James K. Sutter

Adhesive/cohesive properties of thermally sprayed functionally graded coatings for polymer matrix composites

High-velocity oxyfuel (HVOF) sprayed polyimide/WC-Co functionally graded (FGM) coatings with flame-sprayed WC-Co topcoats have been investigated as solutions to improve the solid-particle erosion and oxidation resistance of polymer matrix composites (PMCs) in the gas flow path of advanced turbine engines. Porosity, coating thickness, and volume fraction of the WC-Co phase retained in the graded coating architecture were determined using standard metallographic techniques and computer image analysis. The adhesive bond strength of three different types of coatings was evaluated according to ASTM D 4541. Adhesive/cohesive strengths of the FGM coating were measured and compared with those of pure polyimide and polyimide/WC-Co composite coatings and also related to the tensile strength of the uncoated PMC substrate perpendicular to the thickness. The FGM coatings exhibited lower adhesive bond strengths (∼6.2 MPa) than pure polyimide coatings (∼8.4 MPa), and in all cases these values were lower than the tensile strength (∼17.6 MPa) of the reference uncoated PMC substrate. The nature and locus of the failures were characterized according to the percent adhesive and/or cohesive failure, and the interfaces tested and layers involved were analyzed by scanning electron microscopy.

Insights into the Damage Mechanism of Teflon® FEP from the Hubble Space Telescope:

Metallized Teflon® FEP (fluorinated ethylene propylene) thermal control material on the Hubble Space Telescope (HST) has been found to be degrading in the space environment. Teflon® FEP thermal control blankets (space-facing FEP) retrieved during the first servicing mission (SM1) were found to be embrittled on solar-facing surfaces and contained microscopic cracks. During the second servicing mission (SM2) astronauts noticed that the FEP outer layer of the multi-layer insulation (MLI) covering the telescope was cracked in many locations around the telescope. Large cracks were observed on the light shield, forward shell and equipment bays. A tightly curled piece of cracked FEP from the light shield was retrieved during SM2 and was severely embrittled, as witnessed by ground testing. A failure review board was organized to determine the mechanism causing the MLI degradation. Density, x-ray crystallinity and solid-state nuclear magnetic resonance (NMR) analyses of the FEP retrieved during SM1 were inconsistent with results of FEP retrieved during SM2. Because the retrieved SM2 material was curled while in space, it experienced a higher temperature extreme during thermal cycling, estimated at 200°C, than the SM1 material, estimated at 50°C. An investigation on the effects of heating pristine FEP and FEP retrieved from the HST was therefore conducted. Samples of pristine, SM1 and SM2 FEP were heated to 200°C and evaluated for changes in density and morphology. Elevated-temperature exposure was found to have a major impact on the density of the retrieved materials. The characterization of the polymer morphology of the as-received and heated FEP by NMR provided results that were consistent with the density results. Differential scanning calorimetry (DSC) was conducted on pristine, SM1 and SM2 FEP. DSC results provided evidence of chain scission and increased crystallinity in the space exposed FEP, which supported the density and NMR results. Samples exposed to simulated solar flare x-rays, thermal cycling and long-term thermal exposure provided information on the environmental contributions to degradation. These findings have provided insight into the damage mechanisms of FEP in the space environment.

Erosion Coatings for Polymer Matrix Composites in Propulsion Applications

Polymer Matrix Composites (PMCs) offer lightweight and frequently low cost alternatives to other materials in many applications. High temperature PMCs are currently used in limited propulsion applications replacing metals. Yet in most cases, PMC propulsion applications are not in the direct engine flow path since particulate erosion degrades PMC component performance and therefore restricts their use in gas turbine engines. This paper compares two erosion resistant coatings (SANRES and SANPRES) on PMCs that are useful for both low and high temperature propulsion applications. Collaborating over a multi-year period, researchers at NASA Glenn Research Center, AADC, and Rolls-Royce Engines have optimized these coatings in terms of adhesion, surface roughness and erosion resistance. Results are described for vigorous hot gas/particulate erosion rig and engine testing of uncoated and coated PMC fan bypass vanes from the AE 3007 regional jet gas turbine engine. Moreover, the structural durability of these coatings is described in long-term high cycle fatigue tests. Overall, both coatings performed well in all tests and will be considered for applications in both commercial and defense propulsion applications.

