Fiber processing for composite applications

The focus of the dissertation was to use precursor methods to process oxide fibers and non oxide coatings on graphite fibers, which have the potential to improve the mechanical properties of composites (PMCs, CMCs, and MMCs).; For oxide fiber processing, YAG (Y{dollar}sb3{dollar}Al{dollar}sb5{dollar}O{dollar}sb{lcub}12{rcub}{dollar}) and spinel (MgAl{dollar}sb2{dollar}O{dollar}sb4{dollar}) fibers were made using carboxylate precursors. The YAG precursor was a mixture of 5Al(O{dollar}sb2{dollar}CH){dollar}sb3cdot{dollar}3H{dollar}sb2{dollar}O/3Y(O{dollar}sb2{dollar}CCH{dollar}sb3)sb3cdot {dollar}4H{dollar}sb2{dollar}O dissolved in H{dollar}sb2{dollar}O with additives, or Y/Al isobutyrates in THF. The spinel precursor was a mixture of 2Al(O{dollar}sb2{dollar}CH){dollar}sb3cdot{dollar}3H{dollar}sb2{dollar}O/Mg(O{dollar}sb2{dollar}CCH{dollar}sb3)sb2cdot {dollar}4H{dollar}sb2{dollar}O dissolved in H{dollar}sb2{dollar}O with additives. As found in previous studies of carboxylate precursors, both the YAG and spinel precursors behaved like separate compounds and decomposed to nanocrystalline, phase pure materials at relatively low temperatures (YAG at {dollar}sim{dollar} 800{dollar}spcirc{dollar}C and spinel at {dollar}sim{dollar}600{dollar}spcirc{dollar}C) without any evidence of phase separation.; Both the YAG and spinel precursors were easily extruded or hand drawn to form well-defined green fibers. The green fibers (dia. {dollar}<{dollar}30 {dollar}mu{dollar}m) are easily converted to fully dense ceramic fibers at temperatures {dollar}>{dollar}1570{dollar}spcirc{dollar}C for YAG and at 1500{dollar}spcirc{dollar}C for spinel. A carefully selected heat treatment schedule is necessary to maintain fiber integrity during pyrolysis, when the green carboxylate fibers transform to oxide fibers. The room temperature mechanical properties of the processed YAG or spinel fibers were estimated using bending tests. The best bend strength for YAG fibers was 1.7 {dollar}pm{dollar} 0.2 GPa (1600{dollar}spcirc{dollar}C sintering), which is roughly three times greater than previously studied polycrystalline YAG fibers. The bend strength for spinel fibers was 1.0 {dollar}pm{dollar} 0.4 GPa (1500{dollar}spcirc{dollar}C sintering).; For processing non oxide coatings on graphite fibers, a TiN amide polymer precursor solution (hexane as solvent) was used to process TiN coatings on graphite fibers. The best TiN coatings were those obtained using a 1 TiN equiv. wt. % hexane solution followed by heating to 900{dollar}spcirc{dollar}C/1 h/Ar. These coatings were relatively free of pores or cracks (thickness of 0.1{dollar}sim{dollar}0.2 {dollar}mu{dollar}m). The TiN coated fibers exhibit a half life of 60-70 min at 700{dollar}spcirc{dollar}C/Air, which is about seven times longer than desized C-fibers.; Modified polymethylsilane solutions (hexane as solvent) were used to process SiC coatings on graphite fibers. The best coatings were obtained with the 2 equiv. wt. % SiC solutions followed by final heating to 1200{dollar}spcirc{dollar}C/1 h/Ar, as determined by the relative absence of pores or cracks, with a coating thickness of 0.2{dollar}sim{dollar}0.3 {dollar}mu{dollar}m. The 2 wt % SiC coated fiber tows have a half life of 50-80 min, which is 5-9 times longer than desized C-fibers.; For both SiC and TiN coatings, the coating thicknesses were controlled by the concentration of the dipcoating solution. If the coatings are too thick ({dollar}>{dollar}0.4 {dollar}mu{dollar}m), the mismatch in the coefficient of thermal expansion between fibers and coatings appear to cause process flaws, i.e., cracks.