July 2004 High-Temperature Fatigue Strength of Crack-Healed Al-O, Toughened by SiC Whiskers 1263 Specimen type O.Heat-treated smooth specitmen900C Heat-treated smooth specimen Crack-healed specimen 900C oHeat-treated smooth specitmen1100C oHeat-treated smooth spec trmen1100c Crack -healed specimen l100C Crack-healed specimen l100c 1400 1400 Static fatigue test 400 20[:nm Healing condition: 1300 C, Ih in air Monotonic 10 10 for fatigue 10 le to Failure, t,(sec) Fig. 5. Results of static fatigue test of crack-healed Al,O,SiC-W at 900 and 1100°C Fig. 6. Results of cyclic fatigue test of crack-healed Al2O: /SiC-Wat mpared with the bending strength(oB) of each specimen at 900C, i.e., UoB> 90%(see also Table I static stresses with a maximum stress equal to the maximum cyclic the case of the cyclic fatigue test at 1100oC, the values of the stress. These fatigue test results are reasonable considering that cyclic fatigue limit of the heat-treated smooth specimen and the clic loading with a short duration of maximum stress during crack-healed specimen are about 700 and 500 MPa, respectively. each cyclic provides a beneficial effect on lifetime in comparison The ratios of cyclic fatigue limit to the bending strength of each to static loading at the same maximum stress. specimen at 1100.C are more than 75%0, i.e., U/oB>75%. The In a previous study, - it was found that the combination of ratio of oa/oB decreases with increasing test temperature. How- crack-healing and whisker reinforcement was very useful for ever, it can be said that the ratio of o o /oB is sufficiently high, even the bending strength and the fracture te at 1100°C Al,O3. The results of this study show that the combination of The values of cyclic fatigue limit of crack-healed specimen are crack-healing and whisker reinforcement is very useful to increase lower than those of heat-treated smooth specimens at 900 and static and cyclic fatigue strengths at high temperatures 1100 C. These reductions are caused by differences in the bending strength, as shown on the left-hand side of Fig. 6 and in Table I Therefore, it is necessary to increase both the bending strength and IV. Conclusions the fatigue limit of crack-healed specimens at elevated tempera tures to the same level as heat-treated smooth specimens. This Al,O3 reinforced by SiC whiskers(Al,O,SiC-W)was sintered matter is the subject of further study. and the specimens were subjected to three-point bending. A The bending strength of the specimens which survived the semielliptical surface crack of 100 um surface length was made on cyclic fatigue tests were also investigated. The bending strength each specimen. These specimens were crack-healed at 1300%C for are shown on the right-hand side of Fig. 6. bending tests were h in air, and cyclic and static fatigue strengths were systemati- rformed at each testing temperature, i.e, 900 and 1100.C. cally investigated at room temperature, 900, and 1100C. The Again, the heat-treated specimens and the crack-healed specimens main conclusions obtained are as follows that survived the cyclic fatigue test showed bending strengths (1 The values of static(c o)and cyclic(up) fatigue limits for similar to monotonically tested specimens the crack-healed specimens were 750 and 850 MPa, respectively, Table I summarizes the bending strength, the static fatigue limit, at room temperature. The static and cyclic fatigue limits were and the cyclic fatigue limit at room temperature, 900, and about 75% and 85%, respectively, in comparison to the bending 1 100.C. The cyclic fatigue limit of crack-healed specimens is strength at room temp rature Moreover. the values of on and higher than that of the static fatigue limit, regardless of the test for the heat-treated specimens were 800 MPa. Thus, the static and temperature investigated in this study, i.e., room temperature, cyclic fatigue limits of the crack-healed specimens are similar to 900, and 1100C. This is due to the following. The cyclic(Uo) those of the heat-treated specimens, indicating that room- and static (o,o) fatigue limit are defined as the maximum applied ature fatigue strength of the crack-healed zone is compara- stress at which a specimen did not fracture up to Ne=2 X 10 ble to that of base materials cycles and t=10s. This Nr of the cyclic fatigue test is equivalent (2) The static fatigue limits of crack simens(co)at to 4x 10 s. Thus, the cyclic fatigue test is shorter than the static 00° and 1100°were550and450 spectively atigue test. Moreover, previous studies- showed that cyclic values were