Tech Briefs

Size Scale and Defect Engineered Nanostructures for Optimal Strength and Toughness

CRM measurements used a 514.5-nm Ar laser focused perpendicularly onto the (111) surface of the Si nanotower. Measurements taken after RTA heat treatments showed a significant blue shift, with the strongest shift for the 1200 °C treatment. Assuming the Si towers are under a biaxial stress from the CTE mismatch with the SiC coating, this blue shift represents a compressive stress of 0.8-2.8 GPa. While the magnitude of this stress is 3-5 times larger than an elastic estimate for concentric cylinders, the trend of increasing stress with increasing RTA temperature was verified by crosssectioning the towers along the radial axis and measuring the stress relaxation of the Si core.

Considering the increased stability of Si II in small volumes, together with the high compressive stresses found in the Si–SiC core-shell composites, a new toughening mechanism for Si–SiC nanocomposites was proposed as follows. First, Si I nanospheres with diameters in the range of 100-1000 nm are dispersed in a SiC matrix and heattreated to 1100-1200 °C. This will leave the Si nanospheres under a compressive stress of close to 3 GPa. The composite is then loaded to a stress exceeding the critical stress needed for the Si I→II transition (~8-10 GPa). Upon unloading, the nanospheres will remain as Si II since the residual compressive stress is higher than the Si II→I transition pressure. When a crack forms in the composite, the tensile stress at the crack tip will destabilize the Si II and cause a transformation back to Si I. The volume expansion during this transformation will then act as a compressive stress to close the approaching crack. If the tensile stress at the crack tip does not transform the Si II, the Si II nanospheres will act as either ductile inclusions for crack pinning or form voids due to the Si I→II contraction that will serve as crack arrest points. Thus, by controlling the length scale of the Si nanospheres, Si–SiC nanocomposites can be toughened by a combination of phase transformation and ductile phase reinforcement mechanisms.

This work was done by William Gerberich of the University of Minnesota for the Air Force Research Laboratory. AFRL-0201

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