Stress relaxation of silicon nitride at elevated temperatures

The stress relaxation behavior of SN88, SN253, and NCX-5102 silicon nitride materials were experimentally determined in tension at 1300°C using buttonhead specimens. Specimens were held at constant strain after being loaded at 10 MPa/s to an initial stress of 276 MPa (40 ksi) or 414 MPa (60 ksi). The subsequent decay in tensile stress was measured as a function of time. A non-negative least squares algorithm used in conjunction with a generalized Maxwell model proved to be an efficient means to define characteristic relaxation modulus spectra and stress relaxation behavior. In the last part of this study, the utility of using short-term stress relaxation testing to predict long-term creep performance was examined.