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    COLLOQUIUM OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS
        Institute for Computational Sciences and Informatics
	                      CSI 898-Sec 001  
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               A New Mechanism for Creep in Silicon Nitride

                          Sheldon M. Wiederhorn
               National Institute of Standards and Technology

Most commercial grades of silicon nitride exhibit a very high dependence of
creep rate on stress.  Although creep data can be fitted to the usual
power-law functions, data often show curvature and systematic variations of
slope with temperature and stress.  In this paper, we present a new approach
to the creep of silicon nitride and other ceramics that are bonded by a
deformable second phase.  In silicon nitride, the second phase is silicate
phase formed as a result of liquid phase sintering.  A review of
experimental data suggests that creep of silicon nitride  is controlled by
the rate of formation and growth of cavities.  The critical step for
deformation is the flow of silicate phase away from the cavitation site.
The parameters that determine the creep rate of silicon nitride are the
effective viscosity of the silicate phase and the number of cavities that
are active at any moment.  By assuming that the hydrostatic stress
distribution between pockets of silicate phase  is normally distributed, it
is possible to derive a creep equation that closely matches the secondary
creep rate of commercial grades of silicon nitride.  The creep rate is
predicted to depend on the viscosity and amount of silicate phase in the
silicon nitride, but not on the size of the silicon nitride grains.   

	                 Monday , November 3 1997
                                 4:30 pm
                        Room 206, Science & Tech. I

Refreshments will be served.
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(http://www.csi.gmu.edu/lcdm/seminar/schedule.html)