·Table of Contents
·Methods and Instrumentation
High Precision Coating Measurement with UltrasoundAuthor: Johannes Büchler, KRAUTKRÄMER GmbH & Co. oHG, D - Hürth
Co-Author: Thorsten Pagel, KRAUTKRÄMER GmbH & Co. oHG, D - Hürth
The coating thickness measurement instrument CTM 20 (Fig. 1) combines the latest technologies for the ultrasonic probe and the instrument to solve all these limitations. A broadband PVDF probe combined with an instrument using digital signal processing on a DSP, increase measurement resolution. Overlapped reflection echoes coming from very thin coatings can be separated and will be displayed as digital values. The measurement range of the system is from 10 µm to 500 µm with a resolution of 1 µm. On a multi-layer system, up to three individual coatings including the total coating thickness can be measured in one pass. The new instrument includes a high precision measuring mode for wall thickness through coating(s) with a range of 100 µm to 8 mm for metals and 200 µm to 3 mm for plastics with a resolution of 1 µm.
|Fig 1: CTM 20|
The CTM 20 allows quick and easy measurements of coatings on many different substrates, such as plastic, wood, ceramic, glass or metal which are not possible up until now. A wide range of applications in the automotive, wood and plastic industries can now be solved. The performance of the instrument can be clearly seen in different applications when measuring single- and multi-layer coatings.
In Fig. 2 reflection signals from a polystyrene coating layer using a 15 MHz schock-wave probe are shown and it is possible for traditional thickness measurements to be made down to approximately 200 µm. The wavelength of a 15 MHz signal in plastic is about 150 µm. So due to the wavelength limitations, the echoes of this probe are superimposed on each other and the time of flight difference of the echoes can not be determined.
|Fig 2: Signals with 15 MHz schock wave probe|
Polyvinylidenefluoride (PVDF) is used as piezoelectric material which has broadband frequency characteristics. The disadvantage of less sensitivity of this material compared to ceramics is compensated by good matching of the acoustic impedance to plastics. Therefore signal losses from the interface probe/coating are reduced as both have similar impedances. The newly developed probe CTF 1 generates shorter echo signals allowing a considerable improvement in thickness measurements (Fig. 3).
|Fig 3: Signals with new PVDF probe CTF 1|
As the generated ultrasonic pulse propagates through the material it is influenced and then received by the probe. A simplified block diagram of the system including the test object shows the main signals (Fig. 4).
|Fig 4: Block diagram of system|
The transfer function of the test material - here the coating including couplant - is represented by the block h(t) (pulse response). The measured echoes y(t) can be described as the convolution of the input signal x(t) and the transfer function h(t). The input signal x(t) can be measured when h(t) is shorted, that means the probe is not coupled to the test material and the interface echo probe/air is stored. When x(t) is stored and y(t) is measured h(t) can be calculated, this is called deconvolution. Deconvolution can be made in the time domain or by using the Fourier transformed signals in the frequency domain [4, 5, 6, 7, 8, 9].
|Z = r * c||(1)|
The reflection R of one echo at the interface Medium 1 to Medium 2 is given by :
Therefore the amplitude of the echo sequence on the interfaces can be calculated due to multiple reflections. Based on the described principle, the reflection model allows to simulate multilayer (L) systems, when the impedance and the thickness (d) of the material (Mat; PS = substrate, PX = coatings) is given (table in Fig. 5). With this model it can be verified if coating measurements are able to be done, when the difference of the impedance is small.
In the example (Fig. 5) the theoretical reflection model of a three layer system on a plastic substrate including the couplant is shown and in Fig. 6 the real measurement is given.
|Fig 6: A-scan of three-layer coating system on plastic and cross section|
6.1 High precision through coating measurement
The wall thickness mode enables the customer to measure the thickness without removing the coating. The measuring range for metallic materials goes down to 100 µm with a resolution of 1 µm, which opens up new applications for ultrasonic based systems (i.e. coated thin metal foils).
6.2 Three layer coating system on plastic substrate
A typical example from the automotive industry is shown in Fig. 6. The A-scan of this three layer system cannot be visually interpreted any more, as the echoes from the three interfaces are superimposed. But using signal processing the position of all echoes can be calculated, even the 2 µm thick couplant (tk) can be measured. The three calculated coating thicknesses of 34 µm (s1), 17 µm (s2) and 20 µm (s3) are confirmed in the cross section picture.
6.3 Glaze of a ceramic tile
The thickness of the glaze of a ceramic tile in Fig. 7 is 272 µm (s1).
|Fig 7: A-sacn of a ceramic tile and cross section|
6.4 Thickness of the adhesive layer on insulation tape
The thickness of the adhesive layer on insulation tape could be easily measured by taping it to a smooth surface (Fig. 8). The thickness of the insulation tape is 108 µm (s1) and for the adhesive layer it is 26 µm (s2).
|Fig 8: A-scan of adhesive tape on plastic substrate|
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