The development of advanced engines for aerospace applications
depends on the availability of strong, tough materials that can
withstand increasingly higher temperatures under oxidizing conditions.
The need for such materials led to the study of an oxide-based
composite composed of an alumina matrix reinforced with zirconia-coated
sapphire fibers. Because the nonbrittle behavior of this system
depends on the interface and its ability to prevent fiber-to-matrix
bonding and reduce interfacial shear stress, the microstructure
of the zirconia must be carefully controlled during both coating
application and composite processing. When it was both porous
and unstabilized, zirconia (which does not react easily with alumina)
was found to be the most effective material tested in reducing
interfacial shear strength between the fiber and matrix.
From initial composite fabrication efforts, processing parameters
to maintain the porosity of the interfacial coatings were determined
at the NASA Lewis Research Center. Low temperatures and pressures
were needed to avoid severe densification of the interfacial coating.
Under these conditions, however, densification of the matrix was
also limited. It was therefore necessary to alter the composite
processing method to increase matrix densification while maintaining
the porosity of the fiber coating.
By adding aluminum metal to the matrix and utilizing the conversion
of aluminum to aluminum oxide in the initial processing stage
of composite formation, it was possible to increase matrix density
while maintaining a weak fiber/matrix interface in a sapphire-reinforced
alumina matrix composite. The reaction formation of alumina led
to an extremely fine grain size in the matrix that increased the
sinterability and strength of the final ceramic. Without aluminum
additions, the matrix density was approximately 62 percent of
the theoretical density of alumina. Addition of aluminum increased
matrix density up to 85 percent of theoretical density. As anticipated,
the composite strengths also increased with matrix density and
aluminum content. Both first matrix cracking (the stress level
at which matrix cracks first appear) and ultimate stress increased
significantly.
Previous articleLast updated April 30, 1997
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
