Chemical modifications of polymer-derived silicon carbide fibers to enhance thermomechanical stability.

摘要

Low-oxygen SiC fibers of a good thermostability were developed at the University of Florida. Those fibers turned out to have a significant level of excess carbon, which led to low elastic modulus and low thermal stability relative to SiC. Two approaches were studied to improve the stoichiometry and the thermostability of SiC fibers: Si particle incorporation and decaborane incorporation. In preliminary studies, it was found that Si, HfSi{dollar}sb2{dollar} and TiSi{dollar}sb2{dollar} particles reacted with excess carbon in polymer-derived SiC structure during heat-treatments and created additional carbides. Carbothermal reactions proceeded greatly at temperatures much lower than their melting temperatures. Also it was found that the decarborane could work as a boron-releasing densification aid.; Studied SiC fibers are classified into four major categories: CP fibers; CPD fibers (with decaborane); CS fibers (with Si particles); and CSD fibers (with Si particles and decaborane). CP fibers, irrespective of compositions, became fragile after 1500{dollar}spcirc{dollar}C heat-treatment whereas they were good up to 1400{dollar}spcirc{dollar}C heat-treatment. Boron incorporation remarkably enhanced the thermomechanical stability of fibers. Surfaces of CPD fibers remained rather dense and more clean even after 1800{dollar}spcirc{dollar}C treatment. Decaborane incorporation assisted the crystallization process in fibers during 1800{dollar}spcirc{dollar}C treatment and greatly increased the elastic modulus of fibers.; Silicon particles incorporation resulted in improved fiber stoichiometry of silicon to carbon. However, the thermomechanical stability of fibers was not improved by Si incorporation. Porous microstructure resulted even after 1300{dollar}spcirc{dollar}C treatment. This was probably due to two factors: oxidation by Si particles and volume shrinkage by carbothermal reduction reactions. Decaborane incorporation to CS fibers improved the thermomechanical stability to a great extent. Though CSD fibers had rough surfaces after 1800{dollar}spcirc{dollar}C treatment, their strength retention was as high as 70%. Elastic modulus as high as 300 GPa was attained by a batch of CSD fibers. Additionally, CSD fibers had fiber density as high as 2.92 g/cm{dollar}sp3{dollar} after 1800{dollar}spcirc{dollar}C treatment.; Fibers with good tensile properties were not obtained from siloxane-based copolymers with/without decaborane. {dollar}alpha{dollar}-SiC particle incorporated SiC fibers had good as-pyrolyzed tensile properties, but poor thermomechanical stability up to 1500{dollar}spcirc{dollar}C due to surface oxidation.