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Article by Pawan Kumar, Technical Project Manager – Brake Disc Coatings, Oerlikon Metco Europe GmbH
In this research, a comparative study is conducted between ferritic nitrocarburized (FNC) brake discs and single-layer coated brake discs (by High-speed laser cladding process), focusing on their performance in battery electric and hybrid vehicle applications.
The primary parameters considered for this analysis are:
- Corrosion Resistance: Evaluating how each treatment protects against rust and oxidation, particularly in electric vehicles where regenerative braking reduces brake use, leading to potential corrosion.
- Wear Resistance: Assessing the durability of the disc surface under repeated braking cycles, which is crucial for extending disc lifespan in EVs and hybrid vehicles.
- Fine Dust Emission (PM and PN): Measuring the levels of particulate matter (PM) during braking, in line with Euro 7 regulations aimed at reducing environmental impact.
About FNC & High-speed laser cladding process:
Ferritic nitrocarburizing (FNC) is a surface hardening treatment widely used to enhance the performance of brake discs, especially in the context of electric vehicles (EVs), where brake discs face unique challenges. FNC is a thermochemical process that diffuses nitrogen and carbon into the surface of the brake disc at relatively low temperatures (500-600°C) in a ferritic phase. This treatment creates a hardened, wear-resistant outer layer while maintaining a tough, ductile core.
Ferritic Nitro Carburizing (FNC) disc consists of three distinct layers:
- Oxide layer: Thickness of 2-3 µm, hardness of 600 HV0.3.
- Compound layer: Thickness of 12-16 µm, hardness of 1250 HV0.3.
- Diffusion layer: Thickness of 20-40 µm, hardness of 200 HV0.3.
The FNC process produces a composite structure with multiple layers of varying hardness and thinner total layer thickness
High-speed laser cladding is an advanced surface modification technique used to enhance the properties of materials by applying a coating or layer onto a substrate. In this process, a laser beam melts a coating material (typically in powder or wire form) while simultaneously heating the substrate surface. The molten material then rapidly solidifies and bonds with the substrate to form a new surface layer with improved characteristics, such as wear resistance, corrosion resistance, or hardness.
Unlike traditional cladding methods, the high-speed laser cladding process operates at faster scanning speeds, significantly increasing efficiency and reducing heat input. This minimizes thermal distortion and reduces the heat-affected zone (HAZ) in the substrate, making it suitable for precision applications. The process can be fine-tuned for various materials, including metals, alloys, and ceramics, to achieve customized surface properties.
Single layer coated disc features a uniform thickness as low as 100 µm with a hardness of 350 HV0.3.
Corrosion performance
FNC disc and single layer coated discs were tested for corrosion performance as per ISO 9227. Discs rotated 180° as shown after every day to have uniform saline concentration
Red rust started forming on the FNC treated discs after 72 hours in NSS chamber, however single layer coated disc was able to withstand corrosion more than 504 hours, this is more than 10 times that of FNC disc. (The test was stopped after 504 hrs)
Table 1 comparison of brake disc corrosion resistance between FNC and single layer coated disc
However, single layer coated disc has a uniform and regular thickness, providing more consistent protection. In addition, Corrosion-Resistant Elements such as Chromium (Cr) and Molybdenum (Mo) are also present that enhance its resistance to corrosion.
Stiction test results
Under highly oxidative conditions, electrochemical processes on the surface of gray cast-iron rotors can produce corrosion products, such as iron oxides. These products can infiltrate the brake pad material through its inherent porosity. When the brake pad is engaged under static conditions, such as when the parking brake is applied, this can result in stiction—a strong adhesion between the pad material and the rotor. This stiction can cause damage to the brake pad when the parking brake is released. In severe cases, stiction can significantly impair the braking system’s performance, potentially rendering the vehicle undrivable.
FNC with NAO pad & single layer coated disc with hybrid pad were tested for stiction as per the standard ISO 6315. Below are the results with observations.
