Modification of Microstructural and Surface-Mechanical Properties of Nickel-Coated Copper by Ultrasonic Shot Peening with Emphasis on Scratch Response

The current research aims to develop a hybrid surface engineering process combining electrodeposition and ultrasonic shot peening (USP) to enhance surface-mechanical properties and coating strength. A thin nickel coating was deposited on the copper substrate through an electrodeposition process and the coating contained few microcracks and pits. The obtained coating was ultrasonically shot peened with different peening times and the number of peening shots (balls). The microstructural evolution, phase analysis, surface hardness, and scratch resistance of nickel coatings and peened coatings were characterized and discussed. The impact of peening based on duration and quantity of shots (ball), which induced grain refinement and compressive residual stress on the surface of the coating has been investigated. The multi-impact shots at a high velocity strike the coated surface and reduce the thickness of the electrodeposited nickel, creating better adhesion. The peening treatment resulted in enhancing the microhardness of the coated nickel from 123 Hv to 328 Hv. The tensile residual stress of coated nickel has been converted into compressive nature. The plastic deformation developed on the coated surface by USP and other factors lowered the coefficient of friction and enhanced the scratch resistance of the coating. Based on the result, it was established that USP has a broad and effective strengthening approach for the nickel coating deposited on soft substrate to increase its compactness and strength.

Graphical abstract