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1327 |
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Wednesday, June 04, 2008, Room 04 11:40 AM Gasturbines 4 Coatings for mobile and stationary turbines, protection against wear, high temperature corrosion and thermal stresses, clearance control coatings for a better efficiency |
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Development of HVOF sprayed nanostructured TiO2 coatings for high temperature applications |
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Rogerio Lima* / National Research Council Canada, Canada Eugenio Garcia / Consejo Superior de Investigaciones Cientificas, Spain Pilar Miranzo/ Consejo Superior de Investigaciones Cientificas, Spain Maria Isabel Osendi/ Consejo Superior de Investigaciones Cientificas, Spain Christian Moreau/ National Research Council of Canada, Canada |
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Titania (TiO2) coatings could be considered as candidates for high-temperature applications in the fields of wear, corrosion and environmental barrier coatings (EBCs), however, high-temperature applications of titania coatings are not pursued due to the usual presence of the anatase phase in the as-sprayed TiO2 coatings. This phase tends to impede the applications of these materials at high temperatures due to the stresses provided by the critical anatase-to-rutile phase transformation at temperatures around 800oC, which would tend to generate cracks in the coating microstructure, leading to premature coating failure. It was hypothesised that this barrier could be overcome by the use of nanostructured TiO2 coatings, due to their known high toughness and resilience levels. Nanostructured TiO2 powders were HVOF-sprayed. The high velocity levels of the HVOF-sprayed particles generated a leak-tight microstructure (i.e., no through-thickness porosity). XRD indicated that as-sprayed coating contained 77% of rutile and 23% of anatase. After heat treatment at 800oC, the coating exhibited approximately 89% of rutile and 11% of anatase, therefore, the ongoing phase transformation was captured. SEM pictures of heat-treated coatings at 800oC deposited on Ti-6Al-4V substrates did not show any significant signs of crack network formation or delamination, which may have been prevented by the high toughness and resilience of these coatings. Therefore, the leak-tight microstructure was preserved and even some pore healing was observed. These coatings were subsequently deposited on SiC substrates and heat-treated at 1400oC to observe possible variations of coating microstructure, including the nano TiO2/SiC substrate interface, and elastic modulus values. The coefficient of thermal expansion of the coating was measured from ambient temperature to 1400oC and compared to that of the SiC substrate at the same temperature range. |