Results of an International Round‐Robin for Tensile Creep Rupture of Silicon Nitride

Abstract

Fourteen laboratories participated in an interlaboratory study to establish the within-and between-laboratory repeatability of tensile creep rupture of silicon nitride. In air at 1375°C at 200 MPa, the times to failure ranged over a factor of 50, and the minimum creep rates ranged over a factor of 20. Despite these large ranges, taken individually, no one laboratory stands out from any other; all produced equally acceptable data. Consumers of silicon nitride tensile creep data must accept this magnitude of variability in reported creep data. The wide variety of specimen shapes and sizes, gripping systems, extensometry techniques, and temperature measurement strategies makes it impossible to assign definitively the root cause of the variability. However, there was a significant specimen size effect. As a group, the smalldiameter specimens lasted roughly five times longer and crept three times more slowly than the large-diameter buttonhead specimens. A possible interpretation of the origin of this difference is that the oxidizing conditions affected more of the volume of the small specimens during the test.

Specimen Size Effect on the Creep of Si3N31

“…Time-to-failure dependence on specimen size for SN88 has recently been reported in another study in which the authors collaborated [10]. Creep strain as a function of time for the test specimens is illustrated in Fig.…”

“…RESULTS & DISCUSSION 111A. Creep Behavior as a Function of Specimen Size'l%esteady-stat ecreepstrain rates tended to decrease and the times to failure tended to increase with decreasing test specimen diameter.Time-to-failure dependence on specimen size for SN88 has recently been reported in another study in which the authors collaborated [10]. Creep strain as a function of time for the test specimens is illustrated in Fig.…”

Microstructure and Creep Behavior of Silicon Nitride and SiAlONs

“…A thorough, interlaboratory study of tensile creep of identical Si 3 N 4 specimens provided quantitative data on this variation. 112 The findings showed that, from laboratory to laboratory, a factor of 20 difference in measured strain rate was common, and was not the result of experimental error in any of the laboratories. Within an individual laboratory, the strain rate measured from specimen to specimen varied by up to a factor of 10.…”

“…Strain rates have been shown to vary considerably both from specimen to specimen and from laboratory to laboratory. A thorough, interlaboratory study of tensile creep of identical Si 3 N 4 specimens provided quantitative data on this variation 112 . The findings showed that, from laboratory to laboratory, a factor of 20 difference in measured strain rate was common, and was not the result of experimental error in any of the laboratories.…”

“…reported a variation in measured strain rates of more than a factor of 10 from specimen to specimen for SiAlON materials. Although no clear source for this variability was found, it is usually attributed to small differences in the volume of intergranular phase 67 and oxidation behavior 112 of each specimen, depending on where it was cut from the larger billet of material.…”