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3D printing in the textile and fashion industry is a new emerging technology. Applications of 3D printing for designing clothes and other wearable accessories require tribological and biological understanding of 3D printing plastics against the complex human skin to mitigate skin-friction related ailments such as calluses and blisters. This study provides tribological insight in search of an optimal 3D printable material that has minimal friction against the skin. Two low friction 3D printable materials, thermoplastic polyurethane (TPU) and polyamide (TPA) were chosen and tribological testing was carried out against a water responsive skin model. The skin model was synthesized using a gelatine based model made with cotton and crosslinked with glutaraldehyde. Tribological testing of TPU/TPA against the skin model in dry and wet conditions were made. The higher coefficient of friction (COF) was observed in the wet condition compared to the dry condition. To overcome the higher friction, TPA/TPU-sodium polyacrylate composites were prepared by heat pressing that significantly reduced COF of TPU and TPA by ∼ 40% and 75%, respectively, in wet conditions.

The aim of this study was to develop a novel set of Al2O3-B2O3-CuO composites and evaluate their tribological performance at varying humidity (10–95% relative humidity) levels. First, the Al2O3-B2O3 composites were prepared using cold press sintering by varying the amount of B2O3 (5–20 wt%). The results revealed that an increase in B2O3 content in the composites increased the amount of aluminum borate in situ phase during sintering. The presence of the aluminum borate phase in the composite enhanced the hardness and wear resistance, whereas the humidity-sensitive alumina phase reduced friction at higher humidity levels. Next, CuO (5 wt%) was added to the Al2O3-B2O3 composites to form Al2O3-B2O3-CuO composites. These composites showed an increase in the percentage relative density by 16–37% and hardness by 1.2–1.9 times. Subsequently, the tribological performance was improved significantly. The underlying mechanism for improved wear resistance was discussed using the crystal-chemical approach and polarization theory to guide the design of these novel Al2O3-B2O3-CuO composites.

Fly ash (FA) is one of the particulate wastes generated during combustion of coal in thermal power plants. Around 110 million tons of FA is generated in the USA every year, and 60% of it is deposited in landfills. Utilization of FA can create value for this waste material and also help the environment. FA is essentially a mixture of metal oxides that can be used as a filler reinforcement in metal and polymer composites. FA as a filler material reduces the amount of metals and polymer, reducing embodied energy. Hollow FA particles, called cenospheres, can provide the advantage of low density in composites as well as higher hardness and strength. FA also reduces the coefficient of thermal expansion. This article provides a brief review to capture the state of the art on the mechanical and tribological behavior of composites reinforced with FA to identify the possible benefits of using this waste material. The corrosion performance of metal matrix FA composites is also explored. Future perspectives in this field are discussed based on the potential applications of FA-filled composites.

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.

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.

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.

This paper presents a review of recent research inventions in the field of supersonic particle deposition (SPD) additive manufacturing (AM) technology. The SPD, also known as cold spray, is a coating technique that has gained popularity recently because of its ability to apply multi-component coatings. The SPD has the potential to revolutionize the global parts manufacturing and logistics landscape. The state-of-the-art, rapidly emerging cold spray manufacturing technology is an alternative to traditional additive manufacturing (AM) based on powder melting. It enables the rapid fabrication of parts that have properties similar to the parts developed by conventional manufacturing. In this paper, the history and process of SPD are explained. A broad background of metallic SPD AM is provided. SPD parameters, including substrate and particle properties, are discussed. Common challenges in creating consistent SPD coatings as well as challenges specific to metal and ceramic SPD are discussed. This paper explores the material science, processes, and performance gain. Several applications of SPD, including nuclear, aerospace, and electrical industries, are discussed.

