Thermal plasma processing is carried out near atmospheric pressure, so that it involves different regimes of density, power, and heat flow from those in the preceding technologies. It must be viewed in the context of much broader trends that exert a strong influence on the development of this technology. Materials and materials processing are attracting increasing attention, a trend that will continue into the next century. This trend will not be restricted to the development of new materials but will also include the refining of materials, the conservation of materials (by hard facing, coating, and so on), and the development of new processing routes that are more energy efficient, more productive, and less damaging to our environment. Thermal plasma processing will play an important role in these developments. Its potential for developing new materials-related technologies is increasingly recognized, and many research laboratories all over the world are engaged in advancing the frontiers of knowledge in this field. An interesting example of the use of plasma processing has been demonstrated recently in connection with a breakthrough in the field of diamond film deposition. Thermal plasmas provide the highest deposition rates among all known diamond film deposition techniques.
In spite of great strides over the past 25 years, the number of successful industrial applications on a broad base has been relatively small. The primary reason for the relatively slow growth of this technology has been the lack of a solid engineering base. Industrial efforts have not been sufficiently paralleled by basic studies at universities and national research laboratories, and, as a consequence, the required engineering base for many processes is still poorly developed. This problem is directly linked with the nature of thermal plasma processing as a highly interdisciplinary field that cannot succeed without extensive interdisciplinary endeavors. Knowledge of plasma physics, gaseous electronics, fluid dynamics, and heat transfer has to be combined with experience in surface chemistry, electrochemistry, and materials science.
The following sections outline research opportunities associated with various thermal plasma applications.
Plasma spraying is considered to be one of the prime candidates for producing high-temperature-resistant coatings (ceramics) for turbine blades and antiwear and anticorrosion coatings for high-temperature applications. Although plasma spraying is a well-established commercial process, its science base is still in the development stage. Arc-flow interactions within the plasma torch and the fluid dynamics of the plasma jet are still poorly understood. In spite of this drawback, extensive efforts are in progress to develop completely automated plasma spray systems using robotics for substrate/torch motion and feedback control for the plasma spray jet.
Wire-arc spraying is an inexpensive thermal plasma coating process in which the material to be deposited is introduced as wires that serve as consumable arc electrodes. A cold or heated gas jet across the arc drives the molten droplets from the electrode tips toward a substrate, forming a coating on the substrate. Eliminating the need for water cooling of its electrodes allows miniaturization of the spray system, making it suitable for many applications, including bore hole coating. In spite of its economic advantages, this technology has found only limited use in the manufacturing industry
because of the lack of a well-developed engineering base for this process. Only in recent years have basic studies of the arc-wire spray process been initiated for improving both our knowledge and technology base.
One of the most promising new developments in thermal plasma processing is the plasma chemical vapor deposition (PCVD) process. As a relatively new film deposition technique, it can deposit high-quality, even epitaxial, films at deposition rates considerably higher than those obtained by competing low-pressure methods. One of the most visible developments of this technology is the deposition of diamond and ceramic films, which is expected to have a strong impact on manufacturing processes. In contrast to the previously discussed coating technologies, PCVD is still in the laboratory stage; that is, present research efforts concentrate on the establishment of the knowledge base for this emerging technology.
In the process of PCVD, a high-energy-density plasma produces high-density vapor-phase precursors for the deposition of relatively thick films. A direct current plasma torch, for example, generates a high-temperature, high-velocity plasma jet that impinges on a cooled substrate. With temperatures close to the torch nozzle exit exceeding 104 K, the precursor material that is injected into the plasma is rapidly vaporized and dissociated and, because of the high velocities of the plasma jet (of the order of 100 m/s), accelerated toward the substrate. In front of the cooled substrate, a boundary layer with steep gradients forms. Such boundary layers in chemically reacting gases attracted strong interest in spaceflight and reentry simulation and have been extensively analyzed in these connections.
As the temperature across the thermal boundary layer drops from the plasma temperature to the substrate temperature, the chemically active species will be rapidly driven across the boundary layer by the extremely steep gradients there. Because of the rapid traverse of the species across the boundary layer, chemical reactions appear to be more or less “frozen,” resulting in a strong chemical nonequilibrium situation in the boundary layer, where the species concentration is determined mainly by diffusion rather than by chemical reactions. Maintaining a high concentration of chemically active precursors across the boundary layer is crucial for achieving high deposition rates.
In spite of impressive progress over the past years, the chemistry in the boundary layer and at the substrate surface during the PCVD process is still poorly understood. Efforts are continuing to establish realistic models for this situation and demonstrate by corresponding experiments their validity.
Destruction of waste, especially of toxic waste, has grown into an increasingly pressing problem. Among various waste destruction processes, thermal plasma waste destruction is considered to be a viable option for certain types of waste.
