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Diamond-Like Carbon

Structure of Diamond-Like Carbon (DLC)

Diamond-Like Carbon (DLC) is a term which refers to hard carbon films with a metastable amorphous microstructure including a small amount of crystalline phase.

Carbon may exist in various allotropes:

  • Graphite consisting of hexagonal rings forming thin parallel plates. The plates are bonded to each other by weak Van der Waals forces. The bonds configuration of graphite is characterized by the sp2 hybridization
  • Diamond - a crystalline, transparent and extremely hard allotrope of carbon having a tetrahedral structure of carbon atoms with strong covalent bonds. The tetrahedral structure of diamond is determined by the sp3 hybridization of the bonds configuration.
  • Nanocarbons - allotropes of carbon consisting of large molecules, in which carbon atoms are arranged in form of spheres (fullerens), nanocarbon tubes, hollow nanocones or nanofoams.
  • Amorphous carbon having no long-range order of the atoms bonded with different types of hybridization (sp2 and sp3).

Atoms in Diamond-Like Carbon are arranged in a short-range order therefore DLC materials are amorphous. The bond network of Diamond-Like Carbon consists of a mixture of sp2 and sp3 sites.
Properties of amorphous carbon are determined by the ratio between the sp2 and sp3 hybridization:
Diamond-Like Carbon (DLC) is formed when sp3 hybridization is dominant.
Graphite-Like Carbon (GLC) is formed when sp2 hybridization is dominant.

Another factor affecting the DLC properties is the content a doping element introduced into the carbon structure. Commonly DLC films contain Hydrogen as a dopant. Other possible dopants modifying the DLC properties are: Nitrogen, Oxygen, fluorine, boron, silicon, chromium, copper, platinum, silver, titanium.

DLC containing hydrogen are called Diamond-Like Hydrogenated Amorphous Carbon (DLCH) or tetrahedral hydrogenated amorphous carbon (ta-C:H).
DLC which are not doped with hydrogen are called Diamond-Like Non-hydrogenated Amorphous Carbon or tetrahedral amorphous carbon (ta-C).

DLC with lower hydrogen content are characterized with a greater modulus of elasticity, strength, hardness, coefficient of friction, density and refractive index.

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Techniques of deposition of Diamond-Like Carbon (DLC)

Diamond-Like Carbon (DLC) is deposited on the surface of a substrate when it it is bombarded by carbon ions or hydrocarbon radicals at an energy 50-400 eV (Chemical Vapor Deposition (CVD) method).

The deposition techniques used for DLC deposition:

  • Plasma enhanced chemical vapor deposition (PECVD) or Plasma assisted chemical vapor deposition (PACVD). PECVD technique utilizes hydrocarbon gases such as methane, ethane, ethylene, acetylene, propane, benzene as a gas precursor. Two versions of PECVD method are used for deposition of DLC:
    • Direct current (DC) discharge. In this technique the substrate is connected to the cathode whereas the anode is grounded. The bias voltage between the anode and the cathode 100-1000 V ionizes the precursor gas converting it into a plasma. The ions (positively charged) are attracted by the cathode (negatively charged). The cathode (substrate) is bombarded by the ions which decompose and form carbon and hydrogen atoms deposited on the cathode surface. Only electrically conductive substrate may be used in DC discharge deposition.
    • Radio frequency (RF) discharge. In RF method the voltage applied between the electrodes is alternating in the radio frequency range. Non-conductive substrate may be deposited by the method.
  • Sputtering. Sputtering technique utilizes argon ions for bombarding a cathodically connected carbon target. Carbon atoms of the target are knocked out by the high energy argon ions and deposit on the substrate surface. Sputtering systems are commonly equipped with a magnetron helping to trap the electrons close to the substrate. Similar to PECVD sputtering also has two different techniques:
    • Direct current (DC) discharge with a constant voltage between the electrodes;
    • Radio frequency (RF) discharge with an alternating voltage between the electrodes.
  • Ion beam deposition. This technique utilizes an ion source producing the plasma ions from either the precursor gas or solid graphite target. The ions are then accelerated forming a high energy beam focused at the substrate surface where they convert into atoms of carbon and hydrogen (or pure carbon).
  • Pulsed laser deposition. In this method a high power laser in high vacuum strikes a graphite target vaporizing carbon atoms, which are deposited on the substrate surface.

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Characteristics of Diamond-Like Carbon (DLC)

  • Wear resistance;
  • High hardness (up to 8000 HV);
  • Low coefficient of friction: <0.01-0.7 (depending on the atmosphere composition and pressure and on the DLC structure and composition);
  • Corrosion resistance;
  • Biocompatibility;
  • Slow diffusion (may serve as a diffusion barrier);
  • Optical transparency;
  • Wide range of electrical resistivity;
  • May be polished to extremely high surface finish.

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Applications of Diamond-Like Carbon (DLC)

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diamond-like_carbon.txt · Last modified: 2013/12/14 by dmitri_kopeliovich
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