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Y. Zhang et al.
Fig. 23.2 Typical frequency spectrum of the AE signal from a surface vertical crack b sliding
interface crack c opening interface crack d substrate deformation [23]
sliding of the TC and BC due to the shear stress. Another interface crack was the
opening interface crack. It was a horizontal crack that occurred at the interface due
to tensile stress. Depending on the external force, the type of interface crack was
different.
23.4.2 The Influence of Structure
It was mentioned above that in the thermal oxidation environment, the appearance of
TGO would change the structure of TBC. Changes in structure can lead to different
failure processes.
Park et al. [24] conducted a four-point bending experiment on TBC with different
cycle times. The parameters of acoustic emission signals were analyzed. It was found
that the number of thermal cycles was positively correlated with the number of events,
while it was negatively correlated with amplitude, cumulative counts, cumulative
energy, and mechanical strength. The AE signals of TBC which had exposed to
oxidative environments for a long time in 4-point bending test can be classified into
three categories in the frequency domain.
Li et al. [25] studied the Vickers-hardness of TBC before and after oxidation,
it was shown that the appearance of TGO reduced the strength of the coatings and
accelerated the destruction of it. Cluster analysis and wavelet were used and got the
same result that there were 3 types of the signals.
The failure process under 3-point-bending test was different between TBC with
and without TGO. Figure 23.3 showed the specific course. The failure began in TC. A
surface vertical crack existed in TC at first, followed by many same cracks. With the
conjunction of surface cracks, interface crack appeared. Finally, after the interface
cracks elongated and merged, coatings failed down. Compared with Fig. 23.3a, b
showed that when TGO was present, only one single vertical crack and one single
interface crack appeared throughout the failure. It was worth caring that the external
stress after oxidation is also at a low level.
Except for TGO, different substrates would also get different signals.
Wang et al. [26] performed 3-ponit bending tests on superalloy/BC/YSZ and
stainless steel/BC/YSZ. The AE signals were distinguished easily. Figures 23.4, 23.5
Y. Zhang et al.
Fig. 23.2 Typical frequency spectrum of the AE signal from a surface vertical crack b sliding
interface crack c opening interface crack d substrate deformation [23]
sliding of the TC and BC due to the shear stress. Another interface crack was the
opening interface crack. It was a horizontal crack that occurred at the interface due
to tensile stress. Depending on the external force, the type of interface crack was
different.
23.4.2 The Influence of Structure
It was mentioned above that in the thermal oxidation environment, the appearance of
TGO would change the structure of TBC. Changes in structure can lead to different
failure processes.
Park et al. [24] conducted a four-point bending experiment on TBC with different
cycle times. The parameters of acoustic emission signals were analyzed. It was found
that the number of thermal cycles was positively correlated with the number of events,
while it was negatively correlated with amplitude, cumulative counts, cumulative
energy, and mechanical strength. The AE signals of TBC which had exposed to
oxidative environments for a long time in 4-point bending test can be classified into
three categories in the frequency domain.
Li et al. [25] studied the Vickers-hardness of TBC before and after oxidation,
it was shown that the appearance of TGO reduced the strength of the coatings and
accelerated the destruction of it. Cluster analysis and wavelet were used and got the
same result that there were 3 types of the signals.
The failure process under 3-point-bending test was different between TBC with
and without TGO. Figure 23.3 showed the specific course. The failure began in TC. A
surface vertical crack existed in TC at first, followed by many same cracks. With the
conjunction of surface cracks, interface crack appeared. Finally, after the interface
cracks elongated and merged, coatings failed down. Compared with Fig. 23.3a, b
showed that when TGO was present, only one single vertical crack and one single
interface crack appeared throughout the failure. It was worth caring that the external
stress after oxidation is also at a low level.
Except for TGO, different substrates would also get different signals.
Wang et al. [26] performed 3-ponit bending tests on superalloy/BC/YSZ and
stainless steel/BC/YSZ. The AE signals were distinguished easily. Figures 23.4, 23.5