Electroless Coatings

1. Current Applications and Development Areas in Electroless Plating

Electroless plating is a technique in which metal ions are obtained through autocatalytic or chemical reduction methods and deposited onto a material surface [1]. Although the electroless plating method has a long history, its widespread industrial use dates back only 20 years. Fundamentally, an electroless plating bath consists of at least one metal salt as a source of metal ions and a reducing agent. However, auxiliary components such as stabilizers, buffer chemicals or surfactants are frequently required, individual reagents regularly assume multiple roles, and solution composition and reactivity change depending on time and conditions. For example, pH changes, consumption of reducing agents, formation of by-products, bath life, and changes in catalytic interfaces significantly complicate electroless plating processes [2]. Among industrial applications today, electroless nickel plating is the most widely used electroless plating process due to its superior properties. The most commonly used types of electroless nickel coatings are Ni-P and Ni-B alloy coatings. Electroless nickel coatings have a very broad field of application in the automotive, oil and gas, aerospace, chemical, mining, plastics and electronics industries due to the advantages listed below. a. Excellent corrosion and wear resistance, b. Uniform coating thickness, c. Coating hardness can be controlled through various processes, d. Solderability, e. Lubricity, f. Magnetic properties, g. Low labor cost, h. Can be applied to materials with active surface properties or surfaces that can be activated (metal, ceramic, glass, plastic, etc.). In this study, current research on electroless plating and development areas have been evaluated.

1.1. Multi-Alloy System Electroless Coatings

Generally, metals that have a more positive standard potential than the metal complex reducing agent and that meet the conditions for the deposited metal to provide autocatalytic growth by being sufficiently active in anodic reducing agent oxidation can be obtained by the electroless plating method (Table 1). [caption id="attachment_144147" align="aligncenter"] Table 1. Metal coatings obtainable by the electroless plating method [3].[/caption]As is known, not only pure metal coatings but also binary alloy system coatings can be obtained by the electroless plating method. However, when service conditions are considered, it becomes necessary to add other elements to the coating metal that have beneficial effects on hardness, friction, wear, conductivity, optical, magnetic, thermal stability and corrosion properties. Currently widely studied and applied ternary alloy systems include Ni-P-B, Ni-Fe-P, Ni-Co-P, Ni-P-Mo, Ni-Zn-P, Ni-W-P, Ni-Pt-P and Ni-Cr-P. The elements Co, Cu, Fe, Mo, Sn and W that can be added to the Ni-P alloy system can also be added to the Ni-B alloy system to obtain nickel boron-based ternary alloy systems. Successful results obtained with ternary alloy systems have paved the way for electroless coatings with quaternary alloy systems, and work in this direction has begun.

1.2. Multi-Layer Coatings

Multi-layer coatings are characterized by the combination of two or more layers in the same coating structure. Multi-layer coating systems obtained by combining coatings with different structures and properties enable improvement of the hardness, wear and corrosion resistance combination of coatings. On the other hand, there is also the possibility of combining layers of the same structure by creating artificial intermediate phases throughout the coating thickness. In this case, the presence of these intermediate phases parallel to the substrate surface can perform the task of stopping defect progression and preventing dislocation movement, increasing mechanical and protective properties beyond the protective limit offered by single-layer coatings [3]. However, to obtain maximum efficiency from multi-layer coatings, the coating sequence, number of layers and thicknesses must be very carefully designed considering the coating composition and properties. There are two different bath approaches for obtaining electroless multi-layer coatings. These are the dual or multiple bath approach (Figure 1) and the other is the single bath approach. In the multiple bath approach, separate plating baths are prepared and the substrate material is subjected to plating processes sequentially depending on the coating sequence and thickness. [caption id="attachment_144149" align="aligncenter"] Figure 1. Dual or multiple bath approach[/caption] In recent years, a single bath approach combining hybrid electroless plating baths in which multi-layer coatings can be obtained in a single bath has been employed. This coating approach consists of a combination of electroplating and electroless plating baths, which is a single plating bath that allows the deposition of different metals present in the same electrolyte. The surface and cross-section view of the Ni-P/Ni-B duplex coating obtained by the dual bath approach is shown in Figure 2. [caption id="attachment_144148" align="aligncenter"] Figure 2. Surface and cross-section view of Ni-P/Ni-B duplex coating applied on powder metal compact material [4].[/caption] 

