| Promising results for the development of a ceramic/ceramic composite to be used for structural applications in the ultra high temperature range (1650-1850{dollar}spcirc{dollar}C) have been obtained for alumina reinforced with titanium or zirconium diboribe coated with molybdenum disilicide. Prior to this achievement, several theoretical and experimental studies of particulate composites with (TiB{dollar}sb2{dollar} or ZrB{dollar}sb2{dollar}) imbedded in (ZrO{dollar}sb2{dollar}, Y{dollar}sb2{dollar}O{dollar}sb3{dollar} or Al{dollar}sb2{dollar}O{dollar}sb3{dollar}) were conducted. Calculations of the compatibility and stability of oxide matrices with the diborides in vacuum showed positive energies of formation, indicating suitability of reinforcement of alumina under vacuum conditions. Also, since the sublimation of vapor species at ultra high temperatures is very critical to the stability of substances still existing in their condensed states, a detailed thermodynamic analysis of the metal-oxygen systems, was carried out using elemental data of the TiB{dollar}sb2{dollar}, ZrB{dollar}sb2{dollar}, ZrO{dollar}sb2{dollar}, Y{dollar}sb2{dollar}O{dollar}sb3{dollar}, Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, MoSi{dollar}sb2{dollar} and their combinations (TiB{dollar}sb2{dollar} or ZrB{dollar}sb2{dollar})/(ZrO{dollar}sb2{dollar}, Y{dollar}sb2{dollar}O{dollar}sb3{dollar} or Al{dollar}sb2{dollar}O{dollar}sb3{dollar}) for comparison, at 1650, 1850 and 2050{dollar}spcirc{dollar}C. A comparison of the combination TiB{dollar}sb2{dollar} and ZrB{dollar}sb2{dollar} in MoSi{dollar}sb2{dollar}/(Al{dollar}sb2{dollar}O{dollar}sb3{dollar} or ZrO{dollar}sb2{dollar}) at 1650{dollar}spcirc{dollar}C was also made. A stability analysis using equilibrium oxygen partial pressures for the TiB{dollar}sb2{dollar} and ZrB{dollar}sb2{dollar} decomposition at 1650, 1850 and 2050{dollar}spcirc{dollar}C was carried out.; A detailed characterization of the powders used for the oxide matrices, the diborides and the molybdenum disilicide are presented. The effects of the powder characteristics and the vacuum hot pressing parameters on the densification of the composites are discussed.; Stability and chemical compatibility of the particulate and ternary composites in their as hot pressed states and following their vacuum and air oxidation treatments were characterized.; Interdiffusion of elemental species across diboride/disilicide and oxide/disilicide interfaces was studies by EDS dot mapping. It is proposed that the growth of the Mo{dollar}sb5{dollar}Si{dollar}sb3{dollar} is interface reaction controlled. It is proposed that diffusion of atmospheric gaseous oxygen through the porous texture left by the initial reaction of the oxide and diboride to form volatile B{dollar}sb2{dollar}O{dollar}sb3{dollar} accounts for the continued erosion of the structure. (Abstract shortened with permission of author.)... |
