Investigation of the structure function for analysis of optically measured surface data (Structure Function II)

• Applicant: Professor Dr. Ralf Bernhard Bergmann
• Subject Area: Measurement Systems
• Term: since 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 288038549

To characterize surfaces, parameters such as the roughness numbers Ra or Sa are conventionally used. However, these previously used parameters often do not describe the surface properties required for the desired applications. With the structure function, we introduce a new analysis method for surface metrology. However, the informative value of the structure function with respect to the nature of a surface is much greater than that of the above-mentioned characteristics, since all height differences are evaluated for all possible distances within the data set in all directions. The structure function can process large volumes of data, which typically occur in optical measurement techniques, efficiently. The structure function also has several advantages over other statistical methods, such as e.g. the autocorrelation function. In contrast, the structure function is much more practicable. The greater sensitivity for comparative roughness on surfaces has proven to be one of the most important advantages. It is intuitively easier to evaluate and, compared to Fourier-based methods, is much more robust compared to variations in surface geometry. It can be applied to almost any aperture or geometry of the surface and allows the comparison of data from different measurement methods as well.In the previous project, correlations between the structure function and roughness as well as ripples could be shown with corresponding simulations. On diamond-turned surfaces, it has been shown experimentally that roughness differences of a few nanometers on profiles or entire surfaces can be quickly and easily determined by comparing structural function evaluations. Since the structure function is suitable for the characterization of very small roughness, it can also be used for the precise assessment of the wear of tools (stamping, forming, deep-drawing tools) of almost any kind, to name only one example of use.In this continuation proposal, the derivation of reliable parameters (descriptors) from the structure function is to be investigated theoretically and experimentally. As indicated above, conventional parameters such as roughness parameters Ra, Rq or Rz from profile data or Sa or Sq from surface data are not necessarily meaningful parameters for quality assurance. In the project, therefore, independent descriptors are to be derived from the structural function with which the usual surface characterization in the direction of new definitions can be meaningfully extended. For the characterization of waviness, a study of aperiodic or anisotropic distributions is required. Finally, it must be determined with which lower limit surface structures such as roughness or waviness can be absolutely characterized. Another goal is to prove that the structure function is also suitable for damage indication for quality control.

• DFG Programme: Research Grants