A low-density, creep-resistant ceramic composite material has been developed as a prototype of such materials for use at high temperatures in the next generation of aircraft engines. The material consists of Nextel (or equivalent) ceramic fibers in a matrix that is, itself, a composite of mullite particles in a mixture of 90 percent alumina with 10 percent silica. In fabricating an article of this material, the matrix is infiltrated into a three-dimensional woven fiber preform by use of sol-gel techniques in a multistep process.
In general, it is desirable to prevent bonding between the matrices and fibers of high-temperature ceramic composite articles to prevent fiber/matrix interactions of a type that can damage the fibers and thereby lead to embrittlement and failure of the articles. To prevent matrix/fiber bonding in the present material, an unstabilized zirconia interfacial layer is deposited on the fibers prior to infiltration with matrix material. Subsequently, the zirconia goes through a phase change with a concomitant volume change as it is heated and again as it is cooled. These volume changes fracture the interfacial layer, thus destroying any matrix/fiber bonds that might have formed.
The infiltration steps provide a matrix of uniformly low density. The initial infiltrating matrix precursor material comprises mullite powder mixed into an alumina/silica sol-gel. The viscosity of the gel keeps the mullite powder in suspension during infiltration. (The viscosity of the gel can be temporarily reduced by an ultrasonic probe to aid infiltration.) The sol is then gelled in situ by use of ammonia gas to cause a change in pH. The gelation step helps to ensure uniform distribution of matrix material throughout the preform by preventing the liquid component of the matrix precursor from migrating away from the interior of the article during drying. The article is then heat-treated above the matrix-precursor transition temperature of 526 °F (274 °C).
Further densification is achieved by repeated steps of infiltration with alumina/silica sol-gel followed by heat treatment above the transition temperature. A final heat treatment is performed at the intended use temperature to stabilize the matrix.
In an experiment, the four-point-flexure strength of a specimen of this material was 3.9 kpsi (27 MPa) before and 3.6 kpsi (25 MPa) after exposure to a temperature of 1,800 °F (980 °C) for 100 hours. The corresponding flexure-strength figures for a specimen with an all-alumina matrix were lower; 3.3 kpsi (23 MPa) before and 2 kpsi (14 MPa) after exposure to the same high temperature.
This work was done by Anna L. Baker of United Technologies Pratt& Whitney for Lewis Research Center. For further information,access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category, or circle no. 113on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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