Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si3N4/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si3N4 matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si3N4 or Al2O3. The results showed that the addition of SiC whiskers in the Si3N4 matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al2O3 or 10 wt% Si3N4 to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si3N4/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si3N4/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.

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