Dynamics and Morphology of Cracks in Silicon Nitride Films: A Molecular Dynamics Study on Parallel Computers
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ABSTRACT Multiresolution molecular dynamics approach on parallel computers has been used to investigate fracture in ceramic materials. In microporous silica, critical behavior at fracture is analyzed in terms of pore percolation and kinetic roughening of fracture surfaces. Crack propagation in amorphous silicon nitride films is investigated, and a correlation between the speed of crack propagation and the morphology of fracture surfaces is observed. In crystalline silicon nitride films, temperature-assisted void formation in front of a crack tip slows down crack propagation.
INTRODUCTION Porous silica has been the focus of many investigations [1]. This environmentally safe material has numerous technological applications: It is used in thermal insulation of commercial and household refrigerators; in passive solar energy collection devices; in particle detectors; and in catalysis and chemical separation. There is an exciting possibility of utilizing it as an embedding framework in optical switches made of quantum-confined microclusters. Since these applications are due to the remarkable porous structure of the system, it is important to understand the size and spatial distributions of pores and the morphology of pore interfaces. Silicon nitride has been at the forefront of research for high-temperature, high-strength materials owing to its outstanding properties [2]. The combination of low thermal expansion and high strength makes silicon nitride one of the most thermal-shock-resistant materials currently available. The strong covalent bonding between the atoms results in a superb resistance of mechanical deformation and chemical corrosion. Additionally, silicon nitride based films have found a number of applications in microelectronics technology. For these applications, the mechanism of fracture is one of the most important issues. The morphology of fracture surfaces has drawn a great deal of attention in recent years [3-7]. It is now well-established that a fracture surface, z(x,y), is a self-affine object in that it remains invariant under the transformation, (x, y, z) -> (ax, ay, aý z), where ý is known as the roughness exponent. A decade ago, Mandelbrot and coworkers reported the first measurements of the roughness exponent for metallic surfaces [3]. Since then there has been a great deal of controversy regarding the value of ý. Bouchaud, Lapasset, and Plan6s carried out measurements 205 Mat. Res. Soc. Symp. Proc. Vol. 408 01996 Materials Research Society
of ý for four aluminum alloys with different heat treatments and in each case they obtained ý = 0.8 [4]. Mailoy et al. made measurements on six different brittle materials and found ý to be 0.87 ± 0.07 [5]. This led them to conjecture that fracture surfaces of brittle and ductile materials had a "universal" roughness exponent, independent of material characteristics and the mode of fracture. Milman et al. questioned the validity of the "universality" of ý, especially at microscopic length scales, by pointing out that their scanning tunneling microscopy dat
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