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Sapphire is an important ceramic material which finds applications in fields such as temperature sensing, optics, electronics, and ceramic bearings. Polishing of sapphire has always been a difficult task for industries and research communities. Hydrodynamic polishing (HDP) is one of the prominent methods used for polishing of hard and profiled surfaces, whereas rigid tool-based methods such as diamond turning, grinding, and honing have many limitations. The HDP process involves deterministic flow of abrasive particles in the slurry between the workpiece surface and a rotating soft tool to obtain the desired surface finish. A novel experimental setup has been fabricated to realize the conformal hydrodynamic nanopolishing on single crystal sapphire cavity. In this study, the experiments were conducted to understand the effect of abrasive particle size, basicity of slurry, and change in temperature of slurry on the polishing of machined sapphire cavity. The effect of the initial surface roughness of the machined cavity on conformal hydrodynamic nanopolishing has also been investigated. A microcrack/pit-free surface has been found after the final polishing of the sapphire cavity. An improvement of 21% is found in surface finish after the final polishing using abrasive particle size of 0.06 µm. Abrasive slurry with higher basicity (pH 13) does not improve the surface finish. By heating the abrasive slurry to a temperature of 70°C–75°C, surface finish improves by ∼26% as compared to improvement of ∼ 21% at room temperature polishing.

Ceramics, being brittle in nature are highly prone to surface damage during machining. However, at depths of cut lower than a critical depth of cut, brittle materials undergo microscopic plastic shearing resulting in a crack-free surface. The transition from ductile to brittle takes place over a finite time and depth variation. The identification and characterization of the transition zone is important to attain damages free surface grinding. This work focuses on the identification of the ductile-brittle transition (DBT) zone in yttria stabilized sintered zirconia via the analyses of the force signatures. An algorithm has been developed for the prediction of the onset and end of the transition zone based on the gradients and fluctuations in the cutting force signals. The visual inspection of the ground surface validates the proposed methodology based on force signal processing. The transition zone characteristics were found to be affected by the process parameters and the tool geometry. The increase in the scratch speed reduced the transition onset depth but had negligible effect on the transition end depth. Smaller tool tip radius resulted in increased surface damage whereas larger tip radius gave a relatively smoother surface finish. This methodology of accurate prediction of DBT zone using cutting/thrust force behavior allows the manufacturers to achieve crack-free surfaces in brittle materials without keeping track of cutting depth during machining. Thus, the prediction of the ductile regime by the analysis of force signals opens new horizons in deciding threshold force and depth values in real time, which should be maintained to achieve a damage free surface during grinding.

Various bifunctional metal-oxide composites have recently been proposed as advanced hydrogen oxidation reaction (HOR) electrocatalysts for anion-exchange membrane fuel cells (AEMFCs). It is postulated that metal and oxide are active sites for the adsorption of hydrogen/proton and hydroxide ions, respectively. Of particular interest are the so-called buried interfaces. To investigate processes governing activity and stability at such interfaces, we prepare model Pd and Pt electrocatalysts which are fully covered by thin CeOxfilms. We investigate how oxide thickness influences HOR activity and dissolution stability of the electrocatalysts. It is found that materials behave very differently and that only Pd exhibits an enhanced HOR activity, while both oxide-protected metals are more stable toward dissolution. A 10-fold decrease in dissolution and 15-fold increase in HOR exchange current density are demonstrated for the optimized Pd/CeOxcomposites in comparison to pure Pd. We assess the mechanism of the electrocatalytic improvement as well as the role of the protective oxide films in such systems through advanced electrochemical and physical analysis. It is highlighted that a uniform, semipermeable oxide layer with a maximized electrocatalyst–oxide interface is crucial to form HOR catalysts with improved activity and stability.

Accomplishment of good dimensional accuracy and acceptable surface integrity in micro drilling of Ni-based superalloys has always been a challenge. The current study seeks to explore these aspects and correlate the same with spindle speed, feed and forces in micro drilling of Incoloy 825 under flood cooling condition. Spindle speeds of 10,000, 20,000 and 30,000 rpm along with four different feeds of 1, 4, 8, 12 μm/rev were considered in the present work. Thrust force exhibited increasing trend with cutting parameters. The dynamic fluctuation (%) for radial forces was considerably higher compared to that of thrust force. A gradual transition from ploughing to shearing was discernible when feed was increased from 1 to 4 μm/rev. Oversize error was found to be closely related with radial forces. Circumferential deformation layer, white layer and plastic deformation zone at sub-surface region along hoop direction showed distinct relationship with spindle speed and feed. However, spindle speed has more pronounced influence on white layer formation than feed. After careful analysis of results, it was observed that optimal results in the form of superior dimensional accuracy, minimum circumferential damage, burr formation, minimum radial force, and reasonably less value of thrust force were achieved under the feed of 4 μm/rev and spindle speed of 10,000 rpm.

