Effects of hard water on tribological properties of DLC rubbed against stainless steel and brass

This paper reports a study on the tribology of a diamond-like carbon (DLC) coating rubbed against stainless steel and brass in a hard water environment at 20, 50 and 80 °C for water hydraulics applications. Hard water designed to simulate tap water in London containing 95 mg/L Ca2+ was used. DLC containing 30 at% hydrogen was deposited on AISI 630 stainless steel using an unbalanced magnetron sputtering system. The results showed that the wear on the DLC and the counter surfaces, in addition to the amount of friction, in hard water was smaller than that in soft water designed to simulate tap water in Tokyo containing a smaller amount of Ca2+ (23 mg/L). These superior tribological properties in hard water are attributed to a Ca-containing tribolayer formed on the steel and brass surfaces. Overall, the results suggest that attention must be paid to the quality of water used in water hydraulics applications.