Elasticity and inelasticity of silicon nitride/boron nitride fibrous monoliths

A study is reported on the effect of temperature and elastic vibration ampl itude on Young's modulus E and internal friction in Si3N4 and BN ceramic sa mples and Si3N4/BN monoliths obtained by hot pressing of BN-coated Si3N4 fi bers. The fibers were arranged along, across, or both along and across the specimen axis. The E measurements were carried out under thermal cycling wi thin the 20-600 degreesC range. It was found that high-modulus silicon-nitr ide specimens possess a high thermal stability; the E(T) dependences obtain ed under heating and cooling coincide well with one another. The low-modulu s BN ceramic exhibits a considerable hysteresis, thus indicating evolution of the defect structure under the action of thermoelastic (internal) stress es. Monoliths demonstrate a qualitatively similar behavior (with hysteresis ). This behavior of the elastic modulus is possible under microplastic defo rmation initiated by internal stresses. The presence of microplastic shear in all the materials studied is supported by the character of the amplitude dependences of internal friction and the Young's modulus. The experimental data obtained are discussed in terms of a model in which the temperature d ependences of the elastic modulus and their features are accounted for by b oth microplastic deformation and nonlinear lattice-atom vibrations, which d epend on internal stresses. (C) 2001 MAIK "Nauka/Interperiodica".