Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Reexamination Certificate
2000-07-08
2003-03-11
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C501S090000, C501S092000
Reexamination Certificate
active
06531423
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to liquid-phase-sintered SiC shaped bodies with improved fracture toughness and a high electrical resistance, and to methods for producing same.
BACKGROUND OF THE INVENTION
Dense sintered SiC is distinguished by a combination of valuable properties, such as high hardness and wear resistance, ability to withstand high temperatures, high thermal conductivity, resistance to thermal shocks, and resistance to oxidation and corrosion. Due to these properties, the solid-state sintered SiC has nowadays become accepted as a virtually ideal material for sliding-contact bearings and axial face seals which are subject to wear in the chemical apparatus and mechanical engineering sectors. Alpha- or beta-SiC sintering powder with particle sizes in the submicron range (mean grain size <1 &mgr;m) and a simultaneous addition of up to 2% carbon and boron have been recognized as prerequisites for the pressureless sintering. As an alternative to boron or boron compounds, it is also possible to use aluminum and aluminum compounds or beryllium and beryllium compounds, the net result being the same. During the sintering, an SiC polytype transformation and grain growth take place, the extent of which processes are dependent on the nature and quantity of the sintering additives and the sintering temperature. Boron-doped beta-SiC powders tend toward secondary recrystallization (exaggerated grain growth), while starting from alpha-SiC powder with boron or aluminum doping results in a fine-grained globular or bimodal microstructure, comprising a fine matrix with plateletlike crystals showing a high aspect ratio. Generally, the solid-state sintered SIC bodies have a transcrystalline fracture mode and, at a relative density of 95-98% of the theoretical density (% TD), reach a room-temperature strength of up to 450 MPa, which is retained even at elevated temperatures of up to 1500° C. However, the brittleness of this SiC ceramic represents an obstacle preventing any broader expansion of its use in industry. Owing to the high fraction of covalent bonding, SiC is extremely brittle, and even small flaws in the microstructure of the material may lead to sudden failure of a component.
Therefore, it is necessary to develop Sic ceramics with an improved fracture behavior and to meet the requirements imposed on the reliability of the components.
Nowadays, dense sintered Sic with an intercrystalline fracture mode, improved fracture toughness and room temperature strength can be produced in a similar manner to silicon nitride (Si
3
N
4
)using a liquid phase sintering process. The addition of suitable metal oxides or nitrides which form low-melting eutectics with SiC and the adhering SiO
2
results, during the sintering operation, in the formation of a liquid phase which makes a decisive contribution to the densification of the ceramic. The recommended sintering additives are mainly mixtures of Al
2
O
3
and Y
2
O
3
(M. Omori et al.: U.S. Pat. No. 4,502,983 (1985) and U.S. Pat. No. 4,564,490 (1986), and R. A. Cutler et al.: U.S. Pat. No. 4,829,027 (1988)),but also Y
2
O
3
or other sesquioxides of the rare earth metals (RE
2
O
3
) in combination with aluminum nitride (AlN) (K. Y. Chia etal.: U.S. Pat. No. 5,298,470 (1994), M. Nader, Dissertation: INAM, University of Stuttgart (1995), I. Wiedmann et al.:“Flüssigphasensintern von Siliciumcarbid” [Liquid-phase sintering of silicon carbide], in: Werkstoffwoche 1996, Symposium 7: Materialwiss. Grundlagen, Ed. F. Aldinger and H. Mughrabi, DGM Informationsgesellschaft, Oberursel (1997), 151-520, H. Kölker et al.: DE 19730770 (1998)). The liquid-phase sintering may be carried out without pressure, i.e. under atmospheric pressure, if appropriate using a powder bed, or by employing the gas pressure sintering technique under elevated gas pressure without an embedding material. A liquid-phase 35 sintered SiC (LPSSiC) which has been introduced into the market by Wacker-Chemie GmbH under the brand name EKasic T exhibits a virtually pore-free, fine grained microstructure (mean SiC grain size approx. 1 &mgr;m), a flexural strength of approx. 600 MPa, a fracture toughness which is 40 to 60% higher than that of solid state sintered SiC (SSiC) and a higher electrical insulating capacity (cf. Table 1).
