Fractography and crack propagation in functional crystals

Abstract

Failure in structural materials is generally well-known and documented with established standards. Unfortunately, for the case of functional crystals that are found in all our smart devices, failure remains poorly understood and studied although being a very common part of our daily lives. Conventional rupture often occurs catastrophically. The propagation of cracks is sudden and can often be disturbed by instabilities of physical nature, by the orientation of the crystal and/or mechanical stresses. The local stress intensity factor (SIF) is a preeminent element in the study of brittle functional crystals. The SIF changes when the topographic characteristics of the fracture surface are modified during propagation, for example when crystallographic facets are formed. At the time of failure, these singularities remain inscribed on the surfaces newly created and are often the only clues still visible and available to investigate the mechanical history of the crystal. Therefore, the study of SIF in correlation with fractographic traces is the most powerful indicator for understanding the fracture mechanics in the processes of rapid crack propagation. On the other hand, no standards exist for the evaluation of the mechanical failure of functional crystals where their study becomes more complex regarding their strongly anisotropic mechanical behavior. To date, there are very few studies to understand and assess the level of stresses in these materials when a failure occurs with a sufficient resolution, without homogenizing the anisotropic elastic properties. The project focuses on the analytical and experimental understanding of crack propagation in silicon (Si) and gallium arsenide (GaAs) single crystals as model semiconductor materials. The goal is the acquisition of fundamental knowledge in fractography to understand the morphology of fracture surfaces and relate them to the applied mechanical stresses. The project integrates the development of innovative methods and procedures for estimating the mechanical properties of GaAs and Si considering the intrinsic anisotropy of single crystals. This work includes the advanced analysis of the fracture surface of single crystals, which is generally found in micro-electromechanical systems (MEMS) whose properties and behaviors are often subject of debate as no established standard reference exists. The crack surfaces are analyzed to identify and characterize the topographic singularities of the experimentally overloaded samples. Preliminary sample preparation work, as well as mechanical testing at different scale lengths, is part of this study. Along with the experimental work, aspects of unstable crack propagation are studied with analytical modeling that includes dynamic instability, anisotropy, the study of stresses in the vicinity of a crack, and the evaluation of physical properties directly related to the release of strain energy in the crystal. The analytical modeling is validated by the experimental results to predict the behavior of functional crystals.

Date	16 Aug 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Ricardo J. Zednik (Supervisor) & Roberto Dugnani (Co-supervisor)