Evaluation of tribological behavior of B4C–hBN ceramic composites under water-lubricated condition

The aim of this study is to evaluate the influence of hexagonal boron nitride (hBN) addition on the tribological behavior of B₄C-based ceramic composites under distilled water lubrication. Water-lubricated sliding tests of hot-pressed B₄C–hBN ceramic composites with different hBN amounts against pure B₄C ceramic were carried out on a pin-disc type wear apparatus. It was found that the addition of hBN into B₄C ceramic matrix resulted in a severe decrease of the friction coefficient from 0.373 for B₄C/B₄C sliding pair to 0.005 for B₄C–20 wt% hBN/B₄C sliding pair. A B₂O₃ tribochemical film formed on the worn surface of the B₄C–hBN specimen protected both B₄C–hBN and B₄C and facilitated the frictional surfaces to smooth. Therefore, the tribological behaviors of the pairs were significantly improved. The formation process of the film and its antifriction mechanism are discussed.     

Preparation,combustion and thermal behaviors of UV-cured coatings containing organically modified α-ZrP

The bisphenol A epoxy acrylate resin containing a small amount of organically modified alpha-zirconium phosphate (α-ZrP) was cured within seconds upon UV irradiation at ambient temperature. The UV–curing behavior was investigated by fourier transformedinfrared spectroscopy (FTIR). The microstructures were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The transparency and the combustion were examined by the UV–vis transmission spectra and Microscale Combustion Calorimeter (MCC), respectively. The ultraviolet–visible spectra of the cured films showed no obvious absorbance over a range of 400–800 nm, which revealed that the cured films were transparent. The results of MCC revealed that heat release rate (HRR) of the films decreased with the contents of organic α-ZrP (OZrP). Thermal behavior of the cured films was studied by thermogravimetric analysis (TGA), which indicated that with increasing the contents of OZrP, the char yields of UV-cured films were enhanced. Viscoelastic property of photocured nanocomposites was evaluated by dynamic-mechanical analysis. The effects of OZrP on the properties of UV curable films were critically evaluated in this paper.     

Formation behavior and microstructure of multi-cation α-sialons containing neodymium and ytterbium or yttrium

Multi-cation α-sialons containing neodymium and ytterbium, as well as neodymium and yttrium, were prepared by hot pressing at the temperatures ranging from 1550 to 1750°C for 2 h with the compositions (Nd_0.18Yb_0.18/Nd_0.18Y_0.18)Si_10.36Al_1.62O_0.54N_15.46. The densification process, reaction sequence, cell dimensions and microstructure were studied in comparison with the corresponding single cation α-sialon. Experimental results have shown that the samples hot-pressed at 1750°C mainly consist of α-sialon phase, whose content (around 95 wt%) is higher than that of counterpart single rare earth doped α-sialon, especially much higher than the one of Nd-α-sialon. On the other hand, the multi-cation α-sialons compositions are beneficial to lower the eutectic temperature in the systems, thus promoting the dissolution of intergranular crystalline melilite phase, and facilitating the precipitation of α-sialon phase. TEM and EDS revealed that the amount of Yb³⁺ or Y³⁺, which possess relatively smaller ionic radii than Nd³⁺, is higher than that of the Nd³⁺ in α-sialon grains and more Nd³⁺ are remained in the grain boundary phase. Small amount of elongated α-sialon grains were observed by TEM and the preferred orientation also occurred under hot-pressing for such a low x value composition (x=0.36) used in this study.     

Cyclic ablation behavior of C/C–ZrC–SiC composites under oxyacetylene torch

Cyclic ablation behavior of C/C–ZrC–SiC composites prepared by precursor infiltration and pyrolysis process was studied using oxyacetylene torch. After repeated 30 s ablation for four times, the composites exhibited better ablation properties than those under single ablation for 120 s because of the lower surface temperature, and their linear and mass ablation rates were −3×10^–4 mm/s and −2.29×10^–3 g/s, respectively. A continuous ZrO₂–SiO₂ layer formed on the surface of center ablation region and acted as an effective barrier to the transfer of heat and oxidative gases into the inner material. Thermal stress induced by repeated impact of oxyacetylene led to some cracks on the ZrO₂–SiO₂ layer; however its destructive power was weaker than that of higher temperature. Stick like silica as grown silica nanowires were generated in the transition ablation region due to the evaporation of silicon oxide at appropriate temperature.     

Anisotropic tribological behavior of LSI based 2.5D needle-punched carbon fiber reinforced Cf/C–SiC composites

C_f/C–SiC composites were fabricated via liquid silicon infiltration with 2.5D needle-punched carbon fiber reinforced C_f/C composites. The effect of surface topography and carbon content of the C_f/C–SiC composites on the tribological properties was researched by the ball-on-disk reciprocating tribometer. The results indicate that different fiber layers and cross-section of the composites have various surface topography and show significant differences in the friction and wear properties. By the wear morphology and model analyses, the reason for the tribological anisotropy of the composites is that the distribution of carbon and SiC phases in the composites are inhomogeneous caused by the difference of the carbon fiber orientation and the relative content in each layer. Moreover, the wear rate of the short-cut fiber web layer was the lowest and there is an obvious linear decrease in coefficient of friction with increase of carbon content. The present work explains why the tribological properties of the composites are inconsistent and provides a way to adjust the friction properties of composite materials by optimizing the friction surface.     

Microstructure and tribological properties of microwave-sintered Ti0.8Ni–0.3Mo/TiB composites

In this study, the Ti–0.8Ni–0.3Mo/XTiB (X = 5, 10, 15, and 20 wt%) composites were prepared using the microwave-sintering assisted powder metallurgy technique, and tribological properties were investigated. X-ray diffraction and scanning electron microscopy (SEM), with the microscope capable of energy dispersive spectroscopy (EDS), were used to characterize the mixed powder. The density and microhardness of the Ti–0.8Ni–0.3Mo/TiB composites were examined. The Ti–0.8Ni–0.3Mo/TiB composites exhibited a hardness of 260 HV, which is a 20% improvement over Ti–0.8Ni–0.3Mo. Tribological properties were studied by conducting experiments using a pin-on-disc wear tester at varying loads, sliding distances, and speeds. The Ti–0.8Ni–0.3Mo/TiB composites exhibited a reduction in wear loss and coefficient of friction values owing to TiB hardness and good bonding with the matrix. The tribological properties of the Ti–0.8Ni–0.3Mo/TiB composites were enhanced by the addition of TiB particles, which resist wear and friction.     

Study on friction behavior of fabric–silicone rubber composites under dry/wet sliding environment

The dry/wet environment's effects on the sliding friction properties of fabric–silicone rubber composites are studied, and the wear resistance of polyester fabric is evaluated. The yarn directions of the fabrics (inner and outer) have significant influence on the coefficient of friction (COF) of fabric–silicone rubber composites during sliding friction due to the difference in fabric texture and yarn modulus. The COF's variation laws of fabric–silicone rubber composites under different vertical loads and sliding rates are observed, respectively. Additionally, the outer fabric is more sensitive to the changes of vertical loads than those of the sliding rates under the wet environment, which can be attributed to the destruction of the fabric original structure by high load, resulting in the fabric fibers' pulling and breaking during the steel ball's sliding. This study provides new ideas for the design of fabric–silicone rubber composites applied in friction conditions and complex environments.     

