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

The application of a micro-indentation technique for the measurement of interfacial mechanical properties in a fiber-reinforced cement composite has been examined. The composite was formed by placing aligned SiC 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 modifying the surfaces of the fibers prior to their incorporation into the matrix. These modifications included either an application of a stearic acid film or 1000-degrees-C gas phase treatments in air, oxygen, or nitrogen for one hour. These four fiber surface treatments served as the independent parameter for the mechanical testing analysis. The interfacial characteristics, including the shear, the residual axial fiber, and debond stresses, were evaluated with a mechanical properties microprobe by measuring the force/displacement curves generated during load-unload cycling. Estimates of these three stress values were obtained by matching the experimental force/displacement curves with data predicted from an existing model. For two of the four systems investigated, relatively high residual axial fiber compressive stresses were required adequately to describe the large fiber displacement recovery obtained after complete unloading. The stresses were believed to have resulted from the large differential shrinkage between the fibers and matrix during processing. The shear and debond stress values were highest for those fibers oxidized in air and lowest for fibers coated with stearic acid. The calculated interfacial shear stress value becomes significantly overestimated when the residual axial stress has a large magnitude and is not taken into account in the calculation.