AVS2012 Session TR+SE-WeM: Tribology and Wear of Low-Friction Coatings and Materials

In the early 1940s, military pilots and those who serviced their planes were surprised by serious problems with ignition systems and diverse electrical troubles in their state of the art aircraft. Further study revealed that the graphite commutator brushes used in the electrical generator on the airplane (similar to the alternator in our automobiles) were the source of these technical difficulties, because they wore out 100 to 1000 times faster than expected. Over 70 years later, sensitivity of materials at contact interfaces to the extreme ambient environments aircraft are subjected to is still limiting aerospace capability. One of the reasons materials scientists haven’t overcome these problems sooner is because it is difficult to do materials science, at least the part where you correlate performance to structure and composition, when there is such a big difference between the operating environment and the environment in which analysis is conducted. All the materials tribologist can really do is post mortem forensics work on materials designed to get hot after they have cooled down to a convenient handling temperature. During that cooling down time, phase changes and grain growth occur, perhaps misleading the researcher on the compounds or phases yielding high or low friction at interfaces in relative motion. In this talk we will review innovative characterization techniques designed to provide the insight necessary to produce environmentally adaptive tribological coatings, especially materials designed to operate over broad ranges of humidity (i.e, earth to space) and temperature (i.e., ignition to supersonic flight). Characterization of temperature adaptive nanocomposite lubricant materials, such as MoN/Ag and VN/Ag via Raman spectroscopy will be reviewed in depth.

Additives are generally added to lubricants, which react with the surfaces to form a boundary lubricating film that can lower friction and/or prevent wear. At the high interfacial temperatures that occur under so-called extreme-pressure conditions, the surface reaction and film growth kinetics are dominated by thermal processes. In contrast, it is postulated that, under mild conditions, where the surface temperature rise is low, a surface film can be formed by a shear-induced, surface-to-bulk transport mechanism. This effect is investigated by studying the tribologically induced surface reactions of model sulfur- and boron-containing additives on copper surfaces in ultrahigh vacuum (UHV) where the background pressure is ~1×10^-10 Torr. The nature of the initial surface species that are formed by exposure of the copper surface to the model lubricant additive is investigated using a range of surface analytical techniques such as temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). This enables the nature of the initial surface species to be identified, and their thermal stability to be measured in some detail. The effect of rubbing these adsorbate-covered surfaces is investigated, also in UHV, by measuring the friction coefficient and contact resistance during rubbing. The chemical composition of the wear track is followed using high-spatial-resolution Auger spectroscopy to follow the fate of the surface species caused by rubbing and to test the above postulate.

In addition, the effect of low-coordination sites formed on the surface by rubbing on the reactivity of gas phase lubricants is explored and finally, this strategy is used to understand the tribochemistry of borate esters on copper under mild rubbing conditions.

In this study, the tribological and compositional properties of annealing temperatures on electrolessly deposited nickel boride coatings were investigated. All samples were coated in the same bath and had approximately the same starting composition: 31 atomic % Ni, 19 atomic % B, 42 atomic % O, and 8 atomic % C . Samples were annealed at temperatures of 250°C, 400°C, 550°C, and 700°C under a constant flow of oxygen. The tribological properties of the samples were tested using a novel pin-on-disc tribometer. The pin-on-disc tribometry was performed in air under ambient RH conditions, keeping conditions the same throughout all tests that were performed. The influence of the annealing temperature on surface composition was studied by X-ray photoelectron spectroscopy (XPS), used to identify and relatively quantify the elements present. Raman spectroscopy was used to differentiate between chemical species that were indistinguishable in XPS. Increasing annealing temperatures were seen to influence the tribological properties of the coatings to a large degree; with the samples with higher annealing temperatures having a significantly lower coefficient of friction, μ≈0.16 for the sample annealed at 550°C and μ≈0.06 for the sample annealed at 700°C, than the lower temperature annealed samples, μ≈0.5 for samples annealed at 250°C and 400°C. Similarly, the chemical nature of the coatings were strongly affected by the differing annealing temperatures; with the higher annealing temperature samples showing heavy oxidation and migration of boron to the surface, while the lower annealed samples remained largely unchanged. The lowering of the friction coefficient in the samples annealed at 550°C and 700°C was attributed to the migration and subsequent oxidation of boron forming low friction B₂O₃ at the surface.

