AVS 69 Session SE-TuP: Advanced Surface Engineering Poster Session

FeCrAlY is commonly utilized because of its exceptional oxidation resistance, whereas alumina ceramics are valued for their high fracture toughness, resistance to wear, and corrosion¹. Therefore, developing Al₂O₃-FeCrAlY composites is a promising approach for engineering applications requiring high temperature and strength, such as engines, solid oxide fuel cells, thermal barrier coatings, and catalytic supports^1-2. The production of these composite powders involves using multiple materials or phases subjected to physical or chemical treatments such as ball milling and fluidized bed chemical vapor deposition to synthesize homogeneous powders. These fabrication processes have produced many functional materials, such as ceramics, metals, and composites, which possess improved mechanical, thermal, or electrical properties¹. However, these methods are very time-consuming and expensive.

In this study, we investigated a new method to synthesize spherical composite powders consisting of FeCrAlY powders with Al₂O₃ coatings, which are used as feedstock materials for metal additive manufacturing. Chitosan is used as a binder to hold FeCrAlY and Al₂O₃ together, and then the mixture is spray-dried into spherical composite powders. We tailored the sizes and microstructures of the FeCrAlY-Al₂O₃ composite by controlling various parameters such as material content, mixing, flow rates, and reaction temperature. By engineering these composite powders, we can improve the uniformity of material distribution and reduce powder denudation while printing complex structures using metal additive manufacturing technologies. This approach creates a bi-continuous FeCrAlY and Al₂O₃ structure after laser melting and annealing. Furthermore, Al₂O₃ acts as a templating material, which can be selectively etched away, resulting in a highly porous FeCrAlY structure with a large surface area. The composite powders' and printed structures' structure and surface properties were characterized using X-ray diffraction, X-ray 3D tomography, Brunauer-Emmett-Teller analysis, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy techniques.

References

(1)Li, J., et al. “Al₂O₃–FeCrAl Composites and Functionally Graded Materials Fabricated by Reactive Hot Pressing.” Composites Part A: Applied Science and Manufacturing, vol. 38, no. 2, Feb. 2007, pp. 615–620.

(2)Kim, Do Hyung, et al. “A Study on FeCrAl Foam as Effective Catalyst Support under Thermal and Mechanical Stresses.” Surface and Coatings Technology, vol. 209, Sept. 2012, pp. 169–176.

Nowadays, nanoindentation has become a routinely technique for the mechanical characterization of thin films and small-scale volumes. Thanks to the development of friendly analysis software and advances in high sensitive instrumentation, it feels like the measurement and calculation of hardness and elastic modulus can be easily done by just “the pushing of one button.” However, the consequences of easy procedures have led many researchers to multiple publications with erroneous data.

Recently, we have reviewed the nanoindentation hardness of materials at macro, micro, and nanoscale (E. Broitman, Tribology Letters, vol. 65, 2017, p. 23). Some misconceptions in the nanoindentation technique were highlighted, and solutions to errors were proposed. In this presentation, five typical mistakes in the measurement and data analysis during the nanoindentation of thin films will be critically reviewed, and the possible ways to avoid them will be discussed: 1) the wrong area selection to calculate instrumented indentation hardness; 2) the wrong data conversion from Vickers microindentation to Berkovich nanoindentation; 3) the confusion of thermal drift with creep and viscoelastic effects; 4) the wrong correlation of hardness with tip penetration; 5) the preconceptions about a direct relationship between elastic modulus and hardness.

The origins of the aforementioned mistakes will be elucidated from the lack of understanding on contacts mechanics theory, the limits and validation of the Oliver and Pharr’s method, and preconceptions transmitted from generation to generation of nanoindenter users. At the whole, it will be stressed that it is not enough to know “how to push the start button of the nanoindeter” in order to measure the nanoscale mechanical properties of coatings.

Most current nano-electronic devices are using final composited SiNx-SiOy layer to prevent moisture penetration in final devices prior to chip packaging (1). Silicon nitride (SiNx) as a dielectric plays an important role in the semiconductor industry for many years. Si-Rich SiN have been evaluated over the years in electronic device application (2,3). With device scaling to sub-5nm dimension, both passivation and capping layers also needs to scale down to the sub-5nm thickness without compromising the following properties: 1) Excellent Oxidation and diffusion Barrier. 2) High Electrical Breakdown and Low Leakage Current. 3) High stability under thermal stress with low Hydrogen content. 4) Positive performance impacts to the passivating electronic devices including electrical and mechanical properties. 5) Conformal step coverage over high topological structure at low deposition temperature.

This work is focusing on the Si-rich SiNx as an encapsulation film and its ultrathin thickness limitation as oxidation barrier and encapsulation layers. Low-temperature (200⁰ C -300⁰C) plasma enhanced chemical vapor deposition (PECVD) process was used to deposit Si-rich SiNx for encapsulation/capping of temperature sensitive nano-electronic devices. Deposition temperature impact on the Si-Rich SiNx capping film’s electrical, conformality, oxidation and barrier properties of the films are evaluated with completed material compositional analysis vs film’s performance. To obtain a good process at low temperature for encapsulation/capping of temperature sensitive nano-device, novel cyclic multilayers deposition/plasma treatment approach was developed. Ultrathin (3-4 nm) Si-Rich Silicon Nitride film with excellent conformality, oxidation barrier and passivation performance are achieved for passivation/capping with novel cyclic multilayer deposition/plasma treatment approach at low deposition temperature of 200 C and with > 80% conformality over high aspect ratio nanodevice structures. Carbon doped ultrathin (4 nm) SiN (SiCN) was also evaluated and achieved similar robust cap performance for nanodevice fabrication. These high-performance Si-Rich SiN and SiCN films have excellent potential for logic and memory nanodevice as passivation and capping layers.

