Nanoscale Friction Research Articles

Investigating temperature effects on the shear behavior of clays: Molecular dynamics simulations

The shear behavior of clays is critically important for the stability and safety of nuclear waste repositories and clay gouges. These contexts expose clayey geomaterials to high pressures and temperatures. Under such varying conditions, the shear behavior of these materials becomes complex, necessitating thorough investigations. This study aims to elucidate the effect of temperature on the shear behavior of three clayey materials: kaolinite, illite, and montmorillonite—through Molecular Dynamics. The research replicates a geotechnical shear setup at the molecular scale, varying the temperature (including sub-freezing temperatures below 300 K and elevated temperatures 400-500 K and hydrostatic pressures. The results reveal stick-slip behavior, enabling the calculation of nanoscale cohesion, friction angle, and shear modulus across different temperatures. Thermal effects are notably significant for illite and kaolinite, both exhibiting a marked decrease in shear properties with increasing temperature. Kaolinite demonstrates a high shear modulus of up to 40 GPa, indicating substantial shear strength. Illite displays the highest friction angle among the studied materials. For montmorillonite, thermal effects on shear behaviour is comparatively less pronounced. This study provides critical insights into the mechanical behaviour of clays under varying thermal conditions, contributing to the broader understanding of geomaterial stability in high-stake environments.

Thickness-dependent nanoscale friction across the first-order charge density wave phase transition in 1T-TaS2

The layered charge density wave (CDW) phase transition material 1T-TaS2 has garnered significant attention due to its modulable bandgap and electrical transport properties. These unique properties make 1T-TaS2 highly promising for applications in fields such as optoelectronic devices and microstructure physics devices. In various micro-/nanodevices made from quasi-two-dimensional 1T-TaS2, it is often utilized in thin layers, making the understanding of its frictional properties crucial for practical applications. However, the layer-dependent frictional properties of 1T-TaS2 have not been thoroughly investigated. In this article, we examine the CDW phase transition between the nearly commensurate (NCCDW) and commensurate (CCDW) phases in 1T-TaS2 around 183 K using atomic force microscopy, focusing on the number of layers in the samples. Our results indicate that for thicker samples with more than approximately 17 layers, a friction peak is observed during the NCCDW–CCDW phase transition. In contrast, thinner samples do not exhibit this friction peak, and their friction continuously increases as the temperature decreases. This behavior is attributed to the suppressed NCCDW–CCDW phase transition in thinner samples. These results enhance our understanding of the frictional behavior of 1T-TaS2 in the context of micro-/nano-electromechanical systems. Furthermore, our observations offer a straightforward method to identify the NCCDW–CCDW phase transition, providing an alternative to traditional, more complex techniques such as electrical resistance measurements and Raman spectroscopy.

Nanoscale friction and wear of graphite surface in ambient and underwater conditions

The tribological properties of graphite are extremely sensitive to surrounding environment, which affects its longevity and friction reduction performance. This study aims to develop a comprehensive understanding of the effects of a liquid environment on interactions at the sliding interface, thereby influencing the nanoscale tribological properties of graphite surfaces. By combining atomic force microscopy experiments and molecular dynamics simulations, we conducted a comparative analysis of friction and wear behavior of graphite under two different environmental conditions: ambient (air) and underwater condition. Our investigations explored both the step edge and interior (step-free) graphite surfaces. The experimental results revealed a notable contrast in the frictional and wear behavior of graphite at nanoscale in these two environments. The interior surface exhibited a friction coefficient (COF) of approximately 0.003 and 0.006 against a diamond-coated surface in ambient and underwater conditions, respectively. Interestingly, the underwater environment not only increased friction but also significantly compromised the wear resistance of graphene layers near the graphite surface compared to the ambient environment, as evidenced in both step edge and interior step-free regions. The interior region sustained up to ∼7400 nN load in ambient condition but failed at ∼1500 nN under water. Similarly, the step edge failed at ∼375 and ∼187.5 nN in ambient and underwater conditions, respectively. Our simulations revealed that the increased friction in underwater condition is due to resistance of surrounding water molecules during tip sliding. The presence of water at tip-graphite contact interface generated substantial localized stress, leading to the initiation of wear and revealing the pronounced effect of water on the wear characteristics of graphite in underwater condition.

