Electrostatic Spray Deposition Technique
Electrostatic spray deposition (ESD) is a process to deposition both thick and thin layers of a non-volatile coating on various substrates, usually metal. The process is known for its high-quality results, which can range from a Matt finish to a gloss. The most common substrates that are used for this process include metal, wood and ceramics. Other than the usual steel, other materials may be used as well, including plastic, textiles and plastics. It is one of the most widely used methods worldwide in many sectors such as aerospace and automotive, power generation, chemicals, and building and construction.
The electrostatic spray deposition process consists of four main steps. The first step is preparation of the base material to be deposited, which is commonly a metallic or tungsten inert coating. The second step is the mechanical attachment of the deposited thin films, which may involve mechanical roll forming, or the use of energy-based techniques such as vapor compression, electromagnetic induction and mechanical cutting. The third and last step is the encapsulation of the thin films, which is achieved through mechanical positioning or vacuum-based methods.
The electrostatic spray deposition technique uses a two-step process. The first step is known as the preparation of the base material. This includes the cleaning of any surface impurities such as loose dyes, deposits, or grease, and the preparation of the desired layer of the electrostatic coating. In some cases, ionization of the precursor is also required. For instance, the preparation of the nickel-zinc alloy requires the ionization of an electrochemical catalyst.
The second step in electrostatic spray deposition is the mechanical attachment of the deposited thin film. Two types of mechanical methods are used – conductive and contact. In the case of conductive methods, the electrode materials are placed on the collars and then the material, which is to be coated, is passed through the electrode with the help of an electric current. In the contact method, the substrate, which is the base material, is passed through the electrodes, which are electrically charged, and is then coated by the ionized compound.
Electrostatic spray deposition uses two types of electrode materials – one to emit the ions, and the other to catch them. The most commonly used is the tungsten electrodes. While there are other metallic oxides, which also perform well, they have drawbacks such as a tendency to build up a lead oxide, which is a health hazard. Tungsten electrodes are not rust proof and hence it is not feasible to place the electrodes in places where there are likely to be metalworking activities.
There are various ways in which electrostatic spray deposition is executed. The mechanical deposition method involves passing of spray through the substrate, which is in liquid state. This is done by making a passage through the material and then spraying the coating that is to be deposited on the surface. This technique yields faster deposition of the electrode materials and produces lesser spatter. The wet deposition technique uses moist suspension and produces better results than the dry process.
For electrostatic spray deposition to take place, there are certain criteria that need to be met. Usually, there are two types of processes – the wet and dry processes. In the wet process, the mixture of the compounds is heated to a particular temperature and then injected into the electrolyte solution. This leads to the generation of electrostatic blends (which is not totally) and precipitates the active components of the compounds.
In the dry process, no heat is applied and the compounds are left to settle out. Solvent is then added and these react with the electronegativities of the precursor compounds. Hence, electrostatic spray deposition occurs when the solvents are present on the face of the electrode in the form of thin layers. The thickness of the layer depends on the thickness of the electrode, which determines the amount of solvents present.