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Abstract: Thin sputtered films of binary (Ni–Ti) and ternary (Ni–Ti–Hf and Ni–Ti–Cu) shape memory alloys have been subjected to nanoindentation over a range of temperature (up to 400 °C), using a small diameter spherical indenter. The load-displacement plots obtained during these experiments have been interpreted so as to reveal whether the imposed strain was being at least partly accommodated by the martensitic phase transformation, ie whether superelastic deformation was taking place. This was done by evaluating the remnant indent depth ratio (depth after unloading/depth at peak load), which is expected to have a relatively small value if superelastic deformation and recovery are significant. It is confirmed that this procedure, which has previously been validated for bulk material, can be applied to these thin films (~2 µm in thickness). The results indicate that ternary alloys with up to about 20 at.%Hf or 10 at.%Cu can exhibit superelastic behaviour over suitable temperature ranges. [Copyright &y& Elsevier]

Abstract: A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m−1 K−1, which may be compared with ∼1.5 W m−1 K−1 for pure alumina PEO coatings and ∼10–15 W m−1 K−1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes. [Copyright &y& Elsevier]

Abstract: Fine holes have been laser-drilled in bundles of carbon fibres, using a pulsed Nd:YAG laser (). Examination of the drilled fibres showed that some were significantly swollen, with diameter increases up to ∼60%. Scanning electron microscopy (SEM) and Raman spectroscopy have been used to investigate the effects of fibre type on the extent of laser-induced fibre swelling. It has been established that extensive swelling only occurred with low-modulus, poly acrylo-nitrile (PAN)-based fibres. Based on the Raman spectra obtained from both as-received and laser-drilled fibres, the swelling mechanism is attributed to simultaneous structural rearrangement and rapid, heating-induced volatilisation of impurities. A prior heat treatment was found to reduce the laser-induced swelling in low-modulus fibres. A 2D numerical heat flow model has been used to investigate the thermal fields generated during laser drilling of carbon fibres, and this information is correlated with the observed effects and the proposed swelling mechanism. [Copyright &y& Elsevier]

Abstract: This paper presents a steady-state, bi-substrate technique for measurement of the through-thickness thermal conductivity of ceramic coatings, with a range of specimen thickness and porosity content. The technique is based on establishing unidirectional steady-state heat flow through the sample, sandwiched between a pair of (metallic) substrates with known thermal properties. Comparison between the heat fluxes passing through the two substrates allows a check to be made about the accuracy of the assumption of unidirectional heat flow. The interfacial conductances must be known and these can be estimated by testing samples of different thickness. Measured conductivities are likely to be more accurate if the interfacial conductance is relatively high. This is assisted by the introduction of a thin interfacial layer of a viscous, thermally conductive compound, or thermal pads of some sort, and by maintaining a suitable pressure across the setup. However, if such compounds (pastes) are used, then care must be taken to ensure that it does not enter the specimen via surface-connected pores, since this could significantly affect the measured conductivity. The reliability of the technique has been confirmed by testing fused silica samples of known thermal conductivity. It has also been applied to sprayed zirconia and plasma electrolytic oxide (PEO) alumina coatings. The values obtained were 1.05±0.10W m−1 K−1 and 1.63±0.35W m−1 K−1, respectively. [Copyright &y& Elsevier]

Phase compositions at various locations within individual thermal sprayed hydroxyapatite (HA) splats were qualitatively determined using Raman spectroscopy analysis and, confirmed through in vitro dissolution exercise. The purpose of the present study was to verify the locations of different phases within a HA splat deposited by thermal spray techniques. The splats were prepared through dc thermal plasma and high velocity oxy-fuel (HVOF) spray processes. Raman spectroscopy studies were performed to determine the locations of various phases that exist within individual splats, and the results were verified with in vitro dissolution tests. Results showed that the Raman spectroscopy is a feasible technique for qualitative determination of the phases within a single HA splat. It was revealed that the thermal decomposition of HA into tricalcium phosphate (TCP), amorphous calcium phosphate (ACP), tetracalcium phosphate (TTCP), CaO and other phases, occurred mainly within the melted portion of the sprayed particle during the thermal spray process. The in vitro tests of the individual HA splats in simulated body fluid established a reasonable correlation between localized dissolution behavior and the phases revealed by the Raman spectroscopy plots. This study demonstrates the feasibility of employing the Raman spectroscopy technique to provide phase distribution information within a single HA splat. Henceforth, a better understanding and control of optimizing the properties of a bulk HA coating through controlling the spray process could be possible. [Copyright &y& Elsevier]

A procedure is outlined for the production of highly porous material by liquid phase sintering of short stainless steel fibres, about 100 μm in diameter. The fibres, which were produced by a melt extraction route, were first electroplated with copper to a thickness of a few μm. The sintering procedure was then completed by holding at about 1100 °C for a few minutes. It is shown that this operation generates strong joints by the migration of liquid copper to the fibre–fibre contact regions, as a result of capillary action. A preliminary study of the mechanical behaviour material produced in this way indicates that its toughness is relatively high, for a highly porous metallic material, and that its fracture energy is broadly consistent with predictions from a model based on evaluation of the work done by plastic deformation and rupture of individual fibres close to the fracture plane. [Copyright &y& Elsevier]