1.3. Electroless Composite Coatings

Although the history of composite coatings obtained by the electroless plating method dates back to the 1960s, with developments in the field of materials science, the increase in diversity and quality of nano-sized particles produced has increased interest in electroless composite coatings and research in this area has accelerated. The process of bonding solid particles added to the plating bath to the coating metal on the surface in electroless plating emerged from the idea of benefiting from their hardness, friction, wear and corrosion resistance. Within the last decade, numerous studies have been conducted in this field, and in these studies electroless Ni-P and Ni-B alloy composite coatings have been obtained using carbides, nitrides, oxides and carbon-based solid particles. Particles commonly used in composite coatings include Si3N4 (Figure 3), Al2O3, graphite, PTFE, WC, MoS2, TiO2, WS2, CNT, SiO2, ZrO2, TiN and BN. The use of different coating alloys in this field and the production of hybrid composite coatings can be considered as open areas for development. Furthermore, particles added to the plating bath can cause deterioration of the plating bath, settling and agglomeration of particles in the bath, and the ability of particles to bond homogeneously with the coating metal at maximum levels, considering the cost of nano particles, emerge as important problems that need to be overcome in this field. [caption id="attachment_144150" align="aligncenter"] Figure 3. Electroless Ni-P-Si3N4 composite coating[/caption]

1.4. Electroless Plating of Micro and Nano-Sized Materials

One of the recently studied subjects is coating processes applied to metal, plastic and ceramic-based particles of different shapes and sizes on micro and nano-sized particles to modify their surface properties. Among the application objectives, the foremost is improving the wetting capability of ceramic particles used as reinforcement in composite materials within liquid metal. In addition, particles of different compositions are being obtained, and the application is also being used to impart conductivity and magnetic properties to the particles. For particles with passive surface properties to be coated, surface activation processes must first be applied. Among the development areas in this field, the foremost are the development of economical surface activation techniques to obtain active surfaces on all surfaces of the particles, prevention of agglomeration of particles added to the plating bath, and minimization of particle losses occurring in activation processes and coating and intermediate washing operations. [caption id="attachment_144151" align="aligncenter"] Figure 4. Electroless Ni-P coated SiC nano fiber.[/caption]

1.5. Nanomaterial Production via Electroless Plating

The electroless plating method can be used not only for coating bulk parts as traditionally encountered but also for producing nanomaterials of different shapes and sizes at the nano scale. Due to the method's ability to provide equal coating thickness regardless of the shape of the substrate material or catalyzed surface and the ability to obtain different metal or alloy structures, electroless plating has become one of the preferred methods. As shown in Figure 5, by promoting coating growth on catalyzed surfaces of materials with different shapes and sizes having passive surface properties, nanomaterials with structures such as particles, nanowires, nanotubes, nanoplates and nanostructures of different sizes (0D: zero-dimensional, 1D: one-dimensional, 2D: two-dimensional, 3D: three-dimensional) can be obtained. [caption id="attachment_144152" align="aligncenter"] Figure 5. Schema of template-assisted electroless nanomaterial syntheses arranged by increasing product dimension [2].[/caption]

2. Conclusions

In this study, in which current research on the electroless plating method and problems encountered in these studies were evaluated, the development of different alloy systems, creation of new coating designs related to multi-layer coatings and development of new multi-layer coating methods, conduct of research on nano particle-reinforced hybrid composite coatings and development of nano particle production through electroless plating process have been evaluated as areas open for development.     References 1. U. Matik, Akımsız Ni-P kaplamalarda Kaplama Parametrelerinin Belirlenmesi ve Mekanik Özelliklerine Etkisi, Gazi Üniversitesi, Fen Bilimleri Enstitüsü, Doktora Tezi, 2010. 2. F. Muench, Electroless Plating of Metal Nanomaterials, Wiley Library, ChemElectroChem 2021, 8, 2993 –3012. 3. F. Delaunois, V. Vitry and L. Bonin, Electroless Nickel Plating Fundamentals to Applications, CRC Press Taylor & Francis Group, 2021. 4. U. Matik, Effect of crystallization on wear and corrosion behavior. of electroless Ni-P/Ni-B duplex coating on ferrous PM compacts, Kovove Mater. 58 2020 247–254.     Assoc. Prof. Ulaş Matik Karabük Üniversitesi TOBB Teknik Bilimler MYO Makine ve Metal Teknolojileri Bölümü