Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2001-06-18
2002-05-21
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S096300, C501S097400
Reexamination Certificate
active
06391811
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to intermetallic materials based on MoSi
2
, intermetallic matrix composites and methods of making the same. More particularly, the invention is directed to a MoSi
2
based material having an engineered micro-structure provided through the use of in-situ reinforcement whiskers.
MoSi
2
is an attractive intermetallic for structural applications due to its excellent high-temperature oxidation resistance, low density and high thermal conductivity. However, it is brittle at low temperatures, weak at high temperatures and suffers from accelerated oxidation at intermediate temperatures. The accelerated oxidation of MoSi
2
at intermediate temperatures causes the material to disintegrate into powder, a phenomenon known as pesting.
Pesting is a general term describing the catastrophic oxidation of intermetallic materials at intermediate temperatures. The accelerated oxidation leads to the disintegration of the material and component failure. For MoSi
2
the temperature at which pesting is most pronounced is approximately 500° C. It has been observed that at 500° C. bulk (i.e., non-composite) MoSi
2
, as well as, composites of MoSi
2
with alumina and aluminum nitride also suffer total disintegration within relatively short time periods, e.g. 100 hours.
The pested samples yield powdery products consisting of MoO
3
whiskers, SiO
2
clusters, and residual MoSi
2
. The MoO
3
whiskers exhibited protruding characteristics and were concentrated at the grain boundaries and cracks. The pesting phenomenon in MoSi
2
has been concluded to be the result of the formation of voluminous molybdenum oxides in microcracks. While not wanting to be bound by theory, the accelerated oxidation apparently involves the simultaneous formation of MoO
3
and SiO
2
in amounts essentially determined by the Mo and Si concentrations in the intermetallic.
The addition of about 30 to 50 volume percent of Si
3
N
4
particulate to MoSi
2
reduced the pesting by forming a protective oxide scale as disclosed in assignee's related U.S. Pat. No. 5,429,997, the teachings of which are hereby incorporated by reference. In addition, improvements in room temperature fracture toughness, reductions in the 1200° C. compressive creep rates and lowered coefficient of thermal expansion were attained. Additional improvements in toughness and elevated temperature strength were achieved by reinforcing the MoSi
2
—Si
3
N
4
matrix with about 30 volume percent of silicon carbide continuous fibers. The use of fiber reinforcement is not entirely satisfactory due to the high costs of the present state-of-the-art techniques for making fiber reinforced, composites.
A further difficulty with the use of fiber reinforcement is the coefficient of thermal expansion mismatch between MoSi
2
and most potential reinforcing materials. MoSi
2
has a relatively high coefficient of thermal expansion as compared to most potential reinforcing materials such as silicon carbide fibers. The coefficient of thermal expansion mismatch between the fiber and the matrix material tends to result in matrix cracking during fabrication and severe matrix cracking during thermal cycling which in turn results in component failure. Possible reinforcing fibers include high strength ceramic fibers such as silicon carbide, single crystal alumina, and ductile, high strength molybdenum and tungsten alloy fibers. Ductile niobium fibers have shown improvements in low temperatures strength and toughness, but a severe reaction between the fiber and MoSi
2
limits its use and, in any case, it does not provide improved high-temperature characteristics. The addition of silicon carbide whiskers has yielded improvements in room temperature toughness, but pesting and coefficient of thermal expansion mismatch continue to be problems.
SUMMARY OF THE INVENTION
It has now been discovered that a MoSi
2
based material may be provided with an engineered micro-structure through processing and composition control. In-situ reinforcement of whisker type &bgr;-Si
3
N
4
grains in a MoSi
2
matrix offers a unique combination of attributes. The in-situ reinforcement is believed to provide a more tortuous crack path with elongated grains or whiskers which lead to crack bridging and deflection resulting in very high fracture toughness.
The invention contemplates a new MoSi
2
based alloy composition which exhibits excellent pest resistance at low temperatures (400 to 600° C.), good coefficient of thermal expansion match with potential fiber reinforcement, excellent oxidation resistance at elevated temperatures and high fracture toughness enabling its use as a monolithic material. Accordingly, a MoSi
2
-based matrix contains a high-volume fraction of randomly oriented in-situ grown long whisker type grains of &bgr;-Si
3
N
4
in the MoSi
2
matrix. The matrix is characterized in part by a toughened micro-structure, lower density, lower coefficient of thermal expansion, excellent resistance to pest attack, and it is much stronger than the binary MoSi
2
material.
The invention also contemplates processing conditions to achieve fully dense alloys with engineered microstructure through the use of sintering aids to grow the long whiskers of &bgr;-Si
3
N
4
in the matrix. During processing, high-temperature and pressure conditions are used to convert the &agr;-Si
3
N
4
particles to randomly oriented &bgr;-Si
3
N
4
long whiskers. Suitable sintering aids include rare earth oxides.
It is presently believed that the improved pesting is related to the formation of more protective silicon oxy nitride, Si
2
ON
2
, and/or mullite/SiO
2
oxide scales that suppress the formation of non-protective MoO
3
. The invention also contemplates forming an outer protective layer of Al
2
O
3
·SiO
2
followed by an inner layer of SiO
2
for better oxidation resistance in reducing or low partial pressure oxygen atmospheres.
The invention also contemplates the use of the MoSi
2
-&bgr;Si
3
N
4
as a matrix in a ceramic fiber reinforced composite to achieve high specific strength, high first matrix cracking stressed and toughness without exhibiting any pesting or cracking during long-term thermal cycling at high and low temperatures. Silicon carbide fibers comprise a preferred reinforcing fiber.
According to a first preferred composition of the invention, there's provided MoSi
2
based matrix materials containing at least about 20 percent by volume &bgr;-Si
3
N
4
whisker type grains based on the combined volume of the MoSi
2
and the &bgr;-Si
3
N
4
. More preferably, the compositionof the invention comprises from about 30 to about 50 percent by volume &bgr;-Si
3
N
4
based on the combined volume of the MoSi
2
and &bgr;-Si
3
N
4
.
The achievement of the whisker type &bgr;-Si
3
N
4
grains is enhanced by the use of rare earth oxide sintering aids. The rare earth oxides are nano sized. Preferred rare earth oxides include Y
2
O
3
and Al
2
O
3
. The sintering aids are used in amounts ranging from 2 to 6 percent by weight based on the combined weight of the MoSi
2
and &bgr;-Si
3
N
4
.
A preferred fiber reinforced composite comprises the above noted MoSi
2
-&bgr;Si
3
N
4
composition as a matrix material and ceramic reinforcing fiber. The reinforcing fiber is interspersed with the matrix material. The matrix material comprises at least about 50 percent, and more preferably about 70 percent by volume of the composite based on the combined volume of the matrix material and the &bgr;-Si
3
N
4
reinforcing fiber. In more preferred arrangements, the reinforcing fiber is a silicon carbide fiber, and it is present in the matrix in an amount of about 30 percent by volume based on the combined volume of the matrix and the reinforcing fiber.
In accordance with the method of the present invention and preferred processing, mixtures of MoSi
2
and Si
3
N
4
are blended and milled to micrometer particle size with the prior addition of sintering aids. The mixtures are formed into thin sheets or plates using vacuum hot pressing to achieve a relatively high green density. This first
Group Karl
Ohio Aerospace Institute
Pearne & Gordon LLP
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