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,OIn 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