February 2018 Archives

#332 Ceramic Spraying - Plasma Spraying

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Ceramic spraying uses ceramics consisting primarily of aluminum oxide, chromium oxide, titanium oxide, or zirconium oxide as spraying materials, in the following methods: plasma spraying, powder flame spraying, and rod flame spraying.

(1)Plasma thermal spraying films

[Table 1] shows the types and symbols of plasma thermal spraying films.

[Table 1] Types and symbols of plasma thermal spraying films
TypeSymbolMain ingredients of the spraying materialsIntended use
Alumina spraying Type
1 A
P-WAO-1AL2O3 98%
or more
Abrasion
resistance
Type
2 A
P-WAO-2Chemical
resistance
Type
3 A
P-WAO-3Heat resistance
and insulation
Type 1 BP-GAO-1AL2O3 94% or more
TiO2 1.5-4.0%
Abrasion
resistance
Type 2 BP-GAO-2Chemical
resistance
Type 3 BP-GAO-3Heat resistance
and insulation
Alumina/titania sprayingType 1 AP-AO-TiO-13-1AL2O3 85% or more
TiO2 12-14%
Abrasion
resistance
Type 2 AP-AO-TiO-13-2Chemical
resistance
Type 1 BP-AO-TiO-40-1AL2O3 58% or more
TiO2 39-41%
Abrasion
resistance
Type 2 BP-AO-TiO-40-2Chemical
resistance
Titania sprayingType 1 AP-TiO-1TiO2 98%Abrasion
resistance
Type 2 AP-TiO-2Chemical
resistance
Chromium oxide
spraying
Type 1 AP-CrO-1Cr2O3 98% or moreAbrasion
resistance
Type 2 AP-CrO-2Chemical
resistance
Type 1 BP-CrO-TiO-3-1Cr2O3 89% or more
TiO2 2-3%
SiO2 4-5%
Abrasion
resistance
Type 2 BP-CrO-TiO-3-2Chemical
resistance
Spinel sprayingType 3 AP-AO-MgO-29-3AL2O3 68%
MgO 28-30%
Heat resistance
and insulation
Zirconia sprayingType 2 AP-ZrO-2ZrO2 98% or moreChemical
resistance
Type 3 AP-ZrO-3Heat resistance
and insulation
Type 2 BP-ZrO-SiO-32-2ZrO2 64% or more
SiO2 31-34%
Chemical
resistance
Type 3 B P-ZrO-SiO-32-3Heat resistance
and insulation
Type 2 CP-ZrO-CaO-7-2ZrO2 90% or more
CaO 6-8%
Chemical
resistance
Type 3 CP-ZrO-CaO-7-3Heat resistance
and insulation
Type 2 DP-ZrO-MgO-24-2ZrO2 73% or more
MgO 23-25%
Chemical
resistance
Type 3 DP-ZrO-MgO-24-3Heat resistance
and insulation
Type 2 EP-ZrO-YO-8-2ZrO2 89% or more
Y2O3 7-9%
Chemical
resistance
Type 3 EP-ZrO-YO-8-3Heat resistance
and insulation
Type 2 FP-ZrO-YO-12-2ZrO2 85% or more
Y2O3 11-13%
Chemical
resistance
Type 3 FP-ZrO-YO-12-3Heat resistance
and insulation

* Note) AL: Aluminum, Cr: Chromium, Ca: Calcium, Zr: Zirconium, Ti: Titanium, Si: Silicon, Mg: Magnesium, Y: Yttrium

