A Comparison of mechanical and tribological behavior of nanostructured and conventional WC-12Co detonation-sprayed coatings

In the present study, WC-12Co coatings were deposited by detonation-spraying technique using conventional and nanostructured WC-12Co feedstock at four different oxy/fuel ratios (OF ratio). The coatings exhibited the presence of phases like W2C and W due to the decarburization of the WC phase, and the proportions of these phases were higher in the nano WC-12Co coatings compared with conventional WC-12Co coatings. Coating hardness and fracture toughness were measured. The tribological performance of coatings was examined under dry sand rubber wheel abrasion wear, and solid particle erosion wear conditions. The mechanical and wear properties of coatings were influenced by degree of decarburization and more so in the case of nanostructured WC-Co coatings. The results indicate that the extent of decarburization has a substantial influence on the elastic modulus of the coating which in turn is related to the extent of intersplat cracking of the coating.

Evaluation of strength of component-laminates in strip-based mechanisms

This paper deals with the evaluation of the component-laminate load-carrying capacity, i.e., to calculate the loads that cause the failure of the individual layers and the component-laminate as a whole in four-bar mechanism. The component-laminate load-carrying capacity is evaluated using the Tsai-Wu-Hahn failure criterion for various lay-ups. The reserve factor of each ply in the component-laminate is calculated by using the maximum resultant force and the maximum resultant moment occurring at different time steps at the joints of the mechanism. Here, all component bars of the mechanism are made of fiber reinforced laminates and have thin rectangular cross-sections. They could, in general, be pre-twisted and/or possess initial curvature, either by design or by defect. They are linked to each other by means of revolute joints. We restrict ourselves to linear materials with small strains within each elastic body (strip-like beam). Each component of the mechanism is modeled as a beam based on geometrically non-linear 3-D elasticity theory. The component problems are thus split into 2-D analyses of reference beam cross-sections and non-linear 1-D analyses along the three beam reference curves. For the thin rectangular cross-sections considered here, the 2-D cross-sectional nonlinearity is also overwhelming. This can be perceived from the fact that such sections constitute a limiting case between thin-walled open and closed sections, thus inviting the non-linear phenomena observed in both. The strong elastic couplings of anisotropic composite laminates complicate the model further. However, a powerful mathematical tool called the Variational Asymptotic Method (VAM) not only enables such a dimensional reduction, but also provides asymptotically correct analytical solutions to the non-linear cross-sectional analysis. Such closed-form solutions are used here in conjunction with numerical techniques for the rest of the problem to predict more quickly and accurately than would otherwise be possible. Local 3-D stress, strain and displacement fields for representative sections in the component-bars are recovered, based on the stress resultants from the 1-D global beam analysis. A numerical example is presented which illustrates the failure of each component-laminate and the mechanism as a whole.

Thermal vibration analysis of monolayer graphene embedded in elastic medium based on nonlocal continuum mechanics

This paper presents the thermal vibration analysis of single-layer graphene sheet embedded in polymer elastic medium, using the plate theory and nonlocal continuum mechanics for small scale effects. The graphene is modeled based on continuum plate theory and axial stress caused by the thermal effects is also considered. Nonlocal governing equation of motion for this graphene sheet system is derived from the principle of virtual displacements. The closed form solution for thermal-vibration frequencies of a simply supported rectangular nanoplate has been obtained by using the Navier's method of solution. Numerical results obtained by the present theory are compared with available solutions in the literature and the molecular dynamics results. The influences of the small scale coefficient, the room or low temperature, the high temperature, the half wave number and the aspect ratio of nanoplate on the natural frequencies are considered and discussed in detail. The thermal vibration analysis of single- and double-layer graphene sheets are considered for the analysis. The mode shapes of the respective graphene system are also captured in this work. The present analysis results can be used for the design of the next generation of nanodevices that make use of the thermal vibration properties of the graphene.

