Brake discs are traditionally made with gray cast iron with lamellar graphite phases. The advantages are discs with good thermal conductivity, high thermal capacity, made at a relatively low cost. The disadvantage is that as the brake discs found in vehicles are continually subjected to high mechanical loads the material is subject to corrosion and wear.
Disc brakes are similar to the brakes found on bicycles. With disc brakes, the brake pads do not squeeze the wheel but the rotor hydraulically transmits the force through a cable. The resulting friction that occurs between the disc and the pads slows the discs down. These processes leave fine particulates to scatter throughout the environment and decrease the usable life span of the brake discs.
The current methods used to protect brake discs include electroplating and thermal spraying. Electroplating involves coating the brake discs with a thin, durable coating of another metal. Thermal spraying involves spraying melted or heated materials over the surface of the brake discs. However, they have proven to be ineffective as well as costly.
However, the Extreme High-Speed Laser Material Deposition (EHLA) process is perfectly suited for coating brake discs. The EHLA process allows coatings to be applied that form a metallurgical bond with the base material used to build the brake discs themselves. Metallurgical bonds occur as a result of chemical bonding between the base material and the coating with no voids or other forms of discontinuities. These metallurgical bonds are extremely strong and therefore do not chip or flake off the brakes when under pressure. Unlike coatings created through traditional processes, those created by EHLA do not have any cracks in the coating, minimizing any possible chipping or flaking. As a result, the brake discs have a longer life span, benefitting the environment and buyers alike.
A further benefit of the EHLA process is that is able to work with a wide variety of materials and therefore has many applications within the vehicle industry. Different coatings can be applied to suit the specific requirements of the diverse vehicle parts, and to suit the environment to which those parts will be subjected.
One advantage of the EHLA process is that the coating material is melted directly with the laser beam, making the coating process much quicker. The process speed of traditional methods is approximately a few meters per minute while the EHLA process speed is 500 meters per minute. Consequently, the material being coated is exposed to heat for a much shorter period of time. This low heat exposure stops the carbon from dissolving from the brake into the melt, preventing the formation of any pores and cracks in the coating. This lack of pores and cracks reduces the chances of any particulates rubbing off the brake discs and entering the environment.
Another advantage is that the coating layer produced by the EHLA process is much thinner than those created by methods that are more traditional. The thickness of the coating is decreased from 500 microns to between 10 and 250 microns. This makes the process more economically feasible as less material is required. Additionally, the thinner coating provides a smoother surface, reducing the likelihood of any flaking or chipping.
A further advantage is that the EHLA process is free of chemicals. This makes the process environmentally friendly, a concept that is becoming increasingly more important to our global society.
HPL Technologies, an Aachen-based company, is currently constructing a system incorporating the EHLA process to produce coatings for gray cast iron conventional brake discs.
Further details can be found here: EHLA: Extreme High‐Speed Laser Material Deposition, Thomas Schopphoven, Andres Gasser, Gerhard Backes, https://onlinelibrary.wiley.com/doi/abs/10.1002/latj.201700020.