Fabrication Of Si₃N₄/BN Composite Ceramics And Their Mechanical Property And Thermal Conductivity

Silicon nitride?Si₃N₄?ceramic is a kind of advanced ceramic with excellentperformance with high strength,good mechanical properties,excellent chemical corrosion resistance and high temperature resistance,and has been widely used in engineering field.However,due to its brittleness and high hardness,Si₃N₄ ceramics are difficult to be fabricated into complex shape parts,which limits their application.Generally,boron nitride?BN?has two different crystalline phases:hexagonal boron nitride?h-BN?and cubic boron nitride?c-BN?.The crystal structure and properties of h-BN is similar to graphite.Boron and nitrogen atoms are bonded together by strong sp² covalent bond,and adjacent layers are bonded together by weak van der Waals force.This special crystal structure enables h-BN a series of unique performances,including low dielectric coefficient,low tangent loss,high decomposition temperature up to about3000°C?non-oxidizing atmosphere?,excellent thermal shock resistance,high thermal conductivity and good processability.In Si₃N₄/BN composite ceramics,with the increase of BN content,the cleavagebehavior of layered h-BN particles leads to the good machinability of the composite.In this thesis,c-BN and?-Si₃N₄ were used as the starting powders,and hot press sintering method was used to prepare Si₃N₄/BN composite ceramics in nitrogen atmosphere at a temperature of 1800°C.The c-BN phase changed into h-BN phase with accompanied volume expansion.The densification and mechanical properties of the composite ceramics were improved.?1?The effects of different type of sintering aids?Si O₂-B₂O₃?Y₂O₃-Al₂O₃ and Y₂O₃-Al₂O₃?and starting powders?c-BN and a-BN?on the mechanical properties of Si₃N₄/BN composite ceramics were studied.It can be found that the composite ceramics prepared using Mg O-Y₂O₃ as sintering aid and c-BN/?-Si₃N₄ as the starting powders have higher density and better mechanical properties than the others.After that,Mg O-Y₂O₃was used as sintering aid to investigate the influence of BN content?wt.%?on the mechanical properties of Si₃N₄/BN composite ceramics.With the increase of BN content,the strength and toughness of Si₃N₄/BN composite ceramics first increase and then decrease.Compared with pure Si₃N₄ phase,the strength and toughness increase from 852.43 MPa and 7.4 MPa·m^1/2 to 878.26 MPa and 11.2 MPa·m^1/2,respectively.When 30wt.%BN is added,Si₃N₄/BN composite ceramics keep high toughness of 7.01MPa·m^1/2.The hardness of Si₃N₄/BN composite ceramics decreases rapidly with the increase of BN content.When the content of BN increases from 0wt.%to 30wt.%,the hardness of composite ceramics decreases from 16.42 GPa to 7.57 GPa.?2?The toughening mechanism of BN particles was studied.It is believed that thetoughening mechanism of the composite ceramics is mainly improved by the self-toughening of columnar?-Si₃N₄ grains,the embedding toughening of BN sheets,the lamellar fragmentation toughening of BN particles and the compressive stress toughening.Among them,the self-toughening and compressive stress toughening of?-Si₃N₄ grain are the primary toughening mechanism,the BN sheet embedding toughening mechanism and the lamellar fragmentation toughening of BN grains are the second toughening mechanism.?3?The effect of BN content on the thermal conductivity of Si₃N₄/BN compositeceramics was studied.The effect of anisotropy and boron nitride grain size on the thermal conductivity of Si₃N₄/BN composite ceramics was analyzed by ANSYS method.The thermal conductivity of the composite ceramics decreases firstly and then increases with the increase of BN content.When 30%c-BN is added,the thermal conductivity perpendicular to the hot-pressing direction still reaches 75.1 W·K^-1·m^-1,which is similar to that of pure Si₃N₄(76.9 W·K^-1·m^-1).The increase of BN content will improve the anisotropy of the thermal conductivity of Si₃N₄/BN composite ceramics.The effect of anisotropy and boron nitride grain size on the thermal conductivity of Si₃N₄/BN composite ceramics is studied by finite element analysis.It is found that the larger the BN grain size,the greater the anisotropy of the thermal conductivity.Under the ideal conditions without considering the glass phase between the particles,the effect of BN grain size on the thermal conductivity is not obvious.The smaller the grain size,the lower the thermal conductivity of Si₃N₄/BN composite ceramics.