Microstructure, Thermal Shock and Plasma Erotching Resistance of Bn-SiO_2-Al_2O_3 System Ceramic Composites

Abstract: Ceramic composites of BN-SiO_2-Al_2O_3 system were hot pressed at 1650℃under pressure of 30MPa for 1h in argon atmosphere using h-BN, amorphous silica, and alpha alumina powders as starting materials. The composites were characterized by DSC, XRD, SEM, TEM and three-point-bending test, water quenching and plasma etching techniques to investigate the effect of BN content on the microstructure, mechanical properties, thermal shock resistance and plasma sputtering resistance.Mullite was formed through the interaction between the amorphous SiO_2 and Al_2O_3. Three kinds of phases including mullite, h-BN, and residual Al_2O_3 existed in the as-fabricated composites with 10vol.%~40vol.%BN. The residual spherical alumina particles were embeded in mullite matrix, which led to the formation of high content of dislocations in mullite grains. The content of mullite and dislocation density reduced as BN increased. In contrast, in the composites with 50vol.%, 60vol.%BN, amorphous SiO_2 tend to retain instead of Al_2O_3, and equiaxed mullite grains in smaller sizes and nearly free from dislocations distributed between amorphous SiO_2 and h-BN,. Large amount of layered dislocations were observed in BN particles.With the increasing of BN content, density, Vicker’s hardness of the composites decreased, and the apparent porosity of them slightly (<0.5%) increased. Flexural strength, Young’s modulus and fracture toughness first increased and then dropped, achieving a peak value, 284.3MPa, 257GPa and 3.99MPa?m1/2, respectively, for the composite with 40vol.%BN.Specific heat monotonically increased for the composites with 20vol.% and 30vol.%BN, but first increased and then decreased for other compoites. Well, the specific heat of composites with 30 vol.%BN at 1000℃reached a maximum value of 1.4J·g-1·K-1. Thermal expansion coefficient reached a maximum value and a minimum one for the composites with 10vol.%BN and 60vol.%BN, respectively, and nearly had similar values for the composites with 20vol.%~50vol.%BN. Flexural strength of the composites declined apparently after thermal shocking except for the composite with 60vol.%BN, whose residual flexural strength was 134.5% and 125.8% of its original value for thermal shock temperature difference of 800℃and 1000℃, respectively.The composites exhibited excellent plasma sputtering resistance, though slightly lowered linearly with the increasing content of BN as well as the decline of its relative density…
Key words: BN-SiO_2-Al_2O_3 system ceramic composites; microstructure; mechanical properties; thermal shock resistance; plasma sputtering resistance

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