A 2D carbon fabric (T300, manufactured by Toray) wasused for the preparation of the carbon-fiber-reinforced CMCs.Polysilazane (HTT1800, AZ-EM, Wiesbaden, Germany) and apolysilazane-based SiHfBCN precursor (which was prepared asdescribed in21) were considered as preceramic precursors forthe PIP fabrication of the composites. The carbon fabric wascut into rectangular pieces with dimension of 44 mm × 16 mm.After performing the infiltration with the preceramic precur-sor, 16 infiltrated pieces of carbon preform were stacked ontoeach other within a Teflon mold (Fig. 1a) in a hand lay-up process in order to provide the fiber preform with ca.3.5 mm thickness. After a curing step, the composites werepyrolyzed in a tube furnace (1100◦C, Ar) to furnish Cf/SiCNand Cf/SiHfBCN-based CMCs (Fig. 1b). The obtained sampleswere re-infiltrated and pyrolyzed up to 7 times. The specimenswere cut and machined into rectangular coupons with dimen-sions of 44 mm × 3 mm × 3 mm. In order to push down the openporosity, the specimens were re-infiltrated with HTT1800 orSiHfBCN precursor by using one more infiltration prior to thehydrothermal corrosion experiments.