Hypersonic missile, aircraft, and reentry vehicle technologies require radomes (windows) to protect sensitive guidance electronic equipment, including antennas. Hypersonic RF radomes should have high flexural strength (>50 MPa), low dielectric constant (<9, 5 preferred), low loss tangent (<0.1), and high resistance to thermal shock while providing transparency for RF communications (0.3 – 30 GHz) at temperatures ranging from 1000 to 1700°C. Three of the most investigated materials include silicon nitride (Si3N4), silica (SiO2), and alumina (Al2O3), however none intrinsically exhibit all required properties. Another material of interest is hexagonal boron nitride (h-BN) which has excellent thermal shock resistance and good machinability, but most importantly a dielectric constant of 4.3 – 4.4. Often called “white graphite” due to its similar structure to graphite, B and N atoms are bonded by covalent bonds in the same layer and weak van der Waals forces between layers. Although h-BN has high temperature resistance, high thermal shock resistance, and low dielectric properties, literature states h-BN has not been further explored due to its lower mechanical strength as temperature increases, difficulty self-densifying, and its oxidation in temperatures beyond 800oC.

 

As is well known, sintering is the process of transforming a bulk ceramic powder into a dense part through heating and is typically accomplished through hot pressing, spark plasma sintering (SPS), or pressureless sintering. Since h-BN has weak interfacial bonding and poor dispersion, it is difficult to densify through sintering, therefore low melting point or second phase sintering aids are usually added to promote liquid phase sintering and improve properties. Commonly used sintering aids include MgO-Al2O3-SiO2 (MAS), yttria (Y2O3), boria (B2O3), calcium oxide (CaO), and aluminum nitride (AlN). Overall, research conducted for h-BN has some commonalities, including: 1) orientation of grains during processing and/or sintering increases flexural strength and crack deflection, 2) porosity can decrease dielectric constant, but inhibit complete densification, 3) size of h-BN grains affects sinterability and 4) liquid phase can promote sintering. However, there are a few disparate conclusions from the literature leading to a need for further investigation for hypersonic applications of h-BN. These include identifying a best forming and sintering method, ideal sintering temperature, and sintering aid package for RF radome production and survival in the hypersonic environment