Nanotribological properties of precision-controlled regular nanotexture on H-passivated Si surface by current-induced local anodic oxidation

Nano-sized textures resulted from localized electrochemical oxidation by using atomic force microscopy (AFM) were fabricated on H-passivated Si surface. In this paper, the fabrication and nanotribological properties of nanotexture by local anodic oxidation (LAO) on H-passivated Si surface are presented. A special attention is paid to find the relation between the size of oxide nanotexture and operational parameters such as tip-sample pulsed bias voltage, pulsewidth, and relative humidity to fabricate oxide nanotexture. The nanotribological properties were investigated by a colloidal probe. The results indicate that the nanotextures exhibited low adhesion and greatly reduced friction force at nanometer scale.     

Reducing Friction Force of Si Material by Means of Atomic Layer-Deposited ZnO Films

With excellent lubricating property, zinc oxide (ZnO) films are promising candidates to act as protective coatings in Si-based microelectromechanical system devices for the purpose of decreasing friction forces of silicon (Si) material. In this paper, the nanotribological behavior of ZnO films prepared by atomic layer deposition on a Si (100) substrate is investigated by an atomic force microscope. The ZnO films have various thicknesses ranging from 10.0 to 182.1 nm. With the increase of film thickness, the root-mean-square roughness of the films increases, while the ratio of hardness to Young’s modulus (H/E) decreases. Due to their large surface roughness, the thick ZnO films are low in adhesion force. The friction force of the ZnO films is smaller than that of the Si (100) substrate and is greatly influenced by their adhesion force and mechanical property. In a low-load condition, the friction force is dominated by the adhesion force, and thus, the friction force of the ZnO films decreases as film thickness increases. While in a high-load condition, the friction force is dominated by plowing. Films with higher H/E possess smaller friction force, and thus, the friction force increases with the decreasing film thickness.     

Ni nanodot-patterned surfaces for adhesion and friction reduction

Surface nano-patterning with Ni nanodot arrays was investigated for adhesion and friction reduction of contacting interfaces. Self-assembled anodized aluminum oxide (AAO) templates in conjunction with thermal evaporation was used to fabricate nano-patterned surfaces with ordered Ni nanodot arrays on Si substrates. Surface morphology of the Ni nanodot-patterned surfaces (NDPSs) was characterized by scanning electron microscopy (SEM). Adhesion and friction studies on a Ni NDPS and a baseline smooth Si(100) surface were conducted using a TriboIndenter employing a diamond tip with 100 μm nominal radius of curvature. The results show that the ordered Ni nanodot-patterning reduced the adhesion forces and coefficients of friction up to 92 and 83%, respectively, compared to those of the smooth silicon surface. Surprisingly, the nanoscale multi-asperity contact between the diamond tip and inhomogeneous Ni NDPSs under low loads follows a continuum contact mechanics model.     

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5-85 nm sputtered on Si(1 0 0) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200 nm and contact (normal) loads between 10 and 1200 μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50 μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150 μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.     

Nanotribology of a Silica Nanoparticle-Textured Surface

Tribological properties of a silica nanoparticle-textured (SNPT) surface were investigated at the nanoscale using a nanoindenter. The sample was fabricated by spin coating chemically synthesized silica nanoparticle solution onto a silicon substrate and then annealing the substrate in an N₂ environment. Environmental scanning electron microscopy (ESEM) and scanning probe microscopy (SPM) were used to characterize the morphology of the SNPT surface. Adhesion and friction experiments were performed with a diamond tip of nominal radius of curvature of 5 μ m, under contact forces of 750-1500 μ N, and with sliding speed of 0.1-2 μ m/s. The nanotribological properties of the SNPT sample were compared to those of a smooth silicon oxide film (SOF)-coated sample. The adhesion performance of the SNPT surface was found to be much better than that of the SOF surface. The coefficient of friction (COF) reduced up to 26%.

     

Investigation of nanotribological properties of self-assembled monolayers with alkyl and biphenyl spacer chains (invited)

Understanding the relationships between molecular structure and nanotribological performance of self-assembled monolayers (SAMs) are quite important for molecular tailoring for efficient lubrication. For this purpose, SAMs, having alkyl and biphenyl spacer chains with different surface terminal groups (-CH3, -COOH, and -OH), and head groups (-SH and -OH), were prepared. The influence of spacer chains, surface terminal groups, and head groups on adhesion, friction and wear properties were investigated by contact mode atomic force microscopy (AFM). The relative stiffness of SAMs was determined by force modulation mode AFM and indentation experiments using load-displacement curves. The friction properties of SAMs are explained using a molecular spring model in which local stiffness governs the friction properties. Micropatterned SAMs specimen were fabricated and studied to verify the molecular spring model. The influence of relative humidity, temperature and velocity on adhesion and friction was studied. The failure mechanisms of SAMs and substrates were investigated by wear and continuous microscratch AFM technique. Based on these studies, the adhesion, friction and wear mechanisms of SAMs at molecular scale are discussed.     

Effect of novel phosphazene-type additives on the tribological properties of Z-DOL in a steel-on-steel contact

The effect of hexakis(1,1,5-trihydroperfluoropentoxy)cyclotriphosphazene (X-100) and a novel synthetic bridged alkoxycyclotriphosphazene (denoted LA-2P) as additives in Z-DOL, a perfluoropolyether-type lubricant, on the friction and wear behavior of a steel-on-steel system was investigated. Thus, the friction and wear test of a steel disc sliding against the same steel counterpart ball was carried out on an SRV oscillating friction and wear tester. The chemical features of the worn steel surfaces were analyzed by means of X-ray photoelectron spectroscopy. The results indicate that both additives are effective in improving the tribological performance of Z-DOL and in preventing its catalytic decomposition. Z-DOL/LA-2P shows the best anti-wear ability. Moreover, both additives also increase the thermal stability of Z-DOL in the presence of AlCl₃ as a catalyst. In particular, the Z-DOL/LA-2P mixture shows the highest complete thermal degradation temperature.     

改性碳纤维增强纸基摩擦材料摩擦磨损性能

采用双-[γ-(三乙氧基硅基)丙基]四硫化物(Si69)和乙烯基三乙氧基硅烷(A151)对PAN基碳纤维(CF)进行表面改性处理,利用SEM、FTIR、EDX对改性前后的CF进行表征,测量接触角和表面能、力学性能和界面性能;通过湿法成形技术,制备不同改性CF增强聚酰亚胺纸基摩擦材料,并测试其孔隙率和摩擦学性能。结果表明:与未改性CF相比,Si69和A151能够有效地增加CF表面粗糙度,且新基团的引入使接触角变小,提高了CF表面活性,改善了纤维与树脂之间的结合力,使得A151-CF表面能增加了37.3%,Si69-CF表面能增加了109.4%,A151-CF/聚酰亚胺复合材料界面性能增加了19.1%,Si69-CF/聚酰亚胺复合材料界面性能提高了45.3%;相比未改性CF,Si69改性CF使纸基摩擦材料孔隙率下降了20.2%,A151改性CF使纸基摩擦材料孔隙率下降了8.8%;表面改性CF能够提高纸基摩擦材料的摩擦学性能,其中Si69改性CF增强纸基摩擦材料摩擦学性能优于A151改性CF增强纸基摩擦材料。     

Wetting study of patterned surfaces for superhydrophobicity

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water-repellent properties. A number of studies have been carried out to produce artificial biomimetic roughness-induced hydrophobic surfaces. In general, both homogeneous and composite interfaces are possible on the produced surface. Silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values were fabricated. We show how static contact angles vary with different pitch values on the patterned silicon surfaces. Based on the experimental data and a numerical model, the trends are explained. We show that superhydrophobic surfaces have low hysteresis and tilt angle. Tribological properties play an important role in many applications requiring water-repellent properties. Therefore, it is important to study the adhesion and friction properties of these surfaces that mimic nature. An atomic/friction force microscope (AFM/FFM) is used for surface characterization and adhesion and friction measurements.     

Nanofriction of silicon oxide surfaces covered with thin water films

A thin water film present on surfaces plays a central role in defining the micro- and nanotribological properties of a system. This paper presents a quantitative examination of the nanotribological effects of thin water films in ultra high vacuum (UHV) on OH-terminated (hydrophilic) and bare (no OH terminations, hydrophobic in vacuum) silicon oxide surfaces. Water film thickness was controlled by varying the water partial pressure in UHV. Friction was measured by scanning force microscopy (SFM) as a function of an external applied load. The surface energy and the shear stress of the nanotribological contact was then approximated by fitting the friction-load curves using the Derjaguin-Muller-Toporov (DMT) model. The surface energy as well as the adhesion force of the OH-terminated hydrophilic sample first decrease and later increase significantly at higher water partial pressures. No such dependence could be deduced from the friction-load curves at varying water pressures for the bare hydrophobic silicon oxide surface. However, at relatively high normal loads (pressures) and water partial pressures the bare hydrophobic silicon oxide is transformed to an OH-terminated surface. This transformation appears to occur only in the area of contact leading to the conclusion that it is friction-induced. This work shows that the chemical composition of the topmost surface layer defines the frictional behavior of the tribosystem.     

Adhesion and Friction Studies of a Selectively Micro/Nano-textured Surface Produced by UV Assisted Crystallization of Amorphous Silicon

This paper presents a novel methodology of producing selectively micro/nano-textured surfaces for applications in micro/nano-electro-mechanical systems, and friction and adhesion/stiction studies on the micro/nano-textured surfaces. The selective textures were produced by ultraviolet-assisted aluminum-induced crystallization of plasma-enhanced chemical vapor deposited amorphous silicon. Friction and adhesion/stiction studies were conducted using a TriboIndenter. The results show that the surface texturing technique significantly reduces both adhesion/stiction forces and coefficients of friction.     

Microtribological studies of unlubricated sliding Si/Si contact in air using AFM/FFM

Tribological properties of Si/Si contacts were measured on a microscale by using an atomic force/friction force microscope. Friction forces and pull-off forces between a Si tip and a polished surface of a Si(100) wafer were studied as a function of applied normal load and relative humidity of the surrounding air. The results show that pull-off forces and friction coefficients increased and were strongly influenced by capillary forces with increasing humidity. Tribological interactions during 20 passes of overlapping sliding contact at 50% relative humidity and very small loads of 70 nN were confined to the layer of adsorbates and chemical reactions, without measurable solid damage on the Si(100) wafer.     

Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM

Molecularly thick perfluoropolyether (PFPE) films are considered to be good protective films for micro/nanoelectromechanical systems (MEMS/NEMS) to reduce stiction, friction, and improve their durability. Understanding the nanotribological performance and mechanisms of these films are quite important for efficient lubrication for MEMS/NEMS devices. These devices are used in various operating environments and their effect on friction, adhesion and durability needs to be clarified. For this purpose, mobile and chemically bonded PFPE films were deposited by dip coating technique. The friction and adhesion properties of these films were characterized by atomic force microscopy (AFM). The effect of rest time, velocity, relative humidity, and temperature on nanotribological properties of these films was studied. Durability of these films was also measured by repeated cycling tests. The adhesion, friction mechanisms of PFPE at molecular scale, and the mechanisms of the effect of operating environment and durability are subject of this paper. This study found that adsorption of water, formation of meniscus and its change during sliding, viscosity, and surface chemistry properties play a big role on the friction, adhesion, and durability of the lubricant films.     

Tribological Properties of Patterned NiFe-Covered Si Surfaces

The tribological properties of patterned surfaces were investigated under lubricated conditions. Micropatterns were fabricated on a Si surface using a combination of photolithography and plasma etching. NiFe film with a 150 nm thickness was then deposited on the patterned Si surface. We prepared four kinds of patterned surfaces: dimple, grating, bump, and mesh patterns. The dimensions of the patterns were: size 30–40 μm, pitch 120 μm, and depth 10–12 μm. Friction tests were carried out using a pin-on-plate tribometer. The pin specimen was made of cast iron and had a flat end. The normal load was varied from 9.8 to 98 mN, and the average sliding speed from 1.0 to 5.0 mm s⁻¹. Slideway lubricating oils or a gear oil were used as the lubricant, and the ISO viscosity grades of these oils were VG32, VG68, and VG320. The results showed that the friction coefficients of the two reverse patterns showed very similar tendencies and that circular patterns had a lower friction coefficient than did the rectangular patterns at a high bearing characteristic number. The surface geometry of the Si surface did not affect the friction coefficients at a low bearing characteristic number.     

Nanotribological Behavior of Ultra-thin Al2O3 Films Prepared by Atomic Layer Deposition

Si-based nano/micro-electromechanical system (NEMS/MEMS) devices with contacting and rubbing structures cannot run reliably due to their poor tribological performance. A thin alumina (Al₂O₃) film is a promising candidate for the protective coating in the applications of NEMS/MEMS devices. In this study, nanotribological behavior of ultra-thin Al₂O₃ films prepared by atomic layer deposition on a Si (100) substrate was investigated in comparison with that of Si (100). X-ray photoelectron spectroscopy was used to determine the composition of Al₂O₃ films. Atomic force microscopy with different tips was employed to measure the scratch resistance, adhesion and friction forces of various samples. The results show that Al₂O₃ films have larger scratch resistance than that of Si (100). In addition, the adhesion and friction forces of Al₂O₃ films are smaller than that of Si (100). Thus, the Al₂O₃ films are capable of a wide application in Si-based NEMS/MEMS devices. The improved tribological performance of Al₂O₃ films is attributed to their hydrophobic properties.     

Surface charges and adhesion measured by atomic force microscope influence on friction force

A good correlation has been found between friction force measured using a ball-on-disc tribometer (normal load 200 mN) and adhesion hysteresis measured by atomic force microscopy. Both adhesion and friction forces were investigated in liquid media (water, ethanol, formamide, ethylene glycol) and involved interactions between silicon nitride and several materials (Si(1 0 0), Si(1 1 1), silica glass, DLC and TiN coatings). Despite the difference between the two scales of measurement, comparison between the measured friction force and the dissipated energy during the adhesion process has shown that the two quantities follow the same trend. Additional experiments were conducted in NaCl 10⁻³ M at various pH values in order to investigate surface charge effect on adhesion and friction.     

Effect of surface structure on frictional behaviour of a tongue/palate tribological system

The effect of surface structure on the friction and lubrication properties of a model tribological system representing the tongue/palate contact was investigated under dry conditions and in the presence of oil and aqueous solution having the same viscosity at mouth temperature. To this end, several soft silicone surfaces with well-defined surface structures, based on hemisphere pillars of different dimensions in the sub-millimetre range, were fabricated by a moulding technique in order to cover the different scales roughness of the human tongue. The friction experiments were performed on a reciprocating motion sliding tribometer involving contact between a relatively hard ball (steel or PCTFE) representing the palate and one of the soft silicone surfaces simulating the tongue. Test conditions were designed to represent those encountered in the mouth when thin films of food residues coat the oral mucosa surfaces. The results show that the frictional behaviour of the investigated model tribosystem is strongly affected by the topographical structure of the contacting surfaces. Under dry conditions, the coefficient of friction decreases significantly with increase of hemispherical pillar density. For lubricated surfaces, higher pillars with an optimal high density increase the friction coefficient. Further, it was observed that a minimal wetting of at least one contacting surface is essential for establishing effective lubrication.     

Lubricant Effects in the Transition from Boundary to Microelastohydrodynamic Lubrication

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement.     

Assessing Nanotribological Performance and Surface Energies of Inconel-ZrN,Cr-ZrN,Nb-ZrN,and ZrN Thin Films

The nanotribological performance for three groups of metal-ZrN, including Inconel-ZrN, Cr-ZrN, Nb-ZrN, and polycrystalline ZrN thin films has been investigated and results were correlated with surface energy evaluations. Metal-ZrN and ZrN thin films were deposited using direct current (DC) unbalanced magnetron sputtering and their elemental composition was investigated using X-ray photoelectron spectroscopy (XPS). Both nanomechanical and nanotribological properties were evaluated using a triboscope interfaced with an atomic force microscope (AFM) and the surface energies were calculated from the contact angle measurements. The present research reports for the first time on the nanowear behavior, surface roughness, and friction coefficients correlated with surface energies of metal-ZrN and ZrN thin films.

All metal-ZrN thin films showed improved nanotribological performance compared to the polycrystalline ZrN. Results indicate that several of the Inconel-ZrN thin film compositions have both superior nanotribological behavior and good wettability and thus have high potential use for wear resistant applications.     

Nanotribological behavior of tethered reinforced polymer nanolayer coatings

We fabricated molecularly thick thermoplastic elastomeric films with organized microdomain structure and intriguing nanotribological properties. Molecular films from poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) were obtained by a melt/solution grafting to a functionalized silicon surface modified with epoxy-terminated self-assembling monolayers. We varied the thickness of grafted block–polymer films from 1.35 nm (disordered polymer layer) to 9 nm (well defined nanophase structure) and tested their friction, adhesion, shear and wearing properties on a microscale with scanning probe microscopy. Tethered SEBS monolayers, composed of a rubber matrix reinforced by a two-dimensional net of glassy polystyrene (PS) microdomains, possess a friction coefficient as low as 0.02 and shear strength in the range 0.15–1.5 GPa. Chemically tethered SEBS monomolecular films are much more stable under shear stresses than conventional molecular coatings.