Friction in Amorphous Diamondlike Carbon

Since Enke et al. [1] first reported in 1980 that diamondlike carbon (DLC) films deposited from
acetylene have very low friction coefficients, DLC films are considered in general as materials
with very low friction coefficients and as potential solid lubricants. However, this generalization
is oversimplified. A compilation of friction coefficients (m) of DLC films [2] shows that they
span a large range of values, from m=0.002-0.4 in vacuum below 10 -4 Pa, to m=0.05-1.00 in
ambient air, at relative humidities of 20%<RH<60%. Most typical, however, are the ranges
m=0.007-0.02 in vacuum and m=0.1-0.4 in ambient air. The large spread in the values of the
friction coefficients can be understood if one takes into account the fact that the DLC term
describes a large group of materials whose composition, structure and properties are affected by
the deposition system, i.e. the used precursors, deposition method, and deposition conditions.
The measured friction coefficients are, therefore, strongly dependent on the system used for the
deposition of the specific films and are affected by the testing conditions. This dependence is
illustrated in the diagram shown in FIGURE 1. The deposition system determines the properties
of the bulk of the film and of it surface. The tribological system, which includes the counterpart
used in the friction and wear measurement, the Hertzian pressure, the relative speed between the
sliding parts and the ambiance, determines the testing conditions, which in turn affect the
measured friction values.
Detailed reviews of the tribology of DLC and its modifications can be found elsewhere [3,4],
therefore the following will present only an updated summary of the frictional properties of DLC
materials. The friction between surfaces in contact is affected by a variety of factors, however, it
is controlled mainly by the formation and breaking of chemical bonds at the interface between
the moving parts [5]. Crystalline diamond is characterized by a low friction coefficient which
depends strongly on the environment and is mainly controlled by surface dangling bonds.
Diamond has a low friction coefficient in humid air or dry nitrogen, due to a contaminant layer of
low shear strength. In contrast, clean diamond surfaces can interact strongly producing high
friction coefficients. Thus, the friction coefficient of diamond against diamond in vacuum can
increase to values as high a 1.0 when the surface hydrogen is desorbed, leaving interacting
dangling bonds [5]. Saturation of the dangling bonds by atomic hydrogen or other adsorbates
reduces the friction coefficient of diamond to m=0.02. Water [6,7] and atomic hydrogen [8] act
as lubricants also for graphite and improve its wear and lubricating behavior.
DLC films deposited by plasma enhanced chemical vapor deposition (PECVD) have several
common tribological characteristics [3,4]: (i) they display a behavior similar to diamond in
ultrahigh vacuum (UHV) but not in humid atmosphere; (ii) the friction coefficient of
hydrogenated DLC is low in humid nitrogen or oxygen, extremely low in dry nitrogen or UHV,
and, sometimes, very high in dry oxygen; (iii) loss of hydrogen through annealing at high
temperature causes a marked increase in the friction coefficient of DLC in UHV, but not in
humid environment; (iv) both friction and wear of DLC are usually affected by a transfer layer
which forms in most cases during friction