Chengbiao Wang

Synergistic Effects between Plasma-Nitrided AISI 52100 Steel and Zinc Dialkyldithiophosphate Additive under Boundary Lubrication

Plasma nitriding is applied to many industrial parts running in the boundary (or mixed) regime. It is necessary to understand the synergistic effect on tribological behavior between the nitrided steel surface and lubricant additive to achieve an effective performance. However, there has been very little work concerning the effect of plasma-nitrided steel on the tribochemical interactions of zinc dialkyldithiophosphate (ZDDP) additive and the composition of ZDDP tribofilm. In this work, the interactions and synergistic tribological effects between the plasma-nitrided AISI 52100 steel surface and ZDDP additive under boundary lubrication were investigated. The tribological behavior of plasma-nitrided and untreated steels under lubrication with different ZDDP concentrations was examined on a four-ball friction and wear tester. The chemical characteristics of the tribofilms generated on the worn surfaces were analyzed by X-ray photoelectron spectroscopy (XPS). The results showed that the obvious synergistic effects of better friction reduction and wear resistance between plasma-nitrided steel and ZDDP were attributed to the hard nitrided layer and the shorter chain phosphates tribofilm with greater carbon and phosphate contents.

C Fibers@WSe2 Nanoplates Core–Shell Composite: Highly Efficient Solar-Driven Photocatalyst

Recently, WSe2 as a typical transition metal dichalcogenide compound has attracted extensive attention due to its potential applications in electronic and optoelectronic devices. However, WSe2 alone cannot be directly used as a photocatalyst due to its inferior performance possibly caused by the strong recombination of photogenerated electron-hole pairs. Here a novel C fibers@WSe2 nanoplates core-shell composite (NPCSC) was successfully synthesized via facile, one-step thermal evaporation, in which numerous WSe2 thin nanoplates were in situ, densely and even vertically grown on the surface of the C fibers. Such composite presents highly solar-driven photocatalytic activity and stability for the degradation of various organic aqueous dyes including methylene blue and rhodamine B, and highly harmful gases like toluene, showing the great potential for environmental remediation by degrading toxic industrial chemicals using sunlight. Under simulated sunlight irradiation, comparing with commercially available WSe2 powder, the as-synthesized C fibers@WSe2 NPCSC presents significantly enhanced reaction rate constants by a factor of approximately 15, 9, and 3 for the degradation of aqueous methylene blue, aqueous rhodamine B, and gaseous toluene, respectively, due to the effective separation of photogenerated electron-hole pairs promoted by the rapid transfer of photogenerated electrons through C fibers. Moreover, this one-step thermal evaporation is an easy-handling, environmentally friendly, and low-cost synthesis method, which is suitable for large-scale production.

Mechanical and biomedical properties of copper-containing diamond-like carbon films on magnesium alloys

The high susceptibility to corrosion of magnesium alloys may lead to their rapid degradation in the human body and thus greatly jeopardizes their applications as potential biodegradable bone implant materials. To improve their mechanical properties and biocompatibility, Cu-DLC films were coated on Mg alloys in a mid-frequency dual-magnetron system. The anti-corrosion properties, frictional properties, and degradation behaviours of these coated films were then investigated as a means of evaluating their protective effects on the Mg alloys. The results exhibited a rather low coefficient of friction (COF) μ in simulated body fluid conditions for the film doped with a:C-Cu8.7% when compared with the Cu-DLC films doped with other contents. This is mainly because the a:C-Cu8.7% film gives higher hardness, lower intrinsic stress, and more superior adherence. Tests also suggested that the hemocompatibility and protective-ability of the Mg alloys deposited with the a:C-Cu8.7% film were greatly improved when compared with their uncoated counterparts. Analysis of the mechanism involved with the biocompatibility of Cu-DLC films revealed that adequate diffusion of nanosized Cu crystallites into the DLC films on the Mg alloys can result in a high ID/IG ratio, a relatively smooth surface, and a low coefficient of friction. Such diffusion effectively suppresses the release of Mg ions from the Mg alloys, and subsequently improves the film hemocompatibility and protective-ability.

Study of the Regenerated Layer on the Worn Surface of a Cylinder Liner Lubricated by a Novel Silicate Additive in Lubricating Oil

Some special silicate particles as additives in lubricating oil have shown a certain self-repairing function for the rubbing pairs of industrial equipment in recent R&D of extreme pressure antiwear additives. This article introduces an investigation on the regenerated layer on the worn surface of a practical cylinder liner lubricated by lubricating oil with a silicate additive using some advanced techniques like transmission electron microscopy (TEM), atomic force microscopy (AFM), nano-hardness tester, scanning electron microscopy (SEM), auger electron spectroscopy (AES), and Raman spectroscopy. The basic formula of the mineral in the silicate additive is Al4[Si4O10](OH)4. Through some macro- and microanalyses, it was found that the silicate additive showed an obvious improving effect on their friction surface and self-repairing function. The roughness of the worn surface could be decreased greatly to several tens of nanometers, and its hardness was still above 10 GPa. The worn surface with some pits and cracks had been covered by a transparent regenerated layer, and the wear of cylinder liners was maintained at almost zero-wear level on average. The mechanism of the self-repairing function was approached. It was revealed that the silicate additive was acting as a catalyst to promote a series of complex tribochemical reactions to form a regenerated layer with amorphous carbon structure on the worn surface under high-friction temperature and pressure in the friction and wear process.

