A new technology for fast and non-destructive measurement of thermal spray coating thickness, enabling early process control to detect and correct deviations.
When the eight traditional gray cast iron inserts in the crankcase of the cylinder engine were replaced with a hot melt sprayed iron-based coating, the porosity and mechanical properties of the aluminum castings in the cylinder bore area were very high.
The actual production process mainly includes four steps: fine boring (fine boring), roughening, coating and post-processing. Between the above steps, tests must be performed to ensure quality. Must be 100% tested based on part quantity, application and critical quality parameters.
The purpose of fine boring is to align the hole with the crankshaft axis and enlarge the porosity according to the desired coating thickness after final machining. At this point, shape and position tolerances are determined, and cylindricity is also produced. Regarding the coating thickness, only a very limited positional tolerance can be corrected subsequently, which is a few hundred microns thick. Therefore, the holes need to be set in place prior to the roughening and coating process.
After replacing the eight traditional gray cast iron inserts in the crankcase of the cylinder engine with a hot melt sprayed iron-based coating, the requirements for the porosity and mechanical properties of the aluminum castings in the cylinder bore area are very high.
The purpose of roughening the cylindrical surface is to produce the desired coating adhesion. This step can be done by sandblasting corundum, high pressure or pulsed low pressure fluid jet or by pure machining. For roughening the surface to be coated, corundum sandblasting and fluid jetting processes can be carried out in separate equipment.
Mechanical roughening of the crankcase can usually be done after fine boring in the same clamping. Thus, a roughened profile can be introduced without offset. Surface defects are then detected and classified for reprocessing using optical image recognition technology. In the case of corundum blasting, such surface defects can be embedded in the corundum particles; in the case of fluid jets, it is mainly the exfoliated phase and expanded pores.
In order to measure the coating thickness, the measuring optics are inserted into the cylinder bore (upper bore opening), so that the coating thickness can be reliably measured at any position on the circumference and depth in the cylinder.
When mechanically roughening, the groove profile is cut into the aluminum. The shape of this profile, grooves and metallic glossy surface make it unsuitable for inspection with camera systems.
Increase the service life of the engine
The thermal spray process is characterized in that the coating material is melted by a heat source, such as a plasma flame, and sprayed onto the part by a swirling airflow. Liquid particles suddenly solidify and delaminate on contact with the surface, resulting in a coating. Almost all materials can be processed by powder-based atmospheric thermal spraying. The layer spectrum ranges from thermoplastics, metals and carbides, to ceramic layers. Iron-based coatings have the required tribological properties and are available as wire or powder. Wire arc spraying, plasma transferred arc (PTWA) and rotating single wire (RSW) are examples of wire coating processes.
Powder-based atmospheric plasma spray coatings offer the option of spraying ceramic materials in addition to metals. Low-alloy carbon steels are mainly used for coating the running surfaces of gas and diesel engines. The coating is non-uniform and consists not only of molten and solidified particles. Inside the coating, oxides and carbides, as well as voids, build up to a frequency between 1% and 4%, depending on process parameters and process choice. These holes act as lubricant reservoirs and hold the lubricant in place after honing. Therefore, the friction force between the piston ring/piston and the cylinder wall is reduced, thereby reducing fuel consumption and increasing the service life of the engine.
Enables coating thickness measurements within one second
The thickness of the thermally sprayed coating that is still unfinished must follow a narrow allowable range. Any deviation from the specified tolerances may result in subsequent rework and destruction of the honed tool during subsequent machining.
Measuring the coating thickness early in the process saves any subsequent processing steps in the value-added chain, such as a coating that is too thin. For rough sprayed surfaces, traditional coating thickness measuring instruments have low repeatability and are therefore not suitable for quality assurance. Random inspection via photomicrographs is time-consuming and does not allow for seamless and non-destructive inspection of the coating process.
The measuring instrument is designed based on the process of thermal coating testing. The device's light source heats the surface of the sprayed coating from milliseconds to several degrees Celsius. It measures the film thickness of thermally sprayed coatings based on photothermal methods and the difference in thermal conductivity between the coating and the substrate.
The coating thickness within the working surface of the cylinder is measured from bottom to top at opposite points (a, b).
Measuring points 1-15 are crankcase 1, and measuring points 16-30 are crankcase 2
The light source used by this device is similar to a camera flash and does not pose any hazard to humans or the environment. Each measurement process analyzes over 100,000 temperature readings and then determines coating thickness. It can measure at a distance of 1 meter from Z, and can measure areas from 2 to 50 mm in diameter on the surface. The error of a single measurement is usually less than 1%. Coating thickness can be recorded at frequencies up to 2 Hz. Through the optical measuring probe, each measuring point distributed on the entire working surface of the cylinder is automatically measured, and the measured thickness is automatically recorded.
Surface roughness reduction
In the final honing process, the rough structure on the sprayed surface needs to be removed, so the surface roughness needs to be reduced to an average roughness in the range of 5 microns.
The above is the control method of the thermal spray coating process introduced by the metal spraying manufacturer. I hope it will help you!
