Size Scale and Defect Engineered Nanostructures for Optimal Strength and Toughness

These thin coatings can be used in environments involving severe wear, such as automotive, aircraft, electronics, manufacturing, and biomedical.

Realistic combinations of ceramics and/or semiconductors have been developed that simultaneously achieve high hardness (>40 GPa) and toughness (>400 MPa*m1/2). To achieve this goal, there were three primary objectives: (1) the development of physically based dislocation models to understand the deformation of brittle materials, (2) the synthesis of model ceramic nanocomposites that demonstrate high hardness and toughness, and (3) the detailed understanding of the arrangements and types of dislocation structures in small volumes. This involved the uniaxial compression of Si nanovolumes (spheres and towers) using a combination of TEM in situ indentation and molecular dynamics simulations for objective (1), the deposition of Si-SiC core-shell nanotowers for objective (2), and the HR TEM analysis of deformed Si nanovolumes for objective (3).

One of the major components of this project was the deformation of silicon nanospheres and nanotowers through TEM in situ nanoindentation experiments. These consisted of a series of uniaxial compressions of hypersonic plasma particle deposition (HPPD) synthesized single-crystal Si nanospheres (d=63-349 nm) on Al2O3 substrates and VLS grown Si(111) nanotowers (d=231-415 nm). In the nanospheres, the oxide shell prevents the release of dislocations and leads to the formation of a back stress due to dislocation pile-up.

Si nanospheres and nanotowers were also fractured in situ with a quantitative measurement of the fracture stress. Indents were run in displacement control, with 10 nm/s loading and unloading rates and a 5-second hold at the peak load. Five spheres (d=113-349 nm) and three towers (d=231-415 nm) showed signs of fracture at failure. To determine the fracture toughness of the spheres and towers, a work-per-unit fracture area method was used since it accounts for dislocation plasticity and closely matches finite element modeling of the fracture toughness in Si nanotowers.

A second component of this research was the development of nanocomposite materials based on the small-scale strengthening and toughening mechanisms described earlier. The HPPD technique was used to deposit nanocrystalline (<20 nm grain size), 3C-SiC coatings with thicknesses of 100-200 nm on Si(111) nanotowers. The Si(111) towers were vapor-liquid-solid grown using chemical vapor deposition by collaborators. Based on the DF TEM imaging, the composite structures show the conformal nature of the coating, with good evidence that the appropriate adhesion has been achieved.

The composite towers were then heattreated using rapid thermal annealing (RTA) for 20 seconds at 900-1200 °C in an Ar environment. Due to the mismatch in the coefficients of thermal expansion for the Si core and SiC shell, the Si core was left in residual compression following the RTA treatment. This compressive stress was measured using DF TEM strain contours, confocal Raman microscopy (CRM), and atomic force microscopy (AFM). Oscillations in DF TEM contrast originate from the base of the tower and show axial symmetry. This is consistent with an elastic strain profile for a capped annular coating, where the highest strain is at the top of the Si tower and relaxes at the base due to less confinement.

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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Size Scale and Defect Engineered Nanostructures for Optimal Strength and Toughness

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