Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-12-06
2002-08-06
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S212000, C428S304400, C428S318400, C428S408000, C428S698000, C428S704000
Reexamination Certificate
active
06428885
ABSTRACT:
The present invention relates to a support body having a coating comprising at least 95% by weight titanium boride wherein said coating has an oxygen content of less than or equal to 1% by weight, a metallic impurities content of less than or equal to 0.5% by weight and a specific electrical resistance of less than or equal to 10 &mgr;&OHgr;·m at room temperature.
It is known that coatings on a support body often serve to increase the life of the support body in a certain application, to improve the properties for a certain application, and/or to open up new fields of use for the support material. A coating allows the possibilities for use of the support body to be improved and its resistance to be increased.
Special requirements are made of this coating. It is known that in the case of components which are employed in various technical fields, different types of stress may arise, often in combination. For use in metal melts and/or salt melts, for example, high corrosion resistance and erosion resistance are necessary. Furthermore, it is necessary to ensure good adhesion to the component and good cohesion within the coating system. In addition, the protective coatings must have low internal compressive stresses, and high hardness and load-bearing properties. In certain applications of the components, furthermore, low specific electrical resistance of the protective coating must be ensured.
DE-A-35 13 882 discloses a protective coat which consists of an adhesion layer applied to the support, an interlayer applied to the adhesion layer, and a top layer applied to the interlayer. The adhesion layer corresponds in its composition essentially to the support material, so that there is not too great a difference in the thermal expansion coefficient. The interlayer is a mixture of the materials of the adhesion layer and of the top layer. The top layer can be, inter alia, titanium diboride. This protective coat is intended to protect the support material against corrosion, oxidation, abrasion, erosion, chemical attack and radiation, insulate it electrically at the same time, and, by virtue of heat insulation, to protect it in the short term against overheating.
A coat for protection against high-temperature corrosion is described in GB-A-1 104 840 for a metallic body comprising refractory metal such as, for example, tantalum, tungsten or molybdenum. The coat consists of a mixture of from 85 to 99% by weight zirconium boride/titanium boride and from 1 to 15% by weight silicon. When the materials were selected it was taken into account that the thermal expansion coefficients of substrate and coat must be comparable.
In U.S. Pat. No. 5,368,938, protection against oxidation is achieved by specially pretreating the surface of the carbon body, applying an interlayer to this pretreated surface and then applying a top layer. The pretreatment of the carbon body consists in etching the surface and then reacting the resultant porous surface layer with boron oxide. This produces a boron carbide layer having a porosity of about 50%. The porosity of this base layer enables the glass-forming interlayer applied over it to form a connection with the base body. Then a top layer comprising refractory materials is applied to this base layer. The layers are applied by way of CVD. The complicated treatment of the substrate and the application of interlayers serve to seal cracks which occur in the outer top layer.
This aim is also pursued in U.S. Pat. No. 5,536,574, in accordance with which the protection against oxidation of a carbon substrate is intended to be achieved by way of a boron-containing SiC interlayer and a top layer of glass ceramic consisting of TiB
2
, colloidal SiO
2
and SiC. The layers are applied by applying the various materials in paste form in succession and then sintering them.
As protection against corrosion by liquid aluminum, DE-A-23 05 281 describes a coating or a covering of melted or highly sintered, dense, refractory hard material on a cathode or a cathode element made of carbon. The hard material referred to comprises the borides, nitrides, carbides and silicides of transition metals of groups four to six of the Periodic Table. Present in addition to the hard material is a small proportion of carbon, which forms a binary system. This melt coating can be obtained either by heating at temperatures from 2200 to 2300° C. or by plasma spraying.
Furthermore, DE-A-12 51 962 discloses a cathode or a carbon cathode with a coating that consists of a mixture of a hard material such as titanium boride or titanium carbide and at least 5% carbon. This cathode is calcined at temperatures of preferably from 1600 to 2000° C.
The prior art described above shows by way of example the problems of applying a coherent, jointless coating, preferably of titanium boride, to a support body with good adhesion when the coating and the support body have different thermal expansion coefficients. The approach described in GB-A-1 104 840, namely to use only materials having a similar expansion coefficient, in practice entails too great a restriction in the selection of material. In many cases, interlayers are applied in order to harmonize the different thermal expansion coefficients of coating and support body, the material of said interlayers having to be selected such that there is gradual adaptation of the expansion coefficients between support body and top layer. One disadvantage of this procedure is a complex coating, which becomes relatively expensive and also requires the presence of suitable materials having the correct thermal expansion coefficients.
In order to seal the support material effectively against the external medium, the coating must be so impermeable that it has no open pore channels which connect the support body with the environment outside the coating. This means that the coating must have a sufficiently low porosity and a certain thickness. Pure titanium diboride, as a high-melting material (T
m
at approximately 2900° C.) compacts very poorly on sintering. In order to obtain compaction at more readily achievable temperatures, additives are frequently added in order to reduce the sintering temperature. Titanium diboride and carbon, for example, form a binary eutectic for which the eutectic temperature for the composition 85% TiB
2
and 15% C is about 2287° C. The addition of the low-melting silicon, which lowers the sintering temperature by more than 1000° C., must be seen from the same standpoint. The addition of other metals and alloys, such as Fe, Ni, Cr and Mo, for example, produces the same effect. Likewise, with oxygen, a very low-melting boron-containing phase (glass phase) is formed. The addition of additives is advantageous for the greater ease of compaction of the titanium diboride; however, the formation of low-melting secondary phases is accompanied to the same extent by a reduction in the high-temperature resistance of the material. A further disadvantage of adding additives is the change in a number of material properties of the sintered body, such as, for example, the specific electrical resistance. An oxygen content of a few percent by weight leads to an increase in the specific electrical resistance by a factor of from 100 to 1000. The oxygen content of the coating is preferably less than 0.6, in particular less than 0.3, % by weight.
The formation of vitreous or glass-ceramic interlayers allows for compensation of the stresses which occur on heating owing to the different thermal expansion coefficients, since these amorphous layers are relatively flexible. However, even these glass-ceramic layers have the disadvantage that their high-temperature stability is low.
The majority of the coatings known to date exhibit poor adhesion to the support body, especially at relatively high temperatures such as, for example, at 900° C. if heating and cooling are conducted in alternating cycles. In many cases cracks are formed, and the coating begins to become detached. Some coating compositions, and their production methods, are highly complex and therefore of no economic intere
Hiltmann Frank
Hornung Michael
Kühn Heinrich
Seitz Katharina
Süssbrich Stephan
Aventis Research & Technologies GmbH & Co KG
Connolly Bove & Lodge & Hutz LLP
Turner Archene
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