Nanoindentation,AFM and tribological properties of thin nc-WC/a-C Coatings

Instrumented indentation, AFM (atomic force microscopy) and tribological studies were performed on PE CVD (Plasma Enhanced Chemical Vapor Deposition) nanocomposite WC–C coatings to investigate the effects of surface roughness on the reliability of nanoindentation data and the possibilities of different AFM modes in nanomechanical testing, which can be used as a feedback to optimize deposition technology from the viewpoint of their mechanical properties. It was confirmed that surface roughness below 30 nm is necessary to keep the scatter of indentation modulus, E_IT, and hardness, H_IT, below 15%. PF QNM (Peak Force Quantitative NanoMechanical) mode was successfully applied for qualitative mapping of the elastic modulus of coatings with the stiffness above 300 GPa. LFM (lateral force microscopy) mode showed only weak contrast and quantitative measurements in both AFM modes require precise calibration. Coefficients of friction of the studied WC–C coatings were below 0.2 at RT, but increased to 0.7–0.8 at 450 °C due to the formation of a transfer film. Optimization of the deposition conditions based on nanoindentation resulted in the increase of E_IT from ～220 GPa to 350 GPa and H_IT from ～17 GPa to ～29 GPa.     

Impact of nanotube density and alignment on the elastic modulus near the top and base surfaces of aligned multi-walled carbon nanotube films

The mechanical compliance of vertically aligned carbon nanotube (VACNT) films renders them promising as interface materials that can accommodate thermal expansion mismatch. Here we study the relationship between the detailed morphology and elastic modulus of multi-walled VACNT films with thicknesses ranging from 98 to 1300 μm. A systematic analysis of scanning electron micrographs reveals variations in nanotube alignment and density among samples and within different regions of a given film. Nanoindentation of both top and bottom film surfaces using an atomic force microscope with spherical indenters with radii between 15 and 25 μm provides evidence of the modulus differences. The top surface is shown to have a higher modulus than the base, with out-of-plane modulus values of 1.0–2.8 MPa (top) and 0.2–1.4 MPa (base). The indentation data and microstructural information obtained from electron microscopy are interpreted together using an open cell foam model to account for differences in nanotube alignment and density, which are generally lower at the base and yield predictions that are consistent with the modulus data trends. This work shows that microstructure analysis complements property measurements to improve our understanding of nanostructured materials.     

Direct three-dimensional observation of the microstructure and chemistry of C3S hydration

Disagreements about the mechanisms of cement hydration remain despite the fact that portland cement has been studied extensively for over 100 years. One reason for this is that direct observation of the change in microstructure and chemistry are challenging for many experimental techniques. This paper presents results from synchrotron nano X-ray tomography and fluorescence imaging. The data show unprecedented direct observations of small collections of C₃S particles before and after different periods of hydration in 15 mmol/L lime solution. X-ray absorption contrast is used to make three dimensional maps of the changes of these materials with time. The chemical compositions of hydration products are then identified with X-ray fluorescence mapping and scanning electron microscopy. These experiments are used to provide insight into the rate and morphology of the microstructure formation.     

Nanomechanical properties of zirconia- yttria and alumina zirconia- yttria biomedical ceramics,subjected to low temperature aging

Samples of ZTA composites {80 wt%Al₂O₃+20 wt%TZ-3Y}, ATZ composites {20 wt%Al₂O₃+80 wt%TZ-3Y}, tetragonal polycrystalline zirconia (3Y-TZP), and cubic stabilized zirconia (8Y-CSZ) were prepared to study the nanomechanical properties by nanoindentation before and after low temperature aging or degradation. Moreover, structural properties and crystalline present phases were evaluated by X-Ray Diffraction (XRD) and Electron Diffraction Patterns from transmission electron microscopy (TEM). The 8Y-CSZ ceramic showed the best mechanical behavior among all the analyzed materials (ZTA, ATZ and 3Y-TZP), the 8Y-CSZ sample did not showed any phases transformations when subjected to low temperature degradation (LTD). Absolute nanohardness, Young's modulus and fracture toughness after the LTD were carried out in different samples, the obtained results, in a decreasing order were: 8Y-CSZ>ZTA>3Y-TZP>ATZ, 8Y-CSZ>3Y-TZP>ZTA>ATZ and 8Y-CSZ>3Y-TZP>ATZ>ZTA, respectively. The 8Y-CSZ ceramic, did not showed any variations in nanomechanical properties due to the absence of anisotropic behavior, manifesting high hardness, elastic modulus and relative values of fracture toughness. Perhaps this material could be candidate for biomedical applications.     