Enhanced tribological properties of MWCNT/Ni bulk composites – Influence of processing on friction and wear behaviour

The friction and wear behaviour of MWCNT-reinforced Ni matrix bulk composites is investigated and compared to the response of a pure bulk reference sample. Several effects are observed in the composites such as microstructural refinement, leading to improved hardness, coefficient of friction and wear resistance. Said refinement might also be responsible for a non-trivial behaviour of hot-pressed composites under a 300 mN load, due to higher oxidation rates. The worn volume is analysed and discussed from a microstructural and morphological point of view. In this context, a direct correlation is shown between two relevant wear parameters, namely, the wear constant K and the cutting efficiency f_AB. In addition, the tribochemistry is investigated by Raman spectroscopy and linked to the friction and wear behaviour. Finally, the wear volume reduction peaked at 94.1% in HUP samples. This clearly indicates the high potential of these composites for tribological applications.     

Molecular dynamics simulation study on π-π stacking of Gemini surfactants in oil/water systems

Whereas the π-π stacking interactions at oil/water interfaces can affect interfacial structures hence the interfacial properties,the underlying microscopic mechanism remains largely unknown.We reported an all-atom molecular dynamics(MD) simulation study to demonstrate how the Gemini surfactants with pyrenyl groups affect the interracial properties,structural conformations,and the motion of molecules in the water/n-octane/surfactant ternary systems.It is found that the pyrenyl groups tend to be v...     

Effect of blend ratio on the dynamic mechanical and thermal degradation behavior of polymer–polymer composites from low density polyethylene and polyethylene terephthalate

Microfibrillar polymer–polymer composites (MFCs) based on low-density polyethylene (LDPE) and polyethylene terephthalate (PET) were prepared by cold drawing-isotropization technique. The weight percentage of PET was varied from 5 to 45 %. Microfibrils with uniform diameter distribution were obtained at 15 to 25 wt% of PET as evident from the scanning electron microscopy (SEM) results. Dynamic mechanical properties such as storage modulus (E′), loss modulus (E″) damping behavior (tan δ) were examined as a function of blend composition. The E′ values were found to be increasing up to 25 wt% of PET. An effort was made to model the storage modulus and damping characteristics of the MFCs using the classical equations used for short-fiber reinforced composites. The presence of PET microfibrils influenced the damping characteristics of the composite. The peak height at the β-transitions of loss modulus was lower for MFCs with 25 % PET, showing that they had superior damping characteristics. This phenomenon could be correlated with the PET microfibrils morphology. The thermal degradation characteristics of LDPE, neat blends and microfibrillar blends (MFBs) were compared. The determination of activation energy for thermal degradation was carried out using the Horowitz and Metzger method. The activation energy for thermal degradation of microfibrillar blends was found to be higher than that for the corresponding neat blends and MFCs. The long PET microfibrils present in MFBs could prevent the degradation and enhance the activation energy.     

Microstructure and tribological behaviors of C/C–BN composites fabricated by chemical vapor infiltration

In this paper carbon fiber reinforced carbon–boron nitride binary matrix composites (C/C–BN) were prepared by chemical vapor infiltration (CVI). The infiltration of BN in the CVI process was controlled by the diffusion of BCl₃, and BN matrix was distributed homogeneously in the porous carbon fiber reinforced carbon matrix composites (C/C) due to the good infiltration ability of BN. The as-received C/C–BN composites were composed of 92 vol% C and 8 vol% BN. Both the friction coefficient and wear rate of C/C composites decreased significantly by the incorporation of BN. After heat-treated at 1600 °C, the interlayer spacing of CVI BN decreased to 3.36 Å, and CVI BN with high crystalline degree displayed the excellent lubricating effect, leading to the decrease of friction coefficient and wear rate. The improvement of the tribological properties also was partially attributed to the improved oxidation resistance and the formation of friction film by the incorporation of BN matrix.     

