Effects of the Amorphous Oxide Intergranular Layer Structure and Bonding on the Fracture Toughness of a High Purity Sili
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Effects of the Amorphous Oxide Intergranular Layer Structure and Bonding on the Fracture Toughness of a High Purity Silicon Nitride A. Ziegler1, C. Kisielowski2, M. J. Hoffmann3 and R. O. Ritchie2 1
Lawrence Livermore National Laboratory, University of California, Livermore, CA 94551, USA Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA 3 IKM, University of Karlsruhe, D-76131 Karlsruhe, Germany 2
ABSTRACT The microstructural evolution and structural characteristics and transitions in the thin grainboundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt% Y2O3 ceramic for ~200 hr at 1400°C in air, the fracture toughness can be increased by ~100%, coincident with a change in fracture mechanism from transgranular to intergranular. Structural phase transformations occur in the thin grain boundaries during oxidation that are revealed by XRD, EDX, Raman and EELS analyses. They affect the local bonding of atoms. It is concluded that only specific crystal “building blocks”, i.e., tetrahedra, are transformed along the grain boundary and the resulting difference in the atomic structure of the oxide interface is seen directly to alter the macroscopic fracture behavior. INTRODUCTION Minute structural changes and transitions along the thin grain-boundary films in silicon nitride ceramics can have a marked influence on macroscopic behavior. Indeed, post-sintering heat treatment of Si3N4 ceramics has been used to improve their mechanical properties, specifically by fully or partially crystallizing the amorphous triple junction regions [1,2], although the mechanism by which, for example, the fracture toughness is increased is uncertain. Triple junctions and thin grain boundaries are interconnected and thus their respective transformations and chemical balances can influence each other. However, it is not clear what happens in terms of atomic structure and bonding at the very local level of the thin grain boundaries. One can propose that the local bonding character and specific structural transitions along the thin two-grain interface are responsible for the marked changes in the mechanical response. During an oxidative heat treatment, it is known that oxygen diffuses into the bulk material while sintering additive cations, and possibly also nitrogen, diffuse outward to the surface. It is also probable that oxygen replaces nitrogen along the thin grain boundary, consistent with calculated binding energies of Si-clusters with various O/N compositions, which show that Si-O is the more favorable bond to Si-N [3]; this implies that a low N:O ratio strengthens the atomic bonding along the boundary. However, Becher et al. [4] has observed that a decrease in N:O ratio apparently reduces the interfacial debonding energy for b-Si3N4 whiskers embedded in a Si-Al-Y oxynitride glass. Furthermore, other
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