Observations of Stick-Slip Friction in Velcro®

Friction, and in particular stick-slip friction, occurs on every length scale, from the movement of atomic force microscope tips at the nanoscale to the movement of tectonic plates of the Earth’s crust. Even with this ubiquity, there still appears to be outstanding fundamental questions, especially on the way that frictional motion varies generally with the mechanical parameters of a system. In this study, the frictional dynamics of the hook-and-loop system of Velcro^® in shear is explored by varying the typical parameters of driving velocity, applied load, and apparent contact area. It is demonstrated that in Velcro^® both the maximum static frictional force and the average kinetic frictional force vary linearly with apparent contact area (hook number), and moreover, in the kinetic regime, stick-slip dynamics are evident. Surprisingly, the average kinetic friction force is independent of velocity over nearly two-and-a-half orders of magnitude (~2 × 10^?4 to ~6 × 10^?2 m/s). The frictional force varies as a power law on the applied load with an exponent of 0.28 and 0.24 for the maximum static and kinetic frictional forces, respectively. Furthermore, the evolution of stick-slip friction to more smooth sliding, as controlled by contact area, is demonstrated by both a decrease in the spread of the kinetic friction and the spread of the fluctuations of the average kinetic friction when normalized to the average kinetic friction; these decreases follow power-law behaviors with respect to the increasing contact area with exponents of approximately ?0.3 and ?0.8, respectively. Lastly, we note that the coefficients of friction μ _s and μ _k are not constant with applied load but rather decrease monotonically with power-law behavior with an exponent of nearly ?0.8. Phenomenologically, this system exhibits interesting physics whereby in some instances it follows classical Amontons–Coulomb (AC) behavior and in others lies in stark contrast and hopefully will assist in the understanding of the friction behavior in dry surfaces.     

Influence of surface treatment of elastomers on their frictional behaviour in sliding contact

To reduce friction of elastomer parts moving against a metal counter body in dry conditions, two different surface treatment techniques were applied on elastomer parts: laser cladding and plasma treatment at atmospheric pressure.Polyamide 11 (PA 11) based coatings were produced on thermoplastic polyurethane (TPU) substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS₂ coating: friction of the TPU substrate was reduced with 40%. The incorporation of 15 wt% PTFE in the PA 11 coating resulted in a further decrease of the frictional force. A reduction of 80% of the frictional force of the TPU substrate was measured. The surface of the coatings before and after tribotesting was analysed.The plasma treatment of HNBR was done using a Plasmaspot^® to form a plasma polymerised coating based on two different types of siloxanes. A reduction of 74–80% of the initial friction coefficient was measured in two different tribotest rig configurations: ball-on-disc and disc-on-disc. The resulting wear tracks were analyzed by SEM and EDX.     

Friction and Wear Properties of TiAl-Ti3SiC2-MoS2 Composites Prepared by Spark Plasma Sintering

TiAl matrix self-lubricating composites (TMC) with various weight percentages of Ti₃SiC₂ and MoS₂ lubricants were prepared by spark plasma sintering (SPS). The dry sliding tribological behaviors of TMC against an Si₃N₄ ceramic ball at room temperature were investigated through the determination of friction coefficients and wear rates and the analysis of the morphologies and compositions of wear debris, worn surfaces of TMC, and the Si₃N₄ ceramic ball. The results indicated that TMC with 10 wt% (Ti₃SiC₂-MoS₂) lubricants had good tribological properties due to the unique stratification subsurface microstructure of the worn surface. The friction coefficient was about 0.57, and the wear rate was 4.22 × 10^?4 mm³ (Nm)^?1. The main wear mechanisms of TMC with 10 wt% (Ti₃SiC₂-MoS₂) lubricants were abrasive wear, oxidation wear, and delamination of the friction layer. However, the main wear mechanisms of TMC without Ti₃SiC₂ and MoS₂ lubricants were abrasive wear and oxidation wear. The continuous friction layer was not formed on the worn surfaces. The self-lubricating friction layer on the frictional surface, different phase compositions and hardness, as well as density of TMC contributed to the change in the friction coefficient and wear rate.     

Complex approach to investigate mechanical and tribological characteristics of microcontacting objects for MEMS

Abstract

A complex approach based on atomic force microscopy (AFM) is developed to establish influence of nanoscale layer thickness on its elastic, adhesive and frictional properties of polymeric coatings for microelectromechanical systems. Thermoheating element was applied to perform AFM measurements with thermal effects in the temperature range from 20 to 120°C. Friction coefficients at high velocities of sliding and dependences of friction coefficient on the temperature of heated films at low velocities of sliding are defined. This study concludes that the Young’s modulus of ultrathin polymeric films on silicon substrate is reduced when thickness or temperature is increased.     

