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Surface topography represents a critical barrier to the advancement of additive manufacturing (AM). Because some internal features cannot be polished and because of the growing trend of in situ process monitoring, it is important to understand the as-built surface topography of AM components. Here we highlight the challenges of using industry-standard surface-measurement techniques on binder-jet-printed parts. We measured the topography of binder-jet-printed Inconel alloy 625 samples in their green state and over the course of sintering; this system allowed the investigation of identical starting materials undergoing systematic changes in topography. Specifically, we compared the results from industry-standard surface-measurement techniques—optical interferometry, 3D microscopy (by fringe projection), and stylus profilometry—against the “true topography,” as revealed by cross-sectional scanning electron microscopy. While the true topography changed significantly with sintering, the industry-standard techniques detected no change in the root-mean-square height because of complex surface features, including multi-scale topography, overhangs, and steep surface slopes. While these findings do not invalidate the use of industry-standard techniques for binder-jet-printed samples, they demonstrate a challenge in their application, and they motivate the development of new metrics and new techniques to more accurately describe surface topography in AM. 

Abstract For predicting surface performance, multiscale topography analysis consistently outperforms standard roughness metrics; however, surface-characterization tools limit the range of sizes that can be measured. Therefore, we evaluate the use of scanning electron microscopy (SEM) to systematically measure small-scale topography. While others have employed SEM for similar purposes, the novelty of this investigation lies in the development and validation of a simple, flexible procedure that can be applied to a wide range of materials and geometries. First, we established four different options that can be used for sample preparation, and we measured quantitative topography of each using the SEM. Then the power spectral density (PSD) was used to compare topography among the four preparations, and against other techniques. A statistical comparison of PSDs demonstrated that SEM topography measurements outperformed AFM measurements at scales below 100 nm and were statistically indistinguishable from (highly labor-intensive) TEM measurements down to 16 nm. The limitations of SEM-based topography are quantified and discussed. Overall, the results show a simple generalizable method for revealing small-scale topography. When combined with traditional stylus profilometry, this technique characterizes surface topography across almost seven orders of magnitude, from 1 cm down to 16 nm, facilitating the use of physical models to predict performance.