High-performance varistors simply by hot-dipping zinc oxide thin films in Pr 6 O 11 : Influence of temperature

High-performance ZnO-Pr6O11 thin-film varistors were fabricated simply by hot-dipping oxygen-deficient zinc oxide thin films in Pr6O11 powder. The films had a composition of ZnO0.81 and a thickness of about 200 nm, which were deposited by radio frequency magnetron sputtering a sintered zinc oxide ceramic target. Special attention was paid on the temperature dependence of the varistors. In 50 min with hot-dipping temperature increased from 300–700 °C, the nonlinear coefficient (α) of the varistors increased, but with higher temperature it decreased again. Correspondingly, the leakage current (IL) decreased first and then increased, owing mainly to the formation and destroying of complete zinc oxide/Pr6O11 grain boundaries. The breakdown field (E1mA) decreased monotonously from 0.02217 to 0.01623 V/nm with increasing temperature (300–800 °C), due to the decreased number of effective grain boundaries in the varistors. The varistors prepared at 700 °C exhibited the optimum nonlinear properties with the highest α = 39.29, lowest IL = 0.02736 mA/cm2, and E1mA = 0.01757 V/nm. And after charge-discharge at room temperature for 1000 times, heating at 100 or 250 °C for up to 100 h, or applying at up to 250 °C, the varistors still performed well. Such nanoscaled thin-film varistors will be very promising in electrical/electronic devices working at low voltage.

Measurement of Residual Stresses Using Nanoindentation Method

Instrumented indentation, which is also known as nanoindentation or depth-sensing indentation, is increasingly being used to probe the residual stresses of materials including bulk solids, thin films, and coatings. The residual stresses are proved to have significant effects on various nanoindentation parameters such as hardness, loading curve, unloading curve, pile-up amount around indentation, and true contact area. By analyzing these parameters, numerous methods are developed to evaluate the residual stresses of materials in recent years. This article reviews six commonly used models which determine residual stresses from analyzing load-depth curves, as well as indentation fracture technique which is based on the classical fracture mechanics. Emphasis is placed on the principle, application and limitation of each nanoindentation method.

The Effect of FeS Solid Lubricant on the Tribological Properties of Bearing Steel under Grease Lubrication

In order to improve the tribological properties of 52100 steel under grease lubrication, FeS solid lubricant was used in two ways. Low-temperature ion-sulfurization technology was utilized to prepare solid lubricant iron sulfide (FeS) films on the surface of 52100 steel, and FeS particles were mixed into the lithium grease as additive. The friction and wear properties were examined systematically on a “ball-on-disc” testing machine. The results showed that the tribological properties of bearing steel under grease lubrication can be improved either by using ion-sulfurization technique or by adding FeS microparticles into the grease. The tribological performance of sulfurized surface lubricated by grease is better than that of a plain surface lubricated by grease containing FeS microparticles at lower load and speed. The plain surface lubricated by the grease containing FeS micropaticles possesses better antiwear property under harsher conditions. The mechanism of the experimental results is discussed in detail.

Thermal stability of ultrahard polycrystalline diamond composite materials

Thermal stability of the ultrahard polycrystalline diamond (UHPCD) composite material developed by the reinforcement of the polycrystalline diamond (PCD) with chemical vapor deposition (CVD) diamond has been investigated in a flow of argon at 1200°C. The indentation, Raman spectra and wear test have been performed to compare hardness, C-C structure and wear resistance of untreated and thermal treated UHPCD. It has been shown that the hardness of CVD diamond in UHPCD attains 133 ± 7 GPa after high pressure and high temperature (HP-HT), while after thermal treatment the hardness decreases to 109 ± 3 GPa, and the wear resistance of the thermal treated UHPCD decreases from 0.17 to 0.6 mg/km. The narrowing of full width at half maximum (FWHM) and shift of Raman peak to lower frequencies of CVD diamond in thermal treated UHPCD imply a decrease of crystal structural defects and compressive stresses, which results in a drop of the hardness of CVD diamond in a thermal treated UHPCD. The higher wear rate of thermal treated UHPCD is due to the lower hardness.

ELECTRICAL CONDUCTING AND MECHANISM OF OXYGEN-DEFICIENT TIN OXIDE FILMS DEPOSITED BY RF MAGNETRON SPUTTERING AT VARIOUS O2/Ar RATIOS

Oxygen-deficient tin oxide thin films were prepared by radiofrequency magnetron sputtering with a sintered non-stoichiometric tin oxide ceramic target under an atmosphere of various ratios of O2/Ar from pure Ar to 1:1. X-ray diffraction analysis showed that the thin films were polycrystalline with relatively strong (1 1 0), (1 0 1) and (2 1 1) diffraction peaks. Scanning electron microscopy observation revealed that the thin films prepared at different O2/Ar ratios were all of relatively dense and homogeneous structure. With increasing O2/Ar ratio, the grain size of the films decreased slightly, and their chemical composition became close to the stoichiometric SnO2; but the deposition rate as well as film thickness increased first and then decreased sharply. It was revealed that the main defect in obtained films was oxygen vacancy (VO), and as the O2/Ar ratio increased, the concentration of VO fell down monotonously, which would lead to an increased electrical resistivity.

Influence of copper content and nanograin size on toughness of copper containing diamond-like carbon films

Abstract Five copper containing diamond-like carbon films (Cu-DLC), each with a particular content of Cu, were deposited on Si (100) substrates in an ion beam assisted deposition system. The influence of Cu content and nanograin size on film toughness has been investigated along with verifying through the microstructural, mechanical and sliding tribological behaviours of the films. Variation of Cu content and nanocrystallites dispersing in DLC matrix induces corresponding modification in film toughness as well as hardness, intrinsic stress and sliding frictional behaviour, resulting in improvement of film toughness for the combined tribological performances. The film toughness was investigated by scratch crack propagation resistance from the critical load data obtained from scratch test. Results revealed that doping Cu with nanograin sizes, especially at a suitable content of 10·5 at-%, will significantly improve the crack initiation resistance and propagation resistance of crack during scratch test, demonstrating the improved toughness.