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Process Control for Thermal Spray Coatings
Release time:
2020-08-11 21:48
Source:
Process Control for Thermal Spray Coatings
(A new technology that enables rapid, non-destructive measurement of thermal spray coating thickness, allowing for early process control, timely detection of deviations, and prompt corrective actions.)
After replacing the eight conventional gray cast iron inserts in the crankcase of the cylinder engine with thermally sprayed iron-based coatings, the requirements for porosity and mechanical properties of the aluminum castings in the cylinder bore area became extremely high.
The actual production process mainly consists of four steps: precision boring (fine boring), roughening, coating, and post-processing. Between each of these steps, testing must be carried out to ensure quality. Testing must be performed at 100% coverage, based on the number of parts, the application, and critical quality parameters. The purpose of fine boring is to align the holes with the crankshaft axis and enlarge the hole diameters to the required coating thickness after final machining. At this stage, both shape and positional tolerances must be determined, and cylindricity is also achieved. As for coating thickness, only very limited positional tolerances can be corrected afterward—tolerances that are measured in hundreds of micrometers. Therefore, it is essential to precisely position the holes before the roughening and coating processes.
Figure 1 After replacing the eight conventional gray cast iron inserts in the crankcase of the cylinder engine with thermally sprayed iron-based coatings, the aluminum castings in the cylinder bore area must meet extremely high requirements for porosity and mechanical properties.
The purpose of roughening the cylindrical surface is to achieve the desired adhesion for the coating. This step can be accomplished through sandblasting with corundum, high-pressure or pulsed low-pressure fluid jetting, or purely mechanical machining. Separate equipment is available for performing corundum sandblasting and fluid jetting processes specifically designed to roughen surfaces intended for coating.
Typically, after precision boring in the same clamping setup, the crankcase can undergo mechanical roughening. Consequently, the roughening profile can be introduced without any offset. Subsequently, optical image recognition technology is employed to detect surface defects and classify them for rework. In the case of alumina sandblasting, these surface defects may become embedded within the alumina particles; whereas in the case of fluid jetting, the primary defects are delaminated layers and expanded pores.
Figure 2: To measure the coating thickness, the measurement optical component is inserted into the cylinder bore (upper opening). As a result, the coating thickness can be reliably measured at any location along the circumference and at any depth within the cylinder.
When mechanical roughening is performed, grooves are cut into the aluminum material. The shape of this profile, along with the grooves and the metallic-glossy surface, makes it unsuitable for inspection using camera systems.
Extend the service life of the engine.
The thermal spraying process is characterized by the coating material being melted by a heat source—such as a plasma flame—and then sprayed onto the component via a rotating gas stream. As the liquid particles come into contact with the surface, they suddenly solidify and undergo layering, thereby forming the coating. Using atmospheric thermal spraying based on powders, nearly any material can be processed. The resulting coatings span a wide spectrum, ranging from thermoplastic plastics and metals and carbides to ceramic layers. Iron-based coatings possess the desired tribological properties and can be used either in wire or powder form. Wire arc spraying, plasma transferred arc (PTWA), and rotary single-wire (RSW) are examples of wire-coating processes.
In addition to metals, powder-based atmospheric plasma spraying coatings also offer the option of coating ceramic materials. Low-alloy carbon steels are primarily used for coating surfaces in gas and diesel engines. These coatings are non-uniform, consisting not only of melted and solidified particles. Within the coating, oxides, carbides, and pores accumulate, resulting in porosity ranging from 1% to 4%, depending on the process parameters and the specific process chosen. After honing, these pores serve as lubricant reservoirs, effectively retaining the lubricant in designated locations. As a result, friction between the piston rings/pistons and the cylinder walls is minimized, thereby reducing fuel consumption and extending the engine's service life.
Achieve coating thickness measurement within one second.
The thickness of unprocessed thermal spray coatings must adhere to a narrow tolerance range. Any deviation from the specified tolerances could result in subsequent reworking or scrapping of the honing tools during subsequent machining processes.
Measuring coating thickness early in the process can eliminate any subsequent processing steps in the value chain—such as those required when the coating thickness is too low. For rough-surfaced spray coatings, conventional coating-thickness measurement instruments suffer from low repeatability, making them unsuitable for quality assurance. Random inspections using microscopic photographs are extremely time-consuming and do not allow for seamless, non-destructive inspection of the coating process. In contrast, Winterthur’s device, CoatMaster, can measure coating thickness over surface areas with diameters ranging from 1 to 3 millimeters. The results show that even on highly rough surfaces, this instrument achieves high repeatability—between 1% and 2%—and completes measurements in less than one second. Consequently, during mass production, it is possible to test multiple measurement points for each hole.
This measuring instrument is designed based on the thermal coating testing process. The device’s light source heats the sprayed coating surface by several millikelvins to several degrees Celsius for a few milliseconds. It measures the thickness of thermally sprayed coatings by utilizing the photothermal method and taking advantage of the difference in thermal conductivity between the coating and the substrate.
Figure 3: The coating thickness within the working surface of the cylinder is measured from bottom to top at the opposite points (a, b).
Measurement points 1–15 correspond to crankcase 1, and measurement points 16–30 correspond to crankcase 2.
The light source used by this device is similar to a camera flash and poses no harm to humans or the environment. During each measurement, the device analyzes more than 100,000 temperature readings before determining the coating thickness. Measurements can be taken from a distance of up to 1 meter, covering surface areas with diameters ranging from 2 to 50 millimeters. The error in a single measurement typically remains below 1%. Coating thickness can be recorded at frequencies as high as 2 Hz. Using an optical measurement probe (Figure 2), the system automatically measures and records the thickness at each point across the entire working surface of the cylinder. “CoatMaster enables precise and rapid measurement of thermal spray coatings directly during the manufacturing process in a non-destructive manner,” said Dr. Peter Ernst, Head of Running Surface Coating Technology at Oerlikon Metco. “It allows for quick detection and correction of process deviations that may affect coating thickness.”
Reduction of surface roughness
In the final honing process, the rough structure on the sprayed surface must be removed, so the surface roughness needs to be reduced to an average roughness within the range of 5 micrometers.
Relevant Information
The complete set of plasma-sprayed equipment has been shipped to the customer’s site in Thailand.
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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