ARTIFICIALLY LAYERED NANOCOMPOSITES FABRICATED BY JET VAPOR-DEPOSITION

Novel jet vapor deposition (JVD) processes offer considerable promise for the inexpensive synthesis of functionally graded (composite) materials (FGMs). Here, we explore microstructure-mechanical property relationships for a model Al/Cu metal-metal system and an Al/Al2O3 metal-metal oxide multilayered nanocomposite system fabricated by the JVD process. The 10 mu m thick Al/Cu multilayers were deposited on silicon wafers at a substrate temperature of similar to 140 degrees C. The Al and Cu layers were of approximately equal thickness and were systematically varied from similar to 20 to similar to 1000 nm. The 20 mu m thick Al/Al2O3 multilayers were deposited on glass slides at similar to 250 degrees C. The oxide layer thickness was held constant in the similar to 2-6 nm range, whilst the Al layer thickness was systematically varied from similar to 3 to similar to 50 nm. The structure of the Al/Cu multilayers was polycrystalline and had a strong (111) texture, whereas the Al/Al2O3 multilayers consisted of amorphous aluminum oxide layers and polycrystalline metal layers with randomly oriented grains. The yield strength of the Al/Cu multilayers exhibited an inverse dependence upon layer thickness when the layer spacing exceeded similar to 50 nm. When the Al/Cu layer spacing was thinner than similar to 50 nm, the strength was better predicted by a Koehler image force model. A similar phenomenon was also found in the Al/Al2O3 multilayers. In this case the critical metal layer thickness for the transition from an Orowan to a Koehler type behavior was approximately 25 nm. This is consistent with theoretical predictions which indicate that the critical layer thickness of the low modulus consistuent decreases as the difference in shear moduli between the two constituent layers increases.