Plasma Electrolytic Oxidation (PEO) Process-Processing, Properties, and Applications

Abstract

Plasma electrolytic oxidation (PEO) is a novel surface treatment process to produce thick, dense metal oxide coatings, especially on light metals, primarily to improve their wear and corrosion resistance. The coating manufactured from the PEO process is relatively superior to normal anodic oxidation. It is widely employed in the fields of mechanical, petrochemical, and biomedical industries, to name a few. Several investigations have been carried out to study the coating performance developed through the PEO process in the past. This review attempts to summarize and explain some of the fundamental aspects of the PEO process, mechanism of coating formation, the processing conditions that impact the process, the main characteristics of the process, the microstructures evolved in the coating, the mechanical and tribological properties of the coating, and the influence of environmental conditions on the coating process. Recently, the PEO process has also been employed to produce nanocomposite coatings by incorporating nanoparticles in the electrolyte. This review also narrates some of the recent developments in the field of nanocomposite coatings with examples and their applications. Additionally, some of the applications of the PEO coatings have been demonstrated. Moreover, the significance of the PEO process, its current trends, and its scope of future work are highlighted.

Keywords: additives; corrosion; nanocomposite coating; plasma electrolytic oxidation; tribology.

Figure 1

Number of papers published in the research area of plasma electrolytic oxidation treatment in the period from 2000 to 2021 (data taken from the web of science).

Figure 2

Comparison of common surface treatment techniques, a limitation for valve metals and alloys. Adapted from [12].

Figure 3

A schematic diagram explaining PEO coating formation on a 1060 Al substrate. Adapted from [40]. Copyright Elsevier, 2020.

Figure 4

A schematic diagram explaining primary stages of an oxide layer generation in anodizing and PEO process. Adapted from [38]. Copyright IntechOpen, 2012.

Figure 5

Processing conditions in corrosion resistance of PEO coatings. Adapted from [39].

Figure 6

Comparison of coating thickness obtained from electrolytes versus time. Reproduced with permission from [101]. Copyright Springer Nature, 2019.

Figure 7

Graphs of voltage versus time for a PEO process pertaining to (a) AJ62 Mg alloy and (b) AM50 Mg alloy. Reproduced with permission from [12]. Copyright IntechOpen, 2014.

Figure 8

Snapshots during PEO coating depicting size and color of micro discharges produced at durations of (a) a few seconds, (b) 1 min, (c) 15 min, (d) 45 min, and (e) 100 min. Reproduced with permission from [104]. Copyright Elsevier, 2007.

Figure 9

Depiction of surface morphology and distribution of elements for PEO coating produced by (a) silicate electrolyte, (b) phosphate electrolyte, and (c) mixed electrolyte of silicate and phosphate. Reproduced with permission from [40]. Copyright Elsevier, 2020.

Figure 10

EDS spectra for (a) Si-coating, (b) P-coating, and (c) mixed Si-P coating. Reproduced with permission from [40]. Copyright Elsevier, 2020.

Figure 11

SEM images obtained after PEO process on a titanium sample in the presence of (a) K₃PO₄ electrolyte and (b) K₄P₂O₇ electrolyte. Reproduced with permission from [108]. Copyright Elsevier, 2011.

Figure 12

Potentiodynamic polarization curves of PEO coatings developed. Reproduced with permission from [129]. Copyright MDPI, 2018.

Figure 13

Heat flux of AZ31 Mg substrate and PEO coatings with various CNT concentration. Adapted from [129].

Figure 14

Illustration of the Ca₃(PO₄)₂ layer formation on the PEO coating after being immersed in an SBF solution where (a) is without ZnO nanoparticles and (b) is with the incorporation of ZnO nanoparticles. Reproduced with permission from [130]. Copyright Elsevier, 2019.

Figure 15

FE-SEM surface morphology for samples Z0 (a–c), Z1 (d–f), Z2 (g–i) and Z3 (j–l). Reproduced with permission from [130]. Copyright Elsevier, 2019.

Figure 16

Discharge image captured by the high-speed camera when applied on (a) thick coating and (b) thin coating. Reproduced with permission from [144]. Copyright Elsevier, 2015.

Figure 17

Description of events taking place during single discharge: (a) break down at beginning, (b) formation of plasma channel through coating, (c) beginning of bubble growth and generation of oxide, (d) enlargement of bubble and heating of adjacent region, (e) decrease of bubble area due to cooling, and (f) last stage signifying quenching and removal of liquefied oxide from discharge channel. Reproduced with permission from [144]. Copyright Elsevier, 2015.

Figure 18

Sketch of the PEO experimental setup with stainless steel as a cathode and AM50 alloy as anode. Reproduced with permission from [151]. Copyright Elsevier, 2016.

Figure 19

Graphs depicting (a) average current density on the front and back sides of the substrate versus electrode distance, and (b) average coating thickness produced against electrode distances of 10, 20, 40, 60, 80, 100, 120, and 240 mm for both front and back sides. Reproduced with permission from [151]. Copyright Elsevier, 2016.

Figure 20

SEM micrographs of PEO coating on 6082 Al alloy developing coating thickness of (a) 5 μm and (b) 40 μm. Reproduced with permission from [152]. Copyright Elsevier, 2005.

Figure 21

Variation of (a) friction coefficient and (b) wear rate. Reproduced with permission from [174]. Copyright Elsevier, 2015.

Figure 22

Friction versus sliding time graph for samples with addition of MoS₂ additive. Reproduced with permission from [174]. Copyright Elsevier, 2015.

Figure 23

Bar charts comparing (a) COF for AZ91 with and without graphite, (b) COF for AZ80 with and without graphite, (c) wear scar depth of AZ91 with and without graphite, and (d) wear scar depth for AZ80 with and without graphite. Adapted from [175]. Copyright Elsevier, 2018.

Figure 24

A graph comparing average friction coefficient and wear rate when sample experience different voltages. Adapted from [176].

Figure 25

SEM micrograph of coating (a) before thermal shock test (b) after thermal shock test. Reproduced with permission from [192]. Copyright Elsevier, 2014.