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Low aspect ratios and sp3 hybridization of carbon nanoparticles benefit both, small agglomerate sizes in ethylene glycol and nickel composites.

The use of lubricants (solid or liquid) is a well-known and suitable approach to reduce friction and wear of moving machine components. Another possibility to influence the tribological behaviour is the formation of well-defined surface topographies such as dimples, bumps or lattice-like pattern geometries by laser surface texturing. However, both methods are limited in their effect: surface textures may be gradually destroyed by plastic deformation and lubricants may be removed from the contact area, therefore no longer properly protecting the contacting surfaces. The present study focuses on the combination of both methods as an integral solution, overcoming individual limitations of each method. Multiwall carbon nanotubes (MWCNT), a known solid lubricant, are deposited onto laser surface textured samples by electrophoretic deposition. The frictional behaviour is recorded by a tribometer and resulting wear tracks are analysed by scanning electron microscopy and Raman spectroscopy in order to reveal the acting tribological mechanisms. The combined approach shows an extended, minimum fivefold longevity of the lubrication and a significantly reduced degradation of the laser textures. Raman spectroscopy proves decelerated MWCNT degradation and oxide formation in the contact. Finally, a lubricant entrapping model based on surface texturing is proposed and demonstrated.

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Abstract:In this study, nickel matrix composites reinforced by carbon nanotubes (CNTs) are compared to unreinforced CNT-coated (by drop-casting) bulk nickel samples in terms of their friction and wear behavior, thus gaining significant knowledge regarding the tribological influence of CNTs and the underlying tribo-mechanism. It has been shown that the frictional behavior is mainly influenced by the CNTs present in the contact zone, as just minor differences in the coefficient of friction between the examined samples can be observed during run-in. Consequently, the known effect of a refined microstructure, thus leading to an increased hardness of the CNT reinforced samples, seems to play a minor role in friction reduction compared to the solid lubrication effect induced by the CNTs. Additionally, a continuous supply of CNTs to the tribo-contact can be considered isolated for the reinforced composites, which provides a long-term friction reduction compared to the CNT-coated sample. Finally, it can be stated that CNTs can withstand the accumulated stress retaining to some extent their structural state for the given strain. A comprehensive study performed by complementary analytical methods is employed, including Raman spectroscopy and scanning electron microscopy to understand the involved friction and wear mechanisms.