Accession Number : ADA334604

Title :   Transformation Weakening of Ceramic Composite Interfaces

Descriptive Note : Final rept. 15 Sep 96-31 May 97

Corporate Author : ILLINOIS UNIV AT URBANA DEPT OF MATERIALS SCIENCE AND ENGINEERING

Personal Author(s) : Kriven, Waltraud M.

PDF Url : ADA334604

Report Date : 31 OCT 1997

Pagination or Media Count : 121

Abstract : A new concept for achieving graceful failure in oxide composites is studied. It is based on debonding of a weak interphase between a matrix and an interphase in a laminated composite. The interphase can be thermally or shear stress induced by transformation weakening, which results from an accompanying significant volume contraction and/or unit cell shape change, on cooling from a high temperature to low temperature crystal structure. Mullite/cordierite laminates with a beta --> alpha-cristobalite transformation weakened interphase were investigated in order to demonstrate interphase debonding behavior. The laminate showed fracture behavior dependent on a critical size effect. The grain size of polycrystalline beta-cristobalite was controlled by annealing. With increasing annealing time, the strength decreased due to the formation of internal microcracks in the cristobalite layer which occurred spontaneously during thermally-induced transformation. A hot-pressed laminate, annealed for 10 h at 1300 deg C, had an average grain size of 4.2 micrometers and a strength of 131 MPa. Its work of fracture was 2.38 kJ/sq m with a non-catastrophic fracture behavior. The indentation response indicated crack deflection along the cristobalite debonding interphase.

Descriptors :   *INTERFACES, *COMPOSITE MATERIALS, *LAMINATES, *CERAMIC MATERIALS, *OXIDES, *TRANSFORMATIONS, MAGNESIUM, ANNEALING, LOW TEMPERATURE, ALUMINUM COMPOUNDS, CRYSTAL STRUCTURE, MODIFICATION, HIGH TEMPERATURE, CRACKS, SHAPE, DEFLECTION, GRAIN SIZE, PHASE STUDIES, COOLING, FRACTURE(MECHANICS), IRON, QUARTZ, SHEAR STRESSES, HEATING, SILICON COMPOUNDS, LOW STRENGTH, REFRACTORY MATERIALS, MINERALS, POLYCRYSTALLINE, MICROCRACKING.

Subject Categories : Ceramics, Refractories and Glass

Distribution Statement : APPROVED FOR PUBLIC RELEASE