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Y. Zhang et al.
problem. Coatings are used in a wide range of applications especially in defense,
machinery, and biology. As a material to protect the substrate, once the coating fails,
the substrate will soon be ruined. In coatings, stress causes the initiation, propagation
and spallation of cracks, which is the main inducement of coating failure. In the past,
many research methods on the failure mechanism of coatings used time-sampling
methods, which means take out a sample at regular intervals in a test. However,
accidental introduction of new cracks and further expansion of original cracks during
sampling make the results unreliable.
Acoustic emission (AE) is a type of non-destructive testing technology with a wide
range of uses, which has made many contributions to the development of industrial
manufacturing, petrochemical and aerospace since the birth of this technology. The
distinguishing features of this method include dynamic, passive, and in-situ monitoring, which make it can reflect some physical damage of materials or parts in order
to judge the internal state of them and even predict their life [1–5]. AE can be a suitable tool to detect the occurrence, development, and failure of cracks, which could
help us study the failure process of coatings. Among a series of coatings, metal coatings, ceramic coatings and thermal barrier coatings have been thoroughly studied by
AE. A large number of studies have shown that AE has great potential in the study of
coating failure process. This article summarized some of the current status, existing
problems and application prospects.
23.2 Metallic Coatings
Metallic coatings refer to the coatings of metal or alloy on the surface of the product
by spraying, electroplating and other methods, which play a role of wear resistance
and corrosion resistance.
Zhang et al. [6–10] used AE to systematically study the fatigue failure process
of supersonic sprayed iron-based alloy coatings. The abrupt changes in the energy,
amplitude and RMS (root mean square) of the acoustic emission signal were regarded
as the basis for judging the critical failure state of the coating. According to the
trend of the parameters and SEM photos of cross section, the entire failure process
was divided into four stages: crack initiation, stable crack propagation, crack instability propagation, and spalling. A clear acoustic emission signal of fatigue damage
was obtained by Empirical Mode Decomposition (EMD) and wavelet analysis. By
changing the magnitude of contact stress, it was found that the amplitude of the
acoustic emission signal has nothing to do with the magnitude of stress, but only
with the failure mode. When the value of contact stress was low, pitting effect was
likely to occur, and the amplitude of acoustic emission signal was also small; when
the number was higher, spalling failure occurred, and the amplitude of AE signal
was higher too; When the contact stress continued to increase, stratification failure
occurred, and the amplitude of AE signal was at the same level as that of spalling
failure.
Y. Zhang et al.
problem. Coatings are used in a wide range of applications especially in defense,
machinery, and biology. As a material to protect the substrate, once the coating fails,
the substrate will soon be ruined. In coatings, stress causes the initiation, propagation
and spallation of cracks, which is the main inducement of coating failure. In the past,
many research methods on the failure mechanism of coatings used time-sampling
methods, which means take out a sample at regular intervals in a test. However,
accidental introduction of new cracks and further expansion of original cracks during
sampling make the results unreliable.
Acoustic emission (AE) is a type of non-destructive testing technology with a wide
range of uses, which has made many contributions to the development of industrial
manufacturing, petrochemical and aerospace since the birth of this technology. The
distinguishing features of this method include dynamic, passive, and in-situ monitoring, which make it can reflect some physical damage of materials or parts in order
to judge the internal state of them and even predict their life [1–5]. AE can be a suitable tool to detect the occurrence, development, and failure of cracks, which could
help us study the failure process of coatings. Among a series of coatings, metal coatings, ceramic coatings and thermal barrier coatings have been thoroughly studied by
AE. A large number of studies have shown that AE has great potential in the study of
coating failure process. This article summarized some of the current status, existing
problems and application prospects.
23.2 Metallic Coatings
Metallic coatings refer to the coatings of metal or alloy on the surface of the product
by spraying, electroplating and other methods, which play a role of wear resistance
and corrosion resistance.
Zhang et al. [6–10] used AE to systematically study the fatigue failure process
of supersonic sprayed iron-based alloy coatings. The abrupt changes in the energy,
amplitude and RMS (root mean square) of the acoustic emission signal were regarded
as the basis for judging the critical failure state of the coating. According to the
trend of the parameters and SEM photos of cross section, the entire failure process
was divided into four stages: crack initiation, stable crack propagation, crack instability propagation, and spalling. A clear acoustic emission signal of fatigue damage
was obtained by Empirical Mode Decomposition (EMD) and wavelet analysis. By
changing the magnitude of contact stress, it was found that the amplitude of the
acoustic emission signal has nothing to do with the magnitude of stress, but only
with the failure mode. When the value of contact stress was low, pitting effect was
likely to occur, and the amplitude of acoustic emission signal was also small; when
the number was higher, spalling failure occurred, and the amplitude of AE signal
was higher too; When the contact stress continued to increase, stratification failure
occurred, and the amplitude of AE signal was at the same level as that of spalling
failure.