Erosion/Oxidation Resistant Coatings for High Temperature Polymer Composites

Thermally sprayed coatings are being studied and developed as methods of enabling lightweight composites to be used more extensively as structural components in propulsion applications in order to reduce costs and improve efficiency through weight reductions. The primary goal of this work is the development of functionally graded material (FGM) polymer/metal matrix composite coatings to provide improved erosion/oxidation resistance to polyimide-based polymer matrix composite (PMC) substrates. The goal is to grade the coating composition from pure polyimide, similar to the PMC substrate matrix on one side, to 100% WC-Co on the other. Both step-wise and continuous gradation of the WC-Co loading in these coatings are being investigated. Details of the processing parameter development are presented, specifically the high velocity oxy-fuel (HVOF) combustion spraying of pure PMR-II matrix material and layers of various composition PMR-II/WC-Co blends onto steel and PMR-15 composite substrates. Results of the HVOF process optimization, microstructural characterization, and analysis will be presented. The sprayed coatings were evaluated using standard metallographic techniques - optical and scanning electron microscopy (SEM). An SEM + electron dispersive spectroscopy (EDS) technique has also been used to confirm retention of the PMR-II component.

An evaluation of elastic properties and coefficients of thermal expansion of graphite fibres from macroscopic composite input data

In this work, a methodology has been presented for the evaluation of stiffness properties and temperature–dependent coefficients of thermal expansion of continuous fibres from the macroscopic properties of either unidirectional or woven composites. The methodology was used to determine the stiffness and thermal properties of T650–35 graphite fibres from the macroscopic input data of unidirectional and woven composites based on the same fibres embedded in a PMR–15 polyimide matrix. In the first part of the analysis, the fibre properties were determined directly from the unidirectional composite macro data using the inversed Eshelby–Mori–Tanaka approach. Subsequently, certain fibre properties were additionally evaluated indirectly from the woven composite, using the finite–element method and the concept of a representative unit cell.

It has been shown that the temperature–dependent coefficients of thermal expansion of the fibres can be estimated from the unidirectional composite macro data with significantly smaller errors than in the case of the elastic properties. It has also been shown that the errors in the evaluation of the elastic properties of the fibres from the macro unidirectional composite data could be significantly reduced if the fibres were placed in a stiff matrix material: much stiffer than the polyimide resin. The longitudinal and transverse coefficients of thermal expansions and the shear modulus of the T650–35 fibres determined from the unidirectional composite analysis were successfully verified by investigating the woven composite.