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Porous materials have recently attracted significant attention in the aerospace and biomedical fields for addressing issues related to friction and wear. Porous materials are beneficial in applications where continuous lubrication is not feasible or for components that operate under extreme conditions, such as high speeds, elevated temperatures, and heavy loads. The pores can serve as reservoirs for liquid lubricants, which are gradually released during the operation of the components. The tribological properties of these materials depend on their porosity, the lubricants used, and any additional additives incorporated into the porous materials. This review article provides insight into common fabrication techniques for porous materials and examines their tribological performance for all three classes of materials—polymers, metals, and ceramics. Additionally, it discusses design criteria for porous self-lubricating materials by highlighting the critical properties of both the substrate and lubricants. [ABSTRACT FROM AUTHOR]

In this work, a self-lubricating composite was manufactured using a novel hybrid 3D printing/in situ spraying process that involved the printing of an acrylonitrile butadiene styrene (ABS) matrix using fused deposition modeling (FDM), along with the in situ spraying of alumina (Al2O3) and hexagonal boron nitride (hBN) reinforcements during 3D printing. The results revealed that the addition of the reinforcement induced an extensive formation of micropores throughout the ABS structure. Under tensile-loading conditions, the mechanical strength and cohesive interlayer bonding of the composites were diminished due to the presence of these micropores. However, under tribological conditions, the presence of the Al2O3 and hBN reinforcement improved the frictional resistance of ABS in extreme loading conditions. This improvement in frictional resistance was attributed to the ability of the Al2O3 reinforcement to support the external tribo-load and the shearing-like ability of hBN reinforcement during sliding. Collectively, this work provides novel insights into the possibility of designing tribologically robust ABS components through the addition of in situ-sprayed ceramic and solid-lubricant reinforcements. [ABSTRACT FROM AUTHOR]

The present work reports the utilization of laser treatment for the structuring of a B2O3 coating on ceramic substrates. The two ceramic substrates, pure Al2O3, and Al2O3-20wt% B2O3, were prepared in-house using cold pressing and sintering route. The coatings were prepared by applying the ethanol-B2O3 slurry on the ceramic substrate using a dip coater followed by drying at 100 °C. The dried coatings were subjected to the continuous laser at 22, 29, and 36 W of laser power to achieve the final coating. Coating characterization by X-ray diffraction (XRD) revealed the importance of laser power on controlling the phases of B2O3, which are H3BO3, HBO2, and amorphous B2O3. The crystallinity index (CI) of the coatings calculated using XRD spectrum decreased with an increase in laser power. The coating processed at 36 W showed the lowest CI that reveals higher amorphous B2O3 content. Sliding tests conducted in a controlled humid environment at three sliding velocities suggested a direct relationship between tribological performance and CI value—coating with lower CI value provided consistent lower COF. Additionally, a strong influence of the substrate on tribological performance was also observed—the coatings on the Al2O3-20wt%B2O3 substrate performed superior compared to the coating on the pure Al2O3. The effect of substrates is discussed in terms of thermal conductivity to achieve higher amorphous B2O3 content and the initial surface roughness of the substrates. [ABSTRACT FROM AUTHOR]

