The mechanics underlying ice–skate friction remain uncertain despite over a century of study. In the 1930s, the theory of self-lubrication from frictional heat supplanted an earlier hypothesis that pressure melting governed skate friction. More recently, researchers have suggested that a layer of abraded wear particles or the presence of quasi-liquid molecular layers on the surface of ice could account for its slipperiness. Here, we assess the dominant hypotheses proposed to govern ice–skate friction and describe experiments conducted in an indoor skating rink aimed to provide observations to test these hypotheses. Our results indicate that the brittle failure of ice under rapid compression plays a strong role. Our observations did not confirm the presence of full-contact water films and are more consistent with the presence of lubricating ice-rich slurries at discontinuous high-pressure zones (HPZs). The presence of ice-rich slurries supporting skates through HPZs merges pressure-melting, abrasion and lubricating films as a unified hypothesis for why skates are so slippery across broad ranges of speeds, temperatures and normal loads. We suggest tribometer experiments to overcome the difficulties of investigating these processes during actual skating trials.

Austenitic stainless steel is used in several industrial branches due to its mechanical and thermal properties, and to its good corrosion resistance. With low cost and biocompatibility, it is used to manufacture prostheses and devices for bone fixation. However, direct contact with body fluids may cause corrosion. Thin films of FeAlCr intermetallic alloy can be used to increase service life of prostheses and avoid replacement surgeries. The aim of this work was to cover the austenitic stainless steel to study the effect of target–substrate distance on the film characteristics. Coatings were performed using the magnetron sputtering technique with the substrate positioned at different distances from the target. The influence on film thickness, morphology, roughness, and adhesion to the substrate was investigated. The thin films of FeAlCr (160 nm thick deposited at 100 mm far from the substrate) were formed by smaller particles (11.2 nm long), densely packed (551,000 particles/mm2), with flat and regular appearance, and greater adherence to the substrate.

In the present study, the effect of Al addition on microstructure evolution, mechanical properties, and wear performances of a newly developed Ni–Si-containing complex brass was studied. The results showed that with increasing Al content from 0 to 3 wt%, the corresponding strengthening phase evolves from δ-Ni2Si to [Ni(Al)]2Si phases. Simultaneously, it is of great interest that the increasing Al addition also brings about a remarkable change in morphology of the secondary strengthening phase from dendrite and thin rod to regular block. Additionally, the hardness, yield strength and tensile strength of complex brass effectively increase with increasing Al content, and the fracture mechanism transforms from cleavage failure and microvoids accumulation fracture to cleavage failure. It was also found that the brass with adding 3 wt% Al exhibits the solidification microstructure with the uniform distribution of the block-shaped strengthening phase and has the best wear resistance. This present study provides a potential strategy for further improving the comprehensive performance of existing complex brass.

A liquid-state pyroelectric energy harvester is described and a remarkable capacity to convert a thermal gradient into electrical energy is demonstrated.

Increasing the sustainability of energy generation can be pursued by harvesting extremely low enthalpy sources: low temperature differences between cold and hot reservoirs are easily achieved in every industrial process, both at large and small scales, in plants as well as in small appliances, vehicles, natural environments, and human bodies. This paper presents the assessment and efficiency estimate of a liquid-state pyroelectric energy harvester, based on a colloid containing barium titanate nanoparticles and ferrofluid as a stabilizer. The liquid is set in motion by an external pump to control velocity, in a range similar to the one achieved by Rayleigh–Bénard convection, and the colloid reservoir is heated. The colloid is injected into a Fluorinated Ethylene Propylene pipe where titanium electrodes are placed to collect electrical charges generated by pyroelectricity on the surface of the nanoparticles, reaching 22.4% of the ideal Carnot efficiency of a thermal machine working on the same temperature drop. The maximum extracted electrical power per unit of volume is above 7 mW/m3 with a ΔT between electrodes of 3.9 K.

Intelligent control of friction and adhesion has attracted much attention for use in soft robotics, human-sensor interfaces, and bionics. Here we introduce a shape memory photonic crystal (SMPC) polymer that can be programmed and recovered by solvent to realize switchable surface friction. Micro sliding test show that the friction coefficient on this SMPC in the programmed and recovered state can vary by three times. We also show that the mechanism behind this switchable friction coefficient is the surface roughness related adhesion.

