Diamond-like carbon coatings prepared in an asymmetric bipolar pulsed d.c. plasma

High quality diamond-like carbon (DLC) coatings can be routinely produced in r.f. (13.56 MHz) based capacitively coupled plasma-assisted chemical vapour deposition processes. In practice, this process is limited to relatively small batch volumes (<1 m(3)). Bipolar pulsed d.c. processes may offer an alternative to way forward to large-scale processing. In this work, DLC coatings were deposited from CH(4) using an asymmetric bipolar pulsed d.c, power supply. The positive pulse was fixed at a low voltage of +75 V, while the negative pulse amplitude was varied between -300 V and -1100 V. The following coating properties were characterized: C-H and C-C bonding structure using Fourier transformed IR and Raman spectroscopy, hardness using depth sensing indentation and internal stress using the bending beam technique. Substrate heating during deposition was evaluated using the annealing characteristics of hardened 100Cr6 steel pins. These properties are correlated with the major variables of the power supply: pulse frequency (120-250 kHz) and pulse amplitude. Under optimised plasma conditions, coating hardnesses as high as 18 GPa could be obtained. (C) 1999 Elsevier Science S.A. All rights reserved.