Triboluminescence is the spontaneous emission of light that results from the mechanical force applied to certain materials. This spontaneous light emission has been observed for hundreds of years. However, there are few practical applications of triboluminescence. Mechano-luminescence is a form of triboluminescence, and sometimes the terms are used interchangeably. Triboluminescence has the potential to use in many fields. This paper is a review of the history and mechanics of triboluminescence, and some applications are discussed. The types of triboluminescence covered in this paper include elastic-stress mechano-luminescence, plastic-stress mechano-luminescence, and fracture mechano-luminescence. Materials and methods in developing future applications have also been included. The research reviewed in this paper gives an insight into past and current advances in the field of triboluminescence and explains the potential for future applications.

In recent years, various solar cells have been developed that have involved silicon wafer-based systems, and various thin films. Solar cell efficiency highly depends on the surface characteristics of the solar cells. Other than efficiency, reflectivity and scattering depends on the surface parameters. In this review article, various synthesis techniques for solar cells are discussed that include vacuum-assisted and non-vacuum techniques. Surface characteristics, such as surface roughness and textures developed during synthesis or by chemical etching and lithography techniques, are discussed that affect the solar cell performance. The article highlights the importance of surface engineering for solar cells that can guide the building of multifunctional composite films for self-cleaning, antireflective, antifogging and invisibility cloak coating.

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.

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 × 10−6mm3/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.

Additive manufacturing (AM) by laser powder bed fusion (LPBF) has gained significant research attention to fabricate complex 3D Inconel alloy components for jet engines. The strategic advantages of LPBF-based AM to fabricate jet components for aerospace applications are well reported. The jet components are exposed to a high degree of vibration during the jet operation in a variable aqueous environment. The combined vibration and the aqueous environment create a tribological condition that can accelerate the failure mechanism. Therefore, it is critical to understand the tribocorrosion behavior of the Inconel alloy. In the present work, tribocorrosion behavior of the LPBF fabricated standalone coating of Inconel 718 in the 3.5% NaCl aqueous solution is presented. The LPBF fabricated samples are analyzed to determine the impact of porosity, generated as a result of LPBF, on the triobocorrosion behavior of AM Inconel 718. The study includes potentiodynamic tests, cathodic polarization, along with OCP measurements. The corrosive environment is found to increase the wear by 29.24% and 49.5% without the initiation of corrosion in the case of AM and wrought Inconel 718, respectively. A corrosion accelerated wear form of tribocorrosion is observed for Inconel 718. Additionally, the corrosive environment has a significant effect on wear even when the Inconel 718 surface is in equilibrium potential with the corrosive environment and no corrosion potential scan is applied. This study provides an insight into a critical aspect of the AM Inconel components.

A primary wear mechanism for sliding metal pairs is oxidative wear in which the passive oxide film is mechanically removed when two surfaces are rubbed together. The freshly exposed layer undergoes further oxidation, and the cycle continues. This mechanism of material and lubricant degradation is referred to as wear–corrosion synergism, and the phenomenon is termed tribocorrosion. The chapter introduces the process and mechanism of tribocorrosion followed by techniques available to measure and analyze tribocorrosion behaviors.

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) &gt, 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.

Wear and friction properties of Al2O3 composite reinforced with in-situ formed aluminum borate (9Al2O3&middot, 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&middot, 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 &deg, 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&middot, 2B2O3 and h-BN that improved the tribological properties. By comparing the tribological performance of different composites, it was found that the hard 9Al2O3&middot, 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.

Graphene is one of the most emerging material in the field of nanotechnology, and it is gaining a tremendous amount of research interest in recent years. Apart from its unique electronic, electrical, mechanical, and thermal properties, graphene also possesses lower wear rate and friction coefficient. The superior tribological properties of graphene have attracted many researchers to synthesize self-lubricating nanocomposite materials that can have several applications in the automobile, aerospace, and marine industries. Graphene is also proven a competitive nanomaterial as an additive in the lubricant oil because of stable suspension and improved anti-wear properties of the component. This article aims to present the review of the advancement in the field of graphene synthesis; tribological properties of graphene; the role of graphene as reinforcement in polymer-matrix, metal-matrix, and ceramic-matrix nanocomposites; and also graphene as an additive in lubricant oil. The results reveal that addition of graphene to the composites can lead to decrease both wear rate and coefficient of friction for all three class of materials. Moreover, when graphene used as a lubricant additive, it improves the load carrying capacity of lubricant oil along with decreased friction and wear. Performance of graphene-reinforced composites and graphene suspended lubricants depend on various tribological and materials parameters which are discussed in this article. The limitation of present knowledge and future research scope are also highlighted.

