| Because of its outstanding structural stability, creep resistance and oxidationresistance, Si-B-C-N ceramic attracts great attention in recent years, and maypossess extensive applications in aircraft, metallurgy, energy, information, etc.Methods used to prepare Si-B-C-N ceramic mainly include organic polymerpyrolyzing, physical vapor deposition and mechanical alloying. Much attention ispaid to the mechanical alloying method, due to its cheap and innocuous rawmaterials, simple process and advantage in the preparation of dense bulk ceramic.However, the sintering and crystallization process or mehanisms of the amorphousSi-B-C-N powder, and the high-temperature properties of the hot pressed nanoceramic, etc., are still needed to be further investigated. Aiming at these problems,amorphous2Si-B-3C-N powder was high-energy ball milled in this subject, and wasthen hot pressed to prepare bulk ceramic. Methods, including X-ray diffraction,SEM, TEM, TG-DTA, mechanical property testing, etc., were employed to carefullystudy the microstructure, sintering process, crystallization and property of2Si-B-3C-N powder or bulk ceramic. Main results are as follows.The mechanically-alloyed amorphous2Si-B-3C-N powder consists of hardaggregates, with near spherical shapes and an average size of about5μm. Theamorphous2Si-B-3C-N powders, prepared employing different milling parameters,have similar size distribution, surface structure and thermal stability. When therotation speed of pot is higher than600rpm, or the ball to powder mass ratio islarger than20:1, nano SiC crystals with a size of about1-5nm appear in theamorphous matrix. When milling time is30h or40h, silicon atoms are ready toreact with carbon atoms, but crystals are hard to be found. At temperatures higherthan1000℃, SiC nano wires may grow on the surface of the heated2Si-B-3C-Npowder. At about1350℃in nitrogen, carbothermic reaction occurs on the surfaceof2Si-B-3C-N powder and hence CO or CO2is released.Sintered at a temperature higher than1830℃（at80MPa）, the2Si-B-3C-Nceramic body takes rapid volume shrinkage and its density is greatly enhanced. Thebulk density, relative density, room-temperature fracture strength, Young’s modulus, fracture toughness and Vickers hardness of the as-prepared2Si-B-3C-N ceramic,hot pressed at1900℃/80MPa/30min/8bar N2, are about2.52g/cm3,88.7%,331.1MPa,139.4GPa,2.81MPa·m1/2, and5.65GPa, respectively. Whentemperature is lowered to1800℃, or pressure is decreased to50MPa, the densityand mechanical property of the prepared ceramic sharply degrade. The addition ofZrO2or AlN （5mol%） reduces the temperature, at which notable sintering occurs,to about1720℃and1760℃, respectively, and the density and fracture strength ofthe prepared cramic are considerablely elevated.Hot pressed under a pressure of80MPa, the amorphous2Si-B-3C-N powderstarts to crystallize at about1485℃. The growth of SiC and BN（C） grains relies onthe stacking of （111） or （0002） planes, or the incorporation of adjacent grains. The2Si-B-3C-N ceramic, hot pressed at1900℃/80MPa/30min/8bar N2, is composedof uniformly distributed β-SiC、α-SiC and BN（C）, with an average grain size lessthan100nm. BN（C） has a turbostratic structure and consists of unevenly distributedt-BN, t-carbon, boron adopted t-carbon and carbon adopted t-BN.The nano2Si-B-3C-N ceramic prepared here has outstanding structuralstability, creep resistance and oxidation resistance. Annealed at1800℃in1bar N2for3h, the ceramic shows little change in its microstructure. However, due to thedecomposition of ceramic, the sample has a mass loss of about3.55%. Under thepressure of125MPa, the2Si-B-3C-N ceramic exhibits a steady-creep rate of about5.5×10-10/s at1400℃,3.4×10-8/s at1500℃and8.5×10-8/s at1600℃in vacuum.In static and dry air, the ceramic possesses excellent oxidation resistance attemperatures lower than1200℃or at1600℃. However, the ceramic rapidlyoxidizes at1400℃. The addition of ZrO2or AlN （5mol%） greatly degrade theoxidation resistance of the2Si-B-3C-N ceramic.Current results give further comprehension on the preparation process,microstructure, thermal stability, crystallization, and high-temperature properties ofthe mechanically alloyed amorphous2Si-B-3C-N powder or the hot pressed2Si-B-3C-N ceramic, providing new theoretical insights or experimental data forthe further research of Si-B-C-N ceramic or related materials. |
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