Several types of silicon based nanocomposite ceramics have been produced by hot isostatic pressing (HIP) mixtures of ultrafine SiC, CSiC and SiSiC powders (particle size below 50 nm) with conventional carbon, boron carbide or silicon carbide powders. Compared to pressureless sintering grain growth was substantially reduced by this technique. Samples have been characterized with respect to microstructure. Thermal conductivity measurements at room temperature indicate a reduction of thermal conductivity compared to conventional materials. High heat flux experiments in the electron beam test facility JUDITH (Juelich divertor test equipment in hot cells) with energy densities up to 11 MJ/m 2 have been performed. Although the thermal conductivity is relatively low the materials prepared from SiSiC/C-mixtures show lower erosion rates compared to conventional composites. The negative effect of low thermal conductivity on thermal shock resistance seems to be overcompensated by the microstructural improvements in the nanocomposite materials. A model, which calculates the erosion rates and takes account of microstructural features like chemical composition and grain size, is presented.