Tribological behavior of nanostructured nickel

Abstract

The present study reports the effects of electroplating parameters on the microstructure, and thus the mechanical and tribological properties, of nanostructured nickel. Electroplating was conducted in a Watt's type bath with a pH of 3.0 carried out using direct current in galvanostatic mode at current densities of 30 mA/cm2 and 15 mA/cm2 in electroplating bath temperatures of 30°C and 50°C. Grain size and distributions were determined from TEM (Transmission Electron Microscope) and SEM (Scanning Electron Microscope) micrographs. Tribological tests were carried out on a pin-on-disc type tribometer. The same tests were conducted on Ni-200 for the purpose of comparison. Wear rates were calculated for the nickel surfaces using optical profilometry and for the alumina pins using optical microscopy. Nano-indention techniques provided the nanohardness, stiffness, and reduced modulus values for all samples. Microhardness readings were also recorded to further study the surface properties. Results show that electroplating produced thick, dense and uniform nickel deposits with grain size down to 10's of nm and a length/width ratio around 1.8. Lower current densities were found to produce smaller grain sizes while temperature showed a minor effect with higher temperatures producing a broader grain distribution. The grain size and distribution were found to significantly affect hardness and elastic properties with the smallest grain size possessing a hardness that was at least three times higher than that of bulk nickel. Nanostructured nickel showed lower coefficients of friction and wear rates compared to that of bulk nickel and the nanostructured nickel with the smallest grain size exhibited the lowest coefficient of friction and wear rate. These differences were attributed to different wear mechanisms. Bulk nickel showed extensive cracking and evidence of material removal under a wear fatigue mechanism. On the contrary nanostructured nickel exhibited a fine abrasive wear mechanism. This study presents results that suggest a consistent relationship between processing parameters, grain size and distribution, hardness, and wear behavior in electroplated nickel. Understanding of this relationship can be applied to tailor properties and improve behavior of MEMS components.