Mechanical properties degradation of polyimide films irradiated by atomic oxygen

https://doi.org/10.1016/j.polymdegradstab.2009.05.013Get rights and content

Abstract

Mechanical properties of polyimide films are degraded by exposure to a low earth orbit environment. The main environmental factor for that degradation is atomic oxygen (AO). Using tensile tests, AO-irradiated surface topography observations, and fracture surface analyses, this study investigated the degradation behavior of polyimide films’ mechanical properties by increased AO fluence and its accompanying degradation mechanisms. Tensile strength and elongation of polyimide films were reduced concomitantly with increased AO fluence. Furthermore, AO-irradiated polyimide films fractured from the AO-irradiated surfaces, of which roughness became marked as AO fluence increased. These results reflect that reduction of mechanical properties is attributable to the roughness increase in AO-irradiated surfaces. Polyimide films coated with indium tin oxide (ITO) were also evaluated to confirm the degradation behavior of AO protective films. Surfaces of ITO-coated polyimide films remained smooth even after AO irradiation. However, undercut cavities were formed at ITO coating defect sites. Rupture of ITO-coated polyimide films initiates from the undercut cavities, engendering large reduction of tensile strength and elongation. The degradation of the mechanical properties of ITO-coated polyimide films increased substantially until the undercut cavities fully penetrated the film.

Introduction

Deployable structures such as large flexible solar arrays and deployable antennas are necessary for spacecraft. Polymer thin films are suitable as a construction material for these lightweight and flexible deployable structures. For application as a construction material, adequate mechanical properties in a space environment are especially required for polymer thin films. Degradation of their mechanical properties can engender deformation and rupture, possibly resulting in spacecraft mission failure.

Mechanical properties of polymer materials, however, are highly susceptible to many space environmental threats: high vacuum, orbital thermal cycling, high-energy ultraviolet (UV), and various types of radiation (protons, electrons, and X-rays). For instance, polymer materials under a high vacuum volatilize the plasticizer added during the production process, rendering it brittle. Thermal cycling accumulates the heat strain in polymer materials, causing delamination and cracks. Furthermore, UV and radiation can alter the chemical structures of polymer materials through decomposition and cross-links of polymer bonds, embrittling such materials. In a low earth orbit (LEO), atomic oxygen (AO), the predominant atmospheric component in a LEO environment, is an additional threat that can cause degradation of polymer material properties. Polymer materials exposed directly to a LEO environment collide with AO at spacecraft velocities of about 8 km/s. The translational energy of the AO collisions is approximately 5 eV, which is sufficient to break the polymer bond and induce oxidative decomposition. The oxidative products of most polymer materials are gaseous matter. Consequently, the polymer surfaces are substantially eroded by AO: the polymer materials lose their mass, become thinner, and exhibit a rough texture [1]. Thinned polymer materials have a reduced load that they can withstand. Moreover, their rough texture not only degrades the optical properties seriously; it might also cause crack initiation and tearing of thin polymer films [2]. Understanding these effects of space environmental threats on their mechanical properties is indispensable for application of polymer thin films as a spacecraft construction material.

Polyimide has been applied as a base film for flexible solar arrays of the International Space Station (ISS) and some artificial satellites [3], [4], [5]. Because polyimide has considerable resistance to high temperatures and high tolerance against radiation among polymer materials; it also has high specific strength and rigidity, high dimensional accuracy, and a low rate of thermal expansion. Mechanical properties’ durability of polyimide films in a LEO environment has been assessed using material space exposure experiments. The polyimide films exposed to a LEO environment exhibited a large reduction of mechanical properties; AO attacks were considered as the main cause of degradation of mechanical properties [2], [6], [7]. However, the number of the samples is insufficient to support statistically significant results of analyses. The mechanism causing degradation of mechanical properties has remained unexplained.

Polymer films coated with an inorganic material such as indium tin oxide (ITO), silicon oxide, and aluminum have a high resistance for AO because the inorganic coatings can protect the underlying polymer from AO erosion. The AO protective coatings, however, commonly have some defects, or non-coated spots, caused by surface irregularities on the underlying polymer itself, contaminant particles on the polymer surface during the coating process, and microscopic scratches and cracks during shipment and handling. The coating defects can provide pathways for AO attacks. Consequently, AO can erode the underlying polymer through defects in the protective coatings, then deep cavities are formed at the defect sites after AO irradiation. This phenomenon, which is known as “undercutting erosion”, produces cavities: “undercut cavities” [8], [9]. The undercut cavities are expected to have a potential to serve as a crack initiation, causing a degradation of mechanical properties.

This study is intended to elucidate the relation between mechanical properties degradation of polyimide films and AO fluence, to identify the degradation mechanism, and to examine the effects of undercut cavities on mechanical properties of AO protective polyimide films. Polyimide films and ITO-coated ones were irradiated by AO using a ground simulation facility. Then, their mechanical properties, after evaluation using tensile tests, were compared to those of pristine control samples. Additionally, the surface topography of the AO-irradiated area and fracture surface morphology were observed using scanning electron microscopy (SEM). These evaluations can provide significant insight into the degradation mechanism.

Section snippets

Materials

The tested materials were 125 μm-thick polyimide films (UPILEX-S; UBE Industries Ltd.) and 25 μm-thick ITO-coated polyimide films (ITO/UPILEX-S; UBE Industries Ltd.). The sample dimensions are presented in Fig. 1. The samples, which were punched out from a sheet using a die, have a dumbbell-shape resembling that of the “Type IV” specimen of the American Society for Testing and Materials (ASTM) Standard D-638-03 [10].

Polyimide sheets are prepared via desolvation and thermal imidization of

Mass and thickness loss, and erosion yield

The mass loss per unit area of polyimide films and ITO-coated ones as a function of AO fluence is presented in Fig. 2. After AO irradiation tests, polyimide films showed a large mass reduction. The reduction increased in almost direct relation to the AO fluence. The averages of thickness losses at each AO fluence of 0.30 × 1021, 0.85 × 1021, and 1.30 × 1021 atoms/cm2 were, respectively, 8.0, 21.5, and 27.1 μm. Erosion yields of the polyimide films were 1.7–3.0 × 10−24 cm3/atom, which is

Conclusions

Rupture of AO-irradiated polyimide films initiated from AO-irradiated surfaces, whereas pristine control samples fractured from the film inside. The AO-irradiated surfaces became rougher concomitantly with the AO fluence. Thereby, their tensile strength and elongation were reduced with the AO fluence increasing. The ITO-coated polyimide films indicated a high durability for AO erosion: slight mass loss and no rough surface, even after AO irradiation. However, undercut cavities were formed at

Acknowledgments

The author gratefully acknowledges the experimental support of the Advanced Engineering Services staff.

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