How to choose coating material and spraying process correctly

The performance and quality of the thermal spray coating are mainly determined by the coating material you choose, the spraying process and its process parameters.

The selection of coating materials should be carried out according to the following principles:
First of all, it must meet the application requirements of the coating, that is, meet the requirements of the actual operating conditions of the coating and meet the requirements for the service life of the coating.
Comprehensive consideration of the applicability, processability and economy of the material, that is, the material can not only meet the application requirements, but also meet the requirements of the spraying process, and it is more economical.
When a single material cannot meet the requirements, the principles of multi-material compounding and multi-layer coating compounding can be adopted. In some working conditions, multiple materials can be combined to form a composite coating; when a single-structure coating cannot meet the requirements, a multi-layer structure can be used.

Since the composite coating should have the same function throughout the coating thickness, the selection of composite coating materials should pay attention to:
The comprehensive performance of multiple materials must meet the requirements of use.
It is not only compatible with the substrate material, but also has good compatibility among various materials in the coating.
The deposition efficiency of different materials when spraying should be considered.

When selecting materials for multi-layer coatings, attention should be paid to:
Choose according to the different performance requirements of the material.
Bonding bottom material should consider its bonding performance and re-covering performance; surface layer material is mainly selected according to the function required by the workpiece for the coating.
Compatibility of base material and surface material.

Comprehensive analysis of applicability, processability and economy in material selection:
Since the application requirements of materials are often multifaceted, the main application requirements should be highlighted when selecting materials, taking into account other performance requirements. For example, coatings that require abrasion resistance often also require corrosion resistance. When selecting, the main function is wear resistance, and the corrosion resistance requirements are taken into account.
Good processability, good fluidity, easy to transport, controllable, high deposition efficiency.
It is not necessary to one-sidedly emphasize the service life of the coating, and it is best to have the same service life as the workpiece.

The choice of coating material should also take into account the influence of the spraying process on the material:
The material changes after the spraying process. Such as materials will produce oxidation, carbon loss and phase change. Its performance will also change. For example, carbon steel will be oxidized and lose carbon after spraying, titanium oxide materials will lose oxygen after spraying, and carbide ceramic materials will lose carbon and phase change, reducing the hardness. After spraying zirconia, a reversible phase change will occur, accompanied by a certain volume change.
The deposition efficiency of different materials is different. For composite coatings, the composition ratio will change after spraying.
Different from materials made by conventional methods such as casting and forging, the coating is a layered structure composed of countless deformed particles, contains certain pores, the material is anisotropic, and there are various complex stresses in the coating.

After the coating material is determined, the spraying process should be determined according to the application requirements. Selection principle of spraying process:
The spraying process is determined according to the application requirements of the coating and the coating material.
The selected spraying process should be suitable for coating the workpiece itself.
When there are two or more process methods that can meet the requirements, the one with the lowest cost should be selected.

When choosing a spraying process, you can generally make the following choices:
Determine the spraying process according to the application requirements of the coating. As long as the requirements can be met, low-cost processes are preferred. If the technical requirements for the coating are not strict, and the metal or alloy material with a melting point below 2000°C, flame spraying can be selected.
For high melting point ceramic materials, such as zirconia, chromium oxide, etc., the working conditions of the coating are relatively harsh, and the workpiece is a more expensive key component, so plasma spraying can be considered. Because the plasma spraying material has a wide range of adaptability, the melting point of the material is not limited, the heat source is non-oxidizing, the process parameters can be finely controlled, and the reproducibility of the coating is good.
For large-scale metal spraying projects with a large amount and a wide area, the arc spraying process can be selected. For example, large-scale aluminum spraying and zinc spraying anti-corrosion projects adopt arc spraying process. Due to the high efficiency, low cost and relatively good quality of arc spraying, it is widely used in nickel-chromium alloy spraying of boiler "four tubes" and stainless steel spraying of paper machine drying cylinders.
For metals, alloys and certain cermet coatings that require high bonding strength and low porosity, hypersonic flame spraying (HVOF) or detonation spraying can be used.