We deposited amorphous diamond-like carbon (DLC) films using an unbalanced magnetron sputtering method on M2 tool steels. The deposition process was controlled using a closed-loop optical emission monitor (OEM), which regulated the flow of reactive gases of N-2 and C2H2 via a fast-responding piezo valve. By tuning the OEM settings for N-2 and C2H2, we were able to deposit a compound DLC coating consisting of Ti, TiN, TiCN, TiC and a-C:H/Ti in sequence. Excellent mechanical and wear performance was achieved. Microstructure and tribological properties of DLC coatings were characterized using transmission electron microscopy, electron probe microanalysis, Raman spectroscopy, Vickers, and wear tests. The Raman intensity ratio I-D/I-G of the characteristic G and D bands decreases and the G line position moves toward 1550 cm(-1) with the decrease of OEM settings, corresponding to a higher sp(3) content and higher microhardness. Wear tests demonstrate that the average friction coefficient of DLC films reduces from 0.33 to 0.14, and wear life increases from 900 to 24 000 m with the decrease of OEM settings. The Ti content, corresponding to enhanced wear properties, in DLC increases with OEM settings. Finally, we discovered that the phase transformation from TiC to DLC is strongly influenced by OEM settings, and is demarcated at an OEM setting of 20%.