Sliding Friction Measurements Research Articles

Friction and Contact Temperatures in the Cleaving of Bone and Wood Using Stone Tools – A Case Study in Palaeolithic Tribology

During archaeological fieldwork wedge-shaped quartz stones that show clearly visible "glossy patches" composed of high quartz have been found. It is generally accepted that these tools have been used to cleave or punch wood and bone materials. For the transformation from quartz to high-quartz to occur, the temperature should exceed 574 °C. The hypothesis tested in this manuscript is that the phase change in the stone tool results from frictional heating during the cleaving action. Dry sliding friction measurements were carried out on a reciprocating tribometer using four types of stone, representing the punch tool, and pine, oak and bovine bone, representing the work piece. Measured coefficients of friction were approximately 0.1 on oak, 0.2 on pine and up to 0.35 on bovine bone, with some minor fluctuations for the different types of stone. These coefficients of friction were inserted into a computational model describing the flash temperatures in a moving contact, from which it was shown that the hypothesis might hold in the case of lydite-bone contact. This means that the glossy patches on the stone tools may have been caused by frictional heating during the cleaving of bone.

Fluid-dependent shear slip behaviors of coal fractures and their implications on fracture frictional strength reduction and permeability evolutions

During coalbed methane (CBM) formation stimulation, large volumes of fracking fluids are injected, and these fluids include water, hot steam, CO2, N2, foams, and their mixtures. These injected fluids can potentially generate new fractures and/or reactivate pre-existing faults/cleats. Coal, unlike other subsurface rocks, has systematic cleats that cause deterioration of the coal's mechanical strength. Unique to coal, fluid sorption-induced swelling and softening are commonly observed. With these unique properties, friction-based fracture sliding in stressed coal exhibits differing mechanisms. To better understand the frictional sliding behavior and permeability evolution in well-cleated coal, sliding friction measurements were conducted using artificially generated coal fractures. Various fluids (helium, carbon dioxide, water, and moisturized methane) were injected into coal to investigate the dynamic characteristics of friction coefficients and permeability evolution during faulting. Unlike the traditional view of stress-based fault reactivation, our results demonstrated that the initial sliding reactivation stress on the coal was not only pressure dependent but also fluid-type dependent. The friction strength of the coal fracture significantly dropped after injecting sorbing fluids, which could lead to fault reactivation at a low pore pressure since the friction failure curve can approach Mohr's circle envelope. Fracture permeability during sliding is sensitive to the softening effect of injected fluids; and water-involved injections of fluids may lead to an abrupt increase of friction due to the dilation caused by interactions between surface teeth and gouge particles. Both the velocity increase and decrease steps showed an increase in velocity strengthening when investigating the rate-state-friction parameters, indicating a continuous friction increase with sliding distance.

Sliding tribocorrosion behavior of Fe-based bulk metallic glass under corrosive environments

Sliding tribocorrosion behavior of non-ferromagnetic Fe-based bulk metallic glass (BMG) was investigated under corrosive environments. Electrochemical results revealed that Fe-based BMG exhibited excellent corrosion resistance in 3.5 wt% NaCl, 1 mol/L HCl and 1 mol/L HAc solutions. Sliding friction measurements indicated the presence of aqueous solutions significantly exacerbated the material loss. The tribocorrosion mechanism of Fe-based BMG involved the abrasive wear associated with corrosive wear. The synergy effect of wear and corrosion was dependent on the corrosive condition, Cl−-containing solutions could ease the material loss within a certain range by forming fragmentized passive products as the lubricants during the tribocorrosion sliding process. Results hinted the potential of Fe-based BMGs as structural materials for the anti-tribocorrosion applications.

Promoting rotation, friction, and mixed lubrication for particles rolling on microstructured surfaces.

We investigate how the aspect ratio of micropillar or microwell arrays patterned on a surface affects the rolling and slipping motion of spheres under flooded conditions at low Reynolds numbers. We study arrays of rigid microstructures with aspect ratios varying over two orders of magnitude for surface coverages ranging from 0.04 to 0.96. We investigate how the surface features (dimensions, surface coverage, and geometry) individually impact the motion of the sphere. We find that increasing microstructure height results in higher rotational velocities on all studied surfaces. We then model the motion of the spheres using two physical parameters: an effective separation and a coefficient of friction between the sphere and the incline. We find that a simple superposition of resistance functions, previously shown to accurately predict the motion of spheres for different surface coverages and geometries, indeed shows good agreement with experimental outcomes for all microstructure heights studied. We also perform separate sliding friction measurements via a force microscope to measure the coefficient of friction between the sphere and incline, under identically flooded conditions. A comparison of the sliding friction measurements at different Hersey numbers suggests that the effect of the microstructure on the coefficient of friction becomes more important as the Hersey number increases.

