Microstructural evolution in Cu–Sn coatings deposited on steel substrate and its effect on interfacial adhesion

• Microstructural evolution in Cu–Sn coatings on steel substrate was investigated.
• The presence of Cu3Sn precipitates in Cu–Sn coatings was confirmed.
• SnO2 barrier layer formation was predicted in Cu–Sn alloy coated steel interface.
• Cu3Sn and SnO2 checked the Cu-sulfide formation in vulcanized Cu–Sn coated steel.
• Optimally thick Cu-sulfide resulted in best adhesion in Cu–5 wt.% Sn coated steel.

The objective of the present work is to understand the microstructural evolution in Cu–Sn coatings with varying Sn content (3–6.5 wt.%) deposited on steel substrate via immersion coating method and its effect on interfacial adhesion with styrene butadiene (SBR) based bead rubber. The phase formation prediction in different Cu–Sn alloy systems from Pourbaix diagrams constructed using FactSage revealed the formation of higher amount of SnO2 with an increase in Sn content in the coatings. Quantitative depth profiling by glow discharge optical emission spectroscopy, microstructural characterization by transmission electron microscopy and phase analysis by grazing incidence X-ray diffraction confirmed the presence of Cu3Sn precipitates with increasing volume fraction as Sn content in coatings increases. Adhesion strength measured by performing pull-out test on the SBR rubber vulcanized Cu–Sn coated steel wire samples exhibited maximum value for Cu–5 wt.% Sn coating. The formation of Cu3Sn and SnO2 played crucial role in controlling the Cu activity at the coating–rubber interface to form optimally thick Cu-sulfide layer in Cu–5 wt.% Sn coating and thus provided the maximum adhesion strength.