Scorpion-inspired multi-scale contact enabled multifunctional piezoresistive pressure sensor based on hybrid nanofiber decoration

Inspired by the slit organs of scorpions, an array narrow orifice configuration (ANOC) piezoresistive foam sensors are developed by decorating hybrid polypyrrole (PPy)/ carbon nanotubes (CNT) nanofibers on the foam strut. The developed ANOC composite foam exhibits a unique multi-scale contact sensing mechanism enabled by the macro-scale internal dome structures, the micro-scale struts of the porous foam, the nano-scale PPy/CNT hybrid conductive networks, and the nano-scale friction of the rough CNT/PPy hybrid nanofibers (NFs), which rendered the ANOC composite foam an ultra-high sensitivity of 1.8 kPa−1 at low pressure within 1.5 kPa. When used as a piezoresistive pressure sensor, the ANOC composite foam exhibits a wide detection range (0 %-70 %), fast response/recover time (120/90 ms), and long-term stability and fatigue resistance (over 80000 s), and demonstrated capability to monitor various human activities. A smart cushion integrated with an array of foam sensors can accurately identify the human sitting positions and posture rectifying. Moreover, owing to the highly porous conductive network and the doped ions, the ANOC composite foam can generate energy from saline water and serve as a water-induced energy generation (WIEG) device for direct energy supply. This research paves the way for the development of multifunctional high-performance.

Laser surface structuring of diamond-like carbon films for tribology

The paper overviews experimental findings of the direct laser processing and surface microstructuring (texturing) of various diamond-like carbon films (a-C:H, ta-C, DLN, metal-doped DLN), aimed at improvements of their tribological and nanotribological properties. The nanosecond UV and femtosecond IR/visible pulsed lasers were applied in microprocessing of the films, focusing on high precision surface structuring with fs-laser pulses. The studies were concentrated on the following tasks: (i) surface graphitization in laser microstructuring of the films under different irradiation conditions, (ii) lubricated friction performance of DLN films micropatterned with UV ns and visible fs pulsed lasers, and (iii) nanoscale friction of laser-structured DLN and metal-doped DLN films examined with contact-mode atomic force microscopy. The important findings of our studies are related to fabrication of highly-precise microgroove/microcrater patterns on DLN films and improvements of frictional properties of the laser-structured films at the macro, micro and nanoscale. The surface microstructures improved the film properties under oil-lubricated sliding in dependence on their geometrical parameters (size, depth, period) and ambient temperature. The nanoscale friction behavior of laser-structured films was shown to be controlled by the surface graphitization, nanoscale roughness, capillary forces and wear of AFM tips during friction force imaging.

Substrate deformability and applied normal force are coupled to change nanoscale friction.

Amonton's law of friction states that the friction force is proportional to the normal force in magnitude, and the slope gives a constant friction coefficient. In this work, with molecular dynamics simulation, we study how the kinetic friction at the nanoscale deviates qualitatively from the relation. Our simulation demonstrates that the friction behavior between a nanoscale AFM tip and an elastic graphene surface is regulated by the coupling of the applied normal force and the substrate deformability. First, it is found that the normal load-induced substrate deformation could lower friction at low load while increasing it at high load. In addition, when the applied force exceeds a certain threshold another abrupt change in friction behavior is observed, i.e., the stick-slip friction changes to the paired stick-slip friction. The unexpected change in friction behavior is then ascribed to the change of the microscopic contact states between the two surfaces: the increase in normal force and the substrate deformability together lead to a change in the energy landscape experienced by the tip. Finally, the Prandtl-Tomlinson model also validates that the change in friction behavior can be interpreted in terms of the energy landscape.

Nickel phosphorous trisulfide: A ternary 2D material with an ultra-low coefficient of friction