Effects of self-ligating brackets on the surfaces of stainless steel wires following clinical use: AFM investigation

In orthodontic treatment, the frictional force between the archwire and bracket reduces the effectiveness of orthodontic treatment. The frictional force is affected not only by the geometry of the self‐ligating brackets but also by physical changes between the bracket slots and archwire surfaces during sliding movement. This study examined quantitatively the effect of self‐ligating treatments on the surfaces of stainless steel (SS) archwires during tooth movement in vivo by atomic force microscopy. Orthodontic 0.019″ × 0.025″ SS archwires after clinical use with the first bicuspid‐extraction treatment were employed using the Damon 3MX^® SS self‐ligating brackets, Clippy‐C^® ceramic self‐ligating brackets, and Kosaka^® SS brackets. Intact SS archwires were used as the control group. All SS archwires after clinical use showed severe scratches and significantly higher roughness caused by frictional interactions between the brackets and archwires (p < 0.0001 vs. control). The descending order of surface roughness was the SS archwires treated, with ceramic self‐ligating brackets, with conventional SS brackets, and with SS self‐ligating brackets (p < 0.001). These findings suggest that an orthodontic treatment with SS self‐ligating brackets may require smaller orthodontic forces than that with ceramic self‐ligating brackets or conventional SS brackets.     

Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition

Abstract

The current work evaluates the wear and frictional performance of ultrahigh molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) sliding against different metal counterfaces, stainless steel(SS), mild steel (MS) and aluminium (Al), under dry contact condition. The experiments were conducted using pin on disc machine at different sliding distances (0–40·32 km), 15 N applied load and 2·8 m s^–1 sliding velocity. Interface temperatures and frictional forces were measured simultaneously during the sliding, while specific wear rates were determined for every 1·68 km sliding distance. Based on the optical microscopy of the worn surface and wear track, frictional and wear results were analysed and discussed. The experimental results showed that the type of counterface material significantly influences both frictional and wear performances of the selected polymers. This was mainly due to the film transfer characteristics. Higher temperature and friction coefficient for UHMWPE and HDPE were evident when sliding took place against Al counterface. Sliding the polymers against stainless steel showed low friction coefficients compared to other counterfaces.     

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.     

Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Friction and Wear Characteristics of Single Crystal Ni-Based Superalloys at Elevated Temperatures

The purpose of this study was to investigate the friction and wear behavior of single crystal superalloys at elevated temperatures. Pin-on-plate experiments were conducted using a custom-built high-temperature fretting/wear apparatus. Measurements were performed on two single crystal Ni-based alloys and Waspaloy^® (used as a baseline material). The coefficient of friction for the single crystal materials (i.e., during running-in and steady state) was lower compared to the Waspaloy^®. In addition, the experiments showed that the friction coefficient of the single crystal is dependent on the crystallographic plane; the friction coefficient was lower for the tests on the {100} plane compared to the {111} plane. The wear behavior was aligned with the friction behavior, where the single crystal Ni-based alloys showed slightly higher wear resistance compared to the Waspaloy^®. Ex situ analysis by means of FIB/SEM and XPS analysis revealed the formation of Co-base metal oxide layer on the surface of the single crystal alloy. Similarly, a Co-base oxide layer is observed on the counterface providing a self-mated oxide-on-oxide contact and thus lower friction and wear compared to the Waspaloy^®.     

Investigation of adhesive and frictional behavior of GeSbTe films with AFM/FFM

Frictional force microscope (FFM) was used to investigate the nanoscale frictional behavior of GeSbTe films deposited by magnetron sputtering. The effects of relative humidity, scanning velocity and surface roughness on friction were taken into account. Besides, the frictional behavior of GeSbTe films with different compositions was analyzed. Experimental results show that the coefficient of friction of GeSbTe films is almost independent of scanning velocity, while the frictional force decreases with increasing velocity. Both the relationship of friction vs. normal load and that of friction vs. RMS keep relatively linear, and the coefficient of friction increases with the increase in RMS. The influence of humidity on adhesion between the tip and the GeSb₂Te₄ film is more significant than that between the tip and the Ge₂Sb₂Te₅ film.     

An investigation of the tribological properties of thin KCl films on iron in ultrahigh vacuum: modeling the extreme-pressure lubricating interface

The frictional properties of thin KCl films deposited onto clean iron are measured in ultrahigh vacuum using a tungsten carbide tribotip, where the observed initial rapid decrease in friction coefficient with film thickness is proposed to be due to the formation of a complete KCl monolayer where the friction coefficient of this film is ∼0.27. A 1800 Å thick KCl film shows a hardness and friction coefficient similar to those for bulk KCl when the width of the surface height distribution of the tribotip measured by atomic force microscopy (AFM) is 2000–3000 Å. This implies that the KCl film behaves like the bulk material when the film thickness exceeds the roughness of the interfaces.     

