Mechanical and dielectric properties of porous and wave-transparent Si3N4-Si3N4 composite ceramics fabricated by 3D printing combined with chemical vapor infiltration

Porous Si3N4-Si3N4 composite ceramics were fabricated by 3D printing combined with low-pressure chemical vapor infiltration (CVI). This technique could effectively improve the designability of porous Si3N4 ceramics and optimize the mechanical and dielectric properties. The effects of process parameters including the deposition time and heat treatment on the microstructure and properties of porous Si3N4-Si3N4 composite ceramics were studied. The study highlights following: When CVI processing time was increased from 0 to 12 h, the porosity decreased from 68.65% to 26.07% and the density increased from 0.99 to 2.02 g/cm(3). At the same time, the dielectric constant gradually increased from 1.72 to 3.60; however, the dielectric loss always remained less than 0.01, indicating the excellent electromagnetic (EM) wave-transparent performance of porous Si3N4-Si3N4 composite ceramics. The maximum flexural strength of 47 +/- 2 MPa was achieved when the deposition time attained 6 h. After heat treatment, the porosity increased from 26.07% to 36.02% and the dielectric constant got a slight increase from 3.60 to 3.70 with the dielectric loss still maintaining lower than 0.01. It has been demonstrated that the porous Si3N4-Si3N4 composite ceramics are a promising structural and EM wave-transparent material suitable for high temperature service.