Mitigating friction and wear by pre-designed or tribo-induced heterostructures: an overview

Frictional sliding creates distinct microstructural and chemical discontinuities between the subsurface layer and base materials, which dictate the friction and wear properties. Traditional methods for lowering friction and wear in metals primarily rely on strategies that lower the interfacial shear strength between the contacting discontinuities. Contrary to conventional wisdom, we propose an alternative strategy: introducing tribo-induced or pre-designed heterostructures to suppress tribo-induced strain localization, thus yielding superior tribological properties unachievable with their conventional homogeneous counterparts. Citing recent experimental examples, this review explores the multiscale heterogeneities contributing low friction and wear, by interlinking surface deformation mechanisms and applied tribological conditions. GRAPHICAL ABSTRACT


Introduction
One-fourth of the global energy usage is spent on overcoming friction.For instance, in a standard car engine (Figure 1(a)), frictional energy losses in critical moving components such as pistons, cylinder walls, bearings, and camshafts were found to be among the highest and consumed about 28% of the total fuel energy.With harsher and more stringent conditions for future engines [1], the quest for significant reduction in friction and wear has triggered extensive interest in the tribology and materials science communities.
To alleviate friction and wear in metallic components, our first task is to comprehend the genesis of tribolayers of material originating from a complex interplay of mechanical mixing, chemical reactions, and material transfer between contacting interfaces [2,3].As depicted in Figure 1(b), grain refinement transpires within the tribolayer of limited thickness owing to friction-induced large plastic deformation, in tandem with the formation of debris and material transfer.Debris and material transfer have a propensity to roughen the worn surface, resulting in a high friction coefficient [4,5], an occurrence frequently reported in traditional metals exhibiting a homogeneous coarse-grained (CG) structure [6,7].Concurrently, the brittle tribolayer composed of refined grains is mechanically unstable and susceptible to cracking and delamination under repeated sliding contact, leading to the destruction of the tribolayer.The ceaseless formation and destruction of this unstable tribolayer culminate in unavoidable high friction coefficients and high wear rates in conventional metals and alloys.
Intriguingly, the character of the tribolayer depends not only on external factors but also on the microstructure and physical properties of the material itself.At times, it assumes an active role in modifying the material's tribological properties, courtesy of the heterostructure generated within.Tracing to the origin, friction-induced gradient plastic strain results in the creation of a gradient structure beneath the contact surface of metals at room temperature (Figure 1(c1)).Additionally, under extreme conditions, such as elevated temperatures, oxidation begins to play a part, culminating in the formation of a resilient surface oxide film atop the heterostructured (HS) tribolayer.Given that the heterostructure is better equipped to accommodate frictioninduced plastic strain and bolster the mechanical stability of the tribolayer [8][9][10][11][12][13], it aids in inhibiting wear damage, which is generally initiated from the metal surface.
Acknowledging the efficacy of the heterostructure formed during the friction process, we can transform the passive state of friction and wear into an actively manageable state through the application of pre-designed heterostructures.Indeed, the utility of heterostructures to enhance wear resistance has historical precedents.For instance, in the 6th century AD, the Damascus steel [14,15] was produced by forging together layers of different steels for sword blades, demonstrating superior wear resistance.Similarly, the classical technique of carburization, which dates back to the late nineteenth century, has been validated as a profoundly effective method for enhancing the friction and wear resistance of steels.This enhancement is largely attributable to the heterogeneous distribution of carbon, whereby the surface layer is naturally enriched with carbon, significantly augmenting the material's surface hardness and wear durability.In recent years, various heterostructures, such as the gradient structure [16][17][18][19][20][21][22][23][24] (Figure 1(d1)), layered structure [25][26][27][28][29][30][31] (Figure 1(d2)), and their complex mixed heterostructure [32][33][34] (Figure 1(d3)), have been successfully synthesized via diverse methods including surface severe plastic deformation, electrodeposition, accumulative roll bonding, and additive manufacturing.Contrary to conventional homogeneous-structured CG materials, a notable attribute of HS materials is that their structural heterogeneity frequently induces heterogeneous deformation, such as heterogeneous stress and strain [8][9][10]13,35].This can confer a set of mechanical properties superior to those of their CG counterparts, encompassing friction and wear properties.As we deepen our understanding of various wear mechanisms through the continuous enrichment of heterostructure types, we are setting a robust foundation for the future application of heterostructures to mitigate friction and wear.
In this review, we focus on the alterations in the tribological behaviors of metallic materials induced by various heterostructures.To begin with, we examine the friction-induced heterostructure and its implications on the subsequent friction and wear of metallic materials.Next, grounded in the concept of active control, we review recent progresses in our fundamental understanding of various pre-designed heterostructures that contribute to low friction and wear.The design guidelines are discussed by interlinking tribological behavior, surface deformation mechanisms and applied tribological conditions.Furthermore, we summarize the influence of various heterostructures on reducing friction and augmenting wear resistance, and we outline the outstanding issues and challenges that demand attention for the optimization of the tribological properties of heterostructures.

