Heinrich J.G., Aldinger F. (Eds.) Ceramic Materials and Components for Engines
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(a), 40
N,
n=20
(a),
IOON,
n=20
(b),
100
N,
n=1000
Fig.
6
Optical micrographs of the surface tested in
lubricated condition
(b),
IOON,
n=1000
Fig.
7
SEM
images
of
wear debris
3.4.Crack initiation under sliding contact
No
tiastic behavior was observed on the load-
displacement curve. The ring crack
was
clearly seen
on the surface
as
shown
in
Fig.
8.
The
maximum
value of the applied load was approximately
400
N,
which was considerably greater than the applied load
for the friction experiments. It was confirmed that the
radius
of
the ring crack was about
15
%
larger than that
of
the contact area predicting from the Hertzian contact
theory
(7).
It
was also observed that several ring
cracks formed outer region to the primary ring crack
when the applied load exceeded the critical value
predicted from the AE signal.
Assuming that the contact condition between the
alumina sphere and the glass bars was elastically, the
critical stress
(T,
for the crack initiation was given
as
following equation
(8),
(3,
=
yp
2nx
Where, P,
v
and
x
are maximum load, Poison’s ratio of
the glass
(=0.22)
and the radius of the ring crack,
respectively. Critical stresses of the ring crack
initiation was approximately
400
MPa and
was
greater
than those obtained by conventional 4-point flexural
test
(=70
MPa).
Fig.
8
Optical images of ring crack
3.5.
Inert strength
No
plastic behavior was observed on the load-
displacement curve. The residual strength as a
function of the number
of
friction cycle was shown
in
Fig.
9.
The nominal strength before the friction
experiment (n=O) was
70
MPa,
In
dry condition, the
strength degradation was larger at lower friction cycles
and at greater normal loads.
In
the lubricated
condition, The degradation was relatively small
172
compared with the results tested
in
dry condition.
Particularly, at 40
N
The strength degradation after
20
friction cycles where the crack was not observed on the
surface was negligible.
0
strength
A
a"
E
60
'O
t
I
10
100
I000
Number ofCycles,
n
Fig.
9
Residual strength after the friction test
4.
DISCUSSION
4.1.Effect of shear stress on the crack initiation
Relationship between the maximum tensile stress
(T
and coefficient of friction
p
was shown
in
Fig.
10.
The maximum stress
(T
occurred at the trailing edge
and defined as following equations
(8).
o
=o,
+o,
Where, a
is
the radius of the contact area and Po is the
maximum normal stress.
(T
n
is the same as given in
sect. 3.4. The value
of
the maximum stress
is
given at
x=a, because the contribution
of
(T
t on
(T
is greater than
that of
(T
n
and decreases with the increase
in
x at x>a.
The value of the coefficients
of
friction
p=l
and
p=O.l
correspond to the dry and the lubricated
condition at the initial stage of the friction test. Under
the static contact (i.e.
p=O),
the stress for the crack
initiation was 400 MPa. The maximum stress
increases with the increase
in
the coefficient of friction,
and exceeds the critical stress of the crack initiation
(
400 MPa )even in dry condition at a load of
15
N.
The
results of the friction experiments show good agreement
with Fig.
10.
Therefore, this suggests that the
tangential traction results
in
the decrease
in
the normal
loads for the crack initiation.
4.2.Crack growth below the wear scar
Figure
11
shows fracture surfaces. crack formed
by the sliding contact below the wear scar was seen.
In
the lubricated condition at lower normal load, Crack
growth was small (Fig.
1
la). At higher normal load,
The crack path became complicated. It was confirmed
that the area of the crack growth increased with the
increase
in
the normal load and the number of cycles.
Considering the strength degradation (Fig.
9),
the crack
mainly grows at initial stage then becomes complicated
path.
Coefficient of Friction,
p
Fig.
10
Maximum stress
vs.
coefficient of friction
(a),
40
N,
n=20,
in
lubricated condition
(b),
100
N,
n=1000,
in
dry
condition
Fig.
11
Optical micrographs of fracture surfaces
5.
