Stock material or miscellaneous articles – All metal or with adjacent metals – Laterally noncoextensive components
Reexamination Certificate
2001-10-18
2004-03-02
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
All metal or with adjacent metals
Laterally noncoextensive components
C428S364000, C428S394000, C428S367000, C428S381000, C428S607000, C428S634000, C428S299100
Reexamination Certificate
active
06699589
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority of German Application 100 51 901.6, filed Oct. 19, 2000, the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention relates to carbon reinforcements, in particular for use in carbon-reinforced materials, and to a method of generating internal protection against oxidation in carbon reinforcements.
b) Description of the Related Art
Carbon reinforcements are currently used in many materials, use being made e.g. of whiskers and carbon fibers. Carbon fibers, also known as coal fibers, are thin fibers of pure carbon, embedded in plastics, metals and glass for reinforcement thereof. This yields high-strength materials, such as e.g. carbon-fiber reinforced plastics (KFK).
Carbon-reinforced materials are used, among others, in particular in aircraft construction and in space technology, and in many cases in the form of carbon-reinforced silicon carbide materials. In these materials, carbon reinforcements, and in particular carbon fibers, are incorporated in a silicon carbide matrix. In contrast to monolithic silicon carbide, carbon- reinforced silicon carbide materials are more elastic, due to the embedded C fibers, and tolerate more damage in terms of their rupture behavior.
The carbon-reinforced silicon carbide materials, which have only a very low specific weight and are also known as C/SiC materials, are used predominantly at high temperatures, in particular in combination with light-weight designs in aircraft construction, and, in this connection, very often in re-entry technology.
A problem has been encountered in connection with C/SiC-materials in that they degrade at temperatures of from about 500° C. by oxidation of the carbon reinforcements, in particular at the silicon carbide matrix/carbon reinforcement interface as well as at those locations where the carbon reinforcement is not protected by the SiC matrix, e.g. at the material surface. Said oxidation, also referred to as “fiber burn off” in connection with the use of carbon fibers for reinforcement, leads to a significant deterioration of the properties of the material, e.g. its strength.
It has hitherto been known, in practice, to suppress oxidation of the carbon or C reinforcements in materials, e.g. by coating C/SiC materials on the outer surfaces with a &bgr;-SiC coating by means of a CVD method. However, thermal or mechanical stresses cause cracking in such a &bgr;-SiC layer, allowing the oxidizing gas to freely enter the material and thus also the C reinforcement. Thus, once such cracks have formed, undesirable oxidation of the C reinforcement may occur again.
The published German patent application DE 44 43 789 describes a further external anti-oxidation coating for carbon composite materials. Said anti-oxidation coating is provided such that it consists of hafnium-containing aluminum borosilicate glass comprising a boron-containing filler material and/or high melting point SiC or Si
3
N
4
filler materials, which may be applied on an SiC surface to be coated.
Up to about 1600° C., said coating shows continuous, thermoviscous plastic behavior and thus “seals” any cracks formed in the SiC coating. The low oxygen permeation through said coating is obtained by silicium-containing boron compounds, which allow dissolved oxygen to be bound while forming SiO
2
and B
2
O
3
, and thus have a so-called “oxygen getter function”.
However, said thermoviscous coatings are disadvantageous in that they solidify upon cooling while, in turn, forming cracks again. Thus, if there are no more “oxygen getters” present upon binding of all boron compounds, the oxidizing gases may penetrate again through the cracks thus formed to the C reinforcement, consequently causing an undesired oxidation, which leads to a considerable deterioration of the material.
According to DE 196 16 217 C2, an increase in the functionality of an external anti-oxidation coating may be obtained such that said protection coating contains refractory ceramic powder particles having a degree of emission of at least 0.70, and that, by incorporating compounds having the general elemental composition of Me
x
B
z
, wherein Me is Si, Al, Zr, Hf or Y, and x and y each represent a number from 1 to 6, a vitreous-amorphous phase of the general type Me
2
O
3
—MeO
2
—B
2
O
3
is formed during a short heating period.
In an oxidation protection coating as described in said document, the refractory ceramic powder particles may form a disperse substance component, and thus the mechanothermally stabilizing structure of a protective coating as well as the compound Me
x
B
z
may form the thermoviscous matrix component which functions as the in situ-binding phase for the stabilizing structure consisting of ceramic particles. This means that, in this case too, a coating is formed as an oxidation protection layer.
It is known that future re-entry aircraft to be used in space flight will be provided with novel control surfaces formed entirely of a C/SiC material and, moreover, being re-usable. In addition to immobile structural elements, such control surfaces also contain mobile elements, such as e.g. sliding bearings, whose contact surfaces can not be provided with thermoviscous layers, since said contact surfaces are subject to wear and thus can hardly be permanently protected on the outside with the help of a protective coating against oxidation, since such coating will be worn off first due to wear. This also applies to re-usable screw connections made of a C/SiC material, which are indispensible in the construction of large control surfaces on re-entry aircraft.
Further, thermo-viscous layers for protection against oxidation are generally inadmissible in a tribological system, since they solidify upon cooling and permanently connect the surfaces with each.
It is known from D. W. McKee, “Effect of absorbed phosphorus on the oxidation behavior of graphite”, Cabon, 1972, Vol. 10, pp. 491-497, to effect oxidation protection of carbons by infiltrating them with aluminum phosphate or with POCl
3
. The mechanism involved therein is believed to be the chemical adsorption on the semi-crystal layers of the carbon lattice.
However, these so-called impregnating tests with aluminum phosphate or POCl
3
are permanently effective only up to about 1000° C. due to the tendency of phosphates to decompose at elevated temperatures, so that, at elevated operating temperatures, such as those found, e.g., in re-entry technology of aircraft, no permanent oxidation protection can be obtained by said infiltration of carbons.
OBJECT AND SUMMARY OF THE INVENTION
Thus, it is a primary object of the present invention to provide carbon reinforcements, in particular for use in a carbon-reinforced silicon carbide material, which are oxidation-resistant up to high temperatures, even if subjected to tribological stresses.
According to the invention, this object is achieved in carbon reinforcements, in particular those for use in carbon-reinforced materials, by sulfur complex-forming substances being chemically adsorbed, at least in part, at potential oxidation locations of an hexagonal carbon lattice in the carbon reinforcements for internal oxidation protection.
Carbon usually reacts with oxidizing gases, such as e.g. oxygen or carbon dioxide contained in the air, to form CO and CO
2
. However, such oxidation is undesirable since it causes a marked deterioration of material properties.
For carbon reinforcements, carbon is used in its graphite modification, which has an hexagonal crystal structure, wherein the hexagonal base layers are arranged in layers perpendicular to one another in the sequence . . . ABAB . . . Within said base layers, the atoms are covalently bound and the Van-der-Waals bond determines cohesion between the base layers in a vertical direction.
The reaction of the carbon with said oxidizing gases preferably takes place at potential oxidation locations, i.e. at the ends of the so-called prism surfaces of the hexagonal carbon lattice, the planes perp
Dogigli Michael
Woydt Mathias
Dixon Merrick
Reed Smith LLP
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