Note: Descriptions are shown in the official language in which they were submitted.
RD-8586
366
The present invention relates to silicon-based ceramic
composites exhibiting a reduced tendency to form metallic
silicides when used in contact with a metallic surface at an
elevated temperature.
Prior to the present invention, as shown by R.L. Mehan
and D.W. McKee, Interaction of Metals and Alloys with
Silicon-Based Ceramics, Journal of Materials Science II
(1976) 1009-1018, silicon based ceramics have been found to
chemically interact with a number of metals and alloys at
temperatures in the vicinity of 1000C in air. Included
among the reaction products are silicides, silicates and
carbides. The significance of the finding of Mehan et al
is that when a silicon-based ceramic, for example, a
silicon carbide-silicon matrix composite, such as taught
f B in U.S. Patent No. 4,/4~ ~4~ dated ~ yc~ 79
of Laskow and Morelock, and assigned to the same assignee
as the present invention, when usea in the form of a
turbine bucket or aircraft engine blade, may interfere
with the performance of the gas turbine or aircraft engine
based on the formation of the metallic silicides when the
silicon based ceramic comes in contact with a high perfor-
mance metal or metallic alloy at elevated temperatures.
It would be desirable, therefore, to reduce the tendency
of such silicon-based ceramic to interact with metallic
surfaces at elevated temperatures.
The present invention is based on the discovery that
silicon-based ceramic exhibiting a reduced tendency to
interact with metallic surfaces at elevated temperatures
can be made, if the surface of the silicon-based ceramic is
etched to remove surface silicon to a depth of at least
about 0 001 inch, and thereafter the cavity of the etched
thereafter the cavity of the etched ceramic is filled with
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an inorganic oxide mixture, such as an aluminum oxide-
silicon oxide blend, which is fired to produce a surface
ceramic substantially replacing the silicon removed from
the ceramic during the etching step.
There is provided by the present invention, a method
of reducing the tendency of a silicon-based ceramic from
forming reaction products, such as silicides, when in
contact with a metallic surface at elevated temperatures
which comprises:
(l) etching the surface of the silicon-based
ceramic to effect the removal of surface
silicon,
(2) substantially filling the cavity resulting from
the removal of silicon-based ceramic from the
surface of the silicon composite of step (l)
with an inorganic oxide mixture and
(3) firing the treated silicon-based ceramic of
(2) to a temperature of up to 1250C resulting
in the conversion of the inorganic oxide mixture
to an adherent ceramic coating.
An examp~e of a silicon-based ceramic which can be
treated in accordance with the method of the present invention
is the silicon carbide-silicon matrix composite shown in
Hillig and Morelock Canadian patent application Serial No.
272,280 filed February 18, 1977 and assigned to the same
assignee as the present invention, Additional examples of
silicon-based ceramic composites which can ~e
treated in accordance with the present invention are
the silicon carbide composites shown in Wakefield
U.S. patent No. 3,459,842 issued December 26, 1967 and Geiger
U.S. patent No. 2,431,327 issued November 25, 1947.
C
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3366
Included by the inorganic oxide blends which can be
used to treat the etched surface of the silicon-based
ceramic composites of the present invention are, for example,
blends of aluminum oxide and silicon oxide, such as Kyanite,
Bell Clay, Kaolin, etc. The inorganic oxide blend can be
used as a paste when blended with Al(H2PO4)3 H2O, etc.
Etching solutions which can be used are, for example,
mixtures of hydrofluoric and nitric acid.
In the practice of the invention the silicon-
based ceramic is treated with an etchant to effect the
removal of at least .001 inch to .Oln inch of surface
silicon from the silicon-based ceramic. After the etchant
has been rinsed from the surface of the silicon-based
ceramic, it can be treated with an inorganic oxide mixture
and the resulting silicon-based ceramic is fired to convert
the inorganic oxide to an adherent ceramic coating.
The silicon-based ceramic can be etched by various
standard techniques, such as immersion, swabbing, spraying,
etc., with a suitable etchant as previously defined. De-
pending upon the nature of the etchant and the depth towhich the surface silicon is to be removed from the silicon-
based ceramic, the time for treating the silicon-based
ceramic with the etchant can vary widely, such as a few
minutes or less to several hours or more. After sufficient
gilicon has been removed from the surface of the silicon-
based ceramic, the etchant can be rinsed therefrom.
The silicon~based ceramic is then treated with the
inorganic oxide mixture which can be applied with an
applicator in the form of a paste, or it can be sprayed,
painted, etc. Depending upon the specification desired in
the silicon-based ceramic, excess inorganic oxide mixture
can be removed by stand- are techniques, such as brushing,
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llQ~366
etc., until the desired surface thickness is achieved.
The silicon-based ceramic can then be fired at a temperature
of from 1250C to 1400C to convert the surface inorganic
oxide to an adherent enamel or smooth coating which sub-
stantially resembles the original silicon-based ceramic prior
to etching. However, after firing, the color of the surface
of the silicon-based ceramic can vary widely, based upon
the compositions of the inorganic oxide mixture employed.
The silicon-based ceramic composites, and preferably
the silicon carbide-silicon matrix composites which can be
treated in accordance with the practice of the present
invention can be used in a variety of applications, such
as turbine buckets aircraft engine blades, abradable coat-
ings in the form of rotary sealants, etc.
