The Preparation And Research Of High-Performance Hexagonal Boron Nitride Ceramic Materials

Hexagonal boron nitride ceramics have high chemical stability,excellent lubricity,excellent high temperature stability and thermal shock resistance,high thermal conductivity,low dielectric constant and dielectric loss,which have a wide range of applications as structural and functional materials at high temperatures.However,due to its flake structure and stable chemical properties,it's difficult to sinter hexagonal boron nitride powder into a dense ceramic by existing processes.Hexagonal boron nitride ceramics were generally prepared by hot-press sintering in previous work,and sintering aids were need to boost the density because it's very difficult to consolidate high-density hexagonal boron nitride ceramics by hot-press sintering.However,the sintering aids will seriously affect the high-temperature stability and dielectric properties of hexagonal boron nitride ceramics.The comprehensive performance of previously prepared hexagonal boron nitride ceramics is far from satisfactory.We have proposed a new self-densification sintering strategy to prepare a high-density,high-performance hexagonal boron nitride ceramics by spark plasma sintering.Cubic boron nitride are used as sintering aid and reinforcing phase.During the sintering process,the cubic boron nitride particles are transformed into hexagonal boron nitride onions with a significant volume expansion,which fill the voids between the hexagonal boron nitride flakes.Thus,high-performance hexagonal boron nitride ceramics with a relative density of 97.6%can be prepared at a relatively low sintering temperature(1700?).The prepared ceramics are pure hexagonal phase boron nitride materials without any sintering aids,which have the best comprehensive performances at present:the prepared high performance hexagonal boron nitride ceramics are 2-3 times stronger than the conventional hexagonal boron nitride ceramic,and possess high thermal conductivity and excellent dielectric properties,and dielectric loss tangent is as low as 2.48×10-4.This sintering strategy of phase transformation induced volume expansion provides new ideas and scientific basis for the research of other ceramic systems.