Anomalous cyclic fatigue-crack propagation behavior of small cracks in monolithic, grain-bridging ceramics C.J. Gilbert a , Y.S. Han b , D.K. Kim b , R.O. Ritchie a, * a Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720, USA b Department of Material Science and Engineering, Korea Advanced Institute of Science and Technology, Yusong-Gu, Taejon 305-701, South Korea Received 29 July 1999; received in revised form 7 September 1999; accepted 17 November 1999 Abstract Cyclic fatigue properties of two monolithic, high-toughness SiC ceramics were examined, with emphasis placed on dierences between long- (>3 mm) and small-crack (<250 mm) behavior. Studies were performed on two microstructures in which sintering additives were systematically varied. Small cracks were initiated with Vickers indents placed on the tensile surfaces of beams, and crack extension was monitored optically under an applied cyclic load. For small cracks, high growth rates which exhibited a negative dependence on the far- ®eld driving force were observed. Such behavior was rationalized both in terms of indent-induced residual stresses and the relative size of cracks compared to bridging zone lengths. # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: C. Fatigue; C. Fracture; C. Mechanical properties; D. SiC 1. Introduction Over the past decade, considerable research eort has focused on improving the toughness of monolithic ceramics [1]. A particularly successful strategy involves the promotion of highly elongated, heterogeneous microstructures (often referred to as in situ toughening). Silicon nitride is the earliest example of this approach [2], and more recent work on silicon carbide [3±5] has demonstrated its broad applicability. As cracks propagate through such microstructures, provided that fracture is intergranular, the elastic and frictional tractions gener- ated from the contact of opposing crack faces (generally termed ``grain bridging'') act to reduce the driving force in the immediate vicinity of the crack tip. As one might expect, the degree of shielding is enhanced in larger, more elongated microstructures. Indeed, long-crack fracture toughnesses approaching 10 MPa p m have been achieved in optimally designed Si 3 N 4 and SiC [2,5]. An important limitation associated with grain bridging involves the length scales over which it operates. In order for a saturated bridging zone to develop, the peak (or long-crack) fracture toughness is not reached until cracks are substantially larger than microstructural dimensions. This scaling eect gives rise to the ubiqui- tous ``resistance-curve'' always associated with tough ceramics [1]. Because the saturation distance (or bridging zone length) is typically on the order of 1 mm, long- crack toughnesses do not apply to microstructurally small cracks. Cracks in this size domain are known to dictate important mechanical properties such as strength, wear, and machinability, and in most struc- tural components the initial ¯aw distribution is indeed physically small. Moreover, cyclic-fatigue crack growth rates measured using long-crack specimens may over- estimate resistance to subcritical crack propagation. Recent work has demonstrated that microstructurally small cracks propagate at substantially higher growth rates than long cracks at equivalent driving forces under both quasi-static and cyclic loading conditions in a number of monolithic ceramics and their composites [6±9]. Therefore, the primary objective of the present study is to quantify cyclic fatigue-crack growth rates of small cracks in several high-toughness SiC microstructures. Particular emphasis is placed on the dierences between long-crack (through-thickness cracks with initial lengths >3 mm) and small-crack (indent-initiated surface cracks with initial lengths <200 mm) growth rates. 0272-8842/00/$ - see front matter # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(00)00010-9 Ceramics International 26 (2000) 721±725 * Corresponding author. Tel.: +1-510-642-0417; fax: +1-510-486- 4995. E-mail address: roritchie@lbl.gov (R.O. Ritchie).