Traditionally the tribological behavior of PTFE (polytetrafluoroethylene) has been studied in the steady state after many sliding cycles. Steady state studies neglect events that occur during the first contact and sliding cycles, as well as the early transient behavior of PTFE. In-situ surface plasmon resonance (SPR) tribology experiments allow for the observation of initial transfer after the first contact and first sliding cycle of PTFE. SPR showed transfer of PTFE after the first cycle of sliding, contrary to previous models of films formed by delamination wear debris. Surface enhanced raman spectroscopy (SERS) verified the chemical identity of the transfer film. Friction behavior of PTFE was shown to differ in the first cycle from the behavior seen after a transfer film was created and to depend on contact pressure and sliding speed.

Since microelectromechanical systems (MEMS) are critically-limited by interfacial phenomena such as friction and adhesion, strategies have been developed to reduce friction in these systems. One common strategy is to coat MEMS surfaces with molecularly-thin self-assembled monolayer (SAM) coatings. Silicon MEMS are most commonly coated with silane-based SAMs, such as octadecyltrichlorosilane (OTS). Continued development of MEMS technology may require new material systems to be employed. Therefore, in this study, we investigate the frictional properties of octadecylphosphonic acid (ODP) monolayers deposited on aluminum oxide surfaces. Measurements using an atomic force microscope (AFM) and separately using a nanoindenter-quartz crystal microbalance system were performed each with a microsphere-terminated probe, allowing for a comparative study between different velocity regimes using contacts with similar sizes, surface roughnesses, and interfacial chemistries. AFM colloidal probe friction measurements indicate that while the frictional properties of aluminum oxide can be reduced with the use of an ODP monolayer, a more dramatic rubbing-induced modification is clearly evident for both bare and SAM-coated aluminum oxide substrates. These modification effects depend on scanning duration and environmental conditions such as humidity. We hypothesize that a tribochemical reaction occurs between aluminum oxide and ambient water, which has also been observed in macroscopic tribological studies of aluminum oxide published in the literature.

There has been recent interest in the use of solid lubricant nanoparticles in coatings and as boundary additives in liquid lubricants. Examples include nanoparticles of MoS₂ and WS₂. The formation of these nanoparticles is nontrivial, requiring techniques such as gas phase syntheses and electric discharges. We are exploring simpler syntheses using wet chemical techniques. In particular, the synthesis of MoS₃ nanoparticles involves hydrothermal reaction between molybdate salts and sodium sulfide under controlled pH conditions.¹

The use of MoS₃ as a tribological material has not been explored beyond its use as an oil additive.² We are investigating its potential for use in solid lubricant coatings. Bonded MoS₂ coatings experience widespread usage for lubricating mechanisms on virtually every spacecraft (i.e., using micron-sized MoS₂ particles). We formulated resin-bonded coatings using MoS₃ nanoparticles as the lubricating pigment, and compared their tribological performance to commercial bonded MoS₂ coatings. Surprisingly, the MoS₃-formulated coatings performed similarly to the MoS₂-based coatings. Specifically, they showed similar coefficients of friction (i.e., 0.04 to 0.06) and endurances in dry nitrogen (<0.1% RH). We will present results of surface analyses on worn coatings to reveal changes in composition and chemical state of the MoS₃ that might explain the measured low friction. In addition, results of tribological performance in humid air atmospheres will be presented.

We have also explored syntheses of MoS₂ nanoparticles using nanosize MoS₃ as a starting material. A recent study purported to create MoS₂ nanoparticles with the addition of an aqueous reducing agent during hydrothermal synthesis of MoS₃.³ However, X-ray fluorescence analysis of the product of our synthesis showed that the S:Mo remained at 3.0 after using the reducing agent. Other conversion methods including vacuum reduction of the MoS₃ nanoparticles will be discussed.

References:

1. P. Afanasiev, “Synthetic approaches to the molybdenum sulfide materials,” Comptes Rendus Chimie, 11(1–2) (2008) 159–182.

2. O.P. Parenago, V.N. Bakunin, G.N. Kuz’mina, A.Yu. Suslov, and L.M. Vedeneeva, “Molybdenum Sulfide Nanoparticles as New-Type Additives to Hydrocarbon Lubricants,” Doklady Chemistry, 383(1-3) (2002) 86-88.

3. Y. Tian, X. Zhao, L. Shen, F. Meng, L. Tang, Y. Deng, Synthesis of amorphous MoS₂ nanospheres by hydrothermal reaction,” Materials Letters 60 (2006) 527–529.

This work was funded in part by The Aerospace Corporation's Sustained Experimentation and Research for Program Applications program.

© The Aerospace Corporation 2012