Composites have tremendous applicability due to their excellent capabilities. The performance of composites mainly depends on the reinforcing material applied. Graphene is successful as an efficient reinforcing material due to its versatile as well as superior properties. Even at very low content, graphene can dramatically improve the properties of polymer and metal matrix composites. This article reviews the fabrication followed by mechanical and tribological properties of metal and polymer matrix composites filled with different kinds of graphene, including single-layer, multilayer, and functionalized graphene. Results reported to date in literature indicate that functionalized graphene or graphene oxide-polymer composites are promising materials offering significantly improved strength and frictional properties. A similar trend of improved properties has been observed in case of graphene-metal matrix composites. However, achieving higher graphene loading with uniform dispersion in metal matrix composites remains a challenge. Although graphene-reinforced composites face some challenges, such as understanding the graphene-matrix interaction or fabrication techniques, graphene-reinforced polymer and metal matrix composites have great potential for application in various fields due to their outstanding properties.

Ionic liquids (ILs) represent a new class of green lubricants that exhibit superior lubrication performance compared to conventional lubricants. Further, solid particle additives, particularly lamellar solid particles to enhance the tribological properties of base oils, are well known. In the current investigation, various sizes, shapes, and types of lamellar solid particles and multi-walled carbon nanotubes were added in Phosphonium-based ILs. The tribological properties of these lubricant mixtures were evaluated using a pin-on-disk test. It was revealed that within ILs, particle size was independent of particle material, where the nano sized particles exhibited improved tribological properties. The influence of particulate additives was empirically modeled by a fractal function using a fractal dimensions based on the particle size and the surface roughness.

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.

Rock drilling performance of O1 tool steel was simulated at laboratory scale using mortar specimens. Mortar specimens were prepared with cement-to-sand ratio of 1:2 and cured for 7 and 30 days to simulate soft and hard rocks, respectively. Rock drilling performance of tool steel was measured in terms of corrosion, tribocorrosion, and wear of mortar specimens in the presence of 3.5 wt% NaCl solution. The tribocorrosion properties were measured by modification of a tribometer and adding a three-electrode setup where tool steel pin was used as a working electrode counterpart against mortar sample. The effect of mortar on corrosion behavior of tool steel was measured by potentiodynamic polarization tests, while the tribocorrosion was monitored by open-circuit potential tests. It has been found that the mortar specimens negatively affect the corrosion and tribocorrosion, and thus improve the corrosion performance of tool steel. This effect was more prominent against 7-day-cured mortar specimen compared to 30-day-cured specimen. Various factors, such as mechanical properties of rock material, cutting fluid, drilling mechanism, and dissolution of minerals, which can affect the tool life are discussed.

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.

The present study investigates the influence of laser shock peening (LSP) on surface energy (SE) 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 SE was found to be least at low laser intensity LSP. Lower interfacial SE 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 SE 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.

