Preforms (essentially, shaped mats) of single-crystal yttrium aluminum garnet (YAG) fibers can now be readily fabricated in net size and shape, with tailored orientation of the fibers. These preforms can be used as fiber reinforcements in ceramic- and metal-matrix composite materials that withstand temperatures as high as 1,700 °C. The development of these preforms and composites is a continuing effort in conjunction with other efforts to develop lightweight components for aircraft turbine engines.

The single-crystal form of YAG is needed for creep resistance; the polycrystalline forms of YAG and other oxide fiber materials do not resist creep adequately at the contemplated high operating temperatures. It would be expensive and impractical to construct preforms by weaving, braiding, or laminating premanufactured single-crystal fibers, and handling of the fibers during such construction would weaken them.

Figure 1. A Scanning Electron Micrograph shows a random porous single-crystal YAG.

A proprietary process is being developed to overcome the above-mentioned limitations. In this process, reinforcement preforms are formed to net-shape, eliminating the need for operations such as weaving and braiding of single filaments. The net-shape preforms can have random, oriented, or mixed reinforcement in selective areas. Reinforcement volume-fraction shape and orientation can be controlled. Thin complex parts such as airfoils can be fabricated. Single-crystal porous preforms with volume percent in the range of 30 to 54 percent and strut dimensions in the range of 10 to 125 micrometers have been produced.

Figure 1 shows a scanning electron micrograph of porous single-crystal YAG reinforcement. Figure 2 shows a Laue x-ray diffraction pattern from this porous YAG preform. The Laue x-ray pattern is characteristic of single-crystal YAG, thus proving its single-crystal nature. High magnification microscopy indicated the absence of grain boundaries and confirmed the single-crystal nature. Reinforcement preforms with random, unidirectional and [0/90] reinforcement have been fabricated so far. Sufficient permeability of the preforms was demonstrated by conducting infiltration of the preform by polymer and metal matrices by pressure infiltration. Work is underway to optimize the preform rein forcing efficiency and infiltration with ceramic and intermetallic matrices.

Figure 2. A Back-Reflection Laue X-Ray Diffraction Pattern from the preform in Figure 1 confirms its single-crystal nature.

Porous single crystals made by this proprietary approach also have applications such as selective emitters, filters, and photonic band-gap materials due to their unique physical properties. Porous single crystals of sapphire, YAG, eutectic YAG, doped sapphire, and ytterbia have been fabricated by this process and potential exists for fabricating porous single crystals of many other materials.

This work was done by Prashant G. Karandikar, Ronald Roy, and Uday Kashalikar of Foster-Miller, Inc., for John H. Glenn Research Center.Inquiries concerning rights for the commercial use of this invention should be addressed to

John H. Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135.

Refer to LEW-16665.

NASA Tech Briefs Magazine

This article first appeared in the May, 1999 issue of NASA Tech Briefs Magazine.

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