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

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. Film cooling technique requires rows of small holes through 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 .5 mm diameter in the gage section. The holes were precision drilled using diamond coated drill bits. The presence of diamond‐drilled holes noticeably lowered tensile properties and degraded creep performance of the CMC. Our earlier study considered the effects of small holes drilled using a CO2 laser on the tensile properties and tensile creep resistance of this composite. The presence of laser‐drilled holes also considerably lowered the creep resistance of the Nextel 720/alumina CMC. In both cases the reductions in tensile strength and creep resistance are due to damage caused to composite microstructure by hole drilling. However, different 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.