Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

5-12
(a)
) and a 
ductile deformation (
Figure
5-12
(b)),
indicating a fracture toughness increase due to energy 
consumption during crack growth. Crack bridging of Ni particles, which has been reported in 
[137], could not be found in the current study.
Figure 5-12: (a) Intergranular failure mode of Ni-8YSZ and (b) ductile deformation of Ni 
particles. 
Electrolyte 
Anode 
Anode 
Anode 

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Results and discussion 
88 
d) Re-oxidized NiO-8YSZ tested at 800°C 
The channelling crack-type failure of the electrolyte through thickness can be seen in 
Figure
 
5-13
. Further crack propagation into the substrate was not found. With respect to the failure 
mode, the re-oxidized NiO showed an intergranular failure mode (
Figure
 5-13
). 
Figure 5-13: As in case of oxidized NiO, the re-oxidized NiO also showed an intergranular 
failure mode. The SEM image also confirms that the channelling-type cracks in the electrolyte do 
not propagate into the re-oxidized NiO-8YSZ composite. 
5.1.2.8.
Summary – fracture toughness 
The section reported and discussed the fracture toughness results of the two most common anode 
substrate material variants, NiO-3YSZ and NiO-8YSZ, at room temperature and at a typical 
stack operation temperature of 800°C. Different types of warm pressed and tape cast NiO-8YSZ 
substrates were characterized in oxidized and reduced states. The study considered both, 
oxidized and reduced material states, where also an outlook is given onto the behavior of the re-
oxidized state that might be induced by malfunctions of sealants or other functional components. 
Overall, the results confirmed superior fracture toughness of 3YSZ based composites compared 
to 8YSZ based composites in the oxidized state, whereas in the reduced 3YSZ based composites 
showed similar fracture toughness at room temperature, but a higher value at 800°C compared to 
8YSZ based composites. Complementary microstructural analysis aided the interpretation of 
mechanical characterization.