 |
Hard Carbon Coating (DLC: "Diamond-Like Coating")
DLC is perhaps the world's most rugged optical coating, offering uncanny resistance to abrasion, salts, acids, alkalis, and oil. Most commonly applied to Silicon and Germanium windows deployed on military vehicles and outdoor thermal cameras, DLC protects the outer optical surfaces from high velocity airborne particles, seawater, engine fuel and oils, high humidity, improper handling, etc.
A single layer of hard carbon, deposited using highly specialized coating methods, offers an attractive combination of highly protective properties coupled with good anti-reflective performance. DLC adheres extremely well to Germanium and Silicon.
Throughout the infrared wavelength region in which it is employed, DLC exhibits moderate absorption and scattering. With a refractive index of approximately 2.0, DLC delivers good BBAR performance on both Ge and Si. DLC, because it consists of a single layer, is optimized for a specific wavelength region (most commonly the 3-5 µm and 8-14 µm bands) by adjusting the layer thickness during the coating process.
Tab. 1 Transmission of a Silicon window (thickness 2 mm). One side coated with a standard BBAR infrared coating optimized for lowest reflective loss at 3-5 µm.
The second side with DLC layer also optimized at 3-5 µm.
| Wavelength, µm |
Transmission, % |
Wavelength, µm |
Transmission, % |
Wavelength, µm |
T, % |
| 3.00 |
87,07 |
3.68 |
96,29 |
4.36 |
96,70 |
| 3.04 |
88,06 |
3.72 |
96,52 |
4.40 |
96,65 |
| 3.08 |
88,81 |
3.76 |
96,70 |
4.44 |
96,59 |
| 3.12 |
89,41 |
3.80 |
96,84 |
4.48 |
96,52 |
| 3.16 |
89,94 |
3.84 |
96,94 |
4.52 |
96,45 |
| 3.20 |
90,46 |
3.88 |
97,00 |
4.56 |
96,36 |
| 3.24 |
90,98 |
3.92 |
97,03 |
4.60 |
96,25 |
| 3.28 |
91,52 |
3.96 |
97,04 |
4.64 |
96,12 |
| 3.32 |
92,08 |
4.00 |
97,03 |
4.68 |
95,97 |
| 3.36 |
92,64 |
4.04 |
97,01 |
4.72 |
95,80 |
| 3.40 |
93,21 |
4.08 |
96,98 |
4.76 |
95,60 |
| 3.44 |
93,77 |
4.12 |
96,94 |
4.80 |
95,37 |
| 3.48 |
94,30 |
4.16 |
96,90 |
4.84 |
95,11 |
| 3.52 |
94,80 |
4.20 |
96,87 |
4.88 |
94,82 |
| 3.56 |
95,25 |
4.24 |
96,82 |
4.92 |
94,50 |
| 3.60 |
95,65 |
4.28 |
96,78 |
4.96 |
94,15 |
| 3.64 |
96,00 |
4.32 |
96,74 |
5.00 |
93,76 |
|
Fig. 1 DLC+BBAR coating at Ge window 2mm.
Characteristics of DLC coatings
| Chemical properties |
| Composition |
Carbon, hydrogen |
| Structure |
Mixture of sp3 (tetrahedral diamond type) and sp2 (trigonal graphitic) and amorphous |
| Reactivity |
Generally inert to acids, alkalis, solvents, salts, water, and other reagents at ambient temperature |
| Physical properties |
| Density |
1.8-2.1 g/cm3 |
| Thermal conductivity |
10 W/cm x K |
| Coefficient of expansion |
9 x 10-6/C |
| Electrical resistance |
Several MΩ x cm |
| Dielectric constant |
About 4-11 |
| Adhesion |
5000 psi std. pull strength |
| Permeability |
Barrier to hydrogen and other gases |
| Optical properties |
| Optical transparency |
From NIR to FIR (see table VII-2) |
| Refractive index |
1.85 - 2.0 (see table VII-2) |
| Thickness |
Depends on operation wavelength. For example at 10.6 µm DLC layer thickness is 1.4 µm |
| Surface roughness |
Depends on chosen substrate |
| Stresses |
Not measurable |
| Other properties |
| Deposition temperature |
10 C approximately |
| Operational temperatures |
-60 C ÷ +400 C |
| Biocompatibility |
Maintains cell integrity, no inflammatory response |
| Durability performance |
Adhesions, severe abrasion and mechanical strikes, high humidity, high and low temperature influence, thermal shock, salt spray fog, salt solubility, water solubility and resistance to atmospheric precipitation, dust and sand impact, resistance to some acids, resistance to oil and diesel fuel as per the following standards (where applicable): MIL-C-675C, MIL-STD-810 |
|
Tab. 2 DLC refractive index and absorption coefficient vs. wavelength.
| Wavelength, µm |
Refractive index, n |
Coefficient of absorption, α, cm-1 |
| 0.4 |
2.05 |
0.28 |
| 0.5 |
2.04 |
0.22 |
| 0.6 |
2.02 |
0.14 |
| 0.7 |
2.01 |
0.08 |
| 0.8 |
2.0 |
0.04 |
| 0.9 |
1.99 |
0.03 |
| 1.0 |
1.98 |
0.027 |
| 1.1 |
1.97 |
0.026 |
| 1.2 |
1.965 |
0.025 |
| 2.0 |
1.95 |
0.025 |
| 3.0 |
1.945 |
0.025 |
| 4.0 |
1.943 |
0.025 |
| 5.0 |
1.941 |
0.025 |
| 6.0 |
1.94 |
0.025 |
| 7.0 |
1.938 |
0.025 |
| 8.0 |
1.935 |
0.024 |
| 9.0 |
1.933 |
0.024 |
| 10.0 |
1.931 |
0.024 |
| 11.0 |
1.93 |
0.023 |
| 12.0 |
1.93 |
0.023 |
| 13.0 |
1.93 |
0.023 |
| 14.0 |
1.93 |
0.023 |
|
| |
|
| |
|
