Mehran Jaberi Zamhari - Ceramics Department, Materials and Energy Research Center, Karaj, Iran
Mohammad Zakeri - Ceramics Department, Materials and Energy Research Center, Karaj, Iran
Mansour Razavi - Ceramics Department, Materials and Energy Research Center, Karaj, Iran

In this study, the UHTC-based composite layers were applied on the graphite substrates using the SPS method to protect them against ablation. The protective layers had some defects and problems such as crack, fracture, separation, melting, and weak adhesion to the substrate. Several factors such as the thickness of the composite layer, the number of protective layers, the SPS conditions (temperature, applied pressure, soaking time and mold), the chemical composition of the layers, the type of the substrate and the mismatch between the thermal expansion coefficients of the substrate and the applied layer(s) affected the quality and connection of the protective layer to the graphite substrate. The amount of additive materials influenced the melting phenomenon in the composite layer; for example, further MoSi۲ in the layer led to more melting. The mismatch between the thermal expansion coefficients of the graphite substrate and the composite layer caused stresses during the cooling step, which resulted in cracks in the applied layer. Hence, proximity in the thermal expansion coefficients seems to be necessary for the formation of an acceptable adhesion between the layer and the substrate.

Carbon materials, Spark plasma sintering, Ultrahigh temperature ceramics, Protective layer, Thermal expansion coefficient