(1)Sean King; Journal of Vacuum Science & Technology A 29, 041501 (2011); doi: 10.1116/1.3584790

(2)Son.V. Nguyen and S. Fridmann; J. of Electrochemical Society, V. 134, No# 9, p.2324-2329 (1987).

(3)H. Kim et al ; Journal of Vacuum Science & Technology A 35, 01A101 (2017); https://doi.org/10.1116/1.4964889

Current optoelectronic applications such as touchscreen displays, photovoltaic cells or lenses require surfaces that can possess multiple functions, for example, easy optical and/or electrical tunability, high transparency, self-cleaning properties, and antimicrobial properties, to name a few. Borrowing from designs found in nature such as the eyes of nocturnal insects or the lotus leaf, much progress has been made towards bio-inspired surfaces. Yet developing mass-scalable, and cost-efficient methods for fabricating multifunctional optical surfaces is still a major challenge due to the limitation of the existing nanofabrication methods that rely on traditional optical and e-beam lithography. In this talk, we review recent efforts from our group in developing optical surfaces based on the use of ultrathin metal films (UTMF) and solid-state dewetted nanoparticles (DNPs) as a scalable and lithography-free approach to confer functionalities such as high transparency, broadband and omnidirectional antireflection effect, self-cleaning properties, and antimicrobial properties.

Solid-state thermal dewetting of ultrathin metal films (Ag, Cu, Ni) has emerged as a viable strategy to obtain features down to few nanometers, therefore, it has great potential to be implemented as a fast and low-cost method in industrial scale nanofabrication. The dewetted nanoparticles on glass surfaces serve as a mask in creating nanopillars or nanoholes using dry etch process. Initial metal films thickness, temperature, and duration of the dewetting, and dry etching times are the parameters that give us morphological and optical control on the nanostructures. The structured surfaces present omnidirectional broadband antireflection effect with low scattering and have self-cleaning properties. Importantly, structures maintain their optical and wetting properties after repeated abrasion making them attractive to be used in consumer display devices. Moreover, when high aspect ratio nanopillars in fused silica are fabricated, we observe increase of the optical emissivity in the infrared range, which can be exploited for passive radiative cooling. The versatility of dewetted nanoparticles is also shown by using Cu DNPs to make transparent, antimicrobial surface.

During cold spray, powder particles undergo severe plastic deformation upon impact with the substrate. This results in particle flattening, oxide breakage, and metallurgical bond formation at particle-particle and substrate-particle interfaces. At smaller length scales, heterogeneity of the bond coating can create property differences, which are yet to be explored. Thus, a comprehensive understanding of local interfacial bond strength at this heavily deformed interface would assist in designing optimal cold spray processes. In this study, we investigated the microstructure and mechanical properties of zinc (Zn) cold sprayed on AZ91 magnesium (Mg) substrates via correlative microscopy and in situ micro-tensile testing.Micro-tensile dogbones fabricated using Plasma Focused Ion Beam (PFIB) were tested in a displacement-controlled mode to estimate the interfacial strength and live deformation behavior.

The accretion of ice has resulted in adverse impact on a variety of household, industrial, and polar research station activities. Despite significant efforts being made to prevent ice adhesion with different surfaces by developing various passive anti-icing coatings, it is still essential to enhance overall performance and durability. Herein, we report the designing of icephobic coatings through self-formation of 3D porous micro-nanostructure utilizing siloxane/fluoropolymer/silica nanoparticles (NPs). The spin coating method was used for coating PDMS/PTFE composite on aluminium (Al 6061) substrate. The excellent miscibility and adhesion in PDMS and PTFE were observed due to the secondary electrostatic interactions between H and F atoms. These interactions were supported by the density functional theory (DFT) calculations and structural studies. Moreover, the controlled addition of PTFE powder to PDMS improved the water-repellency properties, mechanical strength, and adhesion strength of the coating. The self-formation of superhydrophobic Cassie Baxter state of PDMS/PTFE composite was achieved by sprinkling SiO ₂ NPs on it. The PDMS/PTFE/SiO ₂ NPs composites, surface morphology images showed the formation of porous 3D micro-nanostructure that produces a highly textured surface with several trapped air pockets. These air pockets minimise the surface contact area with water droplets or ice, which enhanced the water contact angle (WCA) and reduced the ice adhesion strength (IAS). Additionally, we experimentally demonstrated that the freezing at low temperatures can be delayed by controlling the heat flow rate, interfacial contact area, and surface texture.These results suggest the feasibility of the method for a wide range of promising anti-icing applications.

*Corresponding author: hbrlee@inu.ac.kr [mailto:hbrlee@inu.ac.kr]

Keywords: Superhydrophobicity, Icephobicity, PDMS/PTFE composite, SiO₂NPs, freezing delay time