Optical EngineeringInelastic effects in a thermoplastic adhesive used for bonding a diamond disk
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A 1-in.-diam polished diamond disk was bonded to a disk of zinc sulfide (ZnS) using a high-temperature thermoplastic adhesive to form an optical structure that may be used for infrared applications. A postbonding thermal-stress-induced deformation in the structure at room temperature was observed to decrease exponentially over time. This curvature decrease may be attributed to viscoelastic stress relaxation within the adhesive layer. The relaxation behavior of the adhesive layer was characterized over a range of temperatures above room temperature by observing curvature changes in the structure heated to these temperatures. This characterization has led to knowledge of the deformational behavior of this structure during manufacturing and service conditions. In addition, relaxation and approximate physical properties of the adhesive were extracted from the observed behavior.