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Quality Inspection Methods for Thermal Spray Coatings
Release time:
2019-12-12 17:01
Source:
Quality Inspection Methods for Thermal Spray Coatings
The quality of a coating directly determines the success or failure of surface engineering applications, as well as the service life and safety performance of components—even entire devices. Therefore, it is essential to conduct quality inspections on coated surfaces. For quality inspection of coated surfaces, there are three basic methods: visual inspection, non-destructive testing, and destructive sampling inspections.
1. Visual inspection
(1) Purpose of visual inspection
Apart from protecting the substrate and achieving certain functional specifications, coatings almost always require a certain level of aesthetic quality and must be free from obvious defects. Moreover, visual inspection can, to some extent, reveal deficiencies in the coating process itself. Therefore, visual inspection of coatings is the most basic and common type of inspection; if the appearance fails to meet the required standards, there is no need to proceed with testing other parameters.
Inspecting surface coatings with the naked eye or a low-magnification magnifier is one of the most fundamental methods. For some thicker thermal spray coatings, the naked eye can readily detect surface defects such as unevenness, porosity, oxidation, flaking, spatter, surface cracks, and delamination.
(2) Check conditions
To facilitate clear observation of coating surface defects and prevent interference from external factors, visual inspection of coating surface defects should be conducted on a designated appearance inspection workbench or in an appearance inspection booth.
When performing visual inspection under natural lighting, the specimen should be placed on a white platform free of reflected light and examined using naturally scattered light that shines clockwise.
When using artificial lighting for visual inspection of workbenches and inspection boxes, the illuminance should be approximately 300 lux, closely resembling natural daylight (equivalent to the illuminance at 500 mm from a 40W fluorescent lamp). During visual inspection, the distance between the specimen and the naked eye should be no less than 300 mm. For critical coatings and specimens with special requirements, magnifying glasses with magnification factors of 2 to 5 may be used for inspection.
The specimens used for inspecting coating appearance defects must have their coated surfaces cleaned before inspection.
2. Nondestructive Testing
Thermal spray coatings can also be quality-tested using nondestructive testing (NDT) techniques.
NDT technology detects internal and surface defects in various engineering materials, components, structural parts, and more by leveraging changes in responses—such as thermal, acoustic, optical, electrical, and magnetic properties—that arise from anomalies or defects within the material's internal structure, all without causing damage to the object being inspected. It also enables the identification and evaluation of defect types, characteristics, quantities, shapes, locations, dimensions, distributions, and their changes.
There are many types of NDT techniques. Among them, ultrasonic testing, radiographic testing, eddy-current testing, magnetic particle testing, and penetrant testing are the five most commonly used and most mature inspection methods, often referred to as conventional nondestructive testing techniques. In addition to these five conventional NDT techniques, there are several other NDT methods that are also widely applied, such as neutron radiography, computed tomography (CT) scanning, acoustic emission testing, noise-based diagnostic techniques, magnetoelastic testing (including Barkhausen noise detection and magnetoacoustic emission testing), leakage magnetic field detection, industrial endoscopy, laser holography, microwave testing, and leak detection techniques.
Each nondestructive testing method has its own specific application range and cannot fully substitute for one another. No single method is universally applicable, and no single testing method can provide all the information required. Therefore, it is essential to select one or two NDT techniques based on the specific type of defect being examined.
In terms of defect types, they are generally classified into two categories: volumetric defects and area-type defects. Table 1 lists various volumetric defects and the corresponding NDT methods that can be used to detect them; Table 2 lists various area-type defects and the corresponding NDT methods that can be used to detect them.
