Examination of Fracture Interfaces in Silicon Nitride

1975 ◽  
Vol 33 ◽  
pp. 60-61
Author(s):  
Nancy J. Tighe
Keyword(s):  
Silicon Nitride ◽  
Grain Boundary ◽  
Crack Growth ◽  
Subcritical Crack Growth ◽  
Slow Crack Growth ◽  
Ceramic Materials ◽  
Grain Boundary Sliding ◽  
Boundary Phase ◽  
Turbine Engines ◽  
Boundary Sliding

Silicon nitride is one of the ceramic materials being considered for the components in gas turbine engines which will be exposed to temperatures of 1000 to 1400°C. Test specimens from hot-pressed billets exhibit flexural strengths of approximately 50 MN/m2 at 1000°C. However, the strength degrades rapidly to less than 20 MN/m2 at 1400°C. The strength degradition is attributed to subcritical crack growth phenomena evidenced by a stress rate dependence of the flexural strength and the stress intensity factor. This phenomena is termed slow crack growth and is associated with the onset of plastic deformation at the crack tip. Lange attributed the subcritical crack growth tb a glassy silicate grain boundary phase which decreased in viscosity with increased temperature and permitted a form of grain boundary sliding to occur.

Microvoid formation in the grain boundary phase of a sintered Si-Al-O-N ceramic after static fatigue testing

1986 ◽  
Vol 44 ◽  
pp. 490-491
Author(s):  
M. R. Hughes ◽  
T. A. Nolan ◽  
J. Chang
Keyword(s):  
Silicon Nitride ◽  
Elevated Temperatures ◽  
Fatigue Testing ◽  
Slow Crack Growth ◽  
Boundary Phase ◽  
Turbine Engines ◽  
Static Fatigue ◽  
Gas Turbine Engines ◽  
Sintering Aid ◽  
Intergranular Phase

Sintered silicon nitride materials are currently being considered for use in hot flow-path components of gas turbine engines because of their good thermal shock and oxidation resistance as well as strength at high temperatures. These materials, however, have been shown to be susceptible to slow crack growth (SCG) and creep at elevated temperatures. The high-temperature properties are largely determined by the intergranular phase which is composed of the sintering aid residue and may be either amorphous or crystalline depending on sintering and annealing parameters. The silicon nitride examined in this study had reportedly been sintered with Y2O3 (5.86%) and Al2O3 (2.2%) to produce a composite of β'Si3N4 crystals in an amorphous Y-Si-Al-O-N matrix. Static fatique tests performed on test bars of this material resulted in failures originating, via SCG and creep within the intergranular phase, above certain stress loads at 1000°C. These sites and other areas through the cross section of the test bars were examined by SEM and AEM to determine the microstructure and chemistry related to these failure phenomena.

Microstructure and microanalysis of silicon nitride ceramics in the Y-Si-Al-O-N and Y-Si-O-N systems

1990 ◽  
Vol 48 (4) ◽  
pp. 1072-1073
Author(s):  
Michael K. Cinibulk
Keyword(s):  
Silicon Nitride ◽  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Liquid Phase Sintering ◽  
Boundary Phase ◽  
Full Density ◽  
High Temperature Strength ◽  
Transmission Electron ◽  
Structural Applications ◽  
Nitride Ceramics

Silicon nitride ceramics are among the leading candidate materials for use in structural applications at high temperatures. Due to the highly covalent nature of the Si-N bond and therefore low self-diffusivity, processing Si3N4 to full density requires the use of additives to provide a medium for liquid-phase sintering. When exposed to temperatures above ∼1000°C the resulting amorphous grain-boundary phases soften, leading to grain-boundary sliding and the eventual failure of the ceramic. The objectives of this work were to modify the grain-boundary phase composition and then attempt to devitrify the resulting intergranular phase to a refractory crystalline phase, producing a sintered Si3N4 with improved high-temperature strength and oxidation resistance. Transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDS) were used to characterize these materials. This paper describes these results.

Creep Crack Growth in an Austenitic Cr-Ni-Mn Steel

1990 ◽  
Vol 112 (3) ◽  
pp. 353-357 ◽  
Author(s):  
M. Tanaka ◽  
H. Iizuka ◽  
F. Ashihara
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Crack Size ◽  
Grain Boundary Sliding ◽  
Creep Crack ◽  
Boundary Sliding

Effects of grain-boundary sliding and strenthening by serrated grain boundaries on creep-crack growth are investigated using an austenitic 21Cr-4Ni-9Mn steel at 973 K in air. In surface notched specimens, crack growth rate is lower in specimens with serrated grain boundaries than in those with normal straight grain boundaries at the same stress intensity factor (K1), but the difference in crack growth rate between these specimens decreases with crack growth. In smooth bar specimens, growth rate of the largest surface crack is correlated with the average amount of grain-boundary sliding, although a unique relationship is not found between them. Grain-boundary sliding controls the crack growth when the crack size is small, but total creep deformation becomes important and crack linkage occurs with increasing crack length, thus reducing the strengthening effects of serrated grain boundaries.

INTERNAL FRICTION PEAKS CONNECTED WITH GRAIN BOUNDARY SLIDING IN Al

1983 ◽  
Vol 44 (C9) ◽  
pp. C9-759-C9-764
Author(s):  
E. Bonetti ◽  
A. Cavallini ◽  
E. Evangelista ◽  
P. Gondi
Keyword(s):  
Internal Friction ◽  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Boundary Sliding
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