Preparation And Properties Of Laminated Si₃N₄ Based Ceramic Materials

As an advanced structural ceramic,Si₃N₄had excellent properties:obviously high bending strength,good creep property and high hardness.However,traditional silicon nitride ceramics have low heat loss tolerance and poor reliability due to poor fracture toughness.Inspired by the bionic structure in the nature,this experiment mainly takes Si C as the interface layer and Si₃N₄as the matrix layer to prepare laminated Si₃N₄/Si C ceramics by tape-casting and hot-pressing sintering.The basic mechanical properties,oxidation resistance and thermal shock resistance of the samples were studied.The cast sheets were prepared by tape-casting.When the binder polyvinyl butyral content in the slurry was 20 wt.%,the plasticizer polyethylene glycolcontent was 5 wt.%,the dispersant glycerol content was 2 wt.%,and the solid content was 0.33 g/ml,the prepared matrix cast sheets had a smooth surface and no obvious defects.For Si C interfacial cast sheets,the optimal ratio was:the content of binder and plasticizer were 15wt.%and 10 wt.%,respectively,solid phase content was 0.1 g/ml.According to the different ratio of interface layers,two kinds of laminated Si₃N₄/Si C ceramics were prepared,which named LSN-1?the ratio of interface layer was Si C-20vol.%Si₃N₄?and LSN-2?the ratio of interface layer was Si C-20 vol.%Zr B₂?.The thickness of LSN-1 matrix layer and interface layer were 250 and 30?m,respectively and LSN-2 were 300 and 40?m.The relative densities of LSN-1,LSN-2 and SN were 98.4%,99.1%and 99.4%,respectively.The flexural strengths of LSN-1,LSN-2 and SN were 365MPa,371 MPa and 706 MPa,respectively.The interface layers were relatively thick and had certain pores,which reduces the overall density of the laminated ceramics.Hence,the strength was reduced.The fracture toughness of laminated LSN-1 and LSN-2 ceramics were 13.3 MPa·m^1/2and 11.6 MPa·m^1/2,respectively,which were 48.8%and 41.3%higher than that of monolithic SN ceramic.The increase in toughness was due to the deflection of the crack during the fracture of the sample,which extended the propagation path and increased the fracture work.At the same time,the R-curve behavior of the laminated sample increased obviously,which improved the safety and reliability.The oxidation weight gain of laminated LSN-1 and monolithic SN samples increased with the increase of the oxidation time at 1300°C.When the oxidation time was 5 h,the oxidation weight gain rates of LSN-1 and SN were 0.957 mg/cm²and 0.762 mg/cm²,respectively,the weight gain of the whole was less.When the oxidation time of LSN-1 was0.5 h,the flexural strength and fracture toughness were 385 MPa and 13.9 MPa·m^1/2,respectively,which increased by 4.7%and 5.1%.This was due to the formation of Si O₂glass phase,a protective film was formed and the surface cracks were healed.When the oxidation time is 5 h,the flexural strength and fracture toughness decreased by 24.2%and39.8%,respectively.Because the holes existed in the sample,which eliminated the stress concentration.With the increase of thermal shock temperature,the residual strength of laminated LSN-1 and monolithic SN ceramics showed a decreasing trend.Under the thermal shock temperature difference of 400°C and 800°C,the surface of LSN-1 and SN has no obvious holes,defects and traces of oxidation.The residual strength retention rates were above 80%and 65%,respectively.When the thermal shock temperature difference was 1000°C,LSN-1 still maintained a good residual strength.Because the holes existed in the sample,the stress concentration was eliminated.