Abstract
Carbon-fiber-reinforced plastic (CFRP) has a higher strength-to-weight ratio and forming flexibility than metals, making it suitable for fabricating lightweight x-ray mirrors. However, CFRP has the disadvantages of print-through and deformation due to moisture absorption, which have prevented its use in optical mirrors. To expand the application of CFRP, we studied the formation of a moisture barrier layer on CFRP substrates. We formed a flattening layer a few micrometers thick on a CFRP substrate, following which we coated the substrate with SiOx as a moisture barrier. The effect of moisture absorption was then evaluated using accelerated aging tests. We found that the diffusivity of the CFRP substrate at 60°C and a relative humidity of 100% was ∼2 × 10 − 6 mm2 h − 1, which is 1/500th that of the barrier-less substrate. In the tests, the moisture absorption rate increased after ∼800 h. As we observed cracks on the flattening layer after 600 h, the rate increase could be associated with these cracks. Considering the damage to the barrier layer, we propose a modified model for the time profile, which is congruent with the observed time profile of the moisture content.
Highlights
With the advent of telescopes, it has become possible to detect weak light and detailed structures in the universe that cannot be observed by the human eye
The design of x-ray telescopes is roughly classified into two types based on the mirror configuration: one type focuses on achieving a high spatial resolution,[1,2,3] while the other focuses on achieving a large effective area.[4,5,6,7,8,9,10,11,12]
We previously found that the time profile of moisture absorption in a Carbon-fiber-reinforced plastic (CFRP) substrate with a barrier layer cannot be reproduced by a simple diffusion model.[31]
Summary
With the advent of telescopes, it has become possible to detect weak light and detailed structures in the universe that cannot be observed by the human eye. There are known disadvantages of using CFRP as a substrate for high-precision optics[18,19,20,21,22,23,24,25,26,27,28] and, in particular, x-ray mirrors.[29] One is the microscale deformation of the substrate surface, which is called print-through. One method that reduces the effect of moisture absorption utilizes a barrier layer on the CFRP surface. We previously coated a thin CFRP substrate with functional sheets having a low watervapor transmissivity and found that such coating is effective for reducing moisture absorption.[31]. In the present study, we coated a moisture barrier layer on the CFRP substrate, instead of using functional sheets, and evaluated its moisture absorption effect by using accelerated aging. To evaluate the effect of the moisture barrier, in the present study, a model characterizing the time profile of the moisture absorption is proposed to represent the time profile considering damage to the moisture barrier
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