Design strategy of phase and microstructure of Si_(3)N_(4) ceramics with simultaneously high hardness and toughness

Aiming to achieve silicon nitride( Si_(3)N_(4))ceramics with high hardness and high toughness,the relationships among phase composition,microstructure,and mechanical properties of Si_(3)N_(4) ceramics prepared by spark plasma sintering(SPS)at temperatures ranging from 1500 to 1800℃were investigated in this study.Two stages with different phase and microstructure features were observed and summarized.Theα-βphase transformation occurs first,and the development and growth of grains lag behind.During the first stage,the average grain size remains basically unchanged,and the hardness maintains at a value of~20.18±0.26 GPa,despite theβ- Si_(3)N_(4) phase fraction increases from 7.67 to 57.34 wt%.Subsequently,the equiaxed grains transform into rod-like grains with a high aspect ratio via the reprecipitation process,resulting in a significant increase in the fracture toughness from 3.36±0.62 to 7.11±0.15 MPa·m^(1/2).In the second stage of sintering process,the fraction ofβ- Si_(3)N_(4) phase increases to 100.00 wt%,and the grain growth also rapidly occurs.Thus,the fracture toughness increases slightly to 7.61±0.42 MPa·m^(1/2),but the hardness reduces to 16.80±0.20 GPa.The current results demonstrate that the phase contents ofβ- Si_(3)N_(4) and the microstructure shall be carefully tailored to achieve high-performance Si_(3)N_(4) ceramics. Si_(3)N_(4) ceramics with a finegrained bimodal microstructure,consisting of the mainα-andβ-phases,can exhibit the optimized combination of hardness and toughness.

Dense and pure high-entropy metal diboride ceramics sintered from self-synthesized powders via boro/carbothermal reduction approach

Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.

Optimal preparation of high-entropy boride-silicon carbide ceramics

High-entropy boride-silicon carbide(HEB-SiC)ceramics were fabricated using boridebased powders prepared from borothermal and boro/carbothermal reduction methods.The effects of processing routes(borothermal reduction and boro/carbothermal reduction)on the HEB powders were examined.HEB-SiC ceramics with>98%theoretical density were prepared by spark plasma sintering at 2000℃.It was demonstrated that the addition of SiC led to slight coarsening of the microstructure.The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed a fine-grained microstructure and higher Vickers9 hardness but lower fracture toughness value as compared with the same composition prepared from borothermal reduction powders.These results indicated that the selection of the powder processing method and the addition of SiC phase could contribute to the optimal preparation of high-entropy boride-based ceramics.

On the existence of the compound“Ce_(3)NbO_(7+δ)”prepared under air atmosphere

The solid state synthesis of the phase"Ce_(3)NbO_(7+δ)",from stoichiometric quantities of CeO_(2)and Nb_(2)O_(5),was investigated under air atmosphere,as previously described by Zhang et al.(Journal of Rare Earths,2007,25,730-733),A combination of powder X-ray diffraction,scanning electron microscopy with energy dispersive X-ray analysis,and laboratory Ce L_(3)X-ray absorption spectroscopy,demonstrates the product of the reaction to be a mixture of CeO_(2)and CeNbO_(4).These data,and reexamination of the published X-ray diffraction data of"Ce_(3)NbO_(7+δ)",are consistent with this phase in fact being CeO_(2).No evidence for the formation of a phase of composition"Ce_(3)NbO_(7+δ)"is found,by solid state synthesis under air atmosphere.