Chapter 33. Elevated Temperature Behavior of Sintered Silicon Nitride Under Pure Tension, Creep, and Fatigue
- John B. Wachtman Jr.
Published Online: 28 MAR 2008
Copyright © 1993 The American Ceramic Society
Proceedings of the 17th Annual Conference on Composites and Advanced Ceramic Materials, Part 1 of 2: Ceramic Engineering and Science Proceedings, Volume 14, Issue 7/8
How to Cite
Sankar, J., Krishnaraj, S., Vaidyanathan, R. and Kelkar, A. D. (1993) Elevated Temperature Behavior of Sintered Silicon Nitride Under Pure Tension, Creep, and Fatigue, in Proceedings of the 17th Annual Conference on Composites and Advanced Ceramic Materials, Part 1 of 2: Ceramic Engineering and Science Proceedings, Volume 14, Issue 7/8 (ed J. B. Wachtman), John Wiley & Sons, Inc., Hoboken, NJ, USA. doi: 10.1002/9780470314180.ch33
- Published Online: 28 MAR 2008
- Published Print: 1 JAN 1993
Print ISBN: 9780470375266
Online ISBN: 9780470314180
- mechanical reliability;
- electron microscopy;
Mechanical strength properties and the long-term mechanical reliability of advanced ceramics at high temperatures are important to their future utilization in heat engines. Pure tensile, tensile creep, and tensile cyclic fatigue/creep interaction data are reported for a sintered silicon nitride (GTE SNW-1000) that is being investigated as a candidate material for advanced heat engine applications.
Pure uniaxial tensile test data are reported for room temperature, 900°, 1100°, and 1200°C. General observations indicated that the tensile strength was not sensitive to temperatures below 900°C. The decrease in tensile strength was rather moderate, by about 10%, as the test temperature increased to 1100°C. However, the material exhibited a sharp decrease in tensile strength, by 50%, as the test temperature was further increased to 1200°C.
Tensile creep tests were performed at 1100° and 1200°C. Creep strains were monitored with a noncontact laser extensometer. Results showed that the steady-state creep rate was dependent on both the temperature and the applied stress, the effect of temperature being more dominant than the applied stress. The results also indicated that even at lower applied stress levels, the steady-state creep rates were higher for specimens tested at 1200°C than at 1100°C. Scanning electron microscopy results indicated that after creep, the edges of pores in the material had softened and fissures extended beyond the pores, indicating the possibility for the linkage of pores during creep. Energy dispersive spectrum analyses performed in the pore areas of specimens subjected to creep indicated a difference in yttrium and silicon contents. The diffusion of yttrium into the pores was more along the longitudinal (stress) direction.
The effect of cyclic loading on creep behavior and residual tensile strength of the material after cycling were also studied at 1200°C. The specimens were cycled at different fatigue stress levels and two different patterns: constant amplitude triangular waveform, and low to high stress levels in multiple steps known as coaxing. Test results showed that precycling can dramatically modify the primary creep behavior and enhance the resistance to further creep deformation. Tests also indicated a significant increase in tensile strength of the material after precycling at 1200°C.