Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2002-04-09
2004-04-20
Zimmerman, John J. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of metal
C428S471000, C428S697000, C428S699000, C428S701000, C428S702000, C501S108000, C501S112000, C501S118000, C501S119000, C501S120000, C501S121000, C501S123000, C501S125000
Reexamination Certificate
active
06723442
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to a ceramic material for use in various coating techniques, such as for example the different variants of thermal spraying. The invention also relates to a method for producing a ceramic material of this type. Furthermore, the invention relates to a use of the ceramic material and to a layer made from the ceramic material on a metallic or ceramic body.
BACKGROUND OF THE INVENTION
Different variants of thermal spraying are flame spraying, plasma spraying, high-speed spraying, detonation spraying, and coating by way of laser or powder plasma weld surfacing. These methods are used to coat highly stressed components which are exposed to abrasive or erosive wear, corrosion, high temperatures or a very wide range of combinations of such loads. Components of this type are used, for example, in automotive engineering, in mechanical engineering, in power engineering, in chemical or petrochemical installations and numerous other sectors of the economy.
In the thermal spraying method, meltable material, such as metal or ceramic, is softened or melted by heating and is applied onto a surface which is to be coated. The heated particles of the material come into contact with the surface, on which they cool and, as a result, adhere to the surface. The heating of the material to be sprayed, including the acceleration of the heated material toward the surface, usually takes place in a spray gun for thermal spraying. The material to be heated is fed to the spray gun in powder form. The mean grain size of a powder of this type is usually between 2 &mgr;m and 150 &mgr;m.
The powder is accelerated in the spray gun by way of a gas stream. This gas is generally one of the operating gases of the spray gun, which generate the combustion or plasma flame in the spray gun. For a plasma spray gun, operating gases of this type are usually nitrogen or argon, on the one hand, and helium, on the other hand. In this case, the nitrogen or argon simultaneously serves as carrier gas for the powder.
Various ceramic materials, or materials which resemble sintered carbides, are in widespread use as coating powder for thermal spraying in engineering. A ceramic material is used in particular if the component to be coated is to be protected against corrosion or thermal influences. One example of such an application is that of protecting against wetting by metallic or oxidic melts. Particularly with components of this type, the problem arises that high mechanical stresses are generated in the ceramic coating under thermal loads. These stresses readily lead to cracks in the coating or to the coating becoming detached from the coated component. Mechanical stress occurs when the coefficient of thermal expansion of the ceramic coating material differs significantly from the coefficient of thermal expansion of the material of the component. Therefore, it is preferable to select a material for thermal spraying which has a coefficient of thermal expansion which is similar to that of the material of which the component to be coated consists.
To coat a metallic component, it is particularly appropriate to use a ceramic material whose coefficient of thermal expansion is close to that of the metal. Since metals generally have a coefficient of thermal expansion which is greater than 10*10
−6
/K, only a few oxides may be used for coating purposes. A preferred spraying material is zirconium oxide, which is used with a stabilizing additive of 7 to 9% by weight of yttrium oxide, for example in internal-combustion engines. The coefficient of thermal expansion of zirconium oxide layers of this type is in the region of 11*10
−6
/K. However, the resistance of zirconium oxide to attack from metallic or oxidic melts is lower than that of a number of other materials.
MgO has a satisfactory resistance to melts, and its coefficient of thermal expansion of 13.6*10
−6
/K means that it is also a suitable coating material for metals. However, MgO is not a suitable material for use in a thermal spraying process, since MgO decomposes at the high temperatures which occur in such processes, and the decomposition products are volatile.
Ceramics which are produced from a mixture of MgO and Al
2
O
3
have good properties for use in combination with various metals. Sintered ceramics produced from MgO and Al
2
O
3
are commercially available. They have the advantages of being highly resistant to chemical, thermal and mechanical attacks and of having a coefficient of thermal expansion which lies in the region of 11*10
−6
/K. However, ceramics of this type have only limited suitability as coating material, since in practice they are not suitable for coating by way of a thermal spraying method. In these ceramics too, the MgO of the ceramic evaporates at the high temperatures which occur during thermal spraying.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a stable ceramic material which is suitable for a coating operation by way of a thermal spraying method and which has a coefficient of thermal expansion which is matched to a metal.
A further object of the present invention is to provide a method for producing a material of this type. A further object of the present invention is to describe a use of the ceramic material. Furthermore, the invention has an additional object of providing a layer on a metallic body, which is able to withstand thermal loads.
The first object is achieved by a ceramic material which, according to the present invention, includes 10 to 95% by weight of MgAl
2
O
4
, 5 to 90% by weight of MgO, up to 20% by weight of Al
2
O
3
, remainder standard impurities, and which has grains of MgO with a mean diameter of 0.1 &mgr;m to 10 &mgr;m that are embedded in a matrix of MgAl
2
O
4
in spinel form.
The present invention is based on the consideration that, unlike in a sintered ceramic including MgO and Al
2
O
3
, in which MgO and Al
2
O
3
adjoin one another, in a compound formed from MgO and Al
2
O
3
the evaporation of MgO during thermal spraying can be prevented or greatly restricted. An example of such a compound is MgAl
2
O
4
. This compound or ceramic has proven to be a suitable material for a thermal spraying process in a number of tests. Moreover, it is highly chemically and mechanically stable. The drawback of a ceramic of this type is its low coefficient of thermal expansion of approximately 8.5*10
−6
/K, which is lower than that of most metals.
Furthermore, the present invention is based on the consideration that MgO has a coefficient of thermal expansion of approximately 13.6*10
−6
/K. Therefore, the introduction of MgO into MgAl
2
O
4
leads to an increase in the coefficient of thermal expansion of the ceramic material which forms. Depending on the amount of MgO added, the coefficient of thermal expansion can be set in a defined way and can be specifically matched to the coefficient of thermal expansion of the metal to be coated,or at least the difference between the coefficients of expansion can be reduced.
In a third step, the present invention is based on the consideration that the MgO has to be incorporated in the ceramic material in such a manner that it does not decompose or sublime in the hot flame of the spray gun used for thermal spraying. It is incorporated in this way if the material has areas of MgO which are embedded in a matrix of MgAl
2
O
4
. The MgO present inside such areas, which can also be referred to as grains, is surrounded by MgAl
2
O
4
. The MgAl
2
O
4
is preferably in the form of a homogeneous matrix which does not include MgAl
2
O
4
grains which have been sintered together with spaces between them, but rather includes homogeneous, pore-free MgAl
2
O
4
. This MgAl
2
O
4
is sufficiently thermally stable to preserve the covering which surrounds the MgO even during the thermal spraying operation. In this way, the area of MgO remains enclosed during the thermal spraying, and the MgO cannot sublime or evaporate.
In the range between 0° C. and 1000° C., the ceramic material has a coeffic
Decker Jens
Jansing Thomas
Schürholt Günter
Blackwell-Rudasill G.
Siemens Aktiengesellschaft
Zimmerman John J.
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