Hierarchical adaptive nanostructured PVD coatings for extreme tribological applications: the quest for nonequilibrium states and emergent behavior

Adaptive wear-resistant coatings produced by physical vapor deposition (PVD) are a relatively new generation of coatings which are attracting attention in the development of nanostructured materials for extreme tribological applications. An excellent example of such extreme operating conditions is high performance machining of hard-to-cut materials. The adaptive characteristics of such coatings develop fully during interaction with the severe environment. Modern adaptive coatings could be regarded as hierarchical surface-engineered nanostructural materials. They exhibit dynamic hierarchy on two major structural scales: (a) nanoscale surface layers of protective tribofilms generated during friction and (b) an underlying nano/microscaled layer. The tribofilms are responsible for some critical nanoscale effects that strongly impact the wear resistance of adaptive coatings. A new direction in nanomaterial research is discussed: compositional and microstructural optimization of the dynamically regenerating nanoscaled tribofilms on the surface of the adaptive coatings during friction. In this review we demonstrate the correlation between the microstructure, physical, chemical and micromechanical properties of hard coatings in their dynamic interaction (adaptation) with environment and the involvement of complex natural processes associated with self-organization during friction. Major physical, chemical and mechanical characteristics of the adaptive coating, which play a significant role in its operating properties, such as enhanced mass transfer, and the ability of the layer to provide dissipation and accumulation of frictional energy during operation are presented as well. Strategies for adaptive nanostructural coating design that enhance beneficial natural processes are outlined. The coatings exhibit emergent behavior during operation when their improved features work as a whole. In this way, as higher-ordered systems, they achieve multifunctionality and high wear resistance under extreme tribological conditions.     

NIR-red reflectance-based algorithms for chlorophyll-a estimation in mesotrophic inland and coastal waters: Lake Kinneret case study

A variety of models have been developed for estimating chlorophyll-a (Chl-a) concentration in turbid and productive waters. All are based on optical information in a few spectral bands in the red and near-infra-red regions of the electromagnetic spectrum. The wavelength locations in the models used were meticulously tuned to provide the highest sensitivity to the presence of Chl-a and minimal sensitivity to other constituents in water. But the caveat in these models is the need for recurrent parameterization and calibration due to changes in the biophysical characteristics of water based on the location and/or time of the year. In this study we tested the performance of NIR-red models in estimating Chl-a concentrations in an environment with a range of Chl-a concentrations that is typical for coastal and mesotrophic inland waters. The models with the same spectral bands as MERIS, calibrated for small lakes in the Midwest U.S., were used to estimate Chl-a concentration in the subtropical Lake Kinneret (Israel), where Chl-a concentrations ranged from 4 to 21 mg m⁻³ during four field campaigns. A two-band model without re-parameterization was able to estimate Chl-a concentration with a root mean square error less than 1.5 mg m⁻³. Our work thus indicates the potential of the model to be reliably applied without further need of parameterization and calibration based on geographical and/or seasonal regimes.     

Creep of the eye sclera

 Irreversible stretching (creep) of the eye sclera, considered as a cause of the high myopia, has been studied experimentally. Both dramatic acceleration and significant deceleration of the sclera creep have been observed in the presence of some enzymes. Creep of various connective tissues caused by static and cyclic mechanical stresses (gravity, blood pressure, etc.) can manifest in many age and pathological changes in human body. Authors express the hope that investigations in this area and finding appropriate inhibitors may replace surgery (eye, cosmetic) by prophylactic therapy. Received: 21 July 1998 / Accepted: 28 August 1998     

New hard ternary Hf–Ir–B borides formed by reaction hafnium diboride with iridium

A variety of the ternary Hf–Ir–B phases formed via the reaction between iridium and hafnium diboride at elevated temperatures was found. The data on the phase and elemental composition, as well as crystal structure, obtained by powder and single-crystal X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectrometer, and time-of-flight neutron diffraction analysis unambiguously confirm that HfIr₃B_x solid solution, two known ternary borides (HfIr₃B₄, Hf₂Ir₅B₂), as well as two novel ternary HfIr_2.1B_1.3 and HfIr_5.7B_2.7 phases, are formed at elevated temperatures. This result is fundamentally different from that previously obtained by us for the Hf–Ir–C system in which only one binary intermetallic compound, HfIr₃, was produced. The measured Vickers microhardness for all the aforementioned ternary borides (13–19 GPa) allows us to consider them hard. The coefficients of thermal expansion of ternary borides were measured by in situ high-temperature X-ray analysis.     

