Wettability of Amorphous Diamond-Like Carbons Deposited on Si and PMMA in Pulse-Modulated Plasmas

Pulse-modulated direct-current methane plasmas are used to deposit amorphous diamond-like carbon films on Si and dentistry-use polymethyl methacrylate (PMMA) substrates as a function of the negative pulse voltage applied to the substrate (V_max). The films on PMMA show a transition from diamond-like to more graphitic carbon in the Raman spectra with increasing V_max, dissimilar to those on Si. This is attributed to easy deformation of PMMA, leading to the low compressive stress of the films (1 to 2 GPa). The contact angle of water for the films on both Si and PMMA is large, ranging from 79° to 94° almost independent of V_max, confirming that the films are hydrophobic despite the difference in carbon bonding state. The large dispersion component (41-43 mJ/m²) of the surface free energy of the films measured from the contact angle of water and 1-bromonaphthalene indicates the high mass density of the films. The small polar component (0.2-3.5 mJ/m²) is attributed to hydrogen saturation of the surface sites forming nonpolar C-H bonds and, thus, responsible for the hydrophobic behavior.