TABLE 1
Comparison of properties of SSiC and LPSSiC
Sintering
Resis-
Fracture
SiC
Sintering
additive
Color
tivity
tough-
material
mecha-
contents
(polished
Ohm ·
ness*
variant
nism
% by weight
surface)
cm
MPa · m
1/2
EKasic
Solid
<1.5 (B + C)
black
10
3
-10
4
2.2
BM
phase
(SSiC)
EKasic
Solid
<1.5 (Al +
black
10
−2
-
2.5
D
phase
C)
10
1
(SSiC)
EKasic
Liquid
5-6 (AlN +
black
>10
7
3.5***
T
phase
YAG**)
(LPSSiC)
*measured using the bridge method (sharp crack), cf. K. A. Schwetz et al.: Journal of Solid State Chemistry 133, 68-76 (1997)
**YAG = Y—Al garnet (Y
3
Al
5
O
12
)
***Corresponds to a value of 8.4 MPa · m
1/2
measured using the SENB method with a 0.5 mm notch width
The high electric resistance of EKasic T is caused by the presence of a continuous vitreous grain-boundary phase which surrounds the SIC grains in an insulating manner in the form of a thin film (approx. 1 nm). Together with the sintering aid Y—Al garnet, at the triple junctions of the SiC grains this continuous grain-boundary phase forms the so called binder phase, which joins the SiC hard-material grains to form a strong composite. Since the microstructural development and the properties of liquid phase-sintered SiC are decisively influenced by the selection of the composition (type and amount of the SiC and the binder phase) and the specific sintering parameters (gas atmosphere, pressure, temperature, time), it is not surprising that it is already possible for a number of SiC materials with very different properties to be produced from the sintering additive system AlN—Y
2
O
3
(or RE
2
O
3
or YAG).
According to the method which is known from DE 3344263 (corresponds to U.S. Pat. No. 4,569,922, Inv.: K. Suzuki/Asahi Glass), silicon carbide powders are sintered, together with sintering additives based on 3-30% by weight AlN, 0-15% by weight oxides of the IIIa transition metals (in particular yttrium, lanthanum and cerium) and 0-20% by weight SiO
2
, Al
2
O
3
or Si
3
N
4
, under an argon or nitrogen atmosphere, without pressure or under gas pressure, at 2000-2200° C. for from 2 to 10 hours to form SiC shaped bodies with a density of over 95% of the theoretically possible density, which exhibit flexural strengths of >800 MPa both at room temperature and at 14000° C. The microstructure of these SiC sintered bodies exhibits elongate and/or platelike grains (mean grain length 3-5 &mgr;m) of an SiC—AlN mixed crystal and a crystalline grain-boundary phase. The composition of the sintered bodies essentially comprises SiC with 2-20% by weight Al, 0.2-10% by weight N, 0.2-5% by weight 0 and 0 to 15% by weight of a metal from group IIIa. With sintering additives comprising less than 3% by weight AlN, only deficient sintered densities are obtained.
According to the process for the liquid-phase sintering of starting powder mixtures which, in addition to SiC, contain 1-10% by weight AlN, 1-15% by weight Y
2
O
3
and up to 8% by weight SiO
2
as sintering additives, which process is known from Japanese patent application No. 59-051384 (Publication No. 60-195057, 10.03.1985, Inventor: S. Nagano/Kyocera), sintered bodies with densities of between 95 and 99% TD which, due to their low resistivity of 0.5 ohm·cm, can be machined by spark erosion, are obtained after pressureless sintering for 2 3 hours under anargon atmosphere in the temperature range 1800-1950° C.
According to the method which is known from U.S. Pat. No. 5,298,470,dated Mar. 29, 1994 (corresponds to EP 419,271, Inv.: Chia et al./Carborundum), once again starting powder mixtures which, in addition to silicon carbide, contain 0.5-15% by weight AlN, 0.1-15% by weight Y
2
O
3
(or other RE
2
O
3
) and up to 10% by weight SiO
2
as sintering additives are sintered without pressu
Kempf Thomas
Schwetz Karl-Alexander
Sigl Lorenz
Victor Georg
Collard & Roe P.C.
Group Karl
Wacker-Chemie GmbH
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