Ferritic nitrocarburizing (FNC) has emerged as a powerful, low-distortion heat treatment solution for automotive components, especially brake rotors, thanks to its wear resistance, corrosion protection, and compatibility with tight dimensional tolerances.

In this Technical Tuesday installment, adapted from a recent Heat Treat Radio conversation, Mike Holly, a retired GM metallurgical engineer, shares how FNC reshaped brake rotor technology at General Motors (GM), and what its future may hold for commercial and in-house heat treaters alike.

This informative piece was first released in Heat Treat Today’s August 2025 Annual Automotive Heat Treating print edition.

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A Brake Rotor Problem Worth Solving

Ferritic nitrocarburizing (FNC) may not be new to the thermal processing world, but its potential is still unfolding, especially for heat treaters working with automotive components. With over four decades in the automotive and heavy truck industry, 32 of those with GM, Mike Holly’s perspective is forged in real-world experience. In the mid-2000s, he and a colleague tackled a costly issue: brake rotor warranty claims due to pedal pulsation and corrosion. The solution? FNC, if it could be controlled tightly enough to meet the dimensional demands of finished machined rotors.

From Concept to Production

Working with Kolene (in salt bath) and later Woodworth (in gas), the GM team optimized the process to FNC rotors after finish-machining without subsequent grinding. Stress relieving the castings ahead of machining helped eliminate dimensional shifts. This process innovation led to multiple patents and debuted in the 2009 Cadillac DTS and Buick Lucerne. Today, it’s used widely across GM platforms and by other OEMs.

Initially, implementation required close collaboration with external heat treaters and careful process validation. GM chose to pilot the technology on low-volume, premium platforms before expanding to high-volume vehicles like trucks.

“Eventually,” said Holly, “the best setup was to co-locate FNC operations with machining and casting facilities. That reduces shipping costs and protects dimensional tolerances.”

Why FNC Works

“FNC is a thermal chemical case hardening process,” Holly explained. “It diffuses nitrogen and carbon into ferrous materials at subcritical temperatures, typically 560 to 590°C (1040°F to 1090°F). That creates a hard, wear- and corrosion-resistant surface with minimal distortion.”

The benefits are compelling: compressive residual surface stress improves fatigue life; resistance to adhesive and abrasive wear boosts durability; and there’s virtually no hazardous waste. For heat treaters managing precision parts or looking to support sustainability goals, FNC checks several boxes.

Unlike carburizing or carbonitriding, FNC operates below the critical transformation temperature of steel. This means fewer dimensional changes and minimal phase transformations, making it ideal for components that have already been finish-machined. Parts emerge from the process with a compound layer and a diffusion zone that enhances performance without requiring post-processing.

Still, it’s not for every application. “The shallow case depth (10 to 20 microns) limits FNC’s use in high-load or rolling contact fatigue applications, like ring and pinion gears,” said Holly. “But for lightly loaded gears, brake backing plates, clutch discs, and now brake rotors, it’s a great fit.”

To make FNC viable for finished parts like brake rotors, precise control over fixturing and orientation is essential. “Vertical orientation in the furnace is crucial,” said Holly. “And you must stress relieve parts, at least where the geometry or casting process warrants it.”

Fixturing strategies are typically proprietary to commercial heat treaters, but OEMs require dimensional accuracy to be statistically validated. Proper stress relief, careful racking, and consistent atmosphere control are all part of ensuring tight tolerances and minimizing scrap.

Applications Beyond Brakes

FNC isn’t just for brake rotors. It’s used in numerous applications where wear, corrosion resistance, and dimensional control are critical. Lightly loaded gears, hood struts, locking mechanisms, clutch pack discs, and brake backing plates all benefit from FNC.

In non-automotive sectors, FNC has been applied to hydroelectric gates, military components, and even stainless steels where added surface strength and wear resistance are needed.

Opportunity for Heat Treaters

For in-house and commercial heat treaters, the expanding use of FNC presents an opportunity. Whether supplying OEMs or developing in-house capability, the process can offer a competitive edge in applications requiring low distortion and enhanced surface properties.

However, Holly emphasizes that consistency and precision are paramount. OEMs, he explained, don’t need to know the proprietary fixturing methods used by commercial heat treaters, just that the finished parts meet all dimensional specifications.

“Show us statistically that your lateral runout, your thickness and your wheel mount surface meet our specs,” he said.

In-house heat treat operations should prioritize tight process control, consistent dimensional outcomes, and scrap minimization. As with most thermal processes, success lies in mastering the details.

Preparing for Euro 7

The process’s low temperature and environmental profile are key drivers. And now, its role in upcoming regulatory changes could make FNC even more relevant. The European Union’s Euro 7 regulation, expected to begin rolling out in 2026, includes strict limits on brake dust emissions. Holly sees two viable solutions on the horizon: FNC and laser cladding.

“FNC with niobium alloying is the low-cost alternative,” said Holly. By alloying gray iron brake rotors with niobium before FNC, Holly’s team is developing enhanced surface hardness through niobium carbides, without post-process grinding or exotic feedstocks. It’s a scalable path for meeting wear and emission goals while keeping costs in check.

Alloying for the Future

In response to Euro 7 and increasingly aggressive friction materials, Holly is working with clients to improve the case properties of FNC rotors through alloying. Niobium, a known carbide-former, is added during the casting phase to enhance both case and core properties.

“There is a heavy truck rotor application that was niobium alloyed for many years, and that was advertised as a 1-million-mile rotor,” said Holly. “In the heavy truck industry, it’s all about uptime — keeping the trucks out of the shop and on the road.”

This alloying strategy leverages existing infrastructure and doesn’t require major capital investment, a significant advantage over competing technologies like laser cladding, which demands specialized equipment, multiple process steps, and expensive materials.

Cladding vs. FNC: The Economic Equation

While laser cladding offers impressive wear resistance, it brings substantial cost and environmental considerations. The materials involved (nickel, chromium, molybdenum) are expensive and volatile. Post-process grinding generates nickel-laden swarf, which requires special handling and disposal.

FNC, by contrast, uses common gases like ammonia and natural gas. It’s performed in batch processes that are well-suited to high volumes and heavy parts. And it can be integrated into existing facilities without massive investment.

While cladding may be necessary for extreme-duty applications, FNC, especially with niobium alloying, offers a highly competitive solution for most mainstream automotive needs.

The Road Ahead

As regulatory pressure increases and OEMs push for performance and sustainability, FNC is well-positioned to meet the challenge. Holly continues consulting with clients in North America, Europe, and South America, supporting FNC development and publishing papers at industry events like SAE Brake Colloquium and EuroBrake.

For heat treaters, whether in-house or commercial, staying ahead means understanding not just the metallurgical fundamentals but also the evolving regulatory and performance landscape. FNC presents heat treaters with a scalable, efficient, and regulation-ready solution to meet evolving performance and emissions demands.

About The Author:

Mike is currently a consultant with Mike Holly Metals LLC, specializing in heat treatment, coating, casting, metal forming and joining operations. He has 42 years of experience in industry, including 32 years at the General Motors Materials Engineering department where he was assigned to support automotive and truck chassis applications. He holds 15 patents and was key in the development of Ferritic Nitrocarburizing Brake Rotors. Mike has a bachelor’s degree in metallurgical engineering from Wayne State University and a master’s degree from Purdue University.

For more information: Contact Mike Holly at mike.holly72@att.net.

To listen to Heat Treat Radio Episode 117: How GM Started & Grew FNC for Brake Rotors click here.

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