Conventional Electroplating Research Articles

Electroplating Additively Manufactured Honeycomb Structures to Increase Energy Absorption Under Quasi-Static Crush

Honeycomb (HC) has been used in energy absorption applications due to its high stiffness and low density. Metallic HC are used for energy absorption applications, however, these metallic structures can be challenging to manufacture if complex geometric features designed to improve energy absorption are used, which motivates the use of additive manufacturing (AM). Metal AM methods include powder bed fusion (PBF) and direct energy deposition (DED). In addition to capital equipment cost, these processes possess challenges that include a required inert environment, powder handling, final part porosity, residual stresses, and nonuniform surface finish. These concerns can be alleviated through the use of polymer AM, however, polymeric parts exhibit brittle failure and have a lower stiffness than metallic HC structures. In this study, a low-cost 3D polymer printing method, stereolithography (SLA), is combined with a conventional electroplating process to fabricate a metal-plastic composite HC structure with energy absorption capability much greater than of a plastic HC structures of the same nominal volume. SLA parts have a smooth surface, so that the surface finish is at least as uniform after electroplating as the SLA part. The energy absorption characteristics of the electroplated HC is studied to determine how these energy absorbing materials can be manufactured at reduced cost. Our study confirms that the metal-plastic composite HC increases both the crush strain range and the mean crush stress of these samples, resulting in metal-plastic composite HC structures with substantially increased energy absorption. This study also examines how buckling initiators (BIs), or diamond shaped holes located at 50, 75, and 100% of the height of the hexagonal cell vertices, can influence energy absorption performance. This study shows that it is feasible to fabricate electroplated HCs, using an SLA preform, to achieve a substantial increase in energy absorption over using SLA alone.

Copper metallization of carbon fiber-reinforced epoxy polymer composites by surface activation and electrodeposition

The application of polymer metallization in lightweight and high-strength materials for the sporting goods, automotive, aerospace and construction sectors has attracted considerable attention. The present study aims to deposit a thin, uniform copper (Cu) layer on an epoxy‑carbon fiber (epoxy-Cf) composite material for lightweight, high-frequency radio reflector applications (Ka-Band and higher frequencies) required in space missions. In this work, a surface-activated electroplating technique in which the surface of the substrate is activated with a Sn/Ag system, followed by conventional electroplating is studied. Surface activation deposits layers of conducting metal ions on the surface of the epoxy-Cf composite, which significantly improves the electrical conductivity of the composite surface. The subsequent electrodeposition takes place from a CuSO4 solution with a pH value of 4 at three different current density values of 0.05 A/dm2, 0.5 A/dm2 and 1 A/dm2. The presence and abundance of metallic Cu over epoxy-Cf composite were confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The morphology and elemental composition of the coating were characterized by scanning electron microscopy equipped with energy dispersive spectroscopy. With current density and coating thickness, morphology of the deposit changed from cauliflower-like to spherical along with grain refinement. Microhardness test shows a hardness of 330 HV and the pull-off adhesion test gave a bond strength of 1.69 MPa for 27.56 μm thick copper deposit. A major challenge encountered during the deposition of Cu is the oxidation of the metal followed by immediate tarnishing. This issue has been effectively addressed by employing benzotriazole solution as a protective agent.

Environmentally-Friendly Electroplating Process Using Solid Electrolyte Membranes

