Amorphous (a-C) and hydrogenated amorphous carbon (a-C:H) films have high hardness, low friction, electrical insulation, chemical inertness, optical transparency, biological compatibility, ability to absorb photons selectively, smoothness, and resistance to wear. For a number of years, these economically and technologically attractive properties have drawn almost unparalleled interest towards these coatings. Carbon films with very high hardness, high resistivity, and dielectric optical properties, are now described as diamond-like carbon or DLC, table 1.

Table 1. Properties of diamond and DLC materials.




Thin Film


Bulk

Property


CVD Dia.


a-C


a-C:H


Diamond


Graphite

Crystal Structure


Cubic
ao=3.561Å


Amorphous
Mixed sp2 and sp3 bonds


Amorphous
sp3/sp2


Cubic
ao=3.567Å


Hexagonal
a=2.47

Form


Faceted crystals


Smooth or rough


Smooth


Faceted crystals




Hardness (Hv)


3000-12000


1200-3000


900-3000


7000-10000




Density (g/cm3)


2.8-3.5


1.6-2.2


1.2-2.6


3.51


2.26

Refractive Index


-


1.5-3.1


1.6-3.1


2.42


2.15

Electrical Resistivity (Ω/cm)


>1013


>1010


106-1014


>1016


0.4

Thermal Conductivity (W/m.K)


1100


-


-


2000


3500

Chemical Stability


Inert


Inert


Inert


Inert


Inert

Hydrogen Content (H/C)


-


-


0.25-1


-


-

Growth Rate (µm/hr)


~1


2


5


1000 (synthetic)


-
Methods for Producing DLC Films

Several methods have been developed for producing diamond-like carbon films:

· primary ion beam deposition of carbon ions (IBD)

· sputter deposition of carbon with or without bombardment by an intense flux of ions (physical vapour deposition or PVD)

· deposition from an RF plasma, sustained in hydrocarbon gases, onto substrates negatively biased (plasma assisted chemical vapour deposition or PACVD).

Until recently, the work on DLC worldwide has not yielded the expected benefits in the field of wear resistance and general mechanical performance. Most of the success has been in applications for magnetic storage media and optical coatings. The reasons for this are:

· only thin coatings (< 1µm) have been used

· the 2D aspect of most of the deposition routes

· the difficulty in gaining good adhesion to metallic substrates.