers inden￾tations were introduced on the polished surface by applying an indent load of 98 N for 20 s. The material has the following properties: Young’s modulus, E 380 GPa, and Vickers hardness, HV 19.6 GPa. The effect of crack-healing was investigated using specimens having dimensions of 3 mm  4 mm  23 mm. Normally, two or three specimens were used in crack-healing and subsequent bend￾ing tests. As an exception, one or five specimens were used. A tensile surface (4 mm wide side) of each test specimen was mirror-finished. Semielliptical surface cracks were made at the center of the tensile surface of each test specimen with a Vickers indenter, using a load of 19.6 N. By this method, semielliptical cracks of about 100 m in surface length were made. The ratio of depth (a) to half the surface length (c) of the crack (aspect ratio) was a/c 0.9. Specimens containing indent cracks were subjected to heat treatment at temperatures between 1000° and 1300°C for 1 h in air to investigate the effect of healing temperature on the bending strength at room temperature. The bending strength of crack￾healed specimens was also measured at elevated temperatures ranging from 400° to 1300°C in air. For elevated temperature tests, precracked specimens were subjected to heat treatment at 1300°C for 1 h in air. The specimens were heated at a rate of 10°C/min, and cooling was spontaneous in the furnace. The bending strength of smooth specimens (without indent cracks) was also investigated. It has been reported that small surface flaws caused by polishing decreased the bending strength of ceramics.27 Thus, all smooth specimens were heat-treated at 1300°C for 1 h in air before the bending test to heal preexisting surface flaws. The specimens which were in￾dented have not received this heat treatment before indentation because the polishing flaws were much smaller than the indenta￾tion cracks. Bending tests were conducted on a three-point loading system with a span of 16 mm at both room and elevated temperatures. The crosshead speed in the bending tests was 0.5 mm/min. The specimen surfaces and fracture surfaces were analyzed using a scanning electron microscope (SEM). The X-ray diffraction method was used to investigate the surface oxidized layer. The radiation used in X-ray diffraction was CuK at 50-kV accelerated voltage and 60-mA electric current. III. Results and Discussion (1) Fracture Toughness The average values of KIC for five data evaluated by the SEPB method and IF method are KIC 5.7 0.2 and 5.6 0.2 MPa
m1/2, respectively. The scatter of the KIC values is quite small. The values of KIC for Al2O3/SiC-W are larger than that of monolithic alumina (KIC 3–4 MPa
m1/2). Therefore, SiC whis￾kers apparently contribute to increasing the fracture toughness of Al2O3/SiC-W. (2) Effect of Crack-Healing Temperature on the Bending Strength of Crack-Healed Specimens Figure 1 shows the effect of healing temperature on the bending strength of the crack-healed specimens at room temperature. The average value of
B for the heat-treated smooth specimens (solid circles) is about 1000 MPa. The
B of the precracked specimens with 2c 100 m (open triangles) is 440 MPa, which is a 56% reduction from
B of the heat-treated smooth specimens. The solid triangles indicate the
B of crack-healed specimens. The average values of
B for the specimen crack-healed at 1200° and 1300°C are 970 and 980 MPa, respectively. These bending strengths are almost the same as that of the heat-treated smooth specimens (1000 MPa). Thus, the surface cracks with 2c 100 m can be healed completely at 1200° or 1300°C for 1 h in air. However, the value of