Micromilling process finds applications in biomedical and aerospace industries to produce complex 3D micro-scale features and components. One of the key limitations of micromachining is the low flexural stiffness of the micro-tool due to small diameter. For machining of difficult-to-cut materials like Ti alloys, high rotational speeds are typically used to reduce the chip load and cutting forces. High speed micromilling of Ti alloys generates high temperature in cutting zone due to low thermal conductivity which can lead to a variation in the cutting forces and excessive tool wear. The friction at the tool-chip interface between the workpiece and the cutting tool also affects the cutting forces. It may be noted that abrasion resistant coatings on cutting tool can play important role in reduction of the tool wear and friction. In addition, the solid lubricant coatings over the abrasion resistant coating can further reduce the friction and heat generation between the surface and the cutting tool. This study is focused on investigating the effect of coatings on micro-tool in high speed micromilling of Ti-6Al-4 V. Experiments were carried out at a rotational speed of 50,000 rpm and different depth of cut using uncoated, anti-abrasion coating of TiAlN and TiAlSiN and anti-friction coating of solid lubricant WS2. The cutting forces and specific cutting pressures were compared for machining with different coated tools and uncoated tools. A significant reduction in the cutting forces was found if dry lubricant coating of WS2is used with TiAlN. The machined surface finish shows an improvement if the solid lubricant coatings are used. A comprehensive assessment of the tool damage was done to understand the progression of wear with different coatings. Stability limits have been predicted and compared for diiferent tool coatings. Dry lubricant coating over TiAlN coated tool shows a maximum increase of ∼47% in stability limits.

This work presents a comprehensive analysis of the mechanical, metallurgical and tribological responses of the various micro-scale laser hardened patterns of EN-8 steel at different process conditions. A 100 W continuous wave fibre laser with ~200-900 µm beam diameter was used for micro-scale hardening. EBSD was used to investigate the micro-structure and the hardness values were measured using micro hardness tester (Shimadzu-HMV). Additionally, to measure the tribological response, four different hardened patterns were analysed using the 'pin on disk setup' (DUCOM wear and friction monitor TR-20LE). The preliminary results revealed improved surface hardness values with increase in percentage overlap due to the formation of martensite. Furthermore, it was observed that the average wear rate of hardened specimen decreases by ~91% as compared to the unhardened specimen. The results revealed an inverse relationship between friction force and percent-hardened area, with minimum friction force obtained for a fully hardened pattern.

The micromilling process is increasingly being used in biomedical, defence, electronics and aerospace industries for fabrication of miniaturized products of complex shapes. For difficult-to-cut materials like Ti alloys, limited stiffness of the micro-tool is a major impediment. This limitation can be overcome by using high rotational speeds which leads to reduction in chip load and, therefore, the cutting forces. However, high spindle speeds and low tool stiffness can render the process unstable due to dynamic variation in cutting forces. High speed micromilling of Ti6Al4V generates high temperature in cutting zone due to low thermal conductivity of Ti alloys which can also lead to a variation in cutting force and, hence, dynamic instability. Cutting fluid can play an important role because of its capacity to reduce friction and its ability to dissipate the heat generated between the micro tools and the workpiece. This study is focused on studying the effect of lubrication on the cutting forces and dynamic instability called chatter in high speed micromilling of Ti6Al4V. Experiments were carried out at different spindle speeds and feeds in both dry and lubricated condition. A significant reduction up to 38% in the cutting forces has been found in lubricated machining as compared with the dry machining. Two distinct regimes, lubrication sensitive (at rotational speeds >47,000rpm) and lubrication insensitive (at rotational speeds <47,000rpm) have been observed in this study. A critical spindle speed of 47,000rpm has been identified which corresponds to the onset of lubrication dominant regime. To capture the effect of process mechanics, mechanistic force model using velocity-chip load dependent coefficients has been used to predict the forces and stability boundary. The predicted stability boundary limits have been compared with the experimental onset of chatter at four different spindle speeds. FFT spectrum analysis of acceleration of workpiece has been done to detect the process instability.

Microdrilling can be used to create micro holes (<100μm) in turbine blades, guide vanes and medical implants made of Ti6Al4V which has high strength to weight ratio and excellent biocompatibility. Microdrills have limited flexural stiffness which can lead to catastrophic tool failure. The undesirable microburr formation is another major impediment. To counter this lack of stiffness, high rotational speeds can be used to reduce the chip load and forces. It is expected that lower forces could reduce the burr formation as well. It may be noted that macro-drilling models have been reported in the literature but these models do not adequately represent the microdrilling regime where the chip thickness is of the order of microns and cutting velocities are very high. Consequently, in this paper, a mechanistic drilling force model based on specific normal and friction forces has been developed as a function of the feed rate and spindle speeds. The forces were determined by summation of specific cutting forces over the cutting edge and using oblique cutting model to evaluate forces in the feed direction. Experiments have been carried out to validate the force model using microdrills of 300μm, spindle speeds of 60,000, 80000 and 100000rpm and the feed of 0.0167, 0.025 and 0.03μm/rev. Specific normal forces (Kn) was calculated for different feed rate and cutting velocity. The model is in good agreement with the experimental results. The burr model has also been developed which is based on energy balance where the net deformation work is used in shear, elongation and bending. The burr height prediction model is in good agreement with the experimental results.

