Preparation And Performance Of Precursor Derived Wave-transparent Nitrides Composites

The requirements of wave-transparent, carrying capacity and high-temperature resistence with long time using for the wave-transparent materials have been put forward by precision guided aircrafts and missles which need flying with high speed and long time. Taking these materials as the research target, the fabrication processes and integrative properties of nitrides wave-tansparent composites are investigated in this dissertation. The preparation processes and intergrative properties of Si₃N₄-BN composites fabricated by die-pressing-PIP method and gel-casting-PIP method are investigated systematically, and the pilot study of the fabrication of Si₃N₄-BN composites radome is developed. The preparation and properites of BN-BN and Sialon-BN composites are also investigated.The optimization investigation of the preparation of Si₃N₄-BN composites by die-pressing-PIP is developed. Under the experimental conditions, the densities and mechanical properties of the composites elevates along with the increasing of pressing pressure, pyrolysis temperature and PIP cycles. Crack between layers can be generated easier when the pressing pressure is higher than 100 MPa. The densification of the composites can be realized after 3 PIP cycles. The density, porosity, flexural strength, elastic modulus and fracture toughness of the composites pyrolyzed at 1750°C are 2.33 g·cm-3, 14.11%, 219.1MPa, 75.5GPa and 2.62MPa·m1/2, respectively. The optimizied process parameters include the pressing pressure of 100 MPa, the pyrolysis temperature of 1750oC and 3 PIP cycles.The integrative properties of Si₃N₄-BN composites fabricated by the optimized die-pressing-PIP are investigatied and the results are illustrated as follows. After been exposed to the air at 1500oC for 15 min, the flexural strength of the composites is 160.8MPa and the residual ratio is 73.40%. The dielectric properties exhit good stability at the frequency of 7¹8GHz and the temperature of 25¹200oC, the dielectric constant is around 5.0 and the loss tangent keeps below 5.0×10-3. The linear and mass ablation rates are 0.20mm·s-1 and 0.021g·s-1, Si₃N₄ is oxided to Si O2 while BN is oxided to B2O3 and then vaporized. The chemical moisture asorption is happened with the reaction production（NH4）2O·x B2O3·y H2 O when the pyrolysis temperature is below 1400°C, and the phemonen disappears after the temperature becomes higher than 1400°C. The coefficient of thermal conductivity is 7.62⁹.04 W·m-1·K-1（0³00°C）. The coefficient of thermal expansion increases along with the rising of environment temperature and 3.77×10-6·K-1 is achieved at 1600°C.The phenomenon that the adoption of pyrolysis BN has hindered the phase transition of Si₃N₄ in the Si₃N₄-BN composites is found. The pyrolysis BN hindered the touching, fusion of Si₃N₄ grains and the growth of β-Si₃N₄ seed grains by the mode of steric hindrance. The phase transition amount of Si₃N₄ increases slightly along with the rising of pyrolysis temperatures and achieves 3.90 wt.% at 1750oC.The fabrication of porous Si₃N₄ preform by the process of gel-casting with acrylamide（AM） and N,N′-methylenebisacrylamide（MBAM） as monomers is investigated, and the preforms with the solid loading of 30⁴5vol.% are prepared. The optimized process parameters of preparing Si₃N₄ slurry are chosed as the p H value of 10.3¹1.0, the dispersant agent amount of 2.0vol.% and the ball milling time of 18²4h. The degassing process in vacuum, decarbonization process at 600°C for 2h with a heating-up rate of 1°C/min and drying process with relative humidity and temperature control by steps are also employed.The Si₃N₄-BN composites are fabricated by gel-castinig-PIP route, and the integrated properties of the composites are similar with the samples prepared by die-pressing-PIP route. The composites are also composed by h-BN, α-Si₃N₄ and β-Si₃N₄, and phase transition of Si₃N₄ is limited too. The mechanical properties of the composites increase with the rising of the solid loading. The flexural strength, elastic modulus and fracture toughness of the composites with the solid loading of 45 vol.% are 231.4MPa, 79.6GPa and 2.83MPa·m1/2, respectively. The dielectric constant of the composites are increased from 4.31⁴.44 to 5.01⁵.06（7¹8GHz） with the rising of solid loading from 30 vol.% to 45 vol.%. The loss tangent of the composites are all below 3.31×10-3（7¹8GHz） with different solid loadings.The effect of sintering additives on the properties of Si₃N₄-BN composites is investigated. When the composites are prepared by die-pressing-PIP precess, the adoption of sintering additives realizes the increase of both phase transition of Si₃N₄ and mechanical properties of the composites at room temperature, but the high temperature mechanical properties at1000oC and dielectric properties decrease slightly. The method of gel-casting-sintering-PIP to prepare Si₃N₄-BN composites is brought forward considering the hinderance of phase transition of Si₃N₄ by pyrolysis BN. As the sintering process and sintering additives are adopted, the phase transition of Si₃N₄ is increased significantly and β-Si₃N₄ becomes the main phase, which possesses crystal grains with high slenderness ratios. And accordingly, the mechanical properties of the composites elevate obviously. However, mechanical properties of the composites at high temperatures are decreased, the flexural strength of 127.1MPa with a residual ratio 49.82% is gained after being exposed to the air at 1500oC for 15 min when the solid loading is 35 vol.%. The dielectric properties of the composites degrade too. The dielectric constant increases from 4.94⁵.03 to 5.54⁵.56（7¹8GHz） with the rising of solid loading from 30 vol.% to 45 vol.%. Better mechanical properties can be achieved by this preparing method, but dielectric properties degrade accordingly, and bigger volumetric shrinkage happens meantime. The gel-casting-sintering-PIP method is suitable for the application with more demanding for mechanical properties and less requirements for the dielectric properties and shape formings.Bulk BN-BN composites are fabricated by die-pressing and PIP route. The composites are composed of high pure h-BN phase, and the microsuture transition from pyrolysis BN binding to piece like particle stacking is ocuured along with the increasing of pyrolysis temperature. The density, porosity, flexural strength, elastic modulus and fracture toughness of the composites pyrolyzed at 1600°C are 1.81 g·cm-3, 20.16%, 39.8MPa, 31.8GPa and 1.46MPa·m1/2, respectively. The dielectric constant and loss tangent of the composites at 15 GHz are 2.7 and 0.0052. The BN-BN composites exhibit unfavorable mechanical properties but excellent dielectric properties, which are suitable for the wave-transparent application with less mechanical property demanding.The integrated properties of Sialon-BN composites fabricated by gel-casting-PIP method are investigated. The existing phases of the composites are Si3Al3O3N5 and h-BN. The Sialon particles are coated and binded by the pyrolysis h-BN. The density, porosity, flexural strength, elastic modulus and fracture toughness of the composites pyrolyzed at 1750°C are 2.16 g·cm-3, 24.21%, 178.6MPa, 75.5GPa and 4.54MPa·m1/2, respectively. The dielectric constant and loss tangent of the composites are 3.51³.55 and 1.0².5×10-3（7¹8GHz）.The Si₃N₄-BN composites cone-shaped radome with small size is fabricated by gel-casting-PIP method and the wave transimittance of basically above 60% at 7¹8GHz is achieved. The gel-casting-PIP method has proved the route for the fabrication and application of Si₃N₄-BN composites radomes with big sizes.