Ultra-low friction is crucial for the anti-friction, anti-wear, and long-life operation of nanodevices. However, very few two-dimensional materials can achieve ultra-low friction, and they have some limitations in their applications. Therefore, exploring novel materials with ultra-low friction properties is greatly significant. The emergence of ternary two-dimensional materials has opened new opportunities for nanoscale ultra-low friction. This study introduced nickel phosphorous trisulfide (NiPS3, referred to as NPS), a novel two-dimensional ternary material capable of achieving ultralow friction in a vacuum, into the large nanotribology family. Large-size and high-quality NPS crystals with up to 14 mm × 6 mm × 0.3 mm dimensions were grown using the chemical vapor transport method. The NPS nanosheets were obtained using mechanical exfoliation. The dependence of the NPS nanotribology on layer, velocity, and angle was systematically investigated using lateral force microscopy. Interestingly, the coefficient of friction (COF) of NPS with multilayers was decreased to about 0.0045 under 0.005 Pa vacuum condition (with load up to 767.8 nN), achieving the ultra-low friction state. The analysis of the frictional dissipation energy and adhesive forces showed that NPS with multilayers had minimum frictional dissipation energy and adhesive forces since the interlayer interactions were weak and the meniscus force was excluded under vacuum conditions. This study on the nanoscale friction of a ternary two-dimensional material lays a foundation for exploring the nanoscale friction and friction origin of other two-dimensional materials in the future.

Influence of cutoff radius and tip atomic structure on energy barriers encountered during AFM tip sliding on 2D monolayers

In computational studies using the Lennard–Jones (LJ) potential, the widely adopted 2.5 σ cutoff radius effectively truncates pairwise interactions across diverse systems (Santra et al 2008 J. Chem. Phys. 129 234704, Chen and Gao 2021 Friction 9 502–12, Bolintineanu et al 2014 Part. Mech. 1 321–56, Takahiro and Kazuhiro 2010 J. Phys.: Conf. Ser. 215 012123, Zhou et al 2016 Fuel 180 718–26, Toxvaerd and Dyre 2011 J. Chem. Phys. 134 081102, Toxvaerd and Dyre 2011 J. Chem. Phys. 134 081102). Here, we assess its adequacy in determining energy barriers encountered by a Si monoatomic tip sliding on various two-dimensional (2D) monolayers, which is crucial for understanding nanoscale friction. Our findings emphasize the necessity of a cutoff radius of at least 3.5 σ to achieve energy barrier values exceeding 95% accuracy across all studied 2D monolayers. Specifically, 3.5 σ corresponds to 12.70 Å in graphene, 12.99 Å in MoS2 and 13.25 Å in MoSe2. The barrier values calculated using this cutoff support previous experiments comparing friction between different orientations of graphene and between graphene and MoS2 (Almeida et al 2016 Sci. Rep. 6 31569, Zhang et al 2014 Sci. China 57 663–7). Furthermore, we demonstrate the applicability of the 3.5 σ cutoff for graphene on an Au substrate and bilayer graphene. Additionally, we investigate how the atomic configuration of the tip influences the energy barrier, finding a nearly threefold increase in the barrier along the zigzag direction of graphene when using a Si(001) tip composed of seven Si atoms compared to a monoatomic Si tip.

Frictional mechanisms of hydrated montmorillonite under normal loading

The friction behavior of clay particles plays an essential role in controlling its mechanical properties, but remains unclear in microscale. As one of the major clay particles, the nanoscale friction properties of hydrated montmorillonite have been studied using steered molecular dynamics (SMD) simulation method, considering the coupling effect of water content of 0 ∼ 30 % and normal load of 1 atm ∼ 10 GPa. The SMD friction along the y-direction has been performed to investigate the frictional mechanism of hydrated montmorillonite and its structural properties of interlayer water. The evolution of average friction load with water content, friction coefficients, and the distribution of interlayer water molecules were obtained. Furthermore, the SMD pulling along the z-direction was conducted to study the effect of interlayer water molecules on the interaction between montmorillonite layers. All simulation results showed that more energy and a higher friction load were required to slide at a higher normal load. The friction coefficient of 0.058 ∼ 0.17 and internal friction angle of 3.3°∼9.6° for hydrated montmorillonite agreed well with previous studies in microscale, showing the weak friction properties. The evolution of friction coefficient with water content was different at different normal loads, where the friction coefficient of 5 % hydrated montmorillonite was the highest at 3 ∼ 10 GPa. Moreover, the increasing normal load could change the distribution of interlayer water molecules.

