Creep of a Nextel™720/alumina ceramic composite containing an array of small holes at 1200°C in air and in steam

Abstract

Tensile stress-strain and tensile creep behaviors of an oxide-oxide composite containing an array of small circular holes were evaluated at 1200°C. The composite consists of Nextel™720 alumina-mullite fibers in a porous alumina matrix. Test specimens contained an array of 17 holes with 0.5-mm diameter drilled using a CO2 laser. The presence of holes caused reduction in tensile strength and modulus. Tensile creep tests were conducted at 1200°C in air and in steam at creep stresses ranging from 38 to 140 MPa. Primary, secondary, and tertiary creep regimes were noted in air and in steam. The presence of the laser-drilled holes accelerates the steady-state creep rates. Creep run-out, defined as 100 hours at creep stress, was attained for stress levels <60 MPa in air and for stresses <40 MPa in steam. The presence of the laser-drilled holes significantly degrades creep resistance of the composite. The retained tensile properties of all specimens that attained run-out were determined. Composite microstructure was examined; the damage and failure mechanisms were considered. The degradation of tensile properties and creep resistance are attributed to damage caused to composite microstructure by laser drilling.