Plasma power can make better powders It may be a truism to say that round powders pack better, but spheroidisation of powder particles is one of the successful commercial applications of induction plasma technology and can play a key role in substantial improvement of powder quality and fluidity... The last decade has seen significant technology transfer from laboratory to industrial scale application of induction plasma processing. Meanwhile, a number of newer subjects for the induction plasma process, such as plasma-particulate interaction, heat and mass transfer, plasma reactor mixing pattern modes, particulate nucleation and growth mechanisms have been widely studied in the laboratory. The successful industrial application of the induction plasma process depends largely on fundamental engineering support. For example, industrial plasma torch design, which allows high power levels of between 50kW and 600 kw and long periods of processing - sometimes as much as three shifts of 8 hours a day. Another example is the powder feeders that convey large quantity of solid precursor (1kg/h to 60 kg/h) with reliable and precise delivery performance. Tekna Plasma Systems, one company bridging the gap between academic laboratory and industry, has developed many induction plasma processes in various industrial applications. The company has promoted and developed powder particle spheroidisation/densification for commercial applications using induction plasma technology. The need for powder spheroidisation occurs in very different industrial fields, from powder metallurgy to thermal spray applications. The most pressing need is for an industrial process to turn agglomerated powders produced by spray drying or sintering techniques and angular powders produced by crushing of the process feed material into spherical form powders. At least one of the following benefits of spheroidisation are sought. Improve powder "flowability". Spheroidisation of particles provides a homogeneous, free-flowing character to the subject powder. This facilitates powder handling and allows precise control of powder feed rates in a wide range of applications, including powder metallurgy and in various thermal spray processes. Hall flow test results have demonstrated that a material, which initially has poor flow-ability, could have its Hall flow time reduced by half as a result of plasma spheroidisation. Increase powder packing density. Spherical particles provide denser packing of powders, increasing overall bulk tap density. Eliminate particle internal cavities and fractures. The melting of individual powder particles offers the means for eliminating the internal porosity of individual particles, consequently increasing particle hardness and overall powder bulk density. Change particle surface morphology. The macroscopic surface is made smoother. This effect benefits applications requiring lower inter-particle friction coefficients and low material contamination during pneumatic gas or other means of transport. Enhance powder purity. The melting process can also be favourably used to enhance powder purity through the reactive vapourisation of impurities. Through proper control of plasma medium chemistry, induction plasma melting can provide significant increases in the purity of initial powder materials, by a factor of 10 to 100, lowering impurities to the ppm range or less. Spherical powders are also ideal for injection moulding work, as well as applications for thermal spray coating or the forming of near net-shape parts. For instance, in the field of thermal spraying, the quality of coatings (density, microstructure, etc.) can be significantly improved by the use of spherical, dense powder particles as the starting material. Metal injection moulding (MIM) applications can benefit from spherical powder's use through the improvement in flowability of the material. 16 MPR May 2004 0026-0657/04 2004 Elsevier Ltd. All rights reserved.
Powder Plasma gas RF electrical supply (MHz) Magnetic coupling Figure 1: Principle of operation of the induction plasma torch Since its incorporation in 1990, in Sherbrooke, Québec Canada, Tekna Plasma Systems has been recognised as a world leader in induction plasma technology development, with sales in Europe, Asia and North America. Tekna's core technology combines the latest laboratory research with modern industrial processing and technology. How it works Currently, Tekna is specialised in the design, development and manufacture of "turn-key" plasma systems for a wide range of material processing and surface treatment applications, such as powder spheroidisation, nanopowder synthesis, near net-shape form deposition and plasma coatings. The company offers "turnkey" plasma systems that can be tailored to specific customer needs. Plasma is usually referred to as the fourth state of matter. The notion is based on the fact that if sufficient energy is supplied, solids can be melted to liquids, liquids can be vapourised to gases, and gases are then ionised to form a plasma. Plasmas are partially ionised gases, containing ions, electrons, atoms and molecules, all in local electrical neutrality. The overall temperature of a thermal plasma is typically around 10,000 o K or higher. Thermal plasmas can be generated at atmospheric pressure or under soft vacuum conditions for a wide range of gases, providing an inert, oxidising or reducing atmosphere for the needs of materials processing. Typical examples of thermal plasmas include various forms of DC arcs and high frequency induction plasma discharges. Induction plasmas are generated through electromagnetic coupling of the input electrical energy into the discharge medium. As schematically represented in Figure 1, when an AC current of radio frequency (RF) type passes through a suitable coil, the oscillating magnetic field thereby generated will couple to a partially ionised gas load flowing within the discharge cavity, providing for its ohmic heating in order to sustain the plasma. The plasma so generated is called an Figure 2: Induction plasma generated by high frequency discharge metal-powder.net May 2004 MPR 17
