Inorganic nanoparticles of layered [two-dimensional (2D)] compounds with hollow polyhedral structure, known as fullerenelike nanoparticles (IF), were found to have excellent lubricating properties. This behavior can be explained by superposition of three main mechanisms: rolling, sliding, and exfoliation-material transfer (third body). In order to elucidate the tribological mechanism of individual nanoparticles in different regimes, in situ axial nanocompression and shearing forces were applied to individual nanoparticles using a high resolution scanning electron microscope. Gold nanoparticles deposited onto the IF nanoparticles surface served as markers, delineating the motion of individual IF nanoparticle. It can be concluded from these experiments that rolling is an important lubrication mechanism for IF-WS 2 in the relatively low range of normal stress (0.96±0.38 GPa). Sliding is shown to be relevant under slightly higher normal stress, where the spacing between the two mating surfaces does not permit free rolling of the nanoparticles. Exfoliation of the IF nanoparticles becomes the dominant mechanism at the high end of normal stress; above 1.2 GPa and (slow) shear; i.e., boundary lubrication conditions. It is argued that the modus operandi of the nanoparticles depends on their degree of crystallinity (defects); sizes; shape, and their mechanical characteristics. This study suggests that the rolling mechanism, which leads to low friction and wear, could be attained by improving the sphericity of the IF nanoparticle, the dispersion (deagglomeration) of the nanoparticles, and the smoothness of the mating surfaces.fullerene-like nanoparticles | tribology | rolling friction | nanotribology F riction causes wear and energy dissipation, and is responsible (directly or indirectly) for about one-third of the world's energy resource consumption (1,2). Therefore, friction and wear reduction are key factors in any future energy conservation plan. Similar in many ways to carbon fullerenes and nanotubes; hollow polyhedral nanoparticles (NP) of the inorganic compounds WS 2 , MoS 2 , were first reported in 1992 (3). These NP, known as inorganic fullerene-like (IF) structures and inorganic nanotubes, were shown to reduce friction and wear either as a pure solid lubricant; as additives to various fluid lubricants, or as part of self-lubricating coatings (4, 5). The seamless-hollow nanostructures were shown to be inherent to the layered structure of the material. The IF nanoparticles are common to layered metal dichalcogenides, MX 2 (M ¼ W , Mo, Ti, Nb, Hf; X ¼ S, Se) compounds as well as to numerous other inorganic compounds with layered two-dimensional (2D) structure. Each layer in the MX 2 material consists of three covalently bonded sheets with the metal atoms sandwiched between two chalcogenide sheets. In WS 2 (MoS 2 ) each tungsten (molybdenum) atom is bonded to six sulfur atoms in a trigonal prismatic coordination mode. Weak van der Waals forces are responsible for stacking the planar X-M-X layers together. Both graphi...

The elastic moduli of yttria (Y 2 O 3 ) samples that were made from powders with various particle morphologies were studied by means of ultrasonic measurements. The soundwave velocities in the longitudinal and transverse modes were measured. The elastic moduli were calculated from the sound velocities and density. For the high-purity, highdensity (>5000 kg/m 3 ) Y 2 O 3 that was prepared in the present study, the average density and elastic moduli (and their standard deviations) were as follows: density () of 5020 ± 18 kg/m 3 , Young's modulus (E) of 179.8 ± 4.8 GPa, shear modulus (G) of 69.2 ± 2.0 GPa, bulk modulus (B) of 148.9 ± 3.0 GPa, and Poisson's ratio () of 0.299 ± 0.004. The average longitudinal and transverse soundwave velocities (V l and V t , respectively) were 6931 ± 65 and 3712 ± 49 m/s, respectively. The elastic moduli of lanthana-strengthened yttria (LSY) were ∼6% lower than those of high-purity Y 2 O 3 , and the value for LSY was ∼0.304. It has been argued that soundwave velocity is better than density, in regard to predicting the elastic moduli of fully dense and slightly porous materials. A linear equation that describes the change of the elastic moduli with soundwave velocity alone has been suggested. This equation was applicable to a relative elastic moduli range of 0.75-1.02.

Inorganic layered materials can form hollow multilayered polyhedral nanoparticles. The size of these multi-wall quasi-spherical structures varies from 4 to 300 nm. These materials exhibit excellent tribological and wear-resisting properties. Measuring and evaluating the stiffness of individual nanoparticle is a non-trivial problem. The current paper presents an in situ technique for stiffness measurements of individual WS(2) nanoparticles which are 80 nm or larger using a high resolution scanning electron microscope (HRSEM). Conducting the experiments in the HRSEM allows elucidation of the compression failure strength and the elastic behavior of such nanoparticles under uniaxial compression.

Using an electron beam melting (EBM) printing machine (Arcam A2X, Sweden), a matrix of 225 samples (15 rows and 15 columns) of Ti-6Al-4V was produced. The density of the specimens across the tray in the as-built condition was approximately 99.9% of the theoretical density of the alloy, ρT. Tensile strength, tensile elongation, and fatigue life were studied for the as-built samples. Location dependency of the mechanical properties along the build area was observed. Hot isostatic pressing (HIP) slightly increased the density to 99.99% of ρT but drastically improved the fatigue endurance and tensile elongation, probably due to the reduction in the size and the distribution of flaws. The microstructure of the as-built samples contained various defects (e.g., lack of fusion, porosity) that were not observed in the HIP-ed samples. HIP also reduced some of the location related variation in the mechanical properties values, observed in the as-printed condition.

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