The automobile is a very broad industry, and consists of tens of thousands of parts. Many parts of automobile employ surface treatment to provide several functions such as electronic and anti-corrosion properties. Among several surface treatment process, plating is an important elemental technology for automobile industry. For the industrial perspective, interest in SDGs significantly increases, and efforts to reduce environmental impact are highly urgent. Recently, we have developed novel electroplating process, solid electrodeposition (SED), which is based on ion transfer through solid electrolyte membrane1). This process has unique deposition characteristics that the metal film can be deposited only on the contact area between cathode and the membrane, in a fashion similar to stamping deposition. The SED process has an advantage for reducing amounts of waste effluent and CO2 gas. Moreover, this process has a potential of high deposition rate due to utilization of thin electrolyte membrane, providing lower electrical resistance between electrodes1), 2). Therefore, in the environmental and economical perspective, we believe that SED is promising process to metallize the substrate surface as an alternative to the conventional electroplating, especially in the field of electronics manufacturing. In this study, we report on SED system in details and characterization of deposited copper and nickel films. We also introduce copper fine patterning technique combined with SED using hard mask. Figure 1 shows schematic of the present SED system. The head part composed of anode plate, electrolyte solution and membrane are placed on substrate under controlled temperature and pressure. The membrane mainly plays a role of selective cation transfer from electrolyte solution to the substrate, although a small amounts of electrolyte solution can also be transferred due to pressure difference between internal head part and atmosphere. This solution uniformly spread to the interface between membrane and substrate so that uniform metal deposition is continuously proceeded without adhering the membrane with the substrate. Additionally, the SED system allows one to shorten the distance between electrodes because membrane can act as a diaphragm. Therefore, the power consumption is smaller than that of conventional electroplating, owing to decrease in an internal electrical resistance, providing the decrease of exhaust CO2 gas. The deposited film has relatively uniform thickness as shown in Figure 2, the result of which is due to the short distance between electrodes as well as uniform current feeding to deposit metal films through the back side of the cathode. The deposited copper and nickel films are crystalline and highly conductive, the quality of which are being enough for application of the SED process in the current electronics device manufacturing. By combining the SED process with the conventional screen printing technique, copper circuit patterns with line width of below 50 μm could also be successfully fabricated.1) Akamatsu, S. Nakano, K. Kimura, Y Takashima, T. Tsuruoka, H. Nawafune, Y. Sato, J. Murai, H. Yanagimoto, ACS Appl. Mater. Interfaces, 13, 13896-13906 (2021)2) Narui, Y. Hoshi, I. Shitanda, M. Itagaki, H. Yanagimoto, M. Hiraoka and H. Iisaka, E01-920, 232nd ECS Meeting. Figure 1

Greener electrochemical plating of ABS polymer with unprecedented adhesion via hierarchical micro–nanotexturing

Electrochemical plating is the most widely used coating process for metalizing commodity polymers, such as acrylonitrile–butadiene–styrene (ABS). Conventionally, ABS polymer is pretreated in concentrated sulfochromic acid solution before plating to develop a highly porous surface topography that ensures coating adhesion. Unfortunately, using chromic acid produces extremely hazardous chemical waste, which costs manufacturers millions of dollars in fines. This study proposes a threefold greener route by employing a newly developed in-mold patterning of polymeric parts before plating. We hot-embossed micron-sized dimples on the ABS polymer and acid-etched it for as short as 1 min to obtain a hierarchical micro–nanotextured surface. The coating adhesion on such surfaces appeared to be two times higher as compared to the conventional electroplating, while the etching time was reduced three times. Microscopic characterization of the fracture surfaces revealed that the adhesion strength at the hierarchically structured interface exceeded the strength of the ABS polymer, causing a cohesive crack in the polymer far below the interface when peeling the plated metallic film. These results imply that the use of toxic acids in the electroplating industry can be drastically reduced by the proposed hybrid preparation method without sacrificing the coating quality.

Development of Zinc and Low Carbon Steel Coatings on ABS Plastic by Electric ARC Spraying

<div class="section abstract"><div class="htmlview paragraph">Acrylonitrile Butadiene Styrene (ABS) is a highly sought-after engineering plastic for industrial applications, particularly in modern aircrafts. It offers better balance of impact strength, tensile strength, thermal resistance, dimensional stability, processing ability and affordability, the aircraft interior parts are made up of ABS plastic. However, ABS plastic is not a good choice for heavy-duty industrial applications that require corrosion resistance, abrasion resistance and durability. Several studies have been published on surface metallization of ABS plastic using conventional electroplating and electroless methods, but there is little information is available on the use of thermal spray technique. This opens up a huge opportunity for research in this area to be conducted using most commonly used materials like zinc and low carbon steel. In the light of the above, the current work employs an electric arc spray technique to coat zinc and low carbon steel on ABS plastic. Prior to coating the alumina grit blasting was carried on tensile and bending samples which were prepared as per ASTM D638 and ASTM D790 standards respectively. Comparative studies showed that the zinc coated ABS samples showed better tensile strength, flexural strength and flexural modulus than that of low carbon steel coated ABS specimen. Visual examination showed good surface finish with no surface damage for zinc coated ABS component.</div></div>

Characterization of Ni–Co thin film and its applications to multifunctional substrates of flexible microdevices

