The grain size of in situ Si,N, is varied, and its effects on strength-flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging-zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress (p*) and length (Db) at the moment of the critical fracture. The results show that as grain size increases, p* decreases while D, and the critical bridging zone opening (u*) first increase and then decrease, resulting in a maximum for short-crack fracture toughness at an intermediate grain size. The initial increase of u* and D, with grain size is attributed to an increase in debonding length, while the decrease of p* is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes D, and u* to drop in the large-grain regime. Implications on microstructure design are discussed.

The creep performance of a developmental, in-situ reinforced silicon nitride was evaluated at temperatures between 1300-1425°C in ambient air. The minimum creep rate as a function of tensile stress and temperature was evaluated, and the measured tensile creep performances of two different specimen geometries (buttonhead and dogbonemachined from same billet of material) were compared. This silicon nitride exhibited comparable, or better, creep resistance than other silicon nitrides described in the literature. The measured creep response of the material and lifetime were observed to be geometry dependent; the smaller crosssectioned dogbone specimens exhibited faster creep rates and shorter lives, presumably due to faster oxidation-induced damage in this geometry. The tensile creep rates and lifetimes were found to be well-represented by the Monkman-Grant relationship between 1350 and 1425"C, with some evidence suggesting stratification of the data for the 1300°C tests and a change in dominant failure mode between 1300 and 1350°C. Lastly, values of the temperature-compensated stress exponent and activation energy for tensile creep were found to decrease by approximately 80 and 75% in compression, respectively, illustrating anisotropic creep behavior in this silicon nitride. I. INTRODUCTXON Silicon nitride (Si3N4) remains a leading candidate material for use in structural components (e.g., nozzles and blades [ 11) in advanced gas turbine engines. Si3N4 offers greater operating temperatures than currently used superalloys, is less dense which results in less inertial effects on performance, and offers the possibility of component design without the necessary provisions for cooling. In addition, Si3N4 has good thermal shock resistance and high thermal conductivity, is relatively corrosion-resistant, and possesses good high temperature strength.

The room temperature fracture behavior for in situ reinforced (ISR) silicon nitride is correlated to its microstructure and R-curve behavior. The relation of strength to fracture origin suggests that stable growth of the intrinsic flaw precedes catastrophic fracture. Grainbridging that generates a rising bridging stress behind the crack-tip has been proposed as the cause for stable crack growth, which in turn reduces the strength dependency on initial flaw size. As a result of strong bridging by the acicular β-Si3N4 grains, ISR Si3N4 is characterized for high Weibull modulus. At elevated temperatures, the material's tensile creep rupture behavior follows the Monkman-Grant type plot. A tensile creep rate of -10−9s−1 at 1260°C/250 MPa, 1300°C/180 MPa, and 1350°C/90 MPa has been recorded. This relatively strong creep resistance is related to the sliding-resistance of the acicular grains and the properties of the amorphous film between the grains in ISR Si3N4.