Research On The Preparation Of Polymer-Derived Sibn(C)Ceramic

Among non-oxide advanced ceramics, SiBNC ceramic is a advanced material which can provide high potential characteristics, such as high-temperature stability, high oxidation resistance, excellent high creep resistance etc, it has a great variety of potential applications in national defense, aerospace, energy and other areas. Unfortunately, the direct processing of bulk SiBNC ceramic from its preceramic precursor (PBS-Me) fails owing to the formation of gaseous byproducts during pyrolysis. This leads to the massive volμme shrinkage of the solid phase with concomitant the polymer-derived ceramics (PDCs) route combined with a warm-pressing process to synthesize crack-free bulk SiBNC ceramic has been discussed. Characterizations such as FTIR, EA, gel content test,TGA, SEM, XRD and Vickers hardness test were used to study the structures and properties of polymeric precursor, green bodies and bulk SiBNC ceramic bodies. The major conclusions are as follows:1)As the cross-linking temperature or time increased, the gel content of crosslinked precursor had increased gradually, non cracked ceramic bodies could be achieved at cross-linking temperature between220-260℃after high-temperature pyrolysis, and the bulk densities reached the maximμm value at240℃; When the cross-linking time was between2h and7h, crack-free ceramic bodies can be achieved, and the bulk ceramic density was up to the maximum value when cross-linking time was5h.2)The influences of the warm-pressing temperature and pressure, dwelling time and the particle size of powders on the structure and properties of polymeric precursor, green bodies and bulk SiBNC ceramic bodies were investigated in order to obtain optimum conditions for the preparation of SiBNC bulk ceramics. The bulk densities could reach the maximμm value when the warm-pressing temperature was between200-260℃, or the dwelling time was between40min and90min. The particle size of cured precursor powder was from80μm to240μm, with the increase of particle size, the density of bulk ceramics decreased, but the porosity increased. Generally, with the increase of the pressure, the density of bulk ceramics increased versus the porosity reduced gradually. In this way the pressure performed in the range of50-150Mpa was to consider a reliable value.3) The shaped body were pyrolysised at the temperature between700℃and1500℃under nitrogen or ammonia atmosphere, the main structures and properties of the ceramic bodies were studied, the results are as follows:Upon nitrogen atmosphere, with the increasing of pyrolysis temperature, the density, shrinkage and the hardness of SiBN (C) bulk ceramic increased gradually,while the porosity appeared to increase firstly and then decreased; when the temperature was up to1300℃, the ceramic mainly contained B-N, N-C and Si-N bonds, and it possessed excellent thermal stability and oxidation resistance, the density reached at1.77g/cm3, and the loss in volμme reached at59.4%, while the Vickers hardness (HV) of the bulk material was determined to be13.1GPa. When the green bodies were pyrolysised at1300℃in ammonia atmosphere, the main structures of the bulk ceramic bodies included Si-N, B-N bonds, and the carbon content was about0.21%. The bodies had excellent thermal stability, oxidation resistance, and the density reached at1.78g/cm3.