Adrian Wei Yee Tan

Mechanical and Tribological Properties of Cold-Sprayed Ti Coatings on Ti-6Al-4V Substrates

A cold spray process was used to deposit titanium (Ti) coatings of different thicknesses on commercial Ti-6Al-4V (Ti64) substrates. The hardness of the Ti coatings was measured using a Vickers micro-indenter. It was found that the thicker Ti coatings had higher hardness probably due to the better uniformity and higher density of the coatings. The tribological results showed that the friction and wear of the Ti coatings tested against a steel ball under dry condition became lower with higher thickness probably due to the higher wear resistance of the thicker coatings associated with their higher hardness. The specific wear rates of all the Ti coatings were significantly lower than that of the Ti64 substrate as a result of the higher wear resistance of the Ti coatings associated with their cold-worked microstructures and the formation of high wear resistant oxide layers on their wear tracks during the wear testing.

Muscle-like high-stress dielectric elastomer actuators with oil capsules

Despite being capable of generating large strains, dielectric elastomer actuators (DEAs) are short of strength. Often, they cannot produce enough stress or as much work as that achievable by human elbow muscles. Their maximum actuation capacity is limited by the electrical breakdown of dielectric elastomers. Often, failures of these soft actuators are pre-mature and localized at the weakest spot under high field and high stress. Localized breakdowns, such as electrical arcing, thermal runaway and punctures, could spread to ultimately cause rupture if they were not stopped. This work shows that dielectric oil immersion and self-clearable electrodes nibbed the buds of localized breakdowns from DEAs. Dielectric oil encapsulation in soft-membrane capsules was found to help the DEA sustain an ultra-high electrical breakdown field of , which is 46% higher than the electrical breakdown strength of the dry DEA in air at . Because of the increased apparent dielectric strength, this oil-capsuled DEA realizes a higher maximum isotonic work density of up to , which is 43.8% higher than that realized by the DEA in air. Meanwhile, it produces higher maximum isometric stress of up to 1.05 MPa, which is 75% higher than that produced by the DEA in air. Such improved actuator performances are comparable to those achieved by human flexor muscles, which can exert up to 1.2 MPa during elbow flexion. This muscle-like, high-stress dielectric elastomeric actuation is very promising to drive future human-like robots.

Challenges of using dielectric elastomer actuators to tune liquid lens

Recently, dielectric elastomer actuators (DEAs) have been adopted to tune liquid membrane lens, just like ciliary muscles do to the lens in human eye. However, it faces some challenges, such as high stress, membrane puncture, high driving voltage requirement, and limited focus distance (not more than 707cm), that limit its practical use. The design problem gets more complex as the liquid lens shares the same elastomeric membrane as the DEA. To address these challenges, we separate DEA from the lens membrane. Instead, a liquid-immersed DEA, which is safe from terminal failure, is used as a diaphragm pump to inflate or deflate the liquid lens by hydraulic pressure. This opens up the possibility that the DEA can be thinned down and stacked up to reduce the driving voltage, independent of the lens membrane thickness. Preliminary study showed that our 8-mm-diameter tunable lens can focus objects in the range of 15cm to 50cm with a small driving voltage of 1.8kV. Further miniaturization of DEA could achieve a driving voltage less than 1kV.

High stress actuation by dielectric elastomer with oil capsules

Though capable of generating a large strain, dielectric elastomer actuators (DEAs) generate only a moderate actuation stress not more than 200kPa, which seriously limits its use as artificial muscles for robotic arm. Enhancement of dielectric strength (greater than 500MV/m) by dielectric oil immersion could possibly enable it a larger force generation. Previously, the immersion was done in an oil bath, which limits portability together with DEAs. In this study, we developed portable capsules to enclose oil over the DEA substrate (VHB 4905). The capsules is made of a thinner soft acrylic membrane and they seals dielectric liquid oil (Dow Corning Fluid 200 50cSt). The DEA substrate is a graphiteclad VHB membrane, which is pre-stretched with pure-shear boundary condition for axial actuation. When activated under isotonic condition, the oil-capsule DEA can sustain a very high dielectric field up to 903 MV/m and does not fail; whereas, the dry DEA breaks down at a lower electric field at 570 MV/m. Furthermore, the oil-capsule DEA can produces higher isometric stress change up to 1.05MPa, which is 70% more than the maximum produced by the dry DEA. This study confirmed that oil capping helps DEA achieve very high dielectric strength and generate more stress change for work.

Self-clearing dielectric elastomer actuators using charcoal-powder electrodes

This study found that compliant electrodes using charcoal powder enable self clearing property to dielectric elastomer actuator. Charcoal powder is applied as compliant electrodes by smearing on a 100% bi-axially pre-stretched dielectric elastomer membrane (VHB 9473), with nominal pre-stretched thickness of 62.3 μm. This DEA using charcoal-powder electrodes can sustain up 10 kV without terminal breakdown, while those using graphite or silver grease break down at slightly above 2 kV. It is noted that this DEA using charcoal-powder has maximum areal strain at about 45 % at 4 kV, beyond which the strain does not increase further for reduced electrical conductivity. The dielectric elastomer actuator using the charcoal-powder electrodes generate less actuation strain than that using the graphite. However, the former can produce a large actuation stress as it can driven to a higher driving voltage without pre-mature breakdown.

