An overview is presented of the plasma processing of materials. The principal components of this overview include the definition and classification of plasmas of interest in materials processing, the methods of plasma generation and the basic engineering principles that govern plasma phenomena. Here emphasis is placed on both plasma theory and on the experimental verification of the models. This is followed by a brief description of principal plasma applications, including thermal plasmas used in melting, refining, plasma deposition and plasma synthesis and low pressure plasmas used in synthesis and in the processing of electronic materials.

The review is concluded by a brief discussion of future prospects for plasma technology, where the main developments envisioned are in the area of improved coatings, the processing of electronic materials, melting and refining of refractory metals and superalloys and finally the possibility of more widespread use for plasma systems in extractive metallurgy.

In general, a plasma consists of a mixture of electrons, ions,and neutral species. Although there are free electric charges in a plasma, negative and positive charges compensate each other, i.e. overall a plasma is electrically neutral, a property which is known as quasi-neutrality. In contrast to an ordinary gas, the free electric charges in a plasma give rise to high electrical conductivities which may even surpass those of metals. A hydrogen plasma, for example, at atmospheric pressure heated to temperatures of 106 K, has the same electrical conductivity as copper at room temperature. As the plasma temperature increases, the electrical conductivity increases beyond that of copper. Plasma temperatures of the order of 106 K and above are typical for thermonuclear fusion experiments.

A number of diagnostics problems in the low temperature plasma generators have been analysed with special attention to the plasma spraying process, and a general approach to these problems has been proposed. The advantages, difficulties and limitations of optical nonintrusive methods have been discussed. Examples based on authors experience during last few years, are given.

A review is made of some of the mathematical modelling work carried out over the last few years with the objective of computing the flow and temperature fields for different thermal plasma systems. Typical results are given for d.c. plasma jets, transfered and non-transfered arc plasma reactors and for inductively coupled plasma torches. This is followed by a brief discussion of the important problem of plasma-particle heat transfer.

An overview is presented on several aspects of thermal plasma melting/remelting technology. Included is a brief description of the fundamentals of thermal plasma, basic types of plasma reactors, examples of unique configurations, highlights of some commercial applications, and recent state-of-the-art advances.

It is becoming apparent that plasma technology will revolutionize certain of the conventional operations of extractive metallurgy. The use of plasma arcs in the iron and steel industry is already being demonstrated at powers up to 100 MW and these development will be briefly reviewed. In this connection, the advantages of plasma furnaces over electric furnaces are beginning to be well documented.

It is, however, in the area of the production of the refractory metals, such as molybdenum, zirconium, titanium, niobium, tungsten, tantalum, chromium and vanadium, that plasma technology offers unique opportunities as a result of advances in the design of transferred arc reactors. The work of the authors in this particular field will be described as well as other recent industrial developments.

Rapid solidification processes (RSP) have been reviewed. The structural refinement one may obtain via RSP and the consolidation methods which need to be utilized have also been reviewed. The merits of low pressure plasma deposition (LPPD) or rapid solidification by plasma deposition (RSPD) as a means of combining both the atomization and consolidation steps of RSP is highlighted. The applications and challenges of RSPD are discussed.

The state of the art and the research needs of selected applications of arc technology are described. The selected applications include plasma torch design, plasma synthesis of materials, plasma chemical synthesis and metallurgical processing. The research areas where results can have the broadest impact on the technology are electrode effects, plasma fluid dynamics and non-equilibrium effects.

Combining the reduction of the mineral oxide to a liquid metal and its rapid solidification provides an energetically favorable route tod provide metastable metallic particles or powders. Such an option is available with the Sustained Shockwave Plasma (SSP) reactor in which the mineral oxide is reduced using a carbon-based reductant within the plasma medium and subsequently rapidly solidified. This paper examines the degree of metastability of these metal particles using optical, electron and Auger microscopy and discusses the potential of this processing route.

Rapid heating of coal to temperatures in excess of 1500 K yields lower hydrocarbons which, if conditions are correct, will consist mainly of acetylene. Such conditions involve reaction times of 0.1 to 10 ms, together with rapid rates of quenching. Appropriate conditions have been achieved on a laboratory scale using a d.c.-arc plasma jet.

A unique and efficient plasma jet reactor has been developed and used to study the high temperature production of carbon monoxide from a reaction between powdered carbon and a pure carbon dioxide plasma. The plasma jet reactor was designed to allow the injection of powdered carbon above the arc discharge region rather than into the plasma flame below the arc discharge region. High yields of carbon monoxide, produced at relatively high efficiencies, were a direct result of this technique. The plasma jet was also designed to enable rapid changing and testing of various anode inserts

Average yields of carbon monoxide in the product gases were as high as 80–87% in selected experimental trials. Carbon monoxide was produced at rates exceeding 15,000 1/hr (at STP) with a power expenditure of 52 Kw.

In the process of r.f. plasma sintering of Al2O3 at reduced pressure, a number of puzzling observations have been reported. They include higher heating rates and higher maximum temperature of a sample at increased propagation velocities of the sample through the plasma and a sudden temperature drop of approximately 700°C if the sample motion in the plasma is stopped. An analysis of the sintering process indicates that the change of che surface area and of the catalytic properties of the sample surface during sintering seem to be responsible for these observations. The change from an initially negatively charged co an essentially non-charged surface causes a drastic decrease of the heat transfer from the plasma to the sample. A simulation of the plasma sintering process based on a r.f. argon plasma provides a semi-quancicative explanation of all the observed phenomena.

