Friction loss in engines cannot be avoided and accounts for between 15 to 20 % of the energy expenditure, depending on the size and configuration of the engine. Piston skirt friction, piston rings and bearings account for 66% of total friction losses and the valve train for 20 to 25% at low speed. The remainder of the losses come from crankshaft, transmission and gears. Various ways have been sought to minimize friction loss over the years, but one that is increasingly interesting to the automotive industry is coatings. This is hardly a new solution with surfaces having been coated for years, but what is starting to come to the fore is a process called “Diamond-Like Carbon” (DLC) coatings. It has already been applied to diesel injection systems, as without this treatment they could not sustain the pressure. Now, though, it is being extended into the nether regions of the engine.
Unlike other surface treatments that either increase wear resistance or improve friction behavior, DLC coatings make both properties available in a single surface treatment. At the same time, they possess a low coefficient of friction, a low surface energy approaching that of Teflon, and a hardness that is greater than that of carbide or titanium nitride (TiN). While TiN coatings, for example, have a coefficient of friction of around 0.3 and steel 0.7, the coefficient of friction of DLC type coatings is in the region of 0.1. The result is a coating solution that provides a combination of unique wear and low friction.
There are a number of specialist companies involved in DLC coatings, many as suppliers to race engine manufacturers who adopted this technology a few years ago. However, some are beginning to transition this technology to the mainstream automotive industry, with Bekaert being in the vanguard. This is a €3 billion company employing 17,000 people that is headquartered in Belgium and has facilities located in 120 countries. A world leader in advanced metal transformation, advanced materials and coatings, it is already involved in the automotive industry which accounts for around 40% of its sales. It is number-one in tire reinforcement products, with one in four tires being reinforced with Bekaert steel cord products. It also supplies wire for clutch springs, steel wire and fibers in car seats, and it has developed a coating for flat wiper blades that not only improves their performance but also halves the weight and decreases the number of assembled parts.
For the last 10 years, though, it has also been developing and marketing diamond-like coatings following its acquisition of Sorevi, a French specialist coatings company, and a major supplier of advanced DLC coatings to Formula One and NASCAR engine manufacturers. Its Cavidur coating is a special type of amorphous carbon-based DLC, usually between 2 and 4 microns thick, with a key part of the coatings process being plasma-assisted CVD (chemical vapor deposition). Coatings are applied through a vacuum process at temperatures up to 350ºF. Using this technology, both electrically conductive and non-conductive substrates in a variety of shapes and sizes can be coated homogeneously. “This environmentally friendly technology can be scaled up, enabling Bekaert to offer cost-effective coatings in many industrial sectors,” says Mark Boghe, product market manager automotive of Bekaert. “Any material that can take these conditions without degassing can be potentially coated so even aluminum can be coated. However, because aluminum forms a weak substrate for hard coatings, the application must be oriented more towards reducing technology.”
Another technology used is physical vapor deposition (PVD), where atoms re deposited on a surface by physically removing them from another surface. It allows the design of advanced—or engineered—interlayers to improve the performance of the coatings in very specific applications. To obtain good adhesion, the substrate must be pre-treated by grit blasting, grinding, milling, turning, or a combination of these techniques. Applications include camshafts, finger followers, finger shafts, gudgeon pins, pistons, tappets and valves, plus clutches, brakes and the inner tubes of a racing bike shock absorber.
A spin-off from its DLC Cavidur coating, Bekaert introduced the Dylyn Plus DLC coating last year. Less than 12 months later it announced that it has delivered its 500,000th automotive valve train component to an unnamed European carmaker. “The valve train enables the engine to breathe which, in an engine, means controlling the inlet of a mixture of air and fuel into the cylinder and letting the gases out,” says Michael De Maegt, general manager DLC at Bekaert. “The better an engine breathes, the more efficient it becomes. Coating with Dylyn Plus leads to the reduction of friction in the valve train system, adding wear resistance to the component, extending its life time and allowing the automaker to produce engines with reduced fuel consumption and emissions.” A DLC coating can even allow design changes in the engine, says De Maegt. “Because of DLC’s low coefficient of friction it allows the elimination of bushings so that the piston, for example, can be smaller.”
Another coatings company with roots in motorsport but increasingly supplying the automotive industry is Tecvac, a small UK company based on the outskirts of Cambridge. Following a change in the Formula One engine rules, where engines now have to last two complete race meetings, including practice, rather than when the teams had special engines with different specifications for qualifying and racing, Tecvac found that it was losing business in terms of the volume of heat treatment work it was doing for the engine manufacturers. However, it has now found a new market for DLC on transmission components. “Previously, gearboxes and transmission parts did not represent much of a business for us,” says Mike Morris, Tecvac’s sales manager, “but the low temperatures involved in the DLC process have changed that. So while we have taken a step back with heat treatment, we’ve taken two steps forward with our coating business.”
“The cost for applying a DLC coating depends on numerous factors,” says Boghe. “Most important are the size and the volume of the application, but also significant is the ability to automate, the desired properties and delivery time.”