CONCLUSION
Effect of crack growth on the friction and wear of
ceramics material was evaluated using the soda lime
173
glass bars
as
the model material. Static contact for the
determination of the critical stress for the crack
initiation and flexural test for the estimation of the
crack growth below the wear scar were also were
carried out. Followings are the summarize obtained
results.
1) The addition of tangential traction at the interface
results
in
the decrease in the normal load for crack
initiation.
2) The coefficient of friction and the wear amount
depend on the crack growth and are greater at initial
stage at the test when the stress at the trailing edge
exceeded the critical value of the crack initiation.
3) The crack growth rate
is
greater at initial stage
in
the
friction test.
REFERENCES
I)
G.
L.
Boyd and D. M. Kreiner,
“AGT
10
1
/Advanced Turbine Technology Project”,
Proc. of the Twenty-Sixth Automotive Technology
Development Contractors,2( 1986) 895-899
2)
A. Bennet, “Requirements for Engineering Ceramics
in
Gas
Turbine Engines” Mater. Sci. Technology,
Warrendale, PA
(1
989) 275-280
3)
S.
M. Hsu, Y.
S.
Wang and R.
G.
Munro “Quantative
wear maps
as
a visualization
of
wear mechanism
transitions in cermica materials”, Wear134 (1989)l-11
4)
K.
Hokkirigawa and
M.
Mizumoto “Microscopic
Fractural Aspects of Microtribology” Japanese Journal
ofTribology 37,11(1992)895-901
5)
A.
Yoshida et al, “Fundermental study on applying
fine ceramics to rolling bearings under water
lubrication”
International Tribology Conference Yokohama (1995)
Vol.
1485-490
6)
JIS
R
1601 (1993)
7) B.
R.
lawn, “Indentation of Ceramics with Spheres:
A Century after Hertz”,
J.
Am.
Ceram. SOC., 81,8(1998)
1977- 1994
8) R.
D.
Mindlin, “Compliance of Elastic Bodies
in
Contact”. J. Applied Mechanics Sept. (1944) 259-268
174
DAMAGE DETECTION IN TETRAGONAL ZIRCONIA POLYCRYSTALS
(TZP) BY IMPEDANCE SPECTROSCOPY
S.
Wagner*,
A.
Tiefenbach**,
R.
Oberacker*, M.
J.
Hoffmann",
B.
Hoffmann**
(*)Institut fir Keramik im Maschinenbau, Universitat Karlsruhe,
D-76131 Karlsruhe, Germany
(**)Institut fir Werkstoffe der Elektrotechnik, Universitat Karlsruhe,
D-76131 Karlsruhe, Germany
ABSTRACT
The influence of phase composition and artificial
long cracks on the electrical behaviour of 9 mol% CeOz
stabilised Zr02 (9Ce-TZP) has been investigated by
impedance spectroscopy at room temperature
as
well as
at elevated temperatures. Cracks were introduced and
propagated by the bridge method. The crack
propagation
in
9Ce-TZP is combined with a tetragonal
to monoclinic phase transformation in the process zone
of the crack. These
two
processes correspond to a
change in the electrical properties. It was possible to
correlate the changes in the electrical properties to the
phase transformation
as
well
as
to the crack
propagation. Numerical field simulations were
performed for this purpose. From experimental results
and numerical field simulations a sensitivity analysis
has been derived with respect to the detection of short
cracks under load at room temperature. Closed cracks
are detectable only at elevated temperatures.
INTRODUCTION
In
tetragonal zirconia ceramics (Tetragonal Zirconia
Polycrystals TZP or Partially Stabilised Zirconia PSZ)
subcritical crack growth under static and cyclic load is
observed [1,2]. Phase transformation from the
tetragonal to monoclinic phase, taking place in the
process zone of propagating cracks, acts
as
toughening
mechanism and plays an important role in these
ceramics [3,4,5,6]. Lifetime predictions for TZP
components under static and cyclic loading are
subjected to statistical scatter, due to the
unknown
and
often widely distributed defect population
in
the
material. This limits the use of such components.
Therefore the development of non destructive
evaluation methods
(NDE)
is important for the
detection of fatigue relevant defects prior to
catastrophic failure.
Electrical measurements offer some advantages
over to the more common NDE methods, such
as
local
measurement of the critical component sections and
measurements from room temperature up to high
temperatures. There is, however, only limited
information available on the application of methods
such
as
impedance spectroscopy for this purpose [7].
