Glasses and Multi-Component Sol-Gels for Use as High-Temperature Protective Coatings
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GLASSES AND MULTI-COMPONENT SOL-GELS FOR USE AS HIGH-TEMPERATURE PROTECTIVE COATINGS LAURI J. DeVORE* AND NORA R. OSBORNE** *EG&G Mound Applied Technologies, P.O. Box 3000, Miamisburg, OH 45343 "**University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0172 ABSTRACT Two multi-component sol-gel compositions were developed and compared to several commercially available high-temperature glasses. All were then used and characterized as protective coatings for intermetallic titanium aluminide. The sol-gels were studied as thin film coatings and the commercial glasses were studied as enameled coatings. Attention was given to (1) the effect of the application temperature on the original microstructure of the metal, and (2) the role of interfacial conditions between the glass and metal in cyclic and isothermal thermal cycles between ambient temperature and 760°C (1400°F). TECHNICAL DISCUSSION Several glass coatings were prepared using commercially available materials from Schott Glaswerke and Coming Glassworks. These glasses are readily available, inexpensive, and fully qualified and characterized by these companies. The glasses chosen were Schott's 8456 and 8450, alkaline-earth borosilicates; Corning's 7052, a borosilicate; and Coming code 1720, an aluminosilicate glass. The nominal composition of Corning code 1720 was used as a guide for the aluminosilicate sol-gel formulation, and a four component sol-gel developed by Sandia National Laboratories [2] as a nominal borosilicate sol. Both sol-gels were formulated and consolidated as described in reference [2]. Two process techniques were used to apply the glass compositions to the aluminide substrates. First, a simple air-brushing technique was used. The glass was powdered, suspended in amylacetate, and air-brushed onto all surfaces of the metal substrate. The second method for application of the glass coat was dipping the sample substrate into a sol-gel. Dip coating then allows a continuous thin film (< 0.0511m) to be applied to a surface [3]. EXPERIMENTAL CONDITIONS Ti3 A1 coupons were supplied by Garrett Engine Division of Allied Signal and Rockwell International. These were cut into approximately 1 cm x 1 cm pieces, and each had a 1 inch platinum wire spot welded to a comer. This was done to facilitate the enameling, dipping, and oxidation test procedures. Two different sample preparation approaches were tried for the coupons to be airbrushed. A number were etched using a dilute solution of HF and cleaned by four-step cleaning. Another group was polished to a 600 grit finish and four-step cleaned. After airbrushing the samples where hung in a box furnace heated to a temperature above the glass softening point to form a coherent coat. Table I shows the temperature and time parameters used to form an enamel for the commercial glasses chosen. For the sol-gel samples the titanium aluminide pieces were polished with 600 grit silicon carbide polishing cloth and then four-step cleaned. The sol-gel coatings were applied using the dip-coating technique in a
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