Fracture mode of alumina/silicon carbide nanocomposites
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Fracture mode of alumina/silicon carbide nanocomposites Andre´ Zimmermann Darmstadt University of Technology, Department of Materials Science, Petersenstrasse 23, 64287 Darmstadt, Germany
Mark Hoffman University of New South Wales, School of Materials Science and Engineering, Sydney, NSW 2052, Australia
Ju¨rgen Ro¨del Darmstadt University of Technology, Department of Materials Science, Petersenstrasse 23, 64287 Darmstadt, Germany (Received 7 May 1999; accepted 12 October 1999)
Computer simulations have been designed to elucidate the evolution of microcracking in a nanocomposite using appropriate material values for alumina and silicon carbide. These are compared to a single-phase material using elastic and thermal expansion coefficients for alumina. It is found that the region and the fracture mode where microcracking ensues are determined by the intensity and the length scale of the residual stress fields, which interact. Of specific interest are the region, fracture mode, and length of ensuing microcracks for materials with different inclusion locations (at the grain boundary or within the grain) and with different grain size to inclusion size ratios. I. INTRODUCTION
An important factor governing the fracture properties of a ceramic, or any other material, is the nature of cracks on a microstructural level. Ceramics researchers have long recognized this. For many years the development of structural ceramics was based upon the concept that strength and reliability may be improved by increasing toughness, as explained by traditional Griffith fracture mechanics (f ⳱ KIc /√c, with KIc the fracture toughness and c the crack length). Toughness, in the form of a rising crack resistance curve, may be increased by bridging of the crack by interlocking grains. Full exploitation of this process requires intergranular fracture and large grains to produce large crack bridges.1 This strategy, however, ignores the Orowan–Petch relation, which states an inverse relationship between microstructural dimension, e.g., grain size, and strength.2 Consequently, despite significant recent developments in increasing toughness in ceramics, strength and reliability improvements have been commonly attributed to defect size reductions through improved processing.3 An exception is when a steep and pronounced R-curve at small crack length is observed, as in the case of silicon nitride.4 Occurring concurrently has been the development of nanocomposites where nanometer-sized particles (usually silicon carbide) are distributed throughout a matrix of alumina, silicon nitride, or magnesia with the particulate phase located both within the grain and at the grain boundary.5–7 Nanocomposites have displayed dramatic strength improvements relative to monolithic ceramics. However, in contradiction to previous strategies, the fracJ. Mater. Res., Vol. 15, No. 1, Jan 2000
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ture mode in nanocomposites is predominantly transgranular and toughness increas
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