Heterogeneous diamond phases in compressed graphite studied by electron energy-loss spectroscopy

Chemical mapping imaged by electron energy-loss spectroscopy based on scanning transmission electron microscopy was conducted on a compressed graphite specimen containing different carbon allotropes (hexagonal diamond, cubic diamond, and graphite phases). This imaging process allows visualization of the complex spatial distribution of different diamond phases, and their coexistence was confirmed using dark field (DF) imaging. The chemical mapping images showed spatial distribution of local bonding state for hexagonal and cubic diamond phases in the whole specimen, while the DF images showed only a part of crystalline segments with long-range order. Thus, the chemical mapping method has an advantage for the purpose of observing locally the existence of individual carbon allotropes in the whole specimen. The size distribution of the hexagonal diamond phase is approximately 10–100 nm. These findings indicate that the compressing method can potentially synthesize ~ 100 nm large diamond phases.     

Permeability and elastic modulus of cement paste as a function of curing temperature

The permeability and elastic modulus of mature cement paste cured at temperatures between 8 °C and 60 °C were measured using a previously described beam bending method. The permeability increases by two orders of magnitude over this range, with most of the increase occurring when the curing temperature increases from 40 °C to 60 °C. The elastic modulus varies much less, decreasing by about 20% as the curing temperature increases from 20 °C to 60 °C. All specimens had very low permeability, k < 0.1 nm², despite having relatively high porosity, ? ~ 40%. Concomitant investigations of the microstructure using small angle neutron scattering and thermoporometry indicate that the porosity is characterized by nanometric pores, and that the characteristic size of pores controlling transport increases with curing temperature. The variation of the microstructure with curing temperature is attributed to changes in the pore structure of the calcium–silicate–hydrate reaction product. Both the empirical Carmen–Kozeny, and modified Carmen–Kozeny permeability models suggest that the tortuosity is very high regardless of curing temperature, ξ ~ 1000.     

Mechanical measurements of ultra-thin amorphous carbon membranes using scanning atomic force microscopy

The elastic modulus of ultra-thin amorphous carbon films was investigated by integrating atomic force microscopy (AFM) imaging in contact mode with finite element analysis (FEA). Carbon films with thicknesses of ～10 nm and less were deposited on mica by electron beam evaporation and transferred onto perforated substrates for mechanical characterization. The deformation of these ultra-thin membranes was measured by recording topography images at different normal loads using contact mode AFM. The obtained force-distance relationship at the center of membranes was analyzed to evaluate both the Young’s modulus and pre-stress by FEA. From these measurements, Young’s moduli of 178.9 ± 32.3, 193.4 ± 20.0, and 211.1 ± 44.9 GPa were obtained for 3.7 ± 0.08, 6.8 ± 0.12, and 10.4 ± 0.17 nm thick membranes, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were used for characterizing the chemical and structural properties of the films, including the content of sp² and sp³ hybridized carbon atoms.     

Nanoindentation study on nanomechanical characteristics of a-CN film deposited by shielded arc ion plating

Mechanical properties of the a-CN films including elastic modulus (E_r), hardness (H), elastic recovery (R), contact stiffness (S) and deformation energies were measured by a nanoindentation system. We also evaluated wear resistant behavior of the layered a-CN films in nanometer scale by the same nanoindentation system. All the a-CN films, irrespective of V_b, showed better wear-resistance characteristics than sapphire and quartz. The a-CN (− 300 V)/Si sample showed the best wear-resistance, although its hardness was lower than the a-CN (− 300 V)/a-C (− 100 V)/Si. The wear resistant characteristic of the films can be understood by considering the other mechanical properties including that of R, hardness-to-elastic modulus ratio (H / E_r), and elastic deformation energy (W_e) obtained from the nanoindentation. These various nanomechanical properties certainly govern the wear-resistance of the film.     

Characterization of high strength mortars with nano alumina at elevated temperatures

In this study, the effect of elevated temperatures on chemical composition, microstructure and mechanical properties of high strength mortars with nano alumina was investigated. Mortars with 1, 2 and 3% nano alumina as cement replacement were prepared and then exposed to 100 °C, 200 °C, 300 °C, 400 °C, 600 °C, 800 °C and 1000 °C. XRD, DSC and SEM tests were carried out to identify chemical composition and microstructure changes in the cement matrix after being exposed to elevated temperatures. Residual compressive strength, relative elastic modulus and gas permeability coefficient of samples were also obtained. A brittleness index was defined to monitor changes in brittleness of samples after being exposed to elevated temperatures. Nano alumina enhanced compressive strength of samples up to 16% and improved residual compressive strength. An increase in the relative elastic modulus, higher energy absorption and lower permeability were also observed when 1% nano alumina was added.     

Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics

The structural and morphological evolutions of cordierite-based ceramics produced from stevensite/andalusite mixture sintered from 1150 to 1350 °C were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical behavior was investigated by three-point bending and Brazilian tests. The elastic properties were evaluated using ultrasonic non-destructive testing (NDT). XRD results revealed that the main crystalline phase formed at 1300 and 1350 °C was cordierite with traces of mullite. A linear-elastic behavior followed by brittle fracture was observed in three-point bending test with the presence of multiple discontinuities. Flexural and diametral compression strength values of cordierite sintered at 1300 °C were 39.4±4 and 21.8±2 MPa, respectively. The elastic properties such as Young's modulus (38.7–45.1 GPa), shear modulus (17.90–19.42 GPa) and Poisson ratio (0.08–1.6) of cordierite-based ceramics produced at 1350 and 1300 °C were also determined.     

Influence of silica fume on the microstructure of cement pastes: New insights from 1H NMR relaxometry

¹H NMR has been used to characterise white Portland cement paste incorporating 10 wt.% of silica fume. Samples were measured sealed throughout the hydration without sample drying. Paste compositions and C–S–H characteristics are calculated based on ¹H NMR signal intensities and relaxation analysis. The results are compared with a similar study of plain white cement paste. While the presence of silica fume has little influence on C–S–H densities, the chemical composition is impacted. After 28 days of sealed hydration, the Ca/(Si + Al) ratio of the C–S–H is 1.33 and the H₂O/(Si + Al) ratio is 1.10 when 10% of silica fume is added to the white cement. A densification of the C–S–H with time is observed. There are no major changes in capillary, C–S–H gel and interlayer pore sizes for the paste containing silica fume compared to the plain white cement paste. However, the gel/interlayer water ratio increases in the silica fume blend.     

Resonance tracking atomic force acoustic microscopy quantitative modulus mapping of carbon nanotubes‐reinforced acrylonitrile–butadiene–styrene polymer

This work presents a resonance tracking atomic force acoustic microscopy (RT‐AFAM) quantitative modulus mapping of carbon nanotubes‐reinforced acrylonitrile–butadiene–styrene polymer. RT‐AFAM average local modulus values registered were in good agreement with those measured by nanoindentation test. RT‐AFAM mapping modulus, nanoindentation, and transmission electron microscopy imaging showed that carbon nanotubes reinforcement of acrylonitrile–butadiene–styrene polymer matrix gives an elastic modulus enhancement of approximately 18.3% compared with the polymer matrix alone and showed that this technique provides high spatial resolution and helps to characterize the elastic properties of reinforced thermoplastic polymers and new compound materials at nanoscale. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40628.     

The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement

This study reveals that the nanosilica hydrosols with higher specific surface areas had faster pozzolanic reactivity, especially at early ages; moreover, the results are indicative of the accelerating influence of nanosilicas and silica fume on the hydration of cement. Faster initial and final setting times observed for cement pastes containing nanosilicas are consequence of these mechanisms. However, less hydration degree of cement compared to the plain paste was observed at age of 7 days and after. This can be attributed to the entrapment of some of mix water in the aggregates of nanosilicas formed in cement paste environment, making less water available for the progress of cement hydration. The same mechanism is believed to be responsible for the reduction of flowability of cement pastes.     

Water demand of amorphous nano silica and its impact on the workability of cement paste

This paper addresses the characterization of six different amorphous silica samples with respect to their application in cement paste. Different mixes are compared and analyzed using the mini spread-flow test. Also the granular properties, different void fraction states of packing and distribution moduli q are analyzed and compared using a mix design tool. A deformation coefficient is derived from the spread-flow test, which correlates with the value of specific surface area computed from the particle size distribution, and intrinsic density of the samples. Finally, the thickness of a constant water layer of 25 nm around the particles is computed at the onset of flowing. The granular analysis demonstrated that it is possible to decrease the water demand of the cement paste when nanoparticles are added and the resulting grading follows the modified Andreasen and Andersen curve (q = 0.5), and the concentration of nano silica is less than 5% bwoc.     

