High-performance cold gas coating for wear protection and emission reduction for brake discs
Brake discs are subjected to extremely high loads and are among the most stressed parts of a car. Not only do they have to be replaced at relatively short intervals, due to the high level of wear and tear they create an immense environmental impact through particulate matter. Thanks to improved emission control, a reduction in exhaust gas emissions can be achieved in modern cars. However, it is still neglected how much particulate matter is caused by the abrasion of tires and brakes. A good half of the particulate matter in German road traffic is caused by tire and road abrasion and another quarter by brake pad abrasion. Emissions from brake wear, such as brake dust and particulate matter, are a growing concern for the automotive industry due to more stringent and environmental EU regulations.
Gray cast iron with embedded graphite is the most common material from which brake discs are produced. Gray cast iron brake discs are not only cheap to manufacture, they also have all the necessary mechanical properties that are required. However, poor corrosion resistance and excessive wear of the brake disc during operation are the main problem areas. Since no material can yet compete with gray cast iron for vehicle brake discs from a cost perspective, the coating of cast iron discs represents a practical and cost-efficient solution. But conventional thermal coating processes are still very material- and cost-intensive and the required properties in terms of coating adhesion, corrosion resistance and cracking behavior cannot yet be fulfilled. In contrast to laser cladding, the coldspray process is free of patent protection from any major automotive manufacturer.
With a high-performance cold gas coating with the Impact Cold Spray System EvoCSII, there is no delamination or severe cracking in the coating and the particle emission is reduced drastically. The performance of the coating was evaluated in terms of corrosion resistance, wear resistance, and tensile bond strength between the coating and the brake disc. The cold gas sprayed composite coating of the cast iron brake disc showed a 95% reduction in wear in the SAE J2522 test; In addition, the coating showed excellent adhesion even after extreme bench test runs without any delamination. In addition, 720 hours of salt spray tests were successfully completed without corrosion.
The cold spray process wins over other coating technologies with its simplicity, high performance and low costs. As it is a cold process all process parameters are always constant independent of the dimensions and shape of the brake discs. There is no thermal distortion of the discs, and the layer build-up stands out with its uniformity. Thus, the coating thickness can be reduced to a minimum and the subsequent grinding costs are minimized. A standard brake disc can also be coated in a one-layer system (no intermediate layer) in less than 30 seconds. The present results show a cost-effective and resource-saving solution for the brake disc application with cold sprayed composite coatings.