| Monolithic {dollar}rm Sisb3Nsb4{dollar} ceramics, hot-pressed with either 3% MgO or 6% {dollar}rm Ysb2Osb3{dollar} additives and their respective composites, with additions of 5% and 10% {dollar}rm Sisb3Nsb4{dollar} whiskers, were creep tested at 1250, 1300 and 1350{dollar}spcirc{dollar}C with stresses varying from 40 to 90 MPa. A new creep flexure test, the "dynamic beam profile method", capable of measuring both load-point and center-of-the-beam displacements, was developed. This configuration permitted in-situ profiling of the beam throughout the test, so that the compressive and tensile deformations in the outer fibers, together with neutral axis displacement, could be determined in real time. Analysis of the curvature of the beam was combined with the closed form solution proposed by Chen and Chuang (1990), and a modification of their analysis developed in this research, to obtain creep exponents from the tensile and compressive outer fibers. These exponents were identical to those determined from tensile and compressive creep tests, and the evolution of creep damage in the tensile outer fibers of the creep flexure tests was also identical to that observed in tensile creep tests at the same temperature.; The {dollar}rm Sisb3Nsb4{dollar} whisker reinforcement was found neither to improve the creep rates nor the creep lives compared to the monolithic material. The reliability of the materials was not improved by the addition of whiskers, but depended on the cleanliness of the material. At 1250{dollar}spcirc{dollar}C deformation resulted in the growth of facet-sized cavities. At 1300{dollar}spcirc{dollar}C the facet-sized cavities linked to form microcracks, and at 1350{dollar}spcirc{dollar}C the microcracks linked to form macrocracks. The creep exponents associated with these deformation mechanisms ranged between three and five. The results showed that the new dynamic beam profile method was capable of analyzing both the tensile and compressive creep behavior of ceramic and ceramic composite materials. |
