Fabrication And Properties Evaluation Of β-SiAlON/BN Composite Ceramic

According to the application of the β-SiAlON ceramic in the heat insulation andwave. β-SiAlON/BN composites with different content of BN and various Z valueswere fabricated by the pressureless sintering. Phases and microstructure werecharacterized by XRD and SEM, respectively. The room temperature mechanical anddielectric properties were studied. Moreover, this paper investigated the thermal shockand oxidation resistance of the composites.XRD indicated that the composites consisted of β-SiAlON and h-BN. And SEMresults showed that there are rodlike β-SiAlON and layer BN grain intersecting in thecomposites.The results of property tests showed that the density of the β-SiAlON/BNcomposites increased with the Z values and decreased with the content of BN. Theflexural strength and fracture toughness had the same trend to the density. The flexuralstrength and fracture toughness of the B53composites were up to93.12MPa and1.95MPa m1/2. Similarly, the dielectric constant and tangent loss of the compositesincreases with the Z values and decreases with the content of BN, the dielectricconstant and tangent loss of the B101composites reach to3.18and2.34×10-3.Oxidation tests indicated that the β-SiAlON/BN composites show weight gain,but we can not simply calculation the oxidation rate by the weight of the composites.The bigger Z values and less content of BN, the less weight gain of the composites.The composites showed good oxidation resistance at1000℃, the biggest weight gainfor B51composites was only1.011mg/cm2after20h. With the increase of temperature,the composites generate liquid phase oxidation layer on the hole. After1100℃/20h and1200℃/14h A high density oxide film was found on the surface of the composites andit effiectively inhibit the velosity of oxidation. Oxidation product is mainly composedof glass SiO2, cristobalite and mullite.Thermal shock test results show that, the β-SiAlON/BN composites with lowthermal expansion coefficient, in2~4×10-6K-1. thermal expansion coefficient increaseswith the Z values and decrease with the content of BN. The low content of BNcomposites show that thermal shock temperture diffierence was900℃. And it onlymaintained residual strength ratio50%when thermal shock temperture diffierence was 1200℃. We can saw that the crack on the surface of the composites. The high contentof BN composites thermal shock temperture diffierence up to1100℃. and it stillmaintained residual strength ratio70%when thermal shock temperture diffierence was1200℃and showed good thermal shock resistance. The thermal shock temperturediffierence was mainly influenced by material through ability. The increase of porosityand pore size will make material through ability good, smaller thermal stress., lead tothe thermal shock resistance more better. In addition, under the condtion of hightemperture thermal shock, on the surface of the composites, producing liquid phaseoxide layer, repairing the micro cracks and micro defects. These factors also willimpove the thermal shock resistance.