Applying high voltage cathodic pulse with various pulse durations on aluminium via micro-arc oxidation (MAO)

• Published in: Surface & coatings technology 2018-08, Vol.347, p.278-285
• Main Authors: Yilmaz, Mustafa Safa, Sahin, Orhan
• Format: Article
• Language: English
• Subjects: Aluminium; Aluminum; Aluminum base alloys; Aluminum oxide; Anodic coatings; Cathodic coating (process); Cathodic protection; Coating; Coating effects; Diamond pyramid hardness; Electric potential; Electrolytes; High voltages; Micro-arc oxidation; Oxidation; Phase distribution; Protective coatings; Pulse duration; Surface roughness; Thickness measurement; Transitional aluminas; X-ray diffraction
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2018.04.085

The present study is an investigation of the effect of pulse duration on properties of coatings formed on aluminium alloy in alkaline solution via using of high voltage cathodic pulses. Nine different anodic-cathodic pulse couples with various durations were selected for processing. In order to find out the effect of pulse duration on coating: duty cycle, anodic voltage, cathodic voltage, processing time, electrolyte temperature and electrolyte composition were kept constant. Scanning electron microscope (SEM), profilometer, X-ray diffractometer and indenter were employed to investigate the microstructure, surface roughness, phase distribution and hardness of the coatings. Also, coating thickness measurement was carried out to find out the effect of pulse duration on productivity. Varying of pulse duration caused significant differences among thickness of coating layers. Coating layers with thickness between 45 and 75 μm and surface roughness between 2.5 and 5.5 μm were obtained. Coatings consist of two layers, outer porous layer and inner dense layer. The outer porous layer was γ-Al2O3 and the inner dense layer was a mixture of γ-Al2O3 and α-Al2O3. The hardness of inner dense layers was between 1100 and 1700 Vickers. •Increasing of pulse duration lead to decrease in thickness of dense layer and increase in pore size.•The formation α-Al2O3 was depends on pulse duration.•An inner dense layer of 50 μm was achieved by 20 min of processing time.•α-Al2O3 and γ-Al2O3 phases existed on all coatings.