Tribological behavior of tungsten-doped DLC coating under oil lubrication

Diamond-like carbon (DLC) protective films have received considerable attention in recent years. Beside tools and dies, diamond-like and related coatings are starting to find application in some highly stressed mechanical components, including bearings and gears. Several different gear tests show an increase in carrying load capacity or prolonged lifetime with tungsten carbide-doped DLC (W-DLC)-coated gears. The lubricants have been mostly commercially available gear oils that were primarily developed or tailored for ferrous materials and are probably far from optimal for amorphous W-DLC layers.In the present work, the influence of lubricant chemistry on the friction and wear in W-DLC-coated contacts was investigated using a reciprocating test device. The first six test lubricants were conventional non-inhibited API/ATIEL base stocks followed by selected test blends in combination with different surface-active single additives. Experimental results in coated contact have shown that for friction reduction performance, additives are not necessary, while wear resistance is related with the chemistry of the lubricant. It may be concluded at this stage that different mechanisms of friction-induced interaction between lubricants and W-DLC-coated layers are possible.