In Situ Studies of Cartilage Microtribology: Roles of Speed and Contact Area

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 and 3.2 mm, were used in the study. In situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: (1) at high sliding speed (V = 1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8–3.2 mm) despite the fourfold difference in the resulting contact areas; (2) the contact area was a strong function of the probe radius and sliding speed; (3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 to 5 mm/s; (4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V = 0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid–solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.     

Design of Low Wear Polymer Composites

Multifunctional tribological coatings rely on combinations of materials to improve properties, such as lubricity and wear resistance. For example, some polymer composites exhibit favorable tribological performance as solid lubricants. Here, classical molecular dynamics simulations are used to investigate the tribological behaviors of a mixed system of polyethylene (PE) sliding over polytetrafluoroethylene (PTFE) with the results compared with the sliding of the relevant homogeneous systems. In particular, oriented cross-linked PTFE and PE surfaces are slid in several relative sliding directions such that the surface chains are in-registry or out-of-registry and at various applied normal loads. The simulation results quantify the ways in which the mixed PTFE–PE system behaves differently than either of the homogeneous systems due to the lack of interlocking phases at the interface. These findings are compared with experimental production of polyetheretherketone (PEEK)–PTFE composites that have unusually low wear rates of 7.0 × 10⁻⁸ mm³/Nm, coupled with a steady, low friction coefficient of μ = 0.11 for over two million sliding cycles. The simulation results explain the atomic-scale origins of the frictional properties of this composite.     

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).     

Friction and Wear Properties of Silicon Carbide in Water from Different Sources

Low friction and low wear of SiC sliding against itself in water at room temperature have been well reported in the past 20 years, and some practical applications have been developed. However, the properties of friction and wear in pure, deionized or distilled water have been mainly observed and not in water from sources in nature. In this article, the fundamental properties of friction and wear between SiC ball and disk are observed in water from ground, river, and sea, and the results are compared with those in deionized water in the viewpoints of modes of lubrication and wear and the resultant values of friction coefficient and wear rate. The smallest friction coefficient (μ_ℓ = 0.005) in steady state is observed in deionized water and the largest (μ_ℓ = 0.013) in sea water. The smallest wear rate (w _s = 2.2 × 10⁻⁷ mm³/Nm) is observed in sea water and the largest (w _s = 3.1 × 10⁻⁷ mm³/Nm) in deionized water. The intermediate values of μ_ℓ and w _s between the smallest and the largest ones are observed in ground and river water. The modes of lubrication and wear, which generated observed values of μ_ℓ and w _s, are considered as mixed lubrication and tribochemical wear. The chemical elements of Na, Cl, Mg, and K in sea water observed on wear particles and pits are thought effective to generate the largest value of μ_ℓ and the smallest value of w _s.     

Effect of Applied Load and Surface Roughness on the Tribological Properties of Ni-Based Superalloys Versus Ta<Subscript>2</Subscript>AlC/Ag or Cr<Subscript>2</Subscript>AlC/Ag Composites

The novel Ta₂AlC–20 vol.% Ag (TaAg) and Cr₂AlC–20 vol.% Ag (CrAg) composites were tribologically tested versus a Ni-based superalloy Inc718 (SA) by dry sliding at a sliding speed of 1 m/s at room temperature in air at loads from 3 N to 18 N. The TaAg composites were also tested at 8 and 18 N at 550 °C, and at a 3 N load against the SA with different surface roughnesses at 26 °C and 550 °C. At room temperatures, the coefficients of friction, μ’s, decreased from ~0.8–0.9 to ~0.3–0.4 for both the TaAg and CrAg composites as the applied normal force increased from 3 N to 8 N. Further increases in load to 18 N did not change the μ’s. The specific wear rates, sWR, increased with increased loads for the TaAg composite; they remained almost unchanged for the CrAg composite. This behavior was attributed to the formation of glaze tribofilms—similar to ones observed previously in these tribocouples at elevated temperatures and 3 N—promoted by the increased loads. Preconditioning of the SA surface by sliding against the TaAg composite at 550 °C and 8 N resulted in μ’s of <0.2 and sWR < 10⁻⁶ mm³/N-m in subsequent room temperature sliding at 3 N. Somewhat higher, but stable room temperature μ’s of ~0.3 and sWR of ~3 × 10⁻⁵ mm³/N-m were observed when the TaAg composites were slid versus a sandblasted SA surface at 500 °C and 3 N. It follows that in situ preconditioning of the tribo-surfaces is a powerful tool for improving the properties of the MAX/Ag-SA tribocouples. The relationship between sliding conditions, chemistries of tribofilms, and their properties are discussed.     