Nanoindentation studies on waveguides inscribed in chalcogenide glasses using ultrafast laser

Optical straight waveguides are inscribed in GeGaS and GeGaSSb glasses using a high repetition-rate sub-picosecond laser. The mechanical properties of the glasses in the inscribed regions, which have undergone photo induced changes, have been evaluated by using the nanoindentation technique. Results show that the hardness and elastic modulus of the photo-modified glasses are significantly lower as compared to the other locations in the waveguide, which tend to be similar to those of the unexposed areas. The observed mechanical effects are found to correlate well with the optical properties of the waveguides. Further, based on the results, the minimum threshold values of hardness and elastic modulus for the particular propagation mode of the waveguide (single or multi), has been established.

Carbonaceous nickel oxide nano-composites: as electrode materials in electrochemical capacitor applications

Carbonaceous nickel oxide powder samples have been synthesized from an adducted nickel beta-ketoester complex used as a ``single source precursor'' through a solution-based microwave-assisted chemical route. Comprehensive analysis of the resulting powder material has been carried out using various characterization techniques. These analysis reveal that, depending on the solvent used, either NiO/C or Ni/NiO/C composites are formed, wherein Ni and/or NiO nanocrystals are enveloped in amorphous carbon. As the components emerge from the same molecular source, the composites are homogeneous on a fine scale, making them promising electrode materials for supercapacitors. Electrochemical capacitive behavior of these oxide composites is studied in a three-electrode configuration. With a specific capacitance of 113 F g(-1), Ni/NiO/C is superior to NiO/C as capacitor electrode material, in 0.1 M Na2SO4 electrolyte. This is confirmed by impedance measurements, which show that charge-transfer resistance and equivalent series resistance are lower in Ni/NiO/C than in NiO/C, presumably because of the presence of metallic nickel in the former. The cyclic voltammograms are nearly rectangular and the electrodes display excellent cyclability in different electrolytes: Na2SO4, KOH and Ca(NO3)(2)center dot 4H(2)O. Specific capacitance as high as 143 F g(-1), is measured in Ca(NO3)(2)center dot 4H(2)O electrolyte.

Spark plasma sintering may lead to phase instability and inferior mechanical properties: A case study with TiB2

In contrast to the widely reported beneficial aspects of spark plasma sintering in developing materials with better properties, we report here two interesting aspects recorded with difficult-to-sinter titanium diboride: (a) in situ formation of second phase (TiB) and (b) inferior hardness (by similar to 30%) and elastic modulus (by similar to 20%) for spark plasma sintered TiB2, with respect to hot pressed TiB2. The formation of TiB is discussed with reference to the enhanced reaction kinetics in the presence of pulsed electric field. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Improving GPGPU Concurrency with Elastic Kernels

Each new generation of GPUs vastly increases the resources available to GPGPU programs. GPU programming models (like CUDA) were designed to scale to use these resources. However, we find that CUDA programs actually do not scale to utilize all available resources, with over 30% of resources going unused on average for programs of the Parboil2 suite that we used in our work. Current GPUs therefore allow concurrent execution of kernels to improve utilization. In this work, we study concurrent execution of GPU kernels using multiprogram workloads on current NVIDIA Fermi GPUs. On two-program workloads from the Parboil2 benchmark suite we find concurrent execution is often no better than serialized execution. We identify that the lack of control over resource allocation to kernels is a major serialization bottleneck. We propose transformations that convert CUDA kernels into elastic kernels which permit fine-grained control over their resource usage. We then propose several elastic-kernel aware concurrency policies that offer significantly better performance and concurrency compared to the current CUDA policy. We evaluate our proposals on real hardware using multiprogrammed workloads constructed from benchmarks in the Parboil 2 suite. On average, our proposals increase system throughput (STP) by 1.21x and improve the average normalized turnaround time (ANTT) by 3.73x for two-program workloads when compared to the current CUDA concurrency implementation.

Rheometry of dense granular materials: the crucial effects of gravity and confining walls

We describe here the rheological response of dense, slowly deforming granular materials to shear in a cylindrical Couette cell. All components of the stress on the outer cylinder are measured pointwise as a function of the depth, for different methods of construction of the bed that presumably lead to distinct fabrics. The static stress profiles for the different construction protocols are different, but a stress profile that is independent of construction history emerges when the granular column is sheared for sufficient time, in accord with the predictions of plasticity theories. However the qualitative features of the the stress profile under shear differs radically from the predictions of plasticity theories and data reported in earlier studies. We discuss a hypothesis for the anomalous stress profiles that was proposed recently by us, and the ways in which further experiments may to conducted to verify it.

