Pest resistant molybdenum disilicide type materials

Details Pest resistant molybdenum disilicide type materials Pest resistant molybdenum disilicide type materials

1999-12-22

2001-07-24

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C428S663000, C501S096300

Reexamination Certificate

active

06265080

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to pest-resistant molybdenum disilicide type bulk materials and coatings.
BACKGROUND INFORMATION
Molybdenum disilicide is attractive for high temperature applications because of their strength, oxidation resistance (at most temperatures), and relatively low cost. It's commonly used as heating elements. It's high temperature properties also make it potentially useful in aerospace applications such as gas turbines, particularly as the gas turbine industry strives toward higher engine outputs and efficiencies through designs that operate at higher temperatures and stresses.
The current material family of choice for high temperature applications in gas turbine engines is the superalloy family. Molybdenum disilicide offers potential advantages compared to superalloys, including lower density and potentially higher use temperatures.
A significant limitation encountered by the prior art on the use of molybdenum disilicide has been pesting. Pesting is the rapid oxidation of the material at intermediate temperatures of about 300°-600° C., and is particularly rapid at about 500° C. It is believed that pesting occurs when molybdenum oxides such as MoO
3
form within microcracks in the material. As these oxides grow in volume, they mechanically wedge the cracks apart and eventually cause fracture.
Raj, in U.S. Pat. No. 5,330,590 discusses this phenomena and concludes that molybdenum disilicides are not suited for aircraft engine component applications.
U.S. Pats. Nos. 5,292,691, 5,292,692, and 5,308,806, issued to Maloney et al, disclose molybdenum disilicide composite materials and methods of manufacture. The references teach that composites may be produced from commercially available molybdenum disilicide powders reinforced by metallic or semi-metallic fibers for increased strength, creep resistance, and toughness.
Molybdenum disilicide materials possess a number of useful properties and it is desired to use them in high temperature applications. It is also desired that they be produced at relatively low cost.
It is therefore the object of the present invention to provide a molybdenum disilicide material that does not experience pesting.
Another object of the invention is to provide a relatively low cost molybdenum disilicide material.
Yet another object is to provide a pest resistant coating material based on MoSi
2
.
SUMMARY OF THE INVENTION
The present invention derives from the discovery that partially replacing the molybdenum in molybdenum disilicide with rhenium effectively eliminates pesting. Several mechanisms may explain the suppression of pesting. Rhenium may help form a stable and protective oxide layer, which may suppress further oxidation or it may reduce the volumetric expansion and volatility of MoO
3
. Another potential mechanism may be reduction in anisotropy of thermal expansion of MoSi
2
. Since MoSi
2
with C11
b
crystal structure is non-cubic, thermal expansion is expected to be different in different crystallographic direction. In a polycrystalline aggregate of such a non-cubic material, the resulting differential in contraction upon cool down is likely to cause residual stress in the grain boundaries at lower temperatures. This can promote oxygen diffusion along grain boundaries and lead to pesting. Adding rhenium may reduce the anisotropy of thermal expansion.
The amount of rhenium required to eliminate pesting, on an atomic basis is from 2 to 50% of the sum of the Mo and Re, and preferably from 5 to 40% of the sum of Mo and Re. Expressed as a formula, the broad invention composition range comprises (Mo
a
Re
b
X
c
) (Si
d
Z
e
) where:
a. a+b+c=1
b. d+e=2
c. b=0.02-0.5
d. X is selected from the group consisting essentially of Cr, Ti, Fe, Co, Ni and mixtures thereof.
e. c=0-0.2
f. Z=is selected from the group consisting essentially of B, Ge, Al, Ga, Sn, Sb, Bi and mixtures thereof.
g. d=1.8-2.0
h. e=0-0.2
The invention allows the use of molybdenum disilicide type materials in high temperature applications such as aircraft engines and heating elements without the danger of pesting oxidation damage. The invention material may also be used as a coating.


REFERENCES:
patent: 5429997 (1995-07-01), Hebsur

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