Tribo-induced heterostructures
In the majority of metallic materials, the frictioninduced tribolayer typically manifests as a homogeneous nanocrystalline structure that exhibits pronounced susceptibility to fracture phenomena such as cracking and delamination (reminiscent of Figure 1(b)), contributing to elevated friction coefficients and accelerated wear rates [36][37][38][39].However, the inherent heterogeneity of shear strain, along with the complex external influences such as thermal and some specific chemical interactions during the frictional process, might offer a pathway to transition from the brittle homogeneous nanocrystalline structure to a more favorable heterogeneous structure within the tribolayer.This transition can be effectively engineered through strategic alloying and/or microstructure design, thereby enhancing tribological properties.Specifically, the incorporation of alloying elements characterized by a large negative enthalpy of mixing and significant atomic size difference facilitates the solidstate amorphization of nanocrystalline oxides, culminating in the generation of a nanocrystalline-amorphous heterogeneous structure within the tribolayer [40,41].In a parallel vein, designing columnar-grained architectures promotes the engagement of multiple deformation mechanisms, including dislocation, twinning, and phase transformation, at high-density interfaces, leading to the formation of gradient microstructure in the subsurface layer [42][43][44][45][46]. Subsequent sections will delineate the array of tribo-induced heterostructures, such as the amorphous-crystalline heterostructure, gradient structure, and heterogeneous structures formed at hightemperature and chemically reactive environments, and will elucidate their significant impacts on tribological properties [40,[41][42][43][44][45][46][47][48][49][50][51].

Tribo-induced amorphous-crystalline heterostructure
In the realm of tribological contact, the intense shear stresses, coupled with complex thermal and chemical interactions at the contacting interface, can drive significant structural transformation within the near-surface zones of the materials engaged.Notably compelling is the emergence of a nanocrystalline-amorphous (core-shell) architecture, distinguished by its ultrahigh strength and an innate ability for strain delocalization [40,52].This novel amorphous-crystalline heterostructure was first reported in a CoFeNi 2 medium entropy alloy (MEA) under friction at ambient temperature [40].During friction, the topmost tribolayer, approximately 3 µm thick, manifested nanoscale oxide grains (sub-10 nm) delineated by amorphous boundaries (see Figure 2(a1,a2)).Incorporating oxygen atoms within the alloy matrix, the disordered grain boundaries, and the tribo-induced laminate structure contribute substantially to the genesis of these amorphous boundaries.In addition, Co plays a pivotal role in the formation of compacted nanocrystalline oxides at room temperature due to its good binding strength and excellent sintering ability.Leveraging the amorphous-crystalline (core-shell) heterostructure, the CoFeNi 2 alloy exhibits relatively low friction coefficients (0.37-0.40) and wear rates (7.04 × 10 −7 to 3 × 10 −6 mm 3 N −1 m −1 ), showcasing an anti-wear performance that surpasses other CoCrNi-based high entropy alloys (HEAs, as shown in Figure 2(a3)) by two orders of magnitude.
The tribo-induced amorphous-crystalline dual-phase heterostructure represents another distinct category of interest.It has been reported that a TiNbZr-Ag alloy, characterized by its notably low COF ( ∼ 0.09) at ambient temperature [41], derives its properties from the formation of an amorphous-crystalline dual-phase tribolayer in which a ∼ 20 nm-thick Ag nanocrystalline phase is embedded within the ∼ 400 nm-thick Ti 17 Nb 13 Zr 13 Fe 2 O 55 amorphousphase (see Figure 2 (b1,b2)).It has been pointed out that the environmental oxygen can alter atomic coordination, thereby creating a more negative enthalpy of mixing, which significantly enhances the glass-forming ability of TiNbZr-Ag alloy.Concurrently, the presence of Ag nanocrystals within the amorphous matrix is a consequence of their positive enthalpy of mixing with Nb (refer to Figure 2(b3)).Thus, the tribolayer consisting of this amorphous-crystalline  [41].(c1) A cross-sectional bright-field TEM image for the TaMoNb film after dry sliding against a Si 3 N 4 ball at room temperature.SEM images of the nanopillars composed of nanocomposite (I) and gradient nanostructure (II) before (c2) and after (c3) compression with (c4) typical compressive engineering stress-strain curves [42].(d1) A cross-sectional HAADF-STEM image of the Cu 90 Ag 10 alloy worn against a martensitic steel 440C disk.(d2) Hardness and (d3) spacing between two consecutive Ag layers as a function of depth below the sliding surface [57].
dual-phase heterostructure and renowned for its superior combination of strength and plasticity, along with the low COF, contributing to the enhanced wear resistance (Figure 2(b4,b5)).
Up to now, the tribo-induced amorphous-crystalline heterostructure has been observed in various MEAs/ HEAs, noted for their compositionally complex [40,41].From an empirical standpoint, alloys primed for solidstate amorphization typically comprise three or more constituents, characterized by a large negative enthalpy of mixing and a significant difference in atomic size.Naturally, severe plastic deformation at the contact surface promotes the formation of an amorphous-crystalline heterostructure within tribolayer for MEAs/HEAs [53,54].Commencing with the considerable tribological strain, the friction process invariably leads to the creation of nanostructured tribolayer replete with high-density dislocations and an array of concentrated defects such as nano-lamella [51].This followed by the inter-atomic interactions fostering lattice instability, primarily due to elastic distortion.Culminating in the evolution of partial structural disorder, this progression steadfastly advances until amorphization is achieved.Under these specific conditions, the wear particles generated through friction are ground and mixed, ultimately undergoing transformation into amorphous oxides, a process significantly expedited by the synergistic effects of friction heat and oxygen penetration from the surrounding environment [55,56].Given these observations, the prospect of engineering more compositionally intricate alloys endowed with a heterostructured amorphous-crystalline, wearresistant tribolayer is indeed viable.

Tribo-induced gradient structure
Examining the intriguing domain of heterogeneous structures, the gradient structure emerges as a paramount example, inherently expected to materialize within the tribolayer due to friction-induced stress/strain gradient.Through diligent inquiry, researchers have elucidated gradient subsurface microstructures with varying structural dimensions in metallic materials exposed to tribological loads [38,43,47].Luo et al. [42] observed the formation of a gradient nanostructure in a TaMoNb compositionally complex alloy film subjected to dry sliding against a Si 3 N 4 ball at room temperature (see Figure 2(c1)).This transformation process saw the original columnar grains undergoing refinement, realigning in concert with the sliding direction, while numerous lattice dislocations and stacking faults acted in concert to fragment the severely deformed laminated grains into equiaxed nanograins.Compared to the assputtered columnar-grained structure, the tribo-induced gradient nanostructure exhibited a notable 30% augmentation in the strength, achieving 6.5 GPa, and compressive strain, reaching 45%, as demonstrated in the nanopillar compression test (Figure 2(c2-c4)).Consequently, it is the high strength and remarkable ability to accommodate plastic deformation, inherent to the tribo-induced gradient nanostructure, that confer superior wear resistance.Similarly, Ren et al. [57] reported a unique gradient nanolamellar structure in a Cu 90 Ag 10 alloy when worn against an SS 440C disk (Figure 2(d1)).In the topmost region, extending from the wear surface to a depth of 600 nm, a single-phase Cu-Ag solid solution with equiaxed grain morphology is formed.Delving deeper, from 600 nm to several micrometers, there emerges a chemically layered structure with layer spacing from 25 nm to 250 nm, attributed to the synergistic plastic deformation and little chemical mixing (Figure 2(d2)).This structural evolution is accompanied by a gradient diminution in nanoindentation hardness, tapering from 5.1 GPa to 4.1 GPa with increasing depth (Figure 2(d3)).Thus, it is this tribo-induced gradient nanolamellar architecture that significantly augments the friction reduction and wear resistance of the Cu 90 Ag 10 alloy.
Apart from the tribo-induced gradient structure formed through deformation mechanisms dominated by dislocation activity, other mechanisms such as deformation-induced twinning [43], phase transition [44] and amorphization [45] also contribute to the creation of a gradient structure in tribolayer.Yang et al.'s work [44] reveals a phase gradient in the TiZrHfTa 0.5 metastable refractory high-entropy alloy, post 12,000 sliding cycles, where the bcc to hcp phase transition emerges a mechanism to mitigate strain localization.This phase transition is instrumental in achieving notably low friction coefficients (0.12-0.15) and wear rates ((4.08-9.68)× 10 −5 mm 3 /Nm) at room temperature.Another exploration by Mills et al. [45] into the microstructural evolution of a damage-resistant Ni 56 Ti 36 Hf 8 alloy, particularly post rolling contact fatigue tests, illuminates the intricate interplay between a B2 matrix and nanoprecipitates under substantial contact stress conditions.The sliding process instigates localized slip bands within the B2 matrix, leading to precipitate shear/dissolution, grain refinement, and the eventual formation of confined amorphous bands.Intriguingly, the amorphous phase's volume fraction displays a gradient distribution correlating with the depth within the tribolayer, significantly enhancing tribological performance.
Despite the subsurface layer predominantly manifesting a nanograined structure at the upper echelon and a dynamically recrystallized structure at the lower [58], the confluence of various deformation mechanisms, including dislocation activities, twinning, and phase transformation, alongside tribo-induced gradient deformation, is pivotal in cultivating a tribo-induced gradient structure.Through appropriate alloying design, interfaces with certain mechanical/thermal stability such as grain boundaries, twin boundaries, [43] and phase boundaries [46] can be distributed in a gradient fashion in the tribolayer, leading to increasingly diverse tribo-induced gradient structures can be expected in metallic materials, thereby further optimizing friction and wear properties.

