Brake Rotors - Nitrocarburizing & Automation

New standards like the Euro 7 are requiring the transport industry to reduce particle emissions from brake systems.

How can Nitrex help? Brake rotors treated with our enhanced nitrocarburizing technology exhibit negligible corrosion, which means that they produce less airborne particle emissions from vehicle brakes—a win-win for industry and the environment. Get to know the many ways this ferritic nitrocarburizing technology can help optimize brake rotors. Get the free white paper for more details.

What heat-treating processes are available to improve the corrosion and wear resistance of brake rotors while still meeting the latest emission standards?

In this video presentation recorded during the 2022 SAE Brake Colloquium, you will learn about :

the various techniques of reducing brake rotor corrosion and wear, as well as the most cost-effective alternative
how nitrocarburizing enhances the mechanical properties and corrosion resistance of grey cast iron brake rotors
and get insights into Nitrex’s fully automated heat-treating cell for high-volume GCI brake rotors production

DESIGN & MANUFACTURING

Is post-oxidation requested/required by OEMs? Is ferritic nitrocarburizing (FNC) enough for fulfilling Euro VII regulations?

Post-oxidation is requested by some but not all OEMs. It does not help with braking, but it does help with corrosion to some extent. It is also a matter of aesthetics for certain manufacturers. Zinc-based paint is also used in conjunction with FNC. FNC appears to fulfill Euro VII regulations based on testing performed so far. However, the durability of this solution may vary, as for all solutions, depending on the specific conditions of use of a vehicle.

Why is post-oxidation beneficial?

Post-oxidation improves corrosion resistance but reduces the coefficient of friction. Ideally, oxide layers are composed of magnetite, a very compact iron oxide. Their thickness is usually limited to less than 2 microns (or less than 1 tenth of a thousandth of an inch); therefore, post-oxidized areas are eventually worn out on braking areas. The oxide remaining on other rotor areas helps protect against corrosion.

How durable are brake rotors after ferritic nitrocarburizing (FNC)?

The durability of ferritic nitrocarburizing depends on the type of brake pads use. Non-asbestos organic (NAO) as well as low-metallic and semi-metallic brakes dissipate heat and allow for a much longer service life. On the other hand, ceramic pads tend to be harder on rotors. Braking conditions that generate more heat reduce the durability of the process and the lifespan or rotors.

Does Smart ONC® ferritic nitrocarburizing technology work for all brake applications?

No, it does not. Non-asbestos organic (NAO), as well as low-metallic and semi-metallic brakes, dissipate heat and allow for a much longer service life. On the other hand, ceramic pads tend to be harder on rotors. Braking conditions that generate more heat reduce the durability of the process and the lifespan of rotors. However, most electric vehicle brake systems adopt nitrocarburized brake rotors on the rear wheels.

How does the application of Smart ONC® technology differ between passenger vehicles and heavy-duty trucks?

There is no difference in the use of Smart ONC®, an enhanced ferritic nitrocarburizing process with in-process post-oxidation for passenger vehicles and heavy-duty trucks. In all cases, rotors are subject to a modified post-oxidation cycle.

Is it recommended to deburr the brake rotor after ferritic nitrocarburizing (FNC) due to the higher surface roughness?

No, it is not recommended. The roughness is mostly attributed to graphite and drops fast after a breaking cycle. Moreover, it is possible to control roughness by controlling the initial rotor roughness and optimizing process parameters.

METALLURGY

What is the white layer?

White layer, also called compound zone, forms on the surface of the steel during the nitriding process. White layer is usually composed two types of iron nitrides: Gamma Prime (’) layer with an approximate nitrogen content of up to 6% w/volume, and Epsilon () with a nitrogen content of up to 10% w/volume. The percentage of nitrogen content may vary, but in general white layer is hard, relatively brittle yet with good sliding wear and corrosion resistance.

How much of the white layer is lost during the break-in process of the brake rotor?

It is a great question! While there is no clear answer, experts say that the break-in process for brake rotors typically involves transferring material from the brake pads to the rotor to ensure proper contact and improve overall performance. It should be noted that this process is specific to Non-Asbestos Organic or NAO semi-metallic pads.

At what temperature does the white layer become unstable?

