Miniaturization of an optical surface acoustic wave spectrometer for non-contact material testing (acronym: mOOS)

Funding programme: Programme to promote applied research and development at universities of applied sciences

The “mOOS” funding project aims to further develop an innovative non-destructive and non-contact measuring method for measuring and characterizing relevant near-surface material properties such as hardness gradients, coatings or microstructural changes from bulky and interference-sensitive laboratory structures to a robust, miniaturized and process-suitable online measuring device. The measurement method to be further developed is based on the non-contact excitation and detection of surface acoustic waves (SAW) on the material surface to be characterized via pulsed lasers (laser ultrasound). Due to the thermal expansion and contraction of the material caused by the laser beam, acoustic surface waves in a broadband frequency range are excited on the material surface. The different frequency components penetrate into the material at different depths, so that material information can be obtained over a large depth range. The measured parameter is the surface acoustic wave velocity, which correlates with a variety of material parameters such as material hardness, microstructure or coating properties.

The detection of the surface acoustic waves is also contact-free via an interferometer. Currently, the interferometer used in the ISAT laboratory is based on a two-beam mixture in a photorefractive crystal. The aim of the project is to replace the bulky tabletop structure with free-space optical components (laser source, lenses and interferometer with photorefractive crystal), which have to be adjusted very laboriously and are therefore not suitable for use in industrial environments, with a much more cost-effective compact, mechanically stable microoptical system. The suitability of this miniaturized measuring system for industrial use will then be demonstrated together with the company partners. The innovative development approach is based on a novel combination of surface acoustic waves with planar integrated optical waveguides, fiber optics and micromechanical assembly and connection technology.

Materialeigenschaften, die mit dem mOOS-Verfahren erfasst werden können:                 

  • Materialhärte
  • Härte-Tiefe-Gradienten
  • Eigenschaften der Mikrostruktur
  • Anisotropie
  • Risse
  • Bestimmung von viskoelastischen Eigenschaften von Polymeren, optisch nicht erkennbare Fehlstellen, Charakterisierung von Schichtsystemen z.B. Dicke und Eigenschaften von Beschichtungen (z.B. Polymer, Harteloxal)

Vorteile des mOOS-Verfahrens

  • Kompakter, robuster Aufbau
  • Kombiniert berührungslose, zerstörungsfreie und schnelle Messtechnik mit tomographie-ähnlichen Ultraschallverfahren
  • Härteprofil, Einschlüsse von Partikeln oder Gasblasen, Verspannungen bilden sich im Messsignal der Oberflächenwellen ab.
  • Änderungen der Ausbreitungseigenschaften der Oberflächenwellen sind kontaktfrei erfassbar
  • Bestimmung von viskoelastischen Eigenschaften von Polymeren, optisch nicht   erkennbare Fehlstellen, Charakterisierung von Schichtsystemen
  • Unempfindlichkeit gegenüber Speckle, rauen Oberflächen
  • Unempfindlichkeit gegenüber Erschütterungen
  • Keine spezielle Probenvorbereitung notwendig
  • Messdauer 1 s/Messstelle
  • Tiefenbereich, welcher erfasst werden kann: 50 µm bis 2 mm