In a proposed method of tailoring some of the mechanical properties of metal-matrix, oxide-matrix, and ceramic-matrix composite materials, Kirkendall defects (microscopic pores as described below) would be introduced into thin interfacial layers between the fibers and the matrices. This method could be used in addition or as an alternative to an older method in which one seeks to tailor the mechanical properties of a composite by coating its fibers one or more layer(s) of material(s) distinct from the matrix and fiber materials.
The coatings of the older method are applied as diffusion barriers to isolate the fibers from chemically reactive components of the matrix during processing or service, to protect the fibers from the effects of consolidation into the composite, to provide adequate bonding so that loads can be transferred from the matrix to the fibers, and/or to provide for deflection of cracks or for toughening the composite. In the case of multilayer coatings, each layer is applied in the effort to provide a beneficial effect that will contribute to the overall attainment of the protective effects listed in the preceding sentence.
In the proposed method, the fibers would be coated with an element that diffuses readily in the matrix material. Typically, such an element would be one that has a small atomic radius. The element must be one that is compatible with the matrix material at concentrations to be used.
Examples of techniques for depositing such elements include chemical vapor deposition (CVD), sputtering, and electron-beam physical vapor deposition (EB-PVD). During the thermal processing that is typically done to consolidate the composite, the coating material would diffuse away into the matrix as interstitial contaminants, leaving a porous interface behind. The resulting interfacial pores are instances of the classical metallurgical phenomenon known as "Kirkendall defects."
The initial coating and the subsequent porous interface would provide many of the protective effects mentioned above. In comparison with a composite that lacked the Kirkendall defects but was otherwise identical, the composite would be toughened because the porous interface would have a reduced cross-sectional area and would therefore be weaker.
The initial thickness of the diffusible coating, in conjunction with the processing parameters, would determine the extent of porosity created and, therefore, the degree of toughening. Other coatings could be applied either below or above the diffusible layer to further enhance the overall properties of the composite.
This work was done by Theodore R. Grossman of General Electric Co. for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 - 8
21000 Brookpark Road
Refer to LEW-16733.