Quantitative evaluation of orientation effects in wear and friction characteristics of pure nickel using experiments and atomistic simulations

Nanoscale tribological behavior of Ni for its applications such as MEMS is an important area of study. The present study investigated the nanoscale friction and wear behavior of Ni on different crystallographic orientations using experiments and molecular dynamics simulations. Results showed an anisotropic friction response, with (100) [001] case showing high COF followed by (111) [1¯10] and (110) [1¯10] cases. Analysis showed that such a trend in COF is observed due to variation in activation of the number of slip systems in each case as determined by the Schmid factor calculations. Dislocation analysis showed that (100) [001] case developed the highest number of glissile dislocations which is attributed to high COF value. The wear volume followed an opposite trend of COF order, and the spatial distribution of displaced material is found to be different in each case, indicating an anisotropic ploughing mechanism. Such behavior is observed due to variations in the availability of the number of slip directions, their orientation on the scratch plane, and dislocation evolution during nanoscratch. Overall, this study provides insights into the complex deformation mechanism and wear behavior associated with scratch tests and has implications for the development of wear-resistant materials.

Molecular dynamics simulations into rolling scraping of nickel-copper bilayer film

In order to investigate the nanoscale friction mechanism and deformation behavior of nickel-copper bilayer film under rolling scraping, the effects of different factors during friction process such as the translation velocity, rotation velocity, radius of abrasive grain, contact depth, texture direction and crystallographic orientation are analyzed through molecular dynamics methods in terms of contact force, atomic lattice structure, internal substrate dislocation, kinetic energy and temperature. Results show that the contact force increases with the increase of the translation velocity or the decrease of the rotation velocity. Abrasive grains in a purely rolling state cause the most serious damage to the substrate. The contact force increases with the increase of contact depth or the decrease of abrasive grain radius. The crystallographic orientation has a significant effect on rolling scraping process and there is a crystallographic orientation with the minimum contact force. The degrees of substrate dislocation and the numbers of lattice reconstruction atoms under different factors and levels vary widely.

Why is Superlubricity of Diamond-Like Carbon Rare at Nanoscale?

Hydrogenated diamond-like carbon (HDLC) is a promising solid lubricant for its superlubricity which can benefit various industrial applications. While HDLC exhibits notable friction reduction in macroscale tests in inert or reducing environmental conditions, ultralow friction is rarely observed at the nanoscale. This study investigates this rather peculiar dependence of HDLC superlubricity on the contact scale. To attain superlubricity, HDLC requires i) removal of ≈2nm-thick air-oxidized surface layer and ii) shear-induced transformation of amorphous carbon to highly graphitic and hydrogenated structure. The nanoscale wear depth exceeds the typical thickness of the air-oxidized layer, ruling out the possibility of incomplete removal of the air-oxidized layer. Raman analysis of transfer films indicates that shear-induced graphitization readily occurs at shear stresses lower than or comparable to those in the nanoscale test. Thus, the same is expected to occur at the nanoscale test. However, the graphitic transfer films are not detected in ex-situ analyses after nanoscale friction tests, indicating that the graphitic transfer films are pushed out of the nanoscale contact area due to the instability of transfer films within a small contact area. Combining all these observations, this study concludes the retention of highly graphitic transfer films is crucial to achieving HDLC superlubricity.

Mechanical resistance behind fiber-reinforced polymer pile: Role of clay minerals

Due to the geotechnical design requirements of the fiber-reinforced polymer (FRP) pile embedded in clay, interface friction evolution between the FRP pile and clay plays a crucial role in determining the pile shaft resistance. It is observed from the experimental evaluation that the earth pressure and shear rate affect the pile-soil interface behavior. In this study, a cross-linked epoxy resin is created to contact a siloxane surface of kaolinite in the molecular dynamic simulation. The interfacial accommodation between kaolinite and epoxy resin is studied. In order to investigate the effect of normal stress and sliding velocity on nanoscale friction characteristics, the steered molecular dynamics is utilized to simulate the sliding of the kaolinite model on the surface of the epoxy resin. Simulation results show that the calculated peak interface shear coefficient decreases nonlinearly as the normal stress increases, which follows a logarithmic relationship consistent with the FRP-soil interface shear test. On the other hand, it is observed that two distinct velocity regions, slow pulling mode (5–200 m/s) and fast pulling modes (300–800 m/s), are well fitted by extended Bell theory. The stick–slip motion is only found in slow sliding velocity mode. This increasing tendency of the energy barrier with the normal stresses indicates that higher pulling forces are required for the FRP to slide along the soil at higher normal stress. The present work provides a fundamental understanding of the friction mechanisms between kaolinite and epoxy resin.

