Showing papers by "Gian Carlo Gazzadi published in 2022"

Highly‐Packed Proximity‐Coupled DC‐Josephson Junction Arrays by a Direct‐Write Approach

Abstract: Focused ion beam induced deposition (FIBID) is a direct‐write technique enabling the growth of individual nanostructures of any shape and dimension with high lateral resolution. Moreover, the fast and reliable writing of periodically arranged nanostructures can be used to fabricate devices for the investigation of collective phenomena and to design novel functional metamaterials. Here, FIBID is employed to prepare dc‐Josephson junction arrays (dc‐JJA) consisting of superconducting NbC dots coupled through the proximity effect via a granular metal layer. The fabrication is straightforward and allows the preparation of dc‐JJA within a few seconds. Microstructure and composition of the arrays are investigated by transmission electron microscopy and energy dispersive X‐ray spectroscopy. The superconductor‐to‐metal transition of the prepared dc‐JJA is studied in a direct way, by tuning the Josephson junction resistance in 70 nm‐spaced superconducting NbC dots. The observed magnetoresistance oscillations with a period determined by the flux quantum give evidence for the coherent charge transport by paired electrons. Moreover, the measured resistance minima correspond to two fundamental matching configurations of fluxons in the dc‐JJA, caused by magnetic frustration. The robust properties of the prepared dc‐JJA demonstrate the opportunities for a fast preparation of complex device configurations using direct‐write approaches.

Pre-treatment of Selective Laser Melting (SLM) surfaces for thermal spray coating

Abstract: In this work, we investigated the deposition of thermal spray coatings onto additively manufactured parts obtained by Selective Laser Melting (SLM). SLM is indeed replacing conventional subtractive machining to produce diverse industrial parts; hence, it will become increasingly frequent to apply thermal spray coatings onto SLM surfaces. Whilst grit-blasting is the most widely accepted process to prepare a conventionally machined part for a thermal spray coating, the unique surface texture of SLM surfaces might open new possibilities. As a case study, we chose High Velocity Oxygen-Fuel (HVOF) spraying of WC-10%Co4%Cr onto AISI 316 L SLM surfaces in three different conditions: “as built”, pickled, or grit-blasted. We also examined SLM surfaces grown along different directions: parallel, perpendicular, or inclined with respect to the build platform. The references were a machined and grit-blasted SLM part, and a grit-blasted stainless steel bulk. Dense coatings were obtained on every surface regardless of its roughness and pre-treatment. Electrochemical corrosion testing confirmed the low porosity of the layers. Tensile adhesion/cohesion strength was >70 MPa on both SLM and reference surfaces, but the presence of an oxide scale on as-built surfaces degraded the adhesion strength to some extent. Although TEM analysis showed occasional evidence of diffusion bonding between the coating and the oxide scale, cyclic impact tests revealed that the scale itself broke. Particularly strong adhesion was achieved with pickled surfaces; indeed, failure was only cohesive (i.e. within the coating) in both tensile and impact tests. In addition to mechanical interlocking to the rough surface profile, TEM showed widespread diffusion bonding to the clean metal. An SLM part might therefore need chemical activation but no subtractive machining before application of a thermal spray coating. The coating also exerts a levelling action toward the SLM surface, i.e. the coated surface is smoother than the as built one. • WC-CoCr was HVOF-sprayed onto SLM AISI316 with different surface pre-treatments. • The microstructure and corrosion resistance of the coatings are independent of the substrate surface condition. • There is evidence of diffusion bonding to a clean (pickled, oxide-free) substrate. • Coatings onto pickled surfaces achieve tensile adhesion strength >75 MPa. • The coating has a levelling effect on the roughness of as-built SLM surfaces.