Tribologists have developed a diamond-like film that is generated by the heat and pressure of an automotive engine. The ultra-durable, self-lubricating tribofilm — a film that forms between moving surfaces — can be made to develop self-healing, diamondlike carbon (DLC) tribofilms. The film generates itself by breaking down the molecules of the lubricating oil, and can regenerate the tribofilm as it is worn away.
The researchers coated a small steel ring with a catalytically active nanocoating — tiny molecules of metals that promote chemical reactions to break down other materials — and subjected it to high pressure and heat using a base oil without the complex additives of modern lubricants. Following the endurance test, the ring did not exhibit the expected rust and surface damage; the ring was intact with a blackish deposit on the contact area.
Looking at the deposit using high-powered optical and laser Raman microscopes, the team realized the deposit was a tribofilm of diamond-like carbon, similar to several other DLCs developed at Argonne in the past. Tests revealed the DLC tribofilm reduced friction by 25 to 40 percent, and wear was reduced to unmeasurable values.
Further experiments revealed that multiple types of catalytic coatings can yield DLC tribofilms. The coatings interact with the oil molecules to create the DLC film, which adheres to the metal surfaces. When the tribofilm is worn away, the catalyst in the coating is re-exposed to the oil, causing the catalysis to restart and develop new layers of tri-bofilm. The process is self-regulating, keeping the film at consistent thickness. The film was developing spontaneously between the sliding surfaces and was replenishing itself.
To provide theoretical understanding of the phenomenon, large-scale simulations were run to understand what was happening at the atomic level, and it was determined that the catalyst metals in the nanocomposite coatings were stripping hydrogen atoms from the hydrocarbon chains of the lubricating oil, then breaking the chains down into smaller segments. The smaller chains joined together under pressure to create the highly durable DLC tribofilm.
Manufacturers already use many different types of coatings for metal parts in engines and other applications; however, those coatings are expensive and difficult to apply, and once they are in use, they only last until the coating wears through. The new catalyst allows the tribofilm to be continually renewed during operation. Additionally, because the tribofilm develops in the presence of base oil, it could allow manufacturers to reduce, or possibly eliminate, some of the modern antifriction and anti-wear additives in oil. These additives can decrease the efficiency of vehicle catalytic converters, and can be harmful to the environment because of their heavy metal content.