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The theoretical strength of a material is the minimum stress to deform or fracture the perfect single crystal material that has no defects. This theoretical strength is considered as an upper bound on the attainable strength for a real crystal. In contradiction to this expectation, we use quantum mechanics (QM) simulations to show that for the boron carbide (B 4 C) hard ceramic, this theoretical shear strength can be exceeded by 11% by imposing nano-scale twins. We also predict from QM that the indentation strength of nano-twinned B 4 C is 12% higher than that of the perfect crystal. Further we validate this effect experimentally, showing that nano-twinned samples are harder by 2.3% than the twin-free counterpart of B 4 C. The origin of this strengthening mechanism is suppression of twin boundary (TB) slip within the nano-twins due to the directional nature of covalent bonds at the TB.

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We demonstrate a route to synthesize ultra high-temperature ceramic coatings of ZrB 2 at temperatures below 1,300 K using Zr/B reactive multilayers. Highly textured crystalline ZrB 2 is formed at modest temperatures, because of the absence of any oxide at the interface between Zr and B, and the very short diffusion distance that is inherent to the multilayer geometry. The kinetics of the ZrB 2 formation reaction is analyzed using high-temperature scanning nano-calorimetry, and the microstructural evolution of the multilayer is revealed using transmission electron microscopy. We show that the Zr/B reaction proceeds in two stages: (1) inter-diffusion between the nano-crystalline Zr and the amorphous B layers, forming an amorphous Zr/B alloy; and (2) crystallization of the amorphous alloy to form ZrB 2 . Scanning nano-calorimetry measurements performed at heating rates ranging from 3,100 to 10,000 K/s allow determination of the kinetic parameters of the multilayer reaction, yielding activation energies of 0.47 eV and 2.4 eV for Zr/B inter-diffusion and ZrB 2 crystallization, respectively.