Conclusions
130
800°C. An apparent thickness effects was found for 10 B(Sr), while the sealant 7.5 B(Ba) did not
reveal such an effect within the limits of experimental uncertainty.
Bending tests were carried out for annealed H-Ag and H-F head-to-head specimens. H-Ag
revealed a lower average fracture stress compared to the respective as-sintered sealant; an
opposite effect to the one previously reported for the sealant H-P.
SEM analysis indicated that
for the annealed material, the Ag particles distribute more evenly through the microstructure,
where it appeared that Ag incorporation into the glass-ceramic matrix leads to micro-cracks and
lowers the fracture stress. The annealed sealant revealed also a fracture stress decrease at 800°C;
however, the fracture stress is better preserved than for the as-sintered material. H-F showed an
increase of the fracture stress
with increasing annealing time, indicating that crystallization
enhances this property for this particular material. H-F with 1000 h annealing yielded a rather
temperature independent fracture stress (RT, 700°C, 800°C).
The two Jülich sealant materials have been characterized using
a torsion set-up at RT and
elevated temperatures. The results indicate that, similar as for the bending test,
the obtained
values are rather insensitive to residual stresses that are mainly induced by the thermal expansion
mismatch of sealant and steel. Both sealant materials show a pronounced decrease of the shear
strength above their softening temperature. Ag reinforced sealant material shows a decrease of
the shear strength for longer annealing time, a similar effect was
observed for the bending
strength of head-to-head joined specimens, while such an effect was not observed for the YSZ
fiber reinforced material. Contrary to the Ag particle reinforced sealant,
the H-F material
revealed a change in fracture mode from interfacial to cracking through the sealant at elevated
temperatures, indicating that at RT the weakest position is located at the oxide scale, whereas at
high temperatures the sealant becomes weaker due to the softening of the material. An obvious
implication is that progressive annealing that might enhance the properties of the sealant material,
will not necessarily lead to higher shear strength at RT, where the
properties and the behavior
with respect to long term temperature exposure of the oxide scale dominates. Typically oxide
scales become thicker and weaker for longer annealing time. Note, for bending tests failure
through the sealant was observed at all temperatures. The complex loading
situation in a stack
requires consideration of both, shear and bending/tensile strengths.