Layered double hydroxides (LDHs) are a class of naturally occurring inorganic minerals that are composed of divalent and trivalent metal cations. In this study, three different sized NiAl-LDH nanoplatelets were synthesized by varying crystallization time during the microemulsification process. The layered structure and three-dimensional size of nanoplatelets were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As lubricant additives, their tribological properties in base oil were evaluated by use of a ball-on-disk reciprocating tribometer under three different loads: 50, 100, and 150 N (which created peak Hertz pressures of 1.74, 2.16, and 2.47 GPa). Under contact pressures of up 2.16 GPa, not only did the coefficient of friction (COF) decrease by about 10% after nano-LDHs were added but also the wear performance improved substantially. These improvements resulted from a protective tribolayer formation on the contact interface, as revealed by detailed surface and structure analytical studies. In particular, cross-sectional TEM images revealed that the larger size nanoplatelets (NiAl-24h), rather than the smaller ones (NiAl-6h) showed the best and most stable tribological performance. This was mainly because of their higher degree of crystallinity, which in turn resulted in the formation of a tribofilm with far superior mechanical properties during sliding. Owing to the simple synthetic method and superior tribological properties as oil-based additives, nano-LDHs hold great potential for use in demanding industrial applications in the future.Keywords: LDHs; nanoadditive; oil-based; tribological properties;

•The changes in wear rates and wear mechanisms can be identified by friction forces.•The wear rate of Ti6Al4V against ceramic is higher than against polymer or metal.•The main wear mechanisms of Ti6Al4V are abrasion and adhesion.•The developed method can be used to study the wear behaviors of other materials.Application of titanium and its alloys in the aerospace, biomedical and chemical industry has renewed researchers' interests in their tribological properties. This paper presents a new method for evaluating the kinetic friction and wear performances of Ti6Al4V alloy using a time-frequency approach. The short-time Fourier transform (STFT) was selected to analyze the time-frequency properties of the friction force signals collected in the tribological tests of Ti6Al4V alloy sliding against three different materials. The obtained results showed that the amplitudes of friction force signals were closely related to the differences in wear rates of the tribo-pairs. Furthermore, the frequency amplitudes of the friction force where high energy appeared increased with the sliding time during adhesive wear process and fluctuated significantly during abrasive wear. Meanwhile, the high amplitude at the fundamental frequency of the friction force in STFT indicate the formation of transfer films. To confirm the STFT-based tribological behaviors of Ti6Al4V against different counterbodies in a dry sliding condition, the wear rates and worn surfaces were examined using conventional methods. This study demonstrates that the wear rates and wear mechanisms of the tested samples as well as their changes with time can be identified from the amplitude, distribution and fluctuation of the frequency peaks of the friction force signals. Furthermore, the time-frequency analysis of the friction force is an effective way to study the time-dependent wear process of materials, in particular, changes in the dominant wear mechanism and wear rate.

Saccharides have been recognized as potential bio-lubricants because of their good hydration ability. However, the interfacial structures of saccharides and their derivatives are rarely studied and the molecular details of interaction mechanisms have not been well understood. In this paper, the supramolecular assembly structures of saccharic acids (including galactaric acid and lactobionic acid), mediated by hydrogen bonds O–H⋯N and O–H⋯O, were successfully constructed on a highly oriented pyrolytic graphite (HOPG) surface by introducing pyridine modulators and were explicitly revealed by using scanning tunneling microscopy (STM). Furthermore, friction forces were measured in the saccharic acid/pyridine co-assembled system by atomic force microscopy (AFM), revealing a larger value than a pristine saccharic acid system, which could be attributed to the stronger tip-assembled molecule interactions that lead to the higher potential energy barrier needed to overcome. The effort on saccharide-related supramolecular self-assembly and nanotribological behavior could provide a novel and promising pathway to explore the interaction mechanisms underlying friction and reveal the structure–property relationship at the molecular level.

Poly(vinylphosphonic acid) (PVPA) cross-linked networks on Ti6Al4V show superlubricity behavior when sliding against polytetrafluoroethylene in water-based lubricants. The superlubricity can occur but only with the existence of salt ions in the polymer cross-linked networks. This is different from the phenomenon in most polymer brushes. An investigation into the mechanism revealed that cations and anions in the lubricants worked together to yield the superlubricity even under harsh conditions. It is proposed that the preferential interactions of cations with PVPA molecules rather than water molecules are the main reason for the superlubricity in water-based lubricants. The interaction of anions with water molecules regulates the properties of the tribological interfaces, which influences the magnitude of the friction coefficient. Owing to the novel cross-linked networks and the interactions between cations and polymer molecules, their superlubricity can be maintained even at a high salt ion concentration of 5 M. These excellent properties make PVPA-modified Ti6Al4V a potential candidate for application in artificial implants.