• Diverse surface modification techniques for Mg and its alloys are introduced. • Mechanical, tribological and corrosion behavior of thermal sprayed coatings are presented. • Benefits and restrictions of the cold spray (CS) process are introduced. • The latest development of cold spray (CS) process for industrial applications is presented. • Bonding mechanism at the Mg alloy/cold sprayed layer interface is introduced. Potential engineering applications of magnesium (Mg) and Mg-based alloys, as the lightest structural metal, have made them a popular subject of study. However, the inferior corrosion and wear characteristics significantly limit their application range. It is widely recognized that surface treatment is the most commonly utilized technique for remarkably improving a substrate's surface characteristics. Numerous methods have been introduced for the surface treatment of Mg and Mg-based alloys to improve their corrosion behavior and tribological performance. Among these, thermal spray (TS) technology provides several methods for deposition of various functional metallic, ceramic, cermet, or other coatings tailored to particular conditions. Recent researches have shown the tremendous potential for thermal spray coated Mg alloys for biomedical and industrial applications. In this context, the cold spray (CS) method, as a comparatively new TS coating technique, can generate the coating layer using kinetic energy rather than combined thermal and kinetic energies, like the high-velocity oxy-fuel (HVOF) spray method. Moreover, the CS process, as a revolutionary method, is able to repair and refurbish with a faster turnaround time; it also provides solutions that do not require dealing with the thermal stresses that are part of the other repair processes, such as welding or other TS processes using a high-temperature flame. In this review paper, the recently designed coatings that are specifically applied to Mg alloys (primarily for industrial applications) employing various coating processes are reviewed. Because of the increased utilization of CS technology for both 3D printed (additively manufactured) coatings and repair of structurally critical components, the most recent CS methods for the surface treatment, repair, and refurbishment of Mg alloys as well as their benefits and restrictions are then discussed and reviewed in detail. Lastly, the prospects of this field of study are briefly discussed, along with a summary of the presented work. [ABSTRACT FROM AUTHOR]

The eventual material degradation of steel components in bio-implant, marine, and hightemperature applications is a critical issue that can have widespread negative ramifications from a safety and economic point of view. Stemming from their tribological, corrosion, and erosionbased properties, there is an increasing need to address these issues effectively. As one solution, surface processing techniques have been proposed to improve these properties. However, common techniques tend to suffer from issues spanning from their practicality to their high costs and negative environmental impacts. To address these issues, friction-stir-processing (FSP) has been one technique that has been increasingly utilized due to its cost effective, non-polluting nature. By inducing large amounts of strain and plastic deformation, dynamic recrystallization occurs which can largely influence the tribological, corrosion, and erosion properties via surface hardening, grain refinement, and improvement to passive layer formation. This review aims to accumulate the current knowledge of steel FSP and to breakdown the key factors which enable its metallurgical improvement. Having this understanding, a thorough analysis of these processing variables in relation to their tribological, corrosion, and erosion properties is presented. We finally then prospect future directions for this research with suggestions on how this research can continue to expand. [ABSTRACT FROM AUTHOR]

The tribological behavior of electrical contacts, especially separable type electrical connectors at low contact loads, are considered. The reliability of these connectors has been a major concern due to the fretting phenomenon that can lead to an unacceptable increase in contact resistance. This study analyzes various aspects of the fretting mechanism from a tribological perspective where friction and wear are the primary cause of degradation in electrical components. With the use of precise tribological equipment (high data acquisition rate of 5000 Hz), the electrical contact resistance and coefficient of friction at the contact interface are measured. The measurements were made in-situ for a simulated fretting environment under various constant loading conditions. It was observed that low contact loads (1 N) and low fretting frequency (1 Hz) leads to a high degree of fluctuation in the coefficient of friction. However, for the same conditions, the lowest wear rate and electrical contact resistance were observed. The reason behind this could be due to the lack of continuous electrical contact and a high degree of fretting frequency under low contact loads, ultimately leading to extended periods of an open circuit. Experimental analysis indicates the existence of an optimum loading condition at which the fretting wear effect is at its minimum. Detailed analysis of post fretting surface roughness, coating wear, and wear debris is conducted, as well as transfer film formations to explain the mechanism of fretting observed. [ABSTRACT FROM AUTHOR]

Abstract Both surface texture and multiphase lubricants (additives in base oil) influence the friction and wear performance during sliding. However, the synergy of their interaction is still unknown. In the present investigation, an effort is made to understand the influence of environmental friendly multiphase lubricants on the friction and transfer layer formation during sliding against various surface textures. In this study, unidirectional surface textures were created and sliding tests were conducted against various grinding angles of the unidirectional texture. The sliding tests were conducted in multiphase lubricants consisting of canola oil (biolubricant) and nano-additives (graphene) at various concentrations. The results showed that the coefficient of friction (COF) and transfer layer formation was influenced by the grinding angle. However, it was found that this influence can be minimized by using an optimum concentration of graphene in canola oil. A minimum COF of 0.05 ± 0.01 could be achieved for various grinding angles by utilizing a specific concentration of graphene. It was found that a high concentration of Gr (3 wt%) was required when sliding at 0° and a low concentration of Gr (1 wt%) was required when sliding at 45° to achieve the minimum COF. This was attributed to the variations in asperity and Gr additive interaction when sliding at different grinding angles. The present study details the observed friction and transfer layer formation results and analyzes the mechanism of multiphase lubrication during sliding at various grinding angles. An asperity interaction model for sliding against various grinding angles and in a multiphase lubricant with varying additive concentrations has also been presented. Highlights • Multiphase lubricants (MPLs) can minimize the effect of surface texture on friction. • Minimum friction can be obtained using optimum concentration of Gr in MPLs. • Required optimal concentration of Gr varied as a function of the grinding angle. • Mechanism of asperity and Gr additive interaction was the key for low friction. • Bio-based MPLs have potential as green alternatives for petroleum-based lubricants. [ABSTRACT FROM AUTHOR]

For the first time, Zr702 coatings were deposited onto an Al6061 alloy using a high-pressure cold spray (HPCS) system. In this work, five different N2 process gas temperatures between 700 and 1100 °C were employed to understand the formation of cold sprayed (CS) Zr coatings and their feasibility for enhanced wear resistance. Results indicated that the N2 processing gas temperature of about 1100 °C enabled a higher degree of particle thermal softening, which created a dense, robust, oxide- and defect-free Zr coating. Across all CS Zr coatings, there was a refinement of crystallinity, which was attributed to the severe localized plastic deformation of the powder particles. The enhanced thermal boost up zone at the inter-particle boundaries and decreased recoverable elastic strain were accountable for the inter-particle bonding of the coatings at higher process gas temperatures. The flattening ratio (ε) increased as a function of temperature, implying that there was a greater degree of plastic deformation at higher N2 gas temperatures. The microhardness readings and wear volume of the coatings were also improved as a function of process gas temperature. In this work, the wear of the Al6061 alloy substrate was mainly plowing-based, whereas the Zr CS substrates demonstrated a gradual change of abrasive to adhesive wear. From our findings, the preparation of CS Zr coatings was a feasible method of enhancing the wear resistance of Al-based alloys. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) are composed of multiple elements with equimolar or near equimolar composition that have superior mechanical and tribological properties. In this article, we present a review on the tribological performance of HEAs. The tribological properties of different HEAs systems have been evaluated, and it has been found that the wear rate strongly depends on the crystal structure of the phases. The most common structures are face-centered cubic (FCC), body-centered cubic (BCC), and dual-phase (FCC + BCC) alloys due to the high entropy of mixing instead of forming intermetallic phases. In general, HEAs with a BCC structure showed superior hardness and wear properties compared to FCC and FCC + BCC alloys. The lesser wear rate of HEAs with a BCC structure is attributed to the reductions in ductility, resulting in strong but brittle alloys. In addition to the crystal structure, the effect of temperature on the tribological performance of the HEAs is also discussed, which highlights their potential applications for high temperatures. Moreover, various other factors such as grain size, formation of an oxide layer, and wear mechanisms are discussed. [ABSTRACT FROM AUTHOR]

