Citation: Rodriguez, L.; Paris, J.-Y.; Denape, J.; Delbé, K. Micro-Arcs Oxidation Layer Formation on Aluminium and Coatings Tribological Properties—A Review. Coatings 2023, 13, 373. https://doi.org/10.3390/ coatings13020373 Academic Editor: Fengming Du Received: 24 October 2022 Revised: 25 January 2023 Accepted: 30 January 2023 Published: 6 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Review Micro-Arcs Oxidation Layer Formation on Aluminium and Coatings Tribological ingProperties—A Review Louis Rodriguez 1,2 , Jean-Yves Paris 2 , Jean Denape 2 and Karl Delbé 2, * 1 Galvanoplastie Industrielle Toulousaine, 31270 Cugnaux, France 2 Laboratoire Génie de Production, 65000 Tarbes, France * Correspondence: karl.delbe@enit.fr Abstract: This review proposes to carry out a state-of-the-art associated with micro-arc oxidation. Firstly, the different aspects of the growth mechanisms of the oxides are detailed. Then, the formation of micro-arcs and the case of soft-spark treatment are discussed. Then, the electrolytic reactions involved in the layer construction are outlined. We focused on the influence of aluminium alloys on the appearance of the coating and its characteristics before considering the electrolyte formulation. We have concentrated some of our efforts on silicate-based electrolytes, mainly used in research and industry. The importance of electrical parameters in layer formation is detailed later. The main factors studied in the literature are the current source, current density, treatment frequency and duration, and duty cycle. We have also noted the different phase compositions identified in the literature. Finally, since the process is particularly advantageous for protecting the surfaces of aluminium parts against wear, we conclude this review by presenting work on the tribological properties of this coating. In this final section, we highlight the work on the wear-reducing properties and tribological mechanisms identified in the literature. Particular attention is paid to the relationship between the nature of the substrates used, the role of the electrolyte and the counterpart choice on the friction and wear results. Keywords: aluminium; micro-arc oxidation; tribology 1. Introduction Innovative solutions could reduce energy losses due to friction and wear by about 40% over 15 years [1]. Due to its low density and abundance, aluminium, the world’s most widely used non-ferrous metal, remains essential for reducing the overall mass of systems and the energy consumed to operate them. However, its low hardness makes it susceptible to abrasive wear. Therefore, various processes have been developed to increase the surface hardness of aluminium parts. Processes can be applied to valve metals such as aluminium and its alloys to overcome these weaknesses. These include anodising, chemical conversion coating, chemical or physical vapour, deposition electrolysis, laser surface treatment, organic coating, and thermal or cold spraying [2]. MAO can be used to produce coatings on valve metals [3] and non-valve [4] or mixed alloys such as AlSi [5] or Al6Cu [6]. The advantage of MAO is that, although the power consumption can be high, overall, the POE process, particularly the bath chemistry, is simple, safe and environmentally friendly. It is possible to treat all alloys without distinction. The coating results in a high hardness compared to all other processes. The assembly allows the preparation of very large parts with complex geometry. These coatings meet the need for wear and corrosion resistance, refractory properties, electrical insulation and sometimes for decoration [7]. The applications are numerous, particularly in the transport sector: for the automotive and aeronautical industries and in the biomedical, electronic, and aerospace fields, where it is preferred to other conventional surface treatment processes on titanium, magnesium and Coatings 2023, 13, 373. https://doi.org/10.3390/coatings13020373 https://www.mdpi.com/journal/coatings