B for the specimens crack-healed below 1100°C is lower than that of the heat-treated smooth specimens, indicating that below 1100°C the degree of crack-healing decreased with decreas￾ing temperature. (3) Investigation of Specimen Surfaces and Fracture Surfaces Figure 2 shows the SEM micrographs of the specimen surfaces after heat treatment at temperatures between 1100° and 1300°C for 1 h in air. In Fig. 2, the light regions are alumina and the dark regions are SiC. When the specimens were heat-treated at 1300°C, oxidation products like water droplets originating from SiC whis￾kers (gray phase) were clearly observed, as shown in Fig. 2(c). However, when the samples were heat-treated below 1200°C, the oxidation products were much smaller. On the basis of their morphology, the oxidation products are considered to be a glassy phase. The formation mechanism of the oxidation products will be discussed below. Figure 3 shows a SEM micrograph of the fracture surface of a crack-healed specimen, which fractured from the crack-healed zone. The precrack front is indicated by the white dotted line. Figure 3(b) shows the details of the crack-healed zone. The dark regions in Fig. 3(b) are oxidized; the light ones, unoxidized. Figure 3(c) shows the details of outside the crack-healed zone. Interac￾tions between the crack and the matrix, such as bridging and/or pullouts by whiskers, are clearly observed. Thus, the SiC whiskers contribute to an increase in the fracture toughness of Al2O3/ SiC-W. Kim and Moorhead investigated the oxidation behavior of Al2O3/SiC-W at 1400°C in argon at various levels of oxidation partial pressure.7 They pointed out that in a strongly oxidizing atmosphere, such as air, SiO2 was first formed by oxidation of SiC, after which a reaction between this SiO2 and the matrix Al2O3 Fig. 1. Relationship between the bending strength of Al2O3/SiC-W at room temperature and the crack-healing temperature. 2144 Journal of the American Ceramic Society—Takahashi et al. Vol. 86, No. 12

December 2003 Crack-Healing Behavior of Al,O, Toughened by sic Whiskers 2145 a Vickers indentation 50um 5 um (a b SiC whisker Oxide 5 um (b) c) Oxide 5um OHn Fig. 3. SEM micrographs of the fracture surface of crack-healed A,O, -healed at 1300C for I h in air, JB=90 MPa: (a)crack profile tail of crack-healed zone shown by A in(a); (c)detail of outside k-healed zone shown by B in(a) Fig. 2. SEM m f specimen surfaces after heat treatment in a for A1,O/SiC-W following heat-treatment conditions: (a)1100.C lh;(b)1200°C 1300°C.Ih ould be detected. Therefore, the oxidation products in Fig. 2 are presumed to be the phase consisting of Sio, glass and alumino- urred, forming an aluminosilicate glass and crystalline mullin silicate glass formed by the reaction of SiO, and AL, O,. Chou et w,4and SiC-particle-reinforced Al,0,(AL Oy/SiC-P)sic. the crack-healing of Al 2O /SiC-P after heat treatment at 1300C Similar oxidation behaviors have been pointed out in Al,O,/S Chiu investigated the healing behaviors of thermal-shock for 2 h in air. On the basis of the present observation, it is clear that induced cracks in Al,O,SiC-w. They reported that the shock urface cracks were filled and bonded by the oxidation products nduced cracks were filled by oxidation products after heat formed on the crack surfaces(Fig. 3(b). The bend treatment at 1450%C for 2 h in air. Chou et al. 2 studied the healing cracked specimens after heat treatment in air recovered to a value of indent cracks and subsequent strength behavior of Sic-particle imilar to that of heat-treated smooth specimens due to a substan- reinforced Al2O3 nanocomposites. They pointed out that, after heat tial increase in the bonding force acting across the crack surfaces specimens recovered almost to the value of the unindented strength (4) Effect of Testing Temperature on the Bending Strength of the nanocomposite of Crack-Healed Specimens We examined the oxidation products on the specimen su Figure 4 shows the effect of testing temperature on gB of by X-ray diffraction. However, only crystalline Al,O, and crack-healed specimens Crack-healing was conducted at 1300oC

