Novel polysilazanes as precursors for silicon nitride/silicon carbide composites without “free” carbon

Thermolysis in an argon atmosphere up to 1400 °C of the cross-linked products delivered Si3N4/SiC ceramics in ∼94% yield. The absence of “free” carbon was revealed by mass spectra-coupled thermogravimetric analysis (TGA). These investigations indicated a one-step decomposition in the 250-700 °C range with predominant evaporation of hydrogen. In contrast, elimination of methane and ammonia (1) or methane and methylamine (2) was below 0.5 mass%. Additionally, neutron wide angle scattering proved the absence of a “free” carbon phase. High-temperature properties were investigated using high temperature TGA as well as XRD, EDX and TEM of annealed ceramic samples. The onset of crystallization in both materials was around 1500 and 1300 °C (1 atm N2), respectively, and α- as well as β-Si3N4 formed besides α/β-SiC. Above 1900 °C ceramics derived from both 1a and 2a decomposed to α/β-SiC/β-Si3N4/α-Si composites.