Impact damage as a function of crystal orientation in Ge IR windows employing durable phosphide coatings

The erosion resistance of thin film epitaxial phosphide coatings has been shown to be affected by substrate crystal orientation when deposited on Ge substrates. GaP was systematically studied using optical microscopy, FTIR spectrophotometry, image analysis and x-ray diffraction. Microstructural aspects of the surface fractures indicated a clear crystallographic preference for erosion. X-ray diffraction identified the (111) plane as having the preferred crystal orientation for erosion resistance and variations of the (100) as the erosion prone planes. Removal of the hard carbon coating was observed in a large number of samples resulting in measurable damage to the GaP layer. Crack morphology was distinctly different in (111) and {100} crystals. (111) planes displayed erosion as discrete isolated pits and {100} planes developed very intricate orthogonal crack networks with pits at the intersections. Depth of damage varied as a function of crystal orientation and non (111) coating sections suffered some substrate exposure. Subsections of (111) protected Ge had an average depth of damage (measured vertically) of 0.47 mm and non (111) subsections had an average depth of damage of 2.3 mm. In most specimens, complex crack networks developed within 10 minutes of exposure to the simulated rain environment. The resultant GaP coating loss averaged 18 percent.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.