Impact Innovations
The requirements for emission limits in the automotive industry are becoming increasingly stringent. 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 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. To reduce the immense emissions, poor corrosion resistance and excessive wear, thermal spray coatings are used. Cold spraying, a highly innovative process for metallic coatings, represents here the most advanced of all thermal spray processes.
High-pressure Cold Spray
In comparison to conventional thermal spray processes, cold gas spraying offers special advantages, because the spraying material is neither fused nor melted during the process. Thus, the thermal influence on the coating and the substrate material is minimized. The mechanical properties are thus maintained, there is no delamination or severe cracking in the coating, and particle emission is drastically reduced. 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. The range of coating thicknesses varies from just a few hundredths of a millimeter up to several centimeters. There are mainly metallic coatings produced, which physical and chemical properties barely differ from the properties of the base material. With the latest system technology of Impact Innovations GmbH a process gas - preferably nitrogen or helium - is put into a spray gun with a pressure of up to 50 bar (725 psi) and is heated up to a maximum temperature of 1100 °C (2012 °F) in the gun housing. The subsequent expansion of the heated and high pressurized gas in a convergent-divergent nozzle down to ambient pressure results in the acceleration of the process gas up to supersonic speed and, at the same time, in the cooling down of the gas to a temperature below 100 °C (373 °F). The spray powders are injected in the convergent section of the nozzle by using a powder feeding unit and carrier gas and are accelerated to a particle speed of up to 1200 m/s in the main gas stream. In the highly focused spray jet particles impinge the - in most cases untreated - surface of the component, deform the particles and form a strongly adhesive/cohesive and low-oxide coating.
Impact Innovations
Hard Coatings 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
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 hard metal 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 cold spray 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 by over 85%. 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 even after performing wear tests.
Summary and Outlook
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.
Impact Innovations
In order to achieve other emission limits in the automotive sector, apart from brake abrasion like exhaust emissions, friction in the internal combustion engine must be further reduced. The piston assembly, consisting of piston, piston ring and cylinder bore surface, is responsible for 30-48% of the friction in the engine. In the past, cylinder crankcases were made of gray cast iron. In order to save weight, the crankcases are now made of an aluminum-silicon casting alloy into which gray cast iron liners are then pressed. Conventional cylinder liners are replaced by cold spray coatings to reduce internal friction and make the combustion engine more efficient and lower in emissions. Friction losses and thus fuel consumption are reduced.
Thanks to recent developments, Impact Innovations' cold spray technology can now be used to produce internal coatings for cylinder inner diameters > 70 mm. Benefits include higher wear resistance, longer component life, lower component cost and reduced weight. Cold spraying of internal coatings with a rotating spray gun is characterized by high efficiency and low energy consumption. In addition, no surface preparation (such as roughening or preheating) is required and post-processing steps such as mechanical honing are reduced. Application rates are adaptable to the desired cycle time in the system. The use of an ID coating system for cold spraying of inner diameter coatings is an environmentally friendly and resource-saving process suitable for 24/7 operation.
The still young technology of cold spraying will play an increasingly important role in the future when it comes to saving emissions and reducing weight plus material and fuel consumption. Impact Innovations is already an innovator and pioneer in this field and will continue to work on developing new solutions. The increasing demand for electric-drive cars offers further applications potential as for cooling of batteries and power switches by copper coating hybrid heat sinks.
