EFFECTS OF MICROSTRUCTURAL INSTABILITY ON THE CREEP-BEHAVIOR OF SI-C-O (NICALON) FIBERS IN ARGON

Tensile creep tests of single Si-C-0 fibers (Nicalon, Nippon Carbon Co., Tokyo, Japan) were conducted in argon at 1300-degrees-C and 300 to 700 MPa. Fibers exhibited only primary creep, where the creep strain epsilon and creep time t could be empirically fitted by epsilon = (1/beta) In (1 + betaepsilon0t). The fiber deformation was described by a rheological model for the viscous flow of a concentrated suspension. Under the test conditions, the microstructure of Nicalon was unstable, resulting in weight loss and SiC grain growth. This instability was attributed to the decomposition of the amorphous SiC(x)O(x) phase in the fiber, forming SiC and CO as products. As a result, the viscosity of the fiber increased because of an increase in the SiC volume fraction. The continuous increase in viscosity caused a continuously decreasing creep rate, which made steady-state creep impossible under these conditions. Because of the instability in the microstructure, the chemical environment was found to have a profound influence on the mechanical properties of Nicalon at elevated temperatures.