Load and Time Dependence of Interfacial Chemical Bond-Induced Friction at the Nanoscale.

Rate and state friction (RSF) laws are widely used empirical relationships that describe the macroscale frictional behavior of a broad range of materials, including rocks found in the seismogenic zone of Earth's crust. A fundamental aspect of the RSF laws is frictional "aging," where friction increases with the time of stationary contact due to asperity creep and/or interfacial strengthening. Recent atomic force microscope (AFM) experiments and simulations found that nanoscale silica contacts exhibit aging due to the progressive formation of interfacial chemical bonds. The role of normal load (and, thus, normal stress) on this interfacial chemical bond-induced (ICBI) friction is predicted to be significant but has not been examined experimentally. Here, we show using AFM that, for nanoscale ICBI friction of silica-silica interfaces, aging (the difference between the maximum static friction and the kinetic friction) increases approximately linearly with the product of the normal load and the log of the hold time. This behavior is attributed to the approximately linear dependence of the contact area on the load in the positive load regime before significant wear occurs, as inferred from sliding friction measurements. This implies that the average pressure, and thus the average bond formation rate, is load independent within the accessible load range. We also consider a more accurate nonlinear model for the contact area, from which we extract the activation volume and the average stress-free energy barrier to the aging process. Our work provides an approach for studying the load and time dependence of contact aging at the nanoscale and further establishes RSF laws for nanoscale asperity contacts.

Single-layer MoS2 roughness and sliding friction quenching by interaction with atomically flat substrates

We experimentally study the surface roughness and the lateral friction force in single-layer MoS2 crystals deposited on different substrates: SiO2, mica, and hexagonal boron nitride (h-BN). Roughness and sliding friction measurements are performed by atomic force microscopy. We find a strong dependence of the MoS2 roughness on the underlying substrate material, being h-BN the substrate which better preserves the flatness of the MoS2 crystal. The lateral friction also lowers as the roughness decreases, and attains its lowest value for MoS2 flakes on h-BN substrates. However, it is still higher than for the surface of a bulk MoS2 crystal, which we attribute to the deformation of the flake due to competing tip-to-flake and flake-to-substrate interactions.

Rolling Friction of Single Balls in a Flat-Ball-Flat-Contact as a Function of Surface Roughness

This paper reports on the dependence of rolling resistance on surface roughness of single balls in a flat-ball-flat configuration at sub-Newton loads using an specially designed rolling resistance tribometer. The tribometer setup features an ultra-stable drive and a two-dimensional force transducer with a high ratio between load and friction force sensitivity. Using this setup, the rolling resistance was examined for different materials as well as a self-mated sapphire contact. Results indicate a very strong dependence of rolling resistance on surface roughness for the tested loads. Rolling resistance was found to decrease monotonically with decreasing roughness. A comparison was also attempted between rolling resistance and sliding friction. Sliding friction measurements show that decreased roughness does not necessarily decrease the coefficient of friction. In fact, a friction minimum is reached at a certain roughness. A further decrease of roughness increases the friction. This indicates that sliding friction measurements do not seem to be transferable to rolling resistance in every case, especially when the average surface roughness is in the range of a few nanometres.

Surface modification of chlorobutyl rubber by plasma polymerization

Radio frequency (r.f.) plasma polymerization of vinylidene fluoride (CH2CF2) has been used to modify the surface properties of chlorobutyl rubber. FTIR-ATR spectra of the treated rubbers and transmission spectra of plasma polymer films on NaCl windows indicated that as power increased the F/H ratio decreased. SIMS tests supported the FTIR results, and showed that the decrease in the F/H ratio was due to a decrease in the amount of F and an increase in the amount of H in the plasma polymer. Sliding friction measurements showed a reduction in the coefficient of friction (μ) from 3.7 for the untreated rubber to values ranging between 0.4 and 1.9 for the plasma-treated rubbers. There did not appear to be any correlation between the coefficient of friction and plasma power or monomer flow rate, and the average coefficient of friction for the plasma-treated samples was 0.9, which was lower than a commercially used silicone oil treatment (μ = 1.1–1.3). Repetitive sliding friction tests showed that the plasma- and silicone oil treated-chlorobutyl rubbers had the similar lubricating lifetimes. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1651–1660, 1997

