Special feature

Plasma powercan makebetterpowders

The last decade has seen signifi-

cant technology transfer from

laboratory to industrial scale

application of induction plas-

ma processing. Meanwhile, a number of

newer subjects for the induction plasma

process, such as plasma-particulate inter-

action, 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 sup-

port. For example, industrial plasma torch

design, which allows high power levels of

between 50kW and 600 kW and long peri-

ods 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 pre-

cursor (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 spheroidi-

sation/densification for commercial

applications using induction plasma

technology. The need for powder spher-

oidisation occurs in very different indus-

trial 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 crush-

ing 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 cavitiesand 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 applica-

tions requiring lower inter-particle fric-

tion coefficients and low material con-

tamination 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 increas-

es in the purity of initial powder mate-

rials, 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 appli-

cations 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) applica-

tions can benefit from spherical powder's

use through the improvement in flowabili-

ty of the material.

It may be a truism to say that round powderspack better, but spheroidisation of powder particles is one of the successful commercialapplications of induction plasma technologyand can play a key role in substantial improvement of powder quality and fluidity...

16 MPR May 2004 0026-0657/04 ©2004 Elsevier Ltd. All rights reserved.

metal-powder.net May 2004 MPR 17

Special feature

Since its incorporation in 1990, in

Sherbrooke, Québec Canada, Tekna

Plasma Systems has been recognised as a

world leader in induction plasma technol-

ogy development, with sales in Europe,

Asia and North America. Tekna's core

technology combines the latest laborato-

ry research with modern industrial pro-

cessing 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 plas-

ma coatings. The company offers "turn-

key" plasma systems that can be tailored

to specific customer needs. Plasma is usu-

ally referred to as the fourth state of mat-

ter. 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 tem-

perature of a thermal plasma is typically

around 10,000oK or higher. Thermal plas-

mas 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 suit-

able coil, the oscillating magnetic field

thereby generated will couple to a partial-

ly 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

Powder

Plasma gas

RF electricalsupply (MHz)

Magneticcoupling

Figure 1: Principle of operation of the induction plasma torch

Figure 2: Induction plasma generated byhigh frequency discharge

inductively coupled plasma or an induc-

tion plasma. Figure 2 shows an atmos-

pheric 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 suit-

ed 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 flexi-

ble environment for chemical synthesis

under reducing, oxidising, corrosive or

neutral/inert atmospheres.

The induction plasma powder spher-

oidisation process, as shown schematically

in Figure 3, consists basically of the in-

flight 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 con-

trolled 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 cham-

ber by means of a cyclone and filter collec-

tor 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 par-

ticle becomes increasingly more difficult

for the higher melting temperature mate-

rials and particles of larger size. The dia-

gram shown in Figure 5 provides some

guidance to the plasma temperature need-

ed 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 ener-

gy 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 plas-

ma temperature needs to be considerably

greater than the normal melting tempera-

ture of the material before a 100 or 200

µm particle is able to be successfully

18 MPR May 2004 metal-powder.net

Special feature

Feed powder

Filter

Treated powder (fines)Reactor bottomTreated powder(course)

PL-50inductionplasma torch

Central gas

Sheath gasPowder +Carrier gas

~

Cyclonecollector

Figure 3: Schematic of the process of spheroidization by induction plasma technology.

metal-powder.net May 2004 MPR 19

Special feature

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 combus-

tion-based technology are efficient tech-

nologies for obtaining spherical powders

of the lower melting point materials such

as zinc, aluminum, tin and copper metals

and alloys.

Cemented Alloy Powders

For the higher melting point materials

such as the refractory metals and ceram-

ics, 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 commer-

cial scale.

"Cast" tungsten carbide is a powder

material made from WC-W2C alloy. It is

harder than most steels, has greater mech-

anical 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 wear-

prone 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" associat-

ed with the use of angularly shaped WC

Heat fromPlasma

Powder +carrier gas

Radiation heat loss

Vapourisationheat loss

In-flightparticlemelting

Figure 4: Schematic representation of the basic phenomena involved in the in-flight heating of individual particles

Figure 5: Equilibrium particle temperatures as a function of material, size and the plasma temperature, at 1 atmosphere pressure of Ar/H2 plasma, with additional 10% vol concentration of H2

powder particles, incorporated into a hard

Ni-Cr or Co matrix. The materials hard-

ness of these cemented alloy components

can be considerably increased. The exis-

tence of the commonly experienced micro-

cracks, 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.

Refractory metals

In modern "high-tech" materials

application fields, some of the refract-

ory metals play key roles. The common

feature of all refractory metals is their

high melting point and their sensitivity

to oxygen at high temperatures. Typ-

ical 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

plasma can contribute to the reduction in

the original oxygen content of the precur-

sor metallic material. It is to be noted

that, in contrast with the DC plasma

technologies, there is no electrode conta-

mination associated with induction

plasma processing. This is a strong

attraction for the potential end user seek-

ing supply of higher purity refractory

metal products.

The refractory material powder spher-

oidisation process will also increase the

bulk density and improve the "fluidity" of

flaky powders such that subsequent manu-

facturing 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 induc-

tion plasma spheroidisation processing.

20 MPR May 2004 metal-powder.net

Special feature

Before After

Figure 7: Flaky, interlocking powder particles of Rhenium become dense, separate andspherical after treatment by induction plasma spheroidization processing.

Before After

Figure 8 : Molybdenum powder at 50 µm, treated by induction plasma

Table 1: Some typical powder materials which can be spheroidised by means of

Tekna's integrated plasma systems.

Powder category Powder name

Ceramics Oxide SiO2, ZrO2, YSZ, Al2O3 Al2TiO5, glass

Non-oxide WC, WC-Co

Pure metals Re, Ta , Mo, W, Ni, Cu

Alloys Cr/Fe/C, Re/Mo, Re/W

Figure 6: Cross section of a WC powder, treated by induction plasma and showing itsdense microstructure.

metal-powder.net May 2004 MPR 21

Special feature

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 Molyb-

denum and Tungsten powders, which were

also induction-plasma spheroidised in a

reducing atmosphere.

The challenges for achieving success in

the induction plasma spheroidisation of

oxide ceramics are mostly related to the

poor thermal conductivities and the rela-

tively high melting points of these materi-

als. 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 vapouri-

sation, 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, Al2O3 and ZrO2 pow-

ders 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

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 SiO2

material applications in the semiconductor

industry, which requires a dense and spher-

ical 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 sys-

tem, installed and available at Tekna facili-

ties for demonstration purposes and for the

company's toll processing service.

Unique designs

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 num-

ber of operators to supervise several pro-

duction-scale industrial units.

Tekna offers an on site treatment ser-

vice to demonstrate the technology on pro-

cessing of smaller and larger quantities

and to help the service "end user" intro-

duce new and innovative materials to the

market. The flexibility of this process

makes it possible to treat nearly any mate-

rial under a wide range of conditions. The

spheroidisation of powder materials is an

important application of induction plas-

ma and a solution for the challenging and

demanding requirements of advanced

materials.

Before After

Figure 9: Tungsten powder at 50 µm, treated by the induction plasma spheroidizationprocess

Before After

Figure 10: SiO2 powder spheroidized by air plasma.

Figure 11: 400kW Industrial powder spher-oidization installation available at TeknaPlasma Systems inc. for demonstration pur-poses and for toll operations. This systemproduces spheroidized powders at productionrates of 20 - 40 kg/h or more, depending onthe nature of the powder processed and thedegree of spheