Friction and wear depend critically on surface roughness and its evolution with time. An accurate control of roughness is essential to the performance and durability of virtually all engineering applications. At geological scales, roughness along tectonic faults is intimately linked to stick-slip behaviour as experienced during earthquakes. While numerous experiments on natural, fractured, and frictional sliding surfaces have shown that roughness has self-affine fractal properties, much less is known about the mechanisms controlling the origins and the evolution of roughness. Here, by performing long-timescale molecular dynamics simulations and tracking the roughness evolution in time, we reveal that the emergence of self-affine surfaces is governed by the interplay between the ductile and brittle mechanisms of adhesive wear in three-body contact, and is independent of the initial state.

Wear is the inevitable damage process of surfaces during sliding contact. According to the well-known Archard’s wear law, the wear volume scales with the real contact area and as a result is proportional to the load. Decades of wear experiments, however, show that this relation only holds up to a certain load limit, above which the linearity is broken and a transition from mild to severe wear occurs. We investigate the microscopic origins of this breakdown and the corresponding wear transition at the asperity level. Our atomistic simulations reveal that the interaction between subsurface stress fields of neighboring contact spots promotes the transition from mild to severe wear. The results show that this interaction triggers the deep propagation of subsurface cracks and the eventual formation of large debris particles, with a size corresponding to the apparent contact area of neighboring contact spots. This observation explains the breakdown of the linear relation between the wear volume and the normal load in the severe wear regime. This new understanding highlights the critical importance of studying contact beyond the elastic limit and single-asperity models.

We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme, a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipitation. Both the high-entropy and the multiphase sample are then subjected to simulated particle irradiation. The damage accumulation is analyzed and compared to an elemental Ni reference system. The results reveal that the high-entropy alloy—independent of the initial configuration—installs a certain fraction of short-range order even under particle irradiation. Moreover, the results provide evidence that defect accumulation is reduced in the high-entropy alloy. This is because the reduced mobility of point defects leads to a steady state of defect creation and annihilation. The lattice defects generated by irradiation are shown to act as sinks for Cu segregation.

In this review, we discuss our recent advances in modeling adhesive wear mechanisms using coarse-grained atomistic simulations. In particular, we present how a model pair potential reveals the transition from ductile shearing of an asperity to the formation of a debris particle. This transition occurs at a critical junction size, which determines the particle size at its birth. Atomistic simulations also reveal that for nearby asperities, crack shielding mechanisms result in a wear volume proportional to an effective area larger than the real contact area. As the density of microcontacts increases with load, we propose this crack shielding mechanism as a key to understand the transition from mild to severe wear. We conclude with open questions and a road map to incorporate these findings in mesoscale continuum models. Because these mesoscale models allow an accurate statistical representation of rough surfaces, they provide a simple means to interpret classical phenomenological wear models and wear coefficients from physics-based principles.

The low temperature heat capacity of amorphous materials reveals a low-frequency enhancement (boson peak) of the vibrational density of states, as compared with the Debye law. By measuring the low-temperature heat capacity of a Zr-based bulk metallic glass relative to a crystalline reference state, we show that the heat capacity of the glass is strongly enhanced after severe plastic deformation by high-pressure torsion, while subsequent thermal annealing at elevated temperatures leads to a significant reduction. The detailed analysis of corresponding molecular dynamics simulations of an amorphous Zr-Cu glass shows that the change in heat capacity is primarily due to enhanced low-frequency modes within the shear band region.

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemental copper by atomic resolution imaging. The same grain boundary phases are found by computational grain boundary structure search indicating a first-order transformation. Finite temperature atomistic simulations reveal a congruent, diffusionless transition between these phases under ambient pressure. The patterning of the grain boundary at room temperature is dominated by the grain boundary phase junctions separating the phase segments. Our analysis suggests that the reduced mobility of the phase junctions at low temperatures kinetically limits the transformation, but repulsive elastic interactions between them and disconnections could additionally stabilize the pattern formation.