Nanocrystalline coating by electrophoretic deposition (EPD)

Alumina coatings using sub-micron and nanosized powders on steel substrates have been produced using electrophoretic deopistion (EPD). The sub-micron powder (d(v50) 200nm) gave coherent and cohesive films whereas the nanosized powder gave films with macroscopic defects. Large cracks due to capillary forces during the drying step were the predominant defects. A cellulose derived binder was added to the alcohol/alumina suspensions used in order to reduce the effect. The films had layer thickness between I to 60 mum. The effect of film thickness on the crack formation is also described. For the purpose of comparison the alumina/ethanol dispersion were also slip cast to produce cylindrical pieces. The green densities for the nanosized alumina were very low at 15.9% for both EPD and slip cast bodies. The sintered EPD layer showed a density about 37.1% and the sintered body obtained by slip casting process gave a density about 34.1%. These reasons for these low densities and how to modify them are discussed.