In the present work, the effect of graphite nanoplatelets (GNPs) on tribological properties of the aluminum (Al), and Al/alumina (Al2O3) composite are studied. GNPs are multilayer graphene sheets which were used as a solid lubricant material. Two sets of composites, Al/GNPs and Al/GNPs/Al2O3 with varying amounts of reinforcements, were synthesized by powder metallurgy that involves cold compaction followed by hot compaction. The hardness of the composites increased with the addition of GNPs and Al2O3. The Al/GNPs composite with 1 wt.% of GNPs (Al/1GNPs) showed a 20% increase in hardness whereas Al/GNPs/ Al2O3 composite with 1 wt.% GNPs and 2 wt.% Al2O3 (Al/1GNPs/2Al2O3) showed 27% increases in hardness compared to the pure Al. The coefficient of friction measured at 20 N was observed to be 22% and 53% lesser for Al/1GNPs and Al/1GNPs/2Al2O3, respectively, compared to corresponding alloys without graphene Al. The X-ray diffraction and scanning electron microscopy analysis revealed the presence of GNPs at the worn surface after the tribology tests. The wear rate was also reduced significantly. In comparison with pure Al, the Al/1GNPs and Al/1GNPs/2Al2O3 composites resulted in 5- and 20-times lesser wear rate, respectively. The addition of Al2O3 caused reduction in wear rate due to higher hardness and load carrying ability, whereas composites with more than 1 wt.% GNPs showed higher wear rate due to lower hardness and higher porosity. The Al/1GNPs/2Al2O3 composite exhibited the least coefficient of friction (0.2–0.25) and wear rate (1 × 10−6–4 × 10−6 mm3/N.m) compared to other GNPs and Al2O3 reinforced Al composites. The worn surfaces were further analyzed to understand the wear mechanism by Raman spectroscopy, transmission electron microscopy, and x-ray diffraction to detect the Al4C3 phase formation, chemical bonding, and defect formation in graphene. [ABSTRACT FROM AUTHOR]

High-temperature liquid lubricants are often composed of petroleum-based oils. Recently, the lubrication industry has begun to shift towards environmentally benign lubricants to reduce the use of toxic petroleum-based oils. As a result, there has been a resurgence in the use of bio-based lubricants. These "biolubricants" consist of vegetable oils, synthetic ethers, and polyalkyleneglycols (PAGs) for use as high-temperature lubricants. The largest drawbacks to biolubricants are neither their toxicity nor their lubricity, but rather their thermal instability. Room-temperature ionic liquids (RTIL) are emerging biolubricants that has the potential to replace conventional lubricants at high temperatures. In this investigation 1,3-diakylimidazolium and trihexyl(tetradecyl)phosphonium cations are combined with carboxylate anions to create eco-friendly lubricants for high-temperature applications. Here, these RTILs are compared with bio-based and petroleum-based oils to evaluate their tribological properties through pin-on-disk testing and thermal stability via thermogravimetric analysis (TGA). Additionally, the influence of differing alkyl chain imidazolium cations of the 1-decyl through 1-allyl equivalents and various shape and sized anions are studied. Results indicate that the eco-friendly RTILs exhibit the lowest friction and wear properties as well as superior thermal stability because of their lamellar-like liquid crystal structure, electrostatic potential dipolar, and molecular adsorption layer. • Imidazolium and phosphonium ionic liquids (ILs) are considered for high temperature application. • Aromatic carboxylic anions improves tribological performance of ILs. • Eco-friendly phosphonium-based ILs with carboxylate anion outperformed bio-based and petroleum-based oils. • Thermal stability of ILs evaluated by thermal gravimetric analysis. • Eco-friendly phosphonium-based ILs showed lower onsets of thermal degradation temperature. [ABSTRACT FROM AUTHOR]

In this study, a compact cold sprayed (CS) Ti coating was deposited on Mg alloy using a high pressure cold spray (HPCS) system. The wear and corrosion behavior of the CS Ti coating was compared with that of CS Al coating and bare Mg alloy. The Ti coating yielded lower wear rate compared to Al coating and Mg alloy. Electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic polarization (CPP) tests revealed that CS Ti coating can substantially reduce corrosion rate of AZ31B in chloride containing solutions compared to CS Al coating. Interestingly, Ti-coated Mg alloy demonstrated negative hysteresis loop, depicting repassivation of pits, in contrast to AZ31B and Al-coated AZ31B with positive hysteresis loops where corrosion potential (Ecorr) > repassivation potential (Erp); indicating irreversible growth of pits. AZ31B and Al-coated AZ31B were most susceptible to pitting corrosion, while Ti-coated Mg alloy indicated noticeable resistance to pitting in 3.5 wt % NaCl solution. In comparison to Al coating, Ti coating considerably separated the AZ31BMg alloy surface from the corrosive electrolyte during long term immersion test for 11 days. [ABSTRACT FROM AUTHOR]

