Role of the Interfacial Thermal Barrier in the Effective Thermal Diffusivity/Conductivity of SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride

Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon-coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix-fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1-atm N₂, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial conductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N₂ gas and those inferred from the data for the effective composite thermal conductivity.