High-temperature oxidation resistance and surface electrical conductivity of stainless steels with filtered arc Cr–Al–N multilayer and/or superlattice coatings

The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell (SOFC) stacks has directed attention to the use of metallic alloys with oxidation-resistant coatings. Candidate coatings must exhibit chemical and thermal-mechanical stability and high electrical conductivity during long-term (>40,000 h) exposure to SOFC operating conditions. The high-temperature oxidation resistance and surface electrical conductivity of 304, 440A and Crofer-22 APU steel coupons, with and without multilayer and/or superlattice coatings from a Cr–Al–N system, were investigated as a function of exposure in an oxidizing atmosphere at high temperatures. The coatings were deposited using large area filtered arc deposition (LAFAD) technology [V.I. Gorokhovsky, R. Bhattacharya, D.G. Bhat, Surf. Coat. Technol. 140, (2001), 82-92] and subsequently annealed in air at 800 °C for varying times. Area-specific resistance (ASR) was measured in air as a function of time and temperature using a four-point technique with Pt paste for electrical contact between facing oxidized coupon surfaces. The surface composition, structure and morphology of the sample coupons were characterized using RBS, nuclear reaction analysis, XPS, SEM and AFM techniques. The structure of the CrN/(CrN/AlN) multilayered superlattice coatings was characterized by transmission electron microscopy (TEM). By altering the architecture of the coating layers, both surface electrical conductivity and oxidation resistance [R.J. Smith, C. Tripp, A. Knospe, C.V. Ramana, A. Kayani, Vladimir Gorokhovsky, V. Shutthanandan, D.S. Gelles, J. Mater. Eng., (2004), in press] improved significantly for some of the coated samples tested up to ∼100 h. An order of magnitude decrease in the ASR parabolic growth rate constant was observed in two Crofer 22APU coupons coated with ∼1.5 μm CrN/AlN superlattice coatings with different nanometrical bilayer periods.