Evolution of Oxidation and Creep Damage Mechanisms in HIPed Silicon Nitride Materials

Abstract

Several yttria-fluxed, hot-isostatically pressed (HIPed) silicon nitrides have been tensile creep tested at temperatures representative of those present in expected gas turbine engines. Creep and oxidation assisted damage mechanisms concurrently evolve when these materials are tested at relatively high temperatures and low stresses (i. e., long exposure times at temperature). Atmospheric creep testing results in the creation of oxygen and yttrium gradients across the radial dimension of the specimens. High concentrations of oxygen and yttrium coincide with dense populations of lenticular-shaped cavities near the surface of the crept specimens. The center of the tensile specimens was devoid of oxygen or yttrium; in addition, lenticular cavities were rarely observed. The gradient in lenticular-cavity concentration is coincident with the oxygen and yttrium gradients. Stress corrosion cracking (SCC) also occurs in these HIPed silicon nitrides when they are subjected to stress at high temperatures in ambient air. The size of this damage zone increases when the temperature is higher and/or the applied stress is lower. Stress-corrosion cracking initiates at the surface of the tensile specimen and advances radially inwards. What nucleates SCC has not yet been identified, but it is believed to result from a stress-concentrator (e. g., machining damage) at the specimen’s surface and its growth is a result of the coalescence of microcracks and cavities. The higher concentration of oxygen and yttrium in the grain boundaries near the specimen’s surface lessens the local high temperature mechanical integrity; this is believed to be associated with the growth of the SCC zone. This SCC zone continues to grow in size during tensile loading until it reaches a critical size which causes fracture.

Research sponsored by the Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the Ceramic Technology Project of the Materials Development Program, under contract DE-AC05-84OR21400 managed by Martin Marietta Energy Systems, Inc.