A new plasma reactor system based on a stable expanded conical plasma reaction zone has been used to examine the in-flight extraction of metals from mineral concentrates. The basic principles of the plasma system and details of the equipment are described. Results obtained from the treatment of taconite concentrate, low grade chromite concentrate and a copper-nickel concentrate are presented and discussed.

A novel process is described for making rapidly solidified powders. In its simplest form, powders are melted in a high-temperature plasma and the resulting high-velocity spray is quenched into a fluid medium. Rapid cooling rates are achieved by free fall in inert gases, by quenching into liquid argon or nitrogen, or by impacting against a chilled, moving substrate. Any composition can be processed as long as it or its constituents are available in powder form. When necessary, alloy or composite components are agglomerated to 15–45μm diameter prior to plasma synthesis. The agglomeration is typically achieved by spray drying. Powders with melting points up to and including tungsten have been melted.

Plasma sprayed deposits of alumina are normally of the metastable gamma phase, which transforms to alpha upon heating to an elevated temperature. The poor thermal conductivity of gamma phase is improved by transforming to alpha, but the improvement is restricted by the fragmented nature of the structure, a consequence of alpha being a denser phase than gamma.

Alpha phase has been reported to form when deposits are made on preheated substrates. It was found that reducing the distance between gun and deposition surface, thereby raising the temperature of the latter, resulted in transforming gamma to alpha, and as the temperature continued to rise, alpha formed directly from the liquid. The thermal conductivity of alpha formed directly from the liquid is 0.27 Watt/cm K at 323 K, which approaches the thermal conductivity given for 99.5% pure, 98% dense, polycrystalline alpha alumina.

We report some measurements of unique physical properties that arise in plasma-sprayed alumina. It is suggested their origin is the O- species formed on the surface of the particles during the fabrication process.

Low pressure plasma deposition (LPPD), a recent advancement in plasma spray metallizing, is currently being developed for high performance materials applications. An experimental study of particle melting within the plasma jet was pursued to identify the effect of the process variables and the material properties on the resultant deposit. In tandem the experimental results have been utilized in the development and verification of a mathematical model for the melting of powder particles during the process.

Two binary iron based model alloys - Fe-20 wt% Mn and Fe-20 wt% Cu - were plasma sprayed using Ma 2.4 and Ma 3 guns. Two different methods were used to evaluate the degree of particle melting within the plasma jet. The first method intercepts the particle path in the plasma jet with a glass slide, whereas in the second method the powder particles are collected in free flight using a powder collector (resolidification of powder particles occurs before they impact the collector walls). The droplets which impacted on glass slides and the collected (using the powder collector) powder particles were studied for mode and degree of powder particle melting using scanning electron microscopy. “Sweet spot” deposits (with no relative motion between the plasma gun and the substrate) were also made for the two model alloys in different size ranges using the Ma 2.4 and Ma 3 guns. The resulting deposits were metallographically evaluated. Mode and degree of particle melting injected under different process variables as well as the resultant deposit structures are presented.

A numerical model has been developed to predict the temperature profile of particles injected in a D.C. plasma jet. The equations governing particle melting were applied to spherical powders of binary model alloys. Thermophysical properties of the gas and the powder material have been taken to be temperature dependent. In the proposed model, the latent heat of melting was taken into account by introducing apparent enthalpy as a function of the fraction of liquid formed which can be derived from equations describing non-equilibrium melting. The temperature and velocity profiles of the plasma jet used in this analysis are for a free jet (without target interference) and were calculated using the parabolic Navier-Stokes equation with a K-E turbulence model. Correction factors have been introduced to take into account non-continuum effects encountered in the low pressure environment and the results show that both heat and momentum transfer between the plasma gas and the injected particles are reduced.

A systematic study was made of the characteristics of a microprocessor controlled powder feeder in view of its utilization for the deposition of graded coating. The unit tested had the capability of feeding three different powders at rates which could be continuously varied up to about 40 g/min. MgOZrO2. was deposited on a stainless steel substrate using a series of thin (Ni/Cr)-Zirconia coats as intermediate layers. The results showed that such a technique could provide a valuable means of achieving a precise and effective control on the composition of the intermediate layers which can have important effects on the final quality of the deposit.

Powder Metallurgy (P/M) technology has seen an important series of advances in the past twenty-five or so years. It has progressed from a press + sinter methodology (which still has important uses) to one of being a complete processing technology where, ultimately, the structure and properties of totally new materials can be synthesized. A key step in this evolution was the application of P/M techniques to high performance materials such as nickel-based superalloys. This effort began in the early 1960's and has resulted in several important commercial applications such as P/M gas turbine disks and shafts.Attention has now turned to the potential use of Rapid Solidification Technology (RST) as a means of furthering the span of materials which can be made available for a wide variety of engineering applications. Rapidly solidified nickel, cobalt, titanium, aluminum and copper based materials are all the subject of extensive research and development at the present time (1).