Fig. 1. Energy dissipation from friction and emerging material design strategies for low-friction and wear-resistant metals. (a1) Global energy losses associated with frictional dissipation [7], illustrating the vast societal impact of interfacial mechanical energy waste. (a2) Friction-driven economic losses in the mining industry [8], highlighting the industrial urgency for improved tribological efficiency. (b) Distribution of annual energy consumption expended to overcome friction across major industrial sectors [7], underscoring the cross-disciplinary relevance of friction control. (c) Schematic of the highly heterogeneous subsurface stress field generated under sliding contact, which drives strain localization, and crack initiation. (d) Schematic representation of tribo-layer formation during sliding, capturing the dynamic interplay between structural refinement, cracking, and debris formation. (e) Illustrative architectures—including gradient, laminate, and nanotwinned (NT) microstructures—designed to redistribute deformation, suppress damage localization, and promote stable tribo-layer evolution.

Fig. 2. Microstructural gradient units and frictional response of gradient nanograined (GNG) Cu-Ag alloy and 316 L stainless steel. (a) Hierarchical structural units that constitute gradient materials, illustrating typical grain-size gradients as well as other gradient types involving composition, dislocation density, and twin spacing. Complex gradient couplings—such as grain size + composition, grain size + twin spacing, or even grain size + twin spacing + composition + phase—enable a broader design space for optimizing surface mechanical responses. (b1) Representative longitudinal-section scanning electron microscopy (SEM) image of the as-prepared GNG Cu-Ag sample, displaying a continuous microstructural transition across the thickness [2]. (b2) Depth-dependent variations in longitudinal and transversal grain sizes together with corresponding microhardness profiles [13]. (b3) Evolution of COFs with sliding cycles for CG, NG, and GNG Cu-Ag samples [13], demonstrating the superior frictional stability of the gradient structure. (b4) Calculated distributions of applied stress at the contact center and front along depth in GNG and NG samples [2], showing that the gradient architecture effectively mitigates subsurface stress concentration. (b5) Relationship between COFs and initial hardness across diverse metals and alloys [1], revealing a distinct low-friction regime attainable through gradient design. (c1) Surface SEM and subsurface cross-sectional transmission electron microscopy (TEM) images of the front region in CG, NG, and GNG 316 L samples [41]. (c2) Dependence of steady-state COF on applied load [41]. (c3) Two-dimensional cross-sectional wear profiles showing markedly reduced wear depth in the GNG sample [41].

Fig. 3. Structural architectures and frictional behavior of heterogeneous laminate materials spanning macro- to nano-scale layer spacing. (a1) Electron backscatter diffraction (EBSD) inverse pole figure (IPF) map of Cu/CuZn laminates with a layer spacing of 50 µm [85]. (a2) Variation of the COF with layer spacing for laminates tested under sliding directions perpendicular and parallel to the interfaces [85], demonstrating pronounced frictional anisotropy. (a3) Geometrically necessary dislocation (GND) density mapping of the subsurface region in Cu/CuZn laminates slid perpendicular to the interfaces [85]. (a4) Bright-field TEM image of the worn subsurface CuZn layer under perpendicular sliding [85], showing deformation twins. (b1) Cross-sectional images of Ni-W multilayer coatings with layer thicknesses ranging from 2.5 µm to 0.1 µm [86], illustrating precise control of micro-scale layering. (b2) Comparison of COF values between homogeneous and multilayer coatings [86], underscoring the enhanced frictional stability imparted by laminated interfaces. (b3) Schematic illustration of wear-reduction mechanisms in laminate architectures, where interfaces serve as barriers to crack propagation while underlying soft layers accommodate plastic strain, collectively suppressing surface damage. (c1) Structural schematic of a MoS2/WC superlattice film [87], [88]; inset shows a TEM image of the bilayer unit, confirming its periodic nano-scale alternation. (c2) Long-term anti-wear performance of MoS2/Zn coatings on steel substrates, demonstrating extended wear life through interfacial reinforcement [88]. (c3) Schematic representation of multi-contact interface formation and the associated superlubricity mechanism in MoS2/amorphous-metal superlattice coatings [88], where interfacial sliding and in-situ tribo-film formation cooperatively minimize frictional energy dissipation.

Fig. 4. Structural characteristics and frictional behavior of NT materials. (a1) Variation of the average COF with twin lamella spacing (λ) [123], underscoring the critical role of nano-scale twin architecture in tuning frictional response. (a2) Cross-sectional views of Cu samples with various λ after nano-scratching [123]; white arrows highlight the widening of TBs and the formation of mechanically induced twins. (b1) Comparison of relative wear resistance between NT Al-Ni alloys and other metallic materials [124]. (b2) Schematic illustration of microstructural evolution in NT Al-Ni alloys during sliding, showing the progressive formation of a gradient structure through twin boundary migration and grain refinement/rotation/sliding [124]. (c1) Dependence of COF on surface hardness for the present gradient-nanotwinned (GNNT) materials and reference Cu and Cu-alloy system [67]. (c2) TEM and high-resolution TEM (HRTEM) images showing TB migration and grain-rotation-mediated coalescence in the worn subsurface of the GNNT sample [67]. (d) Scaling laws and dominant deformation mechanisms in NT Ni across nano-scale, micro-scale, and macro-scale frictional loading regimes as functions of λ [43]. At the nano-scale, friction exhibits a nonmonotonic λ-dependent scaling behavior governed by the competition between dislocation-twin interactions and detwinning. At the micro-scale, both COF and wear rate decrease monotonically with reducing λ, driven by the FCC-to-HCP phase transformation. At the macro-scale, friction becomes λ-independent, while larger λ values promote wear reduction through the formation of stable oxide films and near-surface gradient structures.

Fig. 5. A unified paradigm for friction and wear reduction through heterogeneous microstructural design [2], [13], [43], [65], [67], [85], [98], [133], [134], [135], [136], [137], [138], [139], [140], [141], [142], [143]. (a) Literature comparing tribological properties of HS and homogeneous metals and alloys, showing the correlation between wear rate and COF. HS materials occupy a distinct low-friction, low-wear domain, highlighting the synergistic benefits of structural heterogeneity in distributing stress and delaying damage. Because the data are drawn from different studies conducted under varying test conditions (e.g., counterbody material/geometry, normal load, sliding speed/distance, environment, and configuration), direct cross-study comparisons should be interpreted with caution; the plot is intended to highlight overall trends rather than provide a strictly controlled ranking. The corresponding testing conditions and references for each dataset are summarized in Supplementary Table S1. (b) Calculated subsurface shear-stress distributions under varying scratching loads and contact radii, obtained using Hamilton’s model. The results illustrate the transition from nanometer-scale to micrometer-scale contacts and the accompanying changes in stress magnitude and state. (c) Conceptual full-scale design framework for HS metals, linking architectural descriptors—gradient depth and slope, laminate period, interface density, and twin spacing λ—to operational parameters such as load, contact radius, maximum Hertzian pressure, and sliding velocity. This framework connects hierarchical microstructures with tribological loading scales, enabling predictive control of frictional energy dissipation and wear resistance across regimes.