Effects of crystal bonding on brittle fracture
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I. INTRODUCTION Compounds from the IV, III-V, and II-VI columns of the Periodic Table form a series of materials having essentially the same crystal structure but whose atomic bonds possess differing proportions of the ionic and covalent character. Such a series of materials provides us with the opportunity to investigate how bond ionicity affects brittle fracture while minimizing possible complicating contributions resulting from changes in atomic symmetry. Two aspects of brittle fracture in these materials will be considered in this article. The first part of the article discusses the calculation and measurement of the fracture energy for single-crystal compounds. Throughout this article we assume an equivalence between fracture energy, meaning the energy required to grow a crack in the absence of environmental effects, and surface energy, the energy required to separate two planes of atoms to infinity. Implicit in this assumption of equivalence is the assumption that surface relaxation effects can be ignored. An indentation technique was used to measure values of the fracture energy y that were then compared with those calculated using a model developed by previous investigators. '~3 The second portion of the article concerns the determination of those environments that enhance crack propagation. Studies have demonstrated that certain chemical environments increase crack growth rates in some brittle materials well above the rates observed in inert environments.4'5 Recent work6 has shown that the environments that enhance fracture appear to vary depending on the types of bonds fractured. For example,6 acetonitrile, which does not enhance crack growth in mostly covalent (—70%) vitreous silica, does do so in MgF 2 , which is ~ 100% ionically bound. On the other hand, ammonia vapor, which is an ineffective crack growth agent for MgF 2 , enhances fracture in vitreous silica. This work led to the hypothesis that variations in the ionicity of the chemical bond in the solid lead to different crack growth mechanisms and, hence, to variations in which environments would affect crack propaJ. Mater. Res. 3 (3), May/Jun 1988
gation. However, this hypothesis is difficult to evaluate because, in the previous work, other material parameters such as crystal structure have not been held constant. The materials used in this work were chosen to address this difficulty. For the fracture energy determination, the compounds studied were those that crystallize in the diamond cubic structure and its bimolecular equivalent, the cubic sphalerite structure. In addition, two materials, CdS and CdSe, which have hexagonal rather than cubic symmetry, were included because their nearest-neighbor symmetries were the same as those of the sphalerite materials. For the investigation of environmental effects on fracture, other materials such as MgF 9 were included in order both to expand the range of bond ionicity to nearly 1.0 and to compare results with those of the previous investigation.
II. EXPERIMENTAL PROCEDURE A. Materials Single-crystal s
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