Mild steel offers a variety of properties for various applications at lower costs giving the alloy a large application base in industry. However, the increasing complexity and severity of service environments has shifted the focus of many industries to structure modification techniques, like heat treatment, to improve material properties and performance. The focus of this paper was to therefore investigate the effect of heat treatment induced hardness on the sliding wear behaviour of mild steel. The results showed that resistance to dry sliding wear increased with increasing hardness of the mild steel samples. Both abrasive and adhesive wear mechanisms were observed to occur on the samples, however abrasive wear was predominant.

In this study, coatings containing Ca and P elements on Ti6Al4V alloy were fabricated by micro-arc oxidation at different applied voltages. Subsequently, evaluation of the phase structure, morphology, element composition, corrosion mechanism, and tribocorrosion behavior of these coatings was performed. The results showed that the coatings consisted of rutile TiO2 and anatase TiO2. The ratio of rutile/anatase, surface roughness, and hardness increase with the increase of applied voltage. Electrochemical impedance spectroscopy results indicated the corrosion resistance of coatings in simulated body fluid of 400 V > 380 V > 420 V. The open circuit potential of sample 400 V declined during the tribocorrosion test. Sample 420 V possessed the highest wear volume after the tribocorrosion process. The tribocorrosion mechanism of samples 380 and 420 V was mainly confirmed as the wear effect, and the decline of corrosion resistance due to the micro-cracks formed during the abrasive wear of the coating accounts for the tribocorrosion mechanism of sample 400 V.

We report on enhanced mechanical, tribological, and surface-wettability characteristics of polymeric films dispersed with inorganic fullerene (IF)-type tungsten disulfide (WS2) nanoparticles derived through a two-step hydrothermal route. Imaging through transmission electron microscopy suggests the occurrence of polyhedral cage-like structures with a visibly nonspherical hollow ranging 55–75 nm. The mechanical stability of IF-type WS2 dispersed in polyvinyl alcohol (PVA) gets improved with increasing nano-inclusions, and upto 6 wt% loading. As compared with nanosheets, the IF-WS2 in PVA at the critical loading offers nearly 28.6, 33.6, and 42% respective improvements as regards, breaking stress, elongation at break, and toughness. Moreover, Stribeck curves in the mixed lubricating regime have revealed a nearly ∼80% reduction of coefficient of friction (COF) due to inclusion of IF-type WS2 in PVA. In the hydrodynamic region, the COF is drastically lowered from a typical value of 0.55 to 0.15 at the maximal sliding velocity with nanoparticle loading and despite the fact that the tribo feature gives a rising trend for a particular curve. Furthermore, exhibiting a progressive increase in water contact angle, a clear transition from the hydrophilic (∼64°) to hydrophobic (∼107°) surface of the nanocomposite films has been witnessed after inclusion of nano IF-WS2. An increased hydrophobicity and lowered surface adhesion and COF values along with marginal drop in surface energy are ensured in the investigated specimens. Investigation of responsive tribological and wetting–dewetting transition would find scope not only in coating and textile industry but also in smart miniaturized components.

In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed.

The field of in situ nanomechanics is greatly benefiting from microelectromechanical systems (MEMS) technology and integrated microscale testing machines that can measure a wide range of mechanical properties at nanometer scales, while characterizing the damage or microstructure evolution in electron microscopes. This article focuses on the latest advances in MEMS-based nanomechanical testing techniques that go beyond stress and strain measurements under typical monotonic loadings. Specifically, recent advances in MEMS testing machines now enable probing key mechanical properties of nanomaterials related to fracture, fatigue, and wear. Tensile properties can be measured without instabilities or at high strain rates, and signature parameters such as activation volume can be obtained. Opportunities for environmental in situ nanomechanics enabled by MEMS technology are also discussed.

Tribology—the study of contacting, sliding surfaces—seeks to explain the fundamental mechanisms underlying friction, adhesion, lubrication, and wear, and to apply this knowledge to technologies ranging from transportation and manufacturing to biomedicine and energy. Investigating the contact and sliding of materials is complicated by the fact that the interface is buried from view, inaccessible to conventional experimental tools. In situ investigations are thus critical in visualizing and identifying the underlying physical processes. This article presents key recent advances in the understanding of tribological phenomena made possible by in situ experiments at the nanoscale. Specifically, progress in three key areas is highlighted: (1) direct observation of physical processes in the sliding contact; (2) quantitative analysis of the synergistic action of sliding and chemical reactions (known as tribochemistry) that drives material removal; and (3) understanding the surface and subsurface deformations occurring during sliding of metals. The article also outlines emerging areas where in situ nanoscale investigations can answer critical tribological questions in the future.

