Study On Thermal Shock Resistance And Ablation Resistance Of Si₃N₄-BN-MAS Composites

Radome on high speed aircraft is the key structural component to ensure high precision penetration guidance and stability flight.The rapid increase in flight Maher number makes more stringent requirements for the performance of radome materials.Silicon nitride?Si₃N₄?ceramic has excellent mechanical,erosion resistance,thermal stability and chemical stability.It is one of the main research fields of structural ceramics for aerospace applications.However,the machinability and thermal shock resistance of Si₃N₄ ceramics are poor,which limits its application in aerospace industry of some extent.Compared with Si₃N₄,the decomposition temperature of boron nitride?BN?reaches 3000 ?,which has better thermal stability,lower dielectric constant and dielectric loss.The introduction of BN in Si₃N₄ is expected to improve the thermophysical properties,dielectric properties and machinability of Si₃N₄ ceramics.In addition,has great influence on densification and properties of additives on sintering materials,MAS?MgO-Al2O3-SiO2?due to its excellent anti erosion ability of ions and low expansion coefficient,low dielectric constant and low dielectric loss is widely used.Therefore,Si₃N₄-BN-MAS composite ceramics were fabricated by hot press sintering with different content of h-BN as additive,MAS?MgO-Al2O3-SiO2?as sintering aid.A comprehensive analysis of density and mechanical properties,dielectric properties,thermal shock resistance,thermal shock resistance and erosion resistance effect were investigated by means of SEM,XRD and universal mechanical property testing machine,thermal expansion instrument and dielectric analyzer and so on.It is expected to prepare materials that meet the requirements of aerospace wave..The experimental results indicate that in the Si₃N₄-BN-MAS composite ceramics with different BN contents prepared by hot press sintering,the main phase is ?-Si₃N₄ and h-BN.The BN grains of the lamellae are mixed with the short columnar ?-Si₃N₄ grains;With the increase of h-BN content,the density of the composite ceramics decreases,and the variation range is 92.3⁷9.9 %;Bending strength,Vickers hardness and elastic modulus and density to maintain the same trend;The fracture toughness increased first and then decreased with the increase of h-BN content,and reached the maximum value of 7.4 MPa.m1/2 at 20 % of h-BN content,which is 121 % higher than that of pure Si₃N₄,which is due to the lamellar h-BN grains and short columnar Si₃N₄ grains play synergistic toughening effect;the introduction of dielectric properties h-BN and the increase of porosity effectively reduce the dielectric constant of Si₃N₄-BN-MAS composite ceramics.With the test temperature increases,the thermal diffusion coefficient of Si₃N₄-BN-MAS composite ceramics monotonically reduced.With the content of h-BN increas,the thermal expansion coefficient is 3.99².62×10-6?-1.When the content of h-BN is 20 wt%,the ceramic material has excellent thermal shock resistance.This is related to higher fracture toughness and lower elastic modulus.The residual bending strength of Si₃N₄-BN-MAS composite ceramics with different h-BN content is the same trend.When the thermal shock temperature is less than 800 ?,the residual bending strength decreases after thermal shock;When the temperature is 1000 ?,the residual bending strength of the material increases slightly;the thermal shock temperature continues to rise and the residual bending strength of the material decreases significantly.It can be seen from the observation of microstructure that when the thermal shock temperature is higher than 800 ?,the oxidation of the surface begins to occur,and the oxide film containing B2O3 is mainly produced.As the thermal shock temperature increases,despite the thicker oxide film,the greater thermal stress leads to greater thermal stress damage and lower thermal shock resistance.Overall,with the increase of h-BN content,the quality ablation rate and line ablation rate increase,indicating that the ablation resistance decreased.Among them,when the h-BN content is less than 30 wt%,the mass ablation rate and line ablation rate increase is small,and the ablative performance is comparable to Si₃N₄ ceramics without h-BN.The surface of the specimen after ablation can be divided into three regions: ablation center area,ablation transition zone and ablation edge area,Which ablation center area ablation of the most serious;Si₃N₄-BN-MAS composite ceramic ablation mechanism is thermal oxidation ablation,thermal physical ablation and mechanical erosion.Among them,in the central area of the sample and the transition zone is thermal oxidation ablation,thermal physical ablation and mechanical erosion,in the ablation edge area to thermal oxidation ablation and thermal ablation mainly.