Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2001-09-06
2004-08-17
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S336000, C428S408000, C428S312200, C428S312600, C428S319100
Reexamination Certificate
active
06777076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a graphite-coated shaped body made of sintered silicon carbide.
2. The Prior Art
Dense, solid-state sintered SiC has a combination of valuable properties such as high hardness and wear resistance, high-temperature resistance, high thermal conductivity, thermal shock resistance and also oxidation and corrosion resistance. Due to these properties, solid-state-sintered SiC has now been introduced as virtually ideal material for sliding bearings and mechanical face seals subject to wear stresses, e.g. for structural components in chemical apparatus and mechanical engineering.
It is known from U.S. Pat. No. 5,939,185 that SiC is also corrosion resistant toward hot water at a minimized grain boundary content. This results from a bimodal, coarsely crystalline platelet microstructure of the SiC and by the additional presence of graphite which is present as a particulate accompanying phase in the SiC matrix of the seal ring. This graphite reduces the tribochemical grain boundary corrosion which commences at working temperatures of above 200° C. A disadvantage of mechanical face seals made of this coarsely crystalline SiC-material is a very long running-in time (=200 h). In addition, this material when installed in an electrically insulated manner (e.g. as rotating seal ring in a boiler feed pump) has also displayed corrosion phenomena on the SiC ring which could not be explained in chemical or tribochemical terms (FIG.
1
). The contour of the damaged ring shape cannot occur as a result of mechanical and chemical attack. Damage beyond the functional surfaces was also apparent in the absence of mechanical stress. Such corrosion phenomena have been termed electrocorrosion (See, J. Nosowicz and A. Eiletz: “
Operating Performance of Mechanical Seals for Boiler Feed Pumps
”; in:
BHR
-
Conference of Fluid Sealing
, Maastricht 1997, 341-351).
Problems during production, e.g. crack formation during shaping and subsequent sintering of graphite-containing SiC, can be prevented if, instead of incorporating graphite into the SiC microstructure, a graphite layer is applied to the surface of the sintered SiC body.
JP04041590A of NIPPON CEMENT KK discloses the production of a graphite-coated shaped SiC body in which the graphite layer is formed on an open-pored shaped SiC body by chemical vapor deposition (CVD) from hydrocarbons in a mixture with hydrogen. The process is complicated and expensive. In addition, the graphite layer deposited by means of CVD methods is not firmly anchored to the SiC substrate. The CVD-graphite layer serves primarily to seal the open porosity and as lubricant which effects an improvement in the sliding properties via introduction of carbon into the pores of the SiC. To achieve this, the SiC substrate has to be open-pored since the pores act as reservoirs for the graphite lubricant.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a shaped body made of SiC which has been sintered so as to be gastight (=closed porosity), has a density of greater than 90% of its theoretical density and has a graphite layer on its surface, in which body the graphite layer adheres firmly to the SiC substrate and electrocorrosion is prevented.
This object is achieved according to the invention by a shaped body comprising polycrystalline SiC which has a graphite layer which has a thickness of 0.1-100 &mgr;m and which has been produced by thermal surface decomposition of the SiC after it has been sintered to closed porosity.
The graphite layer preferably has a thickness in the range 0.5-20.0 &mgr;m. The graphite layer is preferably single-layered.
The graphite layer preferably has a specific electrical resistance of from 0.5 to 5.0 m&OHgr;cm.
The graphite layer particularly preferably has a specific electrical resistance of from 0.8 to 1.9 m&OHgr;cm.
The graphite layer is preferably present on a tribologically active functional surface and/or on a tribologically inactive outer surface of the shaped body.
On the tribologically active functional surfaces, the graphite layer improves the running-in characteristics and the coefficient of friction of the shaped body of the invention under conditions of mixed friction or of partial dry running.
On the outer surfaces of the shaped body, the graphite layer prevents electrocorrosion. It has been found that electrically insulated installation of a usual SiC slide ring leads to a buildup of potential and as a consequence to corrosion phenomena on the SiC ring. This electrical corrosion can be prevented by discharging the potential via electrically conductive contacting of the SiC slide ring. Since a shaped SiC body does not have sufficient surface conductivity due to the high specific resistance of SiC of about 10°-10
4
&OHgr;cm, the discharge of the potential is not reliably possible in the case of a conventional SiC material. In the case of the shaped body of the invention, the potential is discharged via the firmly adhering electrically conductive graphite layer on the surface of the shaped body.
The shaped body of the invention comprises a conventional solid-state sintered SiC body which has closed porosity and is covered with a graphite layer produced by surface decomposition and having a thickness of from 0.1 to 100 &mgr;m.
The shaped body of the invention preferably consists of 70-99.7% by weight of polycrystalline SiC having an SiC crystallite size of from 1 to 2000 &mgr;m, plus 0.2-5.0% by weight of boron, boron compounds, Al, Al compounds, and also 0.1-25.0% by weight of additional carbon (in the form of amorphous carbon and/or graphite), with the shaped body being able to have a total porosity of up to 10% by volume in the form of independent, closed pores. All percents by weight are based upon the total weight of the shaped body.
The invention also provides a process for producing a shaped body according to the invention.
This process comprises heating a conventional sintered SiC body having closed porosity to a temperature above the decomposition temperature of SiC under a protective gas atmosphere (Ar, He, etc.) ranging in pressure from vacuum to 1 bar. The thermal surface decomposition of the sintered SiC shaped body proceeds according to the following reaction:
SiC
(solid)
=→Si
(gas)
+C
(solid)
The carbon formed remains on the surface as a graphite layer (See FIG.
2
), while the silicon formed vaporizes through the graphite layer and condenses in cold regions of the furnace.
The shaped body used in the process of the invention may have been produced by any solid-state sintering process.
In the process of the invention, preference is given to heating an usual sintered SiC body having closed porosity to a temperature between 1600° C. and 2200° C., preferably between 1800° C. and 2000° C. This temperature is held for a period of from 10 to 180 minutes, preferably from 30 to 90 minutes. The formation of the graphite layer on the surface occurs during this time.
An internal furnace pressure of from 1000 mbar to 10
−5
mbar, preferably from 1 to 50 mbar, is preferably maintained during the process.
The coated shaped body is subsequently cooled to room temperature in a customary manner.
The graphite layer formed according to the invention is predominantly hexagonal 2H-graphite. This can be seen from X-ray diffraction patterns of SiC surfaces decomposed at 1800° C.-2000° C. under reduced pressure by way of the three reflections at 2&THgr;=26.6°/45.4° and 54.70° (CuKa radiation). The thickness of the graphite layer can be set in a targeted manner by varying the process parameters temperature/hold time/pressure within the abovementioned limits. This is illustrated in the bar chart in FIG.
3
.
The specific electrical resistance (
FIG. 4
) of the graphite as a function of the formation temperature does not show a continuously increasing or decreasing behavior. Thus, 2 competing mechanisms occur in the process of the invention and lead to a resistance maximum. The maximum at 1.8 m&OHgr;cm corr
Kayser Armin
Schwetz Karl
Thaler Hubert
Collard & Roe P.C.
Turner Archene
Wacker-Chemie GmbH
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