Ti and Ta/Ti coatings were successfully applied on AZ31B Mg alloy using high pressure cold spray process. These coatings (particularly Ta/Ti coating) with high hardness lowered the wear rate of AZ31BMg alloy. In-vitro bioactivity test, infinite focus 3D measurement and EDS results revealed that the calcium-phosphate (Ca–P) compounds nucleation and growth capabilities on both the coatings surface are higher than AZ31B Mg alloy surface due to the highly passivating nature and rough surface (island-like) of the high pressure cold sprayed Ti and Ta/Ti coatings, which are further conducive to the biological fixation. Electrochemical corrosion and immersion tests disclosed that a dense layer of Ta on Ti coating was able to enhance the corrosion resistance of Ti/AZ31B Mg system in the Hank's balanced salt solution. Rct (charge transfer resistance) and │Z│f=0.01Hz values for Ta/Ti coated Mg alloy have remained slightly stable. This imparted that the rate of electrochemical processes at the electrode/electrolyte interface could remain very slow in the course of immersion time. Likewise, the OCP (open circuit potential) values of Ta/Ti coating remained constant during long term electrochemical corrosion test. This behavior could originate from fairly dense structure of the high pressure cold sprayed Ta layer which considerably impeded the penetration of corrosive solution into the coating interiors. Ta/Ti coating presented the most positive potential, highest Rct, and highest│Z│f=0.01Hz values, suggesting that it is the most thermodynamically stable/least to be corroded in Hank's balanced salt solution over time. In general, cold sprayed Ta/Ti coating might open a new way to broaden the utilization of Mg alloys in the field of biomaterials.

In this research, cold sprayed Al coating layer on AZ31B Mg alloy was coated with cold sprayed Ti and Ta/Ti coatings, which noticeably reduced the wear rate of Al coated Mg alloy. Electrochemical corrosion tests and infinite focus 3D measurement results revealed that the high surface activity of commercially pure-Al coating, diffusion-controlled reactions, and formation of corrosion pits could be mitigated using Ti top coating. However, a dense layer of Ta on Ti coating exceptionally improved the corrosion resistance of Ti/Al coated AZ31B Mg alloy in neutral 3.5 wt.% NaCl solution This work gives a new strategy to considerably raise the wear and corrosion resistances of cold spray Al coated Mg alloys by double layered Ta/Ti coatings.

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.

Titanium and its alloys have become increasingly important in the dental and orthopedic fields due to its good machinability, high yield strength, good ductility, excellent corrosion resistance, and superior biocompatibility compared to other materials. However, an inherent drawback of using pure titanium and its alloys as implant material is the significant mismatch between the moduli of bone and titanium, resulting in the stress shielding effect, fibrous tissue ingrowth, and bone resorption, and therefore reducing the lifespan of the implant. Porous titanium is thus a suitable candidate as implant material due to its ability to be manufactured to a specific Young’s modulus—typically that of bone. Porous titanium has the unique advantage of allowing bone tissue ingrowth into the open space of the implants, thereby accelerating the osseointegration process. The human body as well as the oral cavity is a highly complex environment in which the simultaneous interaction between wear and corrosion, namely tribocorrosion, takes place. Thus, understanding these interactions is of great interest in order to characterize the degradation mechanisms of porous titanium materials used as implants. This paper reviews the state-of-the-art of porous titanium as a viable biomedical implant material. A significant part of this paper is focused on how porous titanium is manufactured and how its parameters are controlled. The following sections focus on the corrosion, wear, and tribocorrosion aspects of porous titanium implant materials. Finally, this review also determines the current limitations in the field and provides future directions in this field.

Nature has developed multi-functional geometric structures, and surface textures with excellent tribological characteristics, such as feet of geckos. Geckos have extraordinary abilities to climb walls and even upside down on the ceiling. Studies have revealed that hierarchical structure of gecko’s feet can bear the weight of two humans and this strong adhesion force is mainly generated by weak van der Waals force. This paper reviews the mechanisms and the forces responsible for gecko’s adhesion, and the effect of humidity on adhesion against different hydrophobic/hydrophilic surfaces. The excellent adhesive and frictional properties of gecko adhesion system have inspired many researchers to develop gecko-inspired synthetic adhesives. In this paper, recent development of gecko-inspired synthetic adhesives has been presented in terms of various fabrication methods, different tip structures, and the effect of counter surface roughness as well as design criteria to avoid bunching of nano-structures. The application of synthetic adhesives is also discussed for wall climbing robots and novel applications in the field of space, biomedical and sports accessories.