Thermal Contributions to the Degradation of Teflon® FEP on the Hubble Space Telescope

Metallized Teflon® fluorinated ethylene propylene (FEP) thermal control material on the Hubble Space Telescope (HST) is degrading in the space environment. Teflon® FEP insulation was retrieved during servicing missions, which occurred in 1993, 1997 and 1999. During the second servicing mission (SM2), the 5 mil aluminized-FEP (Al-FEP) outer layer of multilayer insulation (MLI) covering the telescope was found to be cracked in many locations around the telescope. Teflon® FEP retrieved during SM2 was more embrittled than the FEP retrieved 2.8 years later from a different location, during the third servicing mission (SM3A). Studies have been conducted to understand the degradation of FEP on HST, and the difference in the degree of degradation of FEP from each of the servicing missions. The retrieved SM2 material experienced a higher temperature extreme during thermal cycling (200 °C) than the first servicing mission (SM1) and SM3A materials (upper temperature of 50 °C), therefore an investigation on the effects of heating FEP was also conducted. Samples of pristine FEP and SM1, SM2 and SM3A retrieved FEP were heated to 200 °C and evaluated for changes in properties. Heating at 130 °C was also investigated because FEP bi-stem thermal shields are expected to cycle to a maximum temperature of 130 °C on-orbit. Tensile, density, x-ray diffraction crystallinity and differential scanning calorimetry data were evaluated. It was found that heating pristine FEP caused an increase in the density and practically no change in tensile properties. However, when as-retrieved space samples were heated, the density increased and the tensile properties decreased. Upon heating, all samples experienced an increase in crystallinity, with larger increases in the space-exposed FEP. These results indicate that irradiation of FEP in space causes chain scission, resulting in embrittlement, and that excessive heating allows increased mobility of space-environment-induced scissioned chains. Thermal exposure was therefore found to have a major impact on the extent of embrittlement of FEP on HST.

Assessment of Erosion Resistance of Coated Polymer Matrix Composites for Propulsion Applications

The erosion behavior of tungsten carbide-cobalt (WC-Co) coated and uncoated polymer matrix composites (PMCs) was examined with solid particle impingement using air jets. Erosion tests were conducted with Arizona Road Dust impinging at 20°, 60°, and 90° angles at a velocity of 229 m s−1 at both 294 and 366 K. Noncontact optical profilometry was used to measure the wear volume loss. Results indicate that the WC-Co coating enhanced erosion resistance and reduced erosion wear volume loss by a factor of nearly 2. This should contribute to longer wear lives, reduced related breakdowns, decreased maintenance costs, and increased product reliability.

The Influence of Sizings on the Durability of High-Temperature Polymer Composites

To increase performance and durability of high-temperature composites for potential rocket engine components, it is necessary to optimize wetting and interfacial bonding between high-modulus carbon fibers and high-temperature polyimide resins. Sizings commercially supplied on most carbon fibers are not compatible with polyimides. In this study, the chemistry of sizings on two high-modulus carbon fibers (M40J and M60J, Toray) was characterized as was the chemistry of PMR-II-50 fluorinated polyimide resin. The carbon fibers were characterized using single filament wetting, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopic measurements. The polyimide matrix resins were coated onto glass filaments for characterization by wetting measurements. Surface energy components were obtained by wetting with nondispersive (methylene iodide), acidic (ethylene glycol), and basic (formamide) probes. A continuous desizing system that uses an environmentally friendly chemical-mechanical process was developed for tow level fiber. Composites were fabricated with fibers containing the manufacturers sizing, desized, and further treated with a reactive finish. Results of room temperature tests after thermal aging show that the reactive finish produces a higher strength and more durable interface compared to the manufacturers sizing. When exposed to moisture blistering tests, however, the better-bonded composite displayed a tendency to delaminate, presumably due to trapping of volatiles.