#331 Structure of Thermal Spraying Films-2

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(3) Pores on the films

During thermal spraying in the air, flying particles are exposed to the air. As a result, the surfaces of flying particles will be covered with oxide films, or particles in the semi-solid or melted state will collide with the substrate because they are cooled too fast.Since the thermal spraying films are the layers of these particles deposited, they contain tiny pores.
The porosity changes by the spraying methods, materials, and conditions, but it is usually somewhere between 1% and 8%. The porous films containing these pores have oil-bearing properties, meaning that they retain lubricant and are abrasion resistant. These pores are also known to have adiabatic and soundproof effects.
On the other hand, when porous films are placed in the corrosive environment containing corrosive liquid or gas, the substrate is prone to corrosion, and the coatings may peel off or they are likely to develop swelling when corrosive products enter into these holes.
To prevent such problems, sealing treatment is applied.The sealing treatment includes application of sealer made of silicic acid-based resin, phenol resin, or fluorine-based resin, baking, plating, and other methods.
Another common method adopted for ceramic thermal spraying films is to spray metal on top of them and melt it with heat.

(4) Roughness of the film surface

Right after the thermal spraying application, the surfaces are rough.The degree of roughness will vary based on spraying method, spraying materials, granularity, spraying conditions, and sandblasting particle sizes. For example, using #325 carbide sprayed by the high velocity flame method, the surface roughness will turn out to be around 1 to 3 Ra.
Depending on the intended use, this type of roughness can be reduced by post-processing, such as polishing. To take advantage of the larger friction coefficient resulting from the rough surface, it is used as-is for paper-feeding rolls, or to improve the heat efficiency by the increased surface area, it is used as-is.

(5) Hardness of the films

To measure the hardness of thermal spraying films, the micro Vickers hardness is commonly used for the load 0.1 to 0.3 N. Since the films contain pores, notice that the measured values may be inconsistent.
[Table 1] shows an example of physical properties that the films will have. The films are not only used for improving abrasion resistance but also adopted for the fittings of aircraft parts, because the layers of deposited spray particles are capable of dispersing stress.

[Table 1] Example of thermal spraying films physical properties
Spraying materialsCoating hardness Hv (0.2)Surface roughness (RZ)
Ni-Cr-Mo250 - 350110
Ni-Cr-Mn350 - 480150
Co-Mo-Cr450 - 56030
AL2O3-2.5TiO2700 - 85080
AL2O3-13TiO2650 - 85080
Cr2O3-SiO2-TiO2850 - 110080

#330 Structure of Thermal Spraying Films-1

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(1)Characteristics of the films

The structure of film sprayed in the air is extremely complex and has the following characteristics:

 1. It is a buildup of particles having boundaries.
 2. It has pores.
 3. It contains oxidation products.
 4. Residual stress exists.
 5. The material and film compositions may change before and after thermal spraying.
 6. Both unmelted and semi-molten particles may be present.
 7. The film has a rough surface.
 8. The films are mainly bonded mechanically, but metallurgical bonding may occur depending on the spraying method.

[Fig.1] is the schematic diagram of a film structure.

[Fig.1] Schematic diagram of a thermal spraying film section

(2)onding mechanism of the film

The substrate and spray particles are generally bonded by the combined actions of the following: mechanical bonding (anchor effect) where particles attach to the concave and convex portions of the substrate's rough surface, metallurgical bonding, and/or physical bonding caused by intermolecular attractions, such as Van der Waals force.
The bonding mechanism varies by the substrate materials and surface shapes. For example, when using plasma spraying of Mo, Ta, and W on a steel or aluminum substrate, there will be a localized area that is bonded by metallurgical reactions resulting from diffusion.In addition to the mechanical bonding, the spray particles are bonded by the diffusion of hydrogen ions or metallic ions that are reciprocally generated during collisions of the particles between the oxide films.
One of the methods of increasing the adhesion properties would be to spray in a thin layer of materials that are known to bind strongly between the substrate and the film, such as Mo, W, Ni-AL, Ni-Cr.This method is called "bonding coat". The films formed by this method will alleviate heat expansion between the substrate and the thermal spraying film, while improving corrosion resistance and adhesiveness.The bonding coat is usually applied before spraying oxide ceramics.

WC spraying over a paper roll

#329 Characteristics of Thermal Spraying Method

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(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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