occurred, forming an aluminosilicate glass and crystalline mullite.7 Similar oxidation behaviors have been pointed out in Al2O3/SiC￾W11,34 and SiC-particle-reinforced Al2O3 (Al2O3/SiC-P).35,36 Chiu34 investigated the healing behaviors of thermal-shock￾induced cracks in Al2O3/SiC-W. They reported that the shock￾induced cracks were filled by oxidation products after heat treatment at 1450°C for 2 h in air. Chou et al. 12 studied the healing of indent cracks and subsequent strength behavior of SiC-particle￾reinforced Al2O3 nanocomposites. They pointed out that, after heat treatment at 1300°C for 2 h in air, the bending strength of indented specimens recovered almost to the value of the unindented strength of the nanocomposites. We examined the oxidation products on the specimen surface by X-ray diffraction. However, only crystalline Al2O3 and SiC could be detected. Therefore, the oxidation products in Fig. 2 are presumed to be the phase consisting of SiO2 glass and alumino￾silicate glass formed by the reaction of SiO2 and Al2O3. Chou et al. 12 reported similar results of X-ray diffraction in their study on the crack-healing of Al2O3/SiC-P after heat treatment at 1300°C for 2 h in air. On the basis of the present observation, it is clear that surface cracks were filled and bonded by the oxidation products formed on the crack surfaces (Fig. 3(b)). The bending strength of cracked specimens after heat treatment in air recovered to a value similar to that of heat-treated smooth specimens due to a substan￾tial increase in the bonding force acting across the crack surfaces. (4) Effect of Testing Temperature on the Bending Strength of Crack-Healed Specimens Figure 4 shows the effect of testing temperature on
B of crack-healed specimens. Crack-healing was conducted at 1300°C Fig. 2. SEM micrographs of specimen surfaces after heat treatment in air for Al2O3/SiC-W for the following heat-treatment conditions: (a) 1100°C, 1 h; (b) 1200°C, 1 h; (c) 1300°C, 1 h. Fig. 3. SEM micrographs of the fracture surface of crack-healed Al2O3/ SiC-W, 2c 100 m, crack-healed at 1300°C for 1 h in air,
B 903 MPa: (a) crack profile; (b) detail of crack-healed zone shown by A in (a); (c) detail of outside the crack-healed zone shown by B in (a). December 2003 Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers 2145

2146 Journal of the American Ceramic Sociery-Takahashi et al. VoL 86. No. 12 References e Heat-treated smooth specimens A As cracked(2c=100um) AL2O, SiC Com 124-34 in Proceedings of the Intemational. Strong a Crack-healed at 1300C for Ih in air Adranced mater 2 K. MIhara.“N 1400 ial Issue of the Ceramics Society of Japan), J. Ceram. Soc. Jpn, 99 [10] -82(1991) BJ. Zhao, L C. Stearns, M. P. Harmer, H M. Chan, and G.A. Miller,"Mechanical 12 Behavior of Alumina-Silicon"Nanocomposites,JAm Ceram Soc., 76[21503-l (1993). P F. Becher and G. C. Wei, "Toughening Behavior in SiC-Whisker-Reinforced 1000 Cmm如300 Enforced tcfa- 800 Matrix Composite: Effect of Whisker Surface Treatment on Fracture Toughness, J.Am. Ceram.Soc,73[2]394-402(1990 E. E. Kim and A. J. Moorhead. "Oxidation Behavior and Effects of Oxidati 0600 the Strength of Sic-whisker Reinforced Alumina,J. Mater. Sci., 29, 1656-61 △ F. F. Lange and K. C. Radford, "Healing of Surface Cracks in Polycrystalli 曰400 AL2O3,"JAm. Ceram.Soc,53[0-11420-21(1970) "T.K. Gupta, "Crack Healing and Strengthening of Thermally Shocked Alumina, 200 AloU M. Thompson, H, M. Chan, M. P, Harmer, an nd Stress Relaxation in Al2OrSiC " Nanocomposites,J. Am. Ceram. Soc 78 31 56771(195 J E. Moffatt, W.J. Plumbridge, and R, Hermann, "High Temperature Cr and Alumina-SiC Composite, R.T 400 800 1200 1600 121.A Chou, H.M. Chan, and