Nitrides that compose the white layer become theoretically unstable in the upper range of nitriding process temperatures. The stability of a nitride depends on its chemical composition – in this case, iron, carbon, nitrogen and other trace elements. For instance, at 530-570°C (986-1058°F), excess nitrogen may come out of solution and create porosity. However, the duration for which the white layer is exposed to this temperature is also a factor to consider since braking is typically intermittent, and the rotor has time to cool down.

Is there a correlation between the surface roughness and corrosion resistance of the ferritic nitrocarburized brake rotor?

Yes and no. On grey cast iron, part of the roughness is due to graphite raising out of the surface.

Is there a relationship between microcracks and deep layer diffusion in brake rotors?

It remains unclear what is meant by a "deep" diffusion layer. Most rotors/disks are nitrocarburized to improve surface properties, with the depth of the layer depending on specific process parameters such as time and temperature. It is unlikely that a diffusion layer would cause cracks in this material. However, some suggest that nitrocarburizing can lead to crack problems under harsh braking conditions, while others indicate that the high-temperature corrosion-fatigue properties of cast iron may be influenced. None of the results provide a clear indication of how often or how likely it is for nitrocarburizing to cause cracks in grey cast iron specifically. Nevertheless, it's important to note that the conditions of the nitrocarburizing process, as well as the composition and properties of the cast iron, can have a significant impact on the outcome. So, it is difficult to generalize without specific information.

How does the presence of open graphite channels through the compound layer affect corrosion?

Graphite flakes are embedded and surrounded by nitrides near the surface and in the white layer. They do not lead to graphitic corrosion, as seen in cast iron pipes.

How does ferritic nitrocarburizing modify pitting resistance?

The presence of nitrogen in the modified layer exists as a solid solution in the ferrite phase, which contributes to the improved resistance to pitting corrosion. Furthermore, the nitrocarburized layer is able to reduce the surface energy of the steel surface, which can inhibit the initiation of corrosion pits. Ferritic nitrocarburizing also improves corrosion resistance by reducing the size change and distortion that can occur during traditional treatments, such as carburizing and nitriding.

Does the ferritic nitrocarburizing (FNC) interface layer affect static to dynamic COF ratio?

On grey cast iron, it does. Grey cast iron is considered a self-lubricating material, and the initial static COF versus dynamic is low and remains low. In FNC layers, the static COF is higher due to initial roughness, often resulting from raised graphite flakes. However, the dynamic COF is much lower as the surface becomes smoother, so yes, the ratio is affected.

Does ferritic nitrocarburizing (FNC) affect disc thickness variation (DTV) and lateral runout (LRO) of brake rotors?

DTV and LRO are not affected by the FNC process as long as the castings have undergone proper stress relief prior to machining.

APPLICATION

What is the concern with brake rotor particulate emissions?

Rotor material should almost entirely be iron or iron oxides, which are non-toxic and ubiquitous in the environment. Both iron and carbon are non-toxic and found everywhere in the environment, but breathing particles can cause health problems. However, there is concern for brake rotor particles due to their size and the fact that they become airborne. Their size can vary from several microns to just a few nanometers in diameter, and particles of this size can be inhaled and penetrate deep into the lung tissue, causing pulmonary health problems.

How does the carbon content influence results?

Cast irons have a carbon content of 2.5-4% In grey cast irons the addition of silicone or other graphitizing agent promotes the growth of graphite in the form of flakes. A higher carbon content improves casting but reduces mechanical properties. It is relatively easier to obtain a thicker white layer when carbon content is higher.

Are all grey cast irons comparable?

Cast irons have different mechanical and tribological qualities depending on the mount and form of carbon, as well as on alloying elements. Automotive brake rotors are usually made from grey cast iron.

Would the surface roughness be the same in 1020 steel as grey cast iron (GCI)? Is there an increase in roughness after nitrocarburizing?

If we were to compare identical surface finishes prior to the FNC process for both materials, the grey cast iron would yield slightly higher roughness after the process due to graphite flakes being pushed out. However, since graphite is soft, the higher roughness is not likely to have an impact once the rotor is in service.

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DESIGN & MANUFACTURING

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APPLICATION

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