Wear and Friction of Filled Polytetrafluoroethylene Compositions in Liquid Nitrogen

Wear and friction behavior of slider materials at cryogenic temperature is important to the development of seals and bearings for missile powerplants. Data were obtained in liquid nitrogen (?320°F) with a series of molded and extruded polytetrafluoroethylene (PTFE) compositions containing various filler materials. A 3/16-in. radius rider specimen (PTFE materials) was caused to slide in a circumferential path on the flat surface of a rotating 2½-in. diameter disk specimen (usually type 304 stainless steel). The sliding velocity was usually 2300 ft per min and the load was 1000 grams.

As compared with reference steels and carbons used in conventional seals and bearings, the filled PTFE compositions gave low wear and friction (friction coefficients from 0.06 to 0.13) in liquid nitrogen. Several extruded compositions have particular promise for seal and bearing materials. An extruded glass-filled material gave wear and friction that was essentially unaffected by sliding velocities to 6000 ft per min.     

Effect of base oil polarity on micro and nanofriction behaviour of base oil + ZDDP solutions

Abstract

Ball on disc tribometer and atomic force microscopy (AFM) were used to analyse the effect of base oil polarity on the friction behaviour of steel–steel contacts lubricated with base oil + zinc dialkyldithiophosphate (ZDDP) solutions. Understanding the lubrication properties of the first chemisorbed layer of additives on work pieces yields important information for the optimisation of lubricant formulation, in particular with regard to the type of additive and amount needed. To characterise the influence of base oil polarity, two reference base oils [hexadecane (non–polar) and diethylenglycol (polar)] were blended with different concentrations of C₄-ZDDP, and the solutions were tested. A monolayer of base oil/additive solution was deposited on an ASI 52100 steel plate and scanned in AFM contact mode under various rubbing times and applied load conditions. An AFM technique was developed to estimate the microscopic values of friction coefficients showing how the oil polarity contributes to the differences in friction behaviour of the solution due to the addition of ZDDP. With different base oils [(hexadecane (non-polar base oil) and diethylenglycol (polar oil)] the authors observed significant different friction behaviours (in micro scale and nano scale) due to the addition of ZDDP compared to the base oil alone. This observation may be attributed to the contribution of base oil to transport the ZDDP additive onto the surface which will be discussed in more details in the paper. These results display the importance of base oil polarity on the friction behaviour of formulated lubricants containing additives.     

Oxide scale characterization of ferritic stainless steel and its deformation and friction in hot rolling

The oxidation kinetics of ferritic stainless steel 430 was studied in dry and humid air at 1090 °C by Thermo Gravimetric Analysis (TGA). Different atmospheres and heating times were adopted for reheating to obtain different compositions and thickness of the oxide scale. Hot rolling was performed on a 2-high Hille 100 experimental rolling mill at various reductions. Oxide scale thickness and composition were analysed with optical microscopy (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). The surface profiles were examined by a digital microscope, and the topographic features of the thin oxide scale surface were examined with an atomic force microscope (AFM) before and after rolling. The oxide scale surface and steel/oxide interface roughness were measured after rolling. Inverse calculation of the coefficient of friction was employed to analyse and the effect of oxide scale on friction in hot rolling. The coefficient of friction depends not only on the thickness of the oxide scale, but also on its composition and surface topography before hot rolling.     

Friction Measurements on Contact Lenses in Their Operating Environment

An important issue concerning the use of soft contact lenses is comfort, which, among other factors, has been related to the level of friction between the anterior side of the lens and the inner eyelid. Although several studies have been carried out to investigate the frictional properties of contact lenses, these have not taken the physiological environment of the eye into account. In use, lenses are in contact with proteins present in tears, with corneal cells and with the palpebral conjunctiva (clear membrane on inner eyelid). The focus of this study was to establish a biologically relevant measurement protocol for the investigation of friction of contact lenses that would mimic the eye’s physiological environment. By optimizing parameters such as the composition of the friction counter surface, the lubricant solution, the normal load and the velocity, an ideal protocol and setup for microtribological testing could be established and used to perform a comparative study of various commercially available soft contact lenses.     

