EFFECT OF COATING AND INTERFACIAL REACTION ZONE ON THE RESIDUAL-STRESSES IN FIBER-REINFORCED METAL-MATRIX COMPOSITE

During processing of some coated fiber-reinforced metal-matrix composi tes, a reaction zone develops between the matrix and the coating. Sinc e this reaction zone is usually very thin compared to coating thicknes s, its presence is not accounted for while evaluating the residual str esses generated due to mismatch between the thermal expansion coeffici ents of the constituent phases, during cool down of the composites. Re sidual stress analyses of composites have been reported using at the m ost a four-phase model ignoring the effect of reaction zone laver. Thi s paper presents a comparison of residual stresses predicted from a fo ur-phase model (without considering reaction zone) and five-phase mode l (including reaction zone as a separate phase). As the magnitudes of the stresses obtained from the two models are observed to be significa ntly different, the results are quite suggestive of the need to use fi ve-phase model for the residual stress analysis, incorporating the rea ction zone as a distinct phase with its appropriate properties. A sens itivity study of the residual stresses with respect to coating thickne ss, coating modulus and reaction zone modulus is also included, based on the regression analysis using least square fit. Results of the sens itivity analysis are consistent with the observations made on the basi s of residual stress plots, and convey the need of including the react ion zone as a separate phase. The relative influence of reaction zone modulus, compared to the coating thickness and coating modulus, is obs erved to be very significant on all the stress components in the coati ng as well as in the matrix.