OXIDATION AND CORROSION OF SIC(PARTICULATE) AL/AL2O3 COMPOSITES IN SODIUM-SILICATE AT ELEVATED-TEMPERATURES/

Ceramic-matrix composites (CMCs) fabricated by the directed metal oxid ation process (Dimox(TM)) may have applications in heat exchangers in high-temperature corrosive environments such as those in the glass ind ustry, The oxidation and corrosion properties of such CMCs with and wi thout preformed metal-free surface layers have been investigated in th e temperature range of 1000-1300 degrees C. The untreated CMCs experie nced rapid oxidation in air leading to mass increases of 100 to 140 mg /cm(2) in less than 1 h. This occurred by oxidation of residual metal in the composite to form Al/Al2O3 deposits on the surface. After the i nitial formation of the oxidation product, there is little further rea ction during up to 300-h exposures to oxidizing atmospheres, Experimen tal composite coupons with metal-free surfaces mere resistant to oxida tion except for localized events associated with flaws. Small amounts of sodium silicate (2 to 40 mg/cm(2)) painted on the surfaces produced no corrosive effects on any of the specimens. Dynamic corrosion exper iments, in which a continuous mist of sodium silicate was sprayed onto the surfaces, produced corrosion at 1300 degrees C.