Low-Temperature Synthesis of Ultra-High-Temperature Coatings of ZrB 2 Using Reactive Multilayers

We demonstrate a route to synthesize ultra high-temperature ceramic coatings of ZrB2 at temperatures below 1,300 K using Zr/B reactive multilayers. Highly textured crystalline ZrB2 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 ZrB2 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 ZrB2. 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 ZrB2 crystallization, respectively.