In laser drilling of metals, melt ejection can be a significant mechanism of material removal. Vaporisation within the hole creates high pressure gradients, which expel molten material from the hole. Results are presented for a range of metals drilled with single pulses with durations of 0.1 and 0.5 ms, using a Nd:YAG laser. Power intensities across the focussed beam were of the order of 0.2 MW mm−2. Ejected droplets were collected and characterised, using several experimental techniques. The particle size distribution, angle of trajectory, molten layer thickness and temporal variation of melt ejection were determined. Two complementary methods, high speed photography and a particle stream interruption technique, were used to determine the ejection velocity. The experimental results obtained have been used to gain insight into the overall process of melt ejection. Melt ejection commences with the ejection of small (∼10 μm) droplets, moving at velocities of up to 30 m s−1. This is followed towards the end of the process by the ejection of larger (∼100 μm), slower-moving droplets, with velocities of ∼1 m s−1. Increasing the pulse intensity increases the ejection velocity and decreases the average particle size. This is attributed to the molten layers around the cavity being thinner, as a consequence of the higher thermal gradients. To a first approximation, typical particle diameters appear to be of the order of the molten layer thickness during drilling. [Copyright &y& Elsevier]

UV treatments are commonly employed for removal of pollutants from water in industrial environments. Introduction of a suitable photocatalyst can be beneficial in improving the energy efficiency. This commonly involves creating powerful reactive oxygen species that are able to degrade pollutants. However, the photocatalyst is required to have both high surface area (accessible to irradiation from the light source) and to be mechanically robust. TiO 2 is a well-known photocatalyst, suitable for use under UV illumination. This paper concerns production of photocatalytic TiO 2 via Plasma Electrolytic Oxidation (PEO) of titanium substrates. PEO of Ti commonly results in highly porous TiO 2 coatings that adhere strongly to a robust metallic substrate. Characteristics of the electrical discharges, which are central to the PEO process, have been studied using high-speed photography. These results are correlated with the microstructure of the resultant coating. The photocatalytic performance has been evaluated under UV-C illumination, using urea as a model pollutant, and modelling of liquid flow over the PEO surfaces has been performed using COMSOL. Photocatalytic activity is enhanced on samples with high surface roughness and high surface area, which tend to be produced after intermediate processing times (8 min). This is explained in terms of how fluid flow is disrupted over rough surfaces, reducing the thickness of the hydrodynamic boundary layer and increasing the interaction between pollutant and catalytic surface. [Display omitted] • Correlation between PEO conditions and microstructure/topology of resultant TiO 2 coatings • Demonstrated that rougher coatings lead to more effective photo-catalytic performance • Fluid dynamics modelling relates this to contact between pollutants and photoactive surfaces. [ABSTRACT FROM AUTHOR]

Advanced methods for the repair of single-crystalline (SX) Ni-based superalloys are of special interest for the gas turbine industry. Polycrystalline repair approaches show promising results, while the repair of SX materials is still challenging. Directional annealing experiments resulted in large columnar grains by imposing thermal gradients at the abnormal grain growth temperature of a specific Ni-based superalloy. A numerical model of the Bridgman process is applied to provide an insight into the temperature evolution during zone annealing of the Vacuum-Plasma-Spray (VPS) repair coatings with the aim of promoting grain growth from the SX substrate. The results presented here suggest that this is a promising approach to repair or manufacture SX turbine blades. Unlabelled Image • Abnormal grain growth behavior of vacuum plasma sprayed superalloy coatings. • Directional annealing of fine-grained repair coating results in columnar grains. • FEM simulation of a Bridgeman furnace for directional annealing • Single crystalline repair by simulating grain growth from single crystal substrate [ABSTRACT FROM AUTHOR]

• An iterative FEM procedure is described for reliable extraction of creep strain histories (parameter values in the Miller–Norton law) from creep indentation data. • The key issues that determine the reliability of the procedure are examined in detail. • It is shown that creep strain histories inferred in this way are in good agreement with those obtained via uniaxial testing, for 3 different applied stress levels. A procedure is described for Indentation Creep Plastometery (using a spherical indenter), which is analogous to that developed previously for Indentation Plastometry. As in that case, it is based on iterative numerical simulation of the indentation process, with repeated comparison between an experimental outcome and the corresponding model prediction, systematically varying the values of parameters in a constitutive law until optimal agreement is achieved. The constitutive law used here is the Miller–Norton relationship, which covers both primary and secondary creep regimes (although the transition between them is not well-defined). The experimental outcome is the penetration depth as a function of time, under a constant applied load. An important feature of the procedure is the prior creation of a spherical recess in the sample, having a pre-selected depth and a curvature radius equal to that of the indenter. This allows control over the stress levels created during the indentation creep testing and can be used to ensure that no (time-independent) plastic deformation is stimulated during the test. In the absence of such a recess, this is virtually unavoidable, since the stress levels created during initial contact between a spherical indenter and a flat surface tend to be very high. Such plasticity introduces unwanted complications into creep testing. Confirmation of the viability of the procedure is provided via comparisons between the creep characteristics of pure nickel samples at 750 °C, obtained in this way and via conventional uniaxial tensile testing. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This investigation concerns the mechanisms by which (fine) particles become incorporated into plasma electrolytic oxidation (PEO) coatings when added to the electrolyte. Three different types of particle have been used, covering a wide size range, and processing has been carried out with both Al and Ti substrates. For some of these combinations, the particulate was chemically similar to the expected PEO product, while for others it was different. The power supply was 50 Hz AC, with a pre-selected current density. It has been established that, where such reactions are chemically favoured, phase changes can occur that must have involved the particulate reaching very high temperatures. From this and other evidence, it is concluded that the main incorporation mechanism involved is that of (fine) particulate being swept into the pores associated with active discharge sites, while they are being refilled with electrolyte immediately after collapse of the plasma. They are then likely to become entrapped, and in many cases to be strongly heated as the plasma is created during the next discharge cycle. Typical pore sizes are such that particles (or particulate clusters) above about 10 μm in size would be unlikely to enter them. While particles a few microns in diameter can become incorporated, it takes place more readily with sub-micron particles. It is also concluded that electrophoretic forces are unlikely to play any significant role in the incorporation process. [ABSTRACT FROM AUTHOR]