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The Antares high energy gas laser system uses electroplated copper mirrors to reflect the laser beam through the amplifiers and focus it on the target. These mirrors are either polished or single point diamond machined to meet rigid optical specifications. They are inspected within 24 hours after final machining using a laser interferometer. A routine reinspection of these mirrors after arrival at Los Alamos revealed that some had severe distortions far greater than allowed by the optical specifications. The worst examples exhibited a roughened surface, visible to the eye, similar to an "orange-peel" effect. Metallographic analysis of some of the mirrors showed a highly stressed layer within the electrodeposited copper. This layer had a fibrous or needle-like structure and had a micro-hardness of 170 to 180 DPH. Large, stress-free grains with a micro-hardness of 50 to 55 DPH were mixed in with the fibrous layer in some instances. It soon became evident that the hard, fibrous structure was in varying stages of recrystallization, and the recrystallization was taking place at room temperature. Further investigation revealed that the fibrous layer was the result of excessive levels of organic brightening agent in the electrolyte during the electroplating process. Enough stress was stored in the deposit due to the addition agent to provide an unstable microstructure, which subsequently recrystallized over a period of three to six months. In order to salvage the mirrors, a thermal treatment was developed to force recrystallization. Heating the mirrors to 140 for 90 minutes provides a stable microstructure which machines well. Although some of the grains are large, they have no apparent effect on machineability or reflectivity.
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