The requirements for emission limits in the automobile industry are becoming more and more restrictive. With better emission control, reduced exhaust emissions can be achieved in modern cars. However, the amount of particles caused by tire and brake abrasion is still neglected. A good half of the particles in German road traffic are caused by tire and road abrasion and another quarter by brake abrasion. Emissions due to brake wear, such as brake dust and particles, are an increasing concern for the automotive industry due to stricter European and environmental regulations. To reduce the immense emissions, poor corrosion resistance and excessive wear, thermal spray coatings are used. Cold spraying, a very innovative process for metallic coatings, here represents the most advanced of all thermal spray processes.
High pressure cold spray
Compared to conventional thermal spraying processes, cold gas spraying offers special advantages, as the spray material is neither merged nor faded During the process. Thus, the thermal influence on the coating and the substrate material is minimized. The mechanical properties are thus retained, there is no delamination or severe cracking in the coating, and the emission of particles is considerably reduced. The high kinetic energy of the particles and the high degree of deformation upon impact on the substrate connected to it, allows the manufacture of homogeneous and very dense coatings. The range of coating thicknesses varies from a few hundredths of a millimeter to several centimeters. These are mainly produced metallic coatings, the physical and chemical properties of which hardly differ from the properties of the base material. With the latest system technology from Impact Innovations GmbH, a process gas – preferably nitrogen or helium – is introduced into a spray gun with a pressure of up to 50 bar (725 psi) and is heated to a maximum temperature of 1100°C (2012°F) in the gun housing. Subsequent expansion of the heated, high-pressure gas in a convergent-divergent nozzle to ambient pressure results in the acceleration of the process gas to supersonic speed and, at the same time, the cooling of the gas to a lower temperature at 100°C (373°F). The atomized powders are injected into the converging section of the nozzle using a powder supply unit and a carrier gas and are accelerated to a particle velocity of up to 1200 m/s in the flow main gas. In the highly focused spray jet, the particles strike the – in most cases untreated – surface of the component, deform the particles and form a highly adhesive/cohesive and low oxide coating.
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 need to be replaced at relatively short intervals, due to the high level of wear they create huge environmental impact through the particles
Gray cast iron with integrated graphite is the most common material from which brake discs are made. Gray cast iron brake discs are not only inexpensive to manufacture, they also have all the necessary mechanical properties. However, poor corrosion resistance and excessive wear of the brake disc during operation are the main problems. Since no material can yet compete with gray cast iron for vehicle brake discs from a cost perspective, hard metal lining of cast iron discs represents a practical and economical solution. But conventional thermal coating processes are still very expensive in terms of materials and costs and the required properties in terms of coating adhesion, corrosion resistance and cracking behavior cannot yet be fulfilled. Unlike laser coating, the the cold spray process is exempt from patent protection from any major automaker.
With a high performance cold gas coating with the Impact EvoCSII cold spray system, there is no delamination or major cracks in the coating and particulate emissions are drastically reduced by more than 85%. The coating performance was evaluated in terms of corrosion resistance, wear resistance and tensile strength between the coating and the brake disc. The cold gas spray composite coating of the cast iron brake disc has shown a >95% reduction in wear in the SAE J2522 test; Additionally, the coating showed excellent adhesion even after extreme bench testing without any delamination. In addition, 720 hours of salt spray testing have been successfully achieved without corrosion, even after carrying out wear tests.
Summary and outlook
The cold spray process appeals to 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 formation of layers is distinguished by its uniformity. Thus, the thickness of the coating can be reduced to a minimum and subsequent grinding costs are minimized. A standard brake disc can also be coated with a single layer system (no intermediate layer) in less than 30 seconds. The present results show a cost effective and resource saving solution for the application of brake discs with cold sprayed composite coatings.
In order to achieve further emission limits in the automotive sector, in addition to brake abrasion such as exhaust emissions, the friction in the internal combustion engine must be further reduced. The piston assembly, consisting of the piston, piston ring and cylinder bore surface, is responsible for 30-48% of the friction in the engine. In the old days, cylinder housings were made of gray cast iron. In order to save weight, the housings now consist of an aluminum-silicon casting alloy into which gray cast iron liners are then pressed. Conventional cylinder liners are replaced with cold spray coatings to reduce internal friction and make the combustion engine more efficient and less polluting. Frictional losses and therefore fuel consumption are reduced.
Thanks to recent developments, Impact Innovations cold spray technology can now be used to produce liners for inner cylinder diameter > 70 mm. Benefits include greater wear resistance, longer component life, lower component cost and reduced weight. Cold spraying of interior coatings with a rotary gun is characterized by high efficiency and low power consumption. Additionally, no surface preparation (such as roughening or preheating) is required and post-processing steps such as mechanical lapping are reduced. Application rates are adaptable to the desired cycle time in the system. Using an ID coating system for cold spraying inside diameter coatings is an environmentally friendly and resource efficient process suitable for 24/7 operation.
The still young cold spray technology will play an increasingly important role in the future when it comes to saving emissions and reducing weight as well as material and fuel consumption. Impact Innovations is already a innovator and pioneer in this area and will continue to work on the development of new solutions. The growing demand for electrically powered cars offers other possibilities for applications such as cooling batteries and power switches by copper clad hybrid heat sinks.