Solution-based synthesis of AgI coatings for low-friction applications

Abstract

Thin films of AgI have been synthesized from Ag surfaces and elemental I₂ using a rapid and simple solution-based method. The effect of using ultrasound during the synthesis was studied, as well as the influence of the nature of the solvent, the I₂ concentration, the time, the temperature, and the sonication power. The films were characterized using X-ray diffraction, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy, and found to consist of β-AgI, possibly along with some γ-AgI. It was found that sonication increases the film thickness and grain size. The nature of the solvent has a profound effect on the film growth, with mixtures of water and ethanol leading to thicker coatings than films synthesized using either component in its pure form. Selected coatings were tribologically tested, and the AgI coating was seen to lower the friction coefficient significantly compared to a reference Ag surface under otherwise identical conditions. Long lifetimes (over 30000 cycles) were seen against a Ag counter surface. Tracks and wear scars were studied using SEM and Raman spectroscopy, and it was found that the friction level remains low as long as there is AgI in the points of contact. This method is found to be a simple and fast way to deposit AgI on Ag with large possibilities of tuning the thickness and grains sizes of the resulting films, thereby optimizing it for the desired use.