Tribological Properties of Fluorinated Amorphous Carbon Thin Films

Abstract

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.