Strong, light, and a great performer in harsh conditions, titanium isn’t a very popular metal in car or truck engines because it doesn’t move well. A high coefficient of friction, even when lubricated, makes titanium an acceptable choice for stationary vehicle parts and supports, but not moving components.
Solar Atmospheres hopes that worked conducted with Oak Ridge National Laboratory (ORNL) researchers can change that. The materials treatment company has been working to develop surface treatments that will make titanium more slippery.
Nitriding – introducing nitrogen to heated titanium – has shown great promise in dramatically lowering titanium’s coefficient of friction while maintaining the metal’s wear resistance and durability, says Don Jordan, vice president and corporate metallurgist at Solar Atmospheres.
“Aluminum and titanium are two of the worst metals out there for metal-on-metal moving parts,” Jordan explains. “If you can change that, you open up a whole lot of opportunities for materials and parts in the engine. You can get rid of a lot of the moving weight on those parts and create a much more efficient operation.”
An ORNL study of various treatment processes for titanium parts for potential use in large commercial truck engines supports Jordan’s beliefs. When comparing coatings, heat treatments, chemical treatments, and mechanical treatments, nitriding offered among the lowest coefficients of friction in treated titanium, and wear resistance was close to that of the top performers.
Titanium pros and cons
Researchers have been considering titanium for automotive use for decades, says Dr. Peter J. Blau, one of the authors of the titanium engine parts study who retired from ORNL at the end of 2013.
“ORNL’s interest goes back at least 20 years to the early ’90s, when we were looking at titanium as a possible material for lightweight brakes for heavy vehicles,” Blau says. “It’s very corrosion resistant, and it’s an established technology from aerospace, so a lot is known about the metallurgy. Then in the early 2000s, there were some very promising new technologies for titanium extraction, so the material costs looked like they were coming down. That’s when we took another look at titanium for rubbing components.”
For many harsh conditions, the value of the metal was immediately clear. With commercial truck brakes, road salt could sit on titanium alloy rotors without corroding or pitting, solving a major problem for shipping companies in cold-weather regions. With truck engine pistons, moving from hardened steel to titanium could dramatically reduce the moving mass within the engine, boosting efficiency by removing a big source of parasitic drag on performance.
On the down side, titanium is more expensive than aluminum and cast iron, tougher to machine, and harder to form without going into powder metallurgy or additive manufacturing, Blau explains.
That’s just in part creation. In some motor vehicle uses, such as flame decks, blocks, and brake rotors, there are massive heat problems as well.
“Cast iron in engines has about six times higher thermal conductivity than the titanium alloys we studied,” Blau says. “You need to pay special attention with thermal dissipation.
Titanium disc brake rotors, for example, would heat up more than cast iron or steel, just because thermal conductivity was so low. You couldn’t get rid of the heat fast enough without affecting the surrounding structures.”
One way of handling that would be to let the heat climb, something that could improve thermal efficiency and increase overall fuel economy. But, running truck engines hotter would mean swapping out most metal and rubber parts under the hood for temperature-resistant materials.
Blau explains that truck and engine designers would have to study the tradeoffs between materials costs – both for the titanium parts and the supporting heat-resistant materials – and the efficiency improvements.
Low-friction surface treatment
Still, the biggest challenge for moving engine parts is the poor metal-on-metal movement of titanium. Jordan explains that nitriding could solve that problem, if equipment producers partner with materials experts to develop the heat-treating technology.
For the tests run so far, Solar Atmospheres engineers heated titanium pieces to nearly 1,500°F in a near-vacuum environment. They then introduced a controlled amount of extremely pure nitrogen, bonding those gas molecules to the outer layer of the titanium piece.
“After heating up in the vacuum furnace, we introduce what we call partial-pressure gas. It’s still below atmospheric pressure, but we introduce a small flow of nitrogen gas and it nitrides to the titanium quite nicely and uniformly,” Jordan says.
For engine parts, there are still many variables to be worked out to perfect the process. With nitriding, higher heat and furnace pressures generate harder materials. If the material gets too hard, it can become brittle, losing some of its attractiveness for sliding engine parts. Working out the exact atmospheric conditions and processes will take some time, and Jordan says Solar Atmospheres wants to get started on developing those methods.
“We think we have an answer to how to do this, but we need to develop things. I need the industry to send me samples so we can design the cycles based around what the application performance requirements are,” Jordan states. “I don’t know yet what the ideal temperatures and pressures are to generate the micro-hardness profiles that will perform adequately in their applications.”
Balancing properties
A few racecar companies have expressed interest in Solar Atmosphere’s process, but so far, automotive and commercial trucks companies have stuck to more proven materials.
Blau says there is a need to adjust engine material selection to meet upcoming fuel economy requirements, and adding titanium to the toolbox of solutions is a positive. However, a wide range of technologies are already on the market.
Throughout the years, automotive and trucking companies have considered many alternative materials for powertrains – composite structures, temperature-resistant alloys, and ceramic bearing systems for turbochargers. Blau says looking strictly at the materials side of the equation can be a mistake. With ceramics, for example, the turbocharger rotors worked beautifully because they could handle furnace-like temperatures in turbochargers. But the industry never adopted them, in part due to high costs of testing and quality control.
“Concerned with nitrogen oxide emissions from such hotter-running engines, the producers realized that they should back off the engine temperatures,” Blau says. “They still got most of the performance boosts they needed but could use less expensive, more proven metal alloy technologies instead of ceramics.”
He adds that titanium could be an ideal solution for many applications, but he says that engine designers will approach use of the material very carefully.
“If you can use titanium for the bulk properties that it provides – less thermal expansion, lower mass, corrosion resistance; if you get a full perspective of all of the properties that you need, and you can get titanium to fill most of those boxes, you can probably engineer solutions to its weaknesses,” Blau explains. “It’s an engineering problem. It’s not a home run for any material out there.”
Solar Atmospheres
www.solaratm.com
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.