Nicolas Argibay

Temperature-Dependent Friction and Wear Behavior of PTFE and MoS2

An investigation of the temperature-dependent friction behavior of PTFE, MoS2, and PTFE-on-MoS2 is presented. Friction behavior was measured while continuously varying contact temperature in the range −150 to 175xa0°C while sliding in dry nitrogen, as well as for self-mated PTFE immersed in liquid nitrogen. These results contrast with previous reports of high-friction transitions and plateaus for pure and composite MoS2 at temperatures below about −20xa0°C; instead, we have found persistently weak thermal behavior between 0 and −196xa0°C, providing new insight about the molecular mechanisms of macroscale friction. The temperature-dependent friction behavior characteristic of self-mated PTFE was found also for PTFE-on-MoS2 sliding contacts, suggesting that PTFE friction was defined by subsurface deformation mechanisms and internal friction even when sliding against a lamellar lubricant with extremely low friction coefficient (µxa0~xa00.02). The various relaxation temperatures of PTFE were found in the temperature-dependent friction behavior, showing excellent agreement with reported values acquired using rheological techniques measuring energy dissipation through internal friction. Additionally, hysteresis in friction behavior suggests an increase in near-surface crystallinity upon exceeding the high-temperature relaxation, Tαxa0~xa0116xa0°C.

Linking microstructural evolution and macro-scale friction behavior in metals

A correlation is established between the macro-scale friction regimes of metals and a transition between two dominant atomistic mechanisms of deformation. Metals tend to exhibit bi-stable friction behavior—low and converging or high and diverging. These general trends in behavior are shown to be largely explained using a simplified model based on grain size evolution, as a function of contact stress and temperature, and are demonstrated for self-mated pure copper and gold sliding contacts. Specifically, the low-friction regime (where µxa0<xa00.5) is linked to the formation of ultra-nanocrystalline surface films (10–20xa0nm), driving toward shear accommodation by grain boundary sliding. Above a critical combination of stress and temperature—demonstrated to be a material property—shear accommodation transitions to dislocation dominated plasticity and high friction, with µxa0>xa00.5. We utilize a combination of experimental and computational methods to develop and validate the proposed structure–property relationship. This quantitative framework provides a shift from phenomenological to mechanistic and predictive fundamental understanding of friction for crystalline materials, including engineering alloys.

Grain boundary diffusivity of Ni in Au thin films and the associated degradation in electrical contact resistance due to surface oxide film formation

The low temperature diffusion from a nickel strike layer through varying thickness films of gold, a common electrical contact overlayer, was characterized and correlated to changes in electrical contact resistance (ECR). The diffusivity of Ni in Au was determined by the surface accumulation method (type C kinetics) for Au film thicknesses of 280 and 994u2009nm at an annealing temperature of 150u2009°C over a 32u2009day period. X-ray photoelectron spectroscopy (XPS) was used to determine the rate of Ni surface accumulation. The average product of grain boundary width and diffusivity for Ni in polycrystalline Au was calculated to be δbDb≅2.0u2009×u200910−22cm3s using the Hwang-Balluffi model, and δbDb≅2.5×10−22cm3s using the Ma-Balluffi model. ECR measurements were made in parallel to surface accumulation measurements, revealing a correlation between ECR and Ni-O surface concentration; ECR values for both film thicknesses increased by 1 to 2 orders of magnitude at saturation coverage. Film cross-sections were extracted using f...

Electrical resistivity of Au-ZnO nanocomposite films

The electrical resistivity of electron beam codeposited gold and zinc oxide (Au-ZnO) films was investigated over the full composition range. The electrical resistivity was shown to increase monotonically with increasing ZnO content, with three characteristic regimes of behavior associated primarily with (1) grain boundary electron scattering due to grain refinement at ZnO volume fractions below 0.3, (2) percolation theory for ZnO volume fractions at and above the percolation threshold (fcu2009=u20090.85), and (3) a transition region between these where it was proposed that resistivity was influenced by the formation of Au-Zn complexes due to an oxygen deficiency in the deposited ZnO. The electrical resistivity of the composite films remained below 100u2009μΩu2009cm for ZnO volume fractions below 0.5. A model combining the general effective media equation and Mayadas-Shatzkes grain boundary electron scattering model was shown to generally describe the composition dependence of electrical resistivity for the investigated o...

