Sebastian Suarez

Dispersion analysis of carbon nanotubes, carbon onions, and nanodiamonds for their application as reinforcement phase in nickel metal matrix composites

Dispersions of multi-wall carbon nanotubes, onion-like carbon, and nanodiamonds in ethylene glycol are produced using a homogenizer and an ultrasonic bath, altering the treatment time. The dispersed particles are then used as reinforcement phase for nickel matrix composites. These nanoparticles are chosen to represent different carbon hybridization states (sp2 vs. sp3) or a different particle geometry (0D vs. 1D). This allows for a systematic investigation of the effect of named differences on the dispersibility in the solvent and in the composite, as well as the mechanical reinforcement effect. A comprehensive suite of complementary analytical methods are employed, including transmission electron microscopy, Raman spectroscopy, dynamic light scattering, sedimentation analysis, zeta-potential measurements, scanning electron microscopy, electron back scatter diffraction, and Vickers microhardness measurements. It can be concluded that the maximum achievable dispersion grade in the solvent is similar, not altering the structural integrity of the particles. However, nanodiamonds show the best dispersion stability, followed by onion-like carbon, and finally multi-walled carbon nanotubes. The distribution and agglomerate sizes of the particles within the composites are in good agreement with the dispersion analysis, which is finally correlated with a maximum grain refinement by a factor of 3 and a maximum mechanical reinforcement effect for nanodiamonds.

Long-lasting solid lubrication by CNT-coated patterned surfaces

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.

Influence of Surface Roughness on the Lubrication Effect of Carbon Nanoparticle-Coated Steel Surfaces

In the present study, a systematic evaluation of the influence of the surface roughness on the lubrication activity of multi-wall carbon nanotubes (MWCNT) and onion-like carbon (OLC) is performed. MWCNT and OLC are chosen as they both present an sp2-hybridization of carbon atoms, show a similar layered atomic structure, and exhibit the potential to roll on top of a surface. However, their morphology (size and aspect ratio) clearly differs, allowing for a methodical study of these differences on the lubrication effect on systematically varied surface roughness. Stainless steel platelets with different surface finishing were produced and coated by electrophoretic deposition with OLC or MWCNT. The frictional behavior is recorded using a ball-on-disk tribometer, and the resulting wear tracks are analyzed by scanning electron microscopy in order to reveal the acting tribological mechanisms. It is found that the lubrication mechanism of both types of particles is traced back to a mixture between a rolling motion on the surfaces and particle degradation, including the formation of nanocrystalline graphitic layers. This investigation further highlights that choosing the suitable surface finish for a tribological application is crucial for achieving beneficial tribological effects of carbon nanoparticle lubricated surfaces.

Temperature dependent structural evolution in nickel/carbon nanotube composites processed by high-pressure torsion

Nickel/Carbon nanotube (CNT) composites with varying amounts of CNTs were processed at different temperatures by high-pressure torsion (HPT) with the aim to optimize the process parameters to obtain a homogenous dispersion of CNTs in the metallic matrix. As the CNT distribution has an enormous influence on the composite properties, the structural evolution with increasing strain and the final microstucture of the composites are investigated by scanning and transmission electron microscopy. Microhardness measurements were additionally performed. Microhardness increases up to 800 Vickers (HV) and the mean grain size decreases to an equivalent radius smaller than 40 nm for HPT at room temperature (RT), while the CNTs form rather large agglomerates. HPT deformation at 200 °C shows no significant change in hardness, grain size and CNT agglomerate size. For HPT deformation at 300°C and 400°C grain sizes increase to 60 nm respectively 90 nm, microhardness decreases to 500 HV respectively 400 HV and the size of the CNT agglomerates decreases from more than 5 times the grain size at RT to smaller than the grain size. It could be shown that the optimal HPT processing route to improve the CNT distribution is a combination of deformation at 400°C with subsequent deformation at RT.