Tribo-induced heterostructures under harsh conditions
Considering the progressively harshness of wear conditions, characterized by elevated temperatures, augmented sliding velocities, and chemically aggressive environment, the formation of tribo-induced heterogeneous structures, alongside their ramifications on tribological behavior, have elicited considerable scholarly attention [59][60][61][62][63][64].Typically, a wear-resistant glaze layer, constituted of homogeneous nanograined oxides, materializes on the worn surface during high-temperature wear processes [65,66].With the incorporation of chromium (Cr), an oxidation layer enriched in Cr manifests between the glaze layer and dynamic recrystallization (DRX) layer along the thickness direction, efficaciously preventing further oxidation of the underlying substrate [67].Considering the spinel crystalline structure of certain oxides, conductive to friction mitigation, the intact and thick glaze layer can fulfills the dual function of lubrication and protection [68].However, the integrity of this layer is imperiled by the localized tribological strain, culminating in crack initiation and potential delamination, hence underscoring the imperative for a heterostructure resilient to damage.Liang et al. [63] identified a composite glaze layer of heterostructure in a novel Ni-27Cr-5W-1Co-1Mo superalloy with a relatively low stacking fault energy (SFE) after wear at 800°C.As delineated in Figure 3(a1-a4), numerous regions within the glaze layer, unmarred by oxidation, are enveloped by layers rich in Cr oxide.Concurrently, the DRX regions, devoid of oxidation, are composed of ultrafine grains, densely populated dislocation segments, thus intimating the capacity of these isolated DRX regions to accommodate extensive plastic deformation during the sliding process (see Figure 3(a5-a6)).The presence of hetero-deformation induced (HDI) strengthening and hardening in the glaze layer significantly improves the ductility, subsequently leading to enhanced wear resistance of the Ni-27Cr superalloy.Consequently, vis-à-vis commercial superalloys, the Ni-27Cr variant exhibits a paramount wear resistance across a thermal spectrum ranging from 25°C to 800°C, with wear rate at 800°C documented at 3.1 × 10 −6 mm 3 /Nm (refer to Figure 3(a7)).
Beyond the artificial synthesis of heterogeneous glaze layer, the emergence of a tribo-induced gradient structure represents a profoundly advantageous development, exerting a salutary impact on the elevated temperature tribological behavior.An et al. [61] embarked on the creation of a new multi-principal element alloy (MPEA) with a nanometer-sized pearlite structure.After wear at 550°C, a gradient structure materialized, delineated by an ultrafine nanograined glaze layer, a refined recrystallized layer, a coarse recrystallized layer, and a pearlite substrate layer, extending along the thickness direction (see Figure 3(b1-b3)).The wear rates for the pearlite MPEA at 550°C and 600°C were ascertained to be (2.5-2.9)× 10 −5 mm 3 /Nm, significantly lower than those recorded for high-speed steels, as depicted in Figure 3(b4).The excellent combination of high strength, augmented strain hardening ability, and excellent thermal stability provides substantial resistance against sliding [69], effectively reducing the wear rate of pearlite MPEA at elevated temperatures.Lou et al. [70] designed the WC-CoFeAl composites, integrating a novel B2-CoFe binder phase structure, and examined their tribological behavior against Al 2 O 3 at temperatures ranging from 25°C to 400°C.After sliding against Al 2 O 3 counterfaces at 400°C under vacuum, a top NG tribolayer and an UFG layer formed in the binder area.The tribolayer is characterized by a blend of B2 intermetallic nanograins and WC nano-particles scraped off from the adjacent ceramic phase, exhibiting a degree of interfacial amorphization.Owing to the tribo-induced gradient nanostructures, the B2 intermetallic binders have exhibited improved deformation compatibility with the ceramic constituents and an elevated tolerance to wear-induced damage.
Intriguingly, at elevated thermal regimes, the manifestation of a chemical gradient, in contradistinction to a structural one, becomes distinctly observable within the tribolayer.Dreano et al. [62] investigated the microstructural origin of the wear-resistant Co-based Hayes 25 superalloy post-fretting at 575°C.The topmost layer is characterized by the compositional gradients of Co and Cr elements along the thickness direction (see Figure 3(c1,c2)), whereas the grain size distribution within the glaze layer remains a uniformity (Figure 3(c3)).The emergence of this compositiongradient glaze layer could be attributed to the Crdepletion process, succeeded by Co diffusion and subsequent oxidation.
Investigations into the formation of tribo-induced heterostructures under a spectrum of severe environmental conditions, and their implications on friction and wear attributes, are currently at the forefront of scientific inquiry, albeit remaining in a nascent phase.For instance, an increase in sliding speed initiates partial dislocations, precipitating the emergence of a heterostructure consisting of a high density of deformation twins in the subsurface region [71].Nonetheless, the regulatory mechanisms governing this tribo-induced heterogeneous twin architecture, along with its ramifications on tribological efficacy, are yet to be fully elucidated.Moreover, there are instances where the formation of a heterostructure in the subsurface layer is deemed undesirable.Particularly under high load scratching [72], the formation of a heterogeneous structure ahead the tip of scratch indenter can cause massive delamination, exacerbating the scratch resistance due to pronounced mechanical incompatibility [73,74].Consequently, there is an imperative need to intensify research into the evolution of triboinduced subsurface structures under harsh wear conditions, which remains, to a considerable extent, an embryonic field of inquiry.Furthermore, elucidating the ramifications of their intrinsic properties-namely strength, toughness, and thermal stability-on the resultant friction and wear behavior under harsh wear conditions, represents a critical avenue for future exploration.

