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Efficient Evaluation of Fiber Coatings

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: The objective of this program is development and demonstration of a low cost, rapid method for evaluating the chemistry of fiber coatings used in ceramic matrix composites (CMCs). 

DESCRIPTION: Ceramic matrix composites (CMCs) are beginning to see service in commercial gas turbine engines and are being considered for a range of components in military gas turbine engines due to their potential for weight reduction, reduced cooling, increased operating temperature, and improved durability. Non-oxide CMCs generally require one or more thin coatings (each <1 micron thick) on the reinforcing fibers in order to achieve the weak interface that is required for high toughness. These coatings turn out to be key to the composite performance and also cost and cycle time drivers. Routine evaluation of the coatings is required for quality control. Historically, techniques such as scanning electron microscopy (SEM) and auger depth profiling have been used to assess the coating thickness and chemistry, respectively. These techniques are slow, expensive, destructive, and incapable of sampling meaningfully representative volumes of the coated tow or fabric. Rapid techniques have been developed to evaluate the coating thickness, but evaluation of coating chemistry remains a pervasive need across the community. This topic seeks the development and demonstration of a low cost, rapid, non-destructive technique which is capable of sampling significant areas of coated tows or fabric which routinely have at least 2 distinct coating layers, and may have 4 or more in some situations. Successful development of such a technique will reduce coating cost and cycle time, and improve the consistency and performance of CMCs overall. The ability to assess the morphology and/or crystallinity of the coatings is also of interest, but is secondary to the ability to quantify the coating chemistry. The main chemistries of interest are boron nitride (BN) which may be doped with silicon (Si), silicon carbide (SiC), silicon nitride (Si3N4) and boron carbide (B4C). In all cases the presence of oxygen (O) and in some cases carbon (C), each as a contaminant, is also critical, as is the ability to assess the stoichiometry, for instance, the boron to nitrogen ratio in BN. Techniques exist and are used commercially to characterize thin coatings in various industries. The situation for fiber tow or fabric coatings is more complex due to the substrates being 10 – 20 micron diameter fibers in 500-1000 filament tows often woven into 16-22 tows-per-inch cloth, the coatings often having multiple layers with different chemistries, and the chemistries being more complex, as well as concerns about contaminants and coating stoichiometry. A requirement to evaluate each coating as deposited (i.e. with no other coatings on top of it) is acceptable. The development of an entirely new technique will not be ruled out, but the adaptation of an existing technique or techniques is highly preferred. Destructive (but rapid and low cost) techniques will not be ruled out, but non-destructive techniques are greatly preferred. Teaming with a fiber coating supplier and/or composite manufacturer is encouraged to ensure that suitable techniques are developed and demonstrated on fiber coatings of interest, and in a representative manufacturing environment. 

PHASE I: Develop or adapt the coating chemistry evaluation technique(s) which were proposed. Explore the capabilities and demonstrate the technique on tow or fabric samples with multiple relevant coating chemistries. Estimate the cost and time required for coating characterization and the fraction of the tow or fabric which could be feasibly evaluated. Determine practical layer thickness limits and the number of layers that can be evaluated in a stack. Identify potential improvements for Phase II. 

PHASE II: Make the improvements identified at the end of Phase I and quantify the capability enhancement. Characterize sufficient samples of coated tow and/or fabric to validate the capability (resolution, thickness limits, etc), cost, and cycle time. Validate the results via other techniques as applicable. Demonstrate the technique in an industrial fiber coating or composite manufacturing setting. 

PHASE III: Optimize the coating chemistry evaluation methodology for the chosen fiber coating characterization environment. Modify the existing quality control (QC) specifications for the new methodology. Undertake the evaluation necessary to qualify the new QC methodology in industrial fiber coating and/or composite manufacturing facilities. 

REFERENCES: 

1: "Interfacial Processing via CVD for Nicalon Based Ceramic Matrix Composites," C. L. Hill, J. W.Reutenauer, K. A. Arpin, S. L. Suib, M. A. Kmetz, Ceramic Engineering and Science Proceedings, v 27, n 3, p 253-264, 2006

2:  "Control of Surface Energy of Silicon Oxynitride Films," K. Wang, M. Gu╠łnthner, G. Motz, B. D. Flinn, and R. K. Bordia, Langmuir 2013, 29, 2889−2896

KEYWORDS: Ceramic Matrix Composite (CMC), Fiber Coating, Coating Chemistry, Quality Control, Turbine Engine, Non-Destructive Evaluation 

CONTACT(S): 

Ming Y. Chen 

(937) 255-9821 

ming.chen@us.af.mil 

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