Carbon Nanotube (CNT)-Reinforced Metal Matrix Bulk Composites: Manufacturing and Evaluation

This chapter deals with the blending and processing methods of CNT-reinforced metal matrix bulk composites (Al/CNT, Cu/CNT and Ni/CNT) in terms of solid-state processing, referring mainly to the research works of the last ten years in this research field. The main methods are depicted in a brief way, and the pros and cons of each method are discussed. Furthermore, a tabular summary of the research work of the mentioned three systems is given, including the blending methods, sintering meth‐ ods, the used amount of CNTs and the finally achieved relative density of the composite. Finally, a brief discussion of each system is attached, which deals with the distribu‐ tion and interaction of the CNTs with the matrix material.

High Chromium Cast Irons: Destabilized-Subcritical Secondary Carbide Precipitation and Its Effect on Hardness and Wear Properties

This work evaluates the effect of a destabilization treatment combined with a subcritical diffusion (SCD) and a subsequent quenching (Q) steps on precipitation of secondary carbides and their influence on the wear properties of HCCI (16%Cr). The destabilization of the austenite at high temperature leads to a final microstructure composed of eutectic and secondary carbides, with an M7C3 nature, embedded in a martensitic matrix. An improved wear resistance was observed in the SCD + Q samples in comparison with the Q one, which was attributed to the size of secondary carbides.

Study of the structural defects on carbon nanotubes in metal matrix composites processed by severe plastic deformation

Carbon nanotubes (CNT) have been recently proposed as stabilizers against grain growth that can happen even at low temperature inputs in nano-crystalline and ultrafine-grained materials obtained by severe plastic deformation. In this study, we analyzed the evolution of the structural defects on the nanotubes in CNT-reinforced nickel matrix composites with different reinforcement weight fractions. The composites were processed by high pressure torsion, and we used Raman spectroscopy as the main characterization technique. The results indicate that for CNT subjected to highly energetic processing, it is not sufficient to analyze only the ID/IG ratio (as proposed in the available literature), but it is also necessary to evaluate the shifting of the G band, which traces the amorphization trajectory undergone by the CNT. Furthermore, we observed that the deformation suffered by the CNT is related to the accumulated strain and varies with the partial CNT fractions of these composites. This is related to their capacity to withstand the plastic strain that occurs during deformation. In addition, the defective state reaches a saturation before achieving the saturation in the microstructural refinement. These results will help to efficiently optimize the processing of this type of engineering composites.

Upcycling spent petroleum cracking catalyst: pulsed laser deposition of single-wall carbon nanotubes and silica nanowires

Fluid catalytic cracking (FCC), which currently accounts for half of the worldwide petroleum refining efforts, relies on catalytic, aluminosilicate zeolite particles which slowly deactivate. As of yet, this FCC catalyst residue (FC3R) has no commercial outlet, resulting in abundant amounts of landfill-destined refuse. However, this overlooked waste has the right ingredients for the synthesis of some of todays emerging nanomaterials. High-carbon FC3R, sourced from a Uruguayan refinery, was identified as faujasite particles encased in graphitic carbon shells. We show that pulsed laser ablation of raw FC3R produces simultaneous deposition of single-wall carbon nanotubes and silica nanowires through vapour/solid–liquid–solid self-assembly in distinct zones of an oven-laser apparatus. This is an extreme revalorisation and provides a new untapped resource for research and applications in C- and Si-based nanomaterials and mesoscopic physics.

Influence of Surface Design on the Solid Lubricity of Carbon Nanotubes-Coated Steel Surfaces

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-disk 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.

Influence of Processing Parameters on the Mechanical Properties of HPT-Deformed Nickel/Carbon Nanotube Composites

Nickel carbon nanotube composites with varying amounts of carbon nanotubes are deformed by high pressure torsion at different deformation temperatures to high strains, where no further refinement of the Nickel matrix microstructure is observed. Mean Nickel grain sizes increase with increasing HPT deformation temperature, while the size of the carbon nanotube agglomerates is significantly reduced. Additionally, the distribution of the agglomerates in the metal matrix becomes more homogenous. To investigate the mechanical performance of the HPT deformed composites, uniaxial tensile and compression tests are conducted. Depending on the HPT deformation temperature and the resulting microstructure, either brittle or ductile fracture occurs. Increased HPT deformation temperatures induce a decrease in the anisotropy of the mechanical properties, mainly caused by a shrinking of the carbon nanotube agglomerates. It is shown, that tuning the HPT deformation temperature is the key for optimizing both the microstructure and the mechanical performance.