Functionally graded materials synthesis via low vacuum directed vapor deposition

The spatially distributed microstructures needed to implement many functionally graded material (FGM) designs are difficult to realize affordably with today's materials synthesis/processing technologies. To address this need, a new directed vapor deposition (DVD) technique has been developed and explored as a potential FGM synthesis tool. The technique exploits supersonic helium jets in combination with electron beam/resistive evaporation under low vacuum (10(-3)-10 Torr) conditions to atomistically spray deposit a wide variety of monolithic and composite materials. Two of the most important processing parameters (the carrier gas velocity and the deposition chamber pressure) that control deposition are identified, and their effect upon deposition efficiency for flat and fiber substrates is explored systematically. Under certain conditions, the DVD approach is found to deposit vapor onto fibers with a significantly higher efficiency than traditional high vacuum line-of-sight vapor deposition techniques. It can even deposit material onto surfaces that are not in the line-of-sight of the source. A computational fluid dynamics model has been used to interpret the experimental observations and to identify the role of carrier gas dynamics in controlling deposition efficiency and spatial distribution. (C) 1997 Elsevier Science Limited.