Effects Of ?Si₃N₄?_p And AlN On The Microstructure And Properties Of BN-SiO₂Composite Ceramics

BN-SiO2composite ceramics was fabricated by using h-BN, Si3N4, AlN powdersas starting materials and SiO2as sintering additive, by prepress followed by pressurelesssintering or gas-pressure sintering or hot pressed sintering. Then XRD, SEM, TEMtechniques was applied to characterize the microstructure of BN-matrix compositeceramics, while the mechanical properties, thermal properties, dielectric properties andthermal shock resistance were investigated by Universal Material Testing Machine, etc.,in order to study the influence of the content of Si3N4and AlN on the properties of theBN matrix composite.It was found that, in BN matrix composite ceramics with Si3N4and SiO2, thereexisted h-BN and Si2N2O was formed through the interacion between SiO2and Si3N4.After sintering, Si3N4remained in alpha phase mainly and ?-Si3N4was minor. While inBN matrix composite ceramics with AlN and SiO2, ?-SiAlON(Si5AlON7)andX-SiAlON(Si3Al6O12N2)were fabricated. Most of the SiO2remained its amorphous state,while some of them devitrified.After sintering, the pores in BN-Si3N4composite ceramics scattered disorderly.The density of the composite ceramics was between1.28g·cm-3(pressurelesssintering)and2.27g·cm-3(hot pressed sintering), and increased with increasing Si3N4orAlN addtive content. Composite ceramics which made by hot pressed sintering weredensified better than those produced by pressureless sintering and gas pressure sintering.And ceramics made by gas pressure sintering were a little denser than the onesfabricated by pressureless sintering.The mechanical properties also increased with increasing Si3N4or AlN content.The flexural strength had maximums of22.8±0.7MPa(pressure-less sintering withSi3N4),31.6±0.8MPa(gas pressure sintering with Si3N4),242.9±20MPa(hot pressedsintering with Si3N4)and240.5±21.0MPa(hot pressed sintering with AlN). The flexuralstrength varied with the change of the ratio of BN to SiO2, and when BN:SiO2=6:4, itcame to63.8±3.2MPa(gas pressure sintering)as a maximum. The ceramics produced byhot pressing had the best mechanical properties, and those produced by gas pressuresintering had better mechanical properties than ceramics produce by pressure lesssintering.The dielectric coefficient and tangent loss of BN matrix composite ceramics were2.86~5.90and1.7~7.1×10-3, varying with the change of sintering method, additivematerial and the ratio of BN:SiO2. The comopsite made by hot pressing had betterdielectric properties than those made by pressure-less sintering or gas pressure sintering. And composite with AlN had poorer dieclectric properties than that with Si3N4. Themean thermal expansion coefficients were1.86×10-6~3.19×10-6K-1, and ceramicswithout Si3N4or AlN had a higher value of mean thermal expansion coefficient.After thermal shock, the residual strengths and the critical thermal shocktemperature of BN matrix composite ceramics with Si3N4or AlN adding, were higherthan those of ceramics without Si3N4or AlN. The residual strengths remained more than84.5%and76.8%of the original strength before thermal shock. After a thermal shockwith ?Tc=1200K, the residual strengths of the ceramics were84.7±14.3MPa(withoutSi3N4or AlN),239.1±6.0MPa(with Si3N4only)and212.1±6.5MPa(with AlN only).