Thierry A. Blanchet

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ∼ 10− 3 mm 3 /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ∼ 10−7 mm 3 /Nm level, two orders of magnitude better than the ∼ 10−5 mm 3 /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 304) countersurfaces than those with microparticles, despite the filler being of the same material. In PTFE containing a mixture of both nano- and micro-fillers, the higher wear rate microcomposite behavior predominated, likely the result of the continued presence of micro-fillers and their abrasion of the countersurface as well as any overlying beneficial transfer films. Despite demonstrating such a large effect on the wear rate, the variation of alumina filler size did not demonstrate any significant effect on the friction coefficient, with values for all composites tested additionally falling near the μ = 0.18 measured for unfilled PTFE at this studys 0.01 m/s sliding speed.

Thrust-Washer Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding Contact

PS304 self-lubricating composite coatings were successfully deposited on steel substrates at various plasma spray facilities using mixtures blended from commercially obtained constituent particles. Coatings were evaluated in thrust-washer tests against Inconel X-750 at low contact pressures to 40kPa, sliding speed of 5Amis, and either ambient temperature or 500 °C chosen to simulate conditions in airfoil bearings during startup and shutdown contact. Wear factors for all PS304 coatings tested, regardless of contact pressure and temperature, ranged from 1–3*10−4 mm3/Nm while coefficients of friction of approximately μ =0.5 were measured in all cases. While wear and friction behavior of PS304 in air foil bearings appear to have been simulated, surface roughening was observed in these thrust-washer tests which used continuous sliding contact, as opposed to the evolution of smoother surfaces observed in high-temperature foil bearings experiencing cyclic startup/shutdown. Wear-induced surface smoothening of PS304 was additionally simulated in thrust-washer tests with sliding contact instead imposed intermittently.

Radial distribution of fictive temperatures in silica optical fibers

Abstract The fictive temperature of glass samples influences many glass properties including mechanical strength and fatigue characteristics. A distribution in fictive temperature on a cross-section of a commercial silica glass communications fiber was determined by measuring the profile of the IR reflection band wavenumber of the silica fundamental structural band at ∼ 1122 cm −1 as a function of radial position using an FTIR with a microscope attachment. Results showed a lower peak wavenumber which corresponds to a higher fictive temperature near the surface of the as-received fiber. Upon annealing, this surface characteristic disappeared and a nearly constant peak wavenumber was obtained across the fiber cladding indicating that the high fictive temperature of the commercial fiber surface was produced during the fiber drawing process. The different mechanical fatigue behavior of silica fibers from those of bulk silica glasses may be due to the observed high fictive temperature of the fiber surface.

Tribology of selectively irradiated PTFE surfaces

Selectively irradiated polytetrafluoroethylene (PTFE) surfaces combining the low friction and non-abrasive attributes of an unirradiated polymer with the enhanced wear resistance of wholly irradiated PTFE are demonstrated. Augmented wear resistance, similar to that of filled PTFE composites, is obtained without the accompanying counterface abrasion typical of hard particulate fillers. Friction of these ‘composite’ irradiated/unirradiated surfaces can be less than that of unirradiated PTFE, as irradiated regions limit transfer morphology to only thin oriented films. Spatially distributed irradiated and unirradiated surface regions are patterned by masking 225 keV incident electrons. Under the given contact conditions (6.5 MPa nominal pressure against polished stainless steel) such self-lubricating, wear-resistant composite surfaces had lifetimes of several kilometers sliding distance before giving way to rapid wear of the underlying unmodified PTFE.

Factors affecting the wear of irradiated UHMWPE

Effect of sliding directionality on wear of irradiated UHMWPE (ultra-high molecular weight polyethylene) was studied under serum-lubricated conditions. When exposed to unidirectional sliding, by reciprocation along a line, steady-state wear rates were negligibly small against polished countersurfaces, only reaching measurable levels against roughened countersurfaces. In unidirectional cases, more rapid wear existed during the first million sliding cycles. If instead subjected to multidirectional sliding, by translating countersurfaces through closed paths, unirradiated UHMWPE experienced rapid wear of 8.3 × 10−7 mm3/Nm against polished CoCr. Crosslinked UHMWPE, produced by gamma irradiation to 4 Mrad dose with subsequent 200 °C (melt) vacuum storage, reduced wear three-fold. Vacuum storage allows crosslinking between radiation-induced radicals without competitive oxidative scission, while increased temperature augments crosslinking kinetics. Wear resistance was similarly enhanced using 70 °C (sub-melt) vacuum storage by increasing dose to 10 Mrad. Under multidirectional sliding against polished CoCr, all UHMWPE adopted steady-state wear rates from the onset of sliding. Multidirectional sliding produced higher steady-state wear rates than unidirectional sliding, regardless of whether rectangular or circular translation paths were employed. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 14, 2000

Vapor-Phase Lubrication in Combined Rolling and Sliding Contacts: Modeling and Experimentation

