EFFECT OF PEO DURATION ON MORPHOLOGY AND STRENGTH CHARACTERISTICS OF COATINGS

Abstract

The patterns of changes in the morphology and strength characteristics of oxide coatings for the aluminum alloy AD-0 under conditions of different duration of plasma-electrolytic oxidation in a 1.0 M alkaline electrolyte K 4 P 2 O 7 solution have been established. The changing of the molding voltage has a classic view – four stages, the first three (pre-spark, spark, microarc) contributes to the gradual hardening of the alloy surface, and at the fourth (arc) stage microhardness decreases and becomes instable, as well as delamination of the surface layer occurs. According to the experiment, the best combination of the PEO coating properties (microhardness H V = 109.98 kg/mm 2 , maximum homogeneity, no surface roughness) is reached at an initial current density of i = 5 A/dm 2 , followed, in 9 minutes, reducing up to i = 3 A/dm 2 to maintain the process in the microarc mode; the total duration of treatment is 11 – 13 min. In doing so the hardness of the oxide layer surface in comparison with the unprotected alloy increases up to four times. The study of the influence of temperature and heat treatment time indicates that the resulting oxide coatings are not recommended to be used as hard and wear-resistant at temperatures above 300 °C. The research of the surface morphology of the samples shows that in the process of PEO a fine-grained structure is formed, which with a prolonged processing time tends to enlarge and agglomerate the cells. With a 10 min. duration of oxidation the formed oxide coating has a light gray color, its surface is uniform, which is explained by the incorporation of phosphates from the working electrolyte solution into the defective structure of the surface. However, the transition to the arc mode with a duration of oxidation more than 13 min. leads to substantial roughness and heterogeneity of the coating structure. The aggregate of the identified factors indicates the prospect of the research direction, further work will be aimed at obtaining oxide coatings with desired functional properties for aluminum alloys in various electrolytes with a minimum duration of oxidation.