Reactive Plasma Spray Forming

Research in recent years has demonstrated that the synthesis of advanced materials by chemical routes is more promising than many "conventional" processing methods—casting, forming, and possibly powder metallurgy (Ref 24). Plasma spray forming, a rapid particulate consolidation, high-temperature process, has an inherent suitability for chemical synthesis and is thus a strong candidate for synthesizing multiphase, advanced materials. As described in the literature (Ref 25), plasma spray coating and forming have been practiced for many years and are now practical and economical methods for processing difficult-to-form intermetallic, ceramic, and composite materials (Ref 1, 2, 26). The development of plasma spraying in controlled atmosphere chambers (VPS or LPPS) in the early 1970s (Ref 27, 28, 29) was a key factor behind its advancement from a coating to a forming technology. Controlled-atmosphere plasma spraying, adapted for reactive plasma spraying, has recently been developed to combine controlled dissociation and reactions in thermal plasma jets, for the in situ forming of new materials or to produce new phases in sprayed deposits. Reactive plasma spray forming is consequently emerging as a viable method for producing a wide range of advanced materials. The process, a logical evolution from conventional plasma spraying, allows "reactive" precursors to be injected into the particulate and/or hot gas streams. These reactive precursors may be liquids, gases, or mixtures of solid reactants which, on contact with the high-temperature plasma jet, decompose or dissociate to form highly reactive and ionic species that can then react with other heated materials within the plasma jet to form new compounds. Figure 12 illustrates the reactive plasma spray concept. Chemical reactions rely on plasma-induced dissociation of the injected precursors, for example gaseous methane pn-

(CH4), which decomposes into elemental or ionic species such as 3 , C ~ or even atomic carbon. These species can react with other elements to produce carbides such as TiC and WC, or under certain conditions, diamond or diamondlike carbon (DLC) films. Figure 12 shows the plasma heating of gases, using either a nontransferred electric arc or an inductively coupled plasma (ICP) radio frequency (RF) discharge; injection of reactive precursors into the plasma jet; and injection of powders in a carrier gas stream. Injection locations may vary depending on the materials or reactions desired and can be directly in the plasma generator (torch or gun) or into a reactor located immediately downstream. The primary requirements are that the precursors dissociate into reactive "species" and that the reaction times and temperatures are sufficiently long for the desired products or phases to form. Reactive plasma spray forming has several key components, as shown in Fig. 12:

either on the surfaces of particles or on a the surface(s) on which the products are



Particulate reinforced composite deposit

Fig. 12 Reactive plasma spray process for the synthesis of composite materials

Reactive plasma spray applications include the synthesis of composite materials, shaped brittle intermetallic alloys, reinforced or toughened ceramics, and tribological coatings with in situ formed hard or lubricating phases. Reactive plasma spray forming enables a wide range of materials to be produced, for example, aluminum with AlN, Al2O3, or SiC; NiCrTi alloys with TiC or TiN; intermetallics such as TiAl, Ti3Al, MoSi2, and other ceramics with oxides, nitrides, borides, and/or carbides. All of these have been produced in situ in reactive thermal plasma jets.

Figure 13 shows typical microstructures of some reactively plasma sprayed materials, demonstrating the versatility of the process for the in situ forming of metal/carbide, metal/nitride, and ceramic composites.

Figure 13 shows typical microstructures of some reactively plasma sprayed materials, demonstrating the versatility of the process for the in situ forming of metal/carbide, metal/nitride, and ceramic composites.

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Fig. 13 Microstructures of reactively plasma sprayed (a) Al2O3/SiAlON and (b) MoSi2/SiC

Research indicates, however, that significant technical challenges still remain because, despite the promises and advantages indicated previously, the yields and quality of the final products depend strongly on the starting precursors, their injection methods, and the atmospheres in which they are sprayed (Ref 24, 30). Monolithic SiC, SiN, and AlN powders have also been synthesized in plasma jets (Ref 31, 32). The uniformity of these particular materials has considerable variability, however, and has limited the practical use of many plasma spray processes for forming monolithic compounds, although commercial plasma production of ZrO2 and TiO2 powders in plasma jets has been reported (Ref 9).

Recent investigations have demonstrated that reactive plasma spray forming utilizing incomplete or nonequilibrium reactions in plasma jets to form mixtures of phases can produce functional composites with the reactant products embedded in metallic, ceramic, and even intermetallic matrices. These materials have been shown to be very hard and have potential application as wear-resistant coatings (Ref 33). These reactively plasma sprayed coatings were, in practice, metal-matrix composites with in situ reacted hard phases formed when the reactive gas mixtures reacted with molten metal particles. Fine TiC, WC, TaC, and other refractory carbides, oxides, silicides, or nitrides have been spray formed in situ as particulate phases in near-net-shape metallic, intermetallic, and ceramic matrices, thus eliminating many difficult post-forming operations. Lightweight structural materials such as Ti3Al, TiAl, NiAl, and MoSi2 have also been produced by reactive spray forming. In these cases, aluminum metal powders were injected into TiCl4 plasmas to produce TiAl. Molybdenum particles have also been injected into plasma jets seeded with disilane (Si2H6) to form MoSi2.

The utility and potential of reactive plasma spray synthesis is further illustrated by two high potential examples presented below. Research and development in these areas is receiving increased attention as the potential of thermal plasma processing of these high value materials is realized.

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