Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Utilizing chemically reactive atmosphere other than air – per...
Reexamination Certificate
2001-08-23
2004-09-21
Dixon, Merrick (Department: 1774)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Utilizing chemically reactive atmosphere other than air, per...
C264S667000, C264S345000, C156S089110, C156S169000, C156S184000
Reexamination Certificate
active
06793873
ABSTRACT:
The invention relates to a melt-infiltrated fibre-reinforced composite ceramic containing high-temperature-resistant fibres, in particular fibres based on Si/C/B/N, which are reaction-bonded to a matrix based on Si, and also to a process for producing such a composite ceramic.
Such a process and such a composite ceramic are known from U.S. Pat. No. 5,464,655.
Carbon fibre-reinforced carbon (C/C, also known as CFRC or in German language usage as CFC) is the first industrially successful development in the group consisting of fibre-reinforced composite ceramic materials.
Recently developed high-performance brake systems based on CFRC brake discs with specially developed friction linings, as are used, for instance, in motor racing, can only be produced using numerous impregnation or carbonization and graphitization cycles, so that the production process is extremely time-consuming, energy-intensive and costly and can take a number of weeks or months. In addition, CFRC brake discs have totally unsatisfactory braking properties for use in production vehicles which are not subjected to demanding operating conditions in the presence of moisture and at low temperatures. This manifests itself, inter alia, in decidedly non-constant coefficients of friction as a function of the operating temperature and the surface lining which makes regulation, as has hitherto been customary in 4-channel ABS systems, extraordinarily difficult or even impossible. In view of this background, attempts are being made to develop improved fibre-reinforced composite ceramic materials which can be used, for example, as brake discs for high-performance brake systems in motor vehicles or in railway vehicles. Furthermore, such fibre-reinforced composite ceramic materials are also of interest for numerous other applications, for instance as turbine materials or as materials for sliding bearings.
Although silicon-infiltrated reaction-bonded silicon carbide (SiSiC) containing from 2 to 15% by mass of free silicon has been known since the 1960s and has also been introduced commercially for some applications in heat engineering. Problems in respect of internal stresses (internal stress due to cooling) occur in the manufacture of large and thick-walled components because of a step increase in the volume of the semimetallic silicon when it solidifies in the microstructure of the material. The stressing of the solidified silicon manifests itself, in many cases, in the formation of microcracks and in a reduction in adhesion at internal interfaces, so that the strength of the material is reduced and critical crack propagation under cyclic thermal and mechanical stress can be expected, particularly during prolonged use. In manufacture, the volume expansion on solidification leads to difficulties as have long been known when, for instance, water freezes in closed line systems, i.e. to rupture and breaking of the components and thus to high reject rates. In addition, the manufacture of SiSiC materials is relatively complicated and expensive.
It is therefore an object of the invention to provide an improved fibre-reinforced composite ceramic containing high-temperature-resistant fibres and also a process for producing such a composite ceramic, which makes possible very simple and inexpensive production of mass-produced components such as brake discs, with high thermal stability and hot strength together with sufficient oxidation resistance and thermal shock resistance being prerequisites.
The object of the invention is achieved by, in a melt-infiltrated fibre-reinforced composite ceramic of the type described at the outset, the matrix containing additions of iron, chromium, titanium, molybdenum, nickel or aluminium.
The object of the invention is completely achieved in this manner. According to the invention, it has been recognized that alloying the silicon melt used for the melt infiltration with iron, chromium, titanium, molybdenum, nickel and/or aluminium reduces or even substantially avoids the step increase in volume on solidification of a pure silicon melt. In this way, the problems caused by the stressing of the solidified silicon are avoided, a higher strength, particularly with regard to cyclic thermal and mechanical stress, is achieved and at the same time the production process is simpler and less costly.
SUMMARY OF THE INVENTION
It is thus possible, according to the invention, to obtain a reaction-bonded, melt-infiltrated SiC ceramic (RB-SiC) in which the brittle Si as is present in hitherto customary RB-SiC ceramics is replaced by a phase enriched with Fe and/or Cr, Ti, Mo, Ni or Al, which leads to a significant increase in strength and ductility of the ceramic.
In an advantageous embodiment of the invention, the matrix contains at least additions of iron.
This measure makes it possible, in a particularly inexpensive and environmentally friendly manner, to avoid the volume increase which occurs in the case of pure silicon and the additions of iron at the same time lead to improved braking performance in an application as a brake disc, since an improved friction pairing is obtained with conventional brake linings which are matched to brake discs based on grey cast iron. Brake systems based on such brake discs are thus more readily regulated since, in addition, they are less moisture-sensitive and are insensitive to low temperatures. Furthermore, there are no critical contact pressures which have an adverse effect on regulatability, as in the case of CFRC brake discs. In addition, the production process is simplified and made cheaper by the lowering of the melting point of the silicon melt by the addition of iron.
However, additions of chromium, titanium, molybdenum, nickel or aluminium in a two-material system with silicon also allows the above-mentioned volume increase on solidification of a pure silicon melt to be avoided or at least reduced. Furthermore, there is, in most cases, a lowering of the melting point which makes manufacture simpler and cheaper. Moreover, additions of chromium, titanium, molybdenum, nickel or aluminium can effect the formation of passive layers, so that the oxidation and corrosion resistance is improved.
For this reason, in a further embodiment of the invention, preference is given to adding further additions of chromium, titanium, aluminium, nickel or molybdenum in a suitable ratio as passive layer formers to a matrix based on Si which contains additions of iron.
In this case, different coefficients of thermal expansion of the alloying components lead to stress states in the matrix which compensate for the stresses caused by the fibres on cooling.
In a further embodiment of the invention, the matrix is produced from a silicon alloy containing from 0.5 to 80% by weight of iron, preferably from about 5 to 50% by weight (based on the total mass of the alloy). Since ferrosilicon in comparatively pure form is used on an industrial scale in steel production, with grades having the compositions Fe25Si75 and Fe35Si65 being commercially available, a considerable reduction in the raw material costs compared with the use of pure silicon is achieved. Furthermore, there is a lowering of the melting point from about 1410° C. for pure silicon to about 1340° C. when Fe25Si75 is used and to about 1275° C. when Fe35Si65 is used.
In an additional embodiment of the invention, an additional 5-30% by weight of chromium, preferably about 7-12% by weight of chromium, based on the iron content, is added to the silicon melt which is used for melt infiltration.
This change to a three-material system consisting of Si—Fe—Cr enables the iron-containing phases of the composite ceramic to be protected against corrosion and at the same time allows the melting point to be lowered to less than 1400° C. For this purpose, it is useful to add at least about 7% by weight of chromium (based on the iron content), since from about 7 to 8% by weight of chromium is necessary to effect the formation of a passive layer of chromium(III) oxide, as is known from stainless steels. (Based on the total mass of the alloy, the proportion
Gadow Rainer
Haug Tilmann
Kienzle Andreas
Crowell & Moring LLP
Daimler-Chrysler AG
Dixon Merrick
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