Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
1999-10-04
2002-05-28
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S097200, C501S097300
Reexamination Certificate
active
06395661
ABSTRACT:
This application is a 371 of PCT/EP98/01816 filed Mar. 27, 1998.
BACKGROUND OF THE INVENTION
The present invention relates to sintered silicon nitride (Si
3
N
4
), components made thereof, in particular valves, process for their production and their use.
Materials made of Si
3
N
4
are of proven use in many applications. However, components made of Si
3
N
4
still have inadequacies such as lack or reliability in continuous use, which stand in the way of wide use of Si
3
N
4
components in the said applications, which would be advantageous economically and ecologically. For example, although DE-A 4312251 claims a high-strength Si
3
N
4
material having a defined failure probability, no teaching is given regarding the way in which these failure probabilities are to be achieved. They are merely derived from classical flexural strength determinations and statistical evaluation thereof.
The reliability of ceramic materials is determined from short-term strength, with its spread, and from long-term behaviour under load. In this context, the short-term strength follows the Griffith relationship:
σ
=
K
Ic
c
Y
(
1
)
with:
&sgr; Strength in MPa
K
lc
Fracture toughness in MPa·m
½
c Critical crack length in &mgr;m and
Y Form factor, which describes the shape of the critical crack.
According to this relationship, the strength is directly dependent on the crack or defect length in the material.
The scatter in the short-term strength, which is important for reliability, is described by the Weibull distribution and is characterized by the Weibull modulus according to DIN 51110.
In the case of loading below the stress which leads to catastrophic fracture, however, the “v-k” concept is applicable
v=A·K
n
l
(2)
with
v Crack growth rate in m/s,
K
l
Stress intensity factor in MPa·m
½
in the case of load type l= tensile stress, and
A,n Parameters for subcritical crack growth (life).
This concept is applicable to materials and components which are exposed to varying stresses below the maximum stress, described by the Griffith relationship, which immediately leads to failure, and is therefore relevant to ceramic materials and components for a large number of technical applications, e.g. for valves in reciprocating piston engines.
The crack growth parameter is determined, according to the description of standard draft ENV 843-3 by determining flexural strengths at different loading rates.
The determination of the flexural strength is described in DIN 51 110. The loading rate employed in this case, which is intended to cause fracture in 5 to 10 s, is customarily about 100 MPa/s. The flexural strength determined in this case is referred to as short-term or inert strength &sgr;
c
.
In order to ascertain the crack growth parameter, this measurement is carried out at reduced loading rates. In this case, the cracks which are present have the opportunity to grow, with the result that fracture occurs under lower loads, i.e. so-called subcritical crack growth takes place. If the breaking stress is plotted against the loading rate on a double logarithmic scale, and the median values of the measurement carried out repeatedly for a defined loading rate are joined by a best fit line, then the crack growth parameters n and A are found from the slope of the line and the axis intercept of this line. Typical ceramic materials have n values of 30 to 40 (see Kingery, Introduction to Ceramics, John Wiley & Sons, New York, 1976, page 804) and are therefore apparently to be qualified as subcritical crack growth, so that their life in practical use is limited.
In order to satisfy increasing demands, especially in the automobile industry, a need has arisen for Si
3
N
4
materials and components with improved reliability.
The object of the present invention was therefore to provide sintered Si
3
N
4
and reliable components, in particular valves based on Si
3
N
4
, which have properties meeting this profile and are also straightforward and therefore inexpensive to produce.
It has unexpectedly been found that sintered Si
3
N
4
with a particular chlorine content has improved subcritical crack growth behaviour with high flexural strength and high Weibull modulus at the same time.
DESCRIPTION OF THE INVENTION
The invention therefore relates to sintered Si
3
N
4
which has a chlorine content of 100 to 500 ppm, a subcritical crack growth parameter n≧50, preferably ≧60, a mean flexural strength at room temperature ≧850 MPa and a Weibull modulus ≧18.
The chlorine content of the sintered Si
3
N
4
was in this case determined by pressure digestion with hydrofluoric acid at temperatures between 100 and 120° C. and subsequent potentiometric titration of the chloride by means of silver nitrate.
The sintered Si
3
N
4
according to the invention preferably contains alkaline earth metals, Sc
2
O
3
, Y
2
O
3
, rare earth oxides, TiO
2
, ZrO
2
, HfO
2
, B
2
O
3
and/or A1
2
O
3
as sintering additives, these forming a secondary phase concentration in the sintered material of 7.5 to 20 vol. % in addition to crystalline Si
3
N
4
and/or Si
3
N
4
mixed crystals.
This secondary phase concentration is determined by ascertaining the total oxygen content of the sintered Si
3
N
4
through hot extraction. The known oxygen concentration introduced by the added sintering aids is subtracted from this result. The difference represents the oxygen content of Si
3
N
4
following preparation, which is assumed to be present in the form of SiO
2
. This SiO
2
concentration is added to the sintering aid concentration, which represents the total proportion of oxide constituents in addition to Si
3
N
4
.
For the Si
3
N
4
proportion in the material, its pure density of 3.18 g/cm
3
is employed to calculate the volume fraction, and for the secondary phases which are formed by the reaction of the sintering additives with the SiO
2
in the Si
3
N
4
powder during the sintering, the pure density &rgr;
R
is calculated according to
ρ
R
=
G
-
tot
/
∑
i
=
1
i
=
n
(
G
i
/
ρ
Ri
)
in
g
/
cm
3
(
3
)
with
G-tot=Total weight of the oxide components in g
G
i
=Weights of the individual oxide components in g
&rgr;
Ri
=Pure densities of the individual oxide components in g/cm
3
.
The volume fractions of Si
3
N
4
and secondary phase are thereby determined, the latter being between 7.5 and 20 vol. % for the material according to the invention.
The Si
3
N
4
according to the invention is distinguished by a high packing factor (low porosity) so that, for example, during re-sintering at a temperature up to 50° C. higher than the sintering temperature, neither the density nor the Young's modulus of the material changes.
The invention also relates to a process for preparing the sintered Si
3
N
4
according to the invention where Si
3
N
4
powder, which either contains chlorine in an amount of 500 to 1500 ppm or, as an alternative to this, is used together with a metal chloride, is dispersed in water together with at lest one sintering additive, mixed with organic processing aids,
the aqueous slip is ground to a fineness of 90%<1 &mgr;m,
and subsequently dried preferably by spray drying or fluidized bed drying so that the Si
3
N
4
granules have a moisture content of between 1.0 and 4% by weight, preferably between 1 and 3% by weight and an average granule size of 40 to 80 &mgr;m, and these are subsequently compressed and sintering is carried out after the organic process aids have been baked out under an N
2
pressure of 1≦p≦10 bar.
The compression is preferably carried out axially and/or isostatically.
In a preferred embodiment of the invention, the compression is carried out at pressures <2500 bar, the organic process aids and the moisture are baked out in air, inert gas or vacuum at T≦650° C. and the sintering is carried out under an N
2
pressure of 1≦p≦10 bar at T≦2000° C.
Preferably, the Si
3
N
4
powder used has a Cl content of 500 to 1500 ppm and leads in the sintered Si
3
N
4
to a Cl content of 100
Gugel Ernst
Lindner Hans Andreas
Woditsch Peter
Wötting Gerhard
Bayer Aktiengesellschaft
Eyl Diderico van
Gil Joseph C.
Group Karl
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