Changes in ultrastructure and properties of bracket slots after orthodontic treatment with bicuspid extraction

This study examined the effects of an orthodontic treatment using a bicuspid extraction on the surface roughness and mechanical properties of stainless steel (SS) brackets adjacent to the extraction space. Four experimental groups were employed; groups 1 and 2 used the Archist^® SS brackets before and after the extraction treatment, respectively, and groups 3 and 4 used the Victory^® SS brackets before and after the extraction treatment, respectively. The slot surfaces of the bracket were scanned in air at a resolution of 512 × 512 pixels with a scan speed of 0.8 line/s. The visco‐elasticity of the bracket slot was determined from the force–distance curves of atomic force microscopy. The orthodontic treatment with bicuspid extraction led to a significant increase (p<0.0001) in surface roughness in both groups. In particular, the Archist^® SS brackets showed more changes than the Victory^® SS brackets (p<0.0005). However, there was no significant difference in properties of the Victory^® and Archist^® brackets between before and after treatment. This suggests that the orthodontic treatment with bicuspid extraction is more responsible for the changes in surface roughness than the properties of the brackets. SCANNING 33: 25–32, 2011. © 2011 Wiley Periodicals, Inc.     

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.     

纹理表面滑动摩擦稳态摩擦学性能

基于稳态滑动摩擦系统模型,采用球-盘摩擦副定量分析研究法向载荷、滑动速度、初始表面纹理和摩擦副材料对稳态摩擦因数的影响,得到稳态摩擦因数在不同工况下的变化规律。结果表明：滑动摩擦的稳态摩擦因数与磨损率正相关,周向纹理表面的稳态摩擦因数最大,无纹理表面的稳态摩擦因数次之,径向纹理表面的稳态摩擦因数最小;无论何种初始表面形貌,随着转速的增加,稳态摩擦因数先减小后增大,随着法向载荷的增大,稳态摩擦因数呈增长趋势;较深较宽的表面纹理具有更大的稳态摩擦因数和更大的瞬时波动;稳态摩擦因数也与摩擦副材料的选取有关。     

Penetration of resin‐based materials into initial erosion lesion: A confocal microscopic study

The application of resin‐based materials is an alternative of treatment for eroded lesions. Nevertheless, there are no studies about the penetration of these materials into eroded lesion, which might affect its adhesion. Therefore, this study evaluated the penetration of four resin‐based materials, with and without enamel etching. By using an in vitro protocol, types of treatment were studied at five levels (AdheSE^®, Tetric N‐Bond^®, Single Bond 2^®, Helioseal Clear^®, Icon^®) and types of enamel etching in two levels (with and without). Materials were stained with 0.02 mg/mL ethanolic solution of tetramethylrhodamine isothiocyanate. Bovine enamel samples (4 × 4 mm) were immersed in 0.01 M HCl, pH 2.3, for 30 seconds to produce initial eroded lesions. Afterward, the materials were applied on half of sample enamel surface following the manufacturer's instructions. On the other half of sample, the materials were applied without etching the enamel. Materials penetration into the enamel was assessed by Confocal Laser Scanning Microscopy on reflection and fluorescence modes. The penetration depth (PD) was measured using ImageJ software. Data were analyzed by two‐way ANOVA and Tukey test (P < 0.05). Regardless of the material, etched enamel resulted in higher PD than non‐etched (P < 0.05). Icon^® showed the highest PD in enamel followed by Helioseal Clear^® (P < 0.05), with significant difference between them (P < 0.05) and no difference was found among AdheSE^®, Tetric N‐Bond^®, and Single Bond 2^® (P > 0.05). It can be concluded that prior enamel etching increased the materials penetration into eroded enamel and the Icon^®—infiltrant presented highest penetration. Microsc. Res. Tech. 79:72–80, 2016. © 2015 Wiley Periodicals, Inc.     

Morphological investigation of various orthodontic lingual bracket slots using scanning electron microscopy and atomic force microscopy

Various labial and lingual orthodontic appliances with aesthetic materials have been developed due to an increased demand in aesthetic orthodontic treatment. However, there are few reports regarding the morphology of lingual orthodontic appliances. Therefore, this study evaluates the roughness of slot surfaces of various orthodontic lingual brackets using field emission scanning electron microscopy (FE‐SEM) and atomic force microscopy (AFM). Three types of stainless steel lingual brackets (Stealth^®, 7th Generation^®, and Clippy L^®) and one gold lingual bracket (Incognito?) with a slot size of 0.018 inches × 0.025 inches (0.457 × 0.635 mm²) were selected as representative lingual materials. Both FE‐SEM and AFM examinations showed that the Stealth^® and Clippy L^® brackets had the lowest surface roughness, while the 7th Generation^® bracket had the highest surface roughness. There was a significant difference in surface morphology between the types of lingual brackets, even when composed of the same material. The surface roughness of the bracket slot was dependent on the manufacturing process or surface polishing process rather than the fundamental properties of the bracket materials. There was no significant difference in the mean surface roughness of the slot floor between gold and stainless steel lingual brackets. These findings suggest that, although the gold lingual bracket is very expensive, it has great potential for use in patients with nickel allergy.