Models of Polyimide Spray Coatings

Model calculations have been performed to provide guidance for the development of solution spray techniques for coating NIF scale mandrels with 150 μm thick polyimide ablator layers. The deposition models considered assume independent random placement of the spray droplets on the mandrel surface followed by their spreading to form thin disk-like additions. The dependence on the final surface roughness of the effective thickness of the addition, the size (diameter) of the addition, and the cross-sectional profile of the addition have been explored. In addition, a model that assumes randomly placed, independent additions that cover 50% of the mandrel surface per addition is considered For each model and parameter set the rms surface finish is calculated as well as the surface power spectra. The primary result is that individual, randomly placed coating additions must be very thin, on the order of a few nm at most, if NIF surface specifications are to be met.