Thermodynamic modeling of solid solutions between monosulfate and monochromate 3CaO • Al2O3 • Ca[(CrO4)x(SO4)1-x] • nH2O

In hydrated cement paste AFm-phases are regarded to play an important role in the binding of the toxic contaminant chromate through isomorphic substitution with sulfate. Solid solutions formation can lower the solubility of the solids, thus reducing chromate leaching concentrations.Solid solutions between monosulfate and monochromate were synthesized and characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and inductive coupled plasma optical emission spectroscopy (ICP-OES). Based on the measured ion concentrations in solution total solubility products of the solid solution series were determined.For pure monochromate a logK = ? 28.4 ± 0.7 was determined. Results from solid and solution analysis showed that limited solid solutions exist. Based on XRD diffractograms a solid solution with a miscibility gap 0.15 < Crx < 0.85 with a dimensionless Guggenheim parameter of 2.43 was proposed.     

Time-dependent behaviour of hardened cement paste under isotropic loading

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.     

Densification,microstructure, elastic and mechanical properties of reactive hot-pressed ZrB2–ZrC–Zr cermets

In this study, cermets composed of zirconium diboride and zirconium carbide with intergranular zirconium were sintered by reactive hot-pressing. Relative density exceeding 97% was obtained for the sintered cermets having four distinct compositions varying in concentration of excess Zr. Their densification behaviour was examined by monitoring displacement during sintering. The microstructure was characterized by scanning electron microscopy and X-ray diffraction, and the elastic and mechanical properties were evaluated at room temperature. The effects of Zr concentration on the densification and mechanical properties were assessed. The ZrB₂ and ZrC micron-grains coarsened with increasing amount of Zr starting material. In addition, the cermets exhibited high flexural strength (546–890 MPa) and fracture toughness (6.63–10.24 MPa m^1/2), which simultaneously increased with increasing Zr concentration. However, the elastic moduli and hardness (11–18 GPa) decreased with increasing Zr. The shear modulus and Young's modulus were in the range of 150–190 GPa and 360–440 GPa, respectively.     

Influence of the de-waxing atmosphere on the properties of AlN ceramics processed from aqueous media

The influence of binder burnout atmosphere (air or N₂) on surface chemistry of thermo-chemically treated AlN powders processed in aqueous media, and on the final properties of AlN ceramics was studied. The surface chemistry after de-waxing was accessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD), SEM, high-resolution transmission electron microscopy (HR-TEM), were used to identify the phase assemblage and for microstructural analysis. The effects of the residual carbon and oxygen at the surface on the thermal conductivity and sintered density of AlN samples were investigated. The surface C/O ratios were observed to correlate with the sintering behaviour, the composition and distribution of secondary phases, and grain-boundary composition, as well as thermal conductivity of AlN samples. Thermal conductivities of about 140 W/mK were obtained for the aqueous processed AlN samples de-waxed in nitrogen atmosphere and sintered for 2 h at 1750 °C in the presence of 4 wt.% YF₃ + 2 wt.% CaF₂ as sintering additives.     

Facile synthesis of CdS nanoparticles photocatalyst with high performance

A simple one-step solid-state reaction has been introduced to synthesize CdS nanoparticles. The as-prepared CdS product was characterized by X-ray powder diffraction (XRD), BET surface area measurement, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), particle size distribution (PSD) and UV–vis absorption spectrum. The experiment results reveal that the CdS product was composed of nanoparticles about 60 nm in diameter, of which specific surface area is 78.02 m²/g. The photocatalysis results indicate that the CdS nanoparticles exhibit excellent photocatalytic activity for the degradation of rhodamine B under UV irradiation. Nearly 95% of rhodamine B was degraded after 60 min of irradiation, higher than that of P25, which is due to the large specific surface area and mesoporous structure.     

Effect of the curing temperature on the creep of a hardened cement paste

Uniaxial creep tests have been performed at ambient temperature on a saturated hardened class G cement paste hydrated at 60 °C and 90 °C. The results have shown that creep is enhanced at higher curing temperature. The visco-damage model of Challamel et al. (2005) has been extended and used to analyze the experimental results. The model parameters have been calibrated on the results of the creep tests performed at different stress levels for both curing temperatures. The model correctly reproduces the effect of the curing temperature on time dependent properties of the material. The enhanced creep at higher curing temperature is attributed to the development of more damage in cement paste and to significant weakening of the mechanical properties.