Microstructure And Properties Of Mechanical Alloyed SiBN Powders And Ceramics

Because of the excellent mechanical and dielectric properties and outstanding high-temperature property, SiBN ceramic was regarded as one of the best comprehensive performance thermal-insulation and wave-transparent material with structural and functional integration, especially in the high supersonic aircraft radome application. But the preparation methods for fabricating compact bulk ceramic by the organic precursor method and chemical vapor deposition have become a great limitation in the application of SiBN ceramics. So we prepared amorphous SiBN powders by mechanical alloying using amorphous nano-Si3N4 and h-BN as raw materials and sintered amorphous/nano-crystal SiBN ceramics with great mechanical and dielectric properties by hot pressing in this thesis. The effects of Si/B atomic ratio and sintering temperature in phase, microstructure, mechanical properties and dielectric properties and ablative properties of SiBN ceramics have been investigated.Amorphous SiBN powders with different Si/B atomic ratio by high energy ball milling for 20 h, of which mean particle size is 4~6μm, are made up of spherical nanoparticles. SiBN ceramic by hot pressing is consisted by α-Si3N4, β-Si3N4, Si2N2 O and h-BN phases. Besides those phases, the SiBN with low Si/B atomic ratio contains Si O2. The bulk density and mechanical properties of SiBN ceramics improved with the sintering temperature rising. After hot pressing at 1900 oC, 40 MPa for 30 min, the mechanical properties of the samples reached the max values, of which bulk density, flexural strength, elastic modulus, fracture toughness and Vickers hardness are 2.48g/cm3, 273±10MPa, 137±1GPa, 3.02±0.05MPa·m1/2 and 6.1±0.2GPa, respectively. The mean dielectric constant and dielectric loss tangent value of SiBN also increase to the max value at 1900 oC, which are 5.93 and(6.23~7.83)×10-3 in 21~40GHz range at room temperature, respectively. Further more, the bulk density, elastic modulus, fracture toughness and Vickers hardness of SiBN go down gradually with the decrease of Si/B atomic ratio, but the flexural strength fluctuates. The SiBN ceramic with 1:3 Si/B atomic ratio has good wave-transparent properties, of which mean dielectric constant and dielectric loss tangent value are 4.23 and(0.44~1.61)×10-3 in 21~40GHz range at room temperature, respectively.After the ablative experiments under an oxyacetylene flame for 10 s, SiBN ceramics with different Si/B atomic ratio showed great performances. The sample with 2:1 Si/B atomic ratio has the lowest mass ablative rates which is only 0.0001g/s(the line ablative rate is 0.0042mm/s). At the same time, the line ablative rate of 1:1 Si/B atomic ratio SiBN sample is the minimum value which is namely 0.0031mm/s(the mass ablative rate is 0.0003g/s). The ablative surface of SiBN ceramic include the matrix phases(α-Si3N4, β-Si3N4, Si2N2 O and h-BN) and oxidation products(α-quartz, β-quartz). In addition we detected the Si on the ablative surface of high Si/B atomic ratio sample. The ablative surface consists by the ablative zone, transition zone and heat-affected zone. The cross section in the ablative zone also can be divided into three parts which are Si O2 fusion zone, heat-affected zone and matrix zone. The mechanics of ablative erosion mainly are high-temperature gas scouring and oxidation.