The Indirect Tribological Role of Carbon Nanotubes Stimulating Zinc Dithiophosphate Anti-Wear Film Formation.

Jarosław Kałużny,Andrzej Kulczycki,Bartosz Gapiński,Wojciech Dzięgielewski,Damian Łukawski,Jerzy Merkisz,Oleksandr Stepanenko,Krzysztof Kempa,Tomasz Runka,Michał Mendak,Adam Piasecki

doi:10.3390/nano10071330

Abstract

Experimental studies reveal that the simultaneous addition of zinc dialkyl dithiophosphates (ZDDPs) and multi-wall carbon nanotubes (MWCNTs) to a poly-alpha-olefin base oil strongly reduces wear. In this paper, it is shown that MWCNTs promote the formation of an anti-wear (AW) layer on the metal surface that is much thicker than what ZDDPs can create as a sole additive. More importantly, the nanotubes’ action is indirect, i.e., MWCNTs neither mechanically nor structurally strengthen the AW film. A new mechanism for this effect is also proposed, which is supported by detailed tribometer results, friction track 3D-topography measurements, electron diffraction spectroscopy (EDS), and Raman spectroscopy. In this mechanism, MWCNTs mediate the transfer of both thermal and electric energy released on the metal surface in the friction process. As a result, this energy penetrates more deeply into the oil volume, thus extending the spatial range of tribochemical reactions involving ZDDPs.

Highlights

Carbon nanotubes (CNTs) have been studied intensely since they were first discovered in the 1990s [1]
Earlier experimental studies by the authors of this paper showed that the beneficial frictional behavior of Multi-wall carbon nanotubes (MWCNTs) can be observed only for well-dispersed CNTs that form agglomerate-free nanoliquids
The first mechanism is the adsorption or, chemisorption of additive molecules on lubricated surfaces resulting in anti-wear (AW) film formation; the second mechanism is a chemical reaction of the additive with the material surface forming an extreme pressure (EP) layer

Summary

Introduction

Carbon nanotubes (CNTs) have been studied intensely since they were first discovered in the 1990s [1]. The first mechanism is the adsorption or, chemisorption of additive molecules on lubricated surfaces resulting in anti-wear (AW) film formation; the second mechanism is a chemical reaction of the additive with the material surface forming an extreme pressure (EP) layer These mechanisms have been studied for many decades and have been extensively described in a large number of publications regarding tribochemistry [18,19], for carbon-based tribofilms [20,21,22,23]. The properties of MWCNTs, their elongated shape and electrical conductivity, allow electrons emitted on the rubbing surfaces to deeply penetrate into the oil volume, enlarging the area of ZDDP reactions In this indirect mechanism of tribological action, MWCNTs support strong AW film formation.

Materials and Methods

Results and Discussion

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