No
assessment of the potential of electrical methods
is
possible on the basis of the present knowledge.
However, ionic conductors such
as
zirconia seem to be
suitable materials for this technique.
In this study both, the influence of the tetragonal to
monoclinic phase transformation
as
well the effect of
single cracks on the electrical behaviour of 9 mol%
CeOz stabilised ZrOz (9Ce-TZP) has been investigated
from room temperature up to 55OOC. This 9Ce-TZP
material develops large transformation zones at the
crack tip.
The electrical characterisation of the material result
in restriction only for the room temperature
characterisation, where only capacitive measurements
are feasible. At elevated temperatures, the ionic
conductivity becomes measurable and gives additional
information with respect
to
defects.
EXPERIMENTAL PROCEDURE
Sample preparation
-
3r the sample preparation 9 mol% CeOZ stabilised
,!rij2
powder (9Ce-TZP, Unitec, UK) has been used.
Plates of 65x45~12 mm3 were produced by pressing,
cold isostatic pressing and sintering at 1400°C for
2
hours in air. From the plates four point bending
specimens with a dimension of 48x9~4
mm3
as well as
specimens for the transformation and high temperature
experiments of
1
Ox8x 12
mm3
were manufactured by
diamond cutting and grinding. The grinding induced
monoclinic zone at the surface was removed by
annealing the samples for one hour at 1000°C in air.
By cooling the specimens to -85°C in cryogenic
silicon oil, the transformation of the tetragonal to the
monoclinic (m+t) phase was induced.
A
retransformation process from the monoclinic to the
tetragonal state has been realised by heating to 600°C.
Microstructural characterisation
SEM and TEM were utilised for investigating the
microstructure. The mean grain size was determined
from SEM micrographs taken from polished and
thermally etched specimens by the line intercept
method. Samples for TEM investigations were
prepared by grinding, dimpling and ion milling.
A
Zeiss
EM91 2 Omega microscope with an accelerating
voltage of 120 kV was used for TEM investigations.
The phase analysis was performed at an X-ray
175
difliactometer (SIEMENS model
D500)
using Cu-L-
radiation and a secondary monocromator. The
monoclinic fraction was derived from the (-lll),,,,
(1
1 l),,, and (1 1 l)t
-
peaks according to
[8].
Electrical measurements
The electrodes for the electrical measurements at
room temperature were produced by vacuum deposition
of chromidnickel. For the experiments at elevated
temperatures platinum has been used. At room
temperature the sample capacitance was measured by a
multi-frequency LCR Meter (HP 4275A, Hewlett
Packard) over a frequency range from 10
kHz
to
10 MHz. In the temperature range from 150°C to
550°C an impedance analyser
(SI
1260, Solartron) was
used over a frequency range from 1 Hz to 1 MHz. The
samples were heated by high-intensity infrared line
heaters. All experiments were performed in air.
In
situ impedance spectroscopy experiments
In situ impedance spectroscopy experiments were
performed at an universal testing machine UTS during
four point bending tests. The aim of the in situ
experiments was to monitor the propagation of a crack
and the process zone, which accompanies this crack, by
electrical measurements. The ideal sharp starting cracks
in the specimen was induced by the bridge method.
Figure 1 shows schematically the experimental four
point bending arrangement. The specimens were
stepwise cyclic loaded and unloaded with a successive
increase of the load in each step. The permittivity of the
sample was measured before and after each loading
step. Crack growth was measured by an optical
microscope.
The arrangement was designed for maximum
sensitivity, although it does produce an inhomogeneous
electrical field and thus numerical modelling is required
for quantitative analysis. This modelling was performed
using the software code MAFIA
[9].
RESULTS
AND
DISCUSSION
Microstructure
The material has a mean grain size of 1.1 pm.
XRD
measurements indicate a single phase tetragonal
microstructure in the
as
sintered state. After the cooling
induced tetragonal to monoclinic phase transformation
a monoclinic phase content of 70% has been
determined.
TEM investigations show a good qualitative
correlation with the
XRD
results. TEM micrographs of
the microstructure in the
as
sintered state and after the
cooling induced tetragonal to monoclinic
transformation are represented in Fig. 2.
