Manel Rodríguez Ripoll

Friction and Lifetime of Laser Surface–Textured and MoS2-Coated Ti6Al4V Under Dry Reciprocating Sliding

The use of lasers for creating defined textured patterns on surfaces has steadily gained attention during the past decade. These textures can contribute to friction reduction by acting as a reservoir for lubricant, hydrodynamic bearing and trap for wear debris. In the present work, titanium alloy surfaces were textured using a nanosecond pulsed laser and subsequently coated with MoS2. The samples were tested under dry reciprocating sliding conditions using two different oscillation amplitudes. During the test, the evolution of the coefficient of friction as a function of the number of cycles was measured until coating failure. The influence of the dimple distance on friction and lifetime was evaluated and verified by post-mortem analyses of the samples using optical, confocal and SEM microscopy as well as EDS and XPS analyses. The results show that under certain conditions, surface texturing can reduce friction, extend the lifetime of the coating and provide a progressive coating degradation until failure.

Tribological behaviour of self-lubricating materials at high temperatures

ABSTRACT Self-lubricating materials are becoming more widespread in fields like metal forming or power generation due to the inability to use conventional lubricants in high-temperature (HT) applications. In an effort to summarise the progress done in this field, a detailed literature review has been carried out, ranging from micron-thickness thin films to hardfacings and bulk materials, and classified by the reported solid lubricants. Moreover, the most-cited deposition techniques have been reviewed for each lubricant class in addition to their advantages and limitations. HT friction and wear data for self-lubricating materials have also been examined in order to identify effective lubrication ranges and general trends in their tribological behaviour, which is expected to be useful for researchers interested in this field. Finally, several apparent research gaps have been described, with suggestions for new experimental work that could lead to the development of new high-temperature self-lubricating materials.

Interfacial Microstructure Evolution Due to Strain Path Changes in Sliding Contacts

We performed large-scale molecular dynamics (MD) simulations to study the transient softening stage that has been observed experimentally in sliding interfaces subject to strain path changes. The occurrence of this effect can be of crucial importance for the energy efficiency and wear resistance of systems that experience changes in the sliding direction, such as bearings or gears in wind parks, piston rings in combustion engines, or wheel-rail contacts for portal cranes. We therefore modeled the sliding of a rough counterbody against two polycrystalline substrates of face-centered cubic (fcc) copper and body-centered cubic (bcc) iron with initial near-surface grain sizes of 40 nm. The microstructural development of these substrates was monitored and quantified as a function of time, depth, and applied pressure during unidirectional sliding for 7 ns. The results were then compared to the case of sliding in one direction for 5 ns and reversing the sliding direction for an additional 2 ns. We observed the generation of partial dislocations, grain refinement, and rotation as well as twinning (for fcc) in the near-surface region. All microstructures were increasingly affected by these processes when maintaining the sliding direction but recovered to a great extent upon sliding reversal up to applied pressures of 0.4 GPa in the case of fcc Cu and 1.5 GPa for bcc Fe. We discuss the applicability and limits of our polycrystalline MD model for reproducing well-known bulk phenomena such as the Bauschinger effect in interfacial processes.

The Role of Ferric Oxide Nanoparticles in Improving Lubricity and Tribo-Electrochemical Performance During Chemical–Mechanical Polishing

The role of ferric oxide nanoparticles on the lubricating characteristics of passivating films formed on stainless steel (SS) was discussed in this study. The tribo-electrochemical behavior of mirror-like polished AISI 304 SS, used as an exemplary material, was evaluated in various electrolytes by means of a simulated chemical–mechanical polishing process in laboratory scale. It was clearly demonstrated that a suitable combination of abrasives (ferric oxide nanoparticles) and an oxidizer (nitric acid) can act as an effective lubricant that lowers the friction and wear of the AISI 304 SS surfaces. The excellent lubricating and anti-corrosion properties shown by a slurry containing a high content of ferric oxide nanoparticles at high nitric acid concentrations were attributed to the formation of a stable and robust passive film that was composed of chromium oxide and a mixture of iron oxides. The lack of ferric oxide nanoparticles in two solutions containing nitric acid of different concentrations led to pitting corrosion and abrasive wear. When low concentrations of both ferric oxide nanoparticles and nitric acid were used, wear-accelerated corrosion became the dominant mechanism that was caused by the presence of third-body wear particles in the contact zone.

Close-to-Reality Sliding-Corrosion Test-Rig for Oilfield Application

Sliding-corrosion phenomena play a crucial role in reducing the lifetime of tubings in the oil production industry. The aim of the present work is to develop and apply a close-to-reality test rig to describe the tribological performance of low-alloyed Carbon-steel (C-steel) under sliding-corrosion conditions in O2-free environment. The proposed test rig is highly relevant for the oil production industry and has been designed at the Austrian Competence Center for Tribology (AC²T). The benefit of this equipment is that it allows the setting of the CO2 atmosphere at a certain partial pressure while simultaneously varying the pH-value, Cl− ion concentration, and temperature of the electrolyte solution. Pure corrosion phenomena are investigated within a designed coupon box where conditions of steam, electrolyte injection, static, and dynamic flow are controlled. In addition to the corrosive environment, a tribological reciprocating sliding contact is implemented between tubing segments and coupling samples. Within this study, a commercially available low-alloyed C-steel was selected as base material for the tubing, which was tribologically stressed under reciprocating sliding contact against spray metal coated couplings. The results show a high influence of the steel microstructure on the performance of low-alloyed C-steel tubings under sliding-corrosion. Additionally, the dominant damage mechanisms that are observed in real field applications could be successfully reproduced by experimental simulation using this newly designed close-to-reality test rig.ZusammenfassungIn der Ölfeldindustrie bewirken die Gleitkorrosionsmechanismen eine Verringerung der Lebensdauer der Steigrohre bei Bohrlöchern. Das Ziel dieser Arbeit ist es, das Tribokorrosionsverhalten von niederlegierten Kohlenstoffstählen (C-Stähle) in sauerstofffreier Umgebung mit dem Gleitkorrosionsprüfstand zu beschreiben. Der nachstehende Teststand hat eine sehr hohe Wichtigkeit für die Ölindustrie und wurde am Österreichischen Kompetenzzentrum für Tribologie (AC²T research GmbH) entwickelt. Der Vorteil dieses Teststandes ist es, einen Überdruck der CO2 Atmosphäre realisieren zu können und zusätzlich unterschiedliche Parameter wie pH-Werte, Cl Ionenkonzentration, usw. zu messen. Einflüsse auf die Korrosion wie zum Beispiel Flüssigkeitsbewegung, Dampfkorrosion, usw. können in einer Korrosionsteststandzelle kontrolliert eingestellt und bestimmt werden. In der Gleitkorrosionsteststandzelle werden zusätzlich zu den korrosiven Bedingungen ein Gleitkontakt zwischen Muffe und Steigrohr realisiert. Die Ergebnisse zeigen einen hohen Einfluss der Gefügestruktur auf das Verhalten und Lebensdauer der Steigrohre unter Gleitkorrosion. Die dominanten Schädigungsmechanismen im Ölfeld konnten mit dem entwickelten Gleitkorrosionsprüfstand gut nachgebildet werden und stellen dabei nicht nur einen wirtschaftlichen Vorteil für Unternehmenspartner dar, sondern bilden auch eine wichtige Wissensbasis für weitere Aktivitäten auf dem Gebiet der Gleitkorrosion.