David L. Burris

Tribological Sensitivity of PTFE/Alumina Nanocomposites to a Range of Traditional Surface Finishes

Wear tests were performed with polytetrafluoroethylene (PTFE) + Al 2 O 3 nanocomposites on various manufactured surfaces to determine whether or not the wear resistance of these nanocomposites is a strong function of surface preparation. Four different surface finishes of grade 304 stainless steel counterfaces were used: electropolished (R q = 88 nm), lapped (R q = 161 nm), wet-sanded (R q = 390 nm), and dry-sanded (R q = 578 nm). PTFE + Al 2 O 3 nanocomposites made from powders of roughly 2-20 μm PTFE (matrix) and ∼44 nm Al 2 O 3 (filler) were prepared at filler weight percentages of 0, 1, 5, and 10% and tested on each surface finish. Additionally, 5 wt% 44-nm nanocomposites were compared to identically prepared 5 wt% 80- and 500-nm Al 2 O 3 filled PTFE composites on each surface. Friction coefficients were between 0.12 and 0.19 and wear rates decreased from K = 810 × 10− 6 mm 3 /(Nm) for the 5 wt% 500-nm alumina-filled PTFE on the dry-sanded surface to K = 0.8 × 10− 6 mm 3 /(Nm) for the 5 wt% 80-nm filled composite on the lapped surface. It was found that the minimum wear rate occurred on the lapped counterface for every composite, and the wear rate is a strong function of the transfer film thickness and morphology.

Wear-Rate Uncertainty Analysis

Wear due to relative motion between component surfaces is one of the primary modes of failure for many engineered systems. Unfortunately, it is difficult to accurately predict component life due to wear as reported wear rates generally exhibit large scatter. This paper analyzes a reciprocating tribometer in an attempt to understand the instrument-related sources of the scatter in measured wear rates. To accomplish this, an uncertainty analysis is completed for wear-rate testing of a commercially available virgin polytetrafluoroethylene pin on 347 stainless steel counterface. It is found that, for the conditions selected in this study, the variance in the experimental data can be traced primarily to the experimental apparatus and procedure. Namely, the principal uncertainty sources were found to be associated with the sample mass measurement and volume determination.

Fluid load support during localized indentation of cartilage with a spherical probe

Interstitial fluid pressurization, a consequence of a biphasic tissue structure, is essential to the load bearing and lubrication properties of articular cartilage. Focal tissue degradation may interfere with this protective mechanism, eventually leading to gross degeneration and osteoarthritis. Our long-term goal is to determine whether local contacts can be used as a means to probe local tissue integrity and functionality. In the present work, Hertzian rate-controlled microindentation was used as a model of the more complicated sliding system to directly determine the effects of contact radius and deformation rate on interstitial load support. During localized contact between a steel spherical probe and bovine articular cartilage, the equilibrium and non-equilibrium responses were well-fit by the Hertz model (R(2)>0.998) with a mean equilibrium contact modulus of 0.93 MPa. The effective contact modulus and fluid load fraction were independent of indentation depth, contact radius, and normal force; both increased monotonically with indentation rate. At 21 μm/s indentation rate, the cartilage was effectively stiffened by 6-fold with the fluid pressure supporting 85% of the contact force. The results motivated a simple analytical model that directly links the tribomechanical response (including fluid load support) and the Peclet number to measurable material properties and controllable experimental variables. This paper demonstrates that tribological contacts can be used to probe local functional properties. Such measurements can add important insights into the roles of focal tissue damage and impaired local functionality in the pathogenesis of osteoarthritis.

Sliding orientation effects on the tribological properties of polytetrafluoroethylene

The chemical inertness, high melting point, and intrinsic lubricity of polytetrafluoroethylene (PTFE) have been used to develop solid lubricating parts for operation in extreme environments, from frying pans to satellites. The atomic-level mechanisms associated with friction and wear at PTFE surfaces are elucidated here by systematic investigations of the frictional anisotropy measured with respect to chain orientation. In particular, a combination of atomic-scale simulations, nanometer-scale atomic force microscopy experiments, micrometer-scale microtribometers experiments, and macroscale pin-on-disk experiments are used. Data across these length scales, from both the computational and experimental approaches, provide a consistent view of the mechanisms by which the structural orientation of PTFE contributes to its unique tribological properties.