Table 1: Different Volumetric Defects and Their Applicable Detection Methods
|
Defect Type |
Applicable detection methods |
|
Inclusion, slag inclusion, looseness |
Visual inspection (surface), Penetrant testing (surface), Magnetic particle testing (surface and near-surface), Eddy current testing (surface and near-surface) |
|
Shrinkage cavities, gas pores, corrosion pits |
Ultrasonic testing, radiographic testing, infrared testing, microwave testing, neutron radiography, optical holographic testing |
Table 2: Different Area Defects and Their Applicable Inspection Methods
|
Defect Type |
Applicable detection methods |
|
Layering, poor adhesion, folding |
Visual inspection, ultrasonic testing, magnetic particle testing, eddy current testing |
|
Cold shut, cracks, unfused |
Microwave inspection, acoustic emission inspection, infrared inspection |
When using NDT techniques to perform quality inspections on surface coatings, many factors of the object being inspected should also be considered, primarily including:
(1) The properties of coating materials (magnetic, non-magnetic, metallic, non-metallic, etc.)—the nondestructive testing methods applicable to different coating material properties are listed in Table 9-3.
(2) The shapes of components (tubes, rods, plates, discs, and various complex shapes).
(3) The types of defects that may occur in components (volumetric, areal, continuous, and dispersed).
(4) The locations where defects may exist within components (on the surface, near the surface, or internally).
Table 3: Nondestructive Testing Methods Applicable to Different Materials
|
Detection method |
Main material properties |
|
Penetrant testing |
The defect must extend to the surface. |
|
Magnetic particle testing |
Must be a magnetic material. |
|
Eddy current testing |
Must be a conductive material. |
|
Microwave detection |
Can transmit microwaves |
|
Radiographic Testing |
It varies with changes in workpiece thickness, density, and chemical composition. |
|
Neutron radiography |
It varies with changes in workpiece thickness, density, and chemical composition. |
Generally speaking, radiographic testing is more sensitive to volumetric defects, while ultrasonic testing is more sensitive to surface defects. Magnetic particle testing is applicable only to ferromagnetic materials, whereas penetrant testing is used for detecting surface-opening defects. Eddy current testing, on the other hand, is highly effective for detecting both open and near-surface defects in conductive materials—whether magnetic or non-magnetic. When performing nondestructive testing on the surfaces of thermal spray coatings, it is essential to select the appropriate technique based on the specific circumstances.
3. Destructive spot check
If thermal spraying technology is used to treat parts on a large scale and in batches, it is often necessary to take samples of some parts for destructive sampling inspections to ensure the quality of the thermal spray coating on the part surfaces. Such destructive testing can be carried out using various conventional inspection and testing techniques and methods, including metallographic analysis.
Relevant Information
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Core spraying equipment: SX-80 plasma spray main unit, with a 15,000℃ flame suitable for multiple materials including zirconia and silicon carbide; powder feeding accuracy of ±1%, and coating thickness tolerance ≤ ±5 μm. Environmental protection support system: Customized soundproof booth + pulse dust collector—effectively addressing spray noise pollution while achieving highly efficient dust recovery, compliant with Thai environmental standards. Automated equipment: 6-axis industrial robot + intelligent spray cart, enabling precise spraying of complex workpieces and automated loading/unloading, boosting production efficiency by 40%.
On-site engineers provide hands-on training, focusing on the core operations of plasma spraying equipment such as the SX-80. Sanxin has customized a complete solution for its customers, including an automated spraying robotic arm, a soundproof booth, and a dust removal system—ensuring both high spraying efficiency and compliance with environmental standards.
Guangzhou Sanxin Thermal Spraying Turntable Shipment
Guangzhou Sanxin Company manufactures single-station and multi-station thermal spray turntables, which can be paired with plasma equipment, supersonic equipment, and arc equipment. We offer rapid, customized production tailored to customer requirements and product dimensions. A thermal spray turntable is a high-precision rotary worktable specifically designed for thermal spraying processes—including flame spraying, plasma spraying, and arc spraying. By providing controllable rotational speed and exceptional stability, it ensures uniform coating thickness on the workpiece surface, thereby enhancing spraying efficiency and quality.
SX-80 plasma equipment is being used at a customer site in Russia.
SX-80 plasma equipment is being used at a customer site in Russia.
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