EFFECTIVE ELASTIC-MODULI OF FIBER-MATRIX INTERPHASES IN HIGH-TEMPERATURE COMPOSITES

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature c omposites. The interphasial moduli are calculated from the ultrasonica lly measured composite moduli via inversion of multiphase micromechani cal models. Explicit equations are obtained for determination of inter phasial stiffnesses for an interphase model with spring boundary condi tions and multiphase fiber. The results are compared with the exact mu ltiphase representation. The method was applied to ceramic and interme tallic matrix composites reinforced with SiC SCS-6 fibers. In both com posites, the fiber-matrix interphases include approximately 3-mu m-thi ck carbon-rich coatings on the outer surface of the SiC shell. Althoug h the same fiber is used in both composite systems, experimental resul ts indicate that the effective interphasial moduli in these two compos ite systems are very different. The interphasial moduli in intermetall ic matrix composites are much greater than those in ceramic matrix com posites. After taking the interphase microstructure into account, we f ound that the interphasial moduli measured for the intermetallic matri x composites are very close to the estimated bulk moduli of the pyroly tic carbon with SiC particle inclusions. Our analysis shows that the l ower effective interphasial moduli in the reaction-bonded Si3N4 (RBSN) ceramic matrix composites are due to imperfect contact between the in terphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effec tive moduli give information on the quality of mechanical contact betw een fiber and matrix. Possible errors in the interphasial moduli deter mined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for ass essment of interphasial damage and interphase reactions is discussed.