Smart

Completed Technical Projects - "LASERMON"

CATechnology won a DTI SMART Award to carry out a study on photo-thermal waves generated by solid state lasers within coatings. The project resulted in a working prototype of a non-contacting thickness gauge for coatings.

This project was aimed at providing, for the first time, a non-contacting instrument for measuring the thickness of all types of coating (metals, ceramics, polymer) on all types of substrate (metal, ceramic, polymer). The lack of such an instrument has meant that industry has had to run expensive, destructive test and sampling procedures or run the risk of consequential failure from under/over coated parts.

Figure 1. Schematic of the Photo-thermal System

A scematic of the system is shown in figure 1 above. A modulated solid state laser was used to generate a thermal wave within the coated sample. Using an infra red detector conected to a lock-in amplifier it is possible to measure the phase difference between this and the incident laser source. This phase difference is directly related to the coating thickness. detecting the infra red surafce emissions refleat the coating surface and monitors the phase of a thermal wave propergated through the coating and this phase is propotional to the coating thickness.

Now, for sprayed coatings, there is no absolute definition of what is meant by "thickness"and this needs to be clarifed. A cross-section through a typical thermal sprayed coating is shown in figure 2 and it is clear that coating thickness is not clear cut. Under the microscope, measurement could be made at the maximum, minimum or an estimated mean between the two red lines.

Figure 2. Cross-section Through Coating

Alternatively, micrometers could be used but again the measurement will be dependant upon type of instrument eg ball or anvil. The graph in figure 3 below shows measurements made on four samples using the different methods and, as can be seen, the thickness can vary by a facter of 2 or even 3! However, since the measurement results using either an anvil micrometer and the maximum thickness as observed under the microscope are in close agreement this will be used for comparisson purposes.

Figure 3. Comparisson between different measurement methods

The graphs in figure 4 ad 5 plot the measured phase angle for a range of coating typess and thicknesses. As can be seen there is a good correllation between phase and thickness and these curves can thus be used as a calibration for the measurement of "unknown" samples. The graph below shows the results for a number of coatings.

Figure 4. Phase angle vs Coating Thickness for MCrAlY (a) VPS (blue) and (b) APS (red)

Figure 5. Phase angle vs Coating Thickness for (a) Zirconia (8%PYSZ) (blue) and (b) Platinum (red)

Summary

A system has been developed which can provide non-contact thickness measurements over a range of coating compositions including metals and ceramics. A further patent has been granted and work is continuing.

For more information contact Peter Chandler: peterchandler@btinternet.com

CA Technology Ltd - call us on +44 1794 323126