Friction anisotropy and associated surface deformation mechanisms in heterogeneous copper/bronze laminates

Heterogeneous laminates have demonstrated an extraordinary combination of strength and ductility. However, their response to tribological loading remains elusive. Here, a series of bulk heterogeneous Cu/CuZn laminates with layer spacings from 20 to 200 μm were prepared, to systematically investigate the role of layer spacing and sliding direction on friction and wear. We find that below a critical spacing value or above a tribological stress threshold, the friction coefficient and wear rate during sliding perpendicular to the laminate interface are much lower than the parallel one. When sliding parallel to the interfaces, the formation of a brittle nanostructured tribolayer dominates in the CuZn layers for all sliding cycles. While sliding perpendicular to the interfaces in the early stage, many deformation twins and dislocations form close to the interfaces in the CuZn layer, producing strain gradients and thus alleviating the strain localization. In the late stage of the sliding contact, friction-induced chemical mixing perpendicular to the interfaces is significantly stimulated below the critical layer spacing, mitigating the formation of a delaminating tribolayer. The observed friction anisotropy is intimately related to interface-induced strain delocalization, providing guidelines in designing the heterogeneous laminates with outstanding tribological properties.