Tribological Properties of Fluorinated Amorphous Carbon Thin Films

Abstract

1.1 Amorphous carbon characteristics The peculiar electronic configuration of carbon atoms, 1s2 2s2 2p2, and the small energy difference between their 2p and 2s orbitals, compared to the binding energy of the carbon bonds, allow the electrons to rearrange in s and p mixed orbitals that enhance the binding energy with other atoms. This process is called hybridization and produces three different types of orbitals: sp = s + p, sp2 = s + p + p and sp3 = s + p + p + p. Each different bonding state corresponds to a certain structural arrangement: sp bonding gives rise to chain structures (with two σ bonds and two π bonds), sp2 bonding conforms onto planar structures (three σ bonds and one π bond) and finally sp3 bonding produces tetrahedrical structures (four σ bonds). The p orbitals that form π bonds overlap less than the orbitals forming σ bonds. The reduced overlapping makes π bonds weaker than σ bonds. However, a number of scenarios are possible. Sometimes, as in ethene (C2H4), a σ and π bond combine producing a stronger bond between carbon atoms. This is called a double bond: C=C. Triple bonds consist of a σ bond and two π bonds, as in ethyne (C2H2). Although chemically stronger thanks to double bonds, the mechanical stability obtained with sp2 hybridization in solids is limited, due to the planar geometry. Instead, sp3 hybridization allows the creation of a three dimensional network of σ bonds. Due to this variety of possible bonding configurations, carbon has a number of allotropes: graphene (sheet of sp2 bonded carbons: σ bonds plus delocalized π bonds), carbon nanotubes and fullerenes (graphene sheets rolled over themselves forming cylinders or spheres, respectively), graphite (Bernal stack of graphene sheets), diamond (network of sp3 bonded carbons) and amorphous carbon (cross-linked and non-organized carbon matrix with a mixture of sp2 and sp3 bonds). It is to the modification of the latter with fluorine that this chapter is devoted to. The International Union of Pure an Applied Chemistry (IUPAC) defines amorphous carbon as “A carbon material without long-range crystalline order”. It also states that “Short range order exists, but with deviations of the interatomic distances and/or interbonding angles with respect to the graphite lattice as well as to the diamond lattice.” Depending on the ratio of sp2 and sp3 bonds in the matrix, amorphous carbon (a-C) presents a variety of well-reviewed mechanical properties [Silva, 2003]. Tetrahedral amorphous carbon films (ta-C or TAC) present the highest hardness, with a high degree of sp3 bonding and without hydrogen. It is almost exclusively deposited by filtered cathodic vacuum arc