Chemical, mechanical and electrical properties of CN x-films produced by reactive sputtering and N +-implantation in carbon films

In this work, the N effects on hard C-films properties were investigated. The C-films were prepared at 573 K by rf magnetron sputtering of a graphite target, in Ar discharge. The incorporation of N in the C-films was achieved either by N +-implantation at room temperature, with energies of 30 and 160 keV and at a fluence of 2 × 10 17N +/cm 2, or by reactive sputtering of graphite in a (Ar-50% N 2) discharge. Hardness and elastic properties were examined using a Knoop indenter. Sliding friction measurements were made using a scratch tester. N effects on the electrical conductivity of the C-films were also studied. Atomic film composition was determined by the nuclear reaction analysis. Chemical characterization of the C- and CN x -films was obtained by X-ray photoelectron spectroscopy. The dynamical behaviour of hydrogen was studied by elastic recoil detection. The results show that the CN x -film properties were governed (i) by chemical effects and (ii) by ballistic effects, occurring during implantation, leading to a specific microstructure of the network, associated with a hydrogen redistribution in the nitrogenated films. The chemical effects are relevant mainly in the growth of the CN x -films with reactive sputtering, while the N +-implanted C-film properties are well explained by both chemical and ballistic events.

Frictional characteristics of diamond sliding on as-grown surfaces of large GEM quality single crystal synthetic diamond

Sliding friction measurements are reported on {{111}} and {{001}} surfaces of synthetic diamonds in the as-grown condition. The measurements were done in ambient air, and the lowest friction coefficient observed was 0.004 on {{111}} faces in 〈112〉 directions. The friction coefficient varies with sliding direction, as has been previously reported. The frictional energy loss is primarily associated with adhesion. Polished surfaces of natural diamonds were also measured for comparison. Ultra-fine polishing of a surface reduces the friction compared with a conventional polish. Synthetic diamonds with different dopants were investigated, but no correlation between friction and dopant was found.

Sliding friction measurements of molecularly thin films

Slippage behavior of Xe and Kr monolayers and bilayers adsorbed on gold substrates has been investigated by means of a quartz crystal microbalance. The experiments have been carried out for both solid and liquid layers, and also for films which solidify as the surrounding vapor pressure is increased. Measurements carried out on bilayer films indicate moderate increases in slippage for both liquid and solid films as the second layer condenses. Solid films are observed to exhibit greater slippage than liquids on smooth substrates.

Sliding friction measurements of physisorbed monolayers: A comparison of solid and liquid films

The quality factor and frequency of vibration of a quartz crystal oscillator decreases slightly when a film adsorbs on its surface. Simultaneous measurements of the shifts in quality factor and frequency allow one to quantitatively deduce the degree to which the film ‘‘slips’’ due to the oscillatory motion of the underlying substrate, and also the force which would be required to slide the film along the surface at constant speed. Experiments have been carried out for krypton monolayers undergoing solidification on a smooth gold surface and on an atomically rough silver surface. The force per unit area required to slide a liquid monolayer is roughly the same for each surface. Less force is required to slide a solid monolayer along the smooth surface, while the opposite is true for the rough surface.

Effect of excimer laser alloying of Ti on the sliding friction of AISI 304 stainless steel

Dry sliding friction measurements made on titanium layers evaporated on AISI 304 stainless steel in the as-deposited and excimer laser mixed form show a dependence on the film thickness and the amount of mixing. The effect of laser mixing is dependent on the incident fluence with high fluences and/or large numbers of pulses producing surfaces with poor frictional properties. The optimum total fluence depends on the thickness of the surface layer, a result consistent with thorough mixing of the alloyed layer without the surface damage that results from large numbers of pulses.

Friction and surface chemistry of implantation- modified metal-carbide composites

Dry sliding friction measurements were performed on model carbide- metal composite samples containing 24–42 vol.% carbide. Wear debris, located preferentially on the carbides, were analyzed using scanning electron microscopy-energy dispersive X-ray analysis and Auger scanning microscopy. The friction and wear of the composite samples were found to be influenced by the compatibility (degree of mutual solubility) of the carbides with the steel counterface, as well as by local inhomogeneities in the carbide distribution. Friction tests were also made on the samples following high fluence titanium implantation. Implantation resulted in increased friction and wear, possibly owing to an implantation-induced softening or degradation of the carbides.