THE K2ZRF6 WETTING PROCESS - EFFECT OF SURFACE-CHEMISTRY ON THE ABILITY OF A SIC-FIBER PREFORM TO BE IMPREGNATED BY ALUMINUM

SiC-ceramic materials, either as flat substrates or porous fiber preforms, are spontaneously wetted by aluminum at 700-degrees-C to 800-degrees-C when they have been pretreated with an aqueous solution of K2ZrF6. The wetting enhancement effect results from exothermic chemical reactions occurring at the SiC/Al interface. The first phenomenon thought to occur is a disruption of the alumina film covering liquid aluminum due to a reaction of Al2O3 with K2ZrF6. Then alumina is totally dissolved at low temperatures by potassium/aluminum mixed fluorides, giving rise to a very fluid cryolitic liquid spreading out on the surface of liquid aluminum and to the true SiC/Al interface. Simultaneously, a large evolution of heat occurs, mainly due to the reduction of K2ZrF6 by aluminum and the formation of AlZr. The impregnation of 2D-SiC/SiC preforms by aluminum is modeled, and the effect of both the contact angle decrease and local temperature rise on the impregnation of the preforms, e.g., by gravity casting, is established.