Monitoring the boundary of an adhesive coating to a steel substrate with an ultrasonic Rayleigh wave

1 Introduction

Coating–substrate bonds among the adhesive joints are used in the construction of machinery and equipment [1,2]. Coating application in vehicle construction is important because coatings protect the car body from corrosion, as well as provide a high-quality decorative effect (the visual effect of car varnish) [3,4]. During the implementation of repairs to the car body of means of transport, putty coatings are also used. The coatings smooth the damaged surface of the metal sheet [5]. At the stage of manufacturing adhesive coatings, it is important to control the technological process, especially the adhesion of the coating to the substrate, which determines the quality and durability of the adhesive joint.

Research on adhesive joints has been carried out for many years in various areas. The main directions concern the adhesive joints used in the construction of machinery and vehicles [6–8] as well as coatings applied to various materials [9]. Nevertheless, an important area of research is the use of non-destructive testing methods to assess the condition of an adhesive joint [10,11]. Ultrasound allows detection of defects such as kissing bond [12–14], which causes weakening of the adhesive joint. This can lead, during operation, to progressive degradation of this bond, up to a complete loss of adhesion and strength. Balasubramaniam et al. [15] used ultrasonic-guided waves to monitor debonding in a sandwich composite structure. The algorithm developed as a part of the study allows locating the damages with an error estimation value close to ∼0.8 cm. Ultrasound is also used in the study of sandwich joints to find the defects that lead to cracks due to loading [16,17].

Another area of application for the ultrasonic method is monitoring the condition of structures [18] and adhesive joints [16]. Haldren et al. [19] proposed an ultrasonic phase measurement technique for monitoring adhesive bonded surfaces. The authors obtained constant-frequency ultrasonic phase measurements of the reflectance of an adhesive-glass bond. The degree of curing was controlled by exposing the bond to ultraviolet light. For adhesive coatings, the use of ultrasound allows measurement of coating thickness [20], as well as estimation of coating adhesion to the substrate [21]. Most of the research is conducted using the echo technique. The author of this article proposed a method to monitor the connection between the coating and the substrate with a surface wave that propagates along the boundary of such a connection. It can carry important information about the changes that occur during the operation of the joint and its condition.

The main goal of the research presented in the article was to determine the course of changes of ultrasonic surface wave parameters propagating along the boundary of an adhesive joint during continuous monitoring at the constitution and degradation stage. Changes in ultrasonic wave parameters can be an important diagnostic parameter of the state of the coating–substrate joint. In addition, research tasks to achieve the main goal were defined as:

• determining the course of changes in the gain W of an ultrasonic Rayleigh wave propagating along the boundary of an adhesive joint of the coating–substrate type, during its constitution and degradation process,

• determining the courses of changes of amplitude–frequency parameters, such as resonance frequency f and bandwidth b during the process of constitution and degradation of the adhesive joint of the coating–substrate type.

The above tasks allow to fill the gap in the literature. The main novelty presented in the article is the use of an ultrasonic method to evaluate the manufacturing and degradation processes of the adhesive bond between the coating and the substrate. The presented test results and the method of continuous monitoring of the state of the coating–substrate boundary using surface waves have important practical applications in industry. Having at one’s disposal the waveforms of changes in selected parameters of the ultrasonic wave, it is possible to determine the state of the connection (adhesion) both at the stage of manufacture of the coating–substrate connection and its operation. The proposed method is easy to apply and sensitive to changes occurring at the boundary between the coating and the substrate.

2 Research methodology

The stand for monitoring the boundary condition of coating–substrate joint by ultrasonic Rayleigh wave is presented in Figure 1.

Figure 1

Stand for monitoring degradation of coating–substrate boundary with ultrasonic surface wave; 1 – view of the stand, 2 – ultrasonic flaw detector, 3 – vibration inductor control, 4 – coating strip vibration control apparatus, 5 – instruments for monitoring ambient and inductor parameters and 6 – inductor with ultrasonic transducer mounted on the steel strip.

