Morphology evolution of the porous coatings on Ti–xAl alloys by Al adding into Ti during micro-arc oxidation in Na2B4O7 electrolyte

• Published in: Surface & coatings technology Vol. 395; p. 125948
• Main Authors: Guan, Shiwei, Qi, Min, Li, Yada, Wang, Weiqiang
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15-08-2020; Elsevier BV
• Subjects: Al addition; Alloying effects; Alloying elements; Alloys; Binary alloys; Ceramic coatings; Composition; Electric potential; Electrolytes; Electron microscopy; Evolution; Micro-arc oxidation; Microscopy; Morphology; Morphology evolution; Oxidation; Photoelectrons; Porous coatings; Process parameters; Ti alloys; Titanium base alloys; Voltage; X ray photoelectron spectroscopy; Porous coatings; Ti alloys; Morphology evolution; Micro-arc oxidation; Al addition
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2020.125948

Morphology of the porous ceramic coatings formed on the titanium (Ti) alloys during the micro-arc oxidation (MAO) process are closely related to alloy's composition together with the applied voltage/current, time and other processing parameters. In the present work, MAO of binary Ti alloys with different Al contents (0, 3, 6, 10 and 20 wt%) in an electrolyte of 0.1 M Na2B4O7 were carried out to explore the effect of alloying elements on the morphology of MAO coatings. Morphology, composition, and structure of the coatings were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Results showed that as Al content increased, the ejected oxide, deposited around the discharge channel during MAO, gradually accumulated, and the proportion of the Al/B-containing oxide in the deposition slightly went higher. Meanwhile, the “cortex-like” structure presented on pure Ti gradually evolves into the “crater-like” structure presented on the Ti alloy with 20 wt% of Al. Moreover, with raising Al content, the critical breakdown voltage of the coating as well as the working voltage in the stable stage during MAO increased linearly. Finally, an influence mechanism of Al addition into Ti on the morphology evolution of the porous coating formed by MAO in borate electrolyte was proposed.
•The Al content in Ti-Al alloy greatly affects its coating morphology during MAO.•The dissolution of the sprayed oxide during MAO by Na2B4O7 electrolyte was inhibited.•The “cortex-like” structure gradually evolves into the “crater-like” structure.•The breakdown voltage and the stable voltage gradient both increase with the Al content.