In order that those skilled in the art will be better
able to practice the invention, the following example is
given by way of illustration and not by way of limitation~
All parts are by weight.
Example 1.
A carbon fiber preform was prepared from low modulus
WCA carbon cloth of Union Carbide Corporation using an
aqueous colloidal s~lspension of graphite as a binder. The
density of the fiber was approximately 1.38-1.49 grams/cc
and the total weight of fiber in the preform after it was
machined to a 2.5" diameter disk was about 11 grams.
A 3 inch diameter mold was machined out of Speer
580 graphite having a mold cavity of about 2.5 inches and
a 0.42 inch thickness. Four 0.125 diameter infiltration holes
were drilled into the top half of the mold and 0.125 inch
diameter vent holes were drilled into the bottom half of the
mold. Carbon fiber wicks in the form of ~K braid were in-
serted into the infiltration holes and protruded about
0.125 inches from the top of the mold. The inside surface
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of the mold was treated with a boron nitride powder in a
form of an aerosol spray.
The carbon fiber preform was then placed in the mold,
and the mold was then placed in a supporting structure made
from Armco Speer 580 graphite which had been precision
machined to the specifications of the mold. A charge of
powder silicon was then poured on top of the mold surface.
In estimating the amount of silicon, there was employed
up to about a 15~ excess of that amount of silicon required
to fill the mold cavity in the molten state.
The mold and supporting structure was then placed
in a furnace which was maintained under a vacuum of about
1 x 10 2 torr. A pressure of from 1 x 10 2 torr to 3 torr
also was operable. The furnace was maintained at a temperature
of about 1600C. It was found that the silicon powder con-
verted to molten silicon in about 15 minutes and it was
allowed to infiltrate the carbon fiber prepreg. After
cooling to room temperature, the mold and supporting
structure was removed from the furnace and allowed to cool
under atmospheric conditions. The mold was then opened and
there was obtained a disk which conformed within 0.2~ of the
dimensions of the mold cavity. Based on method or pre-
paration, the disk was a silicon carbide, silicon ceramic
having about 16% by weight carbon in the chemically combined
form, or as a mixture of chemically combined carbon and
elemental carbon and about 84~ by weight of silicon.
The above silicon carbide-silicon matric composite
disk is placed in a crucible on a flat piece of Haynes
718, a nickel-base alloy. The silicon composite disk
and metal strip is then placed in an oven and heated to
1150C under atmospheric conditions. After 150 hours the
silicon ceramic disk and the metal strip are removed from
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the oven and allowed to cool to room temperature. The surface
of the metal strips is then carefully examined under an
optical microscope. It is noted that a siliciding reaction
has taken place on the surface of the metal strip, based
on the appearance of a rough looking surface and a series
of creaters indicating silicon-metal reaction. Those
skilled in the art would know that, in the event a silicon
carbide-silicon matrix composite were used as part of a
power generating apparatus, such as an aircraft engine
blade, and such ceramic part was in continuous contact
with a high performance metal, such as a chromium and
nickle containing alloy, the surface of such metal part
could be adversely affected, eventually resulting in break-
down of the structure.
Another silicon carbide-silicon matrix composite disk
was made following the procedure above which was swabbed
with an etchant in the form of a mixture of hydrofluoric
acid and nitric acid. After the etchant has been allowed
to contact the silicon-based ceramic disk for 15 seconds,
the treated ceramic disk was washed with water to remove
the etchant. Upon examining the etchant surface of the
ceramic disk it was found that the etchant had removed on
average of about .004 inch of silicon from the surface.
A paste consisting of Mullite, a blend of aluminum oxide
and silicon oxide and aluminum hypophosphate was uniformly
applied onto the etched surface of the silicon ceramic
disk employing a spatula. The thickness of the applied
paste was approximately equivalent to the depth of the
cavity resulting from the action of the etchant. The
treated ceramic disk was then placed in an oven and heated
for 15 hours at 1250C under atmospheric conditions. The
ceramic disk was then allowed to cool. It was found that
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the inorganic oxide paste had been converted to a uniform
glaze over the surface of the silicon ceramic disk.
The procedure was repeated with respect to placing
the silicon carbide-silicon matrix composite disk on top of
a metal nickel-chromium alloy strip, except that the ceramic
was placed on the metal strip so that the surface of the
inorganic coating which had been fired on the surface of
the silicon ceramic contacted the metal alloy strip. After
the silicon ceramic had been heated in contact with the
metal strip for the same period of time and under the same
conditions, it was allowed to cool. An examination of the
metal strip shows it is free of any surface reaction with
the silicon ceramic disk. This indicates that the treat-
ment of the silicon-based ceramic disk with the fired
inorganic oxide coating reduces the tendency of the silicon-
ceramic disk to react with the Haynes 715 alloy at tem-
peratures of 1000 C or above after 150 hours.
Although the above example is directed to only a few
of the very many variable which can be used in the practice
of the method of the present invention, it should be
understood that the present invention is directed to the
use of a much broader class of silicon-based ceramic com-
posites, inorganic oxide mixtures, etc., which are shown
in the description preceding this example.
