High-altitude brush problem

Ramadanoff, D.; Glass, S. W.

doi:10.1109/ee.1944.6440571

Cited by 15 publications

(13 citation statements)

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“…The study of humidity effects on the friction and wear properties of graphite dates back to the 1930s, when graphite was used as a solid lubricant for electric brushes [2][3][4]. Systematic researches on the effect of humidity was conducted after it was found that the graphite electric brushes suffered from severe wear in airplanes flying at high altitudes, where the humidity is low [4]. It was found that both the friction and wear of graphite decrease as the water vapor pressure or humidity increases [4][5][6][7][8][9][10][11].…”

Section: Graphite and Graphenementioning

confidence: 99%

“…Systematic researches on the effect of humidity was conducted after it was found that the graphite electric brushes suffered from severe wear in airplanes flying at high altitudes, where the humidity is low [4]. It was found that both the friction and wear of graphite decrease as the water vapor pressure or humidity increases [4][5][6][7][8][9][10][11]. Based on these experimental observations, it was realized that the good lubricity of graphite is not an intrinsic property, but one influenced by environmental factors.…”

Section: Graphite and Graphenementioning

confidence: 99%

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Effect of Humidity on Friction and Wear—A Critical Review

et al. 2018

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The friction and wear behavior of materials are not intrinsic properties, but extrinsic properties; in other words, they can drastically vary depending on test and environmental conditions. In ambient air, humidity is one such extrinsic parameter. This paper reviews the effects of humidity on macro- and nano-scale friction and wear of various types of materials. The materials included in this review are graphite and graphene, diamond-like carbon (DLC) films, ultrananocrystalline diamond (UNCD), transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), boric acid, silicon, silicon oxide, silicates, advanced ceramics, and metals. Details of underlying mechanisms governing friction and wear behaviors vary depending on materials and humidity; nonetheless, a comparison of various material cases revealed an overarching trend. Tribochemical reactions between the tribo-materials and the adsorbed water molecules play significant roles; such reactions can occur at defect sites in the case of two-dimensionally layered materials and carbon-based materials, or even on low energy surfaces in the case of metals and oxide materials. It is extremely important to consider the effects of adsorbed water layer thickness and structure for a full understanding of tribological properties of materials in ambient air.

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Section: Graphite and Graphenementioning

confidence: 99%

Section: Graphite and Graphenementioning

confidence: 99%

Effect of Humidity on Friction and Wear—A Critical Review

et al. 2018

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“…1 It is not the low pressure, but the low humidity, that is primarily responsible for the reduced lubricating ability at high altitude or in space, as a small amount of interlayer water is essential for the easy shear parallel to the graphene hexagonal planes. 2 A complete understanding of this has not been achieved as it is not yet clear how the interlayer water interacts with the graphene layers.…”

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confidence: 99%

Effect of humidity on the interlayer interaction of bilayer graphene

et al. 2019

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The lubricating ability of graphite largely depends on the environmental humidity, essentially the amount of water in between its layers. In general, intercalated molecules in layered materials modify their extraordinary properties by interacting with the layers. To understand the interaction of intercalated water molecules with graphene layers, we performed Raman measurements on bi-layer graphene at various humidity levels and observed an additional peak close to that of the low-frequency layer breathing mode between two graphene layers. The additional peak is attributed to the vibration between an intercalated water layer and the graphene layers. We further propose that the monolayer coverage of water increases between bilayer graphene with increasing environmental humidity while the interaction between the water layer and graphene layers remains approximately unchanged, until too much water is intercalated to keep the monolayer structure, at just over 50% relative humidity. Notably, the results suggest that unexpectedly humidity could be an important factor affecting the properties of layered materials, as it significantly modifies the interlayer interaction.Graphite, the most prominent layered material, has been used as a good lubricant for about 200 years. However, an abnormally high wear rate of graphite brushes in electrical machines aboard aircraft during World War II was reported. 1 Later in the 1960s, NASA found that graphite lost its lubrication in space. 1 It is not the low pressure, but the low humidity, that is primarily responsible for the reduced lubricating ability at high altitude or in space, as a small amount of interlayer water is essential for the easy shear parallel to the graphene hexagonal planes. 2 A complete understanding of this has not been achieved as it is not yet clear how the interlayer water interacts with the graphene layers.Layered materials containing different numbers of layers can now be produced, either by exfoliating from the bulk, 3 or by stacking one layer onto another. 4 Structures made from different numbers of layers may possess unique properties. 5 Intercalated molecules further modify these properties. This vast research area focuses in general on three questions -how, and how many, intercalating molecules can be introduced between the layers, and what effect they have on pristine layered material. The properties of layered materials largely depend on their interlayer interaction. 6 It is therefore important to quantify the effect of intercalated molecules on this interaction.In our work we start with a simple system, bi-layer graphene (the simplest layered structure) with intercalated water (the most common polar molecule), while the amount of water is varied by adjusting the humidity (one of the most common atmospheric conditions).Much work has been done on water in between graphene oxide layers (GO), as it is hydrophilic -water easily comes between the layers and attaches to them via non-covalent bonds. 7 Early work reported interlayer spacings of 0.63 nm for dry...

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“…Graphite has long been recognized to be an effective dry lubricant, but its lowfriction behavior was not well understood until W. Bragg first discovered its lamellar structure by x-ray diffraction in 1928, 1 whence began the long debate on this subject. [2][3][4][5][6][7][8][9][10][11][12] Figure 1 shows the highly anisotropic, layered crystalline structure of graphite, which has strong bonding within the basal planes but weak bonding between these planes as evidenced by the large spacing between them. Bragg attributed the slipperiness of graphite to the shearing of these weakly bonded basal planes.…”

mentioning

confidence: 99%

“…1 This lattice-shear theory was generally accepted until graphite commutator brushes on high-flying aircraft in the late 1930s experienced abnormally high wear rates (10 3 -10 4 times normal) owing to the low humidity at high altitude. 2 It was subsequently shown that in the absence of certain gases and vapors in the environment, such as water, O 2 and CO 2 , graphite would exhibit this high friction and wear behavior, 3 also known as "dusting." Hence, the lubricity of graphite is not an intrinsic property, as implied by Bragg's shear theory.…”

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confidence: 99%

Origin of low-friction behavior in graphite investigated by surface x-ray diffraction

Yen

Schwickert

Toney

2004

Applied Physics Letters

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Contrary to popular belief, the slipperiness of graphite is not an intrinsic property. The presence of vapors, such as water, is required for graphite to lubricate; in vacuum or dry environments, the friction and wear rate of graphite are high. A widely accepted explanation involves weakening of the binding force between basal planes near the surface, thereby allowing these planes to shear easily. This weakening results from proposed chemisorption or intercalation of vapor molecules near the surface, leading to an increase in the interlayer spacing between nearsurface basal planes. Here we use X-ray diffraction from a synchrotron source to show that the basal plane spacing at the surface is the same in vacuum, ambient air, or water vapor saturated air. These results refute this long-held view that the low friction behavior of graphite is due to shearing of weakened basal planes.

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High-altitude brush problem

Cited by 15 publications

References 0 publications

Effect of Humidity on Friction and Wear—A Critical Review

Effect of Humidity on Friction and Wear—A Critical Review

Effect of humidity on the interlayer interaction of bilayer graphene

Origin of low-friction behavior in graphite investigated by surface x-ray diffraction

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