Impact of the multilayer approach on the tribocorrosion behaviour of nanocrystalline electroless nickel coatings obtained by different plating modes

Abstract

This study aims to investigate the tribocorrosion behavior of electroless nickel coatings fabricated by three different means. Multilayered coatings were produced by consecutively stacking three NiP layers with the same phosphorous (P) content. This was accomplished by pausing the plating following each layer growth for sample cleansing and chemical replenishment of the electrolyte. For comparison purposes, two different monolayered coatings were produced in a continuous manner. One of them involved chemical replenishment of the electrolyte (R-monolayer) whereas no action was taken on the other one during the total plating time (Ur-monolayer). The results showed that all the coatings exhibited similar wear volumes under dry wear tests wherein the Ur-monolayer coatings developed fatigue cracks which traversed the coating thickness. Tribocorrosion tests were carried out in a NaCl 3.5 wt% solution and they showed tribolayer formation in all the studied conditions. Moreover, monolayered coatings exhibited fatigue cracks and higher worn volumes than the multilayer ones. The presence of interphases in the multilayer coatings, which originated from pausing the electroless plating, allows enhancing their load bearing capability. Thus, multilayer coatings cope better with the stress derived from the applied normal load and sliding motion during tribocorrosion tests, which translates into a lower plastic deformation of the coatings and 3.5 times smaller worn volumes under the same testing conditions.