Fabrication And Properties Of Textured BN Ceramics

Textured hexagonal BN matrix composite ceramics were fabricated by hot-presssintering technique with different contents of boron nitride and mullite as startingmaterials. The influence of BN raw materials, grain growth, hot pressing pressure,mullite content and compression technology on the grain orientation was studied. Thetexture components, volume fraction and texture sharpness degree were analysed bymeans of pole figure and ODF. Microstructure of hexagonal BN matrix compositeceramics were characterized by SEM and TEM. Using the universal mechanicalproperty testing machine, dilatometer, thermal diffusivity measurements and dielectricinstrument, the anisotropic mechanical, thermal and dielectric properties of the h-BNmatrix composite ceramics were investigated.This study shows that geometrical characteristics of h-BN greatly influence thegrain orientation degree and grain growth has negative effect on the grain orientation.Sintering pressure play a decisive role on the rotation and alignment of the grains,volume fraction increase and texture sharpness degree decrease while increasing thesintering pressure. Appropriately raise mullite content can improve the grainorientation degree. Compression technology make the texture more uniform.The （002） basal planes of h-BN are preferred aligned parallel to the hot pressureaxis through the pole figure, ODF and microstructure analysis. When the sinteringparameter is30MPa,1900℃,1h, we got the largest content of the （002） basal fibertexture. Within the scope of deviating from the ideal orientation±15, the texturevolume content is achieved72.8%, spread width is13.66.The mechanical properties research shows that the material has noticeablyanisotropic mechanical properties. When the loading direction parallel to the boronnitride layers direction, the bending strength and fracture toughness of the material ispoor, only21MPa and0.5MPam^1/2respectively. When the loading directionperpendicular to the hot pressing direction and BN layers direction, the bendingstrength and fracture toughness reach113MPa and2.8MPam^1/2respectively, for the30%mullite content of the BN matrix composite ceramics hot pressing at20MPa.When the loading direction parallel to the hot pressing direction, due to the kink ofcrack, the material shows the characteristics of the pseudo-plastic fracture.Thermal properties of h-BN ceramics show anisotropic characteristics, the h-BNceramic thermal expansion coefficient parallel to the hot pressing direction is positive,while in vertical direction the thermal expansion coefficients is a negative value. At1200℃the value is7.93×10-6K^-1and^-1.3247×10-6K^-1respectively, for the20%mullite content of the BN matrix composite ceramics hot pressing at20MPa.Thermalconductivities measured at room temperatures shows that the thermal conductivity ofthe top surface is15.15W m^-1 K^-1, and the side surface is126.81W m^-1 K^-1, a difference of8.37times, for the30%mullite content of the BN matrix compositeceramics hot pressing at20MPa.Ceramic dielectric constant are related to the content of mullite and materialsstructure. Mullite content increases, the composite ceramic dielectric constant valueincreases. The dielectric constant of the top surface is5.193, while the side surface is4.526for the30%mullite content of the BN matrix composite ceramics hot pressingat20MPa.