Topic: Future coating materials development Objective The new Euro-7-norm limits the particle emission for PM10 by 7 mgr/km independent of the car size for all new vehicles up from middle of 2025. The overall target of the EU Commission is to reduce particles from brakes by 27% compared to Euro-6. Recuperation of hybrid and electric vehicles supports the compliance with the new standard. At least for conventionally powered cars most OEMs plan to apply a wear and corrosion resistant coating on the brake discs to fulfill these requirements. Poor corrosion resistance and excessive wear of the brake disc during operation are additional challenges. Cold Spray Technology / Methodology In comparison to conventional thermal spray processes, cold gas spraying offers special advantages, because the feed stock material is neither fused nor melted during the process. Thus, the thermal influence on the coating and the substrate material is minimized and leads to no thermal distortion or bending of the discs. The mechanical properties are thus maintained, there is no delamination or severe cracking in the coating, and particle emission is drastically reduced by up to 90%. The high kinetic energy of the particles and the high degree of deformation during the impact on the substrate that is connected with it, allows the manufacturing of homogenous and very dense coatings. In contrast conventional thermal coating processes such as laser cladding cannot yet achieve the required properties in terms of coating adhesion, corrosion resistance and cracking behavior and are not free of patent protection from major automotive manufacturer. Results Cold spray coatings for gray cast iron brake discs are already technically qualified by several OEMs like the BMW Group and by foundries for brake disc manufacturing. The Cold Spray technology is currently under final evaluation for the serial production of these companies. High adhesion strength of the coatings to the brake discs of >90 MPa prevent any delamination during operation. The wear is reduced by over 95% and expands this way the lifetime of brake discs. Thermal distortion and corrosion of the discs even after 720 hours in a salt spray test can be avoided completely. Even if corrosion tests are mixed with very harsh crack formation tests no delamination or severe cracking in the coating could be observed. But most important is that the particle emissions for PM10 can be reduced by up to 90% compared to uncoated discs. What is new / different Not only all technical requirements can be fulfilled with Cold Spray coatings, but Cold Spraying also offers high cost saving potentials. The grinding allowance and thus coating thickness can be reduced to a minimum. This saves cycle time, machine capacity and costs for the coating and the grinding process. Supplementary, very low-cost carbide materials as silicon can be used for the hard phase of the top coating. This not only leads to low metal powder costs right now, but also reduces the risk of drastic price increases with the growing demand, when other carbides as tungsten, niobium or titanium become a bottleneck. Conclusion Cold Spray coatings for gray cast iron brake discs provide the highest technical performance of all coating technologies and leads to a high reduction of particle emissions. Next to that the Cols Spray technology offers many cost advantages and is very suitable for high volume manufacturing. The continuous process with constant process parameters that don’t require any adjustments independent of the brake disc design provide a very robust and performant technology.
Dr.-Ing. Sascha Bernhardt, Head of Sales, Impact Innovations GmbH; Dr. Reeti Singh, Principal Scientist, Impact Innovations GmbH