Experimental Studies on the Kinematics of Cutting in Granular Materials

Slow flow in granular materials is characterized by high solid fraction and sustained inter-particle interaction. The kinematics of trawling or cutting is encountered in processes such as locomotion of organisms in sand; trawl gear movement on a soil deposit; plow movement; movement of rovers, earth moving equipment etc. Additionally, this configuration is very akin to shallow drilling configuration encountered in the mining and petroleum industries. An experimental study has been made in order to understand velocity and deformation fields in cutting of a model rounded sand. Under nominal plane strain conditions, sand is subjected to orthogonal cutting at different tool-rake angles. High-resolution optical images of the region of cutting were obtained during the flow of the granular ensemble around the tool. Interesting kinematics underlying the formation of a chip and the evolution of the deformation field is seen in these experiments. These images are also analyzed using a PIV algorithm and detailed information of the deformation parameters such as velocity, strain rate and volume change is obtained.

On the toughness enhancement in hydroxyapatite-based composites

Among various biologically compatible materials, hydroxyapatite (HA) has excellent bioactivity/osteointegration properties and therefore has been extensively investigated for biomedical applications. However, its inferior fracture toughness limits the wider applications of monolithic HA as a load-bearing implant. To this end, HA-based biocomposites have been developed to improve their mechanical properties (toughness and strength) without compromising biocompatibility. Despite significant efforts over last few decades, the toughness of HA-based composites could not be enhanced beyond 1.5-2 MPa m(1/2), even when measured using indentation techniques. In this perspective, the present work demonstrates how spark plasma sintering can be effectively utilized to develop hydroxyapatite titanium (HA-Ti) composites with varying amounts of Ti (5, 10 and 20 wt.%) with extremely high single edge V-notch beam fracture toughness (4-5 MPa m(1/2)) along with a good combination of elastic modulus and flexural strength. Despite predominant retention of HA and Ti, the combination of critical analysis of X-ray diffraction and transmission electron microscopy investigation confirmed the formation of the CaTi4(PO4)(6) phase with nanoscale morphology at the HA/Ti interface and the formation of such a phase has been discussed in reference to possible sintering reactions. The variations in the measured fracture toughness and work of fracture with Ti addition to the HA matrix were further rationalized using the analytical models of crack bridging as well as on the basis of the additional contribution from crack deflection. The present work opens up the opportunity to further enhance the toughness beyond 5 MPa m(1/2) by microstructural designing with the desired combination of toughening phases. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Rheometry of granular materials in cylindrical Couette cells: Anomalous stress caused by gravity and shear

The cylindrical Couette device is commonly employed to study the rheology of fluids, but seldom used for dense granular materials. Plasticity theories used for granular flows predict a stress field that is independent of the shear rate, but otherwise similar to that in fluids. In this paper we report detailed measurements of the stress as a function of depth, and show that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. In the static state, a part of the weight of the material is transferred to the walls by a downward vertical shear stress, bringing about the well-known Janssen saturation of the stress in vertical columns. When the material is sheared, the vertical shear stress changes sign, and the magnitudes of all components of the stress rise rapidly with depth. These qualitative features are preserved over a range of the Couette gap and shear rate, for smooth and rough walls and two model granular materials. To explain the anomalous rheological response, we consider some hypotheses that seem plausibleapriori, but showthat none survive after careful analysis of the experimental observations. We argue that the anomalous stress is due to an anisotropic fabric caused by the combined actions of gravity, shear, and frictional walls, for which we present indirect evidence from our experiments. A general theoretical framework for anisotropic plasticity is then presented. The detailed mechanics of how an anisotropic fabric is brought about by the above-mentioned factors is not clear, and promises to be a challenging problem for future investigations. (C) 2013 AIP Publishing LLC.