Pre-designed heterostructures without changing phase constitution and chemical composition
Upon scrutinizing the extant literature, it becomes apparent that through strategic alloy design or the astute manipulation of microstructure, we are indeed capable of engendering favorable heterostructures within the tribolayer, which in turn, markedly enhances the tribological performance of metallic materials.However, the mere anticipation of heterostructure formed within the tribolayer during the friction process is insufficient.Rather, we must proactively engage in the deliberate architecture of pre-designed heterostructures, whilst maintaining the integrity of the phase constitution and chemical composition (refer to Figure 1(d1-d3)).By adopting this approach, we can synthesize pioneering heterogeneousstructured materials specifically tailored to elicit desired tribological behaviors, thereby fully exploiting the latent potential of heterostructures in augmenting the friction and wear properties of metallic systems.

Friction of gradient nanograined materials
In 2016, Chen et al. [17] were the pioneers in demonstrating that a GNG surface layer within the Cu-Ag alloy enables a significant reduction in COF under high-load dry sliding (Figure 4(a-c)), from 0.64 (CG samples) to 0.29 under a load of 50 N.The scar surface retains its smoothness, devoid of any cracks or material pile-ups, after both single or repeated sliding events (Figure 4(d)), in contrast to the CG and NG samples.The surface roughness along the sliding direction mirrors that of the original sample, which indicates that the sliding-induced plastic strain is well accommodated by the GNG structure without generating strain localization.The unprecedentedly low COF is traced back to the stable gradient nanostructures, which are proficient in accommodating large plastic strains throughout repeated sliding exceeding 30,000 cycles (Figure 4(d)).The authors demonstrated that the superior stability of the GNG structure against sliding-induced surface roughening and the formation of delaminating tribolayers provides a novel strategy for lowering the COFs of metals and alloys.This work inspired a surge of search in low-friction GNG materials, including copper [75][76][77][78] and steels [5,[79][80][81].

Fundamental mechanisms of low-friction in gradient nanograined materials
Chen et al. discerned that plastic deformation behaviors within the GNG surface layer, under sliding contact, diverge fundamentally from those observed in homogeneous structures [17].An intrinsic gradient in the elastic limit is manifest in the GNG layer.During sliding, it is not invariably the case that plastic deformation initiates at the topmost surface layer, given its high strength.Due to the gradient distribution of applied stress under the Hertzian contact, initiation of plastic deformation within the subsurface layer may precede that in the hard topmost GNG surface, as the local stress exceeds its elastic limit.Alternatively, the onset of plastic deformation might occur simultaneously within both the topmost surface and the subsurface layers.Hence, strain localization at the sliding surface layer can be effectively alleviated, suppressing surface cracking and folding, thereby eliminating surface roughening.The exceedingly low and stable COFs observed in the GNG Cu-Ag samples originate from the sustained surface smoothness during sliding (Figure 4(c,d)), which is in harmonious correlation with a stable subsurface gradient nanostructure, characterized by a continuous thin nano-grained topmost layer over a submicron-grained layer [17].
The above explanation is based on the hypothesis that the distribution of applied tribological stress matches the gradient yield stress of the GNG material, engendering strain delocalization beneath the sliding contact surface.A two-dimensional finite element model has substantiated that the diminution in the COF exhibited by gradient-structured materials is fundamentally attributed to the strain delocalization, which results from the co-deformation inherent within the gradient structure [82].Additionally, Chen [75] employed Hamilton's model to elucidate the subsurface stress field of copper under a ball-on-flat contact configuration (Figure 4(e)), thereby substantiating this hypothesis.The shear stress σ xz displays a gradient distribution from the surface down to the bulk interior at the center of the sliding contact.A thick subsurface layer accommodates the plastic strains, as indicated by the comparison between the applied stress and yield stress in the GNG Cu sample (Figure 4(f)).For bulk materials with homogenous structures, such as the CG or the NG samples, the elastic limit maintains uniformity from the surface to the bulk interior.Under the imposition of a high sliding load on a metal surface, the topmost layer incurs a significantly high strain, where the applied stress even exceeds the yield stress of the NG sample (Figure 4(f)).Given that the plastic strain in the confined surface layer is too large to be accommodated, strain localization readily occurs, precipitating surface damage and escalating the COF in the NG sample.Due to the homogeneous CG and NG structures, many metals and alloys exhibit high steady-state COF values, lacking a low and stable friction stage (Figure 4(g)) [17,38,75,[83][84][85][86][87].Nevertheless, the pre-designed GNG structure, with its superior stability, offers a novel approach for enhancing tribological performance.More details on the friction of stable gradient nanograined materials can be found in a recent viewpoint paper [88].