M P Harmer, "Effect of Annealing Environment Br. Cera Trans., 95 [123-29(1996). Test Temperature (C on the Crack Healing and Mechanical Behavior of Silicon Carbide-Reinforced nocomposites,J.An. Ceram. Soc., 81 [5] 1203-208(1998). J Petrovic and L. A Jacobson, " Controlled Surface Flaws in Hot-Pressed SiC, healed zone Soc,59|-2]34-37(1976 hoi and V. Tikare. "Crack Healing Behavior of Hot Pressed Silicon 16M. C. Chu, S. Sato, Y. Kobayashi, and K. Ando, " Damage Healing and h in air. The solid triangles show o of the crack-healed cole mens. The oB of the crack-healed specimens is almost mWmx1sm如0cmaa 7K. Ando, T Ikeda, S. Sato, F, Yao, and Y Kobayashi, "A Preliminary Study on above 1 C and decreases with increasing testing temperature Crack Healing Behaviour of SiaN /SiC Composite Ceramics, Fatigue Fract. Eng. up to 1300.C. Thus, the limit temperature for bending strength is Mater. Struct, 21 [1]119-22(1998) 1100 C. In Fig 4. the solid line indicates the bending strength of IsY. H. Zhang, L. Edwards, and W. J. Plumbridge, "Crack Healing in a Nitride Ceramics,J. Am. Ceram Soc., 81[7 34-37(1998) monolithic Al, O,AL,O,/SiC-W showed on considerably higher 19K. Ando, M. C. Chu, S. Sato, F. Yao, and Y,. Kobayashi, "The Stu than that of monolithic AL,O, in the entire temperature range Healing Behavior of Silicon Nitride Ceramics"(in Jpn ),Jpn. Soc. Mech. tested 623]1936-42(1998) IV. Conclusions 897-903(1999 2K. Ando, M. C, Chu, Y Kobayashi, F. Yao, and S. Sato, "Crack Healing AL,O, reinforced by SiC whiskers(Al203/SiC-w) was hot- Soc. Mech. Eng, 65 (633)1132-39(1999) pressed to investigate the crack-healing behavior. Semielliptical 22K. Ando, K. Tsuji, T. Hirasawa, Y Kobayashi, M. C. Chu, and SSato,Crack urface cracks of 100 um in surface length were introduced on each specimen. These specimens were crack-healed, and their ics"(in Jpn ),J. Soc. Mater. Sci. Jpn, 485]484-94(1999). strength was measured by a three-point bending test at room 2Y. Korous, M. C. Chu, M. Nakatani, and K. Ando, "Crack Healing Behavior of temperature and at elevated temperatures between 400 and SiC Ceramics, J. Am. Ceram Soc., 83[1n2788-92(2000) F. Yao, K. Ando, M. C. Chu, and S. Sato, "Crack-Healing Behavior, High- 1300C. The crack-healing behavior was studied as a function Temperature and Fa strength of Sic-Reinforced Silicon Nitride Composite, of healing temperature. The main conclusions obtained are as J Mater. Sci. Left, 19[ 12]1081-83(2000). follows K. Ando, K. Tsuji, M. Ariga, and S Sato, "Fatigue Strength Properties of Crack 1) The average values of Kic for Al,O,/SiC-W evalua Healed Mullite/SiC Composite Ceramics"(in Jpn ),J. Soc. Mater. Sci Jpn, 48 [10] the SEPB and the IF method were Kic= 5.7+ 1173-78(1999 5.6±0.2 espectively. The value of Kicfor F. Yao, K. Ando, M. C. Chu, and S Sato, "Static and Cyclic Fatigue Behaviour of Crack-Healed Si, N//SiC Composite Ceramics, J. Eur. Ceram Soc., 21, 991-9 than that of monolithic alumina (K 2001) MPam). Thus, as expected, the SiC whiskers also made their "Crack-Healing Behavior under Stress of Mullite/Silicon Carbide Ceramics and the AlO/SiC-W exhibited very interesting crack-healing Resultant Fatigue Strength, J.Am. Ceram Soc., 84[9]2073-78(2001). havior. The cracks with surface length 2c= 100 Hm could b 2K. Ando, M. C. Chu, K. Tsuji, T. Hirasawa, Y. Kobayashi, and S. Sato, "Crack Healing Behavior of Mullite/SiC Composite Ceramics, J. Eur. Ceram. Soc. healed by crack-healing at 1200 or 1300C for I h in air However, the surface cracks could not be healed completely below 2K. Ando, K Houjyou, M. C. Chu, S. Takeshita, K.Takahashi, S. Sakamoto, and the healing temperature of 1100C S. Sato, " Crack-Healing Behavior of Si3N,/SiC (3 The limit temperature for the bending sti healed specimens of Al,O/ SiC-w is 1.C strength of Al2O/SiC-W at elevated temperature is K. Takahashi, S Nakayama, and S Saito, "Crack-Healing Behavior of Mics under Cyclic Stress and Resultant Fatigue Strength at the than that of monolithic Al,O3 Healing Temperature, J Anm Ceram Soc., 85 [9]2268-72(2002)

