Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.. 0 b jective/Scope High temperature alkali corrosion has been known to cause premature failure of ceramic components used in advanced high temperature coal combustion systems such as coal gasification and clean-up, coal fired gas turbines, and high efficiency heat engines. Non-oxide ceramics, such as Sic and Si3N4, are applied in HITAF systems for their well-known and desirable high temperature thermal and mechanical properties. However, these materials are prone to rapid corrosion under some types of high temperature coal combustion conditions. The objective of this research is to apply CMZP and Mg-AI2TiO5 as coatings to Sic to improve corrosion resistance under coal combustion atmospheres as well as to improve high temperature mechanical properties. The research will not only develop and characterize CMZP and Mg-A12Ti05 coatings but will also strive to expand the existing knowledge of the mechanism of coal combustion corrosion of Sic in the temperature range of 1000-1 40OoC. Tee h n i cal Progress 1. Aging Process An aging process was utilized to improve the quality of the CMZP and Mg-A12TiO5 coated samples. The aging process is described below: The CMZP coated samples were fired in large covered crucibles fully filled with CMZP powder at 1200°C for 100 hours. The cover was sealed with liquid K2Di03. This technique results in an atmosphere which is nearly oxygen free. The Mg-AIzTiOs coated samples were fired in air at 1300°C for 100 hours. DISCLAIMER This report was prepared as an a w u n t of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recornmendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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It is the goal of this program to 1) develop coatings for SiC and Si3N4that will enhance their performance as heat exchangers under coal combustion conditions and 2) to conduct an indepth evaluation of the cause and severity of ceramic heat exchanger deterioration and failure under coal combustion conditions. Part 1 Thin film coatings of (C~.GMg0.4)Zr4(P04)G (CMZP) and Mg-stabilized aluminum titanate (Mg-A12Ti05) on dense SiC substrates were investigated using sol-gel coating techniques. The thickness and quality of both CMZP and Mg-A12Ti05 coatings were found to depend on the solution concentration and Iifl rate. Double coatings were applied to obtain homogeneous and crack-free coatings. The quality of double coatings was influenced by different first and second coating thickness. The CMZP coated samples were fired in controlled atmospheres to have the pure CMZP phase. Unhydrolyzed solution of Mg-A12Ti05 was utilized instead of hydrolyzed solution to improve the quality of Mg-A12Ti05 coatings. An aging process was found to improve the quality of CMZP and Mg-A12Ti05 coatings. SiC samples coated with CMZP and Mg-A12Ti05 exhibited good thermal shock resistance and greatly improved the high temperature alkali corrosion resistance. Part 2 Silicon nitride (Si3N4)is a potentially good ceramic material for industrial heat exchangers. However, at elevated temperatures and in coal combustion atmospheres its lifetime is severely reduced by oxidation. To increase its corrosion resistance, the formation of a protective oxidation barrier layer was promoted by the deposition of oxide thin films. Homogeneous and crack-free oxide coatings of CMZP and Mg-doped A12Ti05 were successfully deposited on Si3N4using the sol-gel and dip-coating technique. Coated and uncoated samples were then exposed to a sodium containing atmosphere at 10OO°Cfor 360 hours to simulate typical industrial environment conditions. Structural post-exposure analyses based on weight loss measurements and mechanical tests indicated better corrosion resistance and strength retention for CMZP coated Si3N4compared to as-received and Mg-doped AlzTi05 coated SisN4. This difference was attributed to the protective nature of the corrosion layer which, in the case of CMZP, significantly impeded the inward diffusion of oxygen to the Si3N4surface.

SummarySic samples coated with CMZP and Mg-AI2TiO5, respectively, were tested in a 100-hour slagging combustion test at the Energy and Environmental Research Center at the University of North Dakota. The results of measurements and analysis indicate that CMZP and Mg-AI2TiO5 coatings improved the high temperature alkali corrosion resistance under coal combustion atmospheres. It is clearly shown that it is important to obtain a homogeneous and crack-free coating and good adhesion of the coating to the surface of the substrate for the best corrosion resistance. Some measures to improve the coating procedure are presented.