Fabrication Of Silicon Nitride Nanowires(SNNWs) And SNNWs Reinforced Ceramic Matrix Composites(SNNWs/CMCs)

Silicon nitride nanowires?SNNWs?are ideal ceramic matrix composite reinforcements due to excellent mechanical,dielectric and high-temperature properties.In this dissertation,SNNWs were synthesized and used for reinforcing BN ceramic composites?SNNWs/BN?and SiCN ceramic composites?SNNWs/SiCN?by die-pressing and presursor infiltration and pyrolysis?PIP?process.The structure and properties of SNNWs and the influence of the preparation process on the structure,composition,morphology and properties of composites were investigated.The synthesis of SNNWs was studied.SNNWs with a length of several millimeters and a diameter of 80³00 nm were prepared by thermal evaporation of SiO at 1500oC for 2 h in nitrogen.The SNNWs were composed of?-Si₃N₄,a small amount of?-Si₃N₄and atmosphere SiO_x layer.And the growth mechanism was VS growth.The growth position of SNNWs had an important influence on its morphology,which the SNNWs grown at the distance of 6⁹ cm from the silicon source had the highest quality.The SNNWs can be uniformly mixed with the matrix powder by the high speed shearing method,maintaining a high aspect ratio.The composition,structure and pyrolysis characteristics of polyborazine were investigated,and the SNNWs/BN composites were fabricated.Polyborazine which can be crosslinked by dehydrogenation coupling reaction at 70⁹0oC was composed of the hexatomic ring of B and N,highly active bonds of N-H and B-H.At the temperature of1100oC,the polyborazine almost complete the ceramization,and gradually crystallize into h-BN with the increase of temperature.The results of XRD and XPS showed that the SNNWs/BN composites fabricated at 1200¹600oC consisted of h-BN,B₂O₃,H₃BO₃ and?-Si₃N₄ nanowires and the degree of crystallinity increased with the rise of the fabrication temperature.The density,open porosity and linear shrinkage of the SNNWs/BN composites fabricated at different temperatures were 1.79¹.99 g/cm³,2.3%³.5%and 8.7%¹5.2%,respectively.The density increased with the rise of the preparation temperature and decreased with the increase of SNNWs content.High content of SNNWs can effectively inhibit the contraction of BN matrix during the pyrolysis.The 1200oC,1300oC and 1400oC fabricated SNNWs/BN composites with the same SNNWs fraction displayed a considerable flexural strength of 40.7⁷2.2 MPa.But the flexural strength of 1500oC and 1600oC fabricated composites decreased significantly.The SNNWs/BN composites possessed a elastic modulus of 18.6³0.5GPa and a fracture toughness of 0.85¹.28 MPa?m^1/2.The 1400oC fabricated composites with 3wt%SNNWs owned the highest strength?72.2 MPa?and fracture toughness(1.28 MPa?m^1/2),increasing by 77.4%and 36.2%respectively compared with pure BN ceramics.The SNNWs/BN composites behaved good dielectric properties that the dielectric constant?3.99⁴.33?and dielectric loss(?0.6¹.8?×10^-3)increased with the SNNWs fraction and fabrication temperature.The thermal expansion coefficient of SNNWs/BN composites fabricated at 1400°C is?4.16⁵.54?×10^-6 at room temperature,and increased first and then decreased when the temperature rose.The composition,structure and pyrolysis characteristics of polysilazane were investigated,and the SNNWs/SiCN composites were fabricated.The polysilazane PSZ-50 with Si-N bond-based main chains and C=C bonds had a ceramic yield of 74.5%after cross-linking.The ceramization nearly completed after pyrolyzed at 800oC,and crystalline Si₃N₄,SiC and C appeared with nanowires above 1400oC.SNNWs/SiCN composites were fabricated by die-pressing-PIP process at 1300oC and 1600oC using polysilazane as precursor.After 7 cycles of PIP,the composites fabricated at 1300oC with different contents of SNNWs possessed a density of 2.02².07 g/cm^-3 and open porosity of 7.9⁹.9%and the composites fabricated at 1600oC possessed a density of1.83².00 g/cm^-3 and open porosity of 23.7%²9.9%.Both the flexural strength and elastic modulus of the composites fabricated at 1300oC rose and then decreased with the increasing of SNNWs fraction and reached the top?83.7MPa and 54.0GPa,respectively?with 3wt%SNNWs,increasing by 9%and 64.1%respectively compared with those without SNNWs.For the SNNWs/SiCN composites fabricated at 1600oC,the flexural strength,elastic modulus and fracture strength rose and then decreased with the increasing of SNNWs as well and reached the top(62.2 MPa,27.0GPa and 1.45MPa?m^1/2,respectively)with 3wt%SNNWs,increasing by 57.5%,63.6%and 28.3%respectively compared with those without SNNWs.The SNNWs/SiCN composites displayed fantastic mechanical properties at 1200oC in air.The composites fabricated at1300oC with 7wt%SNNWs had a strength retention ratio of 96.3%and the high-temperature flexural strength of the composites fabricated at 1600oC with 0,1wt%and 7wt%SNNWs was better than that at room temperature.The composites fabricated at 1300oC owned a small coefficient of thermal expansion(<2.2×10^-6 below 490oC)and thermal diffusivity?<0.62 mm²/s?,while the composites fabricated at 1600oC displayed a higher coefficient of thermal expansion(?1.7³.7?×10^-6/K)and thermal diffusivity?1.37².70mm²/s?.