CERAMICS INTERNATIONAL ELSEVIER Ceramics International 26(2000)801-805 Damage resistance and R-curve behavior of multilayer Al2O3 /SiC ceramics Jihong She*, Takahiro Inoue, Kazuo Ueno Department of Energy Conversion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received I September 1999: received in revised form 14 September 1999; accepted 12 January 2000 Damage resistance and R-curve behavior of multilayer Al,O /Sic ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods Due to the crack deflection at the Al2O3 /SiC interfaces, a plateau indentation strength observed to increase from 8.0 to 15.5 MPa mk al resistance to contact-induced damage. Moreover, fracture toughness was ith increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior. C 2000 Elsevier Science Ltd and Techna S.r. l. All rights reserved Keywords: B. Interfaces; C. Fracture; C. Mechanical properties; D. Al203 1. Introduction 2. Experimental procedure In recent years, there has been considerable interest in The fabrication process of multilayer Al2O3/SiC cera- le mechanical behavior of a variety of multilayer cera- mics has been described elsewhere [13]. Briefly, sub mic composites. It has been shown that enhanced micrometer a-Al2O3 powders(0.22 um, TM-D, Taimei strength and toughness in combination with improved Chemicals Co, Ltd, Japan)were extrusion-molded into damage resistance can be achieved for three-layer cera- 0.35-mm-thick sheets, then coated with a 50 vol% SiC mic laminates consisting either of two strong outer lay- containing slurry, and finally hot-pressed in a graphite ers and a tough inner layer [1, 2] or of two compressive die under a 25-MPa pressure at 1500C for I h. The outer layers and a tensile inner layer [3-7. On the other resultant layered structure is shown in Fig. I. As can be hand, increased apparent toughness and fracture energy seen, the Al2O3 / SiC interfaces are smooth and well as well as a non-catastrophic fracture behavior have defined. The thicknesses of the Al]O3 and SiC layers been demonstrated for multilayer ceramic laminates by were determined to be 182.7+10.1 and 12.9+1.4 um introducing weak interfaces [8-ll] or by inserting por- respectively To evaluate the fracture behavior, the hot-pressed billets were machined into the rectangular bars of 3 mm and fracture resistance behavior of these laminates (thickness)x4 mm(width)x40 mm (length), and inden More recently, we have successfully fabricated a mul- ted at the center of one polished tensile surface with a tilayer Al2O3/ SiC ceramic [13]. Due to the deflection of Vickers diamond pyramid indenter under loads ranging through-thickness cracks at the weak Sic interfaces, such from 3 to 500 N. All indentations were made a laminate fails gracefully, and its apparent toughness the indenter contacted the specimen surface for 30 s and fracture energy are up to 15. 1 MPa m and 3335 J/ before it was withdrawn to complete the load cycle m, respectively. In this work, the damage resistance and Care was taken to orient one set of the indentation R-curve behavior are further investigated cracks to be parallel to the longitudinal axis of the rec- orresponding author at present address: Institute of Materials, angular bars. After indentation, the specimens were German Aerospace Center (D tested in three-point bending with a loading span of 2203-69648 mm and a crosshead speed of 0.5 mm /min. Three tests E-mailaddress:jhshe(@hotmail.com(She) were performed at each indentation load 0272-8842/00/S20.00 C 2000 Elsevier Science Ltd and Techna S r l. All rights reserved. PII:S0272-8842(00)00020-1Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics Jihong She *, Takahiro Inoue, Kazuo Ueno Department of Energy Conversion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 1 September 1999; received in revised form 14 September 1999; accepted 12 January 2000 Abstract Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods. Due to the crack de¯ection at the Al2O3/SiC interfaces, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior. # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: B. Interfaces; C. Fracture; C. Mechanical properties; D. Al2O3 1. Introduction In recent years, there has been considerable interest in the mechanical behavior of a variety of multilayer cera￾mic composites. It has been shown that enhanced strength and toughness in combination with improved damage resistance can be achieved for three-layer cera￾mic laminates consisting either of two strong outer lay￾ers and a tough inner layer [1,2] or of two compressive outer layers and a tensile inner layer [3±7]. On the other hand, increased apparent toughness and fracture energy as well as a non-catastrophic fracture behavior have been demonstrated for multilayer ceramic laminates by introducing weak interfaces [8±11] or by inserting por￾ous layers [12] between ceramic substrates. However, little work [12] has been performed on the ¯aw tolerance and fracture resistance behavior of these laminates. More recently, we have successfully fabricated a mul￾tilayer Al2O3/SiC ceramic [13]. Due to the de¯ection of through-thickness cracks at the weak SiC interfaces, such a laminate fails gracefully, and its apparent toughness and fracture energy are up to 15.1 MPa m1/2 and 3335 J/ m2 , respectively. In this work, the damage resistance and R-curve behavior are further investigated. 2. Experimental procedure The fabrication process of multilayer Al2O3/SiC cera￾mics has been described elsewhere [13]. Brie¯y, sub￾micrometer a-Al2O3 powders (0.22 mm, TM-D, Taimei Chemicals Co., Ltd., Japan) were extrusion-molded into 0.35-mm-thick sheets, then coated with a 50 vol% SiC￾containing slurry, and ®nally hot-pressed in a graphite die under a 25-MPa pressure at 1500C for 1 h. The resultant layered structure is shown in Fig. 1. As can be seen, the Al2O3/SiC interfaces are smooth and well￾de®ned. The thicknesses of the Al2O3 and SiC layers were determined to be 182.710.1 and 12.91.4 mm, respectively. To evaluate the fracture behavior, the hot-pressed billets were machined into the rectangular bars of 3 mm (thickness)4 mm (width)40 mm (length), and inden￾ted at the center of one polished tensile surface with a Vickers diamond pyramid indenter under loads ranging from 3 to 500 N. All indentations were made in air, and the indenter contacted the specimen surface for 30 s before it was withdrawn to complete the load cycle. Care was taken to orient one set of the indentation cracks to be parallel to the longitudinal axis of the rec￾tangular bars. After indentation, the specimens were tested in three-point bending with a loading span of 30 mm and a crosshead speed of 0.5 mm/min. Three tests were performed at each indentation load. 0272-8842/00/$20.00 # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(00)00020-1 Ceramics International 26 (2000) 801±805 * Corresponding author at present address: Institute of Materials, German Aerospace Center (DLR), 51147 KoÈln, Germany. Fax: +49- 2203-696480. E-mail address: jhshe@hotmail.com (J. She).