Evolution of microstructure and properties of Si3N4 whisker reinforced composites during densification by polymer infiltration and pyrolysis

Si3N4 whisker (Si3N4w) reinforced composites were prepared by a near-net shaping process, i.e., gel-casting of the Si3N4w preform followed by polymer infiltration and pyrolysis (PIP) densification using polysilazane as precursor. The densification process by PIP was described mathematically, after which several key parameters affecting densification efficiency were discussed. The small pore size (0.04 ∼ 1 μm) of Si3N4w preform can cause filtration effect (low permeability of precursor with a molecular size bigger than pore size), which resulted in the density gradient of the composites. Porosity (P) dependence of flexural strength and elastic modulus of Si3N4w/Si3N4 followed a power law of (1 – P). With increasing density, the response of Si3N4w when confronting cracks transformed from whisker debonding to whisker fracture, which was supposed to be due to the increase of whisker/matrix interface strength. The Si3N4w/Si3N4 developed by us achieved a good balance between high strength and low dielectric constant, making it promising for high-temperature wave-transparent application.