Key factors determining the development of two morphologies of plasma electrolytic coatings on an Al-Cu-Li alloy in aluminate electrolytes

Abstract

The porosity within plasma electrolytic oxidation (PEO) coatings strongly affects their properties, such as the wear resistance. Two typical types of coatings, i.e. bi-layered coatings with large internal pores, which show low wear resistance, and single-layered coatings with excellent wear performance, have been found following PEO of aluminium alloys under pulsed current regimes with an aluminate electrolyte. In this paper, the mechanisms of formation of the different coatings are investigated and discussed based on a thorough investigation of PEO of an Al-Cu-Li alloy using systematic variations of 1000 Hz pulsed current waveforms and electrolyte concentration. Both parameters have important roles in determining the structure of the resultant coatings. The coatings formed in a dilute electrolyte, containing 5 g l-1 NaAlO2, were bi-layered and contained both large pores and pancake structures irrespective of the application of a negative pulse. In contrast, the application of negative current pulse favored the formation of single-layered coatings that contained fewer pancake structures in an electrolyte 32 g l-1 NaAlO2. A more concentrated electrolyte, containing 56 g l-1 NaAlO2, resulted in relatively compact, single-layered coatings independently of a negative current pulse. Similar observations for the coating morphologies were made if the alloy was replaced by high purity aluminium and also for the alloy when the frequency was reduced to 100 Hz. Two different models for the growth of single- and bi-layered coatings are proposed.</p