Choosing a thermal spray material for an application is more complex than selecting a wrought or cast material for the same applications because coating properties are not as predictable as those of conventional materials.
However, now many applications are well established and new ones are continuously being developed. The optimal pairing of base material and surface coating properties is now possible and it allows obtaining a combination of characteristics that would not be possible with homogeneous materials.
Aircraft and aerospace industries have provided an ideal proving ground for testing and integrating a few coating concepts. The technology has advanced to a point where it has increased the credibility and reliability of coatings. They have led to applications in other markets such as paper machines, printing, steel and metal processing, in the textile, chemical, oil, gas, automotive industries, for coatings on plastic, parts of pumps, pneumatic and hydraulic systems, near net shaped parts in rapid prototyping, and parts in the turbine, nuclear, electronic, and electrical industries. However it must be kept in mind that different spray techniques exist which are complementary and not competitive in the majority of cases, i.e., most of the time an optimal spray technique exists for a specific application. The most common functions of thermal sprayed coatings are:
– Wear-resistant coatings against abrasion, erosion, cavitation wear, galling, fretting, friction, etc., and often more than one aspect of wear can be addressed. For example, cermets combine hardness, ductility, and acceptable thermal conductivity. Nonstick materials with low-friction coefficients can be combined with hard coatings. Self-lubricating materials can be deposited (for example, materials containing free carbon, or MoS2). Very hard materials can be obtained with ceramics, which can also be combined (for example, ZrO2 and Al2O3). Corrosion and wear-resistant materials can be combined.
– Corrosion-resistant coatings: many materials are used such as zinc, aluminum, nickel-base alloys, copper–nickel alloys, chemically inert ceramics, plastics, and noble metals. One of the big concerns with thermally sprayed coatings used against corrosion is the interconnected porosity. To make the coatings impervious, high-energy thermal sprays are used, or sealants, of course, depending on service temperature. Coatings can achieve protections against high-temperature sprayed on a superalloy bond coat protecting the substrate from oxidation or corrosion. It is worth to note that corrosion by hot gases decreases through the temperature drop achieved within the ceramic coating.
– Abradable and abrasive coatings: these coatings are used in gas turbine engines for clearance control. Blade tips are designed or coated to make grooves in the relatively soft abradable coating face. The coating thus creates a gas-path seal that prevents gases from bypassing the blades, increasing the engine performance. Nickel/graphite, nickel/bentonite, nickel/polyester, and aluminum/polyester are used or more generally a metal matrix with a nonmetallic filler (graphite, polyester, polyimide, boron nitride, or a friable mineral). The abrasive coatings made of oxides or carbides which can also be imbedded in a metallic matrix are applied to the blade tip to reduce wear as it rubs against the abradable coating.
– Electrically conductive coatings: electrical contacts are made of silver, copper, aluminum, tin alloys, and bronze alloys. Thus electrical conductivity depends on the spray technique used and is generally between 40 and 90 % of that of the bulk material.
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