Friction of heterogeneous laminates
Heterogeneous laminate represents an archetype of heterostructured materials, where the juxtaposition of soft and hard laminates manifests a unique interplay of mechanical properties [13,89].The essence of this interplay lies in the strain gradient and the associated back-stress strengthening at their interfaces, with the optimal spacing between laminates being of paramount significance for achieving superior strain hardening and ductility in these materials [19,25,30].Furthermore, the phenomenon of interface-induced strain delocalization, when combined with a judiciously chosen spacing the remains below a critical threshold, endows these materials with excellent tribological properties [90][91][92][93].For example, the COF and wear rate decrease as the layer spacing decreases for both perpendicular and parallel sliding directions with respect to the interface of the laminates Cu/CuZn [90].In this work, a series of bulk heterogeneous Cu/CuZn laminates with the layer spacing ranging from 20 to 200 µm were prepared, with the interfaces clearly indicated by white dashed lines in Figure 5(a1).Their COFs (Figure 5(a2)) evaluated from the sliding direction perpendicular to the interface are lower than those from the direction parallel to the interface when the layer spacing is below 50 µm (the applied load: 5N; Al 2 O 3 ball: 6 mm).Among the samples examined, a noticeable friction anisotropy was discernible for the laminate with a layer spacing of 50 µm.Subsequent microstructural analysis within the subsurface layer under the initial stages of friction unraveled different deformation mechanisms for the Laminate-50 samples, contingent upon the sliding direction (Figure 5(a3,a4)).Regarding the friction perpendicular to the interface, deformation twins and dislocations were presented in the region of hard CuZn layer nearby the interface, while only dislocations were observed in the soft Cu layer.The presence of high-density twins and dislocations in the CuZn layer serves to mitigate strain concentration near the interfaces.Conversely, sliding parallel to the interface prompts a dislocation-dominated grain refinement across both the Cu and CuZn layers, with the hard CuZn layer near the interface being exclusively composed of dislocations, devoid of deformation twins.The density of geometrically necessary dislocations (GNDs) for the Laminate-50 sample sliding perpendicular to the interface is much larger than that for the parallel one across the entire deformation regions.Besides the different deformation mechanisms, friction-induced chemical mixing at the interface is also observed for the samples with the layer spacing less than the critical value of 50 µm under a maximum Hertzian contact stress of 0.97 GPa, when the sliding direction is perpendicular to the interface [90].This is firstly attributed to the matching of the applied shear stress and the layer spacing.Beyond the critical spacing, the applied tribological stresses fail to instigate soft Cu mass transport across the entire CuZn layer.The observed dislocation activities at the heterogeneous interfaces are believed to facilitate this chemical mixing, with enhanced interfacial dislocation plasticity diminishing strain localization and thus augmenting the chemical mixing between the soft and hard laminates.Similarly, the simulation findings demonstrate that a bimodal structure consisting of soft and hard grains can effectively mitigate strain localization during scratching, thereby reducing the contact depth and achieving a COF as low as 0.399 [94].
Beyond the realm of metal-to-metal laminates, which exhibit relatively strong bonding interfaces, the integration of weakly bonded two-dimensional layers into laminates has also been instrumental in improving the tribological properties of these composite materials [91,95,96].A notable example is the incorporation of a mere ∼ 0.2 vol.% graphene into the copper matrix composite laminate, which precipitated an increase in wear resistance, achieving improvements up to a 100fold [91].The meticulously engineered graphene/copper matrix (Gr/Cu) laminate exhibited a seamless interface connection without porosity or discernible defects (Figure 5(b1)).In the sliding direction perpendicular to the graphene edge (C.S.-V), the lowest specific wear rate was achieved, whereas in the in-plane sliding direction (I.P.), the highest specific wear rate was observed, surpassing even that of the pure Cu counterpart (Figure 5(b2)).The analysis of the sample engaged in C.S.-V sliding contact revealed a pronounced stress concentration near the surface, which diminished progressively with depth, reflecting a gradual decrease in plastic deformation within the lamellar structure oriented toward the sliding surface (Figure 5(b3)).The strategic inclination of graphene layers within the laminate not only facilitated a sustained lubricating effect but also obviated the potential for severe interfacial delamination.The synergic strengthening and lubricating effects of graphene are key to the exceptional wear resistance observed in the sliding direction perpendicular to the graphene edge.

Friction of other heterogeneous materials
Structures consisting of heterogeneous zones with dramatically different mechanical or physical properties are considered heterostructures [8].In addition to the gradient and lamella structures previously mentioned, the core-shell structure emerges as a quintessential example of heterogeneity, featuring an inner core composed of one material encased within a shell of another [97,98].For instance, in the transition-metal carbides and/or nitrides (TMC(N)) core-shell heterogeneous structure, the nanoscale, dislocation-free nanoscale TMC(N) cores act as pristine configurations that suppress the initiation of dislocations and provide 3D constraint interfaces to inhibit dislocation movement under load.Meanwhile, the transition-metal (TM) shells bestow a ductile phase upon the TMC(N)@TM core-shell heterostructure, marrying enhanced hardness with high toughness [99].The synergy of high hardness and high toughness is instrumental in combating abrasion, thereby substantially improve the wear resistance [97,100].Specifically, the ordered, coherent TaC@Ta core-shell heterostructure with TaC nanocrystals surrounded by thin Ta shells (Figure 5(c1)) exhibited low COFs and wear rates under both dry air and wet SBF environment (Figure 5(c2)).Furthermore, the presence of a thin pseudocrystalline Ta encapsulated layer (approximately 1.5 nm) in the TaC@Ta core-shell structure facilitates the formation of a lubricious TaO x phase during sliding (shown in Figure 5(c3)), thereby further lowering the COF.Undoubtedly, core-shell structures offer a novel strategy for enhancing the performance of heterogeneous materials, encompassing improvements in hardness, toughness, friction, wear, and corrosion resistance.

