Optimisation of the plasma electrolytic oxidation process efficiency on aluminium

Abstract

Plasma electrolytic oxidation (PEO) is a surface treatment technology enabling fabrication of adherent thick (50-150 µm) coatings on light metal alloys with significantly enhanced hardness, wear and corrosion resistance compared with other conventional treatments. The technology has the potential to play a significant role in the transport sector for replacement of steel with light-weight materials of improved durability. A main limitation of PEO lies in its relatively high cost, associated with high energy consumption and low coating efficiency. The present work explores possible routes to improve the process efficiency. It is shown that a combination of conventional pre-anodising with sequential PEO treatment reduces specific energy consumption up to five times because of an increase of the coating growth rate, up to 10 µm/min, compared with existing PEO processes. A further approach to improved coating efficiency involves PEO in electrolytes with suspended fine or nanoparticles, which results in the formation of thicker coatings in reduced time as a result of the incorporation of the particles from the electrolyte into the coating. Additionally, melting of the coating material during the micro-arc discharge process leads to formation of stabilised high-temperature phases, such as tetragonal and cubic zirconia, which provide significantly improved microhardness of the coating material and give a potential for thermal barrier applications.

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

“…The PEO of a pre-anodized film also reduces the PEO time with a controlled voltage mode, achieving 100 µm coating in 25 min, instead of 1-2 h in conventional soft sparking, if the anodizing time is not counted. To the best of our knowledge, the energy-saving percentage of this scheme is much higher than the other two proposed schemes, namely, solid particle inclusion [13] and a reduction of the gap between anode and cathode [14].…”

Energy-efficient PEO process of aluminium alloys

“…Recently, use of conventional anodizing as a pre-treatment [11,16,22] has been demonstrated to promote the establishment of "soft sparking" regime in currentcontrolled PEO of high purity aluminium, resulting in greater coating growth rates which helps to overcome the cost limitations of the process. However, many commercial and semi-commercial PEO sources operate in voltage or power-controlled mode [1,19], therefore there is a need to investigate the effectiveness of the precursor approach for those cases.…”

Dielectric breakdown and healing of anodic oxide films on aluminium under single pulse anodizing

“…The lower concentration of oxygen (Fig. 8c) and higher concentration of aluminium ( In PEO of aluminium in alkaline silicate electrolyte, less porous and relatively uniform oxide layer can be formed after transition to "soft" sparking [8,9,12,[15][16][17][18]. The present study reveals that healing open discharge pores occurs more readily during single pulse anodizing in alkaline silicate electrolyte, while such healing is not observed in ammonium pentaborate electrolyte.…”

“…The coatings can range from tens to hundreds of micrometres in thickness, and their improved thermo-mechanical properties, corrosion resistance and wear resistance [1,[5][6][7][8][9] are partly associated with the formation of high-temperature oxide phases, such as -Al 2 O 3 , during PEO processing. The process involves a large number of short-lived microdischarges, caused by dielectric breakdown of oxide coatings at high voltages.…”

“…It has been recently reported that relatively thick intermediate layer with low porosity is formed between an inner barrier layer and an outer loose layer containing large cavities and pores under selected PEO conditions [8,9,12,[15][16][17][18]. The formation of the intermediate layer is correlated with so called "soft" sparking, which is characterized by a change in the optical emission spectra, reduction in the size of microdischarges and reduction in acoustic noise [16,19].…”