Microstructure and ablation behavior of carbon/carbon composites infiltrated with Zr–Ti

Carbon/carbon(C/C) composites infiltrated with Zr–Ti were prepared by chemical vapor infiltration and reactive melt infiltration. Their microstructure and ablation behavior at different temperatures and time were investigated. The results show that C/C composites infiltrated with Zr–Ti have good interface cohesion between carbon fibers, pyrocarbon and carbide. Compared with C/C composites and C/C–ZrC composites, the synthesized sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 exhibits better ablation resistance at 2500 °C due to the newly formed protective layer composed of ZrTiO₂ pinned by ZrO₂ grains after ablation. The ablation resistance of the sample with Zr_0.57Ti_0.43C_1.01 increased gradually with the decrease of temperature from 3000 °C to 2000 °C, whereas the ablation resistance of the sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 first increased obviously and then decreased slightly. In addition, the work indicates that surplus particles or liquid phases of oxides cannot protect the matrix, and that the liquid oxides may even cause severe ablation. Furthermore, a protective layer of oxides tends to be formed with the increase of ablation time.     

Effect of compatibilizer on interfacial tension of SAN/EPDM blend as measured via relaxation spectrums calculated from Palierne and Choi–Schowalter models

Morphology, interfacial tension, and stress relaxation spectra of immiscible SAN/EPDM blend and its compatibilized blend with SAN-g-EPDM (Centrex) was studied. The results showed that the morphology of the blend had a quick response to added Centrex. In the compatibilized blend with 20-wt% compatibilizer (optimized blend) having a droplet-in-matrix type of morphology, the particle sizes were reduced by a factor of 4. The power-law index of EPDM and SAN obtained 0.33 and 0.53, respectively. With increasing of compatibilizer the power-law index decreased. It meant that at the same amount of EPDM its influence in the blend was increased. Also the cross-over point of G′ and G″ curves in the melt of optimized blend decreased which was attributed to increased elasticity. These observations were in good correspondence with the morphological observations. In optimized blend, the number average diameter of EPDM dispersed particles had the lowest value of about 1.8 μm. The interfacial tension of the compatibilized SAN/EPDM blend was determined from the morphological studies and the relaxation time was calculated using the Palierne and Choi-Schowalter models. The optimized blend showed the least interfacial tension about 0.306 (N/m) which was in agreement with the morphological observations.     

Crystallization and melting behavior of polypropylene in β-PP/polyamide 6 blends containing PP-g-MA

In this research, we used a twin-screw extruder to melt and blend PP-g-MA compatibilizer with β-polypropylene (PP)/polyamide 6 (PA6). The influences of the PA6 and PP-g-MA contents in PP/PA6 blends on crystallization and melting behavior of PP phase and morphology were investigated. The results showed that, when PP-g-MA copolymer was added to the β form of nucleated PP/PA6 blends, the anhydride groups in PP-g-MA and PA6 terminal amine groups react to form PP-g-PA graft copolymer in a two-phase interface. This reduces the interfacial tension, improves the interfacial adhesion, and reduces the size of PA6 domains in the blend. The generated PP-g-PA graft copolymer wrapped PA6 phase and buried the anhydride groups of PP-g-MA. When the proportion of PP-g-MA and PA6 was between 0.5 and 0.75, there was no longer interfering to the formation of β-crystals in the PP phase. The content of β-crystal of PP phase in blends was found to reach as large as 85.9%.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Synthesis and electrochemical properties of mixed-phase αx/β1-x-LiVOPO4/C composites

Mixed-phase α_x/β_1-x-LiVOPO₄/C (x = 0.1, 0.3, 0.5, 0.7, 0.9) composites were synthesized by the solid-phase sintering method. X-ray diffraction (XRD) analyses revealed that a series of composites were comprised of pure triclinic α-LiVOPO₄ and orthorhombic β-LiVOPO₄ components. The mixed-phase materials exhibit better rate performance than whether pure triclinic α-LiVOPO₄/C or orthorhombic β-LiVOPO₄/C. The α_0.5/β_0.5-LiVOPO₄/C (α:β = 1:1) composite exhibit superior high-rate capability. When cycled at 2C and 5C, the initial discharge capacities of the α_0.5/β_0.5-LiVOPO₄/C composite are 81.1 mAh g^?1 and 69.8 mAh g^?1 respectively, which are significantly higher than those of the pure α-LiVOPO₄/C (30.6 mAh g^?1 and 18.6 mAh g^?1, respectively) and β-LiVOPO₄/C (56.8 mAh g^?1 and 36.9 mAh g^?1, respectively). The improved electrochemical performance could be attributed to the mixed phase possess an open framework and stable structure.     

