The influence of the Low-Earth orbit (LEO) environment on the mechanical strength of silicon carbide (SiC) was evaluated on two flight experiments as part of the Materials on the International Space Station Experiment (MISSE). SiC samples for modulus of rupture (MOR) and equibiaxial flexural strength (EFS) testing were flown on the Optical and Reflector Materials experiments (ORMatE) as part of MISSE-6 (launched on STS-123, March 2008; returned on STS-128, September 2009) and MISSE-7 (launched on STS-129, November 2009; returned on STS- 134, June 2011). Two different SiC vendors provided material for each flight experiment. The goal of the experiments was to measure mechanical properties of the flight samples and compare them to an equal number of similar samples in control and traveler sample sets. Complete characterization of the strength of brittle materials typically requires many more test specimens than could be reasonably accommodated on the ORMatE sample tray and statistical models based on few samples include large uncertainties. Understanding the results of the mechanical tests of MISSE samples required comparison to results from a statistically valid number of samples. Prior testing by The Aerospace Corporation of material supplied by the same four vendors was used to evaluate the MISSE results, including flight and control samples. The results showed that exposure to LEO over the durations covered by MISSE 6 and 7 (approximately 18 and 20 months, respectively) did not alter the mechanical strength of the silicon carbide for any of the vendors’ materials.
Two types of SiC plates, differing in their manufacturing processes, were interrogated using a variety of NDE
techniques. The task of evaluating the materials properties of these plates was a challenge due to their non-uniform
thickness. Ultrasound was used to estimate the Young's Modulus and calculate the thickness profile and Poisson's
Ratio of the plates. The Young's Modulus profile plots were consistent with the thickness profile plots, indicating
that the technique was highly influenced by the non-uniform thickness of the plates. The Poisson's Ratio is
calculated from the longitudinal and shear wave velocities. Because the thickness is cancelled out, the result is
dependent only on the time of flight of the two wave modes, which can be measured accurately. X-Ray was used to
determine if any density variations were present in the plates. None were detected suggesting that the varying time of
flight of the acoustic wave is attributed only to variations in the elastic constants and thickness profiles of the plates.
Eddy Current was used to plot the conductivity profile. Surprisingly, the conductivity profile of one type of plates
varied over a wide range rarely seen in other materials. The other type revealed a uniform conductivity profile.
The Aerospace Corporation is developing a space qualification method for silicon carbide optical systems that covers
material verification through system development. One of the initial efforts has been to establish testing protocols for
material properties. Three different tests have been performed to determine mechanical properties of SiC: modulus of
rupture, equibiaxial flexural strength and fracture toughness. Testing materials and methods have been in accordance
with the respective ASTM standards. Material from four vendors has been tested to date, as part of the MISSE flight
program and other programs. Data analysis has focused on the types of issues that are important when building actual
components- statistical modeling of test results, understanding batch-to-batch or other source material variations, and
relating mechanical properties to microstructures. Mechanical properties are needed as inputs to design trade studies and
development and analysis of proof tests, and to confirm or understand the results of non-destructive evaluations of the
source materials. Measuring these properties using standardized tests on a statistically valid number of samples is
intended to increase confidence for purchasers of SiC spacecraft components that materials and structures will perform
as intended at the highest level of reliability.
An important space application for structural ceramics is the large (~1m diameter) silicon carbide mirrors used in
telescopes. However, all ceramics have two drawbacks. First, ceramics are brittle and have a low resistance to
catastrophic flaw propagation during service. Second, ceramics have a population of preexisting flaws produced during
manufacturing.
The most modern and successful theory of fracture control is "defect tolerant design", which recognizes that engineering
structures are inherently flawed and which is used to predict the structure's service life. As part of defect tolerant design,
the size of the inherent flaws is controlled by combining nondestructive evaluation (which has a threshold for the
smallest flaw that can be reliably identified) and proof testing (which provides an independent measurement of the
largest preexisting flaw).
We are developing a novel proof test that is specialized for a lightweighted ceramic mirror. The most promising loading
method is identified and an experimental implementation has been proposed and designed for future development.
Over the last few years significant progress has been made in the development of silicon carbide (SiC) for mirror
applications. These improvements include lightweighting techniques, higher production yields, and larger diameter
apertures. It is now necessary to evaluate and address the systems engineering challenges facing this material to ensure
space qualification and integration into future space applications. This paper highlights systems engineering challenges,
suggests areas of future development, and proposes a systematic path forward that will outline necessary steps to space
qualify this new material.
KEYWORDS: Silicon carbide, Databases, Mirrors, Nondestructive evaluation, Data modeling, Space mirrors, Aerospace engineering, Space telescopes, Systems modeling, Telescopes
Production of optical silicon carbide (SiC) for mirror applications continues to evolve and there are renewed plans to use
this material in future space-based systems. While SiC has the potential for rapid and cost-effective manufacturing of
large, lightweight, athermal optical systems, this material's use in mirror applications is relatively new and has limited
flight heritage. This combination of drivers stresses the necessity for a space qualification program for this material.
Successful space qualification will require independent collaboration to absorb the high cost of executing this program
while taking advantage of each contributing group's laboratory expertise to develop a comprehensive SiC database. This
paper provides an overview of the trends and progress in the production of SiC, and identifies future objectives such as
non-destructive evaluation and space-effects modeling to ensure proper implementation of this material into future
space-based systems.
Future large-aperture optical space systems will need to use lightweight materials that meet stringent requirements, and that reduce program and launch costs. Lightweight optical systems produced quickly and cost-effectively, and the resultant lighter payloads, can reduce these costs. Mirrors for future systems have areal density goals of less than 5 kg/m2 and will need to use new materials1. A promising one is silicon carbide (SiC) because of its physical and mechanical properties. These enable the production of low areal density, high quality mirrors, as well as lightweight athermal telescope structures. Athermal structures are desirable because they simplify designs and reduce tolerance requirements to maintain performance during on-orbit temperature changes. The use of SiC to make mirrors and structures is in the developmental stage and has limited space heritage. To ensure the use of this material in space applications, qualification and system performance in the space environment must be addressed. This paper provides an overview of SiC, along with recommendations to further the development of SiC into a mature technology that can be successfully integrated into future large-aperture optical space programs.
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