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Alain Reiser1 Jeffrey Wheeler1 Ralph Spolenak1

1, ETH Zurich, Zurich, , Switzerland

Additive manufacturing (AM) is transforming the way we design and fabricate structures on many scales. At small length scales, additive micromanufacturing is expected to expand the capabilities of microfabrication significantly. This enabling character of microscale AM has been demonstrated by the rise and spread of microstereolithography (MSL) [1].
In the past decade, a multitude of additional microscale AM techniques have been developed. Many of them aim to expand the range of materials available for microscale AM from the organic photoresists of MSL to inorganic materials and metals [2].
Determining the nature and the quality of these synthesized materials is a key aspect for establishing AM in microfabrication, since long proven quality standards have to be met. Additionally, knowledge of a material's microstructure and its influence on the material’s properties is the first step towards engineering and optimizing the material’s performance.
In order to determine the relationships between microscale AM methods and the deposited materials with a consistent methodology, we undertook the first, comprehensive comparison of the microstructure and the mechanical properties of metals fabricated with most of the presently suggested microscale metal AM technique. This work is a collaborative effort of multiple groups in the field of microscale metal AM: the range of techniques studied includes well established methods, e.g., focused electron beam induced deposition and direct ink writing, as well as more novel approaches, e.g., electrohydrodynamic printing and electrochemical deposition. The mechanical performance of the printed structures was evaluated using nanoindentation and microcompression, and the materials’ microstructure was analyzed using cross-sectional electron microscopy.
Both the elastic and plastic properties were found to vary by orders of magnitudes between the individual techniques. We show that these differences can be related to the large variations in microstructure of the deposited materials. These microstructures in turn are coupled with the various physico-chemical principles exploited by the different printing methods. Some microscale AM techniques are demonstrated to deliver materials with dense and crystalline microstructures with excellent mechanical properties, comparable to those of bulk nanocrystalline materials.
This study demonstrates that metallic materials with a wide range of microstructures and properties can be synthesized by contemporary microscale AM techniques. It is intended to provide practical guidelines for future users of these methods and help to establish AM techniques in microfabrication.

[1] J. K. Hohmann, M. Renner, E. H. Waller, G. von Freymann, Adv. Opt. Mater. 2015, 3, 1488.
[2] A. Reiser, L. Hirt, R. Spolenak, T. Zambelli, Adv. Mater. 2017, 201604211, 1604211.

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