Low-Temperature Synthesis of Ultra-High-Temperature Coatings of ZrB₂ Using Reactive Multilayers

We demonstrate a route to synthesize ultra-high-temperature ceramic coatings of ZrB2 at temperatures below 1300 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 nanocalorimetry, 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) interdiffusion between the nanocrystalline Zr and the amorphous B layers, forming an amorphous Zr/B alloy, and (2) crystallization of the amorphous alloy to form ZrB2. Scanning nanocalorimetry measurements performed at heating rates ranging from 3100 to 10000 K/s allow determination of the kinetic parameters of the multilayer reaction, yielding activation energies of 0.47 and 2.4 eV for Zr/B interdiffusion and ZrB2 crystallization, respectively. ■ INTRODUCTION ZrB2 is an ultra-high-temperature ceramic (UHTC) with a unique set of properties, including an extremely high melting point, very high hardness, exceptional resistance to erosion, and excellent thermal and electric conductivity. Due to this combination of properties, ZrB2 has garnered intense attention as a structural material in extreme environments, such as propulsion systems, leading edges of hypersonic flight vehicles, and heat shields for atmospheric re-entry. Practical application of ZrB2, however, has been impeded because of its poor sinterability. ZrB2 is conventionally fabricated using hot pressing (HP), spark plasma sintering (SPS), or pressureless sintering (PS), processes that require a very high temperature (>2200 K) to achieve dense ZrB2 structures. 2 Even with applied pressures as high as 40 MPa, formation of pure ZrB2 requires temperatures in excess of 1950 K to achieve full density. A great deal of research has been conducted in an attempt to enhance the sinterability of ZrB2 by refining the particle size of the raw materials and by including additives such as C, SiC, B4C, WC, B2O3, or MoSi2. 2,4−11 Utilizing these methods, recent studies have demonstrated processing temperatures of dense ZrB2 as low as 1750 K in the presence of additives. Here we demonstrate that it is possible to synthesize crystalline ZrB2 at temperatures below 1300 K through use of Zr/B reactive multilayers. As shown schematically in Figure 1a, a reactive multilayer consists of alternating Zr and B layers with a nominal stoichiometry of 1:2. Upon heating, Zr and B react to form ZrB2 in an exothermic process. We have used nanocalorimetry (Figure 1b,c) combined with transmission electron microscopy (TEM) to explore the nature of the solid-state reaction and the structural changes that occur in the multilayer. Calorimetry has been used extensively to examine the thermodynamics and kinetics of multilayer reactions. Nanocalorimetry sensors can significantly enhance the study of these solid-state reactions because of their extraordinary sensitivity and extremely small thermal mass. These features make it possible to perform measurements on very thin multilayers over a much broader range of heating rates than traditional calorimetry, ranging from isothermal to 10 K/ s. Current nanocalorimetry sensor designs allow measurements at temperatures as high as 1300 K. ■ EXPERIMENTAL SECTION Nanocalorimetry Sensors. All nanocalorimetry measurements were performed using a parallel nanoscanning calorimeter (PnSC). This device consists of a 5 × 5 array of micromachined calorimetry sensors (Figure 1b). Each sensor consists of a thin metal layer that is patterned to serve both as a heating element and a resistance thermometer in a four-point measurement scheme. As illustrated schematically in Figure 1c, this heating element/thermometer is encapsulated in a thin membrane that insulates the heating element from its surroundings. The sample of interest is deposited in the area between the voltage leads as indicated in Figure 1c. To perform a calorimetry scan, an electric current is supplied through the heating element, which results in Joule heating of the sample and addendum. The measured current and voltage are used to determine the power supplied to the sensor and the resistance of the heating element, which is calibrated to temperature. Received: June 15, 2014 Revised: August 7, 2014 Published: August 21, 2014 Article