inductively coupled plasma or an induction plasma. Figure 2 shows an atmospheric pressure, air induction plasma jet at 100 kw. Note the size of the plasma jet in comparison to the shielded operator standing on the right hand side of the plasma chamber. Induction plasmas are particularly suited to powder spheroidisation processes because of their large volume, high purity, axial powders feeding and long particle residence time within the discharge. Flexible environment Induction plasma also provides a flexible environment for chemical synthesis under reducing, oxidising, corrosive or neutral/inert atmospheres. The induction plasma powder spheroidisation process, as shown schematically in Figure 3, consists basically of the inflight heating and melting of individual particles of the powder feed material. The latter could be constituted from sintered or crushed solids. The molten spherical droplets are gradually cooled under "free fall" projection conditions. Depending on the particle size and apparent density of the treated powder, the time of flight is controlled such that the molten droplets have sufficient time for complete solidification before reaching the base of the primary reactor chamber. Finer particles, still entrained in the plasma gases, are recovered downstream of the primary reactor chamber by means of a cyclone and filter collector arrangements. The basic phenomena involved in the in-flight heating of individual particles, as schematically represented in Figure 4, are those of; conductive and convective heat transfer from the plasma to the surface of the particle, and radiation heat losses from the particle surface and the vapour cloud surrounding it. Because of the very rapid increase in radiation energy losses from the surface of the particles to the surroundings, with increases of particle temperature and diameter, the heating and melting of particle becomes increasingly more difficult for the higher melting temperature materials and particles of larger size. The diagram shown in Figure 5 provides some guidance to the plasma temperature needed for melting particles of various melting point materials, and at different particle diameters. The calculations are based on an energy balance between conductive heat transfer between the plasma and the surface of the particle and radiation energy loss from the surface of the particle. The plasma gas composition is assumed in this case to be a mixture of Argon and Hydrogen, maintained at atmospheric pressure. It may be particularly noted that; for the very refractory metals such as Molybdenum and Tungsten, the plasma temperature needs to be considerably greater than the normal melting temperature of the material before a 100 or 200 µm particle is able to be successfully Central gas Sheath gas Powder + Carrier gas PL-50 induction plasma torch ~ Feed powder Cyclone collector Filter Treated powder (course) Reactor bottom Treated powder (fines) Figure 3: Schematic of the process of spheroidization by induction plasma technology. 18 MPR May 2004 metal-powder.net
Powder + carrier gas In-flight particle melting Heat from Plasma Radiation heat loss Vapourisation heat loss Figure 4: Schematic representation of the basic phenomena involved in the in-flight heating of individual particles spheroidised through in-flight melting conducted in a plasma. The graph also underlines the uniqueness of the plasma process since there are presently no other heat sources available to reach such temperatures. The inert gas atomiser and combustion-based technology are efficient technologies for obtaining spherical powders of the lower melting point materials such as zinc, aluminum, tin and copper metals and alloys. Cemented Alloy Powders harder than most steels, has greater mechanical strength, transfers heat quickly and resists much wear and abrasion. The service life of many kinds of machinery parts can be greatly prolonged by the coating of wearprone surfaces with this cemented alloy. It already has wide applications in the construction, pulp and paper industries, and in coal mining, cement production, rock crushing and the agricultural industries. Spheroidised cemented alloy powders significantly increase the qualities of cemented coating layers by overcoming the notorious "corner effect problem" associated with the use of angularly shaped WC For the higher melting point materials such as the refractory metals and ceramics, thermal plasmas offer a unique tool for the densification and spheroidising of these materials in powder form. A great variety of "refractory" metals / metal alloys and ceramics have now been successfully spheroidised / densified, using Tekna's integrated plasma systems. Table 1 presents a partial list of typical materials which can be spheroidised on a commercial scale. "Cast" tungsten carbide is a powder material made from WC-W 2 C alloy. It is Figure 5: Equilibrium particle temperatures as a function of material, size and the plasma temperature, at 1 atmosphere pressure of Ar/H 2 plasma, with additional 10% vol concentration of H 2 metal-powder.net May 2004 MPR 19
