Fracture Resistance of Coated Polycrystalline Silicon Wafers
Sustainable living is a current research priority worldwide with solar energy being the key in achieving long-lasting natural power sources. The main functional units in the solar energy devices are silicon wafers in both single- and poly-crystalline forms. Their inherent defects and brittleness, however, impair their performance and reliability and can lead to premature fracture. This becomes important in applications in which silicon-based devices are subject to cyclic loadings, such as winds (on roofs) and mechanical vibrations (in cars). The wafers may also be subjected to temperature changes due to seasonal and daily variations. Most of the studies on the fracture resistance of silicon wafers have considered the single-crystal type. This project focused on the behaviour of solar-grade, large-grained polycrystalline silicon wafers under cyclic thermal and mechanical loadings. Mainly experimental studies were conducted on the failure of raw and polished wafers, uncoated and coated with nano-sized Si3N4 coatings. The wafers were mechanically characterised. The coating material was also mechanically characterised using instrumental nano-indentation and the effect of the coating thickness revealed. The residual stresses introduced during film deposition were also determined by using curvature measurements. Artificial defects were induced in the samples by means of Vickers and Berkovich indentations, to stimulate fatigue fracture under routine laboratory conditions. A special attention was paid to the resistance of the wafers to cyclic thermal and mechanical loadings. The thermal fatigue tests were conducted by heating the specimens to 300°C and consequently cooling in icy water (2°C). The mechanical fatigue tests were performed under biaxial flexural loading conditions utilising the ball-on-ring configuration. The crack propagation and damage mechanism were frequently examined using optical microscopy, SEM, and laser-confocal microscopy. Evidence was found that uncoated large-grained polycrystalline silicon wafers, although considered inert and fatigue-resistant to Mode I fatigue fracture, do exhibit crack propagation under tensile stresses induced by thermal and mechanical cyclic loads. Coating with Si3N4 appears to be beneficial for the fracture resistance of the wafers since the coated samples did not display crack extension even at a large number of cycles, instead they exhibited a crack arrest phenomenon.
Sustainable living, Solar energy, Silicon wafers, Poly-crystalline, Fatigue resistance, anzsrc-for: 401601 Ceramics, anzsrc-for: 401605 Functional materials, anzsrc-for: 401699 Materials engineering not elsewhere classified, anzsrc-for: 4016 Materials engineering
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