Thermal barrier coatings are designed to protect metal structural components from extreme elevated temperatures, thereby reducing stress and fatigue and increasing the part’s lifespan. In order to provide such a high level of protection, thermal barriers incorporate several key components. Every coating consists of four distinct layers, with each layer adding to the protective thermal properties and enabling the coating to form a unique thermal barrier.
Thermal Barrier Coating Composition
A typical thermal coating simply consists of a metallic layer paired with a ceramic layer. The mismatched thermal expansion coefficients, however, can cause the bond to be compromised where it adheres to the substrate. Sometimes, a “bond coat” is used between the metal substrate and the ceramic layer to aid in adhesion. These types of thermal barrier coatings have four different layers.
The first part of the thermal coating is the metal substrate itself. Usually the metal that thermal coatings work well with is a single or polycrystalline cobalt or nickel alloy mixed with other elements depending on the desired properties of the end-product.
The first layer of the coating is called the bond coat—it’s what enables the coating to bond to the substrate and therefore plays an integral role in forming a thermal barrier. The bond coat is typically a metallic layer made of a nano-structured ceramic-metallic composite that adheres the layer to the metal substrate and is responsible for generating the second coating layer of thermally grown ceramic oxide, which occurs when the coating is subjected to a high temperature.
When nano-particles of aluminum oxides and nitrides are distributed throughout the bond coat or along its surface, the formation of thermally grown oxides is catalyzed. This ceramic layer is responsible for forming a uniform, thermally protective barrier by acting as an oxygen diffuser which prevents the substrate from becoming thermally oxidized.
The last layer of the coating is the ceramic top coat, which is made of yttria-stabilized zirconia (YSZ). The top coat protects the substrate by keeping the other coating layers at a lower temperature than the surface.
Advantages of Thermal Barrier Coatings with a Bonding Layer
When the bonding layer is used to aid in adhesion, several advantages present themselves. The coating itself is harder and more wear resistant, and the stress that can sometimes occur in the top coat from overly expanded thermally grown oxide is reduced. Additionally, thermal shock resistance is increased as is the overall lifespan of the thermal barrier coating.
Thermal barrier coatings can be applied using spray equipment, which can be operated automatically in an enclosure designed to deal with fumes. Sometimes, because of the small production run size or the specialized nature of a product, manual application of the coating is necessary. Either way, whether the application is automatic or manual, there are several elements associated with spray coating application that should be prepared for: noise, light, dust and fumes, and heat and electricity.
Thermal spray application is a loud process because the equipment employs compressed gas. If the process takes place in an enclosure so as to isolate the machine, noise can be greatly reduced.
Different kinds of spray application either use combustion spray equipment or electric arc spraying. In combustion spraying the flame that drives the spray head is very bright and hot. In electric arc spraying, potentially harmful ultraviolet light is created. Ultra-violet absorbent glass should be used in the construction of the enclosure, or in the case of manual application, operators should wear protective gear. Special care should be taken to avoid directly looking at the nozzle of the arc spray gun—special screens to block the nozzle from view are often used.
Dust and Fumes
Because the spray process produces a fair amount of dust and fumes, proper extraction is key to ensure that buildup doesn’t inhibit the process or get trapped in the coating itself. Masks and breathing filters should be worn if manually applying the coating, or if the machine can’t be isolated and contained. Knowing the chemical components present in the coating is also helpful when attempting to prevent exposure to hazardous material. Handle each coating as specific instructions indicate.
Heat and Electricity
The fuel used to power combustion spray guns depends on a combination of gases, namely oxygen and acetylene, to provide force and pressure. Although oxygen is not explosive, it can ignite materials if too much oxygen is present. Therefore, carefully monitoring the machinery for oxygen leaks is critical to running a safe application process. With electric arc guns, it’s not heat but voltage that should be handled with care—although the voltage is low, the current is high.