Table 1: Some typical powder materials which can be spheroidised by means of Tekna's integrated plasma systems. Powder category Powder name Ceramics Oxide SiO 2, ZrO 2, YSZ, Al 2 O 3 Al 2 TiO 5, glass Pure metals Alloys Non-oxide WC, WC-Co Re, Ta, Mo, W, Ni, Cu powder particles, incorporated into a hard Ni-Cr or Co matrix. The materials hardness of these cemented alloy components can be considerably increased. The existence of the commonly experienced microcracks, pores, defects, etc., found in the "angular" cemented powders, derived from the WC powder's process of manufacture, can be reduced considerably, as seen in Figure. 6. Cr/Fe/C, Re/Mo, Re/W plasma can contribute to the reduction in the original oxygen content of the precursor metallic material. It is to be noted that, in contrast with the DC plasma technologies, there is no electrode contamination associated with induction plasma processing. This is a strong attraction for the potential end user seeking supply of higher purity refractory metal products. The refractory material powder spheroidisation process will also increase the bulk density and improve the "fluidity" of flaky powders such that subsequent manufacturing operations undertaken with these materials becomes either easier or indeed feasible at all, especially for the processes of thermal spray forming and (MIM). High purity plasma spheroids Tantalum powder is another example of a refractory material which greatly benefits from the enhanced flow-ability, uniformity and lowered oxygen content of the plasma-processed powder for such applications as tantalum capacitors, now widely used in small portable electronic components and laptop computers, video cameras, games consoles and mobile phones. Figure 7 shows some Rhenium powder, before and after application of the induction plasma spheroidisation processing. Refractory metals In modern "high-tech" materials application fields, some of the refractory metals play key roles. The common feature of all refractory metals is their high melting point and their sensitivity to oxygen at high temperatures. Typical melting points of the refractory metals range from 2800K to 3800K. Induction plasma spheroidisation is the technology of preference for achieving the production of highly spherical and dense, 1-100 µm size refractory metal powders. Their processing with Ar-H2 Figure 7: Flaky, interlocking powder particles of Rhenium become dense, separate and spherical after treatment by induction plasma spheroidization processing. Figure 6: Cross section of a WC powder, treated by induction plasma and showing its dense microstructure. Figure 8 : Molybdenum powder at 50 µm, treated by induction plasma 20 MPR May 2004 metal-powder.net
Figure 9: Tungsten powder at 50 µm, treated by the induction plasma spheroidization process via plasma processing is very often required for special applications of these powders. Silica powder (Figure 10) is also of major interest for use in high purity SiO 2 material applications in the semiconductor industry, which requires a dense and spherical powder. In recent years Tekna has provided its clients with a number of integrated units for the powder spheroidisation operations on a commercial scale (from 50 to 400 kw). Special features of these industrial scale systems for induction plasma processing have included their reliability, their ease of operation, automatic control and real-time data acquisition. Figure 11 illustrates a 400 kw induction plasma spheroidisation system, installed and available at Tekna facilities for demonstration purposes and for the company's toll processing service. Unique designs Figure 10: SiO2 powder spheroidized by air plasma. The spheroidisation efficiency is 100 per cent despite the very high melting point of rhenium (3180 C). Figures 8 and 9 show corresponding micrographs for Molybdenum and Tungsten powders, which were also induction-plasma spheroidised in a reducing atmosphere. Figure 11: 400kW Industrial powder spheroidization installation available at Tekna Plasma Systems inc. for demonstration purposes and for toll operations. This system produces spheroidized powders at production rates of 20-40 kg/h or more, depending on the nature of the powder processed and the degree of spheroidization required. The challenges for achieving success in the induction plasma spheroidisation of oxide ceramics are mostly related to the poor thermal conductivities and the relatively high melting points of these materials. Induction plasma technology provides for the long plasma residence times needed to melt these materials. The homogeneous temperature profile generated within the plasma can also limit the surface vapourisation, often a source of fume generation due to the partial vapourisation of the processed material. Oxide ceramics are efficiently treated under an oxidising atmosphere, using either Air or Oxygen as the plasma gas. As the mainstays of the oxide ceramics materials market, Al 2 O 3 and ZrO 2 powders are both widely used as structural materials. Lately, demand for spheroidised oxide ceramic powders has been increasing. Although some of these powders are "spray-formed" and thus originally assume the spherical shape, as "synthesised" by the spray-drying process, powder densification In terms of industrial engineering development, Tekna has integrated many unique design features into its systems in order to lower operational costs. These include the use of high-energy efficiency, solid-state power supplies and the partial (up to 90 per cent) plasma gases recycling. The high degree of automation achieved also makes it possible for a minimum number of operators to supervise several production-scale industrial units. Tekna offers an on site treatment service to demonstrate the technology on processing of smaller and larger quantities and to help the service "end user" introduce new and innovative materials to the market. The flexibility of this process makes it possible to treat nearly any material under a wide range of conditions. The spheroidisation of powder materials is an important application of induction plasma and a solution for the challenging and demanding requirements of advanced materials. The author Dr Maher Boulos, the author of Powder Densification and Spheroidization Using Induction Plasma Technology, works for Tekna Plasma Systems Inc., Sherbrooke, Quebec, Canada. metal-powder.net May 2004 MPR 21