Coating alumina fibers with sputtered metal has been proposed as a means to enable the fibers to retain more strength when the fibers are incorporated into metal-matrix composites. If the alumina fibers could be made to retain more strength, then it would be possible to manufacture such composites with high volume fractions (20 to 40 volume) percent of fibers to obtain high-temperature properties suitable for such demanding applications as nozzles in engines of supersonic airplanes.
A metal-matrix composite of the type in question is formed by hot pressing or hot isostatic pressing of alumina fibers with a foil or powder of the metal that is destined to become the matrix. The issue of retention of strength arises because it has been observed, in the case of iron- and nickel-base alloy matrices, that the alumina fibers become degraded from their original strength of 425±25 kpsi (2.9±0.2 GPa) to mean strengths <150 kpsi (<1 GPa).
In preliminary experiments to test the proposed strengthening technique, alumina fibers were coated to a thickness of 4.7 µm by sputtering from a target of MA956 alloy. The coatings of some of the fibers were removed, and then the strengths of the fibers were measured and found to be about 360 kpsi (2.5 GPa). Other fibers were heated in a vacuum at a temperature of 1,200 °C prior to removal of coatings, and their strengths were found to be 310 kpsi (2.1 GPa). The sputtered coatings appeared to have resulted in only minimal strength-reducing damage to the fibers. Scanning electron micrographs of the fibers after removal of the coatings showed a surface appearance very different from that typically seen on fibers damaged by incorporation into metal-matrix composites.
Some caution in interpreting the observations in these experiments is in order, in part because the fibers were not rotated during sputtering and therefore the coatings did not extend around their entire circumferences. The fibers were also not coated along their full lengths. Moreover, it is not known whether the partial-strength-preserving effect of sputter coating is repeatable. Further experiments are planned to investigate the effects of full-circumference and full-length coating, to determine repeatability, and to learn more about the chemical and microstructural details of damaging versus nondamaging deposits. Another question to be addressed is whether alternative sputtering-target compositions and alternative coating processes could contribute to retention of strength.
This work was done by M. F. X. Gigliotti, Jr.; M. R. Jackson; and A. M. Ritter of General Electric Co. for Glenn Research Center.
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