Study of tribological and friction behavior of magnesium phosphate coating and comparison with traditional zinc phosphate coating under dry and lubricated conditions

Abstract

In this work, the friction behavior of magnesium phosphate (MP) and zinc phosphate (ZP) coatings in different conditions were evaluated. Initially, the phosphate coatings were applied on AISI 4130 steel substrate and the optimum coating conditions were obtained. The coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and magnetic thickness measurement. The friction behavior of both bare and phosphate coated steels was investigated by means of pin on disc method in three different conditions: dry, continuous lubrication and pre-lubricated. Sodium stearate soap was used as the lubricant. The results indicated that the friction behavior of phosphate coatings is not suitable in dry condition; the average friction coefficient and weight loss is high. In this case, the weight loss of ZP coating is less than MP coating. The best friction behavior is achieved for zinc phosphate coating in continuous lubrication. The average friction coefficient of ZP and MP coatings are 0.09 and 0.12, respectively. However, in pre-lubricated condition, the friction behavior of MP coating is better than ZP coating such that the friction coefficient of ZP and MP coatings are 0.22 and 0.19, respectively. This difference can be attributed to lubricant adsorption capability of the coatings.