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.

Effect of Surface Modification on Adhesion of a Metal Bond Coat to PMR Composites

Graphite-reinforced polyimide composites are being considered for use in advanced turbine engines, which require the application of wear- and oxidation-resistant coatings onto the composites. To ensure in-service adhesion of a hard, protective ceramic or cermet coating to these composites, it is first necessary to apply a metal bond coat This study examines the effect that a silica layer has on the adhesion of a proprietary metal bond coat to PMR-15 and PMR-II-50 composite surfaces. Previous work has shown that radio frequency (RF) oxygen plasma etching was the most effective technique for increasing the surface energy of the composites and plasma-enhanced chemical vapor deposition (PECVD) of silica using tetramethoxysilane (TMOS) produced stoichiometric silica films. In addition, UV/ozone chemical vapor deposition (UVCVD) using tetraethylorthosilicate (TEOS) was also used to deposit silica species on the composite surfaces. The surfaces of PMR-15 and PMR-II-50 samples were modified by the following methods: grit blasting; grit blasting, ultra-violet/ozone (UV/ozone) etch, UVCVD of silic; and grit blasting, RF oxygen plasma etch, PECVD of silica. Adhesion pull tests were conducted and the fracture surfaces were examined using both optical and electron microscopy. PMR-15 samples with PECVD silica showed a significant increase in adhesion when compared with both UVCVD silica and grit blasting only. PMR-II-50 showed no significant difference in the adhesion strength for any surface modification method. Electron microprobe analysis of the fracture surfaces showed no evidence that silica was present on surfaces treated by UVCVD. Analyses also showed that the silica bonded more strongly to both the PMR-15 and PMR-II-50 neat resins than to the graphite fibers. The poor surface quality of PMR-II-50 composites is believed to have a major effect on bond coat adhesion strength.

Out-of-autoclave processing and properties of bismaleimide composites

*This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The emergence of bismaleimide composites has fulfilled some of the increasing demand for higher temperature performance aeronautics and space exploration vehicles. This study examines and evaluates three bismaleimide matrix resins and two bismaleimide adhesives and reports on the processing properties of these resins and composites by out-of-autoclave–vacuum-bag-only oven processing. Measurements were conducted under various cure cycles to characterize and correlate thermal and viscoelastic properties of the materials. Specimens of all three aged matrix resins exhibited an out-time life up to 30 days when stored at room temperature. Solid and sandwich panels were fabricated with the out-of-autoclave–vacuum-bag-only process. Because of tooling limitations in industry practices, composite fabrication of these bismaleimides was restricted to a maximum 177℃ curing, followed by a free-standing postcuring at elevated temperatures in an oven. The adhesive foaming characteristics, composite resin/void content, flat wise tensile strength, and fracture surface features were evaluated. Due to the unique temperature limitations of this work, the resulting panel properties were not necessarily representative of manufacturer specifications or recommendations.

Surface Modification of Polyimide Composites by RF Plasma and UV/Ozone Treatments

Graphite-reinforced polyimide composites are being considered for advanced turbine engine applications, which require that the composite be coated with a wear- and oxidation-resistant coating. For this study, the surface properties of PMR-15 and PMR-II-50 composites were examined and modification techniques investigated to improve coating adhesion. RF plasma and UV/ozone treatments were used to modify the surface chemistry of the polyimide composites, and were optimized to increase the surface energy of the polyimide composites, by etching and deposition of SiO x films. Chemical and physical changes of the composite surface were characterized by contact angle analysis and Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR). Both RF plasma and UV/ozone treatments effectively removed surface contaminants and modified surface chemistry as indicated by increased surface energy. PMR-II-50 and PMR-15 composites responded differently to argon plasma etching and UV/ozone etching because of their structures. Films deposited by plasma-enhanced chemical vapor deposition (PECVD), using tetramethoxysilane, were stoichiometrically closer to SiO2 than the films deposited by PECVD using tetraethoxysilane.