Preparation and properties of BN_mf-Si₃N_4w/Si₃N₄ composites

To prepare a high-temperature wave-transparent material with excellent dielectric and mechanical properties, the BN_mf-Si₃N_4w/Si₃N₄ composite material was prepared by gel-casting (GC) and precursor infiltration and pyrolysis (PIP) process. The effects of the cycles of PIP and the BN_mf content on the mechanical properties and dielectric properties of composite materials were studied. The results show that: (1) As the number of PIP cycles increases, the density of the composite material increases, the porosity decreases, and the mechanical properties improve. The silicon nitride matrix gradually forms a three-dimensional network structure wrapped around the multiphase micro-reinforcement; (2) When the content of BN_mf increases from 4vol% to 12vol%, the bending strength decreases from 175.5 MPa to 139.3 MPa, and the fracture toughness increases from 2.36 MPa·m^1/2 to 2.73 MPa·m^1/2, the dielectric constant decreases from 3.62 to 3.25, and the dielectric loss tangent decreases from 0.012 to 0.007; (3) The strengthening and toughening mechanisms of BN_mf-Si₃N_4w/Si₃N₄ composites are mainly crack bifurcation, crack deflection, and pull-out of BN_mf and Si₃N_4w. Three mechanisms effectively reduce the damage of the main crack to the composite.