Thermal Spray Coating Performance and Coating Design

Coating properties

Your requirements for coating properties in industrial applications, the coating properties you choose should have, the properties of the coating you get, etc. Here's what you should know and master to successfully apply thermal spray technology. Coating properties for thermal spraying include:
General properties of the coating

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Special properties of the coating

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Metallographic evaluation of coatings

 
Porosity

Coating resistance to various types of sports wear.

General assessment.

oxide content

The coating is resistant to corrosion by various media.

Porosity assessment. Oxide content assessment.

Microhardness

The thermal conductivity of the coating.

Phase assessment. Particle deformation assessment.

Bond strength

The conductivity of the coating.

Crack determination.

metallographic microstructure

Machinability of the coating.

Interface contamination assessment.

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Thermal shock resistance of the coating.

Bond strength test.

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Various other specialties of interest.

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Microhardness test.

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Surface roughness inspection.

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Assessed against coating specifications.

coating design

Coating general requirements consist of two aspects: functional requirements and reliability requirements.

According to the main function of the coating, determine its technical requirements. That is, the corrosion resistance of the coating, the wear resistance of the coating, the heat insulation performance of the coating, and the main technical properties that the coating should have when working.

Reliability requirements are what the coating should have, and reliability ensures that the coating can perform its normal function.

The reliability index of the coating is often expressed by some index of the coating, such as: bonding strength, tensile strength, thermal shock resistance, porosity, etc. According to the characteristics of the workpiece coating and according to the requirements of use, a few items are determined as the reliability and functional criteria of the coating. And as the basis for coating test detection. The microstructural characteristics of the coating section can reflect the important characteristics of the coating, such as porosity, bonding state, etc. Taking the metallographic appearance of the coating section as the acceptance standard of the coating quality has been recognized by many well-known foreign companies.

Choice of coating material:

See "How to Correctly Select Coating Material and Spraying Process"

Coating structure design:

There are three forms of coating structure: single structure, mixture structure, multi-layer structure.

Single Structural Coating: Consists of a single material in the thickness direction. A single structure is used in large-area anti-corrosion projects of aluminum spraying and zinc spraying. The process is simple, convenient for large-scale production, and the cost is also low.

Mixture structural coating: The material in the thickness direction is the same, but it is composed of two or more different materials. This is because a single material can no longer meet the requirements of use, and the mixture coating of multiple materials can meet the design requirements. A typical application is the transition layer of thermal barrier coatings, which are generally made of ZrO2 in the surface layer and MCrAlY in the bottom layer in different proportions.

Multi-layer structure: The multi-layer structure can be divided into three types: double-layer, three-layer and multi-layer. Its function is to improve the bonding performance of the coating, disperse the interlayer thermal stress of the coating, and improve the reliability of the coating.

Purpose of structural design:

Improve coating reliability. The principle followed in the structural design is to meet the reliability requirements and ensure the functional requirements, neither of which can be neglected. For example, wear-resistant coating, if the bottom layer + surface layer structure is adopted, the bottom layer has no function, thus reducing the wearable thickness of the wear-resistant surface layer. A single-structure hybrid coating is wear-resistant throughout its thickness.

Coating process design:

The coating process design includes: the surface design of the coated workpiece, the selection and determination of the process method, the design of the spraying tooling of the workpiece, and the post-processing of the coating, etc.

Surface design of coated workpieces: The surface of coated workpieces generally needs to be processed to facilitate coating deposition and post-processing. Rough threading increases contact area and coated grooves increase shear resistance. Surface design also includes surface pretreatment of workpieces.

The choice of process method is determined: each material can be sprayed by different methods. Taking NiCr alloy as an example, it can be sprayed by powder flame, flame wire, arc wire, plasma and HVOF. The cost here is very different, and the performance is also very different. The selection principle of the process method is to choose the one with the lower cost on the basis of satisfying the technical performance index of the coating.

Spraying tooling design: According to different workpieces and different coatings, determine the most suitable spraying process, and then design the tooling for workpiece spraying. During the design, it is required to solve the clamping method of the workpiece, the relationship between the rotation of the workpiece and the movement of the spray gun, the shielding method and shielding parts on the workpiece that do not need to be coated, etc. the

Coating post-processing technology:

If the coating needs to be machined after spraying, the post-processing technology of the coating should be selected correctly.

Coating inspection and quality assurance measures:
Whether the designed coating can finally meet the requirements of use must be assessed by the actual operation results of the coating. But it is also very important to do the necessary tests on the coated samples or coated workpieces.
The reliability inspection of the coating is carried out according to the reliability criterion determined during the design.
The performance index detection of the coating is carried out according to the functional criterion determined during the design.
According to the results of multiple tests, analyze and determine the key quality points in the coating production process. In the subsequent production, the key points of the process are well monitored to ensure the stable and reliable coating quality.