Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Reexamination Certificate
2001-06-18
2004-03-23
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C501S096100, C501S096500, C501S097100, C501S099000, C501S133000
Reexamination Certificate
active
06709999
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a molded part of ceramic material derived from polymers, a process for its production by pyrolysis, and a sliding element having the molded part.
The production of ceramic materials by pyrolysis of suitable polymeric organic precursors is known from numerous publications. Polysilanes, polycarbosilanes, polysilazanes, polysiloxanes or similar compounds are normally preferred as polymeric organic precursors. Examples of many such compounds may be found in a publication entitled “High Yield Synthesis Of Si
3
N
4
/SiC Ceramic Materials by Pyrolysis of a Novel Polyorganosilazane”, by Seyfert et al. in J. Am. Ceram. Soc. (1984) C-132. In that publication it is disclosed that alkylchlorosilanes can be reacted with ammonia in the presence of a strong Lewis base and used as precursors for the production of silicon nitride ceramic materials.
The pyrolysis of siloxanes is described in a publication entitled “Adv. Ceramic Materials” by D. White, 2 [1] (1987) p. 45 ff., and leads in an inert gas atmosphere or in a reducing atmosphere to products containing silicon carbide (SiC).
However, a problem encountered in the production of solid molded parts of such materials was that volatile decomposition products were always released during the pyrolysis. That resulted in high porosities and a drastic shrinkage of the material accompanied by a significant increase in density.
According to the teaching of European Patent Application 0 412 428 A1, molded parts having a high density are produced by pyrolyzing a mixture of organosilicon polymer plus metallic filler. With that method a ceramic transformation accompanied by an increase in density can be achieved, leading to a reduction in the interfering porosity and a reduction in shrinkage, which is likewise unacceptably high in practice. It is possible to produce molded parts in that way by using known molding techniques for plastics materials. However, those molded parts do not have satisfactory dry-running properties enabling them to be used as sliding elements in mixed friction or dry-running applications.
Sliding elements with the best heretofore achieved dry-running properties preferably are formed of carbon graphite, for example EK 2230 grade from SGL CARBON GmbH, Meitingen, Germany, which have an SiC element as a counter-running partner. Those sliding elements can be produced as individual moldings in which running surfaces are also subjected to a final mechanical treatment. The associated production costs are substantially higher than in the case where sliding elements are produced by conventional plastics forming processes. Coal graphite materials are produced by mixing and compressing solid carbon-containing fillers that are at least partially of a graphitic nature (e.g. natural graphite, electrographite) with carbonizable binders such as pitches and resins, and then carbonizing the compressed mixtures at temperatures between 750° C. and 1300° C. under the exclusion of air.
Sliding rings of ceramic materials, such as for example sintered silicon carbide, are also known. However, those materials according to European Patent EP 0 685 437 B1, corresponding to U.S. Pat. No. 5,762,895, may only be used for mixed friction applications, and are based on the fact that the material has a high porosity so that the lubricating film that is normally present between the friction partners is deposited in the pores of the sliding rings and after a brief dry running the lubricating film remaining in the pores is released in order to prevent a permanent damaging dry running. Adding solid lubricants such as preferably graphite according to European Patent Application EP 0 709 352 A1, permits the coefficients of friction of the sliding rings to be improved further, though the use of sliding rings remains limited to the case of mixed friction. The problem of dry friction cannot be solved with such a material, since only the at least partial presence of a lubricating film will permit the good sliding behavior of the material. That behavior is also illustrated further by the results of a reference measurement with SiC given in Table 2.
International Publication No. WO 99/41211 describes a process for the production, from polysilazanes, of crack-free, high density ceramic molded parts based on Si/B/C/N that can be used for tribological purposes. Depending on the composition, the material of the molded parts contains incorporated deposits of various lubricating substances such as for example graphite, boron nitride, titanium oxides and titanium carbide nitride. In that connection, a crosslinked polysilazane powder is first of all compressed hot in a depression at a defined temperature that is higher than the temperature maximum in the TMA diagram of the optimally crosslinked polymer, and is then pyrolyzed. The inexpensive production processes used in plastics technology for shaping, such as for example injection molding and extrusion, cannot be used for the production of the molded parts. The polysilazanes used for the production are furthermore significantly more expensive than other polymeric organic precursors, such as in particular polysiloxanes, which thus additionally increases the production costs of the molded parts. The molded parts produced according to International Publication No. WO 99/41211 have coefficients of friction down to less than 0.1 under dry-running conditions.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a molded part of ceramic material derived from polymers, a process for producing ceramic molded parts and a sliding element having a molded part, which overcome the hereinafore-mentioned disadvantages of the heretofore-known products and processes of this general type and in which the molded parts can be produced by using production methods known in plastics technology, exhibit a low shrinkage of at most 10% during production in addition to a high density of at least 85% of a theoretical value and have satisfactory dry-running properties.
A further object of the invention was to provide molded parts of a ceramic material that can be used as bearings and sliding ring seals for dry-running applications and that can be produced from inexpensive polymeric organic precursors as raw material, in particular from polysiloxanes.
Another object of the present invention is to provide a material for molded parts that is more suitable than known materials as a material for bearings and sliding ring seals in dry-running applications.
With the foregoing and other objects in view there is provided, in accordance with the invention, a molded part of a ceramic material produced by pyrolysis of polymers, comprising a composite body of a single-phase or multi-phase, amorphous, partially crystalline or crystalline matrix selected from the group consisting of silicon carbide (SiC), silicon nitride (Si
3
N
4
), silicon dioxide (SiO
2
) and mixtures thereof, such as for example oxycarbides, oxynitrides, carbonitrides or oxycarbonitrides. The matrix contains graphite inclusions and the ceramic material has a density of at least 85% of a theoretical value.
With the objects of the invention in view, there is also provided a process for the production of ceramic molded parts, which comprises preparing a mixture containing polymer components in an amount of 30 to 80 wt. % referred to a total weight of the mixture, fillers in an amount of 0 to 30 wt. %, and graphite in an amount of 10 to 70 wt. %. The mixture is subjected to a forming process with heating of the mixture to crosslink the polymer components and then to a pyrolysis process for producing the ceramic molded parts.
The polymer components which are used according to the invention are, in particular, polysilanes, polysiloxanes, polysilazanes or polycarbosilanes or similar compounds and mixtures thereof known in the prior art. The inexpensive polysiloxanes which are preferably used in this connection lead to oxidic phases in the matrix of the ceramic composite materials. Methylhydr
Dernovsek Oliver
Greil Peter
Güther Hans-Michael
Wislsperger Ulrich
Marcantoni Paul
Mayback Gregory L.
SGL Carbon AG
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