Passivity of α-brass (Cu∶Zn/67∶33) and its breakdown in neutral and alkaline solutions containing halide ions

The passivation behaviour of -brass (CuZn=6733) in alkaline solutions was studied using cyclic voltammetry and potentiostatic current transient measurements. The recorded cyclic voltammograms exhibited the main features usually observed for pure copper and zinc, and one additional anodic peak on the reverse potential scan. The height, sharpness and location of the different peaks depended greatly on the alkali concentration and the scan rate. The results show that the formation of Cu₂O and Cu(OH)₂ films proceed under ohmic resistance control following a dissolution-precipitation mechanism. The effect of F^–, Cl^– and Br^– ions on the passivity was also studied. The pitting potential was found to decrease with logarithm of halide ion concentration. The current transients in the absence and presence of halide ions were analysed. In the absence of pitting the current, after a few seconds, was found to increase linearly with the reciprocal of the square root of time while in the presence of pitting it was found to fit the Engell-Stolica equation.     

Molecular dynamics simulation for the binary mixtures of high pressure carbon dioxide and ionic liquids简

Molecular dynamics simulation with an all-atom force field has been carded out on the two binary sys- tems of [bmim][PF6]-CO2 and [bmim][NO3]-CO2 to study the transport properties, volume expansion and micro- structures. It was found that addition of CO2 in the liquid phase can greatly decrease the viscosity of ionic liquids （ILs） and increase their diffusion coefficient obviously. Furthermore, the volume expansion of ionic liquids was found to increase with the increase of the mole fraction of CO2 in the liquid phase but less than 35% for the two simulated systems, which had a significant difference with CO2 expanded organic solvents. The main reason was that there were some void spaces inter and intra the molecules of ionic liquids. Finally, site to site radial distribution functions and corresponding number integrals were investigated and it was found that the change of microstructures of ILs bv addition CO2 had a great influence on the orooerties of ILs.     

Dielectric property of polyimide/barium titanate composites and its influence factors (Ⅱ)

Using poly(amic acid) (PAA) as a precursor followed by thermal imidization, the polyimide/barium titanate composite films were successfully prepared by a direct mixing method and in situ process. The influence of processing factors, such as particle size, distribution mode and polymerization method on dielectric properties was studied. Results revealed that the dielectric constant (ɛ) of the composite film increased by using bigger fillers or employing in situ polymerization and bimodal distribution. When the composite film containing 50 Vol-% of BaTiO₃ with size in 100 nm was prepared via in situ process, its dielectric constant reached 45 at 10 kHz. __________ Translated from Journal of Functional Materials, 2008, 39(2): 264–267 [译自: 功能材料]     

Molecular dynamics simulations of α- to β-poly(vinylidene fluoride) phase change by stretching and poling

The mechanism of inducing a phase change from α-poly(vinylidene fluoride) (α-PVDF) to β-PVDF is addressed using molecular dynamics simulations based on a molecular mechanics force field. The effect of applying a strain to the α-PVDF crystal along the axis of the molecules is investigated, as well as poling the crystal before or after stretching. Rather large (at least 10¹⁰ V/m) electric fields that are perpendicular to the axis of the PVDF molecules are required to induce α- to β-PVDF phase change when no strain is applied to the α-PVDF crystal. However, at a strain of 1.0475 (i.e., when the crystal is stretched by 4.75%) α-PVDF changes to a β-PVDF like structure, where the β-PVDF molecules orientate anti-parallel relative to each other. Transformation of the anti-parallel β-PVDF to β-PVDF can be induced by poling (even at the lowest electric field of 10⁵ V/m studied here) or by thermal annealing.