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.

In the present study, surface protection properties of multiwall carbon nanotubes (CNTs) deposited on polished austenitic stainless steel are evaluated. Electrophoretic deposition is used as a coating technique. Contact angle measurements reveal hydrophilic as well as hydrophobic wetting characteristics of the carbon nanotube coating depending on the additive used for the deposition. Tribological properties of carbon nanotube coatings on steel substrate are determined with a ball-on-disc tribometer. Effective lubrication can be achieved by adding magnesium nitrate as an additive due to the formation of a holding layer detaining CNTs in the contact area. Furthermore, wear track analysis reveals minimal wear on the coated substrate as well as carbon residues providing lubrication. Energy dispersive x-ray spectroscopy is used to qualitatively analyse the elemental composition of the coating and the underlying substrate. The results explain the observed wetting characteristics of each coating. Finally, merely minimal oxidation is detected on the CNT-coated substrate as opposed to the uncoated sample.

Topographically designed surfaces are able to store solid lubricants, preventing their removal out of the tribological contact and thus significantly prolonging the lubrication lifetime of a surface. The present study provides a systematic evaluation of the influence of surface structure design on the solid lubrication effect of multi-walled carbon nanotubes (MWCNT) coated steel surfaces. For this purpose, direct laser writing using a femtosecond pulsed laser system is deployed to create surface structures, which are subsequently coated with MWCNT by electrophoretic deposition. The structural depth or aspect ratio of the structures and thus the lubricant storage volume of the solid lubricant is varied. The frictional behavior of the surfaces is recorded using a ball-on-disc tribometer and the surfaces are thoroughly characterized by complementary characterization techniques. Efficient lubrication is achieved for all MWCNT coated surfaces. However, and in contrast to what would be expected, it is shown that deeper structures with larger lubricant storage volume do not lead to an extended lubrication lifetime and behave almost equally to the coated unstructured surfaces. This can be attributed, among other things, to differences in the final surface roughness of the structures and the slope steepness of the structures, which prevent efficient lubricant supply into the contact.

Sucker rods are a key element in certain oil-extraction processes as they link the motor group on the surface with the pumps located downhole. During the transport from the production site toward the extraction well, these components are prone to corrosion. A hydrophobic carbon nanotube (CNT) coating, deposited via electrophoretic deposition (EPD), is proposed as a protective layer, shielding the rods from harsh environmental conditions. Three different coating systems are considered and thoroughly characterized (depending on the additive that is used to deposit the CNT), namely, magnesium nitrate hexahydrate (Mg-Nit), triethylamine (TEA), and a duplex coating (DD). The latter presents an approach which combines the advantages of each additive, mechanical stability from Mg-Nit and strong hydrophobicity from TEA (near superhydrophobic). The former coatings are further processed to overcome their individual shortcomings, resulting in an increase in the coating's stability for TEA coating, as well as transforming the hydrophilic Mg-Nit surface into a hydrophobic surface.

Carbon nanoparticle coatings on laser-patterned stainless-steel surfaces present a solid lubrication system where the pattern’s recessions act as lubricant-retaining reservoirs. This study investigates the influence of the structural depth of line patterns coated with multi-walled carbon nanotubes (CNTs) and carbon onions (COs) on their respective potential to reduce friction and wear. Direct laser interference patterning (DLIP) with a pulse duration of 12 ps is used to create line patterns with three different structural depths at a periodicity of 3.5 µm on AISI 304 steel platelets. Subsequently, electrophoretic deposition (EPD) is applied to form homogeneous carbon nanoparticle coatings on the patterned platelets. Tribological ball-on-disc experiments are conducted on the as-described surfaces with an alumina counter body at a load of 100 mN. The results show that the shallower the coated structure, the lower its coefficient of friction (COF), regardless of the particle type. Thereby, with a minimum of just below 0.20, CNTs reach lower COF values than COs over most of the testing period. The resulting wear tracks are characterized by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. During friction testing, the CNTs remain in contact, and the immediate proximity, whereas the CO coating is largely removed. Regardless of structural depth, no oxidation occurs on CNT-coated surfaces, whereas minor oxidation is detected on CO-coated wear tracks.