Silicon Nitride Ceramics Synergistically Strengthened By Nanoparticle And Texturing

The"brick-clay"structure,which imitates shell pearl layers,is a high-strength,high-ductility laminar microstructure with the potential to improve the brittleness of structural ceramics.The microstructure of highly texture Si₃N₄ceramics is similar to the mimic shell pearl layer structure,which consists of long columnar?-Si₃N₄grains oriented and laminated.In this thesis,a novel strong toughened microstructure-nanoparticles texturing-was designed based on the mimic pearl layer structure.The nanoparticle-coordinated texture-configuration-reinforced Si₃N₄ceramics were prepared using hot-pressure flow sintering method.The effects of compressive strain,intra-and extra-crystalline nanoparticles,and extra-crystalline nanoparticles on the degree of texturing and mechanical properties of Si₃N₄ceramics were investigated.To investigate new methods to improve the brittleness of structural ceramics.The following main findings were obtained from the above-mentioned studies.(1)The effect of compressive strain(0-100%)on the degree of texturing and mechanical properties of Si₃N₄ceramics was investigated.It was shown that the higher the compressive strain,the higher the degree of texturing.The sample YA-25(100%compressive strain)showed the highest degree of texturing with a Lotgering index of 0.986and the highest flexural strength of 1674±60.9 MPa.The highly texture sample YA-25showed a weaker interlayer bond strength.When doing the Vickers indentation test,it is prone to interlayer fracture and the indentation chipping rate reaches 60%.The macroscopic evidence proves that the?-Si₃N₄grain strength is greater than the grain boundary phase strength.(2)In order to strengthen the interlayer bonding of texture Si₃N₄ceramic ceramics,Si C nanoparticles were added to prepare intra-and extra-crystalline Si C nanoparticles to enhance texture Si₃N₄ceramics.The effects of different contents(1-8 wt%)of Si C nanoparticles on the mechanical properties and interlayer bonding strength of the texture Si₃N₄ceramics were investigated.The results showed that the addition of Si C nanoparticles had no significant effect on the degree of Si₃N₄ceramic texturing,and the Lotgering index of all samples reached above 0.96.The maximum flexural strength was 1435±121.85 MPa when the Si C nanoparticle content was 2 wt%.with the increase of nanoparticles,the interlayer bonding strength increased and the indentation chipping rate decreased.When the Si C nanoparticles were 8wt%,the indentation chipping rate was 0.(3)Investigation of the strengthening effect of extra-crystalline nanoparticles on the texture Si₃N₄ceramics:The crystalline phase was precipitated in situ from the glassy phase at the grain boundaries by using mono-and ternary sintering aids and a heat treatment process to obtain extra-crystalline nanoparticles to strengthen the texture Si₃N₄ceramics.With the mono-sintering additive,the sample 10Yb had a low degree of texturing,a Lotgering index of 0.838,and a flexural strength of 823.87±94.35 MPa,and a strength of1069±112.33 MPa after heat treatment,which was improved by 30%.With the ternary sintering additive,the sample has a high degree of RE texturing,with a Lotgering index of0.955,a flexural strength of 1222±15.17 MPa,and a maximum strength of 1366.13±64.08MPa after heat treatment,an increase of 12%.The use of rare earth sintering additives and the crystallization of the grain boundary glass phase after heat treatment can effectively improve the flexural strength of the texture Si₃N₄ceramics.The interlayer bond strength of the texture Si₃N₄ceramics increased after heat treatment crystallization,and the indentation chipping rate of the recrystallized samples were all 0.The sample RERS-1400 showed a short fracture delay during the three-point flexural test,slightly improving the catastrophic fracture of the material.The novel microstructure-extracrystalline nanoparticle synergistic texturing reinforcement provides a new approach to improve the brittleness of Si₃N₄ceramics.