The following factors should be taken into account during monitoring the condition of the adhesive bond between the coating and the substrate:

• the shape and dimension of the adhesive bond, which should be larger than the beam width of the ultrasonic surface wave,

• the surface preparation of the steel substrate and the restoration material – car putty (the car putty manufacturer’s technologies should be used),

• selection and preparation of apparatus for ultrasonic measurements,

• atmospheric conditions during manufacture and degradation of the coating–substrate sample,

• mechanical loading parameters of the coating sample with steel substrate, set on the test bench.

  The most important and main component of test bench is an electrodynamic inductor, which is connected to a generator that produces vibrations of a certain amplitude and frequency. On the exciter there is a system that loads the sample (causing it to vibrate). In addition, the bench is equipped with a cooling system for the exciter and systems that monitor atmospheric conditions both in the area where the exciter and the sample are located and in the room where the tests were conducted. The constancy of the air flow was guaranteed by the use of four fans, and its value was read using an anemometer. The operating parameters of the system were as follows:

• voltage U = 5 V,

• frequency f = 20 Hz.

Important for the reproducibility of the tests and obtaining the best bond between the coating and the substrate is the proper preparation of the surface of the samples. This is one of the most important technological factors in joint production stage (preparation and proper mixing of the components and the application of the coating to the steel substrate). The surface preparation of the steel sample consisted of grinding with P80 sandpaper and careful degreasing. These factors determine the durability of the adhesive bond during vibration of the car body of a motor vehicle. The tests were carried out in a room with constant atmospheric conditions during the experiment. An ultrasonic flaw detector and two transducers generating surface wave were used. The research was carried out using the transmission technique.

In an experiment to control the lifetime period of the joint between the putty coating and the steel substrate by ultrasonic longitudinal wave, a universal putty was used. A coating layer 2 mm thick, 28 mm long and 28 mm wide was applied to the center of a 520 mm × 28 mm steel strip made of ZStE260 steel. The material is used in the construction of body components of one of the world’s automotive vehicle manufacturers.

Ultrasonic testing was carried out using a UMT 12 digital flaw detector, which was built as a card into a PC. This device allows ultrasonic measurements not only in the time domain, but also in the frequency domain. Two heads with a frequency of 2 MHz and the designation INCO 2S7 were used to generate as well as receive the surface wave. The experiment was carried out with identical apparatus settings. Adequate and constant coupling of the ultrasonic heads to the substrate was ensured by using neodymium magnets.

The study of monitoring the lifetime of the adhesive joint the type putty coating – body sheet was carried out at the following settings of the UMT 12 flaw detector, which are invariable, determined experimentally, taking into account the capabilities of the available research and measurement equipment:

Uniform scheme for attaching the ultrasonic transducers to the samples is shown in Figure 2.

Figure 2

Scheme for conducting studies of the constitution and degradation stage of the adhesive bond between coating and substrate by ultrasonic surface wave; 1 – transmitting head, 2 – receiving head, 3 – substrate, 4 – coating and 5 – ultrasonic-surface wave.

The research was conducted using the transmission technique. Two 2 MHz ultrasonic surface wave heads were used, with one working as a transmitter and the other as a receiver. The transmitting head produces a voltage pulse with a value of 50–300 V and a duration shaped at 120–600 ns. The generated pulse causes vibration of the transducer made of piezoelectric material. The use of gel as an acoustic coupling causes the generated vibrations in the ultrasonic head in the form of a wave to be transmitted to the material under test. Wave propagate along the sample, encountering the coating on their way, and are then received in the receiving head. The wave arriving at the receiving head puts the transducer into vibration, which produces an electrical signal with a voltage corresponding to the acoustic pressure. This ultrasonic surface wave is imaged on the screen of the ultrasonic flaw detector (an example of a wave pulse is shown in Figure 3).

Figure 3

Example of an ultrasonic surface wave pulse.

In addition, the UMT 12 flaw detector allows testing in the frequency domain by determining the amplitude–frequency spectrum (Figure 4). The device uses a fast Fourier transform to give the values of the spectrum’s resonant frequency and its bandwidth.