The urea oxidation reaction (UOR) is technologically important for the development of a renewable energy infrastructure. Urea electrolysis (UE) can be used to produce hydrogen much more cost‐effectively than water electrolysis, as it theoretically requires 93% less energy. Urea can also be used as fuel in direct urea fuel cells (DUFCs), instead of H2, and thus serve as an efficient hydrogen carrier. This review addresses the UOR in neutral, acidic, and alkaline electrolytes, with special emphasis on the latter. Recent developments in Ni‐based catalysts for urea oxidation (UO) in alkaline electrolytes are discussed in detail, highlighting proposed reaction mechanisms and intermediates, based on experimental and computational results. Various catalytic designs used to mitigate the UO kinetic barriers, including the use of transition metal oxides and alloys, as well as tailored surface support materials, and discuss their application in UE and DUFCs are presented. The significant challenges impeding advances in urea electrocatalysis, in addition to emerging research areas in this field, are also discussed. Urea electrooxidation (UEO) is central to urea electrolysis and direct urea fuel cells, both of which are important components for the development of a renewable energy infrastructure. This review addresses the mechanistic aspects of UEO, the various types of UEO catalysts reported to date, and energy‐related applications thereof. It also highlights the challenges still remaining in understanding and implementing UEO.

A physician palpates a tissue to detect an embedded tumor nodule by sensing an increase in local tissue stiffness and nodule size. The Hertz contact model, however, is unable to predict the material or physical properties of a tumor nodule embedded in a healthy tissue of finite thickness. In this study, utilizing a hyperelastic material model, we propose a general methodology to analyze the extent to which the stiffness, size, and depth of a nodule embedded in a tissue affect its detectability. Using dimensional analysis, we generate simple power-law relations to predict physical and material properties of tumor nodules embedded in healthy tissue during indentation. Our results indicate that indenter radius and indentation depth are critical parameters in nodule detection and a thin indenter and large indentation depth increase detection sensitivity of an embedded tumor nodule. Our results also show that anisotropic material properties of either a tissue or an embedded nodule render the embedded tumor nodule undetectable using indentation. We define palpation sensitivity maps that can be used to predict material and physical properties of tumor nodules in healthy tissues. The analysis and results presented in this study might increase accuracy and precision in instrumented probe-based laparoscopic or robotic surgeries.

Herein, a unique anion‐exchange membrane fuel cell (AEMFC) containing only affordable and abundant materials is presented: NiFe hydrogen oxidation reaction (HOR) and nitrogen‐doped carbon oxygen reduction reaction (ORR) electrocatalysts. AEMFCs are an attractive alternative to proton‐exchange membrane fuel cells. They can run under alkaline conditions, allowing the use of platinum group metal (PGM)‐free electrocatalysts. Yet, the same alkaline conditions incur an overpotential loss in ORR and also slow the HOR. This can be solved by using PGM electrodes, but then the original advantage disappears. In contrast, the fuel cell is free of both PGMs and critical raw materials (CRMs). Electrochemical studies confirmed that the catalysts are highly active in both HOR and ORR in an alkaline electrolyte. The morphology, composition, and chemical states of the electrocatalysts are characterized by different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), and electron energy loss spectroscopy (EELS). Then, the electrocatalysts’ performance is tested in a fuel cell device. The cell gives a maximum power density of 56 mW cm−2and a limiting current density of 220 mA cm−2. These results are among the best CRM‐free anion‐exchange membrane fuel cells reported to date. Any long‐term sustainable solution to the energy transition challenge will need fuel cells that avoid critical raw materials. Herein, such an option using only nickel, iron, carbon, and nitrogen is presented.

Fracture in cutting of ductile as well as brittle materials can be characterized using parameters such as K, G, R, and J-integral; of these, R has been widely used. To accurately evaluate the contribution of fracture energy in total cutting energy, J-integral would provide a more comprehensive basis as it encompasses several fracture modes, material plasticity, and nonlinear behavior. Therefore, this work adopts J-integral to evaluate the contribution of fracture energy to the size effect during microcutting of AISI 1215 steel. The work uses explicit integration method within ansys/ls-dyna to simulate two-dimensional (2D) orthogonal microcutting. U- and V-shaped cutting edges were used to represent a sharp crack-tip and a blunt crack-tip, respectively. Considering several alternative contours around crack-tip, covering the plastic zone, J-integral was calculated. Upon benchmarking J-integral values with other simulations in the literature, the approach was adopted for microcutting simulations in this work. It is observed that J-integral increases with uncut chip thickness, whereas it decreases with cutting speed, rake angle, and tool edge radius. The term (J/t0) defines contribution of fracture to the size effect in terms of J-integral, which is in the range of 4–24% under various parametric conditions. The corresponding values of R were always found to lie above those of the J-integral indicating that J-integral is relatively more appropriate parameter to quantify the fracture energy during microcutting.