Surface Finishing Tutorial

#329 Characteristics of Thermal Spraying Method

Category : Thermal Spraying

February 2, 2018

(1)General characteristics

General characteristics of thermal spraying method are summarized below.


	1.	There is no limitation on materials or dimensions for the workpieces to be sprayed.
	2.	Films can be formed by using various spraying materials from metals to ceramics.
	3.	Laminated films consisting of different materials can be formed.
	4.	The thermal effect to the substrates can be controlled under 200 °C.
	5.	Spraying can be applied to localized areas where needed.(Masking is required.)
	6.	Spraying is available on site.However, rainy weather must be avoided.
	7.	The film forms quickly.
	8.	Drying time is not necessary.
	9.	The yield ratio is low for some shapes.
	10.	It is difficult to form an even layer over complexly shaped workpieces.
	11.	It is difficult to coat the inner surfaces of a long tube or a tube having a small diameter.
	12.	Dust and noise control measures are needed.

(2)Spraying materials

Most of the spraying materials are in the wire or powder forms. The powder materials must have good liquidity. The particle sizes ranging from 5 to 50µm are used.Examples of metal-based materials are Zn, AL, Ni, Mo, W, Ta, Ni-AL, Ni-Cr, hastelloy, metals such as self-fluxing alloy (a type of brazing filler metal), alloy metals, and composite metals.
The cermet layers have high degree of hardness along with metallic properties because cermet materials are made of oxides, carbines, nitrides, borides, or silicides combined with metals.
As part of green procurement replacing industrial chrome plating using hexavalent chromium, WC-Co and WC-Co-Cr based materials are adopted. "65(ZrO₂MgO)-35(Ni-AL)" is an example of the composite of ceramics and metals.
Ceramic materials are mostly the oxides including AL₂O₃, AL₂O₃-TiO₂, TiO₂, ZrO₂, ZrO₂-Y₂O₃, ZrO₂-MgO, Cr₂O₃, and AL₂O₃-SiO₂.

(3)Processes of thermal spray application

The common application processes are as follows:

	1.	Degreasing (Eliminate grease on the workpiece.)
	2.	Surface roughening (Shot-blasting is generally applied for eliminating the altered layer and achieving the anchor effect.)
	3.	Preheat (The objective is to make the spray material temperature to be closer to the melting temperature.)
	4.	Thermal spraying
	5.	Post-processing (For machine components, machine finish processing is added. To improve rust and corrosion resistance, sealing treatment or coating is applied.)

If it is necessary to apply thermal spraying locally, the area to be excluded from spraying must be covered with a protective material after degreasing.

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