Figure 4

View of the Fourier transform obtained from the pulse of a wave propagating along the boundary of the coating–substrate joint.

Ultrasonic measurements were carried out in two main stages: during the consolidation of the coating’s connection, and then during its degradation. The first period required an increase in the number of measurements, as it is quite dynamic. Therefore, the recording of selected ultrasonic parameters was made every 30 s for 1 h (this time is twice as long as the setting time of the coating with the substrate, which is given by the putty manufacturer). When the combination of the coating with the substrate settles the required properties, primarily related to its durability, the changes at the joint boundary are not as dynamic as during the consolidation stage. Therefore, in the second stage of testing (joint degradation), the recording of ultrasonic surface wave parameters was made every 20 min.

The bench tests for the degradation of the joint between the putty coating and the steel substrate were carried out at an inductor supply voltage of 5 V and a frequency of 20 Hz, which corresponds to an amplitude of vibration acceleration of 40 m/s². Taking into account the above, it should be concluded that the degradation process on the test bench has an accelerated character. The amplitude of vibration acceleration is more than 1,000 times greater than in real conditions. Likewise, the amplitude of deformation of the sheet metal with the applied coating, which was for the set parameters on the test bench of 2 mm, was more than 1,100 times greater than the amplitude occurring in real conditions when a motor vehicle overcomes bumps on the road [22].

3 Result of research

The study of the lifetime of the adhesive bond between the putty coating and the steel substrate by ultrasonic surface wave was conducted from the moment the coating was applied to the surface of the strip until it was completely detached from the substrate. Throughout this period, three selected ultrasonic parameters were recorded, such as the ultrasonic pulse gain, the resonant frequency and the bandwidth. The results are presented graphically as changes in the above parameters as a function of time. In addition to the entire period of existence shown as changes in the ultrasonic surface wave gain, the period of consolidation and final degradation of the joint were also depicted in graphical form for better understanding. Example results of the existence period of the coating–substrate connection are shown in Figures 5–9.

Figure 5

Ultrasonic wave gain waveform during the process of constitution and degradation of the coating–substrate interface: (a) sample 1, (b) sample 2 and (c) sample 3; I – constitution process, II – stable degradation and III – rapid degradation.

Figure 6

Course of ultrasonic wave gain during the process of constitution of the coating–substrate interface – the period of the first 60 min after application of the coating to the steel substrate: (a) sample 1, (b) sample 2 and (c) sample 3.

Figure 7

Course of ultrasonic wave gain during the degradation process of the coating–substrate joint – the last period until the coating completely detaches from the substrate: (a) sample 1, (b) sample 2 and (c) sample 3.

Figure 8

Frequency f of the ultrasonic wave pulse during the process of constitution and degradation of the coating–substrate joint: (a) sample 1, (b) sample 2 and (c) sample 3.

Figure 9

Bandwidth of the ultrasonic wave during the process of constitution and degradation of the coating–substrate joint: (a) sample 1, (b) sample 2 and (c) sample 3.

Figure 5 illustrates the overall course of changes in the gain of the surface ultrasonic wave pulse. Figures 6 and 7 provide an approximation of the period of constitution (formation of the bond – bonding of the coating to the substrate) and the last period of the life of the joint until the complete detachment of the coating to the substrate occurs. These charts show the changes in the ultrasonic wave pulse gain. During the initial period (production), the pulse gain increases, while the opposite trend is observed during the degradation period. The other two graphs (Figures 8 and 9) show the changes in resonant frequency and bandwidth for the entire period of the coating–substrate bonding intimacy. For these ultrasonic wave parameters, one can see spikes in the values, especially in the period before the last lifetime of the coating–substrate joint.

