Nano-crystalline diamond is the hardest diamond-Eke carbon (DLC) with properties very close to true diamond. Among such properties is the low frictional coefficient and high wear resistance. Cemented WC disks were coated by nano-crystalline diamond deposited by cathodic are. These disks were rubbed cyclically by pins made of aluminum-silicon alloy (4032), carbon steel (52100), and alumina ceramics (Al2O3). It was found that frictional coefficient, after the period of "break-in", was significantly reduced when compared with that of uncoated carbides. However, the frictional coefficient (0.2) with steel was more than twice that with Al alloy (0.09) and alumina (0.08). This abnormal high frictional coefficient is possibly due to the chemical interaction between surface atoms of iron and carbon. The wear mechanisms include mechanical attrition, chemical adhesion, and fatigue. The wear loss was the highest for the softer Al alloy, and lowest for 52100 steel. However, the wear of nano-crystalline diamond was the highest when it was rubbed against Al alloy. It was postulated that diamond was worn down primarily by the hardened layers of Al alloy and oxygen formed in situ during the rubbing action.