Abstract
Ceramic matrix composites (CMCs) are leading contenders for use in aircraft engines. Yet even with their high-temperature capabilities, many CMC components will need cooling. External or film cooling technique of a component requires rows of small holes through the component surface. Effects of multiple small holes on tensile stress-strain and tensile creep resistance of an oxide fiber-oxide matrix composite comprising Nextel™720 alumina-mullite fibers and a porous alumina matrix were evaluated at 1200°C. Gauge section of each test specimen included seventeen (17) 0.5-mm diameter holes. The holes were precision drilled using diamond coated drill bits. The presence of diamond-drilled (DD) holes noticeably degraded tensile strength and modulus. Specimens with DD discharge holes were creep tested in air and steam at 1200°C. Applied creep stress varied from 40 to 130 MPa. Endurance of 100 h at creep stress without failure was defined as creep run-out. The existence of diamond drilled holes lowered creep resistance of the CMC as evidenced by higher steady-state creep strain rates and lessened creep lives. An earlier study at the Air Force Institute of Technology considered the effects of small holes drilled using a CO2 laser on the tensile properties and tensile creep resistance of this material system. Geometry of test specimens and test conditions were the same as in the present effort. The earlier study found that the existence of rows of small laser drilled (LD) holes also considerably lowered the creep resistance of the Nextel™720/alumina CMC. In both cases the reduction in tensile strength and creep resistance are due to damage caused to composite microstructure by fabrication of the holes, i. e. drilling. However, different hole drilling techniques result in different microstructure degradation mechanisms. Changes to the CMC microstructure triggered by these two drilling techniques and implications for mechanical performance and durability are discussed.
