Over the years, fretting has been a leading cause of fuel failures for water cooled nuclear reactors. Implementation of liquid lead coolant increases the need for fretting and corrosion-resistant materials. Fe–10Cr–4Al alloy has shown good resistance to thermal aging, oxidation, liquid metal corrosion, and embrittlement in liquid lead environment. The performance of Fe-10Cr-4Al under fretting wear conditions has not been studied previously. This work aims to investigate the fretting wear and friction behaviour of self-mated Fe-10Cr-4Al alloy at varying temperatures, in the presence of a reducing gas environment and liquid lead. The results show that an increase in test temperature from RT to 550 °C decreases the coefficient of friction and promotes the formation of Cr and Fe oxide third body layer. At high temperature in reducing gas atmosphere, the initial plastic deformation and severe adhesion leads to early seizure. Presence of liquid lead has a lubricating effect and allows a third body layer to form on worn surfaces. A detailed description of the initial wear mechanisms of Fe-10Cr-4Al alloy and friction levels under various conditions provides a base for further investigation of this alloy for long-duration fretting wear.

There is a great potential in using different techniques of additive manufacturing (AM) for the production and refurbishment of hot forming dies. In hot stamping of AlSi-coated boron steel, the dies are exposed to a complex combination of abrasive and adhesive wear. This results in tool damage and eventually tool refurbishment is necessary. Laser-metal-deposition (LMD) is an AM technique suitable for tool repairing due to its easier implementation, compared to selective-laser-melting (SLM). However, tribological studies on AM produced tools are still limited, particularly in the context of hot forming. Thus, the aim of this work is to increase the knowledge on the friction and wear behavior of LMD tool materials in hot stamping conditions. Two LMD materials were investigated: a hot-work tool steel (LMD-TS) and a conceptual high-hardness tool material (LMD-HH). Porosity was observed for both materials in the form of lack-of-fusion defects. LMD-TS resulted in a Vickers hardness value of approximately 650 HV, while LMD-HH showed a value of approximately 1100 HV. A high temperature strip drawing tribometer was used to perform friction and wear tests against AlSi-coated boron steel under unidirectional (linear) sliding conditions. Workpiece temperatures during sliding were 600 °C and 700 °C. All tribotests at 700 °C resulted in severe adhesion and AlSi-coating rupture, resulting in higher and more unstable friction. The wear mechanisms observed for LMD-TS were a combination of abrasive and adhesive wear, where ploughing and grooving in the tool material resulted in larger surface defects to accumulate material transfer. At 700 °C, these mechanisms were more severe. LMD-HH did not show significant abrasive damage in any test; the resulting material transfer was thinner and sparser compared to the other LMD material. The change in FeAlSi material transfer for LMD-HH was associated with the reduction in abrasive damage, i.e., ploughing and grooving in the tool material did not take place and thus did not enhance material transfer. The LMD process itself did not seem to have either a positive or a negative effect on the tribological behavior of the tool materials in this tribosystem, and the differences are seen mostly from the hardness perspective. LMD-HH is a promising tool material to reduce wear in hot stamping dies.

Driven by the potential applications of ionic liquid (IL) flow for charging graphene-based surfaces in many emerging technologies, recent research efforts have focused on understanding ion dynamics and structuring at IL–graphene interfaces. Here, graphene colloid probe (GrP) atomic force microscopy (AFM) was used to probe the dynamics and ion structuring of 1-butyl-3-methylimidazolium tetrafluoroborate at graphene surfaces under various bias voltages. In particular, the AFM-measured nanofriction provides a good measure of the dynamic properties of the ILs at graphene surfaces. Compared with the IL at the unbiased graphene surface (0 V), the charged graphene surfaces with either negative (–1, –2 V) or positive (+1, +2 V) voltages favor a reduction in the friction coefficient by the IL. A higher magnitude of the bias voltage applied on the graphene surface with either sign (–2 or +2 V) results in a smaller friction coefficient than that at –1 and +1 V. In combination with the AFM-probed contact stiffness, adhesion forces, and ion structuring force curves with an ion orientational distribution according to molecular dynamics (MD) simulations, we discovered that the unbiased graphene surface (0 V) possesses randomly structured IL ions and that the graphene colloid probe is more likely to become stuck, resulting in more energy dissipation to contribute to a larger friction coefficient. Biasing of the graphene surface under either negative or positive voltages resulted in uniformly arranged ions, which produced a more ordered ion structure and, thus, a smoother sliding plane to reduce the friction coefficient. Electrochemical impedance spectroscopy (EIS) for the IL with graphene as an electrode demonstrated a greater ionic conductivity in the IL paired with the biased graphene than in the unbiased one, implying faster ion movement at the charged graphene, which is beneficial for reducing the friction coefficient.

With growing environmental concerns and resource depletion, developing green lubricants is essential. This study presents a lignosulfonate (LS)-based hydrogel grease, synthesized via a one-step, eco-friendly process at room temperature. Ammonium persulfate (APS) initiated polymerization and crosslinking, with ferric chloride (FeCl₃) as a secondary crosslinker. The 15 % LS hydrogel reduced the coefficient of friction (COF) by 37.5 % and wear volume by 31.25 % compared to the 0 % LS sample. A chemically adsorbed tribofilm, rich in oxygen and sulfur, formed on worn surfaces, enhancing lubrication and wear resistance. This study highlights LS hydrogels as promising green lubricants, offering improved tribological performance and environmental sustainability.

