Mechanism of anodic spark deposition.

Coatings of Al/sub 2/O/sub 3/ up to 12 mil thick were deposited on Al anodes from aqueous solutions of NaAlO/sub 2/ by anodic spark deposition. Although the applied potential ranged from 415 to 540V dc, >97 percent of the potential drop occurred within 2.8 x 10/sup 4/ A/cm/sup 2/. The lifetime of a single spark was approximately 0.17ms. The resulting energy dissipated at a single spark location was approximately 7 x 10/sup -4/cal. O/sub 2/ gas and water vapor were formed at the anode, while H/sub 2/ gas was evolved at the cathode. Anodic spark deposition is believed to be an avalanche process involving local dielectric breakdown of a barrier film and thermal stressing of the neighboring film. The process often involves local melting of the coating and always involves the rapid quenching of the deposition products.