M. S. Bobji

Estimation of hardness by nanoindentation of rough surfaces

The roughness of a surface influences the surface mechanical properties, estimated using nanoindentation data. Assuming a relation between the penetration depth normalized with respect to a roughness scale parameter, and the effective radius encountered by the indenter, a first order model of roughness dependency of hardness is proposed. The practical usefulness of this model is verified by the numerical simulation of nanoindentation on a fractal surface. As the roughness of a surface is increased, the hardness measured at depths comparable with the roughness scale deviates increasingly from the actual hardness. Given the constants related to indenter geometry, the present work provides a rationale and a method for deconvoluting the effect of roughness in arriving at real hardness characteristics of the near surface region of a material.

Time dependence of effective slip on textured hydrophobic surfaces

In this paper, we present results on water flow past randomly textured hydrophobic surfaces with relatively large surface features of the order of 50 µm. Direct shear stress measurements are made on these surfaces in a channel configuration. The measurements indicate that the flow rates required to maintain a shear stress value vary substantially with water immersion time. At small times after filling the channel with water, the flow rates are up to 30% higher compared with the reference hydrophilic surface. With time, the flow rate gradually decreases and in a few hours reaches a value that is nearly the same as the hydrophilic case. Calculations of the effective slip lengths indicate that it varies from about 50 µm at small times to nearly zero or “no slip” after a few hours. Large effective slip lengths on such hydrophobic surfaces are known to be caused by trapped air pockets in the crevices of the surface. In order to understand the time dependent effective slip length, direct visualization of trapped air pockets is made in stationary water using the principle of total internal reflection of light at the water-air interface of the air pockets. These visualizations indicate that the number of bright spots corresponding to the air pockets decreases with time. This type of gradual disappearance of the trapped air pockets is possibly the reason for the decrease in effective slip length with time in the flow experiments. From the practical point of usage of such surfaces to reduce pressure drop, say, in microchannels, this time scale of the order of 1 h for the reduction in slip length would be very crucial. It would ultimately decide the time over which the surface can usefully provide pressure drop reductions. ©2009 American Institute of Physics

Effect of roughness on the measurement of nanohardness—a computer simulation study

This letter reports a simulation study of depth sensing nanoindentation on a rough surface. The estimated mean hardness and scatter are both influenced by roughness, irrespective of whether there is a genuine variation in mechanical property with deformation volume/depth. The scatter due to roughness always decreases with penetration depth. The roughness dependency of hardness is the product of two terms. The first term is independent of property variation with deformation volume/ depth and arises due to the discrete nature of multiple contacts during indentation. The second term is due to property variation with deformation volume/depth, as changing roughness changes the aggregate strength of each contact island.

A miniaturized TEM nanoindenter for studying material deformation in situ

A miniaturized nanoindenter system has been designed and fabricated to carry out localized in situ deformation studies in a high resolution transmission electron microscope (TEM). The coarse positioning is carried out with the help of small inertial drives so that the whole system could fit into the specimen holder of the JEOL 2010 microscope. The fine positioning is achieved with a piezoelectric tube and the force is measured with the help of a four bar flexible hinge spring element. The ability of the system to correlate the force–distance data with the events observed in TEM is demonstrated.

Indentation mechanics of Cu-Be quantified by an in situ transmission electron microscopy mechanical probe

In situ transmission electron microscopy was used to study, in real time, the sub-surface deformation taking place in Cu-Be alloy during nanoindentation. A twinned region of the material was indented with a sharp tungsten tip in a specially developed transmission electron microscopy (TEM) holder. A flexible hinge-based force sensor was used to measure the force on the indenter, and the force-displacement curve for the tip was obtained by tracking the tip in the sequential images of a TEM video of the indentation process. Step-like structures similar to 50 nm in size resulting from the tip surface roughness were observed to generate clusters of dislocations in the sample when they come in contact with the softer Cu-Be. With this setup, the forces and the mean pressure associated with such an individual deformation event in a nanostructured TEM sample were measured.