about 70% in comparison to trength(oB)at stresses extend the lifetime in Al,O, and Si3N4 compared with each temperature.compared with the bending strength (B) of each specimen at 900°C, i.e., f0/B  90% (see also Table I). In the case of the cyclic fatigue test at 1100°C, the values of the cyclic fatigue limit of the heat-treated smooth specimen and the crack-healed specimen are about 700 and 500 MPa, respectively. The ratios of cyclic fatigue limit to the bending strength of each specimen at 1100°C are more than 75%, i.e., f0/B  75%. The ratio of f0/B decreases with increasing test temperature. How￾ever, it can be said that the ratio of f0/B is sufficiently high, even at 1100°C. The values of cyclic fatigue limit of crack-healed specimen are lower than those of heat-treated smooth specimens at 900° and 1100°C. These reductions are caused by differences in the bending strength, as shown on the left-hand side of Fig. 6 and in Table I. Therefore, it is necessary to increase both the bending strength and the fatigue limit of crack-healed specimens at elevated tempera￾tures to the same level as heat-treated smooth specimens. This matter is the subject of further study. The bending strength of the specimens which survived the cyclic fatigue tests were also investigated. The bending strengths are shown on the right-hand side of Fig. 6. Bending tests were performed at each testing temperature, i.e., 900° and 1100°C. Again, the heat-treated specimens and the crack-healed specimens that survived the cyclic fatigue test showed bending strengths similar to monotonically tested specimens. Table I summarizes the bending strength, the static fatigue limit, and the cyclic fatigue limit at room temperature, 900°, and 1100°C. The cyclic fatigue limit of crack-healed specimens is higher than that of the static fatigue limit, regardless of the test temperature investigated in this study, i.e., room temperature, 900°, and 1100°C. This is due to the following. The cyclic (f0) and static (t0) fatigue limit are defined as the maximum applied stress at which a specimen did not fracture up to Nf  2 106 cycles and tf  106 s. This Nf of the cyclic fatigue test is equivalent to 4 105 s. Thus, the cyclic fatigue test is shorter than the static fatigue test. Moreover, previous studies36–38 showed that cyclic stresses extend the lifetime in Al2O3 and Si3N4 compared with static stresses with a maximum stress equal to the maximum cyclic stress. These fatigue test results are reasonable considering that cyclic loading with a short duration of maximum stress during each cyclic provides a beneficial effect on lifetime in comparison to static loading at the same maximum stress. In a previous study,29 it was found that the combination of crack-healing and whisker reinforcement was very useful for increasing the bending strength and the fracture toughness of Al2O3. The results of this study show that the combination of crack-healing and whisker reinforcement is very useful to increase static and cyclic fatigue strengths at high temperatures. IV. Conclusions Al2O3 reinforced by SiC whiskers (Al2O3/SiC-W) was sintered and the specimens were subjected to three-point bending. A semielliptical surface crack of 100 m surface length was made on each specimen. These specimens were crack-healed at 1300°C for 1 h in air, and cyclic and static fatigue strengths were systemati￾cally investigated at room temperature, 900°, and 1100°C. The main conclusions obtained are as follows: (1) The values of static (t0) and cyclic (f0) fatigue limits for the crack-healed specimens were 750 and 850 MPa, respectively, at room temperature. The static and cyclic fatigue limits were about 75% and 85%, respectively, in comparison to the bending strength at room temperature. Moreover, the values of t0 and f0 for the heat-treated specimens were 800 MPa. Thus, the static and cyclic fatigue limits of the crack-healed specimens are similar to those of the heat-treated specimens, indicating that room￾temperature fatigue strength of the crack-healed zone is compara￾ble to that of base materials. (2) The static fatigue limits of crack-healed specimens (t0) at 900° and 1100°C were 550 and 450 MPa, respectively. These values were about 70% in comparison to bending strength (B) at each temperature. Fig. 5. Results of static fatigue test of crack-healed Al2O3/SiC-W at 900° and 1100°C. Fig. 6. Results of cyclic fatigue test of crack-healed Al2O3/SiC-W at 900° and 1100°C. July 2004 High-Temperature Fatigue Strength of Crack-Healed Al2O3 Toughened by SiC Whiskers 1263