Modification of Optical Surfaces Employing CVD Boron Carbide Coatings
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MODIFICATION OF OPTICAL SURFACES EMPLOYING CVD BORON CARBIDE COATINGS
Richard A. Lowden, Laura Riester, and M. Alfred Akerman Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, Tennessee 37831-6063
ABSTRACT Non-reflective or high emissivity optical surfaces require materials with given roughness or surface characteristics wherein interaction with incident radiation results in the absorption and dissipation of a specific spectrum of radiation. Coatings have been used to alter optical properties, however, extreme service environments, such as experienced by satellite systems and other spacecraft, necessitate the use of materials with unique combinations of physical, chemical, and mechanical properties. Thus, ceramics such as boron carbide are leading candidates for these applications. Boron carbide was examined as a coating for optical baffle surfaces. Boron carbide coatings were deposited on graphite substrates from BCI3, CH,, and H2 gases employing chemical vapor deposition (CVD) techniques. Parameters including temperature, reactant gas compositions and flows, and pressure were explored. The structures of the coatings were characterized using electron microscopy and compositions were determined using x-ray diffraction. The optical properties of the boron carbide coatings were measured, and relationships between processing conditions, deposit morphology, and optical properties were determined. INTRODUCTION Space telescopes and other high-performance optical instruments must have baffle and vane surfaces designed to minimize the effect of stray light on system performance.(l-3) Optical baffles are the non-reflective structures and surfaces within an optical device that act to narrow the sighting area by limiting the source from which light is detected, and enhance performance by minimizing offaxis and stray light that might enter the device and reach the detector(s). Optical baffles are essential in the management of light that enters the apparatus and thus play a significant role in determining the performance of an optical system. An optical baffle is typically non-reflective or "black." A variety of "black" coatings have been developed for optical baffle surfaces. (1,4,5) These include paints, anodized surfaces, and etched metal coatings. However, many of the current materials and coatings cannot withstand the extreme environments and severe mechanical and thermal loadings associated with applications such as infrared sighting systems for missiles and projectiles, or telescopes on specialized satellites. High gravitational forces and vibration during launch, and thermal or mechanical loads during service, cause the current materials to fail. The coatings flake and spall diminishing the properties of the baffle surface as well as producing particles that can eventually cover lenses, mirrors, and detectors, rendering the system inoperative. In addition, many of the coatings are fragile and difficult to handle. Simply touching the surface smears or removes the coating, decreasing its effectiveness. Thus, th
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