Results
and discussion
79
based on displacement rate effect described in next section as shown in
Figure
5-5
.)
A similar
value for reduced cells has been reported in [199]. The result indicates a positive effect of ductile
metallic Ni particles on crack growth resistance, which was obviously stronger than the decrease
of the fracture toughness due to the reduction-associated increase in porosity.
Ni-3YSZ specimens tested with a load rate of 1000 µm/min yielded with 3.85 ± 0.27 MPa·m
1/2
a
very similar fracture toughness value as obtained for Ni-8YSZ. The fracture toughness is 26 %
higher compared to the respective material in the oxidized state (3.05 ± 0.17 MPa·m
1/2
),
confirming that Ni particles enhance the fracture toughness. However the effect is stronger in the
case of Ni-8YSZ where the increase in fracture toughness of reduced specimens compared to
oxidized specimens accounts in the case of type 4 material to ~ 230%, indicating that in the case
of Ni-3YSZ either the decrease of the fracture toughness due to porosity has a stronger effect or
any increase due to Ni ductility is less pronounced which might be associated with differences in
Ni-YSZ ratio or grain / pore size.
Table 5-2: Fracture toughness of NiO-8YSZ in type 4 and NiO-3YSZ along with their reduced
state.
Material
K
IC
at RT (MPa·m
1/2
)
NiO-8YSZ
1.76 ± 0.15
Ni-8YSZ
4.12 ± 0.12
NiO-3YSZ
3.05 ± 0.17
Ni-8YSZ
3.85 ± 0.27
5.1.2.3.
Sub-critical crack growth
Both NiO-3YSZ and NiO-8YSZ were characterized with respect to their sensitivity to subcritical
crack growth (SCG) at RT via variation of displacement rates. To have a sufficient range, NiO-
8YSZ was analyzed with loading rates of 10 µm/min, 100 µm/min and 1000 µm/min, while
NiO-3YSZ was additionally also tested with 1 µm/min and 2000 µm/min.
In both cases it was
confirmed that the materials are sensitive to SCG and the fracture toughness evaluation is
affected,
as shown in
Figure
5-5
. Lower fracture toughness were obtained when testing the
specimens
with lower displacement rates, i.e. more time was available for chemical reaction to
take place at the crack tip in the stressed microstructure. The results
generally agree with
Results and discussion
80
previous findings by Selçuk and Atkinson [173] on YSZ material´s SCG sensitivity. This result
also indicates 1000 µm/min is sufficient enough to measure fracture toughness of NiO-3YSZ and
NiO-8YSZ reliably. Post-test SEM inspection of the specimens which were tested with 10
µm/min did not reveal any changes in microstructure related to the SCG.
Testing reduced Ni-8YSZ with different displacement rates resulted in a larger SCG effect. The
material (Ni-8YSZ, type 4) was tested in a range from 100 µm/min to 5000 µm/min (
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