Automotive brake discs never tried to be stars of the car showroom. While the flashier design elements competed for drivers’ attention, these modest gray cast iron parts were content to go about their work largely unnoticed. Their material made them strong, robust and able to withstand extreme service conditions. In testing, they didn’t break down even when heated to more than 500° C and then doused in ice-cold salt water. On the road, they excelled at bringing vehicles to a standstill.
There was just one problem. During their service life, they became worn down by several millimeters on each side, and the material that was ground away created a fine dust that posed health and environmental risks.
The fine particles are not only released to atmosphere but also settle onto all surfaces in the vicinity of the wheel rims. Ironically, although they could fly free while airborne, the particles that latched onto those surfaces clung there stubbornly. Removing them was a challenge, and leaving them in place created aesthetic and maintenance problems. The erosion led to operating challenges, as well: the exposed disc surfaces were more prone to rusting, which took a toll on their appearance and performance.
It was clear that brake discs were in need of a makeover—one that required development of a material that retained all the positive properties of cast iron but resisted wear & corrosion. The solution remained elusive for decades. “The quest for solutions started nearly 70 years ago,” says Christian Bohnheio, Industry Segment Manager — Automotive Marketing at Oerlikon Metco in Wohlen, Switzerland.
Ceramic brake discs offered one potential answer. They met performance requirements and solved the corrosion problem but are very expensive to produce. This makes their use prohibitive outside the luxury car segment.
Additional research and development efforts focused on cost-effective coatings for the cast iron discs. That work led to more than 50 patents, but each innovation fell short of a comprehensive solution. Each coating lacked the durability required to endure extreme braking conditions, particularly those that involved rapid temperature changes. For years, there was not one single coating that achieved market success.
But failure often presents a takeaway that plants the seeds for future success. In this case, those failed coating attempts underscored the importance of pre-treatment to suppress corrosion along the boundary layer. That led to the development of an effective and cost-efficient material and process solution.
“Years of intensive research work performed together with leading car manufacturers and brake suppliers were rewarded when we achieved success by creating a combined coating process fulfilling all requirements,” Bohnheio says.
His team pioneered a two-step process. First, the brake disc is given a thermochemical IONIT OX treatment that envelopes it — including its inner ventilation cavities — in plasma-nitride to optimize resistance to corrosion. Second, a plasma spray process applies an additional top layer of coating that delivers further corrosion resistance and has the properties necessary to withstand all the strains created by braking. The production costs involved in this process are reasonable, making this solution accessible in all automotive segments
When it comes to environmental innovation and progress in controlling fine particle pollution, nothing can stop these brake technologies.