This paper proposes Ni and Co alloy (Ni–Co) as a new material for multifunctional substrates of flexible microdevices and studies its characterization. In the fabrication process, Ni–Co thin films having specific shapes were formed on a Si carrier wafer by conventional electroplating and microfabrication technologies and peeled off from the carrier wafer by a new release process proposed in this paper. Characterization of the Ni–Co thin films was carried out using the following methods: chemical composition, cross-sectional structures, and surface roughness were evaluated using an energy dispersive spectroscopy (EDS), scanning electron microscope (SEM), and scanning probe microscope (SPM), respectively. The mechanical properties were measured using a microscale tensile-testing method and nano-indenter; the thermal properties were investigated using differential scanning calorimetry (DSC) and a thermomechanical analyzer (TMA). Thermal conductivity was estimated using the electrical resistivity measured with a 4-point probe and the Wiedemann-Franz law. Afterward the characterizations, its applicability to multifunctional substrates such as structural substrates and sensory substrates of flexible microdevices was confirmed by implementing various micro devices based on a Ni–Co thin film.

Electrochemical additive manufacturing of graded NiCoFeCu structures for electromagnetic applications

This study introduces the concept of compositionally-graded electrochemical additive manufacturing of advanced metal alloys. The feasibility was demonstrated using a NiCoFeCu system for electromagnetic applications. The effects of grading frequency and voltage on output geometry and elemental composition were studied. Application of constant signals throughout pillar build yielded varying width and elemental composition along the pillar length. Additional grading of voltage from low to high favoured wider pillars and enhanced copper content. Auxiliary plating effects were used to explain compositional deviations from conventional electroplating. Frequency response varied with voltage, suggesting its use as a secondary, fine-tuned parameter.

Molecular plating of actinide compounds on wafer-scale aluminum substrate

A homogeneous thin film of actinide oxides on a metal substrate is very important for nuclear analysis. Compared with physical deposition methods, molecular plating attracts considerable attention because of its wide applicability, high utilization rate, and low consumption of actinide compounds. Distinct from conventional electroplating, molecular plating is generally performed in an organic solvent without additional supporting electrolyte except for dissolved actinide compounds. The potential distribution and the mass transfer processes are crucial for achieving high purity and uniform thickness. Because actinides are rare resources, it is essential to set up a theoretical model to ensure the controllable fabrication of actinide targets. For these, we design a profiled rotating electrode made using platinum-covered titanium wire (Pt-Ti) as the anode, and we simulate the potential and current distribution between the profiled Pt-Ti anode and the aluminum cathode by finite element analysis, considering factors such as the electrode shape, the rotation rate, the anode-cathode distance, etc. The theoretical model is accurate, and the wafer-scale actinide targets are fabricated with high quality and high efficiency.

Polymer-Based 3-D Printed 140-220 GHz Low-Cost Quasi-Optical Components and Integrated Subsystem Assembly

Few examples of individual polymer-based 3-D printed quasi-optical component types have been previously demonstrated above ca. 100 GHz. This paper presents the characterization of polymer-based 3-D printed components and complete subsystems for quasi-optical applications operating at G-band (140 to 220 GHz). Two low-cost consumer-level 3-D printing technologies (vat polymerization and fused deposition modeling) are employed, normally associated with microwave frequencies and longer wavelength applications. Here, five different quasi-optical component types are investigated; rectangular horn antennas, 90° off-axis parabolic mirrors, radiation absorbent material (RAM), grid polarizers and dielectric lenses. As an alternative to conventional electroplating, gold-leaf gilding is used for the polarizer. A detailed investigation is undertaken to compare the performance of our 3-D printed antennas, mirrors and RAM with their commercial equivalents. In addition, a fully 3-D printed, RAM-lined housing with central two-axis rotational platform is constructed for performing two-port measurements of a quasi-optical horn-mirror-polarizer-mirror-horn subsystem. Measured results generally show excellent performances, although the grid polarizer is limited by the minimum strip width, separation distance and metallization thickness. The ultra-low cost, ‘plug and play’ housing is designed to give a fast measurement setup, while minimizing misaligning losses. Its RAM lining is designed to suppress reflections due to diffraction from components under test that may cause adverse multi-path interference. Our work investigates each component type at G-band and integrates them within subsystem assemblies; operating at frequencies well above those normally associated with low-cost consumer-level 3-D printing technologies. This opens-up new opportunities for rapid prototyping of complete low-cost front-end quasi-optical upper-millimeter-wave subsystems.

