Research On The Properties And Sintering Behavior Of Hexagonal Boron Nitride Ceramics Consolidated By Spark Plasma Sintering

Hexagonal boron nitride(h-BN)is regarded as a versatile and promising ceramic material used widely in many fields like metallurgy,chemical industry,electronics and new energy,because of its excellent comprehensive properties such as high melting temperature,good thermal shock resistance,high temperature thermal stability,good corrosion resistance and easy to machinability.However,difficult to densification,low strength and high sintering temperature are the biggest limitations for its further application.In order to solve the above problems,the sintering densification process and mechanical properties of h-BN ceramics and its multiphase ceramics consolidated by spark plasma sintering(SPS)were systematic studied in this paper.The effects of sintering methods and SPS-important parameters(sintering temperature,holding time,sintering pressure and pressure mode)on densification and mechanical properties of h-BN ceramics were firstly systematically investigated.The results show that h-BN ceramics with high density can be prepared by SPS at lower sintering temperature and in shorter time without any sintering aids.Compared with the traditional pressureless sintering and hotpressing sintering,h-BN ceramics prepared by SPS have more uniform and fine grains,higher mechanical properties.The densification temperature is decreased by more than 200?.This further confirmed that pressure and temperature are also the two main factors affecting the densification,grain orientation and mechanical properties of lamella structure h-BN ceramics.The influences of sintering temperature and holding time on grain growth are more significant than on densification.In addition,the green body initial density and pressure mode have a certain degree of influence on the density and mechanical properties.The research for the density and mechanical properties of h-BN ceramics fabricated using different size h-BN powder shows that the smaller grain size,the lower crystallization degree and the higher density and mechanical properties.While the orientation degree of flake h-BN grains can be greatly enhanced by using h-BN powder with larger particle size.Furthermore,the relative density changing curve of h-BN ceramics prepared using the mixture powders with different diameters has the same V shape trends with mechanical properties.In order to promote the densification and increase mechanical properties of SiC/h-BN multiphase ceramics,two sintering additives were adopted like B2O3 and Al2O3-Y2O3 in this paper.The results show that the addition of B2O3 can greatly increase the density of multiphase ceramics,when the B2O3 content is only 5wt%,the relative density is as high as 97%.But the mechanical properties of SiC/h-BN multiphase ceramics are significantly improved after Al2O3-Y2O3 additive is added,and the flexural strength and fracture toughness of SiC/h-BN multiphase ceramics with 8wt%Al2O3-Y2O3 are increased by about 142%and 84%than pure h-BN ceramics fabricated at the same condition,respectively.In order to improve the density and mechanical properties of pure h-BN ceramics,the reinforcing phases like SiC,ZrO2+SiC and Si3N4 were added to h-BN.The research results show that when the SiC content is 25wt%,the relative density of SiC/h-BN multiphase ceramic is as high as 98%under 40MPa sintering pressure at 1600? with 10min holding time.At the same time,the flexural strength and fracture toughness are increased by about 139%and 67%than single-phase h-BN ceramics,respectively.Under the condition of fixed total amount of ZrO2+SiC,the flexural strength and fracture toughness of(ZrO2+SiC)/h-BN multiphase ceramic with 15wt%SiC are increased by about 95%and 76%than pure h-BN ceramics,respectively.Compared with the former two kinds of reinforced phases,add Si3N4 to h-BN can obviously increase the density and mechanical properties of pure h-BN ceramics.The results show that the flexural strength is increased about 2 times,and the fracture toughness is increased by about 1.5 times when Si3N4 content is only 20vol%.In addition,the thermal shock resistance of 25wt%SiC/h-BN multiphase ceramics was studied,and the strengthening and toughening mechanism of reinforcing phases was also analyzed.The research results show that the 25wt%SiC/h-BN multiphase ceramic has good thermal shock stability at below 900?,and the residual flexural strength is improved because of the formation of the silicate glass phase.Nanometer SiC particles can have better filling and dispersion strengthening effect.The content,diameter and length to diameter ratio size of ?-Si3N4 can significantly influence the fracture toughness and bending strength of Si3N4/h-BN multiphase ceramics.