Ultrasound-assisted electrodeposition and synthesis of alloys and composite materials: A review

Highlights

• The application of ultrasound in electroplating materials was reviewed.

• Ultrasound process variables influence the particles deposition.

• Ultrasound-assisted electrodeposition improves alloys corrosion resistance.

• Morphology, composition, and crystallinity are altered by acoustic cavitation.

Abstract

The development of electrodeposited materials with improved technological properties has been attracting the attention of researchers and companies from different industrial sectors. Many studies have demonstrated that the electrodeposition and synthesis of alloys and composite materials assisted by ultrasound may promote the de-agglomeration of particles in the electrolytic solution due to microturbulence, microjets, shock waves, and breaking of Van der Waals forces. The sonoelectrochemical technique, in which the ultrasound probe acts as a working electrode, also has been used for the formation of nanostructures in greater quantity, in addition to accelerating the electrolysis process and eliminating the reaction products on the electrode surface. Regarding the morphological aspects, the acoustic cavitation promotes the formation of smooth and uniform surfaces with incorporated particles homogeneously distributed. These changes have a direct impact on the composition and physical properties of the material, such as corrosion resistance, magnetization, wear, and microhardness. Despite the widespread use of acoustic cavitation in the synthesis of nanostructured materials, the discussion of how process variables such as acoustic power, frequency, and type of ultrasound device, as well as their effects still are scarce. In this sense, this review discusses the influence of ultrasound technology on obtaining electrodeposited coatings. The trends and challenges in this research field were reviewed from 2014 to 2019. Moreover, the effects of process variables in electrodeposition and how these ones change the technological properties of these materials were evaluated.

Introduction

The electrodeposited coatings include nanostructured materials with multilayer and ceramic particles in the metallic, or polymeric matrix. The matrix is usually composed of metal, alloy, ceramic, or polymer, while the particles may have a spheroidal, layered, plate-like, or core-coated shape, with a size order from sub-millimeters to nanometers [1]. These composite coatings are anticorrosive, wear-resistant, hydrophobicity, and have a good tribological performance that make them applicable in the manufacturing process of many industrial sectors [2]. Recent studies have demonstrated the diverse applications of the composite materials [3], [4], [5], [6], [7], [8], as illustrated in Fig. 1. The development of nanometer-scale composites with different morphological structures, composition, and size distribution, can be obtained through the application of acoustic cavitation originated by ultrasound treatment.

Galvanized coatings can significantly improve their physical properties such as hardness, resistance to wear and corrosion when the particles are dispersed in the solution [9]. Promising novel coatings have been developed in last years. Some studies have sought to focus on different operational parameters during the electrodeposition process to increase the concentration of particles in the coating [10]. However, the increase in particle concentrationand the decrease in the concentration of electroactive species were discarded due to instability of the dispersion at high concentrations and problems related to mass transport affected by reduced conductivity of the solution [11]. The agglomeration of particles leads to an increase in the sedimentation rate, impairing thermophysical properties, such as viscosity and thermal conductivity, and pressure drop [12].

The use of ultrasound technology has shown high potential for this purpose, since it can prevent particle agglomeration, promoting uniform surfaces with higher particle content [13]. Also, the use of ultrasound during the electrodeposition leads to the size reduction of porous structures through the hydrogen absorption in the deposit, as well as the formation of a homogeneous microstructure, composed of fine and crystalline granules, with uniform distribution. Ultrasonic waves can play a relevant role in the orientation of crystals and stability related to adhesion between the film and the substrate [14]. This emerging technology has been applied in the fabrication of materials for drug delivery [15], scaffolds for gingival cells growth [16]; environmental process of wastewater treatment [17]; food sector, for the extraction of compounds [18] and dairy products processing [19], and in the gas production, such as H₂ [20].

During the process, acoustic waves are propagated in the liquid medium, causing the vibrational movement of compression and expansion of the molecular structure. Therefore, the distance between the molecules varies according to the oscillation. If this vibrational intensity reaches the state in which the molecular structure becomes unstable, the formation of bubbles occurs, generating the acoustic cavitation phenomenon [21]. Thus, the application of ultrasound in liquid media can alter physical and chemical aspects through the generation of turbulent convection and temperature increase due to the formation and subsequent collapse of bubbles [22]. These changes decrease the thickness of the diffusion layer and improve the mass transport, degassing the electrode surface, and promoting an increase in the reaction rate and generation of hydroxyl radicals [23]. The basic forms of emitting ultrasonic waves and promoting changes in the material can occur directly by converting mechanical energy into acoustic from the transducer to the probe, inducing vibrations in the material; and indirectly, through the cavitation induced in the liquid medium, by the propagation of acoustic waves [24]. However, the influence of ultrasound process parameters on the electrodeposition of composites and metal alloys was poorly studied and requires further studies. In this context, this review presents trends in the application of ultrasound technology in the electrodeposition of metallic coatings and how this emerging technology could improve the technological properties of these materials.