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MATERIALS RESEARCH

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Slow crack-growth behavior of alumina ceramics M.E. Ebrahimi, J. Chevalier, and G. Fantozzi GEMPPM, INSA-Lyon, 69621 Villeurbanne Cedex, France (Received 15 July 1999; accepted 25 October 1999)

The fracture behavior of high-purity alumina ceramics with grain sizes ranging from 2 to 13 ␮m is studied by means of the double torsion method. Crack-propagation tests conducted in air, water, and silicon oil, for crack velocities from 10−7 to 10−2 m/s, show that slow crack growth is due to stress corrosion by water molecules. An increase of the grain size leads to enhanced crack resistance, which is indicated by a shift of the V–KI (crack velocity versus applied stress intensity factor) plot toward high values of KI. Moreover, the slope of the curve is apparently higher for coarse grain alumina. However, if the R-curve effect is substracted from the experimental results, a unique V–KItip (crack velocity versus stress intensity factor at the crack tip) law is obtained for all alumina ceramics, independently of the grain size. This means that the crack-growth mechanism (stress corrosion by water molecules) is the same and that the apparent change of the V–KI law with grain size is a direct effect of crack bridging.

I. INTRODUCTION

For most ceramics, the presence of a reactive environment causes cracks to grow at subcritical stress levels. This phenomenon is commonly referred to as static fatigue or delayed failure. The basic mechanism for this slow crack growth is the stress-enhanced chemical interaction between environmental species (especially water) and the crack tip bonds, the crack growing at a rate determined by the reaction kinetics.1–6 There may, however, exist a subsidiary molecular flow leading up to the reaction stage.4 Thus transport of water molecules can play a significant role in the crack rate. In liquid form, environmental water maintains close contact with the growing crack by means of a rapid capillary action. Only when the crack approaches its terminal velocity does fluid viscosity appears to appreciably retard the transport process.7 Consequently, slow crack growth in the system Al2O3–H2O(l), like SiO2–H2O(l), is governed almost by reaction kinetics. In vapor form, however, the transport of moisture along the crack is inhibited by interfacial diffusion process, particularly under “dilute gas” conditions in which the mean free path intermolecular collision greatly exceeds the typical crack– wall separation.8 This impedance in molecular flow rate to the tip noticeably limits the slow crack velocities in the systems Al2O3–H2O(g) and SiO2–H2O(g). In the literature, these two rate-limiting mechanisms, i.e., reaction at the crack tip for low crack rates and then transport of molecules from the crack mouth to the crack tip for higher crack rates, are denoted region I and region II of crack propagation.4,8,9 A third stage occurs for even 142

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J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 23 Jan 2015

higher crack rates (region