Preparation of highly corrosion-resistant compositionally graded coatings using multi-ion vapor deposition system

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An attempt was made to tailor thin film coatings with a low pinhole defect density by dynamic ion beam mixing (DM) method using multi-ion vapor deposition system. The Cr-Ta binary alloy and the compositionally graded (CrTa-type 304 stainless steel) alloy thin films were sputter-deposited on a type 304 stainless steel (304SS) substrate. The following conclusions were drawn ;(1) The pinhole defect density (R) depends largely on the surface finishing of the substrate.(2) R value for the Cr-Ta coated 304SS decreases with an increase in the film thickness. The lowest R value was obtained for the film of 5 μm in thickness, and is 5x10^<-5>%, Which is about one order of magnitude lower than that for TiN coated 304SS.(3) An increase in acceleration voltage of Freeman Type argon ion source up to 30 kV leads to decrease in R value. Accordingly, DM is quite effective method in enhancing the corrosion resistance of thin film coatings.(4) TEM observation revealed that the Cr-Ta alloy thin films prepared by DM consist of a mixture of bcc microcrystalline Ta in the amorphous matrix, although they are considered to be amorphous single phase by XRD. Furthermore, bubble-shaped particles were observed in the films and in substrate surface in contact with the film. The diameter of the particle is in the range of 2-10 nm.(5) The compositionally graded (CrTa-304SS) thin film coatings shows higher R value compared with the Cr-Ta binary alloy coatings. This may be responsible for significant increase in surface roughness by crystal growth of 304SS layer during deposition.