Fe-, Ni-containing ceramic-like PEO coatings on titanium and aluminum: Comparative analysis of the formation features, composition and ferromagnetic properties

Highlights

• PEO layers are formed on Ti and Al in electrolytes with iron and nickel hydroxides.

• Fe and Ni concentration in the coating pores depends on the substrate metal.

• Fe–Ni crystallites give ferromagnetic properties to Ti-sample.

• Pore-lining Fe–Ni layers are responsible for ferromagnetism of Al-sample.

Abstract

Fe-, Ni-containing coatings have been formed on aluminum and titanium by the plasma electrolytic oxidation (PEO) technique in slurry electrolytes containing colloidal particles of Fe(III) and Ni(II) compounds including hydroxides. Their composition, surface morphology, and magnetic properties were studied and compared. In both cases, the concentration of iron, nickel, and substrate metal is higher, and the oxygen concentration is lower in the pores of the coatings than their average concentration over the surface. In the case of titanium, the electrolyte and substrate metals are concentrated in the pores (at. %: 38.6 Fe, 4.7 Ni, 19.5 Ti) in the form of micro- and nanosized crystallites. In the pores of the coating on aluminum, metals are concentrated in the composition of the layers lining the pores (at. %: 15.3 Fe, 50.6 Ni, 18.8 Al). The values of the coercive force and the saturation magnetization of the ferromagnetic component at 300 K for the aluminum sample (Н_с = 38 Oe, M*_s = 2.8 × 10⁻⁴ emu/g) are significantly lower than those for the titanium sample (Н_с = 73 Oe, M*_s = 7.8 × 10⁻³ emu/g). The discrepancy between the ferromagnetic properties of the aluminum and titanium samples correlates with the difference in the elemental composition of the pores and the presence/absence of crystallites in the pores. More pronounced ferromagnetic properties of titanium samples can be associated with the presence of crystallites in the coating pores.

Introduction

Plasma electrolytic oxidation (PEO) technique is the electrochemical formation of ceramic-like oxide structures on the surface of anodically or anodically-cathodically polarized metals and alloys in electrolytes under spark or microarc electric discharges [1,2]. Along with the processes of electrochemical oxidation, discharges cause plasma- and thermochemical reactions, which makes it possible to form coatings of a certain chemical and phase composition, containing oxides of the substrate metal and electrolyte components, as well as compounds based on them. Depending on the chemical composition, ceramic-like PEO structures can have different properties including protective [3,4], optical [5], catalytic [6], biocide [7], biocompatible [8,9], hydrophobic [10], and sensory ones [11,12]. To date, it has been shown that functional coatings with different magnetic characteristics can be obtained using PEO technique [[13], [14], [15], [16], [17], [18]]. Note that « ferromagnetic oxide coating/paramagnetic metal » composites find various applications in the designs of electromagnetic radiation absorbers [19,20], microwave waveguides [21], in microelectronics [[22], [23], [24], [25]], in devices of catalysts on metal substrates [26], in sensors [23,27], or as implants in medicine [28].

As shown in Refs. [18,[29], [30], [31], [32], [33], [34], [35], [36], [37]], to obtain ceramic-like oxide coatings with ferromagnetic properties, electrolyte-sols can be used, which release negatively charged dispersed particles of transition metal hydroxides, for example, iron and/or cobalt hydroxides during hydrolysis. As a result of the penetration of negatively charged particles of transition metal hydroxides into the channels of electric discharges from the electrolyte followed by their high-temperature transformations, micro and nanocrystallites are formed in the pores of the PEO coatings. It has been experimentally confirmed that the magnetic characteristics of PEO coatings can be controlled by changing the composition and ratio of transition metal hydroxides in the electrolyte, and, consequently, the composition and size of crystallites in the pores.

Among mixed ceramic-like oxide layers containing metals of the iron triad, the magnetic properties of Fe-, Co-containing PEO coatings formed on various valve metals have been studied in detail [[33], [34], [35]]. There are several works on the formation of Fe-, Ni-containing PEO-coatings [34,38]. At the same time, the regularities of the plasma-electrolytic formation of Fe- and Ni-containing coatings, their composition and magnetic properties have been insufficiently studied. The purpose of this work is to carry out a comparative analysis of the features of plasma electrolytic formation of Fe-, Ni-containing ceramic-like oxide coatings on aluminum and titanium, their composition, surface morphology, and magnetic characteristics.