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Finding the right friction formula for braking

08 September 2015

A coatings specialist helps the University of Sheffield Formula Student team overcome problems associated with the hard coating of 3D printed titanium parts.

Formula Student, the annual competition to build a racing car from scratch, attracts over 100 international teams to Silverstone every year. Participating for their sixth time in 2015, the University of Sheffield Mechanical Engineering Department had to overcome a variety of technical challenges.

Material selection for the braking system, in particular, posed a sticky problem that was overcome with help from engineers at the Cambridge site of ultra-hard coating specialist, Wallwork. Sheffield's team principal, Thomas Bloomfield takes up the story:

“The competition exposes students to a raft of real-life engineering problems from the design of components, using computer aided design, through manufacture using additive manufacturing to the final fitting and testing of components on the completed car. A key design objective was to achieve good power-to-weight ratio for race advantage, so titanium alloy was selected for brake components, but this created a new technical challenge.”

“Titanium is very strong, fatigue resistant and most of all light in weight," adds Nathaniel Wellicome, a final year student who led the chassis and vehicle dynamics team. "We were able to translate the design into a manufactured component using an additive process where metal powder is fused by electron beam melting to create the complex component shapes.

"Unfortunately, relative to heavier alternative materials such as steel alloys, titanium is a soft material, so it can bind when under load in metal-to-metal contact. Wallwork have a lot of experience in the application of titanium in aerospace and motorsport so we asked them how to overcome the problems.” Dr Jonathan Housden takes up the story for Wallwork:

“The 3D printed parts were delivered to us with the mating surfaces machine cut to overcome the typical surface roughness that is common in 3D printing. This gave a smooth surface for the coating, but applying a hard coating to this machined surface on its own would be insufficient since the titanium substrate alone is too soft and the coating would break up under load.

“The answer was to use Nitron O, a duplex process that first infuses nitrogen compounds into the metal surface to produce a hard layer to support the subsequent titanium nitride surface coating. This permits the alloy to be used in high-load sliding wear applications, giving endurance and the required frictional properties, without binding.” Thomas Bloomfield again:

“Our students participate in this challenge alongside their degrees so they gain multiple practical skills and develop fantastic time management skills and learn how to deal with realistic high pressure fast-paced environments. This makes them very employable and is why some companies, like Wallwork, are happy to sponsor the team by sharing their expertise.”

Silverstone hosted around 100 international teams from leading UK and overseas universities for the race and technical judging between 9 and 12 July 2015. The results are available here.

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