Euro 7 regulations have been announced that will implement strict restrictions on the allowable amount of brake emissions in consumer vehicles. Brake emissions need to be drastically reduced to comply with the regulation, i.e. by 27% in electric vehicles alongside potentially increased impacts to combustion vehicles. Solutions will need to be realised at production volumes in the millions by around 2026. In addition to this, REACH legislation is becoming increasingly restrictive on potentially harmful elements that are generally well known for aiding against corrosion and wear, such as Ni, Co and Cr. Moreover, the cost and supply chain of those elements are becoming increasingly volatile and restrictive in the market. Grey cast iron is the common brake disc material which needs greater replacing imminently for a lesser wearing alternative. Applying a protective coating is regarded as a potential method to quickly and efficiently resolve the challenge, but maybe current coating materials and processes have major drawbacks. Therefore new coating material choices and coating technology need to be realised to efficiently apply coatings to brake discs at extremely cost-sensitive levels. Recently, thermal spray, specifically high velocity oxy fuel (HVOF), seemed to be one of the most promising technologies for applying coatings to brake discs due to the established nature of the technology and high coating speeds. However, thermal spray can still present significant drawbacks such as limited bonding strength between substrate and coating and low material efficiency. Chrome plating, another established coating technology, has its own challenges including consistency issues, selective surface application, and most critically, is being restricted for its use of chemicals and generation of the carcinogenic element Cr (VI). Here, we propose a novel coating technology that has the potential to apply ‘greener’ coatings onto brake discs. The approach will utilise extreme high-speed laser application (EHLA) to create metallic functional coatings to increase brake performance against wear and corrosion, significantly reducing particle emission. EHLA is an evolution of laser cladding, with a surface coverage rates of up to 100x compared to conventional laser cladding. EHLA melts the metal powder particles in flight, absorbing the majority of the laser energy, while creating a strong metallic bonding between the substrate and the coating, yet minimising the heat input to the substrate with minimal dilution. This work focuses on the applied research of using EHLA as an industrial method for coatings for large brake discs for heavy duty vehicles, including new generation electric models. Existing metallic coating materials as well as new formulation of iron-based material will be evaluated and processed on to grey cast iron brake discs. The objectives of the research is to reduce the harmful elements and especially the quantity of small particles being emitted to the environment during braking and lower the cost of manufacturing greener powder, while providing good wear and corrosion properties to the brake disc. An industrial laser system equipped with a 7 kW diode laser and a coaxial powder nozzle is used for the deposition of the coatings. Optical microscopy and scanning electron microscope (SEM) are used for coating analysis. Hardness measurement and dyno test are used for characterising the coating hardness and the disc performance.
Dr. Yingwei Wu, Project Leader, TWI; Mr. Josh Barras, Principle Project Leader, TWI; Dr. Jhonattan Gutjahr, Project Leader, TWI