Diamond-like or Hard Carbon Coating

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Diamond-like coating (DLC) or Hard Carbon Coating seems to be the most robust of all optical coatings. It is extremely resistant to salts, acids, alkalis, and most organic solvents. High mechanical hardness and low coefficient of friction make this film extremely resistant to abrasion. DLC has very good adhesion to germanium and silicon.

This coating is more often used on the outer surfaces of silicon and germanium optics of thermal imagers used in military equipment and/or operating under extreme climatic conditions. DLC protects external optical surfaces from the abrasive action of dust particles in the air, from sea water and salt, from engine oil and fuel, high humidity, improper handling etc. 

The film is characterized by moderate absorption and scattering levels throughout the entire IR spectral range. Index of refraction of the diamond-like film in the IR is about 1.95. That is why DLC film, applied by special methods, combines outstanding protective properties with a good antireflection effect.

Being a single-layer coating, it can be optimized for a specific wavelength range (usually 3-5 or 8-14 μm bands) by adjusting layer thickness.

Transmission of germanium parts with two-side DLC and DLC-BBAR. 1 mm thickness.

Fig. 1. Transmission of germanium parts with two-side DLC and DLC-BBAR. 1 mm thickness.

Reflection of a DLC-coated germanium part.

Fig. 2. Reflection of a DLC-coated germanium part.

Transmission of silicon parts with two-side DLC and DLC-BBAR.

Fig. 3. Transmission of silicon parts with two-side DLC and DLC-BBAR.

Reflection of a DLC silicon part.

Fig. 4. Reflection of a DLC silicon part.


Chemical Properties
Composition carbon, hydrogen
Structure A composite of sp3-hybridized (tetragonal diamond-like lattice), sp2-hybridized (trigonal graphite-like lattice) and amorphous carbon
Reactivity Inert to acids, alkalis, solvents, salts, water and other reagents at ambient temperature.
Combusts above 400°C
Physical Properties
Density 1.8-2.1 g/cm3
Specific thermal conductivity 10 W/cm × K
Thermal expansion coefficient 9 × 10-6/°C
Resistivity several MOhm × cm
Dielectric constant ~ 4-11
Permeability Impenetrable to hydrogen and other gases
Optical Properties
Optical transparency Near to middle IR
Refractive index 1.85 - 2.0 
Other Properties
Operating temperature range  from -60°C to +400°C
Biological compatibility Does not cause cellular breakdown or inflammation
Durability Good adhesion to substrate, resistant to hard abrasion and mechanical impacts, high humidity, high and low temperatures, heat shock, saline vapors, saline solutions, insoluble in water, resistant to atmospheric condensation, dust and sand, some acids, lube and fuel oils per standards: MIL-C-675C, MIL-STD-810
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