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Thin film coatings of (Ca 06 ,Mgo. 4 )Zr 4 (P04) 6 (CMZP) on Si 3 N 4 , and SiC substrates, including porous SiC used in ceramic filters, were investigated using sol-gel and dip coating techniques. The coating thickness, ranging from 1 to 5 /im, was found to vary with the selected precursors and their concentrations in the solution. When applied to Si 3 N 4 and SiC, CMZP coatings exhibit good thermal shock resistance and greatly improved alkali corrosion resistance. For porous silicon carbide, homogeneous, crack-free coatings were formed on both the outer surface and the interior sections of the pore walls. The CMZP coatings on Si 3 N 4 and SiC exhibit good thermal shock resistance. The CMZP coating greatly improved the alkali corrosion resistance of Si3N4 and SiC.

I. INTRODUCTION Ceramics in general are well known for their thermal stability, and they often exhibit excellent resistance to wear and corrosion as well. Therefore, a great number of ceramic materials are candidates for applications in high temperature, extremely corrosive environments encountered in a wide range of industries. Particularly challenging to the reliability of structural ceramic components are those applications requiring good resistance to alkali attack. These applications include (i) refractories subjected to the action of alkali vapors or slag in glass furnaces, blast furnaces and stove construction, cement kiln linings, combustion chamber boilers, and town gas installations 1 ^; and (ii) advanced high temperature coal conversion and combustion filters, heat exchangers, and other energy systems.5"9 But it has been found that many ceramics may be attacked rapidly by alkali; therefore, the protection of ceramics from alkali corrosion is an urgent need. Currently, many materials in high temperature service are performing at their capability limits. As material requirements become increasingly sophisticated, it is becoming more and more difficult to combine the required structural properties and stability in a single material. The application of high temperature thermal barrier and corrosion resistant materials to substrates possessing the required mechanical properties for a specific application can produce cost-effective composite systems which optimize both corrosion resistance and strength. For example, because nonoxide ceramics such as silicon carbide (SiC) and silicon nitride (Si 3 N 4 ) exhibit outstanding fracture strength at high temperatures and show excellent thermal shock resistance and corrosion resistance in air, they can be fabricated into required 2014

http://journals.cambridge.org

J. Mater. Res., Vol. 9, No. 8, Aug 1994

Downloaded: 16 Mar 2015

shapes and sizes for structural applications at high temperatures. However, Si 3 N 4 , SiC, and porous SiC used in ceramic filters corrode severely in industrial furnace atmospheres containing alkali compounds. Because certain oxide ceramic materials exhibit superior corrosion resistance to alkali,2'3'10'11 these oxides could be applied as coatings to Si3N4 and SiC, creating a co