PROTECTIVE COATINGS FOR COMMERCIAL PARTICULATES

SiC/Al composites are in large-scale production with Al-Si alloy matri ces. The same composites with pure Al or low Si matrices need diffusio n barriers on the SiC reinforcement to control the interfacial reactio n. The present paper describes various approaches taken to obtain prot ective coatings of alumina and zirconia on SiC particulates by sol-gel techniques. Aqueous and organic precursors have been used. The extent of the reaction, i.e., the Si and Al4C3 content in the matrix, was de termined by differential thermal analysis and X-ray diffraction. The r eaction rates of some coated particulates in liquid Al are decreased b y as much as one order of magnitude during the first 15 min of immersi on. Pretreatments of the SiC surface, the composition and thickness of the coating interphase and heat treatments of the coated materials ha ve been studied, and are discussed in relation to their effect on the control of reaction rates.