Conclusions/future perspectives
Heterostructures not only self-organize in response to tribological loading, thereby governing the fate of tribological contact, but can also be specifically pre-designed to modify the tribological properties.The transition from passive to active design of heterostructures holds significant promise for the future, given that friction-induced heterostructures are largely governed by the initial materials themselves.At present, a growing number of HS metals and alloys are being synthesized under the concept of active control to achieve excellent friction and wear properties, which quite often their conventional MS counterparts fail to offer.Referring to Figure 6, most metals and alloys with a homogeneous structure, be it CG or NG structure, exhibit high steady-state COFs exceeding 0.5 and high wear rates in the magnitude of 10 −5 to 10 −3 mm 3 /N•m [17,75,76,79,80,91,97,[101][102][103][104][105][106][107][108][109][110][111][112][113][114].The implementation of heterostructures enables a reduction of COF down to a range of 0.2 to 0.4, and the wear rate to the magnitude of 10 −9 to 10 −6 mm 3 /Nm.Taken as a whole, the outstanding friction and wear performance of heterostructures stems from heterostructure-induced strain delocalization, i.e. suppressing surface roughening and the formation of delaminating tribolayers.It should be noted that the data presented in Figure 6 serve to qualitatively elucidate the tribological properties of heterogeneous and homogeneous structures, thereby illuminating the discernible trend that tribological enhancements are indeed feasible through the strategic tailoring of heterostructures.However, the absence of a standardized methodology for friction and wear testing necessitates the selective incorporation of data in this figure and precludes the possibility of conducting precise, quantitative analyses that require uniform experimental conditions.For instance, although certain nitride coatings have demonstrated a remarkably low coefficient of friction (COF) ( < 0.2) [115,116]-lower than those depicted in Figure 6-their inclusion was omitted from Figure 6.This exclusion was primarily due to the lack of comparative analysis between homogeneous and heterogeneous structures in the studies, rather than any inconsistency arising from variations in test methods.
Despite progress in developing wear-resistant HS metals and alloys, the road ahead for tribology, particularly in the context of heterostructures, is an exciting journey filled with untapped potential and challenges.There are numerous scientific and engineering issues that need to be solved in this new field.The first challenge is how to fabricate/design more complex, hierarchical heterostructure with low COFs and wear rates during dry sliding.As plotted in Figure 6, there exist noticeable holes in the property chart that pair COF and wear rate (i.e. the red area with a low COF of approximately 0.1 and a wear rate less than 10 −6 mm 3 /N•m).Currently, HS metals and alloys primarily occupy the blue area in Figure 6, where the COF ranges from 0.2 to 0.4 and the wear rate is greater than 10 −7 mm 3 /N•m.This scenario may be attributed to the structural simplicity of currently prepared heterostructures, their lack of hierarchical features, and the relatively monotonous nature of the structural units.A thorough examination of natural biological materials uncovers heterostructures coexisting with diverse structural units (i.e.mixed heterostructures in Figure 1(d3)), possessing hierarchical and multi-scale features, and offering excellent multifunctionality.Additive manufacturing is seemingly feasible to offer structural heterogeneity through layer-by-layer design.
Second, the unique appeal of these HS materials lies in their diversity and adaptability, allowing us to customize their tribological properties to suit specific needs, especially under extreme service conditions.These conditions include scenarios like electromagnetic launching, turbomachinery, and electrical contacts, which involves high temperatures, high-speed mechanical impacts, and high current simultaneously.One of the primary challenges in this line of research is recreating these extreme conditions and recording the tribological response of HS metals and alloys in a laboratory setting.The question of whether the heterostructure can lower friction and improve wear resistance under extreme conditions is yet to be verified experimentally.Hence, there is a need for accurate evaluation and measurement of friction and wear properties of HS materials under extreme conditions to explore their applications in harsh environments.
Third, a thorough understanding of the microstructure-friction relationship has not yet been established for HS materials.This is primarily because these properties are not bulk properties and are largely determined by the weakest path of surface and subsurface damage.A broad array of anti-wear mechanisms exists in HS materials, including heterostructure-induced hardening and strengthening, strain delocalization, protective hard-phase structure against wear, soft-phase structure with lubrication functions, and boundary lubrication provided by numerous interfaces.An ongoing research challenge is to precisely control friction and wear of these complex HS materials, which requires a detailed understanding of the origin of friction and wear and fundamental mechanisms of frictional energy dissipation for various heterostructures at the micro-and nano-scale, and the development of advanced computational models and experimental techniques to predict and verify their tribological performance under varying conditions.Above all, the modulation of tribological response using a heterostructure is a nonalloying strategy without changing the material's chemistry and phase constitution.The friction and wear variation windows of a material can be greatly broadened by tuning the quantity, structure, and distributions of heterostructured interfaces.This control over tribological performance by orchestrating heterostructure mirrors an ingenious likeness to the employment of amplified interface effects to secure superior bulk properties in plainified materials [117,118].Being environmentally benign, the plainified heterostructure design demonstrates vast potential in modulating friction and wear of metals and alloys, in such a way to produce tribological property-on-demand components in practical applications.