Nano friction behaviour between magnetic materials and copper considering the inter-diffusion effect

Copper, permalloy, cobalt, and silicon are the materials that have been widely utilised in magnetic devices. When the size of interest is down to the nanoscale, the inter-diffusion between certain materials becomes influential. This paper studies the nanoscale friction characteristics between frictional pairs with and without inter-diffusion properties through the atomic force microscope. The distinct evolution features of nanoscale friction force when inter-diffusion is involved are discovered experimentally, which is also confirmed through theoretical analysis. Firstly, through the thin film deposition method, four pairs of contact materials (Cu–Ni81Fe19, Si–Ni81Fe19, Cu–Co, Cu–Si) are designed for friction tests, in which diffusion occurs at the interface of Cu–Ni81Fe19 pair. Then, the effects of sliding velocity and loading force on the nano friction of each pair are measured. It is found that regardless of the diffusion phenomenon: (1) the adhesion force values exhibit a notable correlation to the values of the friction force; (2) the friction force in all four material pairs consistently increases with the growth of the normal loading force, although the growth rate may differ. In terms of the sliding velocity effect, the friction forces of immiscible materials (Si–Ni81Fe19, Cu–Co, and Cu–Si) are found to increase with the increasing sliding velocity. However, the friction force of Cu–Ni81Fe19, decreases with the increasing sliding velocity. Furthermore, a compositive friction model considering both the velocity and the normal force effect was proposed, which shows good agreement with the experimental results and explains the nano friction behaviour of both miscible and immiscible metals.

Molecular dynamics simulations of the interface friction behavior between fiber-reinforced polymer pile and sand

The interfacial friction performance of the fiber-reinforced polymer (FRP) pile-sand interface plays an essential role in determining the load capacity of the foundation. It is necessary to identify its friction behavior in the marine environment due to the unique pile-soil interaction characteristics, which have not been well established at the nanoscale. The cross-linked epoxy resin and crystalline silica substrate are utilized to investigate the nanoscale friction characteristics at different normal stresses and sliding velocities in the dry, pure water, and salt water systems using the molecular dynamics (MD) simulation. The obtained coefficients of friction in three systems are ranked as dry > salt water > pure water systems. There is a tendency for the coefficient of friction to decrease with increasing normal stress, which is consistent with the experimental results. The interface roughness is validated by an analytical equation based on Archard’s elastic deformation friction theory. The increase in normal stress shortens the distance between the silica and the epoxy and increases the van der Waals force between the two layers, resulting in the increase of maximum friction force in three systems. The higher normal stress induces the more pronounced stick-slip motion of the silica substrate in the dry system, which can be reduced in the pure water system due to the lubrication effect of water, while the NaCl ions weaken the lubrication effect. On the other hand, the lubrication effect of the water molecules has more beneficial at lower sliding velocity because the diffusion of water molecules is more intense, which leads to the reduction of the amplitude of friction force in the stick-slip motion. By considering the effects of sliding velocity, normal stress, water, and NaCl ions, this study provides the fundamental insight for the friction process of FRP pile embedded in sand.

Insight into the hydration friction of lipid bilayers.

Hydration layers formed on charged sites play crucial roles in many hydration lubrication systems in aqueous media. However, the underlying molecular mechanism is not well understood. Herein, we explored the hydration friction of lipid bilayers with different charged headgroups at the nanoscale through a combination of frequency-modulation atomic force microscopy and friction force microscopy. The nanoscale friction experiments showed that the hydration friction coefficient and frictional energy dissipation of a cationic lipid (DPTAP) were much lower than those of zwitterionic (DPPE) and anionic (DPPG) lipids. The hydration layer probing at the surfaces of different lipid bilayers clearly revealed the relationship between the charged lipid headgroups and hydration layer structures. Our detailed analysis demonstrated that the cationic lipid had the largest hydration force in comparison with zwitterionic and anionic lipids. These friction and hydration force results indicated that the difference of the lipid headgroup charge resulted in different hydration strengths which led to the difference of hydration friction behaviors. The findings in this study provide molecular insights into the hydration friction of lipid bilayers, which has potential implications for the development of efficient hydration lubrication systems with boundary lipid bilayers in aqueous media.

