Silicon nitride/boron nitride-based self-sealed laminated structures

Improving mechanical properties such as fracture toughness, fracture strength and work of fracture (fracture resistance) of silicon nitride (Si 3N4) ceramics by designing and producing self-sealed structure of Si3N4/BN (silicon nitride/boron nitride) based multilayered laminates has been investigated.; Bar shaped samples have been produced by a modified slip casting process in a form of square cross-sections, varying the number of layers from 3 to 20 and their thickness from 70 to 1000 microns. The composition of Si 3N4 layers consists of 7 wt. % Y2O3 (yttria) and 3.0 wt. % Al2O3 (alumina). The BN-based interfaces consist of 90 wt. % BN and 10 wt. % Si3N4 in SN-(BN+SN) laminates and 50 wt. % BN and 50 wt. % Al3O 3 in SN-(BN+Al2O3) laminates.; Si3N4/BN based laminates were densified by pressureless sintering at temperatures ranging from 1720°C to 1820°C for 1 h under static N2 gas atmosphere.; The highest density was achieved with samples having 3 Si3N 4 layers in SN-(BN+SN) laminates and 5 Si3N4 layers in SN-(BN+Al2O3) with an average layer thickness of 260 microns and 320 microns, respectively. Also, it was found that samples with the highest density exhibit the highest Young's modulus of 315 GPa in SN-(BN+SN) laminates and 320 GPa in SN+(BN+Al2O3) laminates, respectively.; Systematic studies of the effects of the number, thickness and composition of layers on apparent fracture toughness, strength and work of fracture of Si3N4/BN laminates were performed. It was found that in the both laminates fracture toughness, strength and work of fracture are strongly affected by the chemical composition of BN interfacial layers and their strength, as well as by the number of layers and their thickness. The highest apparent fracture toughness and strength of 22 MPa· m1/2 and 470 MPa, respectively, were found in samples with 7 Si3N4 layers in SN-(BN+Al2O3 ) laminates, while the highest apparent fracture toughness and strength of 19.5 MPa·m1/2 and 515 MPa, respectively, were found in samples with 4 Si3N4 layers in SN-(BN+SN) laminates. The highest work of fracture of 230 kJ/m3 was found in samples having 9 Si3N4 layers in SN-(BN+Al2 O3) laminates, and 320 kJ/m3 in samples having 8 Si3N4 layers in SN-(BN+SN) laminates, respectively.; The microstructure of the Si3N4 layers in both laminates consists of elongated Si3N4 grains uniformly distributed in a matrix of equiaxed or slightly elongated grains and grain boundary phase. The microstructure of the (BN+Al2O3) interface consists mainly of YAG phase with BN and Si3N4 as minor phases, while the microstructure of the (BN+SN) interface consists of BN and Si3N4 as major phases. A much higher level of porosity was observed in the (BN+SN) interfaces than in the (BN+Al 2O3) interfaces.