For oxide-free ceramic matrix composites (CMC), with Si3N4 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 Si3N4 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 Si3N4. The reactivity toward carbon in nitrogen atmosphere is studied in the temperature interval from 20 °C up to 1700 °C. A new ceramic phase – an Yb-carbido-nitiridosilicate, Yb2Si4CN6–is found as product of carbothermal reduction of the Yb-silicate. The carbothermal reduction occurs also with other RE-silicates, RE = Yb, Er, Y, Gd, and Sm while SiC is found as reaction product on carbon fibers coated with pure Si3N4. 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 Si3N4 coated short carbon fibers obtained from the reactive CVD coating process. Such fibers have a high application potential for carbon-fiber reinforced Si3N4-CMC. The role of Yb2Si4CN6 as reinforcement for Si3N4-CMC is discussed based on bond strength comparison of carbides (SiC), nitride silicates (SiAlON), and nitrides (Si3N4).