3D roughness standard for performance verification of topography instruments for additively-manufactured surface inspection

The unique complex topography of additively-manufactured surfaces-and the recent rapid evolution of instruments and techniques to measure them-limits trust in inspection data, in direct conflict with the requirements of application areas such as aerospace, where failure of a part can have serious safety consequences. Topography instrument manufacturers and end-users require measurement standards with a controlled reproduction of representative additively-manufactured surfaces to calibrate, performance-verify and intercompare instruments for inspection tasks, thereby improving confidence in measurement. The design of such a surface texture measurement standard is reported, optimised for optical areal topography-measuring instruments and compatible with x-ray computed tomography instruments. Machined from an additively-manufactured blank, the standard's four sides represent increasing levels of post-processing from the as-deposited surface. Datum features on the measurement standard facilitate direct comparison between topography instruments. Integrated step features support the calibration of an instrument's Z scale. Calibration of a prototype of the standard is reported; the prototype is matched to a typical aerospace Scalmalloy (TM) additive manufacturing process. The calibration is also a trial application of a large-area chromatic confocal microscope; this instrument appears capable of calibrating relatively rough measurement standards if required to do so. Reference Ra and S-parameter measurements and associated measurement uncertainties are presented, correlation between parameters noted, and choice and consequences of filter settings discussed.