Elias P. Koumoulos

Microstructural Study and Mechanical Properties of Dissimilar Friction Stir Welded AA5083-H111 and AA6082-T6 Reinforced with SiC Nanoparticles

Dissimilar friction stir welds were produced in 3 mm thick plates of AA6082-T6 and AA5083-H111 aluminum alloys using SiC as reinforcing material. The optimum weld presents a good distribution of nanoparticles in the weld nugget and mechanical mixing of the two alloys as well as further grain refinement compared to the one without nanoparticles. Higher hardness in the weld nugget is also evidenced, followed by enhanced ultimate tensile strength and elongation values. All specimens, after the tensile test, were lead to fracture at the heat affected zone of AA6082-T6 and specifically at the region of the lowest hardness.

Dissimilar Friction Stir Welding Between 5083 and 6082 Al Alloys Reinforced With TiC Nanoparticles

Dissimilar friction stir welding between aluminum alloys thick plates reinforced with TiC nanoparticles was conducted. The defect-free welds are characterized by good mechanical mixing between the joined materials as well as by good nanoparticle distribution and further grain refinement in comparison with the unreinforced weld. The local mechanical behavior of the produced metal matrix composites was studied and compared with their bulk counterparts and parent materials. Specifically, the measured mechanical properties in microscale and nanoscale (namely hardness and elastic modulus) are correlated with microstructure and the presence of fillers. The hardness, elastic modulus, ultimate tensile strength, percentage of elongation, and yield values increase with the presence of TiC nanoparticles.

Evaluating the Robustness of Top Coatings Comprising Plasma-Deposited Fluorocarbons in Electrowetting Systems

Abstract Thin dielectric stacks comprising a main insulating layer and a hydrophobic top coating are commonly used in low voltage electrowetting systems. However, in most cases, thin dielectrics fail to endure persistent electrowetting testing at high voltages, namely beyond the saturation onset, as electrolysis indicates dielectric failure. Careful sample inspection via optical microscopy revealed possible local delamination of the top coating under high electric fields. Thus, improvement in the adhesion strength of the hydrophobic top coating to the main dielectric is attempted through a plasma-deposited fluorocarbon interlayer. Interestingly enough the proposed dielectric stack exhibited (a) resistance to dielectric breakdown, (b) higher contact angle modulation range and (c) electrowetting cycle reversibility. Appearance of electrolysis in the saturation regime is inhibited, suggesting the use of this hydrophobic dielectric stack for the design of more efficient electrowetting systems. The possible causes of the improved performance are investigated by nanoscratch characterization.

Nanomechanical properties and nanoscale deformation of PDMS nanocomposites

Abstract In this study, nanoindentation technique was utilised to estimate the nanomechanical properties (hardness H, and elastic modulus E) of elastomeric polydimethylsiloxane (PDMS) samples consisting of different nanoclay concentrations (organically modified montmorillonite), namely, 0, 5 and 8 parts per hundred. The PDMS samples were also characterised by Fourier transform infrared spectroscopy, X-ray diffraction and tensile testing. In addition, the surface of the nanocomposites was characterised through scanning probe microscopy, revealing surface modification with increasing nanoclay content in the PDMS matrix. Additionally, several analyses on nanoindentation data were performed, and the exact surface region (with higher values of H and E) was clearly defined. One key problem in using the Oliver and Pharr (O&P) method is the determination of the contact area between the indenter and the sample. It is believed that the contact area is underestimated using the O&P method and Hertzian analysis for soft polymers. Therefore, calculations using the O&P method and Hertzian analysis have been performed and compared. The change in H/E slope revealed that the addition of nanoclay amount strengthens the PDMS–montmorillonite nanocomposite.

Influence of accelerated aging on nanomechanical properties, creep behaviour and adhesive forces of PDMS

Abstract In the present study, the nanoindentation creep behaviour of untreated and ultraviolet (UV) treated polydimethylsiloxane (PDMS) samples was investigated, accompanied with adhesion analysis and Fourier transform infrared spectroscopy (FTIR) characterisation. Different hold times, in the range of 5–2000 s (at 10 μN of applied load), were incorporated into each nanoindentation measurement. The increase in hold time results in an increase in change in depth of the indenter in both samples. The error in hardness/modulus due to creep can be neglected for hold times of ∼400 s or more for untreated PDMS and ∼200 s or more for UV treated PDMS. The FTIR obtained data revealed surface deterioration, while the bulk nanomechanical properties were almost identical. A decrease in adhesive energy in the case of UV treated PDMS was observed, indicating that adhesive forces play a significant role at the nanometre scale in the indentation tests (real contact area determination during nanoindentation measurements).

