Effect of crystal size on the tribological behavior of manganese phosphate coatings under lubricated sliding

Abstract

In this study, the wear and friction behavior of manganese phosphate coatings with different crystal sizes were investigated. Crystal size was controlled modifying the chemical composition of the phosphating bath, particularly the concentration of the activator which modifies the number of nuclei for crystal growth. Activator concentration range used for this work varied from 0 to 0.7 g/L, and crystal size was measured using image analysis software on scanning electron microscopy photographs. Available volume for lubricant retention was determined measuring the phosphated surface with a 3D optical profilometer. At the same time, lubricated wear tests were carried out using a ring-on-block configuration at low speeds (23 mm/s) and high loads (14,500 N). Wear behavior was determined as the sliding distance to failure, which was noticed through signs of removal of the phosphate along with the increase of coefficient of friction. It was found that there is a competition between the availability of volume to hold the lubricant, which increases with the crystal size, and the surface coverage, which diminishes as the crystal size grows. Optimal results were obtained for an activator concentration of 0.3 g/L, which meant an average crystal size of 16 µm.