Pii: S0021-8502(97)10015-5

A new process for the production of nanostructured materials, hypersonic plasma particle deposition (HPPD), is experimentally investigated. In HPPD, vapor phase precursors are injected into a flowing plasma generated by a DC arc. The plasma undergoes a supersonic expansion into a deposition chamber, with the pressure dropping across the nozzle from &500 Torr to &2 Torr. Ultrafine particles nucleate in the nozzle, accelerate in the hypersonic free jet downstream of the nozzle, and deposit by inertial impaction onto a temperature-controlled substrate. The low particle residence time (&50 ks) minimizes particle agglomeration, while the high particle deposition velocity (&1 km s~1) results in the formation of a partially consolidated coating. We have characterized silicon, carbon and silicon carbide coatings produced by injecting vapor-phase SiCl 4 and hydrocarbon (CH 4 and C 2 H 2 ) precursors into an Ar—H 2 plasma. The silicon coatings are polycrystalline, while the carbon and silicon carbide deposits are amorphous and hydrogenated. Both Si and SiC coatings had nanostructured regions with grain sizes on the order of 20—30 nm, reasonably close to the diameters of impacting particles measured using an extractive aerosol probe coupled to a scanning electrical mobility analyzer. ( 1998 Elsevier Science Ltd. All rights reserved I N T R O D U C T I O N There is currently great interest in the synthesis and processing of nanostructured materials, which are materials with grain sizes less than about 100 nm, and are generally obtained by consolidating nanosize powders. Such materials are often found to have properties superior to those of conventional bulk materials. Examples of enhanced properties include greater strength, hardness, ductility, and sinterability; size-dependent light absorption, greater reactivity, etc. Much of the research effort in this field has been directed at determining the properties of nanostructured materials, and their production using a number of techniques including colloidal precipitation, mechanical grinding, and vapor-phase nucleation and growth, the last of which appears to be the most flexible at present. Past developments in this area have been summarized in extensive reviews (Siegel, 1993; Gleiter, 1989). The potential applications of these materials include wear resistant coatings, ductile ceramics, new electronic and optical devices, catalysts, etc. However, in order for these practical applications to be realized, two fundamental engineering problems remain to be addressed, namely, (1) controlled, high-rate synthesis of nanosize powders, and (2) assembly of these powders into useful nanostructured materials. Controlled synthesis implies that the particles are uniform in size, composition and morphology and are contamination free. This is often difficult in gas-phase processes, due to problems such as hard agglomeration or oxidation of particles prior to consolidation. While some of these issues have been partially addressed using in situ powder consolidation and assembly techniques, at present high volume rate production has been established for only a limited class of materials, e.g. ceramic oxides (Parker et al., 1996). This paper describes our attempts at controlled synthesis and in situ assembly of non-oxide nanostructured ceramics using a new process, hypersonic plasma particle