Papers by Author: Maria Ophelia D. Jarligo

Abstract: Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat isolative topcoat. In modern TBCs the ceramic topcoat is further divided into layers with different functions. One example is the double layer system in which conventional yttria stabilized zirconia (YSZ) is used as bottom and new materials as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. Examples of such systems will be shown. In modern gas turbines the increased temperatures and gas pressures lead to an increased fraction of radiative heat flow. Coatings with increased reflectivity can be used to avoid the direct heating of the metallic substrates by this radiation. An effective method to produce such coatings is suspension plasma spraying. These reflective coatings are deposited on top of the TBC system and will lead to a further grading and improved performance of the coating.

Abstract: Lanthania (La2O3) and zirconia (ZrO2) powders in ethanol based suspension were mechanochemically treated in a planetary ball mill for 12 hours at 200 rpm, dried and sintered at various temperatures from 400 to 1500 . Particles in nanometer sizes are produced after milling. X-ray diffractometry results show the formation of single phase lanthanum zirconate on subsequent heat treatment for 1h at 1500 . Phase evolution based on the intensities of the XRD plots and BET surface area analysis indicates three stages of crystallization: below 800 , between 800 and ~1100 , and above 1100 where reflections of La2Zr2O7 with pyrochlore structure are increased with further heating. Only endothermic energy peaks are observed in the differential thermal analysis (DTA) curve of the milled powders, which could be attributed to the reactions involving dehydroxylation, decarboxylation and complete disintegration of ethanol. This indicates that probably, lanthanum zirconate has grown on sintering at high temperatures from the very fine particles produced by mechanochemical activation during milling. Thermogravimetric analysis has recorded a total weight loss of ~9% from the original weight of the milled powder on sintering at 1500 . The values of the surface area of the powders are found to decrease while the crystallite size of La2Zr2O7 are increased with increasing temperature.