On the thermal stability of physical vapor deposited oxide-hardened nanocrystalline gold thin films

We describe a correlation between electrical resistivity and grain size for PVD synthesized polycrystalline oxide-hardened metal-matrix thin films in oxide-dilute (<5 vol.u2009% oxide phase) compositions. The correlation is based on the Mayadas-Shatzkes (M-S) electron scattering model, predictive of grain size evolution as a function of composition in the oxide-dilute regime for 2u2009μm thick Au-ZnO films. We describe a technique to investigate grain boundary (GB) mobility and the thermal stability of GBs based on in situ electrical resistivity measurements during annealing experiments, interpreted using a combination of the M-S model and the Michels et al. model describing solute drag stabilized grain growth kinetics. Using this technique, activation energy and pre-exponential Arrhenius parameter values of Eau2009=u200921.6u2009kJ/mol and Aou2009=u20092.3 × 10−17 m2/s for Au-1 vol.u2009% ZnO and Eau2009=u200912.7u2009kJ/mol and Aou2009=u20093.1 × 10−18 m2/s for Au-2 vol.u2009% ZnO were determined. In the oxide-dilute regime, the grain size reduction of the A...

Highly Oriented MoS2 Coatings: Tribology and Environmental Stability

Molybdenum disulfide (MoS2) coatings have been prepared via nitrogen (N2) spray deposition, a process which deliberately impinges particulates of MoS2 onto a substrate yielding a preferential basally oriented state. Adherent and highly oriented 100- to 300-nm-thick coatings were produced. These coatings exhibited lower initial friction coefficients than sputtered films in dry and humid environments. Such reductions likely stem from a higher degree of basal plane orientation throughout the film as confirmed by XRD. Initial friction in humid air for sprayed coatings (µxa0=xa00.10) was half that of sputtered coatings (µxa0=xa00.21), showing the ability of oriented surface films to produce a low shear strength interface. Aging of these coatings in a humid nitrogen environment also showed the propensity for the films to resist poisoning of their structure which could otherwise result in degraded tribological performance. These results also support the hypothesis that water vapor does not contribute to the oxidation of MoS2.

Achieving Ultralow Wear with Stable Nanocrystalline Metals

Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt-Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt-Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10-9 mm3 N-1 m-1 , comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt-Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.

Temperature-Dependent Friction and Wear of MoS2/Sb2O3/Au Nanocomposites

The temperature-dependent friction and wear of magnetron-sputtered MoS2/Sb2O3/Au nanocomposites was investigated in the range −150 to 150xa0°C using macroscale experiments. We investigate the origin of recent reports suggesting the existence of a relatively high friction (µxa0~xa00.2) transition for these nanocomposites at temperatures below −20xa0°C, contrasting with the characteristic ultra-low friction behavior (µxa0<xa00.01) for pure and composite MoS2 films in vacuum and inert gas environments at room temperature. We present evidence suggesting that the ability to form and maintain basally oriented low-friction surface films is increasingly compromised with decreasing temperature, and show that low friction is achievable at cryogenic temperatures.

Mechanically-induced degradation of metallic sliding electrical contacts in silicone fluid at room temperature

The degradation in electrical contact resistance of a contact pair sliding while submerged in silicone fluid has been investigated. While the contamination of electrical contacts by silicone vapors or migrating species at elevated temperature due to decomposition in electric arcs is well known, the present degradation mechanism appears to arise from chemical reactions in the silicone fluid at room temperature, catalyzed by the presence of the freshly-abraded metal surface. As a result of these reactions, a deposit containing Si, C and O forms in the vicinity of mechanical contact. The specific contact metals present and the availability of dissolved oxygen in the fluid have a dramatic influence on the quantity of reaction product formed. The chemistry of the deposit, proposed formation mechanisms, the impact on electrical contact resistance and mitigation strategies are discussed.