The in situ vapor-phase lubrication of M50 steel, in combined rolling and sliding contacts at 540°C using nitrogen atmospheres containing acetylene, is achieved. Acetylene partial pressures of 0.05 atmospheres are capable of providing continuous lubrication to combined rolling and sliding contacts through pyrolytic carbon deposition. In these tests, friction coefficients as low as μ = 0.01 are found for contacts at 2.0 m/s rolling speed, 10 cm/s sliding speed, 100 N load (1.3 GPa Hertzian contact pressure), and ambient temperature of 540°C, with even lower values observed at more modest sliding speeds. One example of a model for vapor phase lubrication of combined rolling and sliding contacts is developed which predicts the lubricant steady-state fractional coverage of the contact surfaces, and from this makes friction coefficient predictions using a linear rule-of-mixture. Friction coefficient responses to step changes in acetylene partial pressure, sliding speed, and disk wear-track diameter are measured. Increased partial pressure of acetylene and increased area available for deposition are observed to be beneficial, while increased sliding speed is detrimental to lubrication performance. Shapes and trends of steady-state friction coefficient versus acetylene partial pressure, sliding speed, and disk wear-track diameter are described and curve-fit by the model. In combined rolling and sliding this example model predicts large regions of operating conditions over which friction coefficient is independent of rolling speed, as well as regions of independence of vapor partial pressure. In the special case of pure sliding, a region of friction coefficient independence of a ratio of partial pressure to sliding speed and another region of independence of a ratio of partial pressure to the product of sliding speed and normal load are predicted.

A Model for Polymer Composite Wear Behavior Including Preferential Load Support and Surface Accumulation of Filler Particulates

A rule-of-mixtures model is developed for the time-dependent wear and friction behavior of polymer matrix materials containing particulate filler inclusions, based upon the specific wear rates of filler and matrix materials. The model accounts for the accumulation of wear-resistant filler particles within the near-surface region of the composite as sliding proceeds. Account is also made for preferential support of the normal load by filler particles at the sliding surface. Though particle/matrix interfacial shear stress and particle aspect ratio do affect initial transient behavior, wear rate, and friction are independent of preferential load support under steady-state conditions. The model indicates that steady-state composite wear rate can be most affected by the specific wear resistance of the filler particles, as well as the volume fraction filling of particles into the polymer matrix. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Wear resistant irradiated FEP/unirradiated PTFE composites

Wear resistance of fluorinated ethylene propylene (FEP) can be increased through electron irradiation by three orders of magnitude. Unlike polytetrafluoroethylene (PTFE), wear resistance can be maintained in spite of subsequent thermal processing if irradiation is carried out at elevated temperature in inert atmosphere. Composites formed through sintering mixtures of irradiated FEP and unirradiated PTFE particles possess unique combinations of wear resistance and lower friction than that of unirradiated PTFE. Furthermore, irradiated FEP/unirradiated PTFE composites are non-abrasive to mating counterfaces, unlike hard particle-filled composites. Distribution of irradiated FEP particles throughout the entirety of a PTFE body enables these attributes to be maintained indefinitely.

Solid lubrication by decomposition of carbon monoxide and other gases

Abstract Extended duration high temperature (above 500°C) lubrication of silicon nitride sliding and rolling contacts was accomplished by solid carbon deposited and replenished via the decomposition of carbonaceous gas streams directed towards the tribological surfaces. Injection of carbon monoxide-hydrogen mixtures can lead to reductions in high temperature friction and wear of silicon nitride sliding contacts by factors of up to 10 × and 500 × respectively from the unlubricated case. Similar lubrication is attained by various hydrocarbon gas mixtures, while only a ten-fold reduction in wear is attained by a carbon dioxide-hydrogen mixture. Effective lubrication can be modeled as a favorable balance between carbon deposition from the gas phase and removal by wear. This model is based on the dependencies of solid lubricant deposition and removal processes on temperature, sliding speed, normal load, decomposition activation energy, gas flow rate, and tribosystem properties. The model is checked by the mapping of regions where measured friction indicates that lubrication by CO-H 2 mixtures is adequate, and the identification of a boundary representing the onset of marginally effective lubrication.

Experimental Evaluation of a Steady-State Model for the Wear of Particle-Filled Polymer Composite Materials

A model for the steady-state wear behavior of polymer composite materials, including the effects of preferential load support by and surface accumulation of wear-resistant filler particles, is further developed. It is shown that the resultant inverse rule-of-mixtures description of steady-state composite wear rate behavior is independent of the assumed form of filler contact pressure, though preferential load support does affect the degree of surface accumulation of filler particles that occurs. The validity of these descriptions of steady-state wear behavior and surface accumulation as functions of bulk filler volume fraction are investigated by experiments with copper particle-filled PTFE composites for bulk filler volume fractions from 0 to 40 percent. The applicability of the description of surface accumulation for this composite system was limited to bulk filler volume fractions less than 20 percent, a hypothesized result of transition in load-sharing between filler and matrix. The inverse rule-of-mixtures description of steady-state wear rate, however, was maintained over the full range of volume fractions investigated.