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Abstract
Modern day gas turbine blades utilize a ceramic thermal barrier coating (TBC) to reduce the metal surface temperature and a bondcoat between the substrate blade and the TBC to increase the oxidation resistance of the substrate superalloy. The bondcoat oxidizes during high temperature exposures to form an alumina scale that needs to be slow growing and adherent, especially under thermal cycling conditions, in order to protect the underlying substrate from any accelerated oxidation attack. Platinum additions to NiAl bondcoats, typically 6-8 at%, provide improved oxidation resistance of the coating by increasing the adherence of the alumina scale.
This talk intends to first review the effect of Pt on alumina scale adhesion using one set of bondcoat, with or without Pt addition, as an example. Subsequently, experiments that aimed at understanding the observed Pt effects are described. Particular attentions are placed on how Pt affects the interface between the Al2O3 and the alloy. These include a study on the segregation of impurities and alloying elements to the scale/alloy interface, an investigation on the wetting behavior between Ni(Pt)Al and Al2O3 and some first principles calculations on the effect of Pt on the surface energy of NiAl. Results indicate that the most pertinent effect of Pt is to reduce the amount of S segregation to the oxide/alloy interface, which weakens the interfacial bond strength. This reduction, together with an effect to increase the NiAl surface energy, hinders interfacial void formation. However, the extent by which Pt inhibits S segregation is not as strong as that of the reactive elements, such as Hf or Y. Possible reasons, along with some experiments and theoretical calculations, are presented and discussed.
PEGGY Y. HOU
Dr. Peggy Hou was born in Taiwan and immigrated to the United States with her parents in high school. She received her B.S. degree in chemistry with honors from the University of California at Berkeley in 1978. Upon graduation, she worked at SRI International in Menlo Park, California on heterogeneous catalysis for three years. She then earned her M.S. (1983) and Ph.D. (1986) degrees in Material Science and Engineering from UC Berkeley. She joined the Materials Sciences Division at Lawrence Berkeley National Laboratory in 1987 as a staff scientist, and was promoted to principle investigator in charge of the high temperature corrosion research group in 1992.
During her years at LBL, her research concentrated on the fundamental aspects of high temperature corrosion of metals and alloys. Focuses included the adhesion of oxide scales, evolution of oxidation stresses and the scale/alloy interface, erosion/corrosion behavior in simulated fluidized bed combustion systems, and the development of oxidation resistant alloys and coatings for intermediate solid oxide fuel cell applications. Dr. Hou is well known in her field, particularly for her pioneering work on impurity segregation to growing oxide/metal interfaces and its relationship with oxide film development and adhesion.
Dr. Hou is the associate editor of the journal "Oxidation of Metals", and a member on the editorial advisory board of the journal "Materials at High Temperatures". The author and co-author of over 100 journal papers, the author of a chapter in the latest edition of Shreir's Corrosion and the co-editor of four books, she is frequently invited to give keynote and plenary lectures at major technical meetings, to serve on advisory boards of major conferences, to participate in research planning workshops and to chair conference discussion sections. She has been invited as a guest professor to National Tsinghua University in Beijing, China, the Metals Research Institute in Shenyang, China, Ecole National Supérieure d'Electrochimie et d'Electrométallurgie in Saint Martin d'Héres, France, the University of Paris in Orsay, France and Johns Hopkins University in Baltimore, USA. She was the recipient of the Hsun Lee Lecture and Research Awards from the Metals Institute, Chinese Academy of Sciences, and was elected in 2001 as the first woman and Asian scientist to chair the Gordon Research Conference on High Temperature Corrosion.
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