Fig.
2a:
TEM micrograph of tetragonal zirconia a)
tetragonal grain, b) grain boundary c) grain boundary
phase in a triple point
8
Load
Fig.
2b:
TEM micrograph of transformed zirconia with
a) monoclinic grain b) tetragonal grain c) microcracks
at the gain boundary
Electrodes
Fig.
1
:
Arrangement of the in situ impedance
spectroscopy experiments
In the initial state the material has a predominantly
single phase tetragonal microstructure with small
amounts of a grain boundary phase situated in the triple
points. In this state no microcracks were detected. After
the cooling induced t+m transition, monoclinic
as
well
as tetragonal grains are observed. Furthermore,
microcracks at the
grain
boundaries have been
developed due to the volume increase during the t+m
phase transition. After
the
monoclinic to tetragonal
176
retransformation induced by heating the samples, the
material shows a pure tetragonal microstructure with
microcracks at the grain boundaries.
Electrical measurements at room temperature
At room temperature, only the capacitive part
of
the
impedance can be measured for the different materials.
The effective permittivity
of
the
as
sintered material
amounts about 38. There is a reproducible, drastic
decrease in the effective permittivity after the cooling
induced tetragonal
to
monoclinic transformation
as
shown
in
table 1. Differences of -30% compared to the
specimens in the as sintered state have been measured.
After the monoclinic to tetragonal retransformation,
the permittivity is increased again to the initial
permittivity. The cracks, which are still present in the
retransformed 9Ce-TZP obviously cause no significant
differences between the measured permittivities in the
initial state and after the retransformation. This is easily
explained by the small volume fraction of the
essentially closed cracks, as the contribution of a
second phase to the total permittivity is proportional to
its volume fraction.
Table
1:
Permittivity of 9Ce-TZP before and after
phase transformation
9Ce-TZP (microstructure)
tetragonal reference state without
cracks
after cooling induced t+m transition
(70
%
monoclinic phase)
tetragonal state after m+t
retransformation with a network
of
cracks
Effective
Permittivity
E
38,l
26,7
38,O
Detection
of
long cracks under load
Thus, it can not be expected, that closed cracks
would be not detectable by impedance spectroscopy at
room temperature. However, cracks open under
mechanical load. This volume effect result in a
decrease in the effective permittivity of a crack
containing specimen under external load.
To study this effect quantitatively, in situ
impedance spectroscopy measurements were performed
according to Fig.
1.
Figure 3a demonstrates the changes
in measured permittivity
A&
in dependence of the
stepwise applied load. The results under load
as
well
as
after unloading are plotted in this graph. In Fig. 3b, the
corresponding crack length and crack opening are
shown. The crack opening was determined from the
optically measured crack length by the relation
[lo]:
ti=a-%.f(a/W)
E
(1)
where a is the crack length,
uB
the outer fibre stress,
E
the elastic modulus and fa geometric function of the
crack length a /sample width W relation.
Considering only the specimen under load there is a
monotonous decrease in permittivity with increasing
load (Fig. 3a). This is caused by the air volume of the
opened crack
as
well
as
by the mechanically induced
tetragonal to monoclinic phase transformation in the
process zone. Up to about
500N,
crack opening
increases at a stable crack length.
During unloading, the crack closes and this leads to
an increase of permittivity approximately to the initial
value. This confirms the results in table 1, where had
been shown, that closed cracks are not measurable by
impedance spectroscopy at room temperature due to
their negligible volume fraction. Beyond
500
N
can be
recognised, that the permittivity of the unloaded
specimen shows a residual loss, which increases with
increasing the load and the crack length. This is caused
by the tetragonal to monoclinic phase transformation in
the process zone, which has a significant influence on
the permittivity at room temperature (table 1). Contrary
to the reversible contribution
of
crack opening and
closing, there exists an irreversible contribution of
phase transition. This irreversible proportion increases
during crack growth and can be determined in the
unloaded state. Comparing the loaded and unloaded
state it can be recognised, that the main part of the
changes in permittivity is caused by the contribution
of
crack opening.
0
-1
E
-2
-
-3
a
$4
-5
f=100kHz
I
\
s!