Additive manufacturing of Inconel 625 superalloy parts via high pressure cold spray

an emerging additive manufacturing technique, which is used in repair applications of metal components. The benefits of CS process are good metallurgical bonding with less heat-affected zone compared to traditional metal joining processes (i.e. welding, thermal spray etc.) or electron beam melting (EBM) or selective laser melting (SLM) additive manufacturing methods. In this study, Inconel 625 was deposited on Inconel 718 substrate via a high pressure cold spray system. The window of deposition for Inconel 625 particles, gas flow and particle acceleration behavior were investigated by numerical simulations. Powder and coating microstructures were investigated by a combination of optical microscopy and scanning electron microscopy. The bond strength between coating and substrate was tested according to ASTM C633. The hardness tests for both the substrate and the as-sprayed coating were conducted. The results showed that the CS Inconel 625 coatings had a low porosity level and an intimate interface. The bond strength between coating and substrate was greater than the maximum epoxy strength. The good quality of the CS Inconel 625 deposits showed a great application potential for the additive manufacturing of Ni-based superalloy parts.

Tribological and Corrosion Characteristics of Ti6Al4V Coatings Cold Sprayed with Nitrogen and Helium Propellant Gases

The tribological and corrosion characteristics of Ti6Al4V coatings on Ti6Al4V substrates with nitrogen (N2) and helium (He) propellant gases cold sprayed via a high pressure cold spray process were investigated because different masses of the N2 and He working gases could affect the velocities of Ti6Al4V particles during the cold spraying and consequently the properties of the coatings. The more uniform and denser structure of the Ti6Al4V coatings with a lower porosity level was achieved with He working gas. As a result, the Ti6Al4V coatings deposited with the He working gas had a higher hardness than those deposited with the N2 working gas. The tribological results showed that the use of He gas during the cold spraying produced the more wear resistant Ti6Al4V coatings associated with their higher hardness. The denser Ti6Al4V coatings deposited with the He gas had a lower anodic current density in a NaCl solution than those deposited with the N2 working gas. It is clear that the wear and corrosion resistances of the Ti6Al4V coatings were significantly influenced by the propellant gases during the cold spray deposition. Introduction Ti6Al4V alloy is the most widely used titanium alloy for aerospace applications due to its light weight and high strength [1]. The cold spray process is a newly developed coating technique which accelerates powder particles to supersonic speeds in a carrier gas such as nitrogen or helium. Upon impact with a target substrate, the particles plastically deform and adhere to the surface. Subsequent layers are then deposited to build up a coating [2]. Traditionally, thermal spray and welding have been widely used to repair damaged aerospace components. However, many metallic materials are very prone to oxidation, many substrate materials cannot tolerate high temperatures, and many thermal sprayed coatings are subject to high thermal stresses and contain high levels of porosities [3-5]. Thanks to low temperature input during cold spray process, the cold sprayed coatings are free from phase transition, oxides, inclusions and thermal residual stress [6]. The trobological and corrosion properties of the cold sprayed coating were influenced by cold spray process parameters, such the carrier gas used. In this study, cold sprayed Ti6Al4V coatings were deposited by N2 and Helium carrier gas. Their trobological and corrosion behaviors were investigated.

Actuated strains in excess of 100 percent in dielectric elastomer actuators using silver film electrodes

Metallic thin films have not often been used as electrodes in dielectric elastomer actuators (DEAs) as the reported actuated strains have been small. This is especially so when compared to commonly used conductive greases and powders. Here, the use of thin silver films formed by electroless deposition (ELD silver) as electrodes in DEAs is studied. As electroless deposition involves only the use of chemicals, expensive equipment is not needed. That, coupled with the fact that the thin silver electrodes require only a small amount of silver per unit area, means that such electrodes are simple and inexpensive to fabricate. In addition, unlike conductive powders and greases, these silver films adhere well to most substrates that are or have been made hydrophilic. This is especially useful in maintaining structural integrity of the actuator, such as when DEA units need to be stacked up one on top of each other. Most importantly, thin silver film electrodes have the ability to self heal. Self-healing not only averts actuator failure brought about by localised breakdowns, it also enables actuation to resume, even allowing higher driving voltages to be reached. In this paper, we demonstrate that DEAs with corrugated ELD silver electrodes can allow actuated area strains of up to 125% at a relatively low driving voltage of 1.9 kV. This is due to the low stiffening effect that the corrugated ELD silver electrodes have on the dielectric layer, which was found to be close to that of graphite.