Mechanical Properties And Thermal Shock Damage Behaviors Of Si₃N_4f/BN-Si₃N₄ Ceramic Composites

As a novel material of structure-function integration,the nitride-based transparent ceramics have significant applications in aerospace,national defense,communication and other fields.Hexagonal boron nitride ceramics with graphite-like structure possess high-temperature resistance,excellent machinability,dielectric properties and preeminent thermophysical properties.Silicon nitride ceramics take on high strength and good chemical stability.The researches of h-BN/Si₃N₄-based ceramic composites have been gradually carried out owing their complementary advantages.In this paper,the hot-press sintering was adopted and Al PO₄ was used as a solid-phase sintering aid to prepare chopped Si₃N₄ f reinforced h-BN/Si₃N₄?SBS?multiphase ceramics at 1400 1600 ?.The mechanism and influence on microstructure,phase composition,density,mechanical properties,dielectric properties,thermophysical properties and thermal shock resistance were analyzed or studied by adjusting the sintering temperature,fiber content and BN fraction.The thermal shock damage behavior and thermal stress field distribution were analyzed and simulated by FEM.First,the results show that the compositions of SBS composites sintering below 1500 ? are ?-Si₃N₄,h-BN,Al PO₄ and ?-Si₃N₄.?-Si₃N₄ is the main crystal phases under such circumstances,while Sialon phase becomes the main component when sintered at 1600 ?.With the sintering temperature increases,mechanical properties increase and dielectric performance decline.The bending strength,fracture toughness,dielectric constant and loss tangent?tan??of 10SBS-1400 are only 75±2.1 MPa,1.2±0.11 MPa·m1/2,3.9 and less than 2×10^-3,respectively.However,10SBS-1500 has the corresponding properties of 117±12 MPa,2.1±0.53 MPa·m1/2,4.6,and 3×10^-3,respectively.Increasing the BN content from 5% to 30%,the bending strength of the samples were reduced from 176±14 MPa to 119±18 MPa and the dielectric constants were cut down from 5.28 to 4.67.In addition,when the temperature exceeds 1500 ?,the fiber will crystallize and the performances deteriorate.Amorphous BN coating was situ synthesized via PIP method employing boron acid and urea as raw materials on the surface of Si₃N₄ fiber and the thickness was moderate.When the concentration of dipping solution was 0.8 mol/L,the coating would not be easy to fall off.The mechanism of preparing BN coating was further clarified by XRD and XPS analysis.The monofilament test of the fiber showed that the tensile strength after coating decreased and appeared brittle fracture.The XFEM?extended Finite Element Method?based on the traction-separation law and the viscous element were used to simulate the failure behavior of the matrix and interface during stretching process.The thermophysical properties of SBS composites were measured.The thermal diffusion coefficients of samples increased with the promotion of sintering temperature,and decreased with the test temperature increasing.The average thermal conductivity below 1500 ? is 2.75 W/m·K,and the average coefficient of thermal expansion?CTE?is 2.8×10-6/K for S20BS.According to the mechanical properties,the thermal shock resistance factor was calculated.The critical temperature differences of thermal shock are about 456⁵55 ? for the composites with 5%²0% BN content.The thermal shock assessment of ?T=900 ? demonstrates that the strength retention rate is only 19%³0%.Si O₂ and B₂O₃ were formed as BN and Si₃N₄ reacted with oxygen and water during the heat preservation and quenching process.Finite element method of transient complete thermal-mechanical coupling was applied to simulate the macro temperature and stress field during quenching process under mechanical constraints.When the temperature difference surpasses 575 ?,the thermal stress starts to be greater than the strength of the SBS.When the thermal shock temperature difference reaches 1175 ?,the internal stress can be more than 400 MPa.The thermal stress was calculated of the meso-RVE model for fiber-coating-matrix from 600 ? to 1200 ?.The results showed that Si₃N₄ fiber was mainly subjected to tensile stress and the matrix was compressive stress.