I
0
200
400
000
800
1000
F
I
“I
Fig. 3a: Relative permittivity
of
a bending sample in
dependence of the applied load during stepwise loading
and unloading
0 200
400
000
800
I000
F
I
“I
Fig. 3b: Corresponding measured crack length and
crack opening in dependence
of
the applied load
177
For a quantitative discussion of the in situ
experiment results, numerical simulations were
performed
[
111 to correlate the changes
in
permittivity
to the crack geometry and to the tetragonal to
monoclinic transformation in the process zone.
Figure
4
shows a comparison of experimentally
measured and numerically calculated decrease in
permittivity in dependence of crack opening for
different crack lengths. The proportion of the
monoclinic zone in the process zone (irreversible
portion) had been subtracted from the permittivity and
is not considered here. The diagram represents only the
reversible portion.
The curves, representing the numerical results, were
determined for an a/W relation of
0.44
and
0.33
by
varying the crack opening. Although the crack length
measured in the experiment
is
smaller than in the
simulations, the
loss
in relative permittivity is
underestimated in the calculations. That means, the air
volume of the crack is not sufficient to explain
completely the permittivity
loss.
The reason for this
difference could be a partial retransformation of the
monoclinic phase during unloading.
0
-1
-2
g-3
-Sirnulation
for
e0.33
0
4
8
12
16
6,
I
tlJm1
Fig.
4:
Experimentally measured and numerically
determined permittivity
loss
in dependence of the crack
opening for different crack lengths
Sensitivity analysis with respect to short
cracks
The crack length used in the experiments is far
away from practical relevance. Therefore a sensitivity
analysis based on the simulation results has been
derived to extrapolate the capability of the method for
the detection of shorter cracks under load. The stress
intensity factor
K,
at the crack tip is given by:
where
Y
is a geometric fhction.
Using eq.
(1)
for the crack opening it follows:
s=x.,.,
Kl
f
(3)
With E=220 GPa for the investigated TZP ceramic
one obtains crack opening-crack length relations shown
in Fig.
5
for
different stress intensities.
For a sensitivity analysis these curves must be
compared with the changes of permittivity for a
corresponding crack geometry. The results of the
electric field simulations can be fitted by the following
analytical approximation:
(4)
AC
-
=
const
-6
-
6
C
The corresponding results are represented as dotted
lines in Fig.
5
for measurement sensitivities of 0.1
%,
0.05
%
and
0.025
'YO.
With the experimentally realised
sensitivity of
0.05%
and a fracture toughness of
15
MPam0.5, cracks with a minimum length of
30
pm
and an opening
<2
pm should be detectable. This is
valid for a distance of the electrodes of
3
mm.
The
sensitivity can be fiuther increased by decreasing the
distance of the electrodes.
K,
[MPam0'5)
Crack
length
a
trCm1
Fig.
5:
Sensitivity of the room temperature permittivity
measurements in dependence
of
the crack geometry for
different stress intensities at the crack tip
Electrical measurements at elevated
temperatures
Using impedance spectroscopy at room
temperature, phase transformations and opened cracks
can be detected. Damage detection by permittivity
measurements at room temperature prior to catastrophic
failure seems possible only under ideal conditions, e.
g.
positioning of the electrodes close to the damage zone
of
a component.
For
the detection of closed cracks in
TZP
ceramics, impedance spectroscopy at elevated
temperatures has been studied. In the temperature range
up to
55OoC,
TZP is an ionic conductor [12].
Impedance spectra were recorded at temperatures up
to
550°C
on specimens in the initial tetragonal state as
well
as
with a sharp single crack.
In Fig.
6
the Bode plots
of
the impedance spectra
of
9Ce-TZP in the initial tetragonal state are represented
for temperatures from
20°C
to 5OOOC.
178
11
10
5
500'
T
4
-I
Fig.6: Bode characteristics of 9Ce-TZP
in
the reference
state for different temperatures
At room temperature only a capacitive behaviour is
observed, visible by a slope of
(-1)
of the log(Z)/log(f)
plot. lncreasing the temperature ohmic ranges were
detected too. These ohmic ranges can be recognised
from the horizontal parts of the impedance spectra
(slope
0).