An analytical model to predict interstitial lubrication of cartilage in migrating contact areas

For nearly a century, articular cartilage has been known for its exceptional tribological properties. For nearly as long, there have been research efforts to elucidate the responsible mechanisms for application toward biomimetic bearing applications. It is now widely accepted that interstitial fluid pressurization is the primary mechanism responsible for the unusual lubrication and load bearing properties of cartilage. Although the biomechanics community has developed elegant mathematical theories describing the coupling of solid and fluid (biphasic) mechanics and its role in interstitial lubrication, quantitative gaps in our understanding of cartilage tribology have inhibited our ability to predict how tribological conditions and material properties impact tissue function. This paper presents an analytical model of the interstitial lubrication of biphasic materials under migrating contact conditions. Although finite element and other numerical models of cartilage mechanics exist, they typically neglect the important role of the collagen network and are limited to a specific set of input conditions, which limits general applicability. The simplified approach taken in this work aims to capture the broader underlying physics as a starting point for further model development. In agreement with existing literature, the model indicates that a large Peclet number, Pe, is necessary for effective interstitial lubrication. It also predicts that the tensile modulus must be large relative to the compressive modulus. This explains why hydrogels and other biphasic materials do not provide significant interstitial pressure under high Pe conditions. The model quantitatively agrees with in-situ measurements of interstitial load support and the results have interesting implications for tissue engineering and osteoarthritis problems. This paper suggests that a low tensile modulus (from chondromalacia or local collagen rupture after impact, for example) may disrupt interstitial pressurization, increase shear stresses, and activate a condition of progressive surface damage as a potential precursor of osteoarthritis.

Surface and Subsurface Contributions of Oxidation and Moisture to Room Temperature Friction of Molybdenum Disulfide

Molybdenum disulfide (MoS2), a lamellar solid lubricant, is used extensively in space applications due to its exceptional performance in vacuum and inert environments. The friction and wear of MoS2, however, increase in the presence of atmospheric contaminants, such as water. Despite numerous studies of the moisture-sensitive friction response of MoS2 over the decades, important fundamental questions remain unanswered. Two leading hypotheses suggest that water affects friction by causing the MoS2 to oxidize or by physically bonding to edge sites, and thereby disrupting easy lamellar shear. This paper presents a parametric study to (1) isolate the effects of water and oxygen on ambient MoS2 friction, (2) identify the effect of water and oxygen on MoS2 oxidation, and (3) distinguish between the effects of water diffusion and surface oxidation on the frictional response of MoS2 coatings. The experimental findings were used to develop a qualitative model for the effects of environment on MoS2 friction; the model is used to explain transients, hysteretic effects, oxidation effects, and effects of physically bound water.

Investigation of the Tribological Behavior of Polytetrafluoroethylene at Cryogenic Temperatures

Space applications are very demanding and require that lubricants provide low friction and predictable operation over a wide range of temperatures, environments, and contact conditions. Polytetrafluoroethylene (PTFE) is an attractive candidate solid lubricant due to its notably low friction coefficient, wide thermal range, and chemical inertness, but its tribology at space-relevant conditions has not been adequately investigated. This study seeks to gain insight into the cryogenic tribological behavior of PTFE using a well-studied linear reciprocating tribometer. The tribometer was contained within a nitrogen backfilled glove-box and tests were conducted at a constant background temperature of 296 K. Sliding experiments were conducted at a sliding speed of 50 mm/s and a normal pressure of 6.9 MPa, and the temperature of the lapped 304 stainless steel counterface was varied at 2% and 6% RH. Wear rate decreased monotonically with decreased interface temperature below 273 K in the absence and in the presence of ice, presumably due to improved mechanical properties at lower temperatures. The friction coefficient increased monotonically with decreased temperature in a manner consistent with thermal activation over van der Waals–type barriers; it deviated from this trend only during the phase and the glass transitions in the PTFE and after ice deposition occurred at temperatures below the estimated frost point. The data collected here are strikingly consistent with the general PTFE tribology literature and suggest that the friction coefficient of PTFE can be expected to increase by a factor of five as the temperature is reduced from 400 K to 200 K in a space environment.

