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#337 Mechanical Applications of Thermal Spraying-2

Category : Thermal Spraying
March30, 2018

(1)Thermal spraying for improving heat resistance

This sounds like a low-tech example, but the steel chimneys are known to corrode when they are exposed to heat under approximately 550 °C. Corrosion occurs not because of the heat itself but for the air and moisture content combined.To form films that can withstand such conditions, aluminum spraying films of approximately 150 to 200 µm thick are applied by wire flame spraying, followed by sealing treatment using silicone-based materials.In the same manner, to achieve heat resistance up to 900 °C, sealing treatment using asphalt-based materials is applied after forming aluminum spraying films.
To withstand temperature of as high as 750 to 1000 °C without sulfur, sealing treatment using asphalt-based materials is applied after forming nickel-chromium alloy spraying films (200 to 300 µm).For the same temperature range conditions where sulfur is present, sealing treatment using asphalt-based materials is applied over nickel-chromium alloy spraying films (350 to 450 µm) + aluminum spraying films (80 to 130 µm).

(2)Thermal spraying for improving heat insulation

Thermal spraying is also used for preventing heat dissipation from the surfaces of machinery and equipment.For this purpose, thermal spraying films having low heat conductivity are used.Ceramics such as magnesium oxide, and zirconium-based materials containing calcium oxide, are the common materials.
To prevent the films from being peeled off due to the difference of heat expansion rates between the substrate and ceramics, spraying materials having enhanced oxidation and corrosion resistance at high temperature, such as nickel-chromium alloy and nickel-chromium-aluminum alloy, are adopted for the undercoat. In addition, cermet containing both of these materials can be sprayed between the undercoat and the ceramic coatings, as a measure of alleviating heat expansion.

(3)Thermal spraying for biomaterials

As the human life expectancy increases, more people start experiencing problems with body parts and organs.To substitute these, developments of artificial organs, such as heart, and artificial bones are becoming more popular than before.
Dentures are the typical example of artificial body parts that have been around quite a while.For a missing tooth, a bridge is mounted between the existing teeth using metals after drilling the surrounding teeth. A ceramic or dental alloy crown is placed over a chipped or discolored tooth after preparing the tooth to fit into it.
If no tooth is left, full dentures are attached over gum lines.If no root is left due to progressed cavities or periodontal diseases, artificial dental roots need to be formed in the gum.This is called implants.Materials for implants should not be harmful to human body. They also need to be corrosion resistant and be strong enough to withstand the biting force, while being compatible with other parts of the human body.
Implants are artificial tooth roots that are mounted on secure jawbones by drilling a hole in them.Eventually, the jawbones grow around the artificial roots and the implanted roots are secured in place.On the artificial roots, artificial teeth are attached with screws.Titanium is the common material for artificial roots.To improve the affinity with tissues, titanium powder and hydroxyapatite, a component of bones, are sprayed over the implants.Thermal spraying films are also used for artificial joints.