Investigation of MWCNT addition into poly-dimethylsiloxane-based coatings

The purpose of this work is to investigate the effects on the properties of poly-dimethylsiloxane-based coatings with the incorporation of multi-walled carbon nanotubes (MWCNT). For this reason, two types of MWCNT were examined (pristine and functionalised with carboxyl groups). An optimised dispersion method was employed to synthesise coatings with different wt-% concentrations of CNT, using different solvents. The morphology of the nanocomposite coatings was studied under optical and scanning electron microscopy. Moreover, their hydrophobic/hydrophilic behaviour was investigated by contact angle measurements. Furthermore, nanoindentation and nanoscratch tests were conducted to evaluate the nanomechanical properties. The results revealed an optimal threshold concentration of 0.1 wt-% in CNT that combines both acceptable dispersibility and mechanical enhancement of the composite coating. It was proven that the increase in CNT content deteriorates the surface and mechanical properties of the coatings.

Determination of onset of plasticity (yielding) and comparison of local mechanical properties of friction stir welded aluminum alloys using the micro‐ and nano‐indentation techniques

Purpose – The purpose of this paper is to determine if the nanoindentation technique is a reliable method and whether it can be used to measure the surface hardness (H) in friction stir welded aluminum alloys. In order to test the reliability of nanoindentation technique, nanohardness values for friction stir welded aluminum alloys were compared to microhardness values. Additionally, the onset of plasticity (yielding) is investigated.Design/methodology/approach – Nanoindentation experiments were performed for the determination of onset on plasticity (yielding) and comparison of local mechanical properties of both welded alloys. In order to test the reliability of nanoindentation technique, nanohardness values for friction stir welded AA6082 were compared to microhardness values. The specimen was tested using two different instruments – a Vickers microhardness tester and a nanoindenter tester for fine scale evaluation of H.Findings – The results of this study indicate that nanohardness values with a Berkov...

A study on time dependent properties of aluminum alloy by nanoindentation technique

Purpose – Lightweight alloys are of major concern, due to their applicability, in transport and industry applications. The purpose of this paper is to perform a comprehensive analysis of time dependent properties of aluminum alloy by nanoindentation technique, through investigation of creep behavior. Additionally, possible explanations on the time dependent behavior and the influence of the hold period at maximum load and the loading rate on the elastic modulus and hardness results are also analyzed and discussed.Design/methodology/approach – In this work, a comprehensive analysis of time dependent properties of aluminum alloy by nanoindentation technique was performed, by varying the loading rate, the maximum applied load and the loading time. The stress exponent values are derived from the displacement‐holding time curves. The present experimental setup includes three different approaches: variation of loading rate, maximum applied load and loading time. The creep deformation mechanisms of the alloy, wh...

Influence of lignin modification on the mechanical properties of lignin/PEO blends

Purpose The purpose of this paper is to focus on the investigation of mechanical and thermal properties of lignin/poly (ethylene oxide) (PEO) blends, intended to be used as carbon fiber precursor. Design/methodology/approach Softwood kraft lignin was modified via esterification using phthalic anhydride and then blended with PEO. The final lignin/PEO ratios blends were (w/w) 20/80, 50/50 and 80/20 for both unmodified and modified lignin. The structural, thermal and mechanical properties of the blends were investigated by Fourier transform infrared, differential scanning calorimetry and tensile tests, respectively. Findings The results revealed that modified lignin/PEO blend (20/80) exhibited enhanced elongation. Originality/value The structural analysis as well as thermal and mechanical properties of the produced blends are clearly demonstrated.

Nanomechanical Properties and Deformation Mechanism in Metals, Oxides and Alloys

Metals, oxides and alloys are widely used in transport and industry-engineering applications, due to their functionality. In this work, the nanomechanical properties (namely hardness and elastic modulus) and nanoscale deformation of metals, oxides and alloys (elastic and plastic deformation at certain applied loads) are investigated, together with pile-up/sink-in deformation mechanism analysis, subjected to identical condition parameters, by a combined Nanoindenter—Scanning Probe Microscope system. The study of discrete events including the onset of dislocation plasticity is recorded during the nanoindentation test (extraction of high-resolution load–displacement data). A yield-type pop-in occurs upon low applied load representing the start of phase transformation, monitored through a gradual slope change in the load–displacement curve. The ratio of surface hardness to hardness in bulk is investigated, revealing a clear higher surface hardness than bulk for magnesium alloys, whereas lower surface hardness than bulk for aluminium alloys; for metals and oxides, the behavior varied. The deviation from the case of Young’s modulus being equal to reduced modulus is analyzed, for all three categories of materials, along with pile-up/sink in deformation mechanism. Evidence of indentation size effect is found and quantified for all three categories of materials.