The recent developments in aerodynamics and propulsion systems have allowed modern air and space craft to reach extraordinary speeds of up to 10,000 miles per hour. However, these hypersonic flight speeds have exceeded the thermal operational limits of the materials used in the construction of such craft; thus, prompting the demand for new Thermal Protection Systems (TPSs) that can withstand the heat generated. Some of the candidate materials for these TPSs include the refractory diborides of zirconium and hafnium that are classified as Ultra High Temperature Ceramics. These diborides are characterised by high melting temperatures, and excellent thermal and mechanical properties. One drawback of the diborides, however, is their tendency to oxidise at high temperatures. Therefore, a secondary protective material is typically required to provide a defensive scale against further oxygen diffusion. The addition of silicon carbide (SiC) is a prime candidate known to provide a protective scale that can react with the diborides to form protective borosilicate glasses. In this paper we outline the results of combining 10wt%, 20wt% and 30wt% SiC, with both ZrB2 and HfB2, that has been deposited as coatings via plasma spray technology. The results of heat testing by furnace and direct oxyacetylene heating are compared to those of stock ZrB2 and HfB2 coatings to determine the effectiveness of the SiC additions against oxidation of the composite coatings.