SITIS Archives - Topic Details
Program:  SBIR
Topic Num:  AF071-142 (AirForce)
Title:  Nondestructive Evaluation (NDE) for Silicon Carbide (SiC) Optics
Research & Technical Areas:  Materials/Processes

  Objective:  Develop NDE techniques and equipment to detect both defects and internal trapped stresses at the surface and through-the-thickness of polycrystalline SiC-based mirrors.
  Description:  Polycrystalline SiC based materials are being considered for the next generation aerospace (lightweighted) mirror structural substrates because of their low density and coefficient of thermal expansion (CTE) as well as their high strength, stiffness and fracture toughness. These materials can potentially serve as a single wideband optical structural material to replace both Be and glass optics for a range of temperature from cryogenic to high temperatures. The NDE of glass optics usually involves polarized light birefringement measurements that enable both defect identification and internal stress estimations. However, this technique has not been successful for non-transparent substrate like SiC and Be. Currently, these materials can only be screened for reliability by proof testing in an actuated, interferometry test bed. This is very expensive (both in time and money) because it requires a fully finished mirror. If the mirror changes figure during testing, the part is discarded as scrap. In this SBIR topic, new NDE methods are being sought that can inspect SiC based mirror substrates (through thickness) for both defects and internal stresses. The technique should be applicable to inspection at various steps along the production route including as-produced, machined, figured, finished, and coated. In the Phase I proposal, the offeror should present a clear detailed description of their inspection methodology including all advantages and disadvantages. Assumptions should be supported with physics argument and pertinent data from past experience or from the literature. They also need to provide some estimate of the resolution and sensitivity of their technique including minimum defect size and stress level that is detectable as a function of part thickness and lightweighted substrate geometry (honeycomb open or closed backed). Additionally they should compare these figures of merits with those of other methods. The goal of this program is to develop and demonstrate advanced NDE technologies for SiC-based materials to help minimize cost, rejection rates, and increase reliability. In Phase I, the offeror will have to demonstrate feasibility of their method on 1- to 2-inch diameter by ¼-inch-thick SiC flat disks that are processed to contain defects and internal stress. [The offeror will have to procure these from a SiC vendor]. In Phase II, the offeror will have to scale up their process to be able to identify defects in at lease 4- to 6-inch diameter lightweighted SiC mirror structures. This proposed development effort is performance enabling and pervasive for systems such as Space Based Infrared High, the Space Based Surveillance System, and the Space Tracking and Surveillance System. The technology may also be applied to next generation kill vehicles and directed energy systems.

  PHASE I: Investigate potential NDE methods for the characterization of both defects and internal stress contained within poly-SiC-based materials. Demonstrate resolution and sensitivity on samples which contain subsurface damage and trapped stress produced during the fabrication and figuring stages.
  
  PHASE II: Focus on the optimization and scale-up of the technique to inspect small (4- to 6inch-diameter) lightweighted SiC mirror substrates. Track damage during the processing, figuring, finishing, and coating stages. Include design/develop/construction of a breadboard prototype system to demonstrate the innovation and to obtain the operational specifications as well as the limits of the technology.

  DUAL USE COMMERCIALIZATION: Military application: Develop and implement specific NDE technology for characterizing SiC-based optics being considered for imaging telescope systems (STSS and EKV) and directed energy reflectors (ABL and ARMS). Commercial application: SiC are being considered for numerous industrial applications including parts for the jet and diesel engine community. A viable NDE method will be required to assess the reliability of these parts.

  References:  1. E. Brinksmeier, “State-of-the-art of non-destructive measurements of sub-surface material properties and damage,” Precision Engineering, October 1989, Vol. 11, No.4, pp 211-224. 2. J. A. Randi, J. C. Lambropoulos and S. D. Jacobs, “Subsurface damage in some single crystalline materials,” Applied Optics, 20 April 2005, Vol. 44, No. 12 3. N. L. Hecht, P. P. Yaney and G. A. Graves, "The Use of Laser Raman Microprobe Spectroscopy for Investigating Residual Surface Strain in Selected Ceramics," Proceedings of the Conference on Fractography of Glasses and Ceramics III, The American Ceramic Society, Westerville, Ohio, 1995, pp. 367-384.

Keywords:  nondestructive evaluation, NDE, ceramics, Silicon Carbide, SiC, defect detection, internal stress

Additional Information, Corrections, References, etc:
SiC mirror vendors.pdf
SiC mirror vendors.pdf

Questions and Answers:
Q: 1. Could you please provide references for SiC material processing and production route of the mirrors?
A: 1. Please see attached document SiC mirror vendors.pdf.
Q: Any requirements about the accuracy/sensitivity and resolution?
A: Currently we use Sonascan ultrasonic's technique and CAT scan X-ray technique to characterize SiC mirrors. These techniques can resolve void that are approximately 10 to 15 microns in diameter. They cannot resolve sharp cracks, kissing cracks or voids smaller that 10 micron. Nor can they tell us anything about the stress concentrations around the voids or cracks. We would like to identify techniques which could help us resolve voids and cracks less that 1 micron and stress concentrations of 25 MPa or less. We would like to find these sized defects and stress gradients 99% of the time.
A few SiC vendors are Coortek, Poco, Trex, M cubed, SSG, Xinetics and Schafer.
Q: Is the SiC of interest reaction sinteres SiC, sintered alpha SiC, or other?
A: All of the SiC based materials are of interest. You should pick a SiC that will work best for your technique. You should also discuss in your proposal if your techniques has advantages and disadvantages depending upon which SiC material is chosen.
Larry Matson
Q: 1. Could you please provide references for SiC material processing and production route of the mirrors?
A: 1. Please see attached document SiC mirror vendors.pdf.
Q: Any requirements about the accuracy/sensitivity and resolution?
A: Currently we use Sonascan ultrasonic's technique and CAT scan X-ray technique to characterize SiC mirrors. These techniques can resolve void that are approximately 10 to 15 microns in diameter. They cannot resolve sharp cracks, kissing cracks or voids smaller that 10 micron. Nor can they tell us anything about the stress concentrations around the voids or cracks. We would like to identify techniques which could help us resolve voids and cracks less that 1 micron and stress concentrations of 25 MPa or less. We would like to find these sized defects and stress gradients 99% of the time.
A few SiC vendors are Coortek, Poco, Trex, M cubed, SSG, Xinetics and Schafer.
Q: Is the SiC of interest reaction sinteres SiC, sintered alpha SiC, or other?
A: All of the SiC based materials are of interest. You should pick a SiC that will work best for your technique. You should also discuss in your proposal if your techniques has advantages and disadvantages depending upon which SiC material is chosen.
Larry Matson

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