Nanoscale friction of high entropy alloy sulfide thin films in comparison with molybdenum disulfide

We present nanoscale friction measurements performed on sputter-deposited high entropy alloy (HEA) sulfide thin films [(VNbTaMoW)S2] via atomic force microscopy. The results reveal (i) the influence of deposition time on the film morphology and (ii) the presence of isolated areas of low friction on film surfaces. We compare the friction results on HEA sulfide thin films with those on a prototypical solid lubricant, sputter-deposited molybdenum disulfide (MoS2), and find that they are superior in terms of lubricative performance. Variable temperature x-ray diffraction, performed up to 973 K, reveals that HEA sulfide thin films exhibit improved oxidation resistance when compared with MoS2 films. Combined, our results show that HEA sulfide thin films have considerable potential as oxidation-resistant solid lubricant coatings.

Effects of substrate on the nanoscale friction of graphene

In the realm of nanotechnology, atomically thin two-dimensional graphene has garnered attention for its impeccable hexagonal physical structure and chemically inert surface properties. These attributes endow graphene with remarkable mechanical, physical, and chemical characteristics, positioning it as one of the ideal solid lubricants for mitigating friction and wear at contact interfaces. However, the performance of graphene is intricately linked to the substrate it interacts with. Consequently, an in-depth investigation of how substrate variations impact graphene's friction behavior assumes paramount significance in the realm of industrial applications. This study delves into the intricate dynamics of graphene friction through atomic force microscopy experiments, focusing on three pivotal aspects: the binding strength between graphene and the substrate, the Young’s modulus of the substrate, and substrate materials. By subjecting the SiO2/Si substrate to plasma treatment to augment its surface energy, we enhance the interface binding strength between the substrate and graphene, thereby diminishing friction on the graphene surface. Furthermore, we investigate how graphene responds to various substrates, including polypropylene carbonate films of varying Young’s modulus, as well as graphite, h-BN, and SiO2/Si substrates. Graphene demonstrates a pronounced inclination toward increased friction when interfacing with substrates characterized by lower Young's modulus, higher roughness, and adhesion. These findings elucidate the potential for fine-tuning friction in lamellar materials, underscoring the pivotal role of comprehending nanoscale friction dynamics on graphene surfaces.

Nanoscale insight into fluidity improvement of supplementary cementitious materials on concrete: The nano-lubrication process

The workability of concrete is a fundamental property that is frequently influenced by chemical admixtures. In this paper, we provide an interesting perspective to understand the effect of silica fume on the fluidity of cement slurries. By employing molecular dynamics techniques, a nanoscale boundary friction model is proposed to evaluate the flow of cement slurries resulting from cement hydration. According to conventional experimental data, interfacial friction decreases as the water content rises. And additional lubrication of the silica surface is possible. The investigation into the microscopic mechanisms behind interfacial lubrication reveals that water lubricates SiO2 and CSH sheets in various ways at low and high water levels. Throughout the system, non-covalent water-water and water-calcium bonds break down during the lubricating process. It appears that some calcium atoms are split out of the CSH matrix, a sign of a covalent break. In the high water content system, this separation did not happen despite the lubricating action in the low water content system. It is also possible to visually evaluate the impact of additional cementitious materials on the cement paste's fluidity by using the recently discovered mechanism.

A molecular dynamic simulation-based study on nanoscale friction stir welding between copper and aluminium

ABSTRACT The primary aim of this study is to enhance our understanding of friction stir welding (FSW) at the atomic level. To accomplish this, we utilised molecular dynamics simulations to examine the nanoscale fusion welding of dissimilar metals, i.e. aluminium and copper, through the FSW method. Our particular focus was on how the rotation speed of the tool affects structural changes and defect evolution during the nanoscale FSW process. Our research findings revealed that the region subjected to frictional stirring undergoes a phase change as a result of extensive plastic deformation during the FSW operation. Notably, stacking faults and similar defects were predominantly observed on the advancing side as the tool rotated and moved into the friction stir zone. Further, investigation of atomic shear strain snapshots indicated that higher rotational speeds resulted in a broader and more scattered friction stir zone, requiring a longer recovery time compared to slower rotational speeds. Additionally, the changes in atomic concentration during FSW have been studied using displacement vectors, concentration profiles and diffusion coefficient parameters. We also conducted simulation-based tensile and shear deformation tests, which revealed that higher tool rotational speeds led to enhanced material interlocking, consequently improving the mechanical strength of the FSW joints.