Deciphering the discipline

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deciphering the discipline Understanding the mechanism of zirconia to zirconium carbide conversion for ultra-high-temperature ceramic applications Modern hypersonic aircraft have many structural components, such as wing leading edges and nose cones, made from carbon–carbon composites (CCC). At hypersonic speeds, the outer skin of the aircraft may reach temperatures of approximately 2,000°C,1 triggering rapid oxidation of the material, loss of structural material, and, ultimately, failure. Thermal and environmental barrier coatings (EBC) are widely used in such harsh conditions. Yttria-stabilized zirconia is an important EBC material used to protect turbine blades2 because of its versatile thermomechanical properties.3 In a new approach, YSZ is deposited on CCC substrates. The novelty lies in the fact that, as temperature increases beyond 1,657°C, CCC and YSZ react to form zirconium carbide4 at the CCC/ YSZ interface. Because ZrC is a weaker oxygen conductor and has higher thermal conductivity than YSZ, the performance of the protective coating actually improves over time. To understand the carbothermal reduction mechanism, where ZrO2 converts to ZrC, we investigated the reduction reaction in ultra-high-temperature experiments and with thermodynamic modeling. Four composite systems with pressed powder pellets were designed consisting of two parts: an upper half and a lower half. All four had an upper half made from 3 mol% YSZ. Two samples had a lower half consisting of a mix of YSZ and graphite to act as a source of CO. Two others were made from pure graphite. Each of these variants was used with corresponding unsintered and sintered YSZ upper halves and heat treated at 1,800°C in a flowing helium atmosphere. Quantitative X-ray diffraction results established that the carbothermal mechanism involves solid (ZrO2)–gas (CO) and solid (ZrO2)–solid (carbon) interactions. These findings were published in Ceram. Intl., 39 4, 4489–97 (2013). Thermodynamic modeling furthered our understanding of the influence of CO on ZrC formation. Modeling using FactSage 6.3 software5 correlated the activity of carbon due to CO, which ultimately forms ZrC. We plan to augment these findings with new experimental results to better understand how to control the 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.2 0.0 ZrC formed (moles) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Carbon activity, ac (a) 22 20 18 16 14 12 10 8 6 4 2 0 ZrC formed (3 10–4 moles) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 CO partial pressure, PCO (b) Preliminary results of thermodynamic modeling for reaction between 1 mole of ZrO2 with carbon in one case and with CO in another. (a) Influence of carbon activity on amount of ZrC formed at 1,800°C (b) Influence of CO partial kinetics of carbo-thermal reduction of zirconia. Anchal Sondhi is a PhD candidate in the department of Materials Science and Engineering at University of North Texas, Denton, Texas. He also is a member of PCSA Communications Committee. His advisers pressure on amount of ZrC formed at 1,800°C. are department professor Richard F. Reidy and associate professor Thomas W. Scharf. References 1M.M. Opeka, I.G. Talmy, and J.A. Zaykoski, “Oxidation-based materials selection for 2000C+ hypersonic aerosurfaces: Theoretical considerations and historical experience,” J. Mater Sci., 39 19 5887–904 (2004). http://dx.doi. Anchal Sondhi, Richard F. Reidy, and Thomas W. Scharf Guest columnists org/10.1023/B:JMSC.0000041686.21788.77. doi:10.1023/B:JMSC.0000041686.21788.77. 2S. Stecura, “Two layer thermal barrier coating for high-temperature components,” Am. Ceram. Soc. Bull., 56 12 1082–85, 1089 (1977). 3C.A. Daniels, Ceramics: Structure and properties; p. 271. Abyss Books, Washington, D.C., 2002. 4A.W. Weimer, Carbide, nitride, and boride materials synthesis and processing. Chapman and Hall, London, 1997. 5C.W. Bale, E. Bélisle, P. Chartrand, S.A. Decterov, G. Eriksson, K. Hack, I.-H. Jung, Y.-B. Kang, J. Melançon, A.D. Pelton, C. Robelin, and S. Petersen, “FactSage thermochemical software and databases—recent developments,” Calphad, 33 2 295–311 (2009). doi: 10:1016/j.calphad.2008.09.009. n 56 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 6


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