PolyHIPE: IPNs, hybrids, nanoscale porosity, silica monoliths and ICP-based sensors

Typical polyHIPE (porous polymers from high internal phase emulsions) have a cellular structure with volume fractions from 0.2 to 0.04, cell diameters from 15 to 25 μm and intercellular pore diameters from 0.5 to 10 μm. Unique interpenetrating polymer networks synthesized within the polyHIPE exhibited enhanced mechanical properties and an extended temperature range for damping. Hybrid polyHIPE that combine an inorganic polysilsesquioxane network with an organic polystyrene network exhibited superior high temperature mechanical properties and enhanced thermal stability. A nanoscale porosity in the cell walls, produced through the addition of a porogen to the HIPE, reduced the density and significantly enhanced the specific surface area. Porous silica monoliths with silica volume fractions of as low as 0.02 were produced through the pyrolysis of hybrid polyHIPE. PolyHIPE coated with an intrinsically conducting polymer exhibited reversible and repeatable changes in conductivity on exposure to acetone vapor, demonstrating their potential as sensor materials.     

A Large Piezoelectric Anisotropy of a Three-Component Composite with Variable Connectivity

Proposed are two types of three-component piezoelectric composites that change connectivity from 2-2 to 1-3 and contain polarized ferroelectric ceramic and polymer components, i.e., layer 1 reinforced by rods–layer 2–layer 1 reinforced by rods– . . . (type 1) and laminated rods (layer 1–layer 2–layer 1– . . . ) embedded in a matrix (type 2). Some cases of the large anisotropy of piezoelectric coefficients d ₃₃ ^* /d ₃₁ ^* and e ₃₃ ^* /e ₃₁ ^* are analyzed for the composites of the type 1. Original cases of simultaneous reaching d ₃₃ ^* /d ₃₁ ^* 0 and e ₃₃ ^* /e ₃₁ ^* > 10 as well as e ₃₃ ^* /e ₃₁ ^* and d ₃₃ ^* /d ₃₁ ^* at different volume concentrations of the components in the composites of the type 2 are also considered. It is shown that these ratios essentially depend on electromechanical constants of the components, their volume concentrations, microgeometry, as well as on jumps of these constants and internal fields at boundaries between the components.     

Comparison of four global FAPAR datasets over Northern Eurasia for the year 2000

The Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) has been identified as one of several key satellite-derived biophysical datasets. With multiple global FAPAR datasets now available and a lack of in-situ measurements and comparison studies in the far north, this study attempts to provide the reader with an indication of the performance of four global FAPAR datasets (MODIS, CYCLOPES, JRC and GLOBCARBON) over Northern Eurasia in the year 2000 via comparison. Within the year 2000 growing season, both the MODIS and CYCLOPES datasets recorded on average similar but substantially higher values than the JRC and GLOBCARBON datasets. Among three of the four datasets, a high level of agreement in deciduous broadleaf forests and croplands was observed. Largest disagreement occurred among needleleaf forests and grassland/shrubland. Potential reasons for discrepancies among the datasets include different retrieval methods, use of LAI and land cover, snow effects and others. Findings from this study and other published results suggest that overall, JRC best captures FAPAR over northern Eurasia in the year 2000. However, when considering individual landcover types, any one or more of the four products may be suitable. There exists a real need for more in-situ measurements in this region — the lack of such measurements makes evaluation extremely difficult. It appears that areas north of 60° urgently require further investigation.     

Conformational variability of helix sense reversals in poly(methyl isocyanate)

The conformations and energies of several helix sense reversal geometries in poly(methyl isocyanate) (PMIC) have been determined using the PCFF forcefield. In an extension of previous studies, a larger conformational variability for a helix sense reversal has been investigated. In addition to the reversal geometry previously detailed by several authors that results in a relatively small angle deviation from the rod-like polyisocyanate structure, we report the discovery of reversals of similar energy with much larger angle deviations from linearity. The effect of electrostatic interactions as controlled by the value of the dielectric constant, ε, on the conformation and energy of a reversal is also shown to be important. At ε=1.0 (vacuum) the conformations of the reversals with large and small angle ‘kinks’ have similar energies. However, at ε=2.0 (non-polar organic solvent) and ε=3.5 (bulk state) the reversals corresponding to the large angle kinks have lower energies.     

Comparison of the phase behaviour of the liquid-crystalline polymer/poly(methyl methacrylate) and poly(vinyl acetate)/poly(methyl methacrylate) blends

The phase behaviour of blends of a liquid-crystalline polymer (LCP) and poly(methyl methacrylate) (PMMA), as well as the phase state of blends of PMMA and poly(vinyl acetate) (PVA) has been investigated using light scattering and phase-contrast optical microscopy. The blends of LCP and PMMA have been obtained by coagulation from ternary solutions. The cloud point curves were determined. It was established that both pairs demix upon heating, ie have an LCST. In the region of intermediate composition, the phase separation proceeds according to a spinodal mechanism; however for LCP/PMMA blends, the decomposition proceeds according to a non-linear regime from the very onset. In the region of small amounts of LCP, the phase separation follows a mechanism of nucleation and growth. For PMMA/PVA blends, the spinodal decomposition proceeds according to a linear regime, in spite of the molecular mobility that PVA chains develop at lower temperatures. Only after prolonged heat treatment does the process transit to a non-linear regime. The data show a similarity between the phase behaviour of blends of liquid-crystalline and of flexible amorphous polymers. The distinction consists of the absence of a linear regime of decomposition for LCP-PMMA blends. © 1999 Society of Chemical Industry