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

Highlights

• Oxidation of CCC’s preferentially occurs at the fiber/matrix interface.

• Cleaning agent MIL-C-87937 is a catalyst for oxidation of CCC’s.

• Hydraulic fluid MIL-PRF-83282 is not a catalyst for oxidation of CCC’s.

• Presence of Ca explain catalysis, and of Phosphorous explain catalysis inhibition.

• Oxidation evidence of CCC’s are observed morphologically and mechanically.

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.

Introduction

Carbon-Carbon Composites (CCCs) are made of carbon fibers embedded in a carbon matrix. They are widely used in the aerospace industry as rocket nozzles, reentry tips, wing edges, thrust chambers and brake disks as a result of their high thermal stability, low thermal expansion, high thermal shock resistance, high heat capacity, and double the service life with 60% the weight of steel disks [1]. However, CCCs also have disadvantages. Their main one is their low oxidation resistance. Carbon materials react with oxygen at temperatures above 450 °C, while braking temperature rises above 700 °C. This oxidation reaction results in material loss which might deteriorate the mechanical properties of CCCs [2].

Oxidation of CCCs may occur in one of two mechanisms: (1) thermal oxidation, which occurs in an oxygen-containing environment and high temperature (above 450 °C) [3]; (2) catalytic oxidation, which requires a catalyst existence to increase oxidation rate and decrease the temperature necessary for the reaction to occur. Alkali metals and their salts have been found to be catalysts for CCC oxidation either as a powder or aqueous solution (e.g., sodium from marine environment or potassium from cleaning chemicals) [4], [5], [6].

To improve oxidation resistance of CCCs, protection methods have been developed. The protection can be achieved by oxidation inhibitors added to the matrix which act either as internal barriers to oxygen ingress or as oxygen sinks (forming a protective barrier) or by coatings functioning as a barrier for oxygen penetration [7]. “Honeywell” holds a patent for a surface treatment made of oxidation inhibitors in aqueous solution and uses the treatment on airplane brake disks. The surface treatment is a salt solution containing ions based on phosphorous like H₃PO₄ and Al(H₂PO₄)₃, and alkali metal mono, di or tri-basic phosphate. This solution is applied on the composites surface by brushing or spraying and is cured in 500–900 °C to form a stable complex with the active sites in order to prevent oxidation [8].

CCC oxidation reaction and catalysts’ influence on oxidation rate is widely studied, though there is scarce information available on the mechanical behavior of CCC after catalytic oxidation and the relation between the oxidation rate, mechanism and the mechanical properties of CCC. Therefore, in the present study, the oxidation behavior of CCCs was examined after exposure to the hydraulic fluid MIL-PRF-83282 and to the cleaning agent MIL-C-87937. These specific service fluids are widely used in the aviation industry, but their influence on the CCC structure and mechanics have not yet been established. Also emphasized in this work is the correlation between morphological damage and mechanical properties. It was found that oxidation increases CCC tan(δ) and decreases compression strength. Moreover, it was found that cleaning agent MIL-C-87937 is a catalyst for oxidation reaction and therefore it also accelerates the deterioration of the mechanical properties.