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#336 Mechanical Applications of Thermal Spraying-1

Category : Thermal Spraying
March23, 2018

This volume introduces the mechanical application examples of thermal spraying.

(1)Thermal spraying for improving corrosion resistance

To add rustproof and anti-corrosion properties to large structures, such as bridges and steel towers, zinc, aluminum, and zinc-aluminum alloy are used for thermal spraying.In general, a thermal spray layer of as thick as 150 to 300 µm is applied after a pretreatment process consisting of sandblasting which is designed to clean and roughen the surface.
The humidity control is an important factor during this process. If the humidity is not below 80%, the corrosion resistance is significantly degraded.Sealing treatment must be applied immediately after thermal spraying (within four hours).This treatment not only seals tiny holes on porous films but also improves adhesiveness of the topcoat because of the anchor effect.
Coatings that are appropriate for the intended operating period are applied over the sealant.In this way, the recoating interval can be drastically extended by applying thermal spraying as an undercoat.

(2)Thermal spraying for improving chemical resistance

Various thermal spraying methods are adopted for applications requiring higher chemical resistance, such as chemical equipment.Spraying materials having high chemical resistance do not necessary form the films that are strong enough to withstand varying chemical agents. The materials should be evaluated by taking test pieces out and exposing them in the operating environment for a long period.
In general, metals or ceramics are adopted for films requiring high chemical resistance, which are processed by wire flame spraying or powder flame spraying.For large structures, spraying is applied on site.
One of the applications is to use nickel chromium, nickel aluminum, tantalum, or molybdenum as the undercoat over the sandblasted substrate and to apply ceramic spraying on the topcoat, followed by sealing treatment.Another method of the applications is to spray cermet, which is a mixture of undercoat metal and topcoat ceramics, between the undercoat and the top coat.
It is also possible to improve chemical resistance by re-melting the multi-layered films with heat and eliminating the pores.

(3)Thermal spraying for improving abrasion resistance

Abrasion occurs in various forms.However, these varying types of the abrasion, including adhesive wear, abrasive wear, fatigue wear, and corrosive wear, hardly occur by itself; one or more types of abrasion and wear are observed concurrently.When a certain degree of abrasion is expected, lubricants are used.
Since the spray coatings are porous, using oil will improve the lubricating properties.Commonly, spraying materials such as high chrome stainless steel and molybdenum are applied over the shafts and role shaft bearings by wire flame spraying or plasma spraying.
Besides this method, plasma spraying of various metals, ceramics, and cermet is widely used for the industrial purposes.