Ceramic substrates of β-SiC/SiAlON composite from preceramic polymers and Al-Si fillers

Commercial polysilsesquioxanes filled with silicon and aluminum particles were prepared in the form of a thin sheet by tape casting and pyrolyzed in nitrogen atmosphere at various temperatures up to 1500 °C. Two silicone resins with different carbon content were used to prepare slurries containing 30 vol% of Si and Al as filler with volume ratios of 1:1 and 1:3. The crystalline phases that formed in the polymer derived ceramic (PDC) substrates pyrolyzed at different temperatures were identified using x-ray diffraction patterns. Substrates pyrolyzed at 1500 °C were characterized with respect to their density, total open porosity and thermal expansion coefficient. The resulting microstructures were examined using scanning and transmission electron microscopy. The main crystalline phases in substrates pyrolyzed at 1000 °C were identified as Si, β-SiC and AlN. These gradually converted to β-SiC/SiAlON composites at 1500 °C. The higher carbon content favored formation of the phases β-SiC and AlN. The β-SiAlON phase predominated in substrates with higher Si content while SiALON polytypoids were more significant in substrates with higher Al content. The 21R (SiAl6O2N6), 12H (SiAl5O2N5) and 15R (SiAl4O2N4) SiAlON polytypoids were observed in all the samples and were identified by high resolution electron microscopy (HREM) in samples pyrolyzed at 1500 °C.