1260 Journal of the American Ceramic Sociery-Lee et al. Vol 87. No. 7 grain boundaries. The hot-pressed plates were then cut into test IlL. Test Results and discussion specimens measuring 3 mm x 4 mm X 23 mm. The tensile surface of the test specimen was finished. Semielliptical surface (1 Morphology of Surface Cracks before and after acks were made at the center of the tensile surface of the test Crack-Healing specimen using a Vickers indenter at a load of 19.6 N. By this Figures 2(a)and (b) show SEM micrographs of precrack introduced by means of the Vickers indentation method, before nethod, semielliptical cracks of 100 um surface length were and after the crack-healing at 1300C for I h in air,respectively It can be clearly observed that crack closure and rebonding were crack-healed at 1300%C for I h in air. The rising rate of the occurred. In Fig. 2, the light regions are alumina and the dark urnace temperature was 10.C/min. This is the optimized healing lition for a surface crack of 100 um, reported in a previou air, oxidation products originating from Sic whiskers(gray phase) tudy. 2 All smooth specimens were also heat-treated at 1300C were clearly observed, The reaction products are considered to b or I h in air before testing. Bending tests and static and cyclic solidified droplets of a glassy phase atigue tests were conducted using a three-point loading system with a span of 16 mm. The test fixtures used for the bending tests (2) Fatigue Strength Behavior at Room Temperature were made of SiC ceramics. The static and cyclic fatigue tests were The results of the static and cyclic fatigue tests at room rformed in air at room temperature, and at elevated temperatures temperature are shown in Fig 3. On the left-hand side of Figs. 3(a) of 900 and 1100C. The applied stress at which a specimen did and(b), the results of monotonic bending tests are also shown. The ot fracture up to 10%s was defined as the static fatigue limit symbols●,Δ,and▲ show the bending strength of the smooth yclic fatigue tests were conducted at a stress ratio of R= 0.2 and specimens heat-treated at 1300.C for I h in air, as-cracked, and using a sine wave with a frequency of 5 Hz. The maximum applied crack-healed specimens, respectively. The bending strength(B) stress at which a specimen did not fracture up to N=2 X 10 of the heat-treated smooth specimens scattered to a certain extent, cycles was defined as the cyclic fatigue limit. The surface cracks, giving a mean fracture stress of about 1000 MPa. Because of before and after crack-healing, and the fracture surface were precracking of 100 um, the bending strength decreased down to observed using a scanning electron microscope(SEM) 4450 MPa. However, the average value of B for the specimen crack-healed at 1300%C for I h in air was about 1000 MPa. Thus gB of the crack-healed specimens recovered to a level similar to ckers that of the heat-treated smooth specimens indentation SiC-whisker Crack SUum uum Pre-crack XIde forum 50um Fig. 2. SEM micrographs showing a surface crack introduced by Vickers Fig. 1. SEM micrographs of surface cracks: (a) indentation and crack; (b) indentation: (a) surface crack before crack-healing,(b) surface crack after crack-healing at 1300oC for I h in airgrain boundaries. The hot-pressed plates were then cut into test specimens measuring 3 mm 4 mm 23 mm. The tensile surface of the test specimen was mirror finished. Semielliptical surface cracks were made at the center of the tensile surface of the test specimen using a Vickers indenter at a load of 19.6 N. By this method, semielliptical cracks of 100 m surface length were made, as shown in Fig. 1. The ratio of depth (a) to half surface length (c) of the crack (aspect ratio) was a/c
0.9. Specimens were crack-healed at 1300°C for 1 h in air. The rising rate of the furnace temperature was 10°C/min. This is the optimized healing condition for a surface crack of 100 m, reported in a previous study.29 All smooth specimens were also heat-treated at 1300°C for 1 h in air before testing. Bending tests and static and cyclic fatigue tests were conducted using a three-point loading system with a span of 16 mm. The test fixtures used for the bending tests were made of SiC ceramics. The static and cyclic fatigue tests were performed in air at room temperature, and at elevated temperatures of 900° and 1100°C. The applied stress at which a specimen did not fracture up to 106 s was defined as the static fatigue limit. Cyclic fatigue tests were conducted at a stress ratio of R  0.2 and using a sine wave with a frequency of 5 Hz. The maximum applied stress at which a specimen did not fracture up to Nf  2 106 cycles was defined as the cyclic fatigue limit. The surface cracks, before and after crack-healing, and the fracture surface were observed using a scanning electron microscope (SEM). III. Test Results and Discussion (1) Morphology of Surface Cracks before and after Crack-Healing Figures 2(a) and (b) show SEM micrographs of precracks introduced by means of the Vickers indentation method, before and after the crack-healing at 1300°C for 1 h in air, respectively. It can be clearly observed that crack closure and rebonding occurred. In Fig. 2, the light regions are alumina and the dark regions are SiC whiskers. After heat treatment at 1300°C for 1 h in air, oxidation products originating from SiC whiskers (gray phase) were clearly observed. The reaction products are considered to be solidified droplets of a glassy phase.29 (2) Fatigue Strength Behavior at Room Temperature The results of the static and cyclic fatigue tests at room temperature are shown in Fig. 3. On the left-hand side of Figs. 3(a) and (b), the results of monotonic bending tests are also shown. The symbols F, , and Œ show the bending strength of the smooth specimens heat-treated at 1300°C for 1 h in air, as-cracked, and crack-healed specimens, respectively. The bending strength (B) of the heat-treated smooth specimens scattered to a certain extent, giving a mean fracture stress of about 1000 MPa. Because of precracking of 100 m, the bending strength decreased down to
450 MPa. However, the average value of B for the specimen crack-healed at 1300°C for 1 h in air was about 1000 MPa. Thus, B of the crack-healed specimens recovered to a level similar to that of the heat-treated smooth specimens. Fig. 1. SEM micrographs of surface cracks: (a) indentation and crack; (b) fracture surface. Fig. 2. SEM micrographs showing a surface crack introduced by Vickers indentation: (a) surface crack before crack-healing, (b) surface crack after crack-healing at 1300°C for 1 h in air. 1260 Journal of the American Ceramic Society—Lee et al. Vol. 87, No. 7