Traditional lubricating greases are mainly derived from petroleum, which poses major environmental challenges due to their non-biodegradability and pollution issues. This study attempts to synthesize lignin-based green thickeners and explore their potential for developing green and fossil-free greases. A lignin-based gelator was successfully synthesized by reacting malic acid with lignin and epoxidized soybean oil, in which malic acid participated in both the esterification reaction and the ring-opening of the epoxy group. This synthesized gelator was used as the thickener to prepare greases with several different oils, e.g., castor oil, epoxidized soybean oil, rapeseed oil, PAO 15, and paraffin oil. It was found that combining this lignin-based gelator with castor oil and epoxidized soybean oil can be successfully used for preparing greases, while it does not work with other oils. Rheological studied showed that the 35 % gelator-castor oil grease exhibited strong gel-like behaviour, with storage modulus (G', 400 Pa) exceeding loss modulus (G", 40 Pa) and shear-thinning viscosity reducing from 106 to 104 mPa·s under stress. Comprehensive tribological studies on the developed greases show that lignin-based gelators significantly improve lubricant performance (about 20 % lower friction and around 40 % lower wear under a contact pressure of 2.72 GPa, reciprocating speed 0.1 m/s, and 80 °C). XPS analysis further revealed the ability of the developed grease to form a protective film on metal surfaces. This study demonstrates the great potential of lignin as a green thickener and provides new ideas for developing high-performance, green, and fossil-free greases.

Additive manufacturing (AM) of ferrous alloys has achieved a technological maturity level that allows for the production of high-performance steel components. Among these alloys, tool steels and maraging steels can be used in die manufacturing for different hot forming processes as both materials have good mechanical properties and thermal stability. In recent years, several works have successfully produced these alloys through selective laser melting, focusing on the optimization and characterization of their microstructure and mechanical properties. However, few studies look into the tribological aspects of these newly produced materials and even fewer consider high temperature friction and wear performance. Therefore, there is a need to understand the high temperature tribological response of tool materials produced by additive manufacturing. In this context, the aim of this work is to investigate the tribological behavior of a tool steel and a maraging steel, both produced by selective laser melting, at different elevated temperatures. A conventionally produced tool steel was used as reference. A reciprocating sliding tribometer was used to perform tests at 40 °C, 200 °C and 400 °C. The counter-body was a WC-Co cemented carbide. Friction and wear of the AM tool steel and reference tool steel were very similar, thus no signs of the AM route having an impact on the tribological behavior were observed. Both tool steels showed reduced wear volume with increasing temperature, while the opposite was observed for their respective WC-Co counter-bodies. Higher temperature also resulted in increased amount of W transferred onto the tool steel wear scars. AM maraging steel showed higher and more unstable friction level, as well as a much larger wear volume at all temperatures; material transfer onto WC-Co was observed at certain temperatures. Overall, tribolayer formation (or lack thereof) was the dominant aspect in the tribological responses.

The increasing interest in liquid metal cooled nuclear reactors provides technical and scientific challenges such as the understanding, prevention, and prediction of the degradation of materials in liquid lead. Critical components include the fuel rods, heat exchanger tubes, and pump impellers. These functional elements are exposed to mechanical loading (up to 40 MPa), high temperatures (450–550 °C), and fluid-induced vibrations (up to 25 Hz). Under such conditions, fretting wear occurs between e.g., the spacer wire and the outer surface of the fuel or heat exchanger tubes. This work is aimed to establish a laboratory-scale fretting wear test setup and develop test methodology to enable systematic material characterisation in liquid metal environments. The results obtained by using the described methodology indicate that adhesive wear is the dominant degradation mechanism, and 316L stainless steel shows a higher coefficient of friction but a lower wear volume/tribolayer volume compared to 100Cr6 bearing steel. These results are in agreement with those reported in open literature and demonstrates the suitability of the presented method for conducting fretting tests and analysis for various materials and contact configurations in liquid lead environment.

Wear is a complex phenomenon taking place as two bodies in relative motion are brought into contact with each other. There are many different types of wear, for example, sliding, fretting, surface fatigue, and combinations thereof. Wear occurs over a wide range of scales, and it largely depends on the mechanical properties of the material. For instance, at the micro-scale, sliding wear is the result of material detachment that occurs due to fracture. An accurate numerical simulation of sliding wear requires a robust and efficient solver, based on a realistic fracture mechanics model that can handle large deformations. In the present work, a fully coupled thermo-mechanical and meshfree approach, based on the momentum-consistent smoothed particle Galerkin (MC-SPG) method, is adapted and employed to predict wear of colliding asperities. The MC-SPG-based approach is used to study how plastic deformation, thermal response, and wear are influenced by the variation of the vertical overlap between colliding spherical asperities. The findings demonstrate a critical overlap value where the wear mechanism transitions from plastic deformation to brittle fracture. In addition, the results reveal a linear relationship between the average temperature and the increasing overlap size, up until the critical overlap value. Beyond this critical point, the average temperature reaches a steady-state value.