4 Analysis of research result

Analyzing the changes in the gain of the ultrasonic surface wave, it was noted that the changes in this parameter occur in three main stages. In the first stage, an increase in the gain of the ultrasonic wave pulse is noticeable. The first measurement, taken as a reference measurement, was made before the coating was applied to the steel substrate. Depending on the sample, the pulse gain reached a value of 43.75–48 dB. Thereafter, in each of the cases, the gain increased. This phenomenon was associated with the application of the coating to the samples and its subsequent bonding to the steel substrate.

The ultrasonic surface wave propagating along the specimens encountered the putty coating, which caused attenuation of the signal imaged on the flaw detector screen. In order to make the height of the echo obtained on the screen after application of the coating identical to that before application (reference measurement), the signal gain had to be increased.

The pulse gain increased as the coating bonded to the substrate. This phenomenon was observed for about 60 min, rallying through the first of the stages in which continuous monitoring of the state of the bonding by ultrasonic surface wave was carried out. The increase in pulse gain ranged from 1.5 to 4.75 dB from the initial value, depending on the sample.

Also, small changes in pulse gain during the first degradation period, from about 60 to 600 min after the start of the experiment (for two samples) are noticeable. The changes in gain may be indicative of the bonding process of the coating with the substrate, which occurred for longer than declared by the putty manufacturer, that is from about 20 to 30 min.

After the first period of increase in ultrasound pulse gain, a period of stabilization is noticeable. It is longer than the first considered constitution period and amounts, depending on the sample, to about 2,000 min. During this time, small changes in the observed and recorded parameter are noticeable, which can be related to the accuracy of the measuring apparatus.

The stabilization period ends with a rather sharp drop in the ultrasonic pulse gain, which marks the beginning of the final period of joint degradation, until the putty coating is completely detached from the substrate. After the coating detached from the substrate, the gain values were at a similar level as before the coating was applied.

Summarizing the courses of changes in the gain of the ultrasonic surface wave propagating along the boundary of the adhesive joint, it should be said that there is a general trend of an increase in this parameter in the first period of the experiment (joint consolidation), followed by its stabilization and decrease in the final stage of degradation of the coating–substrate joint.

In addition to time-domain studies, monitoring the state of the connection was also carried out on the basis of changes in the parameters of the amplitude–frequency spectrum. The changes in the resonant frequency f are characterized by a trend, that is, for a certain period this parameter is invariant (this applies to both the consolidation period and partially the period of degradation), then its value decreases, and after this decrease it remains constant until the coating detaches from the substrate. For all the samples, the resonant frequency reaches a value of 1.76 MHz. The only change is in its drop to a value of 1.69 MHz during the degradation process. This change occurs, for all samples, earlier than the changes in pulse gain. The change in the considered spectral parameter may indicate the increasing phenomenon of the coating’s detachment from the substrate, which is then suggested by changes in the gain of the ultrasonic wave.

In the second of the studied parameters of the amplitude–frequency spectrum, the bandwidth, b, is characterized by a trend of changes similar to the resonant frequency at the end of degradation stage. The changes occurring in the values of this parameter, decrease similarly to the resonant frequency f.

Changes in amplitude–frequency spectrum parameters can indicate the beginning of the final stage of degradation of the coating–substrate connection, and therefore can be considered as a diagnostic symptom.

A view of the surface of the sample and the coating after the coating has been detached from the substrate is shown in Figure 10.

Figure 10

View of the surface of the sample and coating after the test – coating detached from the substrate, the red rectangle marks the place from which the coating has detached from the substrate.

In the first stage of the lifetime of the connection between the coating and the substrate, there is an increase in the gain of the ultrasonic wave pulse, caused by the binding of the coating to the substrate. The better the adhesion of the coating to the substrate, the greater the gain of the wave pulse because more wave energy will penetrate into the coating material. Next, the stabilization period of the ultrasonic wave pulse gain is visible. At this stage, the adhesion of the coating is unchanged. The last stage – rapid degradation leading to the separation of the coating from the substrate is characterized by a decrease in the gain of the ultrasonic wave pulse. In addition, for the tested joint shortly before the onset of this period there are changes in the amplitude–frequency parameters. This is the information about the approaching final life of the joint, which is associated with its rapid degradation.

