Rain erosion resistance of hard carbon coated germanium.

Amorphous hydrogenated carbon (a-C:H) is a suitable material for durable anti-reflection coatings on Ge infrared windows. For the present investigations a-C:H-films were deposited onto single crystal Ge substrates using a RF glow discharge sustained by benzene vapour. The rain erosion resistance of the samples was tested by using a rotating arm device. The rain erosion caused an optical degradation of the samples, which was characterized by the infrared optical transmission loss at 10 Mym. The erosion behaviour strongly depended on the impact velocity and the impact angle. The a-C:H coated Ge samples always showed a higher rain erosion resistance than the uncoated reference samples. In the 400 to 2000 V range, the value of the bias voltage of the deposition process did not influence the erosion resistance.