Li3MRuO5 (M = Co, Ni), new lithium-rich layered oxides related to LiCoO2: promising electrochemical performance for possible application as cathode materials in lithium ion batteries

We describe the synthesis and crystal structure of Li3MRuO5 (M = Co and Ni), new rock salt related oxides. Both the oxides crystallize in the layered LiCoO2 (alpha-NaFeO2) structure, as revealed by powder XRD data. Magnetic susceptibility data suggest that the oxidation states of transition metals are Li3Co3+(ls)Ru4+(ls) O-5 (ls = low spin) for the M = Co compound and Li3Ni2+Ru5+O5 for the M = Ni compound. Electrochemical investigations of lithium deintercalation-intercalation behaviour reveal that both Co and Ni phases exhibit attractive specific capacities of ca. 200 mA h g(-1) at an average voltage of 4 V that has been interpreted as due to the oxidation of Co3+ and Ru4+ in Li3CoRuO5 and Ni2+ to Ni4+ in the case of Li3NiRuO5. Thus, a different role of Ru ions is played in the isostructural oxides. Finally, in both cases evidence of irreversible behaviour above 4.2 V is observed and interpreted as formation of high valent ions or alternatively oxidation of oxide ions.

Dielectric and electrical conductivity properties of multi-stage spark plasma sintered HA-CaTiO3 composites and comparison with conventionally sintered materials

One of the different issues limiting the wider application of monolithic hydroxyapatite (HA) as an ideal bone replacement material is the lack of reasonably good electrical transport properties. The comprehensive electrical property characterization to evaluate the efficacy of processing parameters in achieving the desired combination of electroactive properties is considered as an important aspect in the development of HA-based bioactive material. In this perspective, the present work reports the temperature (RT-200 degrees C) and frequency (100 Hz-1 MHz) dependent dielectric properties and AC conductivity for a range of HA-CaTiO3 (HA-CT) composites, densified using both conventional pressureless sintering in air as well as spark plasma sintering in vacuum. Importantly, the AC conductivity of spark plasma sintered ceramics similar to upto 10(-5) (Omega cm)(-1)] are found to be considerably higher than the corresponding pressureless sintered ceramics similar to upto 10(-8) (Omega cm)(-1)]. Overall, the results indicate the processing route dependent functional properties of HA-CaTiO3 composites as well as related advantages of spark plasma sintering route. (C) 2013 Elsevier Ltd. All rights reserved.

Strain-controlled fatigue in B-modified Ti-6Al-4V alloys

The strain-controlled fatigue behaviour of Ti-6Al-4V alloy with up to 0.11 wt.% B addition was investigated. Results show significant softening when the strain amplitudes, Delta epsilon(T)/2, are >= 0.75%. B addition was found to improve the fatigue life for Delta epsilon(T)/2 <= 0.75% as it corresponds to the elastic regime and hence is strength dominated. At Delta epsilon(T)/2 = 1%, in contrast, the base alloy exhibits higher fatigue life as TiB particle cracking due to strain incompatibility causes easy crack nucleation in the B-modified alloys. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Small-scale mechanical testing of materials

Small-scale mechanical testing of materials has gained prominence in the last decade or so due to the continuous miniaturization of components and devices in everyday application. This review describes the various micro-fabrication processes associated with the preparation of miniaturized specimens, geometries of test specimens and the small scale testing techniques used to determine the mechanical behaviour of materials at the length scales of a few hundred micro-meters and below. This is followed by illustrative examples in a selected class of materials. The choice of the case studies is based on the relevance of the materials used in today's world: evaluation of mechanical properties of thermal barrier coatings (TBCs), applied for enhanced high temperature protection of advanced gas turbine engine components, is essential since its failure by fracture leads to the collapse of the engine system. Si-based substrates, though brittle, are indispensible for MEMS/NEMS applications. Biological specimens, whose response to mechanical loads is important to ascertain their role in diseases and to mimic their structure for attaining high fracture toughness and impact resistance. An insight into the mechanisms behind the observed size effects in metallic systems can be exploited to achieve excellent strength at the nano-scale. A future outlook of where all this is heading is also presented.