Effects of water environment on tribological properties of DLC rubbed against brass

The effects of the water environment, such as temperature, dissolved ions, and dissolved oxygen, on the tribological properties of diamond-like carbon (DLC) against brass were studied as part of the development of water-lubricated hydraulics, valves and cylinders based on metals (Fe alloys, Cu alloys, etc.). A ball-on-disk type tribotester was used to examine the above various factors. DLC was deposited on stainless steel disks using an unbalanced magnetron sputtering system. Pure water and quasi-tap water, which imitate typical tap water sampled in Tokyo, were used to study the effects of dissolved ions. The water temperature was elevated from 20 °C to 80 °C. The results show that temperature as well as the dissolved ions has a major impact on friction and wear. More specifically, the coefficient of friction was increased at elevated temperatures. Also, in pure water, abrasive wear of DLC was observed due to aluminum condensation on the brass surface, whereas it was prevented in quasi-tap water since aluminum was removed from the contact region due to the dissolved ions. EPMA, XPS and AES indicated that the tribo-layer on the metal surfaces, which consists of carbon and the base metal as well as some elements from water, plays an important role.