Structure and composition of copper-aluminum friction coatings

The properties of a friction coating for machine parts obtained through thermomechanical and diffusion processes can be predicted by establishing a correlation between morphology parameters, elemental distribution, and coating synthesis conditions. To investigate the relationship among these factors, the authors examined the structure and composition of a copper-aluminum coating synthesized by in situ friction cladding on steel substrates. Surface and cross-sectional morphology were studied using scanning electron microscopy (SEM) in backscattered electron mode. Fractal analysis employing the grid method was performed to quantitatively characterize the complex crack system. The depth distribution of Cu, Al, and O, as well as the layer thickness, were determined by Rutherford backscattering spectroscopy (RBS). The obtained copper-aluminum coating exhibits a pronounced multilevel structure. RBS data confirmed the formation of a multilayer system comprising a surface Al₂O₃ oxide film (~5-15 nm), an aluminum-enriched layer with a copper gradient, a transition diffusion zone, and a copper substrate. SEM analysis revealed scale-invariant defect organization, ranging from mosaic polygonization (20-80 μm) to an ultrafine-grained substructure (0.5-2 μm), along with the presence of oxide inclusions (50-200 nm). The fractal dimension of the crack network, measured at 1.917 ± 0.076, indicates a high degree of self-similarity over a scale range of 10-300 μm. The defect density at the micro level reaches 45.7%. Crack formation arises from the combined effect of thermomechanical stresses and the brittleness of intermetallic phases generated under conditions of active interdiffusion. Concurrently, frictional heating activates surface oxidation. Elucidating the fundamental relationship between the parameters of the friction process, the fractal nature of defects, and the coating composition will enable the targeted tailoring of properties in bimetallic systems.

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