Structural Characterization of Sputter-Deposited 304 Stainless Steel+10 wt pct Al Coatings
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NTRODUCTION
THE energy-producing fossil steam turbine and boiler components operate in a severe environment at elevated temperatures. To protect these components from degradation by the operating environment, MCrtype coatings, where M = Fe, Co, or Ni or a combination of these elements, are widely used industrially.[1,2] In particular, thermal-sprayed Fe-Cr or Ni-Cr coatings with or without Al are used predominantly for steam turbine subcritical and supercritical boiler components. These coatings provide corrosion/oxidation protection by the formation of a stable and slowly growing protective oxide scale such as Cr2O3 or Al2O3 on the external surface of the coating, which separates the component substrate in contact with the corrosive environment. A review of field experience of the thermal spray coatings by Sherlock et al.[3] showed that the service life times of these coatings are relatively short, 1 to 3 years, depending on the operating conditions of the boilers, precoating surface preparation, and application procedures. Thus, there is a strong need to develop durable coatings to improve the reliability and availability of the subcritical and supercritical boilers. UMA MAHESWARA RAO SEELAM, Postdoctoral Researcher, and C. SURYANARAYANA, Professor, are with the Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450. Contact email: Challapalli. [email protected] HELGE HEINRICH, Associate Professor, is with the Department of Physics, University of Central Florida. TADAKATSU OHKUBO, Research Scientist, and KAZUHIRO HONO, Director, are with the Nanostructure Analysis Group, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 3050047, Japan. N.S. CHERUVU, Research Scientist, is with the Southwest Research Institute, San Antonio, TX 78228-0510. Manuscript submitted September 5, 2011. Article published online March 20, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Recently, there has been considerable interest in the development of nanocrystalline coatings, which are more resistant to oxidation[4–6] and corrosion[7,8] than their counterpart conventional coarse-grained coatings. The oxidation test results on the FeNiCrAl,[4–6,9] NiCrAl,[10–13] and CoCrAl[14] coatings indicated that an ultrafine grain size in the coating promoted selective oxidation of Al and the formation of a continuous protective oxide scale of alumina (Al2O3) on the external surface of these coatings. This was found to be possible with Al contents as low as 2 wt pct, and the protective oxide layer formed on these coatings during thermal exposure exhibited excellent spallation resistance. The improved oxide spallation resistance of the nanocrystalline coating has been attributed to the fine grain size of the scale formed on the microcrystalline and nanocrystalline coatings after thermal exposure.[4–14] Previous investigations[9,15] have reported that the fine-grained 304 stainless steel (SS) coating consisted of a body-centered cubic (bcc) a-Fe phase and a tetragonal r phase.
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