3650 w Nakao et al. /Journal of the European Ceramic Sociery 25(2005)3649-3655 of a-Al2O3 was 5-6 um. The Sic whiskers were located at grain boundaries between a-Al2O3 grains and were preferer tially oriented within the plane perpendicular to the pressing axis. The density of the sintered plate was 3.83g/cm3.The ic whi sintered plate was cut into the 3mm x 4 mm x 23 mm rect angular specimens bar that were polished to a mirror finish on one face and the edges of specimens were beveled 45to reduce the likelihood of edge initiated failures A semi-elliptical surface crack of 100 um in surface length, as shown in Fig. 2, was made at the center of the tensile surface of specimens with a Vickers indenter, using a load of 196N. The aspect ratio(alc)of the pre-crack which is depth(a) to half the surface length(c)was 0.9. In this invest 5 gation, the pre-crack was healed in air while applying tensile static or cyclic stresses by the three-point loading system shown in Fig 3. The pre-crack was subjected to tensile stress Fig. 1. Microstructure of the SiC reinforced alumina composite, alumina( w). before the crack-healing was started by heating the furnace to avoid the crack-healing under no-stress. Then, the specimen was kept at the above condition for 8 h to finish completely cles. However, oxygen is necessary for the crack-healing, so the crack-healing process. The applied cyclic stress was sinu- em bedded flaws cannot be healed soidal with a stress ratio. maximum stress/minimum stress In conclusion, for higher structural integrity, both high of 0.2 and a frequency of 5 Hz. Moreover, the crack-healing fracture toughness and crack-healing ability should be satis- behavior under stress was also investigated for specimens factorily achieved. From this perspective, alumina reinforced annealed at 1300C for I h in Ar after pre-cracking so as to by 20 mass%SiC whisker, abbreviated alumina( w),was sin- remove the residual stress introduced during pre-cracking by tered and the mechanical properties of alumina( w)were in- using the indentation method vestigated in the previous studies. 4, 5 Alumina(w)exhibited large fracture toughness(6.5 MPam )and excellent crack healing ability. However, the basic crack-healing behavior as a function of crack-healing temperature and time was not well Tension direction investigated. Moreover, crack-healing behavior under stress and the critical stress condition were not investigated at all Therefore, alumina( w) has been subjected to crack-healing under elevated static and cyclic stresses at 1200C, corre sponding to the limiting temperature for the bending strength of alumina( w). The bending strength of alumina( w) crack- Vickers healed under stress has been measured at the crack-healing indentation emperature. From the results, an upper limiting stress that can be applied during crack-healing has been determined for alumina( w) 2. Experimental The a-Al2O3 powder(AKP-20, Sumitomo Chemicals Co td., Japan) used in this study has a purity of 99.999% and a 照 Pre-crack mean particle size of 0. 4-0.6 um. The SiC whiskers (SCw Tateho Chemical Industry Co Ltd, Japan)used have diame ters between 0. 8 and 1.0 um and lengths from 30 to 100 um The mixture of a-Al2O3 powder and 20 mass% SiC whisker was blended well in isopropyl alcohol for 12 h using alumina balls and an alumina mill pot. Then, the slurry was dried Rectangular plates of 50 mm x 50 mm x 9 mm were sintered via uniaxial hot press in Ar at 1850C for I h at 40 MPa. Fig. I shows the microstructure of the sintered SiC whisker Fig. 2. SEM images of (a)surface shapes and(b)cross-sectional shapes of reinforced alumina composite, alumina( w). The grain size the crack and indentation3650 W. Nakao et al. / Journal of the European Ceramic Society 25 (2005) 3649–3655 Fig. 1. Microstructure of the SiC reinforced alumina composite, alumina(w). cles. However, oxygen is necessary for the crack-healing, so embedded flaws cannot be healed. In conclusion, for higher structural integrity, both high fracture toughness and crack-healing ability should be satis￾factorily achieved. From this perspective, alumina reinforced by 20 mass% SiC whisker, abbreviated alumina(w), was sin￾tered and the mechanical properties of alumina(w) were in￾vestigated in the previous studies.14,15 Alumina(w) exhibited large fracture toughness (6.5 MPa m1/2) and excellent crack￾healing ability. However, the basic crack-healing behavior as a function of crack-healing temperature and time was not well investigated. Moreover, crack-healing behavior under stress and the critical stress condition were not investigated at all. Therefore, alumina(w) has been subjected to crack-healing under elevated static and cyclic stresses at 1200 ◦C, corre￾sponding to the limiting temperature for the bending strength of alumina(w). The bending strength of alumina(w) crack￾healed under stress has been measured at the crack-healing temperature. From the results, an upper limiting stress that can be applied during crack-healing has been determined for alumina(w). 2. Experimental The
-Al2O3 powder (AKP-20, Sumitomo Chemicals Co. Ltd., Japan) used in this study has a purity of 99.999% and a mean particle size of 0.4–0.6m. The SiC whiskers (SCW, Tateho Chemical Industry Co. Ltd., Japan) used have diame￾ters between 0.8 and 1.0m and lengths from 30 to 100m. The mixture of
-Al2O3 powder and 20 mass% SiC whiskers was blended well in isopropyl alcohol for 12 h using alumina balls and an alumina mill pot. Then, the slurry was dried. Rectangular plates of 50 mm × 50 mm × 9 mm were sintered via uniaxial hot press in Ar at 1850 ◦C for 1 h at 40 MPa. Fig. 1 shows the microstructure of the sintered SiC whisker reinforced alumina composite, alumina(w). The grain size of
-Al2O3 was 5–6m. The SiC whiskers were located at grain boundaries between
-Al2O3 grains and were preferen￾tially oriented within the plane perpendicular to the pressing axis. The density of the sintered plate was 3.83 g/cm3. The sintered plate was cut into the 3 mm × 4 mm × 23 mm rect￾angular specimens bar that were polished to a mirror finish on one face and the edges of specimens were beveled 45◦ to reduce the likelihood of edge initiated failures. A semi-elliptical surface crack of 100m in surface length, as shown in Fig. 2, was made at the center of the tensile surface of specimens with a Vickers indenter, using a load of 19.6 N. The aspect ratio (a/c) of the pre-crack which is depth (a) to half the surface length (c) was 0.9. In this investi￾gation, the pre-crack was healed in air while applying tensile static or cyclic stresses by the three-point loading system shown in Fig. 3. The pre-crack was subjected to tensile stress before the crack-healing was started by heating the furnace to avoid the crack-healing under no-stress. Then, the specimen was kept at the above condition for 8 h to finish completely the crack-healing process. The applied cyclic stress was sinu￾soidal with a stress ratio, maximum stress/minimum stress, of 0.2 and a frequency of 5 Hz. Moreover, the crack-healing behavior under stress was also investigated for specimens annealed at 1300 ◦C for 1 h in Ar after pre-cracking so as to remove the residual stress introduced during pre-cracking by using the indentation method. Fig. 2. SEM images of (a) surface shapes and (b) cross-sectional shapes of the crack and indentation.