Influence of service fluids on carbon–carbon aircrafts‘ brake disks oxidation

Abstract

Carbon-Carbon Composites (CCC) are widely used in the aviation industry for structural applications. Their main disadvantage is their low resistance to oxidation at elevated temperatures. While the CCCs are in service they might be exposed to different fluids which contain oxidation catalysts. Therefore, this paper examined the influence of service fluids widely used in aviation industry on oxidation and mechanical properties of aircraft brake disks made of 3D CCC. Isothermal oxidation of the CCC was investigated following immersion in either hydraulic fluid MIL-PRF-83282 or cleaning agent MIL-C-87937. The brake disks were evaluated using thermogravimetric analysis revealing that MIL-C-87937 is a catalyst for oxidation of carbon while MIL-PRF-83282 is not. Further investigation was performed by exposure to 700 °C in a cyclic manner followed by morphological characterizations using Scanning Electron Microscope (SEM). Mechanical properties were also characterized using a Dynamic Mechanical Analyzer in a 3-point bending mode and a universal testing machine in compression mode. SEM observations showed amplified oxidation evidence on samples exposed to MIL-C-87937. The morphological changes explain the resultant mechanical degradation: increase in tan (δ) by 220% with oxidation that begins at the fiber–matrix interface and decreased compression strength by 36%. When exposed to MIL-C-87937 cleaning agent, deterioration trend of these properties seems to be accelerated.