Development of corrosion resistant niobium-based PVD coatings.

Niobium is well known for its excellent corrosion resistance based on the formation of a stable passive oxide layer, which protects the metal against corrosion in most aqueous media and makes it an interesting candidate for corrosion resistant coating applications. However, deposition of Nb films is restricted to few technologies and difficulties arise from the toxic nature of the electrolytes employed in electrodeposition of Nb, the high reactivity of the metal with residual gases in vacuum plasma spraying and its high melting point (Tm = 2500 0C) in PVD deposition.The present thesis describes the development of corrosion resistant Nb coatings on stainless steel and brass substrates by the combined steered arc/ unbalanced magnetron sputtering technique. Evaluation of the corrosion behaviour is performed by potentiodynamic polarisation measurements in 3% NaCl. It is shown that corrosion resistant Nb coatings, with passivation characteristics similar to that of bulk Nb, can be produced on stainless steel substrates by unbalanced magnetron sputtering at a low deposition temperature (T = 250 0C) under reduced ion bombardment. However, the ion etching pre-treatment of the substrate prior to deposition has a significant influence on the corrosion resistance of the coating/ substrate system. The employed polarisation measurements reveal that a fully passive and protective behaviour could only be achieved if Nb ions from the cathodic arc source are chosen as the etching species. In contrast, coatings deposited after Cr ion etching from the arc source and inert Ar ion etching, utilising a glow discharge, exhibit localised breakdown and pitting of the substrate. Cross sectional TEM imaging and STEM-EDX analyses reveal that bombardment of the stainless steel substrate by the multiply charged Nb ions generates a compositionally intermixed, very fine crystalline or "amorphised" interface layer, with a thickness of ~3-8nm, depending on the Nb ion energy. It is proposed that this layer acts as an additional barrier against corrosion due to (i) the structural homogeneity achieved by amorphisation and (ii) chemical stabilisation due to the introduction of Nb in the near surface region. The energy of the bombarding Nb ions, i.e. the substrate bias voltage during the etching stage, was found to further influence the corrosion resistance. Best results are achieved with "medium" bias voltages in the range of -600V to -800V, which is believed to be due to an optimum combination of structural and chemical protection mechanisms.The fully passive corrosion behaviour could not be observed in the case of brass substrates. However, the PVD coating systems on brass and on stainless steel are superior, in the employed polarisation measurements, to commercially produced, electroplated Cr, Ni and Ni/Cr coatings on the same substrate materials.Other coating properties investigated in the present study include microstructure, hardness, crystallographic orientation and residual film stresses.