Influence of surface roughness on the scatter in hardness measurements - A numerical study

Indentation hardness is usefully applied in the field of rock comminution, where high contact stresses are commonly encountered. Hardness measurements provide a means for estimating the discontinuity wall strength which is required to understand the shear strength of the discontinuity. The hardness value is also used to estimate compressive strength of rocks with known density. Hardness is measured by penetrating a hard indenter of known geometry onto the rock, either quasi-statically or dynamically. However, the scatter involved in such measurements is quite large. Some of the reasons for the scatter could be attributed to local variation in the composition and grain size of the minerals, bond strength of the matrix, presence of flaws and geometric irregularities (roughness) of the surface under the indentation region. The scatter due to surface irregularities or roughness can be reduced by polishing the surface of the rock. Polishing of rock surface becomes diAcult particularly during in situ measurements. In such situations the study of the eAect of surface roughness on the scatter in hardness measurements becomes imperative. In this paper, the quasi-static indentation of a fractal surface is numerically simulated to characterize the scatter resulting from surface roughness in terms of surface roughness parameters such as fractal dimension, D and the root mean square (rms )o f asperity height.

Springing the trap

Charles Darwin is known the world over as the founder (along with A R Wallace) of modern evolutionary biology. But he wrote a great many books besides The Origin of Species, and all of them illuminate the astonishing ways in which evolution works. In one of those books, Insectivorous Plants, Darwin examined plants that ate animals – in contrast to the usual situation, which is the other way round. Carnivorous plants seem to violate another of nature’s rules: some of them possess the property of thigmonasty or touch-induced movement. Because they display the behaviour without nerves or muscles (though they are not unique in this; see Bonner 1994), carnivorous and sensitive plants, like the familiar Mimosa pudica (touch-me-not), raise the question of where to draw the boundary between plants and animals.

Friction-formed liquid droplets.

The formation of nanoscale liquid droplets by friction of a solid is observed in real-time. This is achieved using a newly developed in situ transmission electron microscope (TEM) triboprobe capable of applying multiple reciprocating wear cycles to a nanoscale surface. Dynamical imaging of the nanoscale cyclic rubbing of a focused-ion-beam (FIB) processed Al alloy by diamond shows that the generation of nanoscale wear particles is followed by a phase separation to form liquid Ga nanodroplets and liquid bridges. The transformation of a two-body system to a four-body solid-liquid system within the reciprocating wear track significantly alters the local dynamical friction and wear processes. Moving liquid bridges are observed in situ to play a key role at the sliding nanocontact, interacting strongly with the highly mobile nanoparticle debris. In situ imaging demonstrates that both static and moving liquid droplets exhibit asymmetric menisci due to nanoscale surface roughness. Nanodroplet kinetics are furthermore dependent on local frictional temperature, with solid-like surface nanofilaments forming on cooling. TEM nanotribology opens up new avenues for the real-time quantification of cyclic friction, wear and dynamic solid-liquid nanomechanics, which will have widespread applications in many areas of nanoscience and nanotechnology.

The formation of carbon nanostructures by in situ TEM mechanical nanoscale fatigue and fracture of carbon thin films

A technique to quantify in real time the microstructural changes occurring during mechanical nanoscale fatigue of ultrathin surface coatings has been developed. Cyclic nanoscale loading, with amplitudes less than 100 nm, is achieved with a mechanical probe miniaturized to fit inside a transmission electron microscope (TEM). The TEM tribological probe can be used for nanofriction and nanofatigue testing, with 3D control of the loading direction and simultaneous TEM imaging of the nano-objects. It is demonstrated that fracture of 10-20 nm thick amorphous carbon films on sharp gold asperities, by a single nanoscale shear impact, results in the formation of <10 nm diameter amorphous carbon filaments. Failure of the same carbon films after cyclic nanofatigue, however, results in the formation of carbon nanostructures with a significant degree of graphitic ordering, including a carbon onion.

Advanced transmission electron microscope triboprobe with automated closed-loop nanopositioning

Here the design and operation of a novel transmission electron microscope (TEM) triboprobe instrument with real-time vision control for advanced in situ electron microscopy is demonstrated. The NanoLAB triboprobe incorporates a new high stiffness coarse slider design for increased stability and positioning performance. This is linked with an advanced software control system which introduces both new and flexible in situ experimental functional testing modes, plus an automated vision control feedback system. This advancement in instrumentation design unlocks new possibilities of performing a range of new dynamical nanoscale materials tests, including novel friction and fatigue experiments inside the electron microscope.