No sticking of pad observed in the new condition, however in used condition the maximum pull off force with FNC & NAO pad is 76N and with single layer & hybrid pad it is 52.5N.
Frictional performance comparison (AK Master test results as per SAE J2522)
NAO pad for FNC and Hybrid pad for single layer was selected based on various trials conducted as per SAE J2522. Below are observations and test results.
μ values are within acceptable range for both single layer and FNC.
Friction values are generally higher with coated rotors compared to FNC.
The initial roughness for FNC is significantly higher (approximately 5 µm) compared to single layer (approximately 0.5 µm).
The final roughness shows a considerable reduction for FNC (around 2 µm), while single layer maintains a consistent roughness (approximately 0.5 µm).
Wear performance (Temperature block wear)
Block wear test was conducted to evaluate the wear performance between single layer coated disc, FNC treated and untreated Gray Cast Iron (GCI) brake discs.
The test identified that after reaching a critical temperature of 300°C, the wear increased in all discs.
Wear Advantages:
- FNC vs. GCI: FNC shows a wear advantage over GCI, reducing wear by a factor of 3.
- Single layer coated disc vs. GCI: single layer coated disc significantly outperforms GCI, offering an 18-fold improvement in wear resistance.
This significantly longer lifespan means that brake discs would require less frequent replacements, leading to reduced manufacturing and disposal needs over time. This directly contributes to conserving resources and minimizing waste and reaching corporate sustainability targets.
Fine dust emission test (WLTP cycle)
The test was performed according to the WLTP (Worldwide Harmonized Light Vehicles Test Procedure) cycle, specifically showing results from cycle 6.
Particulate Matter (PM) levels were extrapolated to vehicle level based on actual results from one rear corner module. The brake system and the test setup were maintained constant and only the brake disc and pads were changed.
Only the relative values are shown for confidentiality reasons. Here GCI and low steel pad is taken as a baseline and the other solutions are compared for the relative change of PM values. The following are the results for PM10.
- FNC & NAO pads: Achieved a 47% reduction in fine dust from the baseline (GCI + low met pad).
- GCI & NAO pads: Reduced fine dust by 27% from the baseline.
- Single layer & Hybrid pad: Reduced fine dust to 14% of the baseline.
Single layer and hybrid solution was able to meet the lower acceptance limit (3mg/km/vehicle) which was set as per the EU commission 2023.
Conclusion
The comparative analysis between Ferritic Nitro Carburizing (FNC) treated discs and single-layer coated discs highlights the benefits of the latter for electric and hybrid vehicle applications.
Corrosion Performance: The single-layer coated disc withstands over 504 hours in the NSS chamber, significantly outperforming FNC (73 hours). The material’s inclusion of elements like chromium enhances its resistance to corrosion.
Wear Resistance: In testing, the single-layer coated disc showed an 18-fold improvement over gray cast iron (GCI) and a six-fold improvement over FNC.
Fine Dust Emission: The single layer solution, when paired with a hybrid pad, achieves a substantial reduction in fine dust emissions (14% of baseline), complying with EURO7 standards.
Note: The single layer was coated with MetcoBrake9501A powder by High-speed laser cladding process
About Metco Joining & Cladding
Metco Joining & Cladding is a leading brand that provides solutions for joining and cladding, including welded overlays, brazing, laser cladding, plasma transferred arc, and hardfacing. Since 1970, we have been offering customizable and comprehensive solutions with a portfolio of materials such as powders, wires, rods, electrodes, braze pastes, and braze tapes. These solutions cater to the critical needs of industries such as aerospace, automotive, power generation, mining, oil and gas, and agriculture. We have pioneered the first serial production solution for brake disc coating. Our single-layer coating solution has been nominated by a major European OEM to reduce fine dust emissions. Our solutions have enhanced offerings available to the automotive sector. With a global presence, Metco Joining and Cladding can offer deep expertise and solutions, in close proximity to our customers, in combination with our broad range of materials. The Metco Joining & Cladding brand is owned by the global Oerlikon Group (SIX: OERL), headquartered in Switzerland.
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