for 1 h in air. The solid triangles show
B of the crack-healed specimens. The
B of the crack-healed specimens is almost constant up to 1000°C. However, it shows a sudden decrease above 1100°C and decreases with increasing testing temperature up to 1300°C. Thus, the limit temperature for bending strength is 1100°C. In Fig. 4, the solid line indicates the bending strength of monolithic Al2O3 13 Al2O3/SiC-W showed
B considerably higher than that of monolithic Al2O3 in the entire temperature range tested. IV. Conclusions Al2O3 reinforced by SiC whiskers (Al2O3/SiC-W) was hot￾pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 m in surface length were introduced on each specimen. These specimens were crack-healed, and their strength was measured by a three-point bending test at room temperature and at elevated temperatures between 400° and 1300°C. The crack-healing behavior was studied as a function of healing temperature. The main conclusions obtained are as follows. (1) The average values of KIC for Al2O3/SiC-W evaluated by the SEPB method and the IF method were KIC 5.7 0.2 and 5.6 0.2 MPa
m1/2, respectively. The value of KIC for Al2O3/ SiC-W is larger than that of monolithic alumina (KIC 3–4 MPa
m1/2). Thus, as expected, the SiC whiskers also made their Al2O3 tougher. (2) Al2O3/SiC-W exhibited very interesting crack-healing be￾havior. The cracks with surface length 2c 100 m could be healed by crack-healing at 1200° or 1300°C for 1 h in air. However, the surface cracks could not be healed completely below the healing temperature of 1100°C. (3) The limit temperature for the bending strength of crack￾healed specimens of Al2O3/SiC-W is 1100°C. The bending strength of Al2O3/SiC-W at elevated temperature is much higher than that of monolithic Al2O3. References 1 K. Niihara, A. Nakahira, G. Sasaki, and M. Hirabayashi, “Development of Strong Al2O3/SiC Composites”; pp. 124–34 in Proceedings of the International Meeting on Advanced Materials, Vol. 4. Materials Research Society, Tokyo, Japan, 1989. 2 K. Niihara, “New Design Concept of Structural Ceramics Nanocomposites” (The Centennial Issue of the Ceramics Society of Japan), J. Ceram. Soc. Jpn., 99 [10] 974–82(1991). 3 J. Zhao, L. C. Stearns, M. P. Harmer, H. M. Chan, and G. A. Miller, “Mechanical Behavior of Alumina–Silicon ‘Nanocomposites’,” J. Am. Ceram. Soc., 76 [2] 503–10 (1993). 4 P. F. Becher and G. C. Wei, “Toughening Behavior in SiC-Whisker-Reinforced Alumina,” J. Am. Ceram. Soc., 67 [12] C267–C269 (1984). 5 G. C. Wei and P. F. Becher, “Development of SiC-Whisker-Reinforced Ceram￾ics,” Am. Ceram. Soc. Bull., 64 [2] 298–304 (1985). 6 J. Homeny, W. L. Vaughn, and M. K. Ferber, “Silicon Carbide Whisker/Alumina Matrix Composite: Effect of Whisker Surface Treatment on Fracture Toughness,” J. Am. Ceram. Soc., 73 [2] 394–402 (1990). 7 H. E. E. Kim and A. J. Moorhead, “Oxidation Behavior and Effects of Oxidation on the Strength of SiC-Whisker Reinforced Alumina,” J. Mater. Sci., 29, 1656–61 (1994). 8 F. F. Lange and K. C. Radford, “Healing of Surface Cracks in Polycrystalline Al2O3,” J. Am. Ceram. Soc., 53 [10–11] 420–21 (1970). 9 T. K. Gupta, “Crack Healing and Strengthening of Thermally Shocked Alumina,” J. Am. Ceram. Soc., 59 [5–6] 259–62 (1976). 10A. M. Thompson, H. M. Chan, M. P. Harmer, and R. F. Cook, “Crack Healing and Stress Relaxation in Al2O3–SiC ‘Nanocomposites’,” J. Am. Ceram. Soc., 78 [3] 567–71 (1995). 11. J. E. Moffatt, W. J. Plumbridge, and R. Hermann, “High Temperature Crack Annealing Effect on Fracture Toughness of Alumina and Alumina–SiC Composite,” Br. Ceram. Trans., 95 [1] 23–29 (1996). 