Nondestructive and contactless determination of layer and coating thickness

ABSTRACT The paper describes some new developments in the field of contactless and nondestructive determination of the layer and coating thickness on different substrates. 1) The first method is based on millimeterwaves which are emitted and re-ceived by a compact integrated radar sens or. 2) Another approach is based on thermal effects induced in the coating by flash heating or laser light heating. The temperature response of the test object is monitored by an infrared sensor. 3) With the help of an airborne ultrasonic transducer the thickness of powder coating, e.g. ceram ics-coating on steel, can be determined contactlessly with an accuracy of better than 5 µm. Keywords: contactless, layer thickness, millimeterwaves, thermography, airborne ultrasound 1. RADAR-LASER TECHNIQUE Normally pipeline tubes made of steel are wrapped by a polyethylene (PE) anti-corrosion layer whose thickness is typi-cally 3 to 5 mm. The entire PE layer generated by an extruder is applied in three layers on the tube while the tube is rotating with a circumference speed of up to 1 m/s. The feed speed parallel to the pipe axis is in the order of magnitude of 0.1 m/s. The industrial producer is searching a method to online monitor the layer thickness. This is important espe-cially at the weld seam since here the layer thickness is normally smaller than at the rest of the tube. A minimum thick-ness must be assured. The accuracy of the existing combined eddy current-laser-method is often not high enough. More-over this system is no more produced. The existing gamma-ray-method requires thorough screening and is often not accurate enough. Therefore an alterna tive combined millimeterwav e-laser-method is now being developed and tested. 1.1. Buildup of the measuring system One part of the system consists in a microwave sensor which works in the millimeterwave range. It is based on a fre-quency m odulated continuous w aves (FMCW) radar sensor with a center frequency of about 94 GHz (W-band). The sensor is supplied by the Fraunhofer-Institute for Applied Solid State Physics (IAF), Freiburg, Germany /Tessmann, 2002/. The heart of the sensor is a single chip MMIC ( m onolithic m icrowave integrated circuit) with a size of about 3 mm * 2 mm which contains all high frequency components (Fig. 1a and b). The microwaves are generated by a voltage controlled oscillator (VCO) whose frequency is swept over a bandwidth of about 5 GHz (Fig. 2) with the help of a function generator. The sensor works in monostatic mode, i.e. with only one antenna for microwave emission and reception. Inside the se nsor the emitted and received microwaves are separated by a Wilkinson directivity coupler. One part of the microwaves generated by the oscillator and the microwaves scattered by the object and received by the antenna are mixed in order to produce a sinus-like intermediate frequency (IF) signal. Because of the frequency sweep the frequency of the IF-signa l is the difference of the frequencies of the emitted and received signal. Its amplitude and phase depend on target properties, e.g. on layer thickness.