•Corrosion of Ru is compared under different slurry pH values.•CMP-chemical experiments are conducted under 3 different conditions.•The corrosion of Ru is a mixed-controlled process under weak alkaline conditions.•Weak alkaline slurry is optimal for it has good mechanical-enhanced chemical effect.Ruthenium (Ru), as one of the most promising barrier layer applied for the 14 nm node and below technology in the integrated circuits manufacturing, the effects of the polishing variables on the chemical mechanical polishing (CMP) process are not intensively investigated. Corrosion investigation methods combined with CMP experiments were utilized to comprehensively reveal the polishing mechanism affected by the potassium periodate (KIO4) concentration and the slurry pH values. The results of both the static and in-situ corrosion investigation show that the static corrosion rate of Ru is the lowest when the slurry is weak alkaline, but it is the most obviously enhanced by the polishing process under this condition. Considering the different oxidant concentration, the corrosion of Ru distinctively accelerates, and turns to be more like a diffusion-limited mass transfer controlled process when the KIO4 concentration increases. Because the corrosion plays a significant role in the whole CMP process, with a full accounting of other variables, the polishing process of Ru is preferred to be carried out under low down pressure in weak alkaline slurries. Under this condition, the mechanical-enhanced chemical effect is predominating, which is helpful to obtain a good surface quality, and a high material removal rate.

•The synergistic effect of mechanical and chemical factors on the CMP performance of TSV was investigated.•Down force and polishing time should be strictly controlled to guarantee high removal rate and low dishing.•A sufficiently high H2O2 concentration is required to prevent via protrusion by modifying the selectivity of Cu/Ti/TEOS.•Material removal models were established to elucidate the mechanism of via dishing/protrusion formation.Despite the fact that substantial research has been conducted on the through-silicon via (TSV) formation process during three-dimensional integration, there is still less understanding on the chemical mechanical planarization (CMP) process and post-CMP via dishing control mechanism in TSV fabrication. In this study, we investigated the synergistic effect of mechanical and chemical factors on the CMP performance of TSV, and analyzed the experimental results from different aspects. A high-resolution surface-profile-measuring instrument, combined with Scanning Electron Microscopy (SEM) and other measurements, was used to demonstrate the mechanism of non-uniformity and via dishing, and the experimental results indicate that via dishing and non-uniformity of wafers are strongly mitigated by combining high down force with low down force. Based on numerical analysis, it can be concluded that the solution of the via protrusion issue requires a sufficiently high hydrogen peroxide concentration in order to modify the removal rate selectivity of copper and barrier/oxide. The polishing time should be strictly controlled to prevent oversize dishing or protrusion. Moreover, material removal models for via dishing, based on several factors mentioned above, were established to elucidate the mechanism of via dishing/protrusion formation, and to illustrate the further optimization of the TSV–CMP process.

•We are the first to introduce basil seed gel in the lubrication/friction felid.•A friction coefficient about 0.003 is obtained between basil seed gel and glass.•The ultralow friction relies on the special characteristics of basil seed gel.In the present work, a plant-based biomaterial named basil seed gel (BSG) is found to have an excellent lubricating property. The lubricating behavior of BSG is evaluated by a specifically designed apparatus without damaging its natural structure. An ultralow friction coefficient about 0.003 is obtained between BSG and the glass plate surface. Further characterizations reveal that the gel of basil seed has a three-dimensional network microstructure, which may provide a favorable condition for holding water molecules and forming hydration layers. The network constitutes of crosslinking polysaccharide sheets, which are easy to shear and easily adsorbed on the glass surface to form a stable nanoscale flat layer. A possible lubrication mechanism is employed that the easy-to-shear and easy-to-adsorb performance of BSG as well as the superior lubricating properties of hydration layer lead to the ultralow friction.