Considering temperatures above 300"C, the
curves have a qualitatively similar behaviour. At low
frequencies there is a first ohmic range, followed by a
weakly pronounced capacitive area and a second ohmic
range. At high frequencies, a pure capacitive behaviour
had been measured. This behaviour corresponds to an
electric circuit consisting of two R-C elements.
In order to find
a
quantitative correlation between
the microstructure and the electrical properties,
modelling is required. Therefore the Brick Layer
model, an idealised and often used model with a
network of two R-C elements
[
131, has been applied. In
this model the microstructure is described as an array of
cubic-shaped grains, separated by plane grain
boundaries. Such a microstructure of grains and grain
boundaries is compatible to the findings of the TEM
investigations (Fig.
2).
In the model, the electrical behaviour of the grains
determines the high-frequency capacitive and ohmic
range of the spectra. Grain boundaries control the low
frequency range of the impedance spectra. In the
investigated 9Ce-TZP they are only detectable at
temperatures beyond 300°C. Using this model specific
conductances and capacitances for the grains and grain
boundary phase were calculated for the undamaged
9Ce-TZP material.
After investigating the undamaged tetragonal state,
a sharp crack was introduced into the specimen and
impedance spectra were recorded. The crack was
propagated by the bridge method up to a/W=0.98 for
successive measurement of impedance spectra. The
corresponding Bode characteristics measured at 400°C
are shown in Fig.
7.
The curves represents only the
influence of the crack. The monoclinic process zone at
the crack tip was removed by an annealing process after
each stable crack growth.
Considering the spectra one can find two ohmic
ranges
in
the damaged state too, similar
to
the
undamaged state. The behaviour of the spectra can be
described by
2
R-C
elements too. In the high frequency
range, there are no differences in the electrical
behaviour between the damaged and undamaged
tetragonal state. The crack only effects the low
frequency behaviour. Here the resulting changes in the
electrical properties can be observed as differences in
the capacitive and resistive parts of the impedance. The
impedance
Z
increases strongly with increasing the
crack length.
5.8
E
2
5.4
3
-5
m
0
m
Fig.
7:
Influence of the crack length on the Bode
characteristics of a 9Ce-TZP specimen
In Fig.
8
the change of the capacitance and the
change of the conductance in the low frequency range
of the impedance spectra are shown as a finction of the
a/W relation. Low frequency capacitance
CLF
and
conductance
oLF
change in a linear manner with
increasing crack length. It can be mentioned, that the
low frequency capacitance is some more sensitive than
the low frequency conductance. The difference is,
however, rather small. With an experimental sensitivity
of
4
for AcLF/cLF and AcJLF/oLF respectively, cracks in
the pm range should be detectable, even if they are
completely closed.
0
-20
n
g
.g
-40
:
-60
5
5
0
3
-80
8
-1
00
0
0.2
0.4
0.0
0.8
I
aMI
Fig.
8:
Chances in low frequency capacitance
CLF
and
conductivity
oLF
caused by sharp cracks of different
length
179
For the interpretation of the electrical behaviour
numerical field simulations has been performed.
Therefore the Brick Layer model has been extended
with a crack
as
an
additional element of microstructure.
This developed model is explained in detail in
[14].
Using numerical simulations impedance spectra can
be estimated. In Fig.
9
the measured
as
well
as
the
simulated spectra are shown for the undamaged and the
damaged state. Comparing the spectra one can see a
very good agreement between the simulated and
measured curves. That means, that the crack geometry
can be derived from the measured spectra using the
numerical simulation model.
Fig.
8:
Comparison of the measured Bode
characteristics with numerically determined spectra
CONCLUSIONS
Using impedance spectroscopy measurements at room
temperature tetragonal to monoclinic phase
transformation of TZP materials can be detected. This
is caused by a drastic difference in permittivity of
30
%
between the tetragonal and monoclinic phase. Closed
cracks, however, do not influence the permittivity. At
room temperature only cracks opened under load are
detectable. For the detection of closed cracks,
impedance spectroscopy at elevated temperatures
seems to have a high potential,
as
the
low
frequency
capacitance and conductance of TZP is very sensitive
to cracks. Impedance spectroscopy is, however, still far
away from being a standard NDE testing method.
