23 Acoustic Emission in Coatings—A Review
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oxidation process, a new layer of thermally grown oxide (TGO) will appear between
TC and BC.
23.4.1 The Influence of Test Condition
A lot of studies have been done to explore the correlation between crack behaviors
and AE signals. Tensile, compression, 3-point-bending, 4-point bending and thermal
cycling were common test methods, especially tensile test.
Yao et al. [19] used the events and amplitude of the acoustic emission signal to
divide the entire stretching process into five stages. According to the characteristics
of the signals in the frequency domain, the signals were divided into three types,
which are the substrate deformation signal, the surface vertical crack signal and
the interface crack signal. The signals began with substrate deformation, then the
surface vertical crack joined in, and finally interface crack, delamination came up.
The quantitative relationship between the number of AE events and the interface
crack size can be given with the help of a mechanical model.
Yang et al. [20–23] studied the failure process of thermal barrier coatings under
many different external loads. In their study, the crack propagation process in thermal
cycle was divided into three parts. At first, vertical cracks appeared on the surface
of the TC layer, and then developed to the interface. At the same time, horizontal
cracks also appeared at the interface. Finally, the interface cracks connected with
each other led to the coating falling off. The quantitative parameters of surface crack
length and interface crack length calculated from thermal stress and residual stress
were in good agreement with the number of AE events. The life prediction was also
consistent with the experimental results.
In the three-point bending test, surface crack and interface crack were dominant
in the yield stage and plastic deformation stage, respectively. And the order and
development of cracks was similar to that in thermal cycle.
As for compress load, the most prominent difference was the type of signal. There
were two kinds of interface cracks. The one was caused by compress named opening
interface cracks, and the other was caused by shear named sliding interface cracks.
Only thermal change and compress load can lead to the former.
As the most dangerous failure mode for TBC, molten calcium-magnesiumalumino-silicate (CMAS) corrosion experiments was monitored by AE with the
help of wave guide. Four kinds of signals were obtained by wavelet analysis, namely
surface vertical crack, sliding interface crack opening interface crack and substrate
deformation. Two types of interface crack predominated in the failure process, which
was the cause of coating spallation.
It was worth noting that even if the external loads causing the coating failure were
different, the crack types were similar and the frequencies of the acoustic emission
signals were approximately the same. Figure 23.2 showed the 4 types of signals.
Surface vertical crack was led by tensile stress in the TC, which existed earliest
and generally in almost all failure processes. The Sliding interface crack was caused
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oxidation process, a new layer of thermally grown oxide (TGO) will appear between
TC and BC.
23.4.1 The Influence of Test Condition
A lot of studies have been done to explore the correlation between crack behaviors
and AE signals. Tensile, compression, 3-point-bending, 4-point bending and thermal
cycling were common test methods, especially tensile test.
Yao et al. [19] used the events and amplitude of the acoustic emission signal to
divide the entire stretching process into five stages. According to the characteristics
of the signals in the frequency domain, the signals were divided into three types,
which are the substrate deformation signal, the surface vertical crack signal and
the interface crack signal. The signals began with substrate deformation, then the
surface vertical crack joined in, and finally interface crack, delamination came up.
The quantitative relationship between the number of AE events and the interface
crack size can be given with the help of a mechanical model.
Yang et al. [20–23] studied the failure process of thermal barrier coatings under
many different external loads. In their study, the crack propagation process in thermal
cycle was divided into three parts. At first, vertical cracks appeared on the surface
of the TC layer, and then developed to the interface. At the same time, horizontal
cracks also appeared at the interface. Finally, the interface cracks connected with
each other led to the coating falling off. The quantitative parameters of surface crack
length and interface crack length calculated from thermal stress and residual stress
were in good agreement with the number of AE events. The life prediction was also
consistent with the experimental results.
In the three-point bending test, surface crack and interface crack were dominant
in the yield stage and plastic deformation stage, respectively. And the order and
development of cracks was similar to that in thermal cycle.
As for compress load, the most prominent difference was the type of signal. There
were two kinds of interface cracks. The one was caused by compress named opening
interface cracks, and the other was caused by shear named sliding interface cracks.
Only thermal change and compress load can lead to the former.
As the most dangerous failure mode for TBC, molten calcium-magnesiumalumino-silicate (CMAS) corrosion experiments was monitored by AE with the
help of wave guide. Four kinds of signals were obtained by wavelet analysis, namely
surface vertical crack, sliding interface crack opening interface crack and substrate
deformation. Two types of interface crack predominated in the failure process, which
was the cause of coating spallation.
It was worth noting that even if the external loads causing the coating failure were
different, the crack types were similar and the frequencies of the acoustic emission
signals were approximately the same. Figure 23.2 showed the 4 types of signals.
Surface vertical crack was led by tensile stress in the TC, which existed earliest
and generally in almost all failure processes. The Sliding interface crack was caused