MoSi2-based composite materials are among the advanced materials undergoing development as potential strong, stiff, lightweight replacements for the nickel-based superalloys now used in aircraft engines. Of the MoSi2-based composites, the most promising ones include SiC-based fibers within matrices that are, themselves, composites of MoSi2 containing 30 to 50 volume percent of Si3N4 particles. MoSi2 exhibits suitable high-temperature oxidation behavior, along with lower density and higher melting temperature relative to superalloys. However, the use of MoSi2 has been hindered by brittleness at low temperatures, inadequate resistance to creep at high temperatures, a coefficient of thermal expansion (CTE) much greater than that of SiC and other candidate fiber reinforcement materials, and a phenomenon called "pesting" (described in the next paragraph) at temperatures in the approximate range of 400 to 500 °C.

This Scanning Electron Micrograph depicts a cross section of a composite of (1) MoSi2 matrix containing 30 volume percent of Si3N4particles and (2) commercial SiC-based fibers.

Pesting, also called "pest oxidation," is usually defined as disintegration into powder. Pesting is considered to result from accelerated oxidation, among other things. In the case of MoSi2, pesting involves the simultaneous formation of MoO3 and SiO2. In most cases, pesting in MoSi2 has been linked to the formation of voluminous Mo oxides in pores or microcracks.

Since 1985, considerable research effort has been directed toward improving the high-temperature performance of MoSi2 by solid-solution alloying, reinforcement by fibers, and reinforcement by discontinuous inclusions. The present line of development of (MoSi2/Si3N4)-matrix/SiC-fiber composites is a logical sequel to previous research that yielded the following findings pertaining to the MoSi2/Si3N4 material system:

  • The addition of 30 to 50 volume percent of Si3N4 particles to MoSi2 increased resistance to low-temperature accelerated oxidation through the formation of a Si2ON2 protective scale, thereby eliminating catastrophic pest failure.
  • The addition of Si3N4 particles also increased compressive strength, fracture toughness, and high-temperature oxidation resistance.
  • The brittle-to-ductile transition temperature of MoSi2 containing 30 volume percent of Si3N4 particles was found to lie between 900 and 1,000 °C.
  • The CTE of MoSi2 containing Si3N4 particles was significantly less than that of pure MoSi2. As a result, unlike the matrices in MoSi2-matrix/SiC-fiber composites, the matrices in (MoSi2/Si3N4)-matrix/SiC-fiber composites did not exhibit cracking, even after thermal cycling.

To fabricate specimens for experiments along the present line of development, mixtures of MoSi2 and Si3N4powders were prepared, then consolidated by hot vacuum pressing followed by hot isostatic pressing to form fully dense plates of matrix-only (MoSi2/Si3N4) material. Specimens of (MoSi2/Si3N4)-matrix/SiC-fiber composites (see figure) were prepared similarly, except that multiple plies of SiC-based fibers in various orientations were interspersed with the mixed MoSi2 and Si3N4powders before pressing. (More recently, tape casting was adopted as the preferred technique for processing the fiber and matrix materials with improved fiber spacing, the ability to use narrower fibers, and lower cost.) The two-step consolidation procedure enabled the use of a consolidation temperature lower than that needed if consolidation were effected by hot pressing alone. The use of the two-step, lower-temperature consolidation procedure resulted in a fully dense material without excessive chemical reactions or damage to the fibers.

The specimens were subjected to a variety of tests to characterize their mechanical, thermal, microstructural, and chemical properties at temperatures ranging up to about 1,400 °C. The results of the experiments agreed with the previous findings and led to the following (among other) additional findings:

  • The addition of Si3N4 to MoSi2 doubled the room-temperature toughness and reduced the high-temperature creep rate by about 5 orders of magnitude.
  • Reinforcement of MoSi2/Si3N4 matrix materials by SiC-based fibers increased room-temperature fracture toughnesses by factors of about 7 and impact resistance by factors of about 5.
  • In general, specimens of (MoSi2/Si3N4)-matrix/SiC-fiber composites exhibited excellent strength and toughness improvements at temperatures up to 1,400 °C.

This work was done by M. V. Nathal of Lewis Research Centerand M. G. Hebsur of NYMA, Inc. Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7-3, 21000 Brookpark Road, Cleveland, Ohio 44135

Refer to LEW-16617.

NASA Tech Briefs Magazine

This article first appeared in the September, 1998 issue of NASA Tech Briefs Magazine.

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