AVS 47th International Symposium
    Surface Engineering Monday Sessions
       Session SE-MoA

Paper SE-MoA3
Influence of the Interface Composition on the Corrosion Behavior of Unbalanced Magnetron Grown Niobium Coatings on Steel

Monday, October 2, 2000, 2:40 pm, Room 201

Session: Coatings for Extreme Environments: Wear Resistant, Lubricious, Anti-corrosive, High Temperature Coatings
Presenter: H. Paritong, Sheffield Hallam University, UK
Authors: C. Schönjahn, Sheffield Hallam University, UK
H. Paritong, Sheffield Hallam University, UK
W.-D. Münz, Sheffield Hallam University, UK
I. Petrov, University of Illinois, Urbana
R.D. Twesten, University of Illinois, Urbana
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In general niobium is well known as a chemically extremely stable material. However the corrosion performance of 0.5 to 1 µm thick , unbalanced magnetron (UBM) grown niobium coatings on steel substrates depends significantly on the in vacuo etching pretreatment of the substrates prior to coating and on the chemical composition of the steel substrate. Corrosion tests, TEM and STEM analyses have shown that a dense fine grained partially implanted 5-10 nm thick niobium interface layer formed during the metal ion etching pretreatment is paramount to protect the steel substrate thoroughly against corrosion in chlorine containing aqueous electrolytes, whereas the pretreatment with Ar @super +@ and Cr @super +@ ions leads to inferior corrosion results. Moreover the energy of the impinging Nb @super +@ ions used during the etching process plays an important role. Maximum pitting potential and minimum corrosion currents were found for bias voltages between -600 and -900 V. In summary the results suggest that the UBM deposited Nb coatings are not completely dense although no evidence for the presence of voids has been found by TEM analysis and that the major justification for niobium as corrosion barrier depends on the existence of the thin interface layer generated by high energetic ion bombardment during the etching step. The UBM deposited coating (U@sub s@=-75V) acts therefore only as an itself chemically stable mechanical spacer protecting the thin interface layer against mechanical damage.