Theoretical Verification of Film Forming in Local Electroplating Process with Electrolyte Suction Tool

Electroplating has many applications, including surface hardening and metal thin film manufacturing. In conventional electroplating, the entire workpiece is immersed in the plating solution. Since the whole surface in contact with the solution is plated, a masking operation to remove the unnecessary plating film is required. This is especially applicable to local plating in the case of ornament plating, surface hardening method, among others. However, these additional processes result in substantial increase in processing time and cost. Earlier, we achieved maskless local electroplating with a smooth surface using a suction tool, in which the electrolyte was retrieved by a suction pump through a suction hole. Even so, formation of film on electroplating is yet to be verified by simulations. Herein, we performed experiments and simulations of the local electroplating by varying the processing time before comparing the results. Finally, the validity of the film forming was examined by discussing the results.

Direct Copper Patterning at High Deposition Rate By Using Environmentally Compatible Electroplating Process

Electroplating has been an important process to produce electronic devices, automotive parts, decorative materials and so on. In contrast to that, plating is endowed with environmental issues originated from exhaust fluids and acid mists. Therefore, it is limited to construct new production line in spite of necessity of plating. Moreover, it is difficult to improve productivity drastically due to completely established plating technology in 21th century. To break down demerits of conventional electroplating, we focus on the fundamental issue regarding with the plating process that can be performed in electrolyte bath. Recently, we have developed novel electroplating process named solid electrodeposition (SED), which is characterized by metal ion electrophoresis through solid electrolyte membrane1). This process has unique deposition behavior that the metal deposition on substrate proceeds only on contact area with the membrane, like a stamping process. The SED has a merit of reducing amount of exhaust fluids as well as direct patterned deposition reflecting the membrane shape. Therefore, we believe that SED is highly promising process to metallize the substrate instead of the conventional electroplating. In this study, we report on copper deposition mechanism and direct fine pattering process by applying SED.We evaluate the limitation of copper deposition rate under controlled process parameters. As a result of that, the maximum deposition rate reaches over 0.63A/dm2 (14μm/min), which should be relatively higher than conventional one. The impressive value is originated from metal transfer mechanism into membrane as well as the device configuration. The metal deposition proceeds on solid-solid interface between membrane and substrate so that no diffusion layer exist near the substrate2). Moreover, the distance between anode and cathode can be shorten because the membrane preserves electrolyte solution. The copper film deposited from electrolyte solution without additives exhibits relatively smooth and minute surface morphology. This may be because membrane contacted on substrate restricts abnormal Cu growth due to contact pressure.SED has a merit for direct patterned deposition reflecting membrane shape, but it is not easy to fabricate fine patterns without masking. Therefore, it is limited to apply high-end parts such as printed circuit boards and sensor devices. To overcome this problem, we have developed direct fine patterning process by combining SED with dry coating and etching technologies. Figure 1 shows schematic diagram and photographs of each step. Firstly, the metal thin layer is formed on substrate by means of sputtering (electrode layer). Secondary, silver nanoparticles are printed as fine circuit patterns and sintered at optimal temperature (seed layer). Thirdly, copper was selectively deposited on seed layer by means of SED, which utilizes the potential difference between electrode and seed layers. The SED proceeds through isotropic growth along vertical direction because the deposition occurs on the surface of seed layer contacted with the membrane. Therefore, SED can produce relative thick and fine patterns following line width of seed layers. Finally, the bear electrode layer is etched away by plasma exposure. We believe that this approach is highly promising process to fabricate circuit pattern in terms of cost (simple step) and environmental (no exhaust fluids) point of view. 1. H. Yanagimoto, R. Mori, K. Okamoto and J. Murai, F01-994, 236nd ECS Meeting.2. Y. Narui, Y. Hoshi, I. Shitanda, M. Itagaki, H. Yanagimoto, M. Hiraoka and H. Iisaka, E01-920, 232nd ECS Meeting. Figure 1

Revisiting the Electroplating Process for Lithium-Metal Anodes for Lithium-Metal Batteries.

