Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

the fracture toughness. 
Table 5-1: Fracture toughness of NiO-3YSZ and NiO-8YSZ was measured under 1000 µm/min at 
room temperature in air. 
Material 
Fracture toughness (MPa·m
1/2
) 
NiO-3YSZ 
3.05 ± 0.17 
NiO-8YSZ 
1.76 ± 0.15 
5.1.2.2.
Ni-YSZ ( reduced material) 
The necessity of pre-cracking was also verified for the reduced material. Tests on the load rate 
effect (see following section) revealed that the load rate of at least 2000 µm/min was necessary 
for Ni-8YSZ type 4 specimens, yielding an apparently higher fracture toughness (4.12 ± 0.12 
MPa·m
1/2
) compared to the oxidized state 

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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 

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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 (