Terahertz: A revolution in thickness measurement
Automotive paint has to withstand a lot: stone-chips, frost or heat. This is why the coatings have become so elaborate. Today at least four different layers are applied to car bodies. However, many car parts are made of plastic, and until now, the paint in these areas could only be tested using destructive methods. With its terahertz technology, Fischer finally makes it possible to measure multiple layers of paint on either metal or plastic substrates – non-destructively.
To ensure that a coating will properly fulfill its function, it must be tested carefully. Only then can an automotive manufacturer, for example, guarantee that the paint finish will last for the entire life of the car while only expending as much material as really necessary. Modern quality assurance is therefore faced with the task of precisely measuring multiple layers of paint.
Typical auto paint finishes consist of several insulating layers, which as yet could only be measured individually using destructive methods
In addition to metal, however, other substrates are also used: from your toaster to the medical devices found in operating rooms, many plastic parts are coated with multiplex paints. To date, it has only been possible to analyze such finishes destroying them with a wedge cut.
But Fischer is developing a technology to finally close this gap. For the first time ever, terahertz (THz) radiation will make it possible to inspect multi-layer coating systems without damaging them – even when they are on plastic substrates. This method is called terahertz TDS (Time Domain Spectroscopy).
Measuring with terahertz – how does it work?
The terahertz radiation in use here is made up of electromagnetic waves with frequencies ranging between 100 GHz and 4 THz. That means: shorter than microwaves but longer than light waves. Only recently efficient emitters and detectors for this frequency range have become available.
The special feature of THz radiation is that it penetrates many kinds of paints and coatings. Fischer harnesses this transparency to measure the thickness of the layers. Similar to measuring thickness with ultrasound, the THz radiation is partially reflected at the transitions between layers: the "echoes" arrive at the detector with a slight delay. From this, the Fischer software can determine the thickness of up to four layers – the measurement itself takes only a second.
The terahertz pulse penetrates the coating and is reflected at the material boundaries. From the delay time of the various reflections, the software calculates the thickness of each layer
How does terahertz compare?
Even now, though still in development, the accuracy of the terahertz method is impressive: on a measuring spot of less than 2 mm, it is possible to measure layer thicknesses of 5-10 μm. In contrast to magnetic induction, which has a similar resolution, terahertz offers 10 times better repeatability, or 1‰.
In measuring a wedge cut, terahertz compares favorably to established methods such as microscopy and magnetic induction
Since the paint layers are transparent to the THz radiation, the rays do not influence the coating. This makes the measurement entirely non-destructive.
Terahertz is much less energy-intensive than X-radiation and therefore harmless. THz devices can thus be used in the open and require no radiation protection.
Unlike magnetic induction and ultrasound, terahertz is non-contact, so it is even possible to measure wet paint.
No calibration needed
While thickness measurement with ultrasound is based on a principle similar to terahertz, it requires complex calibration. Fischer's THz system, on the other hand, can be used on simple coatings without any calibration at all. All the requisite readings are taken in a single step.
Before analyzing multiplex paint finishes, each layer is measured individually. The device saves all the relevant values. Should the same paint be encountered again, there is no need to "re- calibrate" it.
Terahertz – method of the future
The first THz system is being designed specifically to measure multiple layers. Together with automakers and lacquer manufacturers, we are developing solutions for integrating this technology into production processes.
But that does not come close to exhausting the method’s potential! Many substances absorb THz radiation in ways that are characteristic for them alone. This opens up a wide variety of other possible applications in the future.