Specimen Size Effect on the Creep of Si3N31

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.

Section: Discussionsupporting

confidence: 82%

Section: (2) Specimen Size Effectsmentioning

confidence: 99%

Section: (2) Specimen Size Effectsmentioning

confidence: 99%

See 1 more Smart Citation

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

Luecke

2002

“…The creep and slow crack growth resistance can vary dramatically with time, depending upon how the microstructure itself evolves. The creep and slow crack growth resistance may even differ thorough the specimen thickness giving rise to a specimen‐size effect 30 . Such time dependent and spatial variations in creep and slow crack growth resistances will have profound ramifications for reliability and life prediction analyses.…”

Section: Resultsmentioning

confidence: 99%

Depletion of Grain‐Boundary Phase and Elastic Creep Deformation Upon Ultra‐Long Flexural Stress Rupture Experiments of NC‐132 Silicon Nitride

Kleebe

Braue

Quinn

2007

A unique grain‐boundary structure evolution was observed in two MgO‐doped silicon nitride specimens (Norton, NC‐132) that were tested in ultra‐long flexure stress rupture experiments with an applied stress of 266 MPa and fractured at 14 941 and 17 376 h. Transmission electron microscopy showed that, although the starting material had a secondary glass phase both at multi‐grain junctions and along grain boundaries, the tested specimens contained no residual glass phase. Concurrent with the elimination of the secondary glass phase, a continuous network of cracked grain boundaries was observed after long‐term flexure testing consistent with the concept of elastic creep. It is, therefore, concluded that at ultra‐long annealing times, this material is affected by creep deformation via microcrack nucleation and growth due to the depletion of the amorphous siliceous grain‐boundary phase, which is seen as a truly transient, fugitive secondary phase.

“…Although the gauge volume of GPSSN specimens (nominally 100 mm 3 ) was approximately twice as large as those of EXPSN and HIPSN specimens, this difference was found in another investigation 8 to have negligible effect on the creep behavior of these size specimens; however, there could possibly be a size effect for much larger specimens. 10 Moreover, the HIPSN specimens had doubly reduced necks and much smaller radii of neck curvature in the plane of the specimen head faces. 7 The experimental secondary creep-rate data from the HIPSN study 7 and the GPSSN study 12 were analyzed separately in the same manner as that for the EXPSN in the present work ( 12 † Values in parentheses are estimated standard deviations of the fitted parameter.…”

Section: (1) Comparison Of Secondary Creep Ratesmentioning

confidence: 99%

Tensile Creep Behavior of a Gas‐Pressure‐Sintered Silicon Nitride Containing Silicon Carbide

Krause

Wiederhorn

2001

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375°and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs ؊1 , and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.