The present work aims to explore the tribological behavior of the laser texturing process with different patterns on a Co based alloy using a pulsed Nd: YAG laser. Different parameters such as peak power, speed and spot diameter of the laser were explored for the experimental setting. The microstructural analysis was performed using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and nanoindentation tests. The influence of the different textured patterns of the tribological performance tested with a pin-on-disc tribometer under lubricated sliding condition was evaluated. The results showed that typical interdendritic structures in the as-cast CoCr alloy condition were refined due to the heat input and fast cooling rate during the laser treatment. The refined microstructure showed less volumetric loss resulting in a increment of wear resistance and a better tribological performance than as-cast alloy. The correlation among laser parameters, microstructural effect and their influence in wear resistance is discussed.

Since ancient times, lubricants have been used to reduce friction and wear problems of mechanical systems. However, nowadays, there is a constant effort to improve their performance through additives so that they can accomplish properly in this modern world. In that sense, in this study, it was proposed the use of Poly(ε-Caprolactone) (PCL) as a biodegradable additive in Castor oil. The effect that this additive has on the tribological properties of AISI 4140 steel/Al2O3 tribosystem was analyzed. For this purpose, PCL was dissolved in Castor oil at 65 °C for 15 minutes. Later, once the lubricant formulations got into room temperature, friction tests were conducted with a ball-on-disk configuration. Several experiments were systematically carried out in order to study the factors that could influence the performance of the tribological system, for instance: additive concentration, velocity, temperature, and wear track radius. The kinetic friction coefficient was used to analyze the results as an output variable. The parameters in which the best friction behavior was observed were employed again to compare the efficiency of the polymeric additive by profoundly analyzing and comparing the wear response of the system. The PCL additive showed great results by decreasing friction up to 30% compared to the neat Castor oil. Nevertheless, as the opposite effect, this additive increased the steel wear to almost half an order of magnitude. Given the above, this investigation showed that, with further studies, Poly(ε-Caprolactone) could be used as an additive in vegetable oil-based lubricants for the improvement of friction performance.

The aim of this investigation was to study castor, canola, and sesame vegetable oils in order to evaluate their potential use as lubricants in steel mechanical components. For this purpose, densities of each oil were evaluated using the pycnometer method, as well as their dynamic viscosities through a Brookfield DV-II rotational viscometer. Both properties were evaluated at temperatures of 25, 40 and 100 °C. Additionally, viscosity indexes were determined according to ASTM D 2270. These rheological properties were used to estimate the lubrication regime considering parameters of real contact conditions in mechanical components. Friction and wear analyses were carried out to investigate the behaviour of the vegetable oil as lubricants. Such tests were carried out at room temperature on a CSM tribometer with pin-on-disk configuration by using castor, canola and sesame oils as lubricants. AISI 4140 hardened steel against AISI 100Cr6 steel pin was used as a mechanical component. From the rheological study, it was observed that canola and sesame oils behave as dilatant fluids at the evaluated temperatures, while castor oil behaves like a Newtonian fluid at 25 and 40 °C. Castor oil showed the highest density value among oils studied, but it also exhibited the lowest value of viscosity index (271). Contrarily, sesame oil was the least dense, but it exhibited the highest viscosity index (545). On the other hand, the lubrication regime study showed that by using castor oil as a lubricant in the mechanical component (4140/100Cr6), the system worked in a mixed lubrication regime while by using canola and sesame oils the system operated in boundary lubrication conditions. Finally, the kinetic friction coefficients were different for each lubricant obtaining the lowest value with castor oil while the highest value of friction coefficient was exhibited by the sesame oil lubricant.

The present work has been carried out in order to investigate the sliding wear performance of a Ni-Cr based alloy, containing boron, carbon, silicon and iron, deposited onto a SAE 1045 steel substrate. The coatings were thermally sprayed by means of a High Velocity Oxy-Fuel system and post heat-treated for one hour at 760°C in an argon atmosphere. The relative sliding performance was evaluated under unlubricated dry conditions at different applied normal loads and sliding velocities, in order to analyze the influence of these parameters on the wear response. Test results are discussed and interpreted on the basis of both microstructural evaluations and mechanical properties measurements. The surface morphology and topography of the wear scars have been studied through of a Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques. The results have shown an improved wear resistance of the coated steel samples as compared to the uncoated ones.