Figure 1 .
Figure 1.(a) Schematic representation of a typical car engine, highlighting friction-induced wear as the primary cause of failure.(b) Schematic depiction of tribolayer formation under sliding contact.(c) Heterogeneous structures induced by tribo-effects formed under (c1) conventional room temperature conditions and (c2) extreme conditions.(d) Examples of heterogeneous design variants, including (d1) gradient structures, (d2) laminated structures, and (d3) structures integrating mixed features.

Figure 2 .
Figure 2. (a1) The TEM and (a2) magnified HRTEM images in the topmost oxidation layer for the CoFeNi 2 MEA worn at 5 N after 9000 cycles.(a3) Specific COF vs. wear rate maps of tribological CoCrFeMnNi-based HEAs/MEAs, including the present results of CoFeNi 2 MEA [40].(b1) TEM, (b2) STEM and (b3) HRTEM images of the amorphous-crystalline nanocomposite on the surface.(b4) Friction coefficients as a function of sliding cycles for the TiNbZr-based alloys with (b5) 2D cross-sectional profiles of the wear tracks [41].(c1) A cross-sectional bright-field TEM image for the TaMoNb film after dry sliding against a Si 3 N 4 ball at room temperature.SEM images of the nanopillars composed of nanocomposite (I) and gradient nanostructure (II) before (c2) and after (c3) compression with (c4) typical compressive engineering stress-strain curves [42].(d1) A cross-sectional HAADF-STEM image of the Cu 90 Ag 10 alloy worn against a martensitic steel 440C disk.(d2) Hardness and (d3) spacing between two consecutive Ag layers as a function of depth below the sliding surface [57].

Figure 3 .
Figure 3. (a1) Cross-sectional TEM characterization of the worn Ni-27Cr superalloy after 9000 cycles at 800°C with corresponding EDS (a2-a4) mapping results.Magnified TEM observation of the (a5) unoxidized regions and (a6) one DRX grain in the glaze layer.(a7) Summary of wear rates for the Ni-27Cr sample and some commercial Ni-based superalloys from the literature [63].(b1) SEM characterization of the subsurface under the wear track of the pearlite steel after sliding test at 550°C.HAADF STEM images of (b2) fine and (b3) coarse recrystallized region.(b4) Wear rate of various existing steels and present pearlitic multi-principal element alloy [61].(c1) Cross-sectional SEM observation and associated (c2) EDX linescan analysis of the wear track after the glaze layer formation in the Haynes 25 superalloy.(c3) TEM observation of the lamella in bright field [62].

Figure 4 .
Figure 4. Structure and friction of gradient nanograined (GNG) materials [17,75,88].(a) Typical longitudinal-sectional scanning electron microscopy image of GNG Cu-Ag sample.(b) Variation of longitudinal (d l ) and transversal grain sizes (d t ) and microhardness along depth from the surface.Error bars represent the SD of grain size and hardness measurements.(c) Variation of COFs with sliding cycles for the CG, NG, and GNG Cu-Ag samples sliding against WC-Co balls.(d) Measured surface height profiles along the sliding direction in the CG, NG, and GNG Cu-Ag samples after different sliding cycles (as indicated), with corresponding confocal laser microscopy images for surface morphologies after sliding for 18,000 cycles (above).(e) The xz-component of the stress field in the middle of the wear scar along the sliding direction.(f) Variation of applied stress at the center and the front of contact along depth from the sliding surface in the GNG (COF = 0.37) and NG pure Cu samples.(g) Variation of COFs with initial hardness for all sorts of metals and alloys, including the CG, NG and GNG structures.

Figure 5 .
Figure 5. Structure and friction of heterogeneous laminates and other heterostructured materials [90,91,97].(a) Friction of heterogeneous Cu/CuZn laminate: EBSD IPF mappings of the laminates with a layer spacing of (a1) 20 µm, (a2) variations of the COF with the layer spacing for the laminates sliding perpendicular and parallel to the interface under a normal load of 5 N, TEM images of worn subsurface for Laminate-50 under the sliding direction (a3) perpendicular, (a4) parallel to the interface (indicated by the white dashed line).(b) Friction of heterogeneous Gr/Cu laminate: (b1) TEM image for a typical Gr/Cu interface, (b2) COF curves for three sliding directions (I.P. (in-plane surface), C.S.-P, and C.S.-V (parallel and vertical to the lamellae on cross-section surface)) of the Gr/Cu laminates, (b3) finite element simulating results for C.S.-V.(c) Friction of heterogeneous TaC/Ta core-shell-like nanocomposite: (c1) HRTEM image implying the core-shell-like structure of TaC/Ta nanocomposite, (c2) COF values of TaC/Ta nanocomposites (sample # 1 with a Ta layer thickness of ∼ 1.5 nm and sample # 2 with a Ta layer thickness of ∼ 5.5 nm), TaC and Ti-6Al-4V under dry air and SBF solution, (c3) typical XPS core level spectra of Ta4f performed on the worn surface for TaC monolayer and TaC/Ta nanocomposites (sample # 1 and sample # 2).