Wear and friction properties of Al2O3 composite reinforced with in-situ formed aluminum borate (9Al2O3·2B2O3) and hexa-boron nitride (h-BN) have been investigated. The initial constituents for the composites were Al2O3, AlN, and H3BO3. The H3BO3 was used as a source of B2O3, where B2O3 reacted with AlN and Al2O3 to form in-situ h-BN and 9Al2O3·2B2O3. Based on the thermodynamic calculation and phase transformation, four different compositions were selected. First, the powders were mixed by ball milling followed by compaction at 10 MPa. The compacted pellets were sintered at 1400 °C in vacuum. The composites were characterized using X-ray diffraction followed by hardness measurement and reciprocating sliding test against alumina and steel balls. The X-ray diffraction results revealed the formation of in situ phases of 9Al2O3·2B2O3 and h-BN that improved the tribological properties. By comparing the tribological performance of different composites, it was found that the hard 9Al2O3·2B2O3 phase maintains the wear resistance of composites, whereas the coefficient of friction is highly dependent on the counter ball. Against alumina ball, the lowest coefficient of friction was observed for the composites with maximum h-BN concentration and minimum aluminum borate concentration, whereas the opposite trend was observed against the steel ball. [ABSTRACT FROM AUTHOR]

Lightweight high-strength metal matrix nano-composites (MMNCs) can be used in a wide variety of applications, e.g., aerospace, automotive, and biomedical engineering, owing to their sustainability, increased specific strength/stiffness, enhanced elevated temperature strength, improved wear, or corrosion resistance. A metallic matrix, commonly comprising of light aluminum or magnesium alloys, can be significantly strengthened even by very low weight fractions (~1 wt%) of well-dispersed nanoparticles. This review discusses the recent advancements in the fabrication of metal matrix nanocomposites starting with manufacturing routes and different nanoparticles, intricacies of the underlying physics, and the mechanisms of particle dispersion in a particle-metal composite system. Thereafter, the microstructural influences of the nanoparticles on the composite system are outlined and the theory of the strengthening mechanisms is also explained. Finally, microstructural, mechanical, and tribological properties of the selected MMNCs are discussed as well. [ABSTRACT FROM AUTHOR]

The present study investigates the influence of laser shock peening (LSP) on surface energy (S E) of AZ31B Magnesium alloy and its resilience to corrosion and tribocorrosion. AZ31B alloy was treated at different laser intensities. The S E and its interfacial components of treated surfaces were analyzed. The S E was found to be least at low laser intensity LSP. Lower interfacial S E showed decreased wettability, providing enhanced tribocorrosion resistance with respect to untreated surface. However, higher laser intensities increased the surface roughening effect, causing an increase in the interfacial S E and wettability of the surfaces, decreasing the tribocorrosion resistance. The study finds LSP can enhance tribological properties and mitigate the effects of corrosion and tribocorrosion. • Increase in laser intensity increases the surface roughness and surface area. • Increase in surface roughness due to LSP changes the interfacial surface energy (S E). • Solid-liquid interfacial S E component affects tribocorrosion. • Low laser LSP increases the contact angle and thus the tribocorrosion resistance. [ABSTRACT FROM AUTHOR]

The application of tribology is not just limited to mechanical components of engineering systems. As a matter of fact, the understanding of friction and wear can be applied to everyday life. One of the important fields is skin tribology, as human skin interacts with various surfaces of different materials. This paper focuses on the friction behavior of the skin when in contact with the fabric and other materials in relative motion. The excessive friction at the fabric-skin interface may lead to discomfort, blistering, chafing, and pressure ulcers especially in athletes who experience higher friction due to rapid movement for an extended period. Other than understanding the fabric properties, it is equally important to understand the structure and properties of the skin to evaluate its function and interaction with the different fabric materials. The identification of the contributing factors of skin friction can help to design suitable fabric materials. An overview of skin functions and the factors that affect the friction on the skin-textile material interface are presented in this review article.