Polytetrafluoroethylene matrix nanocomposites for tribological applications

Abstract Solid lubricants comprise an important class of materials and find use in applications where the use of more traditional lubrication techniques is undesirable or precluded. Polytetrafluoroethylene (PTFE) is a notable solid lubricant material, being known for its reputably low friction coefficient, high thermal range and chemical resistance, but a high wear rate limits its application in moving mechanical systems. The use of microfillers can reduce the wear rates to more acceptable values, but large particle size, high filler concentration, increased abrasion and increased friction coefficient all contribute to limit the performance of the composite. The use of nanofillers has been shown to provide further improvements in wear rates without introducing detrimental effects on its other beneficial properties. This work outlines recent studies of the wear resistance mechanisms in these novel systems. Several key wear resistance mechanisms have been identified: (1) bonding and strength at the filler/matrix interface, (2) dispersion and mechanical effects of load support and crack deflection, (3) morphological effects of nanoparticles on the matrix, (4) fibrillation and toughening, (5) transfer film coverage, (6) transfer film orientation and (7) chemical degradation. It is found that the ca. 1,000X improvement in wear resistance that trace loadings of nanofillers impart to PTFE is due to a synergism of wear resistance mechanisms that is activated by the small filler size.

Role of Interstitial Fluid Pressurization in TMJ Lubrication

In temporomandibular joints (TMJs), the disc and condylar cartilage function as load-bearing, shock-absorbing, and friction-reducing materials. The ultrastructure of the TMJ disc and cartilage is different from that of hyaline cartilage in other diarthrodial joints, and little is known about their lubrication mechanisms. In this study, we performed micro-tribometry testing on the TMJ disc and condylar cartilage to obtain their region- and direction-dependent friction properties. Frictional tests with a migrating contact area were performed on 8 adult porcine TMJs at 5 different regions (anterior, posterior, central, medial, and lateral) in 2 orthogonal directions (anterior-posterior and medial-lateral). Some significant regional differences were detected, and the lateral-medial direction showed higher friction than the anterior-posterior direction on both tissues. The mean friction coefficient of condylar cartilage against steel was 0.027, but the disc, at 0.074, displayed a significantly higher friction coefficient. The 2 tissues also exhibited different frictional dependencies on sliding speed and normal loading force. Whereas the friction of condylar cartilage decreased with increased sliding speed and was independent of the magnitude of normal force, friction of the disc showed no dependence on sliding speed but decreased as normal force increased. Further analysis of the Péclet number and frictional coefficients suggested that condylar cartilage relies on interstitial fluid pressurization to a greater extent than the corresponding contact area of the TMJ disc.

Viscoelastic behavior of nanotube-filled polycarbonate: Effect of aspect ratio and interface chemistry

A combined experimental and modeling study on the solid-state rheology of multi-walled carbon nanotube (MWNT)/polycarbonate composites as a function, independently, of MWNT aspect ratio and interface chemistry was carried out. Shorter aspect ratio nanotubes lead to greater broadening of the loss modulus peak in frequency space, but there was no effect of aspect ratio on the glass transition temperature. The breadth of the loss modulus peak was found to correlate with the free space parameter, a measure of the spacing between the MWNTs. A new model that accounts for the aspect ratio and distribution in a representative volume element was developed to study these parameters in a controlled setting where morphology was precisely known. Micromechanics modeling was found to correlate well with experimental data. These results shed light on the separate impacts of aspect ratio, dispersion, and interface modification on the solid-state rheology of nanofilled polymers.