General Motors is on a mission to banish corrosion from its brake rotors, while at the same time doubling rotor service life. By 2016 the automaker plans to have highly corrosion-resistant rotors on 80% of its U.S. market vehicles, a move that is enabled by GM’s adoption of a patented surface-treatment process.
The GM-developed process is ferritic-nitro carburizing (FNC), a form of case hardening. FNC is a gaseous process that “diffuses nitrogen and carbon into the metallic part” following production machining of the rotor's friction surface, explained Jim Webster, a GM brake engineer who helped spearhead the program.
The rotors are super-heated at 560°C (1040°F) for up to 24 hours within a nitrogen-rich atmosphere. The nitrogen atoms bond to the surface of the steel rotor, hardening it and increasing its strength.
The FNC process lays down a 10-µm-thick transfer layer across the entire rotor surface, including the center “hat” section and inside the central cooling vanes of ventilated rotors. (Ten microns is roughly 1/10 the width of a human hair.) The resulting friction surface is significantly more durable than that of a rotor without the FNC treatment, while remaining virtually impervious to corrosion and rust.
“We expect FNC will double rotor life from the current 40,000 miles, to 80,000 miles, before it needs to be ‘turned’ [machined to regain factory-spec lateral runout and surface finish],” Webster said. Other benefits include reduced brake dust accumulation on the road wheels and smoother brake-pedal feel over time.
The FNC process, while somewhat similar to carburizing as used in powertrain gear hardening, differs in its nitrogen component, GM engineers said.
Once a rotor is turned, its friction surface loses the FNC layer. But until that point, the FNC rotors are expected to delight various key constituencies. GM dealers and customers will like their aesthetic appeal, particularly due to the increased popularity of polished-aluminum and chromed multi-spoke road wheels that tend to show off the brakes behind them. Owners will appreciate the vastly extended service life and improved pedal feel as the miles rack up.
And GM Design stylists can develop more “open architecture” wheels without worrying that rusty brake rotors will diminish their appeal.
“My partner Ed Welburn [GM Vice President of Design] asked me to fix the rusty-rotor issue,” said John Calabrese, Vice President of Global Vehicle Engineering. He noted that customer surveys have shown that 40% of vehicle owners consider brake-rotor corrosion to be among the most bothersome issues.
GM began investigating solutions for brake-rotor corrosion and extended service life in 2006. Engineers started with salt-bath nitriding, which was first used on the 2008 Cadillac DTS brake rotors. Gaseous FNC then evolved from the salt-bath process, Calabrese said, noting that FNC has helped reduce DTS brake-related warranty claims by over 70%.
The FNC’s on-cost, compared with non-treated rotors, is “slight but manageable,” he said. GM outsources its production brake rotors.
GM has increased its implementation of FNC rotor technology since its 2008 debut. Currently it’s used on the Buick Lacrosse and Regal and the Chevrolet Malibu, Impala, and Volt. Applications thus far are limited to North American and Chinese vehicles, due to those markets' preference (and regulations) for brake-pad materials that are less aggressive than higher-friction materials, particularly those using sintered-metal compositions preferred in Europe, noted Margaret Oswald, GM’s Director of Brake Systems Engineering.
GM claims to be the only automaker currently using a nitriding process to address brake rotor wear and corrosion.