Accession Number:

ADA210739

Title:

Kinetics and Mechanisms of Primary and Steady State Creep in B- and Al- Containing Alpha Silicon Carbide

Descriptive Note:

Final rept. 15 Nov 1985-15 Jul 1989

Corporate Author:

NORTH CAROLINA STATE UNIV AT RALEIGH DEPT OF MATERIALS SCIENCE AND ENGINEERING

Report Date:

1989-07-01

Pagination or Media Count:

12.0

Abstract:

The steady state creep behavior of a number of high temperature structural ceramics have been measured and the results analyzed to determine the controlling mechanism. Pure polycrystalline silicon carbide, devoid of sintering aids, creeps by dislocation motion and climb. Silicon carbide containing B- and AL- sintering aids, creeps by grain boundary sliding controlled by diffusion mechanisms grain boundary diffusion - Coble creep - below 1920K lattice diffusion - Nabarro-Herring creep-above 1920K. The difference in behavior is attributed to the high concentration of vacancies accompanying impurity substitution in the sintered silicon carbide. Experimental measurements of grain boundary sliding offsets on polycrystalline silicon carbide have shown that the primary, transient, creep stage in this material is primarily due to plastic strain within the grains, and that the secondary, steady state, creep stage is primarily due to grain boundary sliding between the grains. The creep of a single crystal and polycrystalline niobium carbide in the 1570-1850 K range is controlled by dislocation glide and climb. The creep of hot pressed silicon nitrate and mullite in the 1470-1800 K range is controlled by grain boundary sliding due to the amorphous phase present as a consequence of Y2O3 and Al2O3 sintering aids. The addition of silicon carbide whisker reinforcement has no beneficial effect on the creep resistance of Si3N4, whereas, in the more easily deformed mullite, silicon carbide whisker reinforcement does result in a reduced steady state creep rate.

Subject Categories:

  • Ceramics, Refractories and Glass
  • Mechanics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE