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.     

Adhesion and Friction Studies of Nano-textured Surfaces Produced by Self-Assembling Au Nanoparticles on Silicon Wafers

This article presents the results of nanoscale friction and adhesion of nanoparticle-textured surfaces (NPTS) using atomic force microscope (AFM). The effects of coverage ratio, texture height, and packing density on the adhesion and friction of the NPTS were investigated. The nano-textured surfaces were produced by self-assembling Au nanoparticles (NPs) with diameters of 20 nm and 50 nm on the silicon (100) surfaces, respectively. Surface morphology of the NPTS was characterized by field emission scanning electron microscopy and AFM. The results show that the NPTS significantly reduced the adhesive force compared to the smooth surface. The adhesion of NPTS is mainly dependent on the coverage ratio of NPs rather than the texture height and higher coverage ratio resulted in smaller adhesive force. The reduced adhesion of textured surfaces was attributed to the reduced real area of contact. The friction of NPTS is mainly dependent on the spacing between asperities. The lowered frictional force was obtained when the spacing between asperities is less than the size of AFM tip, because of the effectively reduced real area of contact between the AFM tip and the NPTS surface.     

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.     

Friction Study of a Ni Nanodot-patterned Surface

Nanoscale frictional behavior of a Ni nanodot-patterned surface (NDPS) was studied using a TriboIndenter by employing a diamond tip with a 1 μm nominal radius of curvature. The Ni NDPS was fabricated by thermal evaporation of Ni through a porous anodized aluminum oxide (AAO) template onto a Si substrate. Surface morphology and the deformation of the NDPS were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), before and after friction/scratch testing. SEM images after scratching clearly showed that, similar to what was assumed at the macroscale, the frictional force is proportional to the real area of contact at the nanoscale. It was found that adhesion played a major role in the frictional performance, when the normal load was less than 20 μN and plastic deformation was the dominant contributor to the frictional force, when the normal load was between 60 μN and 125 μN. Surprisingly, a continuum contact mechanics model was found to be applicable to the nanoscale contact between the tip and the inhomogeneous Ni NDPS at low loads. The coefficient of friction (COF) was also found to depend on the size of the tip and was four times the COF between a 100 μm tip and the Ni NDPS. Finally, the critical shear strength of the Ni nanodots/Si substrate interface was estimated to be about 1.24 GPa.     

Structure and tribological characteristics of steel under melting by plasma flow and simultaneous Mo and W alloying

New findings of studies of the structural, tribological, and physicomechanical characteristics of structural steel 40Kh treated by plasma flow under melting accompanied by either tungsten or molybdenum alloying are presented. Rutherford back-scattering of ions, scanning electron microscopy (with microanalysis), X-ray fluorescent spectral analysis, X-ray phase analysis, wear-resistance tests, measurement of the coefficient of friction, and transmitting electron microscopy with diffraction are the basic research methods. It is found experimentally that a thin layer 5 μm thick saturated with nitrogen and an alloying element (Mo or W) with regularly arranged crystallites arises on the steel 40Kh surface. The crystallites in this layer have a needle- and ribbon-shaped structure. A deeper layer located about 40 μm thick consists of micro- and nanosized grains. Friction and wear studies of the plasma-flow treated (melt) samples show the steel wear resistance to increase 2–2.5 times and the coefficient of friction to decrease from 0.4–0.5 to 0.10–0.15 compared to the untreated samples.     

Temperature-Dependent Atomic Scale Friction and Wear on PbS(100)

Friction and wear on PbS(100) surfaces have been investigated on the atomic scale as a function of temperature with atomic force microscopy. At room temperature and above, the PbS(100) surface exhibited low friction (μ < 0.05) in contact with a silicon nitride probe tip, provided that interfacial wear was not encountered. In the absence of wear, friction increased exponentially with decreasing temperature, transitioning to an athermal behavior near 200 K. An Arrhenius analysis of the temperature dependence of friction yielded an activation energy ∆E = 0.32 ± 0.02 eV for the sliding contact of a silicon nitride tip on PbS(100).     

Small amplitude reciprocating wear performance of diamond-like carbon films: dependence of film composition and counterface material

Small amplitude (50 μm) reciprocating wear of hydrogen-containing diamond-like carbon (DLC) films of different compositions has been examined against silicon nitride and polymethyl-methacrylate (PMMA) counter-surfaces, and compared with the performance of an uncoated steel substrate. Three films were studied: a DLC film of conventional composition, a fluorine-containing DLC film (F-DLC), and silicon-containing DLC film. The films were deposited on steel substrates from plasmas of organic precursor gases using the Plasma Immersion Ion Implantation and Deposition (PIIID) process, which allows for the non-line-of-sight deposition of films with tailored compositions. The amplitude of the resistive frictional force during the reciprocating wear experiments was monitored in situ, and the magnitude of film damage due to wear was evaluated using optical microscopy, optical profilometry, and atomic force microscopy. Wear debris was analyzed using scanning electron microscopy and energy dispersive spectroscopy. In terms of friction, the DLC and silicon-containing DLC films performed exceptionally well, showing friction coefficients less than 0.1 for both PMMA and silicon nitride counter-surfaces. DLC and silicon-containing DLC films also showed significant reductions in transfer of PMMA compared with the uncoated steel. The softer F-DLC film performed similarly well against PMMA, but against silicon nitride, friction displayed nearly periodic variations indicative of cyclic adhesion and release of worn film material during the wear process. The results demonstrate that the PIIID films achieve the well-known advantageous performance of other DLC films, and furthermore that the film performance can be significantly affected by the addition of dopants. In addition to the well-established reduction of friction and wear that DLC films generally provide, we show here that another property, low adhesiveness with PMMA, is another significant benefit in the use of DLC 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%.

     

Frictional Properties of a Mesoscopic Contact with Engineered Surface Roughness

Friction between titanium spheres and an artificially structured silicon surface was measured with a friction force microscope. Two spheres with radii of 2.3 μm and 7.9 μm were firmly glued to the tip of the microscope cantilever. A periodic stripe pattern with a groove depth of 26 nm and systematically increasing groove width from 500 nm to 3500 nm was fabricated from a silicon wafer with a focused ion beam. The sphere substrate friction coefficient shows a strong enhancement at a certain groove periodicity, which is related to geometrical interlocking of the two surfaces. This shows that careful modification of the surface roughness can help to control the tribological behavior of mesoscale contacts.     

Surface Texture Effect on Friction of a Microtextured Poly(dimethylsiloxane) (PDMS)

The effect of surface textures on the friction of a poly(dimethylsiloxane) (PDMS) elastomer has been investigated at both macro and microscales using a nanoindentation-scratching system. Friction tests were conducted by a stainless-steel bearing ball with a diameter of 1.6 mm (macroscale tests) and a Rockwell diamond tip with a radius of curvature of 25 μm (microscale tests) under normal loads of 5, 10, and 25 mN and with a sliding speed of 1 μm/s. Coefficient of friction (COF) on the pillar-textured surface was found to be much lower than that on the smooth surface of the same material, and it was reduced by about 59% at the macroscale tests and 38% at the microscale tests. The reduction of COF can be attributed to the reduced contact areas. The use of the JKR model revealed that the adhesion force has less effect on contacts under higher normal loads. COFs in different sliding directions on the groove-textured surfaces were compared, and a friction anisotropic behavior was identified and analyzed.     

A study of the thermal,dynamic mechanical,and tribological properties of polyphenylene sulfide composites reinforced with carbon nanofibers

The thermal, dynamic mechanical, and tribological properties of polyphenylene sulfide (PPS) composites reinforced with carbon nanofiber (CNF) were studied. Dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) were used to study the viscoelastic properties and thermal transitions. In order to study the tribological properties, friction and wear tests in a pin-on-disk configuration were performed. The changes in melting point, crystallization temperature, and glass transition temperature were found to be small as a result of reinforcement. Steady state wear rates of the reinforced composites sliding against the counterface of roughness 0.13–0.15 μm Ra were significantly lower than that of the unreinforced PPS. When the composites were tested against the smoother counterface of 0.06–0.11 μm Ra, the wear rates were higher. The coefficient of friction in all the cases was not practically affected by the presence of CNF. The transfer films formed on the counterface during sliding were examined by optical microscopy and atomic force microscopy (AFM). The variation of wear is discussed in terms of the texture and topography of transfer film.     

Influence of ethyl acetate and alkyl phosphate adsorption on the frictional properties of VC(100)

This report presents ultrahigh vacuum measurements of the frictional properties of the non-polar (100) surface of vanadium carbide (VC) as a function of the room temperature uptake and reaction of ethyl acetate, triethyl phosphate, and trimethyl phosphate. Atomic force microscopy, employing a silicon nitride probe tip, has been used to determine the changes in friction and interfacial adhesion as a function of adsorbate uptake. Changes in surface morphology have been monitored with scanning tunneling microscopy while the composition of the surface species formed through the reaction of these adsorbates with the VC surface has been determined by X-ray photoelectron spectroscopy. Adsorption and reaction of ethyl acetate leads to an increase in friction with little change in interfacial adhesion. The adsorption and reaction of triethyl phosphate and trimethyl phosphate have no influence on either the friction or adhesion properties of VC. The observed results are discussed in terms of the surface chemical composition, the extent of surface coverage, and the molecular details of the adsorbed species.     

Compositional dependence of the microscopic frictional properties of single crystal metal carbides

The frictional properties of TiC(100), Ti_0.3V_0.6C(100), and VC(100) surfaces in contact with a silicon nitride probe tip have been investigated by atomic force microscopy (AFM) under ambient pressures of dry nitrogen as well as environments of different relative humidities. Calibration of normal and lateral force has permitted the determination of the quantitative frictional properties of the three carbide samples on a nanometer length scale. In these studies, TiC(100) exhibits the lowest friction coefficient, ranging from ∼0.044 to ∼0.082 under the different environments. VC(100) and Ti_0.3V_0.6C(100) have similar friction coefficients (∼0.07) under dry nitrogen conditions, yet VC exhibits a larger friction coefficient (∼0.158) than Ti_0.3V_0.6C (∼0.129) under conditions of higher relative humidity (∼55%). Condensation of water vapor with increasing relative humidity results in an increase in the frictional response for all the three samples. The experimental results demonstrate that the frictional properties of the three carbide samples are correlated to their surface composition and surface free energy.     

The Frictional Response of VC(100) Surfaces: Influence of 1-Octanol and 2,2,2-Trifluoroethanol Adsorption

In this report, we present ultrahigh vacuum (UHV) atomic-scale measurements of the frictional response of the VC(100) surface and the influence on friction through the adsorption of 1-octanol (CH₃(CH₂)₇OH) and 2,2,2-trifluoroethanol (CF₃CH₂OH). Atomic force microscopy (AFM) has been used to determine the changes in interfacial friction and adhesion, while scanning tunneling microscopy (STM) has revealed changes in surface morphology upon adsorption. X-ray photoelectron spectroscopy (XPS) has been utilized to determine the composition of the surface formed through the reaction of these adsorbates with VC. Adsorption of 1-octanol on the VC(100) surface at room temperature causes a 15% reduction in the friction measured between a clean VC surface and a silicon nitride AFM tip. STM images, combined with XPS results, reveal that 1-octanol does not completely cover the surface and that saturation occurs approximately at a 500L exposure. Adsorption of 2,2,2-trifluoroethanol on the VC(100) surface at room temperature produces a significant increase in friction while at the same time producing a decrease in adhesion. These contrasting results are interpreted in terms of differences in interfacial shear strength, chemical composition, and the molecular details of the adsorbed layer.     

Polycrystalline silicon carbide as a substrate material for reducing adhesion in MEMS

The in-use adhesion characteristics of polycrystalline cubic silicon carbide (poly-SiC) films when used as a substrate material in MEMS applications are investigated using micromachined polycrystalline Si (poly-Si) cantilever beam arrays. The detachment lengths greater than 1500 μm are obtained, corresponding to an apparent work of adhesion of less than 0.006 mJ/m². This is to be compared to the detachment lengths of less than 200 μm when poly-Si substrate is used, corresponding to the apparent work of adhesion of greater than 20 mJ/m². To help understand the mechanism leading to the significant reduction in in-use adhesion, the poly-SiC surfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurement. Based on the data, it is suggested that the topography as well as the slower oxidation rate of poly-SiC films may be responsible for the observed adhesion reduction.     

Comparison of bonding procedures for 3D-structured polyimide films on silicon substrates applied to ink-jet cartridges

Established bubble-jet printheads consist of an assembly of three layers, which are a CMOS substrate, a channel layer and a nozzle layer. The aim of the presented work was to simplify the setup of bubble-jet printheads by means of integrating channel and nozzle structures in one single three-dimensional laser-structured polyimide nozzle plate. Different bonding techniques for an assembly onto 1/3 inch standard CMOS printhead substrates are validated. The main challenges are a variety of bonding materials, an alignment accuracy of <5 μm and the prevention of blocking the 20 μm deep fluidic channels in the polyimide which have minimal lateral dimensions of 10 μm and a minimal pitch of 15 μm. In total, three bonding techniques, with and without additional adhesion layers, were developed and evaluated. One method applies a 4 μm thick layer of Epotec 353ND (Polytec), a standard two-component epoxy, in a specifically adapted rolling manner onto the film that is subsequently aligned to the silicon chip using a flip-chip-bonder. Screen-printing and dispensing processes of adhesives were investigated but failed due to insufficient structural resolution. The second method uses photolithographic processes to produce structured adhesion preforms in SU-8 resist. With a layer thickness of 3 μm and an adapted curing schedule, promising results concerning resolution and contour accuracy were obtained. Thirdly, bonding without additional adhesion layers was achieved in a micro-sealing process that takes advantage of the highly defined thermoplastic softening of polyimide KJ (DuPont). The different processes were compared regarding to yield, printing behavior of the assembled printheads and applicability to high volume productions. The hereby developed adhesion technology was applied on the assembly of large one inch printheads for special applications.     

High-Temperature Vapor Phase Lubrication Using Carbonaceous Gases

Following the pioneering work of Prof. James Lauer, the ability to provide continuous solid lubrication through vapor phase delivery of carbonaceous gases has been successfully demonstrated on a pin-on-disk contact at the temperatures of 650 °C. Results from tribological experiments under 2 N normal load and 50 mm/s sliding speed showed an over 20× reduction in friction coefficient. The samples were silicon nitride (pin) versus CMSX-4 (disk) and the experiments when run in a nitrogen environment with acetylene admixtures. Two repeat experiments gave average friction coefficients of μ = 0.03 and μ = 0.02. The process was robust and provided low friction for the entire 500 m of sliding. Using focused ion-beam milling, high-resolution transmission electron microscopy, and confocal Raman spectroscopy, the resulting solid lubricant was found to be oriented microcrystalline graphite.     

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.     

Dynamic Friction of SiC Surfaces: A Torsional Kolsky Bar Tribometer Study

A dynamic tribometer has been successfully developed utilizing torsional Kolsky bar (TKB) technique for tribo pairs subjected to dynamic compression and shear. The dynamic tribometric responses of ground finish (R _a = 0.17 μm) and polished finish (R _a = 0.10 μm) surfaces of silicon carbide (SiC) under compression loading up to 1.5 GPa and shear-sliding velocity to 3.8 m/s have been measured. The experimental results show Coulomb friction behavior. Hardening of shear response is found on both the ground and polished surfaces. Repeating tests on the same tribo-pair demonstrate that steady state shear response on polished surfaces can be achieved by cumulating shear-sliding distance. SEM observation of the tested surface shows that the tested surfaces are microscopically shear-damaged. The similar surface conditions after a relative longer shear-sliding distance on polished surfaces lead to the same steady state frictional coefficient, 0.61, for both finishing surfaces.     

Fabrication,Characterisation and Tribological Investigation of Artificial Skin Surface Lipid Films

This article deals with the tribology of lipid coatings that resemble those found on human skin. In order to simulate the lipidic surface chemistry of human skin, an artificial sebum formulation that closely resembles human sebum was spray-coated onto mechanical skin models in physiologically relevant concentrations (5–100 μg/cm²). Water contact angles and surface free energies (SFEs) showed that model surfaces with ≤25 μg/cm² lipids appropriately mimic the physico-chemical properties of dry, sebum-poor skin regions. In friction experiments with a steel ball, lipid-coated model surfaces demonstrated lubrication effects over a wide range of sliding velocities and normal loads. In friction measurements on model surfaces as a function of lipid-film thickness, a clear minimum in the friction coefficient (COF) was observed in the case of hydrophilic, high-SFE materials (steel, glass), with the lowest COF (≈0.5) against skin model surfaces being found at 25 μg/cm² lipids. For hydrophobic, low-SFE polymers, the COF was considerably lower (0.4 for PP, 0.16 for PTFE) and relatively independent of the lipid amount, indicating that both the mechanical and surface-chemical properties of the sliders strongly influence the friction behaviour of the skin-model surfaces. Lipid-coated skin models might be a valuable tool not only for tribologists but also for cosmetic chemists, in that they allow the objective study of friction, adhesion and wetting behaviour of liquids and emulsions on simulated skin-surface conditions.