W-Co-C amorphous coatings with cobalt content between 18 and 30 at.% were deposited by r.f. sputtering on M2 (AISI) high-speed steel (HSS) disks. They were tribologically tested by pin-on-disk against uncoated mild steel pins with a sliding speed of 0.5 m s-1 and normal load of 4 N. Friction coefficients ranged from 0.38 to 0.49. Wear coefficients between 1 and 2 x 10(-15) m(2) N-1 were measured on the coated surfaces. Very low wear (2 to 20 x 10(-17) m(2) N-1) was obtained on the steel pins. This was related to the formation of a transfer film on the uncoated surfaces. Transmission electron microscopy analysis of this transfer film revealed the presence of various crystalline phases (WC, W2C, WC1-x and CoWO4) in addition to amorphous phase. Formation of the transfer films is discussed in terms of film composition and stress-induced crystallization.