ICMCTF2017 Session F5: Additive-manufacturing-based Methods and Surface Engineering

Alternative materials and fabrication techniques are necessary to revolutionize the performance of large scale flexible electronics. Direct printing of electro-optical devices is a promising new fabrication approach, especially for integration of emerging two dimensional (2D) materials which have uncommon and useful combinations of properties, such as tunable band gaps, mechanical flexibility and optical transparency. However, challenges in conventional printing of suspended 2D particles and low performance of printed circuits remain. Here we describe a printing technique based on a novel a selective molecular attachment (SMA) approach for integration of 2D and other materials into printed circuits or devices. Peptide molecules have been identified via phage display techniques that adhere selectively to semiconducting few-layer MoS₂ flakes and graphene. These peptides are dissolved in ‘ink’ and printed on insulating surfaces to print transistors and other devices. The printed substrates are then dipped in suspensions of graphene and MoS₂ and the particles stick only to the peptide printed surfaces. Simulations of the peptide surface interactions reveal the specific amino acids that bind to substrate and particle. For example we observe that for the HLL peptide adsorbed on MoS₂, the phenylalanine ring had the closest binding position of any amino acid. The other ring residue structures, histidine and the two prolines, were also shown to very close to the surface. The n-term, c-term, and arginine residues were seen to migrate away from the surface. This knowledge enables design of peptides for enhanced adhesion and also for sensing of target molecules in vapor or solution phase. It is anticipated that the exotic combinations of properties found in 2D materials will add unique functionality to devices over standard printed materials and architectures. Here we demonstrate the technique and explore the peptide-surface interactions in SMA processes.

High entropy alloys (HEAs) are relatively a new alloy system comprising of a minimum of 5 principle alloy elements at a concentration between 5-35 atomic percent. Contrary to phase rule prediction many HEA compositions form simple solid solutions. HEAs have attractive properties as bulk or coating materials in elevated temperature applications: high resistance to thermal softening, thermally stable microstructure, low inter-diffusion, and high oxidation resistance. Limited studies have explored the formation of HEA coatings on steel substrates by laser surface melting a static layer of pre-alloyed powders. This technique is restricted to horizontal surfaces, limiting practical implementation. To clad complex shapes an appropriate technique is “blown powder” additive manufacturing (Direct laser deposition, DLD) where powder is delivered to a focused laser on a controllable head. The formation of HEA coatings by this technique is unreported in the literature. This work addresses two main research questions; Can chemically homogenous coatings be manufactured by the DLD of a blend of 5 elemental powders? and Can the DLD process parameters be controlled to eliminate the dilution effect of the substrate?.

Al_xCoCrFeNi (x=0.3, 0.6 and 0.9) HEA coatings of were produced by single-pass DLD on 253MA austenitic steel and Inconel 600 superalloy using a mixture of blended elemental powders. A three-level parametric study was performed on key variables: laser power, scan speed, spot size, powder feed rate, focal offset and hatch distance. Through thickness elemental composition (GDOES) and phase analysis (XRD) was determined. The coating microstructure (SEM, AsB detector), local homogeneity (EDS) and properties (microhardness) were obtained in cross-section. Increasing Al mole fraction from 0.3 to 0.6 and 0.85 resulted in a change in HEA coating crystal structures from FCC, FCC/BCC and BCC. The compositional mixing between the deposited layer and the substrate was quantified using a proposed mixing factor, which was significantly influenced by the deposition variables, chiefly powder feed rate. Homogenous HEA coating compositions were obtained at a relatively higher powder feed rate.