Thickness measurement of non-conductive coatings Introduction

The thickness of non-conducting coatings on conductors is measured by measuring the amount of lift-off the coatings produce. The technique can be applied equally to ferromagnetic or non-ferromagnetic materials, and can also be used to measure the thickness of nonconductors like plastic sheet, by laying them on top of a flat conductor.

Most modern digital eddy current instruments when used in conductivity mode, provide a digital readout of both conductivity and lift off (non-conductive coating thickness).

Comparative measurements can also be made using the impedance plane and are summarised as follows.

Probe and frequency selection.

Any standard absolute or absolute reflection surface testing probe may be used, but to obtain stable and repeatable signals, a spot or spring loaded probe should be used. If a pencil probe is to be used, it should be fitted with a sleeve or shoe so that probe wobble is eliminated (see FIG. 5.22a). If curved surfaces are to be tested, a spring loaded probe in the form of a V block, or a V-block or specially contoured shoe should be used for the same reason (see FIG. 5.22b).

(a) (b)

FIG. 5.22. Methods of obtaining stable and repeatable readings of coating thickness (a) Using a pencil probe with a probe shoe. The probe shoe can be contoured to fit a curved surface. (b) Using a spring loaded probe with V-notches on a curved surface. Convex surfaces can also be tested.

The preferred type of probe is an absolute reflection probe, preferably spring loaded, and with V-notches if curved surfaces are to be tested. Reflection probes are preferred because they show a more linear lift-off curve and they respond to a greater range of lift-off values. Note that differential probes should not be used because they eliminate or reduce the lift-off signal.

The coil diameter is important because the size of the magnetic field increases as the coil diameter increases. The magnetic field from a coil at a distance of one coil diameter from the

coil is only approximately 10% of that adjacent to the coil, and a very low eddy current intensity is produced at this distance. The magnetic fields of small diameter coils may not extend through a thick coating to the substrate. If this is the case, a lift-off signal cannot be obtained and measurement of coating thickness is not possible. In general, therefore, the thicker the coating to be measured, the larger the coil diameter should be. Note that the size of a probe is not necessarily a good guide to coil size. Some spot probes have a small diameter coil in a large diameter housing.

Non-conducting coating thickness measurement may be carried out over a wide range of frequencies, but it is preferable to select a test frequency which give an operating point on the lower portion of the impedance diagram. There are two reasons for this.

(a) The signal for a given amount of lift-off is greater on the lower portion of the impedance curve. FIG. 5.20 shows this effect: for a PC value of 300 (towards the bottom of the curve), the lift-off signal for a lift-off of 5% of the coil diameter (the curve labelled 0.1) is approximately 50% greater than for a PC value of 10 (at the knee of the curve) and approximately 4 times greater than for a PC value of 2 (towards the top of the curve).

(b) When operating towards the bottom of the impedance curve, the change in signal for a given change in conductivity of the substrate is quite small (see FIG. 5.19), so that the effect of minor variations in conductivity of the substrate is reduced. In addition, in the lower portion of the impedance curve, the signals for lift-off and change in conductivity are reasonably close to 90⁰, so that any minor change in the substrate conductivity will have little effect on the lift-off signal For the upper part of the impedance curve, the signals for lift-off and change in conductivity are separated by only a small angle, making them hard to distinguish.

A further consideration when selecting frequency is the depth of penetration of the eddy currents in the substrate. It is preferable to ensure that the effective depth of penetration of the eddy currents is significantly less than the thickness of the substrate, so that variations in conductivity materials therefore require higher frequencies to bring the operating point to a similar location as for high conductivity materials. Generally, as the thickness required to be measured increases, the coil diameter should be increased, as stated above. This moves the operating point further down the impedance curve, and therefore lower frequencies may be used. There is no difficulty with this because, in general, larger diameter coils are designed to operate at lower frequencies. In fact, lower frequencies are recommended for thicker coatings in order to avoid excessive sensitivity to lift off and to obtain a more linear response. For example, for a thickness of 1 mm on an aluminium alloy substrate, a frequency of approximately 80 kHz should be used.

Reference samples

The substrate reference sample for non-conducting coating thickness measurement should have the same conductivity, surface contour and thickness as the material to be tested.

For reference samples for the coating, stiff plastic shims of appropriate thicknesses are metal may be used for calibration. More shim thicknesses may be required if the response is notably nonlinear.

Measurement procedure

(a) Connect the probe to be used to the instrument, switch on the test instrument and allow it to warm up for at least 5 minutes.

(b) Ensure the test area is clean and free from dirt or other contaminants.

(c) Select the frequency required, and set the gain to the middle of its range.

(d) Apply the probe to the reference sample, either bare, or with the thinnest shim placed on it, balance the instrument, and adjust the phase so that the lift-off signal is horizontal to the left.

(e) Adjust the spot to a location on the horizontal centre line near the right hand side of the screen.

(f) Apply the probe to the reference sample with the thickest shim placed on it and, without rebalancing, check whether a signal is displayed. If visible, the spot should lie to the left previously obtained signal. Adjust the gain so that the spot is located towards the left hand edge of the screen. If the spot is not visible, first reduce the gain until it becomes visible, then further adjust the gain to locate the spot towards the left hand edge of the screen.

(g) For some instruments, changing the gain changes the balance point, so steps (d), (e), and (f) should be repeated to ensure that the spot is located on the horizontal centre line, towards the right hand edge of the screen with the probe on the thinnest shim or bare metal, and towards the left hand edge of the screen with the probe on the thickest shim.

(h) The position of the spot when the probe is located on the thickest shim can be marked.

(i) Locate the probe on the other shims in turn, marking the position of the spot.

(j) If measurement of the thickness is required, draw up a graph of the horizontal position of the spot, measured in millimetre or number of scale divisions from one edge of the screen, versus the coating thickness. If the graph is sharply curved, different frequencies should be tried to determine if a more linear response can be achieved. Alternatively, use more reference shims so that an accurate smooth curve can be plotted. Once a

satisfactory graph has been obtained, carry out thickness measurement by applying the probe to the test part, measuring the horizontal position of the spot from the edge of the screen, then using the graph to determine the thickness.

(k) Alternatively, if the measurement is being carried out to verify whether the thickness lies between acceptable limits, move the spot vertically so that signals from the test parts do not overlay the reference shim signals, then apply the probe to the test part. The thickness is acceptable if the spot is located between the marks for the upper and lower thickness limits.

(l) If a large number of measurements are required to be measured, the settings should be checked at least every 30 minutes.

FIG. 5.23. The screen display for calibration to measure non-conducting coating thickness in the range 0 mm to 1 mm. The position of the spot is marked for each reference standard by using the Y position control to change the Y position then return it to its original position.

5.1.5 Thickness measurement of conductors