The stability of the tribological properties of polymer coatings is vital to ensure their long term use. The superlubricity of the poly(vinylphosphonic acid) (PVPA)-modified Ti6Al4V/polytetrafluoroethylene (PTFE) interface can be obtained when lubricated by phosphate-buffered saline (PBS, pH = 7.2), but not when lubricated by deionized water and ethanol. Therefore, the mechanisms for the superlubricity of PVPA coatings affected by lubricant were investigated in detail. The stability of the PVPA coatings and their compatibility with the lubricant are critical factors in realizing ideal tribological properties of PVPA coatings. Robust PVPA coatings are stable under a wide range of pH values (6–10) using PBS as the basic solution, and are also characterized by superlubricity. The hydrolysis kinetics of phosphate anhydride is the main reason for the pH responses. In addition, along with stability, PVPA coatings exhibit different friction coefficients in salt solutions which are composed of various ions, which indicates that the compatibility between PVPA coatings and the lubricant can be used to regulate the superlubricity properties. Based on a fundamental understanding of the mechanism of surperlubricity by considering the effects of the lubricant, PVPA coatings with stability and perfect tribological performance are expected to be applied in more aspects.

•Superlubricity (COF ≈ 0.006) is achieved between steel surfaces with nanodiamonds glycerol colloid solution.•The wear volume is significantly reduced due to the rolling effect of nanodiamonds.•It is the first time that colloid solution is introduced into the field of liquid superlubricity.In this paper, nanodiamonds glycerol colloidal solution is investigated as a lubricant between steel ball and disk, and it is compared with glycerol solution. Although stable superlubricity (coefficient of friction ≈ 0.006) can be achieved with both solutions after a running-in period, the colloidal solution causes much less wear than the glycerol solution. With the content of the nanodiamonds at only 0.01 wt%, the colloidal solution can lead to a reduction of 32.5% in the diameter of the wear scar on the ball, resulting in an increase of 119.3% in the contact pressure between the rubbing surfaces during the superlubricity period. Therefore, a novel superlubricity system based on nanodiamonds glycerol colloidal solution is discovered. Through theoretic calculations, the novel superlubricity system is determined to be in the mixed lubrication regime, but not far from the hydrodynamic lubrication regime. It is believed that the ultralow COF is attributed to the hydrodynamic effect and the hydrogen bond layer. And the reduced wear derives from the rolling effect of the nanodiamonds. This work enriches the field of water-based superlubricity by firstly introducing nanoparticles into this area and has prosperous potential applications.

Liposomes are widely used in drug delivery and gene therapy, and their new role as boundary lubricant in natural/artificial joints has been found in recent years. In this study, the tribological properties of liposomes on titanium alloy (Ti6Al4 V)/UHMWPE interface were studied by a ball-on-disc tribometer. The efficient reduction of friction coefficient and wear on both surfaces under various velocities and loads is found. A multilayer structure of physically adsorbed liposomes on Ti6Al4 V surface was also observed by atomic force microscope (AFM). Except for the hydration mechanism by phosphatidylcholine (PC) groups, the well-performed tribological properties by liposomes is also attributed to the existence of adsorbed liposome layers on both surfaces, which could reduce asperities contact and show great bearing capacity. This work enriches the research on liposomes for lubrication improvement on artificial surface and shows their value in clinical application.

In order to understand lubrication mechanism at the nanoscale, researchers have used many physical experimental approaches, such as surface force apparatus, atomic force microscopy and ball-on-disk tribometer. The results show that the variation rules of the friction force, film thicknessand viscosity of the lubricant at the nanoscale are different from elastohydrodynamic lubrication (EHL). It is speculated that these differences are attributed to the special arrangement of the molecules at the nanoscale. However, it is difficult to obtain the molecular orientation and distribution directly from the lubricant molecules in these experiments. In recent years, more and more attention has been paid to use new techniques to overcome the shortcomings of traditional experiments, including various spectral methods. The most representative achievements in the experimental research of molecular arrangement are reviewed in this paper: The change of film structure of a liquid crystal under confinement has been obtained using X-ray method. The molecular orientation change of lubricant films has been observed using absorption spectroscopy. Infrared spectroscopy has been used to measure the anisotropy of molecular orientation in the contact region when the lubricant film thickness is reduced to a few tens of nanometers. In situ Raman spectroscopy has been performed to measure the molecular orientation of the lubricant film semi-quantitatively. These results prove that confinement and shear in the contact region can change the arrangement of lubricant molecules. As a result, the lubrication characteristics are affected. The shortages of these works are also discussed based on practicable results. Further work is needed to separate the information of the solid-liquid interface from the bulk liquid film.

Poly(vinylphosphonic acid) (PVPA) is a type of hydrophilic polymer that can be used in surface modifications. In our study, PVPA coatings were formed on the surfaces of titanium alloy (Ti6Al4V) using a simple and novel method to achieve efficient lubrication at friction interfaces. The composition and 3D skeletal structure of the PVPA coatings were confirmed by X-ray photoelectron spectroscopy (XPS), focused ion beam/scanning electron microscopy (FIB/SEM), and solid-state nuclear magnetic resonance (NMR). The PVPA-modified Ti6Al4V/polytetrafluoroethylene (PTFE) interface shows a superlow friction coefficient (approximately 0.006) for at least 8 h under a contact pressure of 44.2 MPa (initial pressure), which means it falls into the superlubricity regime. Moreover, wear on the surfaces of both the Ti6Al4V and PTFE after the tribological experiment is superlow. It is proposed that the 3D skeletal structure of the PVPA coating and fluid-like manner at friction interfaces owing to the fast exchange of water molecules are the main factors accounting for the superlow friction and wear. The PVPA-modified Ti6Al4V has the potential uses in artificial cervical discs.Keywords: poly(vinylphosphonic acid); superlow wear; superlubricity; surface modification; Ti6Al4V

Titanium alloy (Ti6Al4V) surfaces are generally modified to achieve some specific surface properties to satisfy requirements of clinical medicine. In our work, hexadecylphosphonic acid (HDPA) films were successfully formed on Ti6Al4V and subsequently confirmed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. The tribological properties of the HDPA-modified Ti6Al4V were investigated using a ball-on-disk tribometer with a linear reciprocating movement. Experimental results indicate that the HDPA-modified Ti6Al4V can maintain a low friction coefficient (approximately 0.06) for 4 h when sliding against polytetrafluoroethylene (PTFE) balls under a load of 0.8 N in comparison to bare Ti6Al4V (approximately 0.2); the friction coefficient of the HDPA-modified Ti6Al4V shows a 70% decline. In addition, the wear rate of PTFE balls sliding against bare Ti6Al4V was almost twenty times that of PTFE balls sliding against the HDPA-modified Ti6Al4V. Moreover, results of tribological experiments for different speeds (from 3 to 24 mm/s) and loads (from 0.8 to 3.2 N) proved that the HDPA-modified Ti6Al4V was not sensitive to both velocity and load. The friction coefficients were still low and stable even under a high load of 3.2 N or at a high speed of 24 mm/s. This indicates that this soft modification is an optional method of improving tribological properties of Ti6Al4V.

•Homogeneous bilayers of Hexadecylphosphonic acid were formed on Ti6Al4V successfully.•Conformation of Hexadecylphosphonic acid bilayers formed on Ti6Al4V was put forward and then proved.•Reasonable formation mechanism of Hexadecylphosphonic acid bilayers on Ti6Al4V was deduced through interfacial analysis.•The Hexadecylphosphonic acid bilayers were especially durable.Titanium alloy (Ti6Al4V) surfaces were successfully modified by self-assembled hexadecylphosphonic acid (HDPA) bilayers (thickness of about 4 nm) using a reproducible method. Conformation of the HDPA bilayers was put forward and then verified by X-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM) and Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) measurements. The first molecular layer contains two kinds of HDPA molecules which interpenetrate each other. Formation of the second molecular layer was associated with reverse vesicles adhesion and then their rupture on the first molecular layer. Mechanism of HDPA bilayers formation on Ti6Al4V was deduced via an observation on the interfacial adsorption of HDPA molecules. High oxide/hydroxyl content on Ti6Al4V surfaces greatly promotes the formation of HDPA bilayers. Annealing after self-assembly enhances the stability of HDPA bilayers due to formation of pyrophosphates and P-O-Ti which can withstand ultrasonic cleaning for a period of 1 h at least. The HDPA-modified Ti6Al4V will have some potential good tribological properties and protein adsorption properties.Formation mechanism of hexadecylphosphonic acid bilayers formed on Ti6Al4V.

The material removal in chemical–mechanical planarization/polishing (CMP) of copper involves both chemical and mechanical effects. The roles of chemical corrosion, abrasive wear, and their synergistic effects on the material removal mechanism were studied by electrochemical analysis and nano-scratching method using atom force microscopy, respectively. Combining with the results of CMP experiments, dominant factors (chemistry and mechanics) in slurries within the range of pH 3.0–10.0 were assessed. Consequently, a removal mechanism map of copper CMP depending on pH values was constructed. In the alkaline slurry, the wear–corrosion effect predominated in the material removal at pH 8.0 and 9.0; while the copper removal mechanism changed to corrosion–wear action in the acidic slurry from pH 4.0 to 6.0, and good surface quality was also obtained. The results and the strategies provide thorough understanding of the material removal mechanism and further optimization of the CMP process.

The poly (N-isopropylacrylamide) brush was covalently bonded on an initiator-coated silicon wafer via surface-initiated atom transfer radical polymerization. The polymer brush was (76.2±0.1) nm in thickness (by ellipsometer) with a grafting density of ca. 0.27 chains/nm2. The tribological properties of the poly (N-isopropylacrylamide) brush were investigated by means of ball-on-disk tests in a rotational mode under water lubrication for tribological application. The experimental results exhibited a low friction coefficient of ca. 0.03. The excellent lubrication property of the brush was due to its amide groups in the polymer chains. It was supposed that the good lubrication property of the brush was attributed to the cross-linked polymer network formed by the hydrogen bond association of N—H…O==C and the water molecular layer adsorbed by the terminal amide groups in the brush. The poly (N-isopropylacrylamide) solution also exhibits a lubrication property due to physical adsorption of the polymer chains.

In this paper, we report the tribological properties of self-assembled molecular (SAM) films of fluoroalkylsilanes and non-fluoroalkylsilanes, with different chain-lengths, adsorbed on Si substrate surfaces by covalent bonds. The SAM films were characterized using a universal ball-disk experimental tester in aqueous solutions. The substrate surface was examined by X-ray photoelectron spectroscopy (XPS), and the SAM films adsorbed on the Si surfaces were inspected by contact angle measurements and XPS. Lubrication studies revealed that several kinds of fluoroalkylsilanes had similar friction coefficients; the small differences were attributed to the chain flexibility. In contrast, differences in the aqueous lubrication properties of SAM films of non-fluoroalkylsilanes were clearly identified. It is suggested that substitution with fluorine atoms and the surface affinities of fluoroalkylsilanes contributed to redistribution of surface changes, causing variations in lubrication behaviors.

In this paper, three kinds of water-soluble fullerene derivatives were synthesized via electrophilic addition reaction and cycloaddition reaction, respectively. The chemical composition characterizations of these derivatives indicated the successful preparation of C60(OH)x, C60(C(COOH)2)x and C60(OH)x(NHCH2COOH)y fullerene derivatives. The aggregation and morphology characterizations showed that the three kinds of derivatives had an ideal spherical aggregating structures and excellent dispersibility in water, especially C60(OH)x and C60(C(COOH)2)x. The lubrication performance of the fullerene derivatives acted as lubricant additives were investigated at different concentrations in the range of 0–1 wt%. The results indicated that the addition of polyhydroxyl and carboxylic derivatives could improve the lubrication properties, which led to the reduction of wear to about 40% at most. It is attributed that the optimized substitutions of fullerene molecules may be of benefit to their distribution properties and lubricating behaviors in water based lubrication.

The tribological properties of perfluoro and non-perfluoro alkylsilane molecular films were investigated and compared detailedly. Their surface properties were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle (CA) measurement. A ball-on-disk tribometer was used to study the frictional properties of these alkylsilane monolayers. The experimental results reveal that the alkylsilane molecular films are good candidates to decrease friction and they have good capability to endure rigorous shear forces. Perfluoro alkylsilane molecular films are bonded better with the Si substrate than the simple hydrocarbon ones. The effects of sliding velocity and normal load on friction coefficient are evident and the friction coefficient increases with the increase of the sliding velocity. However, friction coefficient decreases with the increase of normal load initially and then increases, indicating there exists a critical normal load for the load effect.

Saccharides have been recognized as potential bio-lubricants because of their good hydration ability. However, the interfacial structures of saccharides and their derivatives are rarely studied and the molecular details of interaction mechanisms have not been well understood. In this paper, the supramolecular assembly structures of saccharic acids (including galactaric acid and lactobionic acid), mediated by hydrogen bonds O–H⋯N and O–H⋯O, were successfully constructed on a highly oriented pyrolytic graphite (HOPG) surface by introducing pyridine modulators and were explicitly revealed by using scanning tunneling microscopy (STM). Furthermore, friction forces were measured in the saccharic acid/pyridine co-assembled system by atomic force microscopy (AFM), revealing a larger value than a pristine saccharic acid system, which could be attributed to the stronger tip-assembled molecule interactions that lead to the higher potential energy barrier needed to overcome. The effort on saccharide-related supramolecular self-assembly and nanotribological behavior could provide a novel and promising pathway to explore the interaction mechanisms underlying friction and reveal the structure–property relationship at the molecular level.