Reaction layer formation at the graphite/copper-chromium alloy interface
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I.
INTRODUCTION
CONTINUOUS fiber-reinforced graphite/copper (Gr/Cu) composites offer the potential for improved performance as high heat flux structures for elevatedtemperature applications requiring high thermal conductivity. These composites are candidate materials for applications such as space power radiator panels and as engine heat exchanger materials for hypersonic aircraft. Graphite/copper composites made from pitch-based fibers possess a high thermal conductivity equivalent to copper. They also exhibit a high modulus of elasticity equivalent to beryllium and moderate density similar to titanium.tq The fabrication and performance of composite materials designed for elevated-temperature applications are strongly influenced by the fiber/matrix interface. A key aspect of ceramic/metal interfaces is the ceramic/metal interfacial energy. However, often the ideal fiber/matrix combinations, based upon their mechanical and physical properties, have high interracial energies. [21 The high interfacial energies relative to the surface energies of the fibers result in a lack of wetting between the metal and fibers. This intrinsic lack of wetting causes difficulties in production of the composites. More importantly, the lack of wetting can lead to fiber/matrix debonding and pore formation during the service life of the composites at elevated temperatures, tq
II.
BACKGROUND
To date, limited work has been conducted in the area of Gr/Cu interfaces. Perhaps the most widely referenced studies have been those of Mortimer and Nicholas. t3,4,51 S A N D R A M. D E V I N C E N T , Graduate S t u d e n t , and GARY M. MICHAL, LTV Steel Associate Professor, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204. This article is based on a presentation made in the symposium "High Performance Copper-Base Materials" as part of the 1991 TMS Annual Meeting, February 17-21, 1991, New Orleans, LA, under the auspices of the TMS Structural Materials Committee. METALLURGICAL TRANSACTIONS A
They reported that small additions of Cr reduce the contact angle formed by Cu on HX30 Gr and vitreous carbon in studies conducted in vacuum. Through metallographic examination, they found the presence of reaction products at the interfaces of the Gr/Cu-Cr alloy samples. Four types of reaction products were identified: continuous layers of uniform thickness, discontinuous layers of nearly uniform thickness, flaky layers partly detached from the carbon substrate, and interfacial areas containing many small particles. The reaction product of the Cu-1 at. pct Cr alloy was continuous and quantitatively determined to be Cr3C2. For sessile drop experiments conducted under conditions where a continuous reaction layer forms, the appropriate form of the Young-Dupre equation t31 is as follows: ~gSVcarbide = ~/LValloy
COS
0carbide/al|oy At- ~/SLcarbide/alloy
[1]
where Ysvo~,do is the carbide/vapor interfacial energy, YLV~,oyis the alloy/vapor interfacial energy, 0carbide/alloy is the contact a
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