Study of tribological properties of silicon bronze in different structural states

Introduction. Silicon bronzes are widely used in critical friction units due to their combination of corrosion resistance, machinability, electrical conductivity, and satisfactory mechanical properties. Electron beam additive manufacturing (EBAM) is promising for the production of complex parts, but it forms a large columnar grain structure, leading to anisotropy of properties and limiting their practical application. Methods of severe plastic deformation (SPD), such as multi-axis forging and rolling, are effective for refining the structure, eliminating anisotropy, and increasing strength. However, their effect on the overall properties of silicon bronzes has not been sufficiently studied. Purpose of the work. The purpose of this study is a comparative analysis of the effect of the structural state of silicon bronze on its mechanical characteristics and tribological properties under dry sliding friction. Research methods. Samples in five structural states were obtained by electron-beam additive manufacturing (1), hot rolling (2), multi-axis forging (3), rolling at room temperature (4), and low-temperature annealing after rolling (5). The structure of the samples was investigated by optical metallography and transmission electron microscopy. Mechanical tests were carried out by tensile testing of double-sided dog-bone samples and Vickers microhardness testing. Tribological tests for dry sliding friction against 52100 steel were carried out with a constant load and speed. During friction, the friction coefficient (FC), vibrations in the normal and tangential directions, and acoustic emission (AE) were recorded. A detailed analysis of the surface and subsurface layer of friction tracks was performed using confocal laser scanning microscopy, as well as scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). Results and discussion. The microstructure of the samples after EBAM exhibits large columnar grains, while after hot rolling it shows large equiaxed grains with twins. Multi-axial forging results in the formation of subgrains (lamellae) (<100 nm) with a high dislocation density. Rolling leads to further refinement of the original grains and the formation of elongated submicron grains. Low-temperature annealing forms more equiaxed submicron grains (100–200 nm) with a reduced dislocation density and high-angle boundaries. Samples with a coarse-grained structure have low strength and hardness. After SPD, the strength and hardness increase significantly, and the relative elongation decreases compared to the original material. Low-temperature annealing provides maximum strength with partial restoration of ductility and a decrease in hardness. The smallest and most stable friction coefficient, as well as minimum vibration amplitudes (especially in the tangential direction), were recorded for samples after SPD. The greatest wear occurred for the sample in the as-received condition (hot rolling). SPD reduces wear by 2.1–2.2 times compared to the hot-rolled and EBAM samples. Low-temperature annealing increases wear by 10% relative to the sample after rolling. The predominant wear mechanisms were determined to be: mixed (adhesive-oxidative) for bronze after EBAM; adhesive for hot rolled; and oxidative for samples after SPD. Based on metallographic studies, it was found that the depth of subsurface deformation is maximum for coarse-grained samples (145–155 μm) and decreases by 3.3–4.7 times after SPD. Conclusion. A comprehensive study has revealed a decisive influence of the structural state of 96% Cu-3% Si-1% Mn bronze on its key properties. The use of SPD methods (multi-axial forging and rolling) has proven to be highly effective in dramatically improving the mechanical and tribological properties of silicon bronze, regardless of the original production method (hot-rolled steel or electron beam additive manufacturing).