Thermal plasma reactors offer unique advantages for the destruction of hazardous wastes: (1) the high energy density and temperatures associated with thermal plasmas and the corresponding fast reaction times offer the potential of large throughputs in a small reactor; (2) the high temperatures can also be used to obtain very high quench rates, allowing the attainment of metastable states and nonequilibrium compositions; (3) the high heat fluxes at the reactor boundaries lead to fast attainment of steady-state conditions, allowing rapid start-up and shutdown times compared with other thermal treatment devices such as incinerators; (4) use of electric energy reduces gas flow needs and off-gas treatment requirements and offers control over the chemistry, including the possibility of generating marketable coproducts; and (5) all the characteristics combined allow easy integration into a manufacturing process that
generates hazardous waste, thus permitting the destruction of the waste at the source. The major disadvantage of the plasma process lies in the use of electricity as an energy source, which unfavorably influences the process economics. A further consideration is that plasma processes have more parameters to control than do traditional processes and require, therefore, a higher degree of automation in the process control, which translates into interesting research opportunities.
It also should be emphasized that waste destruction as a new technology is to a large extent driven by government regulations.
Thermal plasma metallurgy comprises melting and remelting technologies as well as extractive metallurgy. The key advantages of thermal plasma approaches, as specifically applied to melting/remelting technologies, include:
The ability to achieve a steady-state, uniform flow of partially ionized gas with bulk gas temperatures well above those obtainable with chemical (combustion) flames or resistance heater systems;
Operation in an inert or reactive environment, thus providing complete control of the atmosphere; and
The possibility of a compact system that can process material in a variety of forms at high throughput rates and with relatively high electrical and thermal efficiency.
Today, a wide variety of arc plasma torches are in use or projected for scrap melting, alloying, iron melting in cupolas, and remelting technologies. These plasma torches operate with direct or alternating current in either the nontransferred or transferred mode and at power levels up to almost 10 MW. In the case of nontransferred arcs, the plasma torch is essentially an arc gas heater producing extremely hot gases, which emanate from the torch in the form of a plasma jet. The more common approach, however, makes use of transferred arcs where the molten pool serves as one of the electrodes and the major energy input is at the arc root of the molten bath surface.
The considerable interest in extractive metallurgy is evidenced by the numerous laboratory and pilot-plant-scale studies reported in the literature (extraction of iron, titanium, molybdenum, and ferroalloys). Two different types of furnaces have been used for plasma extractive metallurgy: transferred arc reactors for ferroalloy production (similar to those used for melting), and reactors in which a reducing gas is preheated and upgraded by using plasma torches. The reduction of the ores to be treated is performed in a furnace filled with coke, which is mainly used as a refractory material for providing a sufficiently long residence time for the injected ore particles to be reduced.
Newer thermal plasma developments concern the extraction of heavy metals from baghouse dust and the recycling of catalytic converters for recapturing platinum. Since thermal plasma metallurgy consumes large amounts of electric energy, economic considerations have been a primary criterion for the development of such technologies. In addition, technical problems associated with electrode lifetime of high-power plasma torches have played a major role.
Over the past years, thermal plasma synthesis of ultrafine and ultrapure powders has been attracting increasing interest, especially in connection with the synthesis of ceramic materials. High-intensity arcs, plasma jets, and high-power radio frequency discharges are the primary sources for producing thermal plasmas required for this emerging technology.
Because of the high temperatures (>104 K) that are typical for thermal plasmas, chemical reactions are much faster than those encountered in conventional processing. Also, quench rates of the product powders are
necessarily very rapid to avoid decomposition of the products. Fast reaction and quench rates result in very short overall processing times—as short as milliseconds. This translates into small reactors with relatively high throughput rates. In spite of this attractive feature, the relatively higher processing costs of using plasma processing must be offset by some superior material properties.
A number of ultrafine oxide powders have been produced in thermal plasmas. Ultrafine oxides have a wide range of uses in surface coatings, high-density ceramics (“high-tech” ceramics), pigments, catalysts, and dispersion strengthening of metals. Three arc-related plasma techniques have been explored: reaction of volatile metal chlorides with oxygen, evaporation and subsequent condensation of oxide powders, and evaporation of bulk oxides. Industrially, TiO2 and high-purity SiO2 are produced by the chloride process. Mixed oxides of chromia and titania or chromia and alumina have been produced by introducing mixed chlorides into the plasma reactor.
In recent developments, the synthesis of nanometer-size particles in thermal plasmas has been demonstrated. Such small particles are considered to be the key for a new generation of engineered materials with unusual properties.
Plasma consolidation includes the processes of spheroidization, densification, and sintering. The first two of these processes are already commercially developed, whereas plasma sintering is still in the laboratory stage.
Frequently, spheroidization and densification occur simultaneously as porous, irregularly shaped agglomerates are injected into a thermal plasma. Equipment similar to that used for plasma spraying is employed for these processes, but the particle size of the agglomerates may be substantially (≫ 100 mm) larger than that used for plasma spraying. As the particles sinter and/or melt in the plasma, they assume a nearly spherical shape and densify at the same time. Commercially, fine particles are spheroidized in a plasma for a variety of applications, including materials with a controlled porosity, catalysts, abrasives, and materials used for powder metallurgy. A wide range of different materials have been spheroidized, including oxides and carbides.
