Layer thickness measurement
The layer thickness measurement deals with the determination of the layer thickness of coatings and metallic coatings on surfaces. The layer thickness is generally the thickness of a coating, e.g. B. paint or varnish that is firmly applied to a base material. This coating can be a single coating, a multiple coating (color on zinc on steel, chrome on nickel on brass, etc.) or a layer system with several polymer layers applied one on top of the other.
Procedure
Various methods are available for determining the layer thickness. At a higher level, these processes are divided into destructive and non-destructive, with preference given to non-destructive processes in industry. Destructive methods are used when the non-destructive measurement methods reach their technical limits, for example when measuring non-ferrous metal layers on non-ferrous base material.
The following methods of layer thickness measurement are common:
- Eddy current testing (coating on a metallic substrate)
- Magnetic inductive measurement
- Ultrasonic testing
- Capacity measurement (insulating layers, capacitive sensor )
- Optical measuring methods (transparent layers, interference , triangulation , confocal technology )
- Radiation and absorption ( X-rays , other types of radiation, e.g. in film and paper production)
- Determination of mass (mass per area or simultaneous measurement during layer deposition with coated quartz crystal , e.g. during vapor deposition , sputtering )
- Measurement based on the radiation from radioisotopes contained in the layer (e.g. asphalt thickness measurement)
- Vertical microscopic measurement by shifting the focal plane , through transparent materials or at the material edge, in particular with confocal technology in profilometers . For measurements through the material, the refractive index must be known and correction factors for the material thickness based on the optical distortions that occur must be determined.
- Lateral microscopic measurement in cross section or cross section
- Microscopic measurement of the micrograph made with a spherical grinder
Which measurement method is suitable for which application is determined by the materials used for the coating and base material. The common methods for measuring layer thicknesses on metallic substrates are the magneto-inductive method for measuring non-metallic coatings on ferromagnetic substrates and the eddy current method for measuring insulating layers on non-ferrous metals, while the ultrasonic method for measuring non-ferrous metals. metallic base material as well as the measurement of multi-layer systems is used.
history
From a historical point of view, the first layer thickness measuring device was developed in Germany in 1952 for measuring paint thickness on steel substrates. It was a mechanical device that uses the magnetic adhesive force to determine the layer thickness. A measuring magnet is placed on the surface and attracted to the magnetic base material. With the help of a spring, the magnet is now pulled away from the surface. This spring is connected to a scale, on which the force that has to be applied to detach the magnet from the surface is displayed as a layer thickness value.
In the decades that followed, the technology of coating thickness measurement was continuously developed. Other methods such as magnetic induction, eddy current and optical methods were added. The devices developed from analog to digital display devices and, in the next step, to digital sensors. With these, the entire measurement value recording and further processing takes place in the electronics of the sensor and only the already processed measurement value is transmitted to the device itself for display, storage and statistical processing.
The current state of the art enables wireless coating thickness measurement, in which a sensor carries out the entire measuring process and then transmits the measured value by radio to a measuring device or alternative device (tablet, smartphone).
literature
- Nasser Kanani: Modern measuring and testing methods for metallic and other inorganic coatings . Expert Verlag, Renningen 2007, ISBN 978-3-8169-2653-5 .
Web links
- Probes for coating thickness measurement (accessed on February 11, 2016)
- Vapor deposition in a high vacuum and layer thickness measurement (accessed on February 11, 2016)
- Layer thickness measurement on printed polymer layer systems (accessed on February 11, 2016)