EVALUATION OF INTERFACIAL MECHANICAL-PROPERTIES IN SIC FIBER-REINFORCED MACRO-DEFECT-FREE CEMENT COMPOSITES

The application of a micro-indentation technique for the measurement o f interfacial mechanical properties in a fiber-reinforced cement compo site has been examined. The composite was formed by placing aligned Si C fiber tows between macro-defect-free (MDF) cement sheets after which the compact was warm pressed at 6.89 MPa (1000 psi) at 80-degrees-C. The interfacial characteristics of the composite were varied by modify ing the surfaces of the fibers prior to their incorporation into the m atrix. These modifications included either an application of a stearic acid film or 1000-degrees-C gas phase treatments in air, oxygen, or n itrogen for one hour. These four fiber surface treatments served as th e independent parameter for the mechanical testing analysis. The inter facial characteristics, including the shear, the residual axial fiber, and debond stresses, were evaluated with a mechanical properties micr oprobe by measuring the force/displacement curves generated during loa d-unload cycling. Estimates of these three stress values were obtained by matching the experimental force/displacement curves with data pred icted from an existing model. For two of the four systems investigated , relatively high residual axial fiber compressive stresses were requi red adequately to describe the large fiber displacement recovery obtai ned after complete unloading. The stresses were believed to have resul ted from the large differential shrinkage between the fibers and matri x during processing. The shear and debond stress values were highest f or those fibers oxidized in air and lowest for fibers coated with stea ric acid. The calculated interfacial shear stress value becomes signif icantly overestimated when the residual axial stress has a large magni tude and is not taken into account in the calculation.