Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Patent
1995-11-08
1997-10-14
Turner, Archene
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
428446, 428457, 428469, 428472, 428697, 428698, 428701, 428702, 428704, 156 89, 427228, 4273977, C23C 2600
Patent
active
056770600
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to protecting products made of refractory material against oxidation.
The term "refractory materials" is used herein to designate, in particular, refractory metals or metal alloys such as alloys based on niobium or based on molybdenum, tungsten, and/or tantalum, or refractory composite materials such as carbon-carbon composite materials or composite materials having a ceramic matrix, e.g. carbon-SiC (silicon carbide) composite materials. Such refractory materials are used, in particular, in the aviation or space industries to make parts that are subjected in operation to high temperatures, such as parts of aero-engines or elements of aerodynamic fairings (space vehicles).
A severe drawback common to the above-mentioned refractory materials is their poor resistance to oxidation, even when exposed to medium temperatures. This considerably limits the possibility of using them in an oxidizing medium at high temperature under static conditions, and makes such use practically impossible under aerodynamic conditions unless protection is provided against oxidation.
The state of the art concerning protecting refractory materials against oxidation is most abundant, in particular for composite materials containing carbon. The term "composite material containing carbon" is used herein to designate a composite material in which carbon is present in the reinforcing elements, e.g. in the form of carbon fibers, or in the matrix, or in an intermediate layer or "interphase" between the reinforcing elements and the matrix.
Generally, a protective coating is formed on the surface of the refractory material, the coating comprising a continuous layer of ceramic that withstands oxidation and that constitutes a barrier against the oxygen of the surrounding medium. The ceramic used may be a carbide, a nitride, a silicide, or an oxide. However, such a ceramic layer is inevitably subject to cracking. Microcracks appear in use because of the mechanical stresses imposed and/or the difference between the thermal expansion coefficients of the refractory material and of the protective coating. Similar defects may even appear while the ceramic layer is being made. The cracks provide the oxygen in the surrounding medium with direct access to the underlying refractory material.
To solve that problem, it is well known to make the coating so that it forms an outer surface layer that has healing properties, or to add such a layer to the ceramic layer. While the material is being used, variations in thermal and mechanical stresses give rise to variations in the shape of the cracks, particularly to their edges moving away from and towards each other. The term "healing layer" is used herein to designate a layer that is capable, under the conditions of use of the refractory material, of stopping, filling, or sealing the cracks while following the movements of the cracks, and capable of doing this without itself cracking. That is why the healing layer is usually made of substances that constitute a glass, or that are suitable for constituting a glass under the effect of oxidation, the glass being selected so as to exhibit viscous behavior at the working temperature of the material.
Thus, it is known that using a protective coating based on silicides provides protection against oxidation at high temperatures because a surface film is formed that is based on silica as a result of oxidizing the silicon contained in the coating. In use, the silica-based film continuously re-constitutes itself, so long as a sufficient quantity of oxygen is supplied. The silica base has a healing function because it passes to the viscous state at high temperatures. It has nevertheless appeared that in the presence of very high energy heat flows at high speed, e.g. in the combustion chamber of a direct air flow hypersonic jet engine, the silica film does not always regenerate quickly enough. The presence of intense heat flows that are localized, particularly in zones having surface defects, at sharp edges, and also in the zones of incidence o
REFERENCES:
patent: 3854892 (1974-12-01), Burgess et al.
patent: 4190493 (1980-02-01), Patel
patent: 5079195 (1992-01-01), Chiang et al.
Bogachkova Olga Petrovna
Goriatcheva Elena Valentinovna
Terentieva Valentina Sergeevna
Societe Europeenne de Propulsion
Turner Archene
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