Aluminum/Copper Nanocomposites Fabricated by The Jet Vapor Depositiontm Process
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ABSTRACT Aluminum and copper nanolaminates have been fabricated at Jet Process Corporation using the novel, proprietary Jet Vapor DepositionTM (JVD)TM process. Laminates with a total thickness of 10 pm were made by depositing alternating layers of' approximately equal thicknesses of copper and aluminum onto preheated silicon wafers at a substrate temperature of -140 'C. The layer thicknesses were systematically varied between 20 nm and I gtm. The microstructure and properties of the laminates were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nanoindentation methods. TEM has shown that the laminates have a strong (1111 texture. The hardness results show that above a critical layer thickness of approximately 50 nm, the yield strength of the composites varies inversely with the layer thickness, while the strength of nanolaminates with layer thicknesses smaller than the critical thickness is better explained by the Koehler model. An alternative model recently proposed by Embury and Hirth fits the data equally well.
INTRODUCTION Novel vapor phase synthesis methods are beginning to emerge with the capability of creating affordable composite structures with independently controlled volume fractions and thickness of two or more constituents [1, 2]. They offer the opportunity to synthesize lamellar composites with nanometer scale dimensions and to vary the layer type and spacing throughout the composite's thickness, thus creating graded microstructure materials. Here, we explore the use of the novel, proprietary Jet Vapor Deposition TM (JVD)TM process developed by Jet Process Corporation, New Haven, CT [3, 4, 5, 6]. The JVD process uses sonic, high purity helium (He) and hydrogen (H2 ) gas jets to entrain atomically dispersed vapor and deposit films/coatings. It is of interest here because several jets can be used sequentially to deposit multilayered materials inexpensively and at high rates. Its potential value for this is examined by depositing and measuring the strength of a model metal-metal system. Background and Theory Materials with thin, multilayered structures are of interest because they potentially offer novel mechanical and physical properties [7, 8]. The movement of dislocations in these structures has been an area of growing interest [9]. Koehler used an image force model to analyze the strength of layered materials of very thin layer thicknesses [10]. He predicted that if one of the layer constituents had a significantly lower shear modulus than the other, dislocations would have to overcome a large repulsive image force in order to move from metal layer B of lower shear modulus 113 Mat. Res. Soc. Symp. Proc. Vol. 382 ©1995 Materials Research Society
(PB) into the A metal layer (with shear modulus PA imately by:
>
PB). This image force (F) is given approx(0)
R9Bb2 sin0 47tr
where R = (PA-PB)/(PA+pB), b is the magnitude of Burgers vector in metal layer B, 0 is the angle between the A/B metal interface and the slip plane, and r is the distance between
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