The Effect of Grain Size on Stribeck Curve and Microstructure of Copper Under Friction in the Steady Friction State

In the present work, the effect of grain size on the friction and wear behavior of a copper (Cu) samples under different lubricant conditions was studied. The structural evolution of Cu subsurface layers under friction in different lubricant conditions was considered. All friction tests were conducted under laboratory conditions using a block-on-ring rig. The effects of sliding velocity and load on the friction coefficient and wear rate of Cu with different grain size (1, 30, and 60 μm) were analyzed. The Cu samples with the average grain size of 1 μm were obtained due to severe plastic deformation (SPD) by equal channel angular pressing (ECAP). The Stribeck curves for Cu samples with different virgin grain sizes were considered. Elasto-hydrodynamic lubrication (EHL) and boundary lubrication (BL) regions were mainly studied in the present work. Similar Stribeck curves were found out for Cu samples with different virgin grain size. A load of the transition from the EHL to BL region was increased with a decrease of the grain size. While the friction coefficients were similar in the EHL and BL regions for the samples with different grain sizes, the wear rate was increased remarkably with an increase the virgin grain size. Flow localization during friction in the BL region led to formation of the vortex structure in subsurface layers. Based on the dependence of the microhardness upon the depth, the degree of hardening (H) was evaluated. A correlation between the coefficient of wear and the deformation hardening of Cu samples with different virgin grain sizes was revealed. In order to take into account the effect of the grain size and to predict the Stribeck curve, a parameter, K, as the ratio between hardness of tested and annealed samples, was incorporated into the lubricant number. The theoretical values of the Stribeck curve calculated for preliminary deformed Cu samples (d = 1 μm) and annealed samples (d = 60 μm) were well coincided with the experimental results.     

Lateral Contact Stiffness and the Elastic Foundation

A model for the lateral contact stiffness for an elastic foundation was developed. The model was evaluated using a low force and low contact pressure microtribometer capable of performing indentation and reciprocated sliding experiments. The slope of lateral force versus the lateral displacement was used to fit the shear modulus. When complementary elastic indentation measurements are made to determine the composite modulus of the elastic foundation, there is sufficient data to fit elastic modulus, shear modulus, and Poisson ratio for these thin films. Using these models, the elastic properties for a thin (~65 μm) vertically aligned multiwall carbon nanotube film were evaluated. The experiments were performed with a silicon nitride indenter (radius = 1.6 mm) over a range in loads from 100 to 800 μN. The resulting values of the elastic modulus, shear modulus, and Poisson ratio were E = 429 kPa, G = 156 kPa, and ν = 0.37, respectively.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

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.     

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.     

Frictional Voltammetry with Copper

This paper presents an experimental study correlating frictional behavior with in situ voltammetry for a unidirectional sliding contact between a hemispherical tipped alumina probe and a flat rotating copper counterface (maximum Hertzian contact pressure of 68 MPa and sliding speed of 10 mm/s). The contact was immersed in an aqueous 0.1 M Na₂CO₃ solution (pH ∼11) where the copper counterface acted as the working electrode in a potentiostat controlled three-electrode cell; a coiled Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) as the reference. Clear and reproducible trends were found between friction coefficient and published data suggesting the onset of particular redox reactions, graphically presented in a frictional voltammetry plot. At anodic potentials primarily associated with the formation of copper(I) oxide (Cu₂O) (V vs SCE ∼−0.25), the measured friction coefficient was in the range μ ∼0.4–0.5. At cathodic potentials primarily associated with the formation of CuO, Cu(OH)₂, and CuCO₃ (V vs SCE ∼−0.10), the friction coefficient transitions to the range μ ∼0.7–1.0. At sustained cathodic potentials associated with reduction of the native copper oxide, Cu₂O, (V vs SCE ∼−0.65), the friction coefficient is observed to fluctuate between μ ∼0.2 and 0.5, arguably a result of exposure of bare copper due to non-uniform reduction (fractional coverage) of Cu₂O.     

