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.

To drill or not to drill? Creep of an oxide‐oxide composite with diamond‐drilled effusion holes at elevated temperature

“…Yet DD holes reduced the UTS by almost 16% and the elastic modulus, by nearly 12%. While these reductions in tensile strength and modulus are smaller than those caused by laser drilled (LD) holes (19% loss in strength and 23% loss in modulus), 39 they are nonetheless a cause for concern.…”

“…Yet, the lessening of tensile strength and modulus due to the presence of the DD holes is unmistakable. In case of specimens with LD holes, the loss of tensile strength and stiffness was readily attributed to microstructural changes owing to laser drilling 39 . Fracture surfaces of all specimens with LD holes were virtually planar, indicating that laser drilling reduced matrix porosity and thus destroyed the crack deflection mechanism in the area surrounding the holes.…”

“…Typically the effective area was approximately 98% of the nominal cross‐sectional area. This approach to calculating engineering stress was also used in our previous study of N720/A specimens with LD holes 39 . Tension tests were conducted in displacement control, and creep tests were performed in load control.…”

“…Recently, we evaluated creep behavior of the Nextel 720/alumina composite perforated with multiple small holes suitable for film cooling 39 . The holes were drilled using a CO 2 laser.…”

Preparation and characterization of Nextel 720/alumina ceramic matrix composites via an improved prepreg process

“…The first one are the reinforcement fibers, which play a crucial role in CMCs concerning reinforcement and damage tolerance. For oxide‐oxide CMCs, common fiber species used in the research and industrial areas are Nextel 610 and Nextel 720 continuous fibers 2,4–14 . Nextel 610 is a high‐purity alumina fiber (>99% Al 2 O 3 ) 15 .…”