Research On Densification And Mechanical Property Enhancement Of SiC-based Ceramics

Silicon carbide（SiC）ceramics possess excellent properties such as low density,high hardness,high strength and high temperature stability,and are widely used in the automotive,mechanical and metallurgical industries.However,SiC ceramics are difficult to densify and possess low fracture toughness,which limits their applications.To address these problems,this thesis investigated the composition,densification,microstructure and mechanical properties of SiC ceramics by regulating the thickness of SiC ceramics,introducing glass sintering additives,adding high entropy carbides and high entropy borides as the secondary phase,and combining with spark plasma sintering（SPS）,in order to improve the densification and mechanical properties of SiC ceramics.The details and results of the study were as follows.（1）SiC ceramics with different thicknesses（50μm,100μm,500μm and 2000μm）were prepared at different temperatures（1700°C,1800°C and 1900°C）using nano-SiC powder as raw material and Al₂O₃-Ce O₂ as sintering aid,and the effects of thickness on the composition,densification,microstructure and mechanical properties were investigated.The results showed that there was a significant size effect on the densification of SiC ceramics.The thinner（50μm）SiC ceramics could be densified at 1700°C.When the thickness was increased to 2000μm,SiC ceramic was sintered at 1900°C to achieve densification.Decreasing the thickness of the SiC ceramics was able to reduce the sintering temperature and improve densification.With the sintering temperature of 1700°C,the nanoindentation hardness of the SiC ceramic with the thickness of 50μm reached 24.8±1.9 GPa due to the higher densification,which was significantly higher than that of the SiC ceramic with the thickness of 2000μm（10.4±1.5 GPa）.When the sintering temperature was increased to 1900°C,although all samples were densified,due to the finer grain size,the nanoindentation hardness of the SiC ceramic with the thickness of 50μm was 30.6±1.4 GPa,higher than that of the SiC ceramic with the thickness of 2000μm（24.7±1.2 GPa）.（2）The SiC ceramics sintered at 1900°C prepared from the mixed Y₂O₃-Al₂O₃-Si O₂ and CaO-Al₂O₃ glass powders by the melt-quenching method as sintering aid were compared with the SiC ceramics prepared from the conventional mixed Y₂O₃-Al₂O₃-Si O₂ and CaO-Al₂O₃systems as sintering aid.The two forms of sintering aids on the composition,densification,microstructure and mechanical properties were investigated.The results showed that the relative density of SiC ceramics in the Y₂O₃-Al₂O₃-Si O₂ system increased from 95.3%to98.9%and the Vickers hardness increased from 16.7±0.6 GPa to 20.4±0.3 GPa when the sintering aids were introducing glass sintering additives instead of the conventional mixed powder.The relative density of SiC ceramics in the CaO-Al₂O₃ system increased from 94.3%to 99.5%and the Vickers hardness increased from 15.1±0.3 GPa to 17.7±0.4 GPa.Therefore,the use of glass powder as a sintering aid could significantly improve the densification and hardness of SiC ceramics.（3）High-entropy carbide of(Ti_0.2Mo_0.2Ta_0.2Nb_0.2Zr_0.2)C was introduced as the secondary phase to prepare SiC-(Ti_0.2Mo_0.2Ta_0.2Nb_0.2Zr_0.2)C composite ceramics at 2000°C.The effect of(Ti_0.2Mo_0.2Ta_0.2Nb_0.2Zr_0.2)C content（0-20 wt%）on the composition,densification,microstructure and mechanical properties were investigated.The results showed that as the(Ti_0.2Mo_0.2Ta_0.2Nb_0.2Zr_0.2)C content increased from 0 to 15 wt%,the relative density of the SiC-based composite ceramics increased from 96.7%to 98.6%,the Vickers hardness increased from 16.8±0.7 GPa to 20.1±0.8 GPa,and the fracture toughness increased slightly from 3.97±0.44 MPa·m^1/2 to 4.27±0.11 MPa·m^1/2.However,the content of(Ti_0.2Mo_0.2Ta_0.2Nb_0.2Zr_0.2)C increasing to 20 wt%,the relative density of SiC-based composite ceramics reduced to 97.7%,the Vickers hardness reduced to 18.6±0.8 GPa and the fracture toughness reduced to 4.16±0.32 MPa·m^1/2.Therefore,the introduction of a moderate amount（15 wt%）of high entropy carbide could improve densification and hardness of SiC ceramics.（4）High-entropy borides of(Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ were introduced as the secondary phase to prepare SiC-(Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ composite ceramics at 1900°C.The effect of(Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ content（0-20 wt%）on the composition,densification,microstructure and mechanical properties were investigated.The results showed that as the content of(Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ increased from 0 to 15 wt%,the relative density of the SiC-based composite ceramics increased from 96.3%to 99.1%,the Vickers hardness increased from 16.6±0.6 GPa to 21.9±0.7 GPa,and the fracture toughness increased from 3.10±0.39 MPa·m^1/2 to 4.88±0.48 MPa·m^1/2.However,the content of(Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ increasing to 20 wt%,the relative density of the SiC-based composite ceramics reduced to 98.5%,the Vickers hardness reduced to 21.7±0.9 GPa and the fracture toughness reduced to 4.16±0.32 MPa·m^1/2.Therefore,the introduction of a moderate amount（15 wt%）of high entropy borides could improve densification,hardness and fracture toughness of SiC ceramics.