High Temperature Behavior of Si₃N₄ and Yb₂SiO₅ Coated Carbon Fibers for Silicon‐Nitride CMC

Abstract

For oxide-free ceramic matrix composites (CMC), with Si 3 N 4 matrix and carbon fiber reinforcement, for extreme high temperature applications, protective coatings of the C-fibers are investigated. Two different coatings are compared: reactive CVD-derived pure Si 3 N 4 coatings to investigate C-fiber-matrix reactions and powder based Yb-silicate coatings to reveal potential reactions with the Yb-silicate additive serving as sintering aid for Si 3 N 4 . The reactivity toward carbon in nitrogen atmosphere is studied in the temperature interval from 20°C up to 1700°C. A new ceramic phasean Yb-carbidonitiridosilicate, Yb 2 Si 4 CN 6 -is found as product of carbothermal reduction of the Yb-silicate. The carbothermal reduction occurs also with other RE-silicates, RE ¼ Yb, Er, Y, Gd, and Sm while SiC is found as reaction product on carbon fibers coated with pure Si 3 N 4 . The oxidation resistance of the coated fibers in air was investigated in the temperature interval up to 1000°C, and the apparent activation energy of oxidation was analyzed based on DTA-EGA results. The oxidation kinetic reveals a significant increase of onset point of oxidation temperature by up to 150 K for Si 3 N 4 coated short carbon fibers obtained from the reactive CVD coating process. Such fibers have a high application potential for carbon-fiber reinforced Si 3 N 4 -CMC. The role of Yb 2 Si 4 CN 6 as reinforcement for Si 3 N 4 -CMC is discussed based on bond strength comparison of carbides (SiC), nitride silicates (SiAlON), and nitrides