Microwave plasma-enhanced chemical vapor deposition was used to grow diamond films on Ti-6Al-4V alloy using various gas phase precursors. The results of four types of experiments with different gas mixtures are compared: (1) 2% CH4 in H2; (2) initial saturation of 5% CH4 in H2 followed by 2% CH4 in H2; (3) a CO-rich mixture with a CO:H2 ratio of 8; and (4) a hybrid mixture of 2% CH4 in H2 followed by a CO:H2 ratio of 8. The substrate temperature during deposition with CH4/H2 mixtures was between 715 and 810°C, and was between 550 and 600°C when CO/H2 mixtures were used. Optical emission spectroscopy was used to monitor the excited gas-phase species in the plasma. The films were characterized by micro-Raman spectroscopy, glancing-angle x-ray diffraction, and nanoindentation. The films grown with the type (1) mixture often exhibited good quality with high hardness (70 GPa) but suffered from poor adhesion to the substrate. The films grown with the type (2) mixture were of slightly lower quality and hardness (58 GPa) but exhibited better adhesion. The films produced from the type (3) mixture were adhered, but exhibited very low growth rates and low hardness (18 GPa). Finally, the films produced from the hybrid type (4) mixture were of poor quality and suffered from poor adhesion to the substrate. The differences in interfacial phases and mechanical properties for each film are discussed and the usefulness of each gas mixture for the diamond growth is evaluated. © 1998 American Institute of Physics.