Nanocomposites for high temperature applications

This paper reviews the research that has been conducted at the Naval Research Laboratory during the past few years on nanocomposites for high temperature applications. The research was inspired by a strengthening theory proposed by N.P. Louat (Acta. Metall., 33 (1985) 59). The theory sought to take advantage of the high strength and toughness of fine-grained metals while at the same time avoiding, through use of composites, the inherent thermal instability of these materials at high temperatures due to thermally activated processes such as creep, grain boundary sliding and grain coarsening. To test this idea, both microcomposites and nanocomposites were synthesized and processed at the Naval Research Laboratory by different techniques that included liquid infiltration, electroless plating, chemical vapor deposition fluidized-bed, inert gas condensation, and ball milling. In all cases, the composites consisted of a hard reinforcing phase embedded in a softer metal matrix phase in which both phases are nearly immiscible. For the case of copper-niobium and brass-niobium microcomposites, both strength enhancement and high temperature strength retention were demonstrated. For physical-vapor deposited copper-niobium nanocomposites, very large increases were observed in the microhardnesses with a peak in the microhardness values around 63 vol.% niobium. Suppression of grain growth at temperatures close to the melting point of copper were demonstrated, as well. Similar results were obtained for silver-nickel nanocomposites. Processing nanocomposite metals has proven to be plagued with two principal challenges:consolidation and oxidation. These two problems with nanostructured metals suggest alternative research directions designed to take advantage both of the strong reactivity and of the large grain boundary surfaces of the nanostructured materials.