Further work is required to investigate the influence
of
microstructural changes, which also contribute to the
impedance spectra and which have to be separated from
the effect of short cracks. A quantitative derivation of
crack geometries from changes in impedance spectra
requires further progress in modelling of the electrical
behaviour.
ACKNOWLEDGEMENT
This work was funded by the Deutsche
Forschungsgemeinschaft DFG (German National
Science Foundation) under contract No. Ho
69311
1-1
2
and Ob
104/4-2.
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180
RELIABILITY AND REPRODUCIBILITY
OF
SILICON NITRIDE VALVES:
EXPERIENCES
OF
A
FIELD TEST
Gerhard
Wetting*,
Jiirgen Hennicke*, Helmut Feuer*,
Karl-Heinz Thiemann**, Dieter Vollmer**, Erwin Fechter**
Frank Sticher***, Andreas Geyer****
(*)
CFI Ceramics for Industry GmbH&Co.KG, Rodental, Germany
(**)
DaimlerChrysler AG, Stuttgart, Germany
(***)
MAHLE Motorventile GmbH, Bad Homburg, Germany
(****)
MAHLE Ventiltrieb GmbH, Stuttgart, Germany
ABSTRACT
Within the last three years a field test with
silicon nitride (Si3N4) engine valves was performed by
DaimlerChrysler with 1628 Mercedes C200-compressor
passenger cars. For 1000 cars Si3N4-valve blanks were
produced by CFI on a pre-production scale and
machined by EuroVal, now
MAHLE.
In order to realize
production volume, the primarily developed gas-
pressure sintered Si3N4-grade had to be down-scaled to
allow low-pressure sintering while still meeting
materials
'
requirements for engine valves. Machining
had to be optimized to avoid damage of components,
effecting life-time and reliability. Fitting in cylinder
heads was adjusted for automatic assemblage. Effects of
the Si3N4-valves on fuel consumption and emissions
were analyzed on special test rigs of DaimlerChrysler
whereas the majority of cars are in normal use by
customers. Details of this development work were
outlined and a preliminary summary on experiences will
be given.
INTRODUCTION
During the last decade a lot of work was done to
develop and realize automotive engine valves made of
silicon nitride (SN). This is documented in various
publications and also contributions to the last
conference of this series [l-61. Daimler-Benz, now
DaimerChrysler
(DC),
were pioneers in this area and
got encouraging results with engines equipped with
SN-
valves on test rigs as well as in passenger cars,
amounting in reductions in NO, up to
30
%,
CO up to
20
%,
fuel up to 6
%,
noise up to 12
%,
and additionally
the observation of reduced coke layer formation on SN-
valves in comparison with metallic ones.
These positive results motivated DC to perform a
field test with the Mercedes C200 passenger car (W202)
equipped with the charged 4 cylinder116
valves-Ml l l-
ML engine with SN-inlet and exhaust-valves. Target
volume was 1,750 cars and CFI was chosen as supplier
of SN-valves for about
1,000
cars.
As
all preliminarily developmental activities and
tests were performed with a gas-pressure sintered SN-
grade of CFI, this decision comprised the problem of a
limited manufacturing capacity for this SN-grade in
reasonable time. Thus, in accordance with DC a new
SN-grade was developed, whch could be sintered under
a low N2-pressure
(10
bar) while still offering sufficient
mechanical properties and reliability. With this sintering
route, the manuf6cturing capacity was increased by a
factor of
3-4
so
that the test could be performed within
the time schedule. Of course, this new material had to
be qualified once more by DC to be a suitable valve
material. Results of respectively experiences made
along the whole manufacturing-, evaluation- and
application-line within this field test are described in the
following chapters.
EXPERIENCE AND RESULTS OF THE
FIELD TEST
GENERAL ASPECTS
One
main
target of this manufacturing- and
application-test was to prove the reliability and
reproducibility of SN-valves. Details of processing and
evaluation are already published [4, 71. In order to
identify origins of possible damage, a tracking
procedure starting from the raw materials along all
processing steps and machining was created by an
individual valve number. This number was also
registered by mounting the valves into the cylinder
heads of the individual engines, and with the engine
number the relation to the car number was given. Thus,
for each valve in any engine the whole route back to the
raw materials and fabrication, sintering and machining
lots is documented.
181