A similar course of ultrasound wave parameters in his study was obtained by Manczak and Josko [23]. The degradation process of the coating–substrate boundary was studied, demonstrating the relationship between the longitudinal and surface waveforms and changes at the adhesive joint. Watanabe et al. [21] used a pulse laser-induced ultrasonic wave for evaluation of the adhesion quality. The results of these examinations are visualized through the use of laser scanning, but these examinations are not continuous (only spot and at a given moment). The method proposed in the article allows continuous monitoring of the entire joint boundary and not just parts of it, which is a definite advantage and gives more diagnostic information about the state of the joint. Online monitoring of process was performed by Rieder et al. [24] for evaluation of the AM process. The authors confirmed the feasibility of monitoring the manufacturing process of simple shaped parts using ultrasonic methods. The sample was produced by melting a layer of powder with a laser beam. It was also found that the critical element of the measurement system affecting the measurement results was the acoustic coupling between the head and the sample fabrication site. The author’s own research presented in this article was conducted on an adhesive joint, and the problem of acoustic coupling stability was solved by bonding the heads with a thin layer of adhesive to a steel sheet. Ultrasonic method was used for evaluation of the additively deposited Stellite 21 coatings on the Inconel 718 substrate [25]. The authors used speed and amplitude of surface wave which was correlated with cracking and warpage of the coating. In ref. [26] the authors studied the possibility of using bottom echo back reflection for evaluation of thermal sprayed coating. It was concluded that the degree of coating wear can be effectively measured by observing the changes in the amplitude of ultrasonic wave echo reflected from the bottom surface of a sample. Wang and coworkers [27] used the ultrasound to measure the adhesion between the epoxy coating and aluminum substrate. The authors used a longitudinal wave and the quality of the joint was checked point-wise based on the decrease in the amplitude of this wave.

5 Conclusions

On the basis of the study of monitoring the lifetime of the adhesive joint of putty coating – body sheet, the following conclusions can be made:

• Ultrasonic surface waves allow monitoring of the adhesive bond between the putty coating and the body sheet at any time during the lifetime of the bond.

• The waveforms of changes in the ultrasonic pulse gain have a reproducible character and are characterized by an initial increase of this parameter (during consolidation of the coating to the substrate), stabilization, and a decrease at the last stage of the joint existence, before the coating is completely detached from the substrate.

• The value of the resonant frequency changes over the experiment, and the nature of these changes is reproducible and may indicate the beginning of the final period of degradation, as confirmed by subsequent changes in the gain of the ultrasonic wave.

• The values of the parameters of the amplitude–frequency spectrum, especially the resonant frequency, can serve as additional information suggesting the formation of changes at the joint between the coating and the substrate.

• Sudden changes in the values of the studied parameters in the final period of degradation may prove the validity of the theory of accumulation of submicro- and micro-dissolution, which leads at a rapid rate to the detachment of the coating from the substrate.

The author plans further work on the study of monitoring the condition of adhesion bonds, especially monitoring the adhesion of the coating to the substrate during the lifetime of the coating–substrate joint. In the case of continuous surface wave monitoring of the state of the coating–substrate interface, the need is to fix the ultrasonic heads permanently to the surface of the sample where there is no applied coating. This limitation can be lifted by using a single longitudinal wave head. Ultrasonic adhesion testing can be performed using the longitudinal wave and determined reflection coefficient |r|. The smaller the value of this parameter, the better the adhesion of the coating to the substrate. Therefore, the author plans to use this parameter based on the ultrasonic wave to determine the changes in adhesion throughout the lifetime of the coating to the substrate, especially to check the changes during the joint constitution period and the final degradation period. Another limitation that can make it difficult to monitor the condition of the coating–substrate boundary is the thickness of the sheet metal on which the coating is applied. The thinner the sheet metal, the higher the frequency of the ultrasonic head to be used. If the frequency of the head is too low then the wave will not be generated in the substrate material making it impossible to monitor the state of the joint.