Availableonlineatwww.sciencedirect.com SCIENCE DIRECT● E噩≈S ELSEVIER Journal of the European Ceramic Society 25(2005)3649-3655 www.elsevier.com/locate/jeurceramsoc Critical crack-healing condition for Sic whisker reinforced alumina under stress Wataru Nakao*, Masato Ono, Sang-Kee Lee, Koji Takahashi, Kotoji Ando Department of Energy and Safery Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received 11 April 2004; received in revised form I September 2004, accepted 12 September 2004 Available online 8 December 2004 Abstract Alumina reinforced by Sic whisker, called here"alumina( w)"was developed with the objective of improving fracture toughness and crack- healing ability. The composites were crack-healed at 1200C for h in air under elevated static and cyclic stresses. The bending strength at 1200C of the crack-healed composites were investigated. The threshold static stress during crack-healing of alumina(w)has been determined to be 250 MPa, and the threshold cyclic stress was found to be 300 MPa. Considering that the crack growth is time-dependent, the threshold stress of every condition during crack-healing of alumina( w) was found to be 250 MPa. The results showed that the threshold stress intensity factor during crack-healing was 3. 8 MPam". The same experiment conditions were applied to specimens cracked and annealed at 1300C for I h in Ar, to remove the tensile residual stress at a tip of the crack. Thus, the threshold stress intensity factor during crack-healing was found to be 3.2 MPa 2 for the specimens crack-healed with annealing. The threshold stress intensity factor during crack-healing of alumina( w)was chosen to be 3.2 MPam"to facilitate comparison with the values of the threshold stress intensity factor during crack-healing. The residual stress was slightly larger than the intrinsic value C 2004 Elsevier Ltd. All rights reserved. Keywords: Crack-healing Composites; AlO3; SiC whiskers 1. Introduction nanocomposite ceramics". Some Al2O3 /SiC nanocompos- Alumina(Al2 O3) has three weak points: (1)low bendir for alumina whose value is 1300C, was achieved by ur strength(oB 400 MPa), (2)low heat-resistance limit tem- formly distributing nanosize SiC particles in alumina grains perature for strength(THR N950C, and(3)low fracture Related to above weakness(3), two possibilities to overcome toughness(KIC 4.0MPam). These weaknesses restrict low fracture toughness are(a)increase in fracture tough- the application of Al2O3 for important components. If these ness by reinforcing whiskers and fibers, and(b)endowing weaknesses were satisfactorily overcome, Al2O3 would be a strong self-crack-healing ability to heal the cracks. As to a leading candidate material for advanced gas turbine and the above(b), monolithic Al2O3 and AlO3/SiC compos- engine, because alumina exhibits much greater resistance to ites have very interesting crack-healing ability. 1, 7-15 That the oxidation than silicon nitride crack-healing behavior of monolithic Al,O3 was induced not For the weakness(1)above, the grain growth of Al2O3 by intrinsic crack-healing but by the secondary sintering, is luring sintering was retarded and strength was increased reported by the previous study , By contrast, Al2O3/SIC by the Hall-Petch effect similar to mullite/SiC by adding composite has been crack-healed by reaction of Sic and the 15-20 mass% nanosize SiC particles. Considering item(2), Oz in the atmosphere. The crack-healing behavior of the pre- Niihara and coworkers-6 have proposed the concept of vious Al203/SiC composite is assumed to be insufficient to overcome weakness(3)above. However, the good self-crack Corresponding author healing ability was introduced as for mullite/SiC2, 16-20 and E-imail address: nakao @ynu ac jp(w. Nakao) Si3 N4/SiC21-29 by adding 15-20 mass% nanosize SiC parti 0955-2219/S-see front matter c 2004 Elsevier Ltd. All rights reserved doi: 10.1016/j- jeurceramsoc 2004.09.021Journal of the European Ceramic Society 25 (2005) 3649–3655 Critical crack-healing condition for SiC whisker reinforced alumina under stress Wataru Nakao∗, Masato Ono, Sang-Kee Lee, Koji Takahashi, Kotoji Ando Department of Energy and Safety Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received 11 April 2004; received in revised form 1 September 2004; accepted 12 September 2004 Available online 8 December 2004 Abstract Alumina reinforced by SiC whisker, called here “alumina(w)” was developed with the objective of improving fracture toughness and crack￾healing ability. The composites were crack-healed at 1200 ◦C for 8 h in air under elevated static and cyclic stresses. The bending strength at 1200 ◦C of the crack-healed composites were investigated. The threshold static stress during crack-healing of alumina(w) has been determined to be 250 MPa, and the threshold cyclic stress was found to be 300 MPa. Considering that the crack growth is time-dependent, the threshold stress of every condition during crack-healing of alumina(w) was found to be 250 MPa. The results showed that the threshold stress intensity factor during crack-healing was 3.8 MPa m1/2. The same experiment conditions were applied to specimens cracked and annealed at 1300 ◦C for 1 h in Ar, to remove the tensile residual stress at a tip of the crack. Thus, the threshold stress intensity factor during crack-healing was found to be 3.2 MPa m1/2 for the specimens crack-healed with annealing. The threshold stress intensity factor during crack-healing of alumina(w) was chosen to be 3.2 MPa m1/2 to facilitate comparison with the values of the threshold stress intensity factor during crack-healing. The residual stress was slightly larger than the intrinsic value. © 2004 Elsevier Ltd. All rights reserved. Keywords: Crack-healing; Composites; Al2O3; SiC whiskers 1. Introduction Alumina (Al2O3) has three weak points:1 (1) low bending strength (σB ≈ 400 MPa), (2) low heat-resistance limit tem￾perature for strength (THR ≈ 950 ◦C), and (3) low fracture toughness (KIC ≈ 4.0 MPa m1/2). These weaknesses restrict the application of Al2O3 for important components. If these weaknesses were satisfactorily overcome, Al2O3 would be a leading candidate material for advanced gas turbine and engine, because alumina exhibits much greater resistance to oxidation than silicon nitride. For the weakness (1) above, the grain growth of Al2O3 during sintering was retarded and strength was increased by the Hall–Petch effect similar to mullite/SiC2 by adding 15–20 mass% nanosize SiC particles. Considering item (2), Niihara and coworkers3–6 have proposed the concept of ∗ Corresponding author. E-mail address: wnakao@ynu.ac.jp (W. Nakao). “nanocomposite ceramics”. Some Al2O3/SiC nanocompos￾ites exhibit excellent strength and THR. The increased of THR, for alumina whose value is 1300 ◦C, was achieved by uni￾formly distributing nanosize SiC particles in alumina grains. Related to above weakness (3), two possibilities to overcome low fracture toughness are (a) increase in fracture tough￾ness by reinforcing whiskers and fibers, and (b) endowing a strong self-crack-healing ability to heal the cracks. As to the above (b), monolithic Al2O3 and Al2O3/SiC compos￾ites have very interesting crack-healing ability.1,7–15 That the crack-healing behavior of monolithic Al2O3 was induced not by intrinsic crack-healing but by the secondary sintering, is reported by the previous study.1,13 By contrast, Al2O3/SiC composite has been crack-healed by reaction of SiC and the O2 in the atmosphere. The crack-healing behavior of the pre￾vious Al2O3/SiC composite is assumed to be insufficient to overcome weakness (3) above. However, the good self-crack￾healing ability was introduced as for mullite/SiC2,16–20 and Si3N4/SiC21–29 by adding 15–20 mass% nanosize SiC parti- 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.09.021