The long-accepted theory to explain why snow is slippery postulates self-lubrication: frictional heat from sliding melts and thereby lubricates the contacting snow grains. We recently published micro-scale interface observations that contradicted this explanation: contacting snow grains abraded and did not melt under a polyethylene slider, despite low friction values. Here we provide additional observational and theoretical evidence that abrasion can govern snow kinetic friction. We obtained coordinated infrared, visible-light and scanning-electron micrographs that confirm that the evolving shapes observed during our tribometer tests are contacting snow grains polished by abrasion, and that the wear particles can sinter together and fill the adjacent pore spaces. Furthermore, dry-contact abrasive wear reasonably predicts the evolution of snow-slider contact area, and sliding-heat-source theory confirms that contact temperatures would not reach 0°C during our tribometer tests. Importantly, published measurements of interface temperatures also indicate that melting did not occur during field tests on sleds and skis. Although prevailing theory anticipates a transition from dry to lubricated contact along a slider, we suggest that dry-contact abrasion and heat flow can prevent this transition from occurring for snow-friction scenarios of practical interest.

In the present work, the TiB2-reinforced AA6061 composites were successfully in situ synthesized by laser surface alloying using a mixture of Ti and AlB2 powders. The microstructure evolution and properties of the composites were systematically studied. The results showed that TiB2 particles displayed a homogeneous distribution in the aluminum matrix with controllable contents and morphologies. By adjusting the molar ratio of alloying powders, phase constitution of the composites was varied. Thermodynamic calculation was used to analyze the phase selection during the solidification. It was found that the morphology of TiB2 particles was converted from hexagonal plate into rod-like structure with an increase of Ti contents. Transmission electron microscopy results illustrated that the in situ synthesized TiB2 particles exhibited a well-bonded interface with the Al matrix. Properties characterization revealed a significant enhancement in microhardness and abrasion resistance compared with the aluminum substrate attributed to the presence of the TiB2 reinforcements. The strengthening and wear mechanism were also discussed.

Increasing the density of data storage is crucial to the future of inexpensive digital technology. The large majority of storage in the “Cloud” consists of magnetic hard-disk drives. The continued evolution of this USD$30 billion industry depends on the commercial introduction of heat-assisted magnetic recording (HAMR). This technology uses heat from a laser beam confined well below the diffraction limit, <50-nm wide, to write to media near 450°C with high magnetic anisotropy that would normally be unwriteable under available magnetic fields. This high anisotropy guarantees thermal stability even for grain sizes around 5-nm diameter, which are necessary for major increases in storage density. In this article and in the articles in this issue, we introduce HAMR requirements and discuss its numerous interdisciplinary materials challenges, including high-temperature/efficient plasmonic materials, low-loss optical materials, highly ordered/thermally anisotropic nanoscale magnetic grains, block copolymers for directed assembly below 10 nm, high-temperature nanometer-thick coatings/lubricants, materials/interfaces to control heat flow at nanometer length scales, and advanced spintronic materials.

The heat-assisted magnetic recording (HAMR) head–disk interface is a unique operating environment that combines nanoscale spacings, high shear rates, high-temperature gradients, and high optical fluxes in a mass-produced device. One of the greatest challenges is to develop materials for the head–disk interface that enable the required head–media spacing while also providing reliability. Traditional head–disk interface materials, engineered and optimized for conventional magnetic recording hard-disk drives, are challenged to provide the needed performance at the high temperatures that HAMR involves. We review some of the primary materials used in conventional magnetic recording, how high temperatures challenge their performance, and some of the current understanding and strategies to develop a reliable HAMR head–disk interface.

Over the last decade, polymer composites reinforced with natural fibers gained interest, both from the academic world and from various industries. Due to the demanding needs for environmentally friendly composites, the automotive industry is now searching for biodegradable and renewable composite materials and products. There are a wide variety of different natural fibers which can be applied as reinforcement or fillers, showing potential as a replacement for inorganic fibers in automotive components. The fact that plastics are often economical to produce implies an advantage especially in very complex shapes, make them promising for obtaining composite materials, achieving short demolding times, as no chemical reaction is required. Moreover, polymers are used increasingly for stressed tribological components, whereby plastic parts replace metallic bearings, gear wheels or sliding elements. In this regard, the objective of this work was to produce composite materials based on natural fibers and to characterize the influence of the addition of different amounts of filler. To do so, composites of high-density polyethylene (HDPE) and peanut shells (PS), at different proportions (2, 4 6, 8 and 10% wt.), were prepared. The composites were produced by injection molding and molded into a particular tension test simple mold. Although the FTIR presented an increment on the O-H vibration and a band around 1600 cm-1, the HDPE structure did not present modification. The mechanical properties of the HDPE were affected with the inclusion of the fibers. The tensile performance of the HDPE decrease with the increment of the fibers inclusion whiles the elastic modulus increases. The sample with 2% of natural fibers presented the lowest wear rate (k) and coefficient of friction (µ).