To drill or not to drill? - Creep of an oxide-oxide composite with diamond-drilled effusion holes at elevated temperature
HARKINS M. 1, RUGGLES-WRENN M. 1
1 US Air Force Institute of Technology, WPAFB, United States
Ceramic matrix composites (CMCs) are prime candidates 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 within the component surface. Effects of multiple small holes on tensile stress-strain and tensile creep performance of an oxide-oxide CMC consisting of Nextel 720 alumina-mullite fibers in a porous alumina matrix were evaluated at 1200°C. Test specimens included 17 holes with a 0.5-mm diameter in the gage section. The holes were precision drilled using diamond coated drill bits. The presence of diamond-drilled holes noticeably degraded tensile strength and modulus. Specimens with diamond-drilled effusion holes were creep tested at 1200°C in air and in steam. Creep stresses ranged from 40 to 130 MPa. Creep run-out was set to 100 h. The presence of diamond drilled holes degraded creep performance of the CMC as evidenced by higher steady-state creep strain rates and reduced creep lifetimes. 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 CMC. Geometry of test specimens and test conditions were the same as in the present effort. The earlier study found that the presence of rows of small laser drilled holes also considerably lowered the creep resistance of the Nextel720/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. Damage to the CMC microstructure caused by these two drilling techniques and implications for mechanical performance and durability are discussed.