An electric vehicle pulls away almost silently. Because the powertrain produces so little noise, acoustic effects that previously went unnoticed in conventional cars have become much more apparent. Clicking, creaking, and metallic knocking are far more noticeable inside electric vehicles, particularly at power transmission interfaces such as the connection between the wheel bearing assembly and the driveshaft.
Adam Dmytryszyn is Wheel Bearing R&D Director Europe at the South Korean automotive supplier and wheel bearing specialist ILJIN. His development team supports projects for European customers and is continually evaluating manufacturing technologies with potential for series production. At ILJIN, one issue that attracted particular attention was the so-called ping-noise phenomenon—a noise, vibration, and harshness (NVH) issue found in many modern vehicles.
As one of the world's largest manufacturers of wheel bearings, ILJIN produces millions of these components each year on highly automated production lines. Based on ILJIN's requirements and in close collaboration with the company, the Fraunhofer Institute for Laser Technology ILT developed an EHLA coating that specifically modifies the contact conditions within the wheel bearing assembly. EHLA (extreme high-speed laser material deposition) is an exceptionally efficient and material-saving coating technology developed at Fraunhofer ILT.
Cause at the wheel bearing unit–drive shaft interface
"The noise does not occur because a component breaks or comes loose, as some people assume when they hear it," explains Dmytryszyn. "It is caused by the extremely high forces acting at the contact surface between the wheel hub and the constant velocity joint." When an electric vehicle accelerates or recuperates energy, the high torque causes the driveshaft to deform slightly. As a result, the two steel surfaces at the interface initially stick together, then suddenly slip apart before sticking again. This stick-slip effect releases energy within fractions of a second, producing vibrations and noise that are clearly audible inside the vehicle.
Technically, it is only a tiny relative movement. In the vehicle, however, it sounds like a hard metallic crack—in other words, like a “ping.” This effect is more pronounced in electric vehicles. They deliver high torque right from a standstill and, at the same time, run so quietly that even brief impulses from the chassis are noticeable.
This is precisely where traditional countermeasures reach their limits. "Greases reduce friction in the short term, but are displaced during operation and gradually lose their effectiveness," explains the wheel bearing expert. "Additional washers with low-friction coatings introduce extra components into an already confined interface. They increase complexity and cost, require additional installation space, and may contain PFAS-based materials that are now considered environmentally harmful."
An approach that only works in testing or masks individual noise disturbances is not sufficient for a wheel bearing that is manufactured in the millions and is also safety-critical. ILJIN sought a solution that not only meets the high quality and cost requirements, but also integrates seamlessly into existing production. The customer’s acoustic problem thus became a question of design, materials, and process: Which functional coating alters the contact surface in such a way that the noise disappears without weakening the component itself?
Fraunhofer ILT and ILJIN launch a development project
Adam Dmytryszyn didn’t get the decisive inspiration from a traditional automotive project, but rather by thinking outside the box. At the Hannover Messe 2023, Dmytryszyn saw an EHLA application presented by Fraunhofer ILT in which a thin metallic coating was applied with high precision and finished by simultaneous machining. The demonstration involved a plain bearing for wind turbines, but what immediately caught his attention was the underlying technology: a thin, durable coating with low surface roughness, a metallurgical bond to the substrate, minimal heat input, and high processing speed. This combination was exactly what was needed to solve the problem at the wheel bearing interface.
The contact made at the trade show soon developed into a joint development project. ILJIN contributed the component, system expertise, and requirements from automotive production, while ILJIN and Fraunhofer ILT jointly translated the functional requirements into specifications for the coating, including the coating material, thickness, heat-affected zone, and surface quality.
At Fraunhofer ILT, Viktor Glushych, group leader of the LMD Coating and Heat Treatment Group, and project manager Eduard Weisser took on the task of developing a robust and efficient EHLA process based on these specifications. Throughout the project, the ILJIN engineering team worked closely with the Aachen researchers, discussing technical questions, building a detailed understanding of the process, and jointly defining the next development steps—from initial prototype components to a process suitable for industrial production.
The feasibility study quickly revealed that the approach did not follow the principle of “select material, apply coating, problem solved.” Applying a metallic coating to a component is only the first step. Crucial to the success was the technical depth with which the Fraunhofer ILT team tailored the process to the specific application: to the component, the surface, the powder material, the process parameters, the coating thickness, and the energy balance. Only through this interplay did a coating emerge that not only adheres but also functions under real-world loads, and is suitable for an industrial process. The Fraunhofer ILT team systematically investigated various coating variants, tested the adhesion to the bearing steel, determined the hardness and heat-affected zone, and transferred the process step by step from a simple sample to the actual wheel bearing.
ILJIN tested and compared the test samples produced by Fraunhofer ILT. To do this, ILJIN operates a test facility in Germany where it conducts tests under conditions similar to those encountered in vehicles. The results of the initial functional tests were incorporated into the final process design.
Precisely because the affected area is safety-critical, ILJIN set particularly high requirements for the coating process. The coating required a flawless, bond-type adhesion, but the process heat was not allowed to damage the underlying steel. At the same time, it had to behave under high surface pressure in such a way that the contact surface would fulfill its function over the entire service life of a vehicle.
Fraunhofer Institute for Laser Technology ILT