Electroplating has been studied for centuries, not only in the laboratory but also in industry for machinery, electronics, automobile, aviation, and other fields. The lithium-metal anode is the Holy Grail electrode because of its high energy density. But the recyclability of lithium-metal batteries remains quite challenging. The essence of both conventional electroplating and lithium plating is the same, reduction of metal cations. Thus, industrial electroplating knowledge can be applied to revisit the electroplating process for lithium-metal anodes. In conventional electroplating, some strategies like using additives, modifying substrates, applying pulse current, and agitating electrolyte have been explored to suppress dendrite growth. These methods are also effective in lithium-metal anodes. Inspired by that, we revisit the fundamental electroplating theory for lithium-metal anodes in this Minireview, mainly drawing attention to the theory of electroplating thermodynamics and kinetics. Analysis of essential differences between traditional electroplating and plating/stripping of lithium-metal anodes is also presented. Thus, industrial electroplating knowledge can be applied to the electroplating process of lithium-metal anodes to improve commercial lithium-metal batteries and the study of lithium plating/stripping can further enrich the classical electroplating technique.

Fabrication of Sub-10-nm Plasmonic Gaps for Ultra-Sensitive Raman Spectroscopy

The past two decades have witnessed the explosion of activities in the field of surface enhanced Raman spectroscopy (SERS). SERS platforms employ nano-structures that excite plasmonic modes with large local electromagnetic fields localized within small gap spaces between each constituting feature. Although the research-oriented SERS platforms yield significant signal enhancements to identify even single molecules, practical SERS-based sensors have not been fully introduced yet. The main reason behind this absence is the need for a cost-effective and reliable manufacturing method for controllable fabrication of plasmonic nano-gaps over large areas. In this article, we introduced a novel manufacturing process that enables fast and scalable fabrication of highly uniform sub-10-nm gaps that could yield large SERS signals. In this process, a conventional electroplating technique is used to produce unique nano-mushroom antenna arrays on a conducting substrate, resulting in controllable gap spaces between mushroom heads. By understanding the nature of mushroom shape antenna formation, we demonstrated the control of inter-metallic gaps down to 5 nm. We showed that the manufactured nano-structures yield Raman enhancements more than 108. Providing such large SERS signals that are uniform over large areas, our cost-effective fabrication technique could be very critical to realize practical SERS devices.

Comparative performances of Ni-SiC composite coatings deposited by conventional and brush electroplating

SiC particles are known as reinforced materials used to improve the coating’s properties and performances. In this paper, Ni - SiC composite coatings were deposited by conventional electroplating from sulfate-chloride bath, and brush electroplating methods from modified chloride bath with different dispersed SiC contents. The plating conditions were investigated and the process’ parameters were defined through electrochemical technique. Scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and micro-hardness test were used to clarify the effect of SiC content on coating’s properties and performances. The hardness of brush electrodeposit reached the highest value of 525 HV when the concentration of SiC in the plating solution was 4 g/L, while the hardness of conventional electrodeposit was only 389.3 HV when the plating bath contained 20 g/L SiC. The characterized results show clear advantages of brush electroplating compared to the conventional method to form the coating with high micro-hardness.

Study on the changes of ultrasonic parameters over supercritical Ni-Co electroplating process

This work discusses the influence of changes to ultrasound (US) parameters over the nickel cobalt (Ni-Co) metal thin film properties produced by supercritical CO2 (SC-CO2) electroplating. Additionally, Ni-Co films were produced by conventional electroplating and silent SC-CO2 and compared against each other. The discussion on metal thin film properties revolves around variations to the bath type ultrasonic power (15 W and 20 W) and frequency (42 k Hz and 72 kHz) during experiments. The properties provided by the three electroplating processes and analyzed include: grain sizes, film elemental content analyses, surface microstructures, film hardness, corrosion resistance, surface roughness, crystalline structure and preferential growth, etc. From the results it was clear that quality of films produced by US-SC-CO2 was improved compared to that of films produced by silent SC-CO2, which itself was better than those produced by conventional electroplating. However, when US power was varied we observed a decline in the mechanical properties of the produced films. The combination of ultrasonic agitation with SC-CO2 allows for improved mechanical properties such as: lower surface roughness, finer grain size and surface morphologies, increased corrosion resistance and film hardness. The ultrasound agitation applied to SC-CO2 electroplating enhanced the formation of alloyed metal as ultrasonic agitation increased the electrolyte flowability during electroplating process resulting in increased mass transfer while at the same time achieving a surface cleaning effect which removed metal ions with poor adhesion and other unwanted particles. Moreover, application of ultrasonic agitation avoids the use of surfactants so only changes to the physical phenomena and no changes to the chemical composition of the deposited thin films were observed, meaning less pollution to the electrolyte and higher purity of the deposited films. The US-SC-CO2 electroplating method described in this work effectively enhanced the mechanical properties of the deposited thin films compared to those produced by both silent SC-CO2 and conventional electroplating processes.