12I. A. Chou, H. M. Chan, and M. P. Harmer, “Effect of Annealing Environment on the Crack Healing and Mechanical Behavior of Silicon Carbide-Reinforced Alumina Nanocomposites,” J. Am. Ceram. Soc., 81 [5] 1203–208 (1998). 13B. S. Kim, K. Ando, M. C. Chu, and S. Saito, “Crack-Healing Behavior of Monolithic Alumina and Strength of Crack-Healed Member” (in Jpn.), J. Soc. Mater. Sci. Jpn., 52, 667–73 (2003). 14J. J. Petrovic and L. A. Jacobson, “Controlled Surface Flaws in Hot-Pressed SiC,” J. Am. Ceram. Soc., 59 [1–2] 34–37 (1976). 15S. R. Choi and V. Tikare, “Crack Healing Behavior of Hot Pressed Silicon Nitride Due to Oxidation,” Scr. Metall. Mater., 26, 1263–68 (1992). 16M. C. Chu, S. Sato, Y. Kobayashi, and K. Ando, “Damage Healing and Strengthening Behaviour in Intelligent Mullite/SiC Ceramics,” Fatigue Fract. Eng. Mater. Struct., 18 [9] 1019–29 (1995). 17K. Ando, T. Ikeda, S. Sato, F. Yao, and Y. Kobayashi, “A Preliminary Study on Crack Healing Behaviour of Si3N4/SiC Composite Ceramics,” Fatigue Fract. Eng. Mater. Struct., 21 [1] 119–22 (1998). 18Y. H. Zhang, L. Edwards, and W. J. Plumbridge, “Crack Healing in a Silicon Nitride Ceramics,” J. Am. Ceram. Soc., 81 [7] 34–37 (1998). 19K. Ando, M. C. Chu, S. Sato, F. Yao, and Y. Kobayashi, “The Study on Crack Healing Behavior of Silicon Nitride Ceramics” (in Jpn.), Jpn. Soc. Mech. Eng., 64 [623] 1936–42 (1998). 20K. Ando, M. C. Chu, F. Yao, and S. Sato, “Fatigue Strength of Crack Healed Si3N4/SiC Composite Ceramics,” Fatigue Fract. Eng. Mater. Struct., 22 [10] 897–903 (1999). 21K. Ando, M. C. Chu, Y. Kobayashi, F. Yao, and S. Sato, “Crack Healing Behavior and High Temperature Strength of Silicon Nitride Ceramics” (in Jpn.), Jpn. Soc. Mech. Eng., 65 [633] 1132–39 (1999). 22K. Ando, K. Tsuji, T. Hirasawa, Y. Kobayashi, M. C. Chu, and S. Sato, “Crack Healing Behavior and High Temperature Strength of Mullite/SiC Composite Ceram￾ics” (in Jpn.), J. Soc. Mater. Sci. Jpn., 48 [5] 484–94 (1999). 23Y. Korous, M. C. Chu, M. Nakatani, and K. Ando, “Crack Healing Behavior of SiC Ceramics,” J. Am. Ceram. Soc., 83 [11] 2788–92 (2000). 24F. Yao, K. Ando, M. C. Chu, and S. Sato, “Crack-Healing Behavior, High￾Temperature and Fatigue Strength of SiC-Reinforced Silicon Nitride Composite,” J. Mater. Sci. Lett., 19 [12] 1081–83 (2000). 25K. Ando, K. Tsuji, M. Ariga, and S. Sato, “Fatigue Strength Properties of Crack Healed Mullite/SiC Composite Ceramics” (in Jpn.), J. Soc. Mater. Sci. Jpn., 48 [10] 1173–78 (1999). 26F. Yao, K. Ando, M. C. Chu, and S. Sato, “Static and Cyclic Fatigue Behaviour of Crack-Healed Si3N4/SiC Composite Ceramics,” J. Eur. Ceram. Soc., 21, 991–97 (2001). 27K. Ando, K. Furusawa, M. C. Chu, T. Hanagata, K. Tuji, and S. Sato, “Crack-Healing Behavior under Stress of Mullite/Silicon Carbide Ceramics and the Resultant Fatigue Strength,” J. Am. Ceram. Soc., 84 [9] 2073–78 (2001). 28K. Ando, M. C. Chu, K. Tsuji, T. Hirasawa, Y. Kobayashi, and S. Sato, “Crack Healing Behavior of Mullite/SiC Composite Ceramics,” J. Eur. Ceram. Soc., 22, 1313–19 (2002). 29K. Ando, K. Houjyou, M. C. Chu, S. Takeshita, K. Takahashi, S. Sakamoto, and S. Sato, “Crack-Healing Behavior of Si3N4/SiC Ceramics under Stress and Fatigue Strength at the Temperature of Healing (1000°C),” J. Eur. Ceram. Soc., 22, 1339–46 (2002). 30K. Ando, K. Takahashi, S. Nakayama, and S. Saito, “Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature,” J. Am. Ceram. Soc., 85 [9] 2268–72 (2002). Fig. 4. Effect of test temperature on the bending strength of a crack￾healed specimen for Al2O3/SiC-W and monolithic Al2O3. 13 The data marked with an asterisk indicate the fracture occurred outside of the crack-healed zone. 2146 Journal of the American Ceramic Society—Takahashi et al. Vol. 86, No. 12

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