Project Title:

Recrystallized Si3N4 Fiber Coating for Toughened Ceramic-Matrix Composites

	  94-1	04.06	4085

	  Recrystallized Si3N4 Fiber Coating for Toughened Ceramic-Matrix 
	  Composites

Abstract:

Ceramic-matrix composites (CMCs) are actively being developed for 
a variety of high-temperature military, aerospace and industrial 
applications. While possessing high specific strength and 
stiffness, high fracture toughness, and potential for exceptional 
oxidation resistance at elevated temperatures, the utility of 
current CMCs are severely limited by their susceptibility to 
oxidation embrittlement and strength degradation when stressed 
at or beyond the onset of matrix cracking and subsequently 
exposed to high-temperature oxidation. CMCs are classified by the 
low linear-elastic strain-to-failure of the matrix constituent 
relative to the reinforcing fiber. For the current state of 
technology, the linear-elastic region represents the "useful" 
design stress-strain region due to the deleterious effects of 
environmental degradation of the compliant fiber coating (i.e., 
carbon, boron nitride) at elevated temperatures following matrix 
cracking.
The objective of this Phase I program is to develop and evaluate 
an oxidation-resistant fiber coating technology for toughened 
CMCs consisting of an engineered recrystallized Si3N4. Laminated 
(0/90kS crossply Hi-Nicalon) SiC fiber-reinforced/SiC matrix 
composite plates will be fabricated incorporating a low density 
(~2.6 g/cm3) amorphous Si3N4 fiber/matrix interface region 
produced by chemical vapor infiltration (CVI). Following the 
complete consolidation with a CVI-SiC matrix, the densified 
composite plates will be subjected to a ~1400@C heat treatment 
process to recrystallize the amorphous S4N4 fiber coating. 
Recrystallization will increase the fiber coating density to ~3.2 
g/cm3, thereby inducing shrinkage and microcracking within the 
Si3N4 fiber coating. The subsequent shrinkage and microcracking 
of the fiber coating may sufficiently weaken the fiber/matrix 
interFacial bond strength and stiffness characteristics necessary 
to impart high composite strength and toughness. SiC/SiC 
composite plates incorporating three (3) different Si3N4 fiber 
coating thicknesses and one (1) baseline pyrolytic carbon fiber 
coating will be produced and evaluated to determine the 
corresponding tensile stress-strain and fracture toughness 
behavioral characteristics at roomtemperature, following high-
temperature isothermal conditioning in air, and in post-stressed 
oxidation environments.

Ceramic-matrix composites are an enabling class of materials for 
a variety of thermostructural applications in aerospace 
propulsion combustors and nozzles, hypersonic airframe thermal 
protection systems, spacecraft re-entry heatshields, land-based 
turbine and power generation components, radiant burner and heat 
exchanger tubes, and other industrial applications.

Key words

Hyper-Therm High-Temperature Composites, Inc.
18411 Gothard Street, Units B & C
Huntington Beach, California 92648