Internal DLC coating of narrow metal tubes using high-density near plasma sustained by microwaves propagating along plasma-sheath interfaces

► Microwave-excited high-density plasma was generated inside a narrow metal tube. ► Diamond-like carbon (DLC) was coated inside the tube using the plasma. ► Friction coefficient between the DLC film and cast iron shaft was measured. ► Minimum friction coefficient obtained was 0.17 at the tube end.