CEP 방식에 의한 니켈 / 나노-알루미나의 복합코팅 특성에 관한 연구

CEP 방식에 의한 니켈 / 나노-알루미나의 복합코팅 특성에 관한 연구

Fabrication of an all-diamond microelectrode using a chromium mask.

We have developed a new method for fabricating all-diamond microelectrodes. The process comprises three steps: masking the tip of an electrode by electroplating with chromium, depositing undoped diamond, which acts as an insulator on the sides of the electrode, and removing the chromium mask to expose the tip of the electrode. The active area of the electrode can be easily controlled in combination only with a conventional electroplating technique.

Effects of Electroplating at Lower Leveler and Suppressor Contents on the Formation of Very Low Resistivity Narrow Cu Interconnects

We investigated the effects of leveler and suppressor contents together on crystal structures of narrow 50 nm width Cu interconnects. Lowering both suppressor and leveler contents led to a bigger decrease in grain size just after plating and bigger increase in grain size after annealing than when just suppressor or leveler content was decreased. It is reported that the finer the grain size is, the larger the grain boundary energy is. We considered that grain size decrease after electroplating was due to nucleation enhancement of Cu by the decreased suppressor or leveler content, and by both decreased suppressor and leveler contents. Furthermore, by the combination of high purity electrolyte (9N) with optimized suppressor and leveler contents, the mean grain size after annealing was increased by about 25% over that of the conventional electroplating conditions, leading to realization of narrow Cu wires with low resistivity. These results were considered to be due to the fact that the very small grain boundaries with much smaller amounts of impurities, obtained by plating using 9N electrolyte with optimized additive contents, enhanced grain growth substantially during annealing. Electric resistivity under 3 μΩ·cm can be expected based on the fitted experimental curve.

Light-Induced Plating of Aluminum on Silicon in an Ionic Liquid

As the consumption of silver for the front electrode in silicon solar cells continues to grow rapidly, a low-cost low-resistivity Earth-abundant metal is likely needed to replace silver. Besides the widely-studied copper, aluminum is another candidate for front metallization in silicon solar cells. Conventional electroplating of aluminum in a nonaqueous ionic liquid has been carried out on silicon solar cells and about 18% efficiency has been demonstrated with an electroplated aluminum front electrode. However, the high-resistivity silicon substrate requires a nickel seed layer to facilitate aluminum electroplating on it. In this talk, we report light-induced plating of aluminum directly on the n side of a silicon p-n junction in an ionic liquid. Without any seed layer, the deposited aluminum has good adhesion to the silicon surface. The resistivity of the aluminum deposits is as low as 4×10–6 Ω-cm, which is only about 1.5 times that of bulk aluminum. The suitable wavelength for the light source is between 600 nm to 1,000 nm. The effect of plating temperature on morphology of the aluminum deposits is analyzed. Our light-induced aluminum plating process enables simpler, lower-cost metallization on silicon solar cells than the copper plating process.

Sample size effect on micro-mechanical properties of gold electroplated with dense carbon dioxide

Micro-mechanical properties of gold electroplated with dense carbon dioxide contained electrolyte (EP-DCE) were investigated by micro-compression tests using micro-pillar specimens for applications as movable components in MEMS devices. The effect of grain size on the micro-mechanical properties was also investigated using a gold film fabricated by conventional electroplating (CONV-EP). The gold film fabricated by the EP-DCE showed a finer average grain size when compared with that of the CONV-EP. Because of the finer grain size, the micro-pillar fabricated from the EP-DCE film showed a higher strength than that of the micro-pillar fabricated from the CONV-EP. An increase in the compressive flow stress from 740 to 810 MPa was observed as dimensions of the micro-pillars decreased from 20 × 20 × 40 to 10 × 10 × 20 μm3, which was a result of the sample size effect.