http://worldcat.org/entity/work/id/137536364

Embrittlement of a Platinum-Iridium Screen

Following successful acceptance testing on a hydrazine Rocket Engine Module (REM), small metallic particles were discovered in a nozzle bag. It was determined that the metallic particles were segments from a Pt-l5wt%-Ir screen that is used to separate coarse and fine mesh catalyst particles within the thrust chamber. One of three screens had numerous brittle, intergranular fractures at bend points of the woven wires. A failure analysis was performed by the Mechanics and Materials Technology Center (MMTC) of The Aerospace Corporation to supplement analyses performed by the spacecraft contractor and the REM manufacturer. Secondary-ion mass spectroscopy (SIMS) was used to determine the presence of any chemical contamination and revealed significant levels of gallium, indium, manganese, and copper on the wire surfaces adjacent to intergranular cracks. Tensile testing of wires extracted from the embrittled screen indicated a ductile fracture mode at 23 and 300 deg C (73 and 570 deg F), but a brittle, intergranular mode at 900 deg C (1650 deg F), indicating that the screen probably fractured during test firing of the REM. Although the cause of the screen failure was not unequivocally determined, it was concluded that contamination of the wire by indium and gallium probably caused the embrittlement. The mechanism of the failure was believed to be either liquid-metal embrittlement or the formation of low-melting-point phases at the grain boundaries.

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