Coating processes – Restoring or repairing
Reexamination Certificate
2001-07-31
2003-10-28
Beck, Shrive P. (Department: 1762)
Coating processes
Restoring or repairing
C427S127000, C427S128000, C427S062000, C427S063000, C427S064000, C427S180000, C427S181000, C427S189000, C427S226000, C427S230000, C427S287000, C427S256000, C427S283000
Reexamination Certificate
active
06638568
ABSTRACT:
The invention relates to a method for curing cracks in ceramic materials, which is especially suitable for improving the mechanical and, where appropriate, also the magnetic properties of a corresponding specimen body.
The method is in principle suitable for all ceramic materials, but is intended for those of fairly high relative density. In this way, it is possible to improve the mechanical properties and, where appropriate, also the surface quality in particular for surfaces on which finishing is carried out. In the case of superconducting materials, it is thereby additionally possible to improve the superconducting and magnetic properties as well. In the same way, it may also be used for non-superconducting ceramic magnet materials. Particular advantages are obtained when textured polycrystalline materials or superconducting materials having only a few magnetic domains are used.
In the case of brittle inorganic materials, it is almost impossible to fully avoid cracks, which here also include microcracks. They can occur, in particular, as a result of mechanical stresses in the component before, during or after the heat treatment, as a result of a heat treatment, as a result of structural stresses—especially when phases with significantly different thermal expansion differences are present—as a result of phase reorganizations or as a result of impact or similar mechanical action.
These cracks may be macroscopically detectable in the components. It is precisely the deeper cracks that extend over larger distances which are often open somewhat wider and are therefore often detectable without visual aids or without a crack test using penetration agents. As a result of many studies, it is known that cracks, including microcracks, deteriorate the mechanical properties. It is already possible to study the effect of cracks according to length, direction and size in flexural or tensile specimens, and to determine their sometimes critical effect. Partial or complete curing of the cracks can therefore significantly improve the mechanical properties of shaped bodies, including components.
Another possible way of detecting cracks is possible in magnetic shaped bodies by recording the remanent induction, especially with a two-dimensional measurement. In this case, separation of magnetic domains often occurs as a result of cracks. If it were possible to join together the structural components, or grain segments, which have been separated by a crack and have an identical or virtually identical alignment, and to cure the areas, respectively neighboring subdomains could be coalesced to form a new domain.
The object was therefore to propose a method by which the cracks in ceramic shaped bodies can be at least partially closed.
The object is achieved by a method for curing cracks in ceramic shaped bodies,
in which a filling material which melts at a lower temperature than the material of the shaped body and/or is flowable at a lower temperature than the material of the latter is applied to the surface of the shaped body at least in the area of a crack and/or is introduced into at least one crack,
in which the shaped body with the applied and/or introduced filling material is heated to a temperature at which the material of the shaped body does not yet melt and/or is not yet flowable, but at which the filling material is in the at least partially molten and flowable state,
in which the filling material consists of nonmetallic or essentially of nonmetallic compounds and is at least partially crystallized, and in which the shaped body with the filling material is cooled.
If the filling material consists essentially of nonmetallic compounds, a noble metal content, especially silver, gold, platinum and/or palladium, of overall up to 25% by weight may also be included. This content preferably constitutes overall up to 10% by weight and occurs especially in the high-temperature superconductor materials.
The method according to the invention is advantageously implemented in such a way that the flowable filling material penetrates at least partially into a crack and/or into an area of the shaped body that has been removed e.g. by a saw cut.
The material of the shaped body is preferably selected from the group comprising silicate ceramics such as cordierites, steatites and porcelains, oxide ceramics such as aluminum oxides, magnesium oxides and titanates, nitride or carbide ceramics such as silicon carbides and silicon nitrides, ceramic magnet materials such as hexaferrites or ceramic superconductor materials, in particular ceramic high-temperature superconductor materials.
The material of the shaped body and/or the filling material preferably essentially consist of chemical elements selected from the group comprising Mg, Ca, Sr, Ba, Sc, Y, La, lanthanides, Zr, Hf, Pt, Pd, Ag, Cu, Hg, Ag, Tl, Pb, Bi, S and O, where proportions of these chemical elements may be substituted by others which have not been mentioned. Relevant lanthanides include all chemical elements of this class.
The material of the shaped body and/or the filling material is preferably selected from the group of materials based on Bi—AE—Cu—O, (Bi,Pb)—AE—Cu—O, Y—AE—Cu—O, (Y,RE)—AE—Cu—O, RE—AE—Cu—O, Tl—AE—Cu—O, (Tl,Pb)—AE—Cu—O and Tl—(Y,AE)—Cu—O, where proportions of these chemical elements may be substituted by others which have not been mentioned and AE stands for at least one alkaline earth element.
The method according to the invention is especially favorable if a filling material is used in which at least two of the lattice parameters of the crystal lattice of one main phase of the filling material are sufficiently similar to those of one of the main phases of the material of the shaped body, in particular so as to permit epitaxial growth.
All forms of shaped bodies are in principle suitable. The preferred and currently usual ones are those which are essentially in the form of plates, round pieces, blocks, solid cylinders, hollow cylinders, rings, tubes or coils, where there may be deviated angles, broken edges, rounded edges, additional recesses such as e.g. bores and grooves as well as additional protruding parts such as extensions, points and similar geometrical variations.
A favorable shaped body is a semi-sintered, sintered or fused ceramic body having a relative density of at least 80%, particularly preferably of at least 95%, although in exceptional cases even less dense shaped bodies may offer advantages when they are treated according to the invention.
The filling material may contain a subsidiary proportion of a metal or of several metals. The main phase or the main phases of the filling material are, however, inorganic nonmetallic compounds.
A suitable filling material is, in particular, a powder or powder mixture which may optionally be mixed with known agents such as water and/or other solvents, plasticizers etc. in a well-known way, and which can be introduced at the surface onto and/or into the cracks, or areas that have been removed. A further especially suitable filling material is a pressed, calcined, sintered or fused shaped body which is put onto the shaped body or is fastened to it. A combination of a powder/powder mixture with a shaped body applied in this way is especially preferred. However, the filling material may also be applied, where appropriate additionally, in the form of a coating, in which case all known coating methods are in principle suitable although attention must be paid to the requisite layer thickness when selecting the coating method.
The filling material preferably has a melting point which is lower than that of the material of the shaped body by at least 5° C., particularly preferably by at least 10° C. and more particularly preferably by at least 20° C. The difference between the melting points must be commensurately greater when the control over the temperature when firing is less accurate and/or when the temperature variations inside the batch space of a kiln are large, in particular during the holding time.
The filling material preferably displays softening behavior, in which the filling ma
Baecker Michael
Bock Joachim
Freyhardt Herbert C.
Leenders Andreas
Ullrich Martin
Beck Shrive P.
Kolb Michener Jennifer
Nexans Superconductors GmbH
Sughrue & Mion, PLLC
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