Plasma densification of presintered agglomerates of metals (e.g., W, Mo) and of carbide-metal mixtures (e.g., WC-Co) has been used to produce spherical, densified powders. Such powders possess excellent flowability, which is beneficial to subsequent thermal coating operations.
Sintering of high-tech ceramics in thermal plasmas has the potential of drastically reducing the time period required for this process, compared with conventional technology. In addition, plasma sintering offers the opportunity for restrained grain growth and for tailoring heat transfer during the sintering process, which may result in desirable structures and properties of the sintered materials. The essential characteristics of plasma sintering and of any other sintering process are an increase in density and strength of a powder compact on heating.
Plasma sintering is a process that may cover a pressure range from 760 to a few Torr. For pressures below 75 Torr, the plasma may no longer be classified as a thermal plasma because of substantial deviations from local thermodynamic equilibrium.
A ROLE FOR NRL
At this time the NRL does not have any research project directly associated with thermal plasma processing. There is, however, a new project, in the planning stage, in which destruction of waste by a thermal plasma process is considered for applications on naval vessels.
The following suggestions take waste destruction into account in addition to recommendations in the field of PCVD. Although no previous experience exists in the thermal plasma area at NRL, the expertise residing at the laboratory is sufficient to branch out into thermal PCVD.
Thermal Plasma Waste Destruction
The NRL has initiated a new research project on thermal plasma waste destruction, geared toward applications on naval vessels. This seems to be a research project that is high on the priority list of the Navy.
Since there is no direct expertise available at NRL, the Laboratory should take advantage of the already existing knowledge in this particular field within the United States. The available literature on plasma waste destruction is at best “spotty,” and therefore it seems advisable to establish contacts with active research groups at universities and national laboratories to prevent “reinventing of the wheel.” An internship at one of the pertinent research institutes, for example, could be of great benefit in bringing this project up to speed.
Plasma Chemical Vapor Deposition
Over the past several years there have been an increasing number of efforts directed toward producing diamond under thermodynamically metastable conditions, driven by the tremendous application potential because of diamond's unrivaled combination of unusual properties. Numerous processes have been suggested and developed over the past decade.
NRL has become internationally renowned for its pioneering work in the deposition of diamond films in the chemical vapor processing group. Modeling of diamond deposition processes at NRL has been combined with sophisticated diagnostics, and the quality of this work has found worldwide acclaim. As pointed out during the NRL site visit, an expanded laboratory facility is being set up combining all experiments presently located in different rooms. The ion/plasma processing group is also involved in diamond film deposition using ECR plasmas. In addition, there is another group at the NRL that is primarily concerned with diamond deposition using flames, and this group seems to be more interested in the application aspects of diamond films.
Considering the expertise and infrastructure at the NRL, it is believed that all the prerequisites are available for addressing both the knowledge and the technology base of this emerging technology. Since NRL's knowledge base is already in an advanced stage, the Laboratory should devote at least some of its resources to developing the technology base. In this context, the highly interdisciplinary nature of this technology should be stressed, making collaboration of all related groups at the NRL, as well as with industry, desirable.
Another aspect relates to the deposition technologies. The chemical vapor processing group should be able to incorporate and evaluate the potential of other deposition techniques such as, for example, thermal plasma deposition, which is currently not used in its laboratory. It is conceivable that broad-based activities of this group may grow into a national center for diamond science and technology.
Cubic Boron Nitride Films
Cubic boron nitride (cBN) is a superhard material, with properties similar to those of diamond—that is, it is chemically inert, optically transparent, thermally conducting, and electrically insulating. Only diamond exceeds the hardness of cBN, but the latter's inertness against ferrous materials is superior even to that of diamond. As an electronic material, its bandgap is even larger than that of diamond. Cubic boron nitride is considered to be an excellent tool material for machining of hard steels, bimetallics, and other exotic materials; for instance, cBN coatings would be of primary interest for cutting tools.
Besides the previously mentioned similarities to diamond, which is a metastable phase of carbon, cBN is also a metastable phase, in this case of boron nitride, with the hexagonal structure as the stable form. The synthesis of cBN films, however, has proven to be much more difficult than that of diamond films.
Although cBN deposition of thin films is a more challenging task, the similarity of the two materials and the wealth of knowledge and experience with diamond deposition residing at
NRL suggest that the Laboratory should embark on this emerging technology. The potential payback would certainly justify the risk involved.
Another exotic material that made the headlines a few months ago is carbon nitride (C3N4). According to predictions, the hardness of β-C3N4 should exceed that of diamond and other properties should be similar to those of diamond. Recently, scientists at Harvard University succeeded in synthesizing CN films and demonstrated structural evidence for the formation of β-C3N4. There is no question that this material offers exciting prospects for both basic research and engineering applications.
NRL should be in a unique position to pursue work along this line. This may not be feasible immediately, but it should be considered in connection with a long-range plan.