Effects of matrix porosity on the mechanical properties of a porous-matrix, all-oxide ceramic composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregn ation and pyrolysis of a ceramic precursor solution. Mechanical tests are p erformed to assess the role of the matrix in both matrix-dominated and fibe r-dominated loading configurations. The results demonstrate a loss in damag e tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are o btained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a co nsequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double -notched shear specimen are broadly consistent with the limited values obta ined by the short beam shear method, although the former exhibit large vari ability. In addition, effects of precursor segregation during drying on thr ough-thickness gradients in matrix properties and their role in composite p erformance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabl ing damage tolerance in porous-matrix composites. Deficiencies in the under standing of the mechanisms that enable damage tolerance in this class of co mposites are discussed.