Ultra-High Tribological Performance of Magnetron Sputtered a-C:H Films in Sand-Dust Environment

The hydrogenated amorphous carbon (a-C:H) films were prepared on AISI 440C steel substrates using a RF magnetron sputtering graphite target in the CH₄ and Ar mixture atmosphere. The friction and wear behavior of a-C:H films were comparatively investigated by pin-on-disc tester under dry sliding and simulated sand-dust wear conditions. In addition, the effects of applied load, amount of sand and sand particle sizes on the tribological performance of a-C:H films were systemically studied. Results show that a-C:H films exhibited ultra-high tribological performance with low friction coefficient and ultra-low wear rate under sand-dust environments. It is very interesting to observe that the friction coefficient of a-C:H film under sand-dust conditions was relatively lower when compared with dry sliding condition, and the wear rate under sand-dust conditions kept at the same order of magnitude (×10⁻¹⁹ m³/N m) with the increase of applied load and particle size as a comparison with the dry sliding condition. Based on the formation of “ridge” layer (composite transfer layer), a transfer layer-hardening composite model was established to explain the anti-wear mechanisms and friction-reducing capacity of a-C:H solid lubrication films under sand-dust conditions.     

Tribological behavior of PEEK components with compositionally graded PEEK/PTFE surfaces

PEEK is a high strength engineering thermoplastic that suffers from a high friction coefficient and a friction induced wear mode. Past studies with 10 μm PEEK and PTFE powders resulted in composite solid lubricant that (at the optimal composition) had a wear rate of k = 2 × 10⁻⁹ mm³/Nm with a friction coefficient of μ = 0.12. A compositional grading of PEEK and PTFE is implemented in this study to create a bulk composite with the functional requirements of component strength, stiffness and wear resistance while providing solid lubrication at the sliding interface. The tribological performances of three functionally graded PEEK components were evaluated on linear reciprocating, rotating pin-on-disk and thrust washer tribometers. Wear rates comparable to samples of the bulk solid lubricant and comparable or improved frictional performance were achieved by compositionally grading the near surface region of PEEK components.     

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.     

Influence of Tool Material and Surface Roughness on Galling Resistance in Sliding Against Austenitic Stainless Steel

Transfer and accumulation of adhered sheet material, generally referred to as galling, is a major cause for tool failure in sheet metal forming. In the present work, the galling resistance of three different tool materials was evaluated in lubricated sliding against austenitic stainless steel using a SOFS tribometer. All tool materials were prepared to four different surface roughnesses, ranging from a polished surface with R _a = 0.05 μm to a ground surface with R _a = 0.3 μm. The overall best performance was obtained for polished nitrogen alloyed powder metallurgy (PM) tool steel, where galling was detected only at the highest load evaluated, 700 N. However, for both the D2 type tool steel and nodular iron, best performance was observed for the surface possessing a surface roughness of 0.1 μm. The improved galling resistance for the rougher surfaces was related to filling of grinding scratches with sheet material during the initial stage of sliding, prolonging the development of protruding sheet material on the tools surface. Similar trend was not observed for the PM steel, which was related to width of the scratches originating from the surface preparation, in relation to tool microstructure.     

Influence of Internal Lubricants (PTFE and Silicon Oil) in Short Carbon Fibre-Reinforced Polyimide Composites on Performance Properties

Short carbon fibre-reinforced thermoplastic polyimides (30 wt%) often show high and unstable coefficients of friction. In this study, the effects of internal lubrication by polytetrafluoroethylene (PTFE) (15 wt%) or silicon oil (15 wt%) have been investigated using a reciprocating cylinder-on-plate test under 50–100 N normal loads and 0.3–1.2 m/s sliding velocities. The sliding mechanisms are discussed by considering mechanical and thermochemical modifications. The PTFE additives provide lowest coefficients of friction completely stabilising after a transition from mechanically into thermally controlled sliding at 120 °C. The sliding mechanisms and homogeneous transfer films are mainly controlled by plasticisation rather than easy-shear of its lamellar structure that is hindered by fibre reinforcement. Thermoplastic lubricants decrease the mechanical strength and therefore cause deformation and highest wear rates under 200 N. Internal oil lubricants do not reduce coefficients of friction at mild to intermediate normal loads and sliding velocities, while they become most efficient at severe sliding conditions, augmenting the pv-limit. While coefficients of friction match uniquely to the pv-conditions, the wear rates are mainly influenced by the load level: the lowest wear rates are provided by PTFE at low to intermediate pv-conditions and by oil lubricants under high normal loads.     

Bridging the Gap Between the Atomic-Scale and Macroscopic Modeling of Friction

A short survey of a modern view on the problem of friction from the physical viewpoint is presented. An atomically thin lubricant film confined between two substrates in moving contact has been studied with the help of molecular dynamics (MD) based on Langevin equations with coordinate- and velocity-dependent damping coefficient. Depending on model parameters, the system may exhibit either the liquid sliding regime, when the lubricant film melts during sliding (the “melting-freezing” mechanism of stick-slip motion), the “layer-over-layer” sliding regime, when the film keeps a layered structure at sliding, or the solid sliding regime, which may provide an extremely low friction (“superlubricity”). Atomic-scale MD simulations of friction, however, lead to a “viscosity” of the thin film, as well as to the critical velocity of the transition from stick-slip to smooth sliding, which differ by many orders of magnitude from the values observed in macroscopic experiments. This contradiction can be resolved with the help of the earthquakelike (EQ) model with a continuous distribution of static thresholds. The evolution of the EQ model is reduced to a master equation which can be solved analytically. This approach describes stick-slip and smooth sliding regimes of tribological systems within a framework which separates the calculation of the friction force from the atomic-scale studies of contact properties.     

Tribological Behaviour of Pure Ti with a Nanocrystalline Surface Layer Under Different Loads

Surface mechanical attrition treatment (SMAT), a novel surface severe plastic deformation method, was carried out for titanium (Ti) to create a gradient-structured Ti (SMAT Ti). The tribological behaviour was studied under different loads and dry sliding conditions. The results showed that the deformation layer of SMAT Ti was about 160 μm. The friction and wear results showed that the wear resistance of SMAT Ti was enhanced compared to the coarse-grained (CG) counterpart. SMAT Ti showed abrasive wear under 1 and 5 N, and exhibited abrasive and adhesive wear under 2 N. While CG Ti showed abrasive and adhesive wear under 1–2 N, and exhibited abrasive wear under 5 N for the work hardening effects.     

Tribological Behavior of Bronze Composite Coatings Obtained by Plasma Thermal Spraying

Bronze aluminum composite coatings containing different amounts of alumina were fabricated by plasma spray process and their tribological properties were investigated using ball-on-disk (BOD) and rubber wheel (RW) tests at room temperature. Main wear mechanisms in pure bronze coatings during the ball-on-disk friction test were abrasion and intersplat delamination. The addition of alumina in bronze coatings clearly enhances their wear resistance. To explain this behavior, this article proposes an additional wear mechanism in the composite coatings that involves the rupture of the alumina lamellae located just below the wear track leading to a uniform distribution of fine alumina particles enveloped by the bronze matrix, which increase the surface hardness and hinder the wear. The deposition of debris on the wear track of composite coatings provokes an enhancement of the wear resistance as well. Bronze coatings show a low and stable friction coefficient of around μ = 0.3. Nevertheless, coatings with reinforcing particles of alumina show an abrupt transition in the friction coefficient from values around μ = 0.4–0.8, related to the modification of the surface contacts on the wear track due to the formation of a compacted debris layer deposited during the tribological test.     

Tribological Properties of Spark-Plasma-Sintered ZrO2(Y2O3)–Al2O3–Ba x Sr1?x SO4 (x?=?0.25, 0.5, 0.75) Composites at Elevated Temperature

Self-lubricating ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have been fabricated by spark plasma sintering (SPS) method. The tribological properties have been evaluated using a high-temperature friction and wear tester at room temperature and 760 °C in dry sliding against alumina ball. The composites exhibit distinct improvements in effectively reducing friction and wear, as compared to the unmodified ZrO₂(Y₂O₃)–Al₂O₃ ceramics. The ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have great low and stable friction coefficients of less than 0.15 and wear rates in the order of 10^− 6mm³/Nm at 760 °C. Delamination is considered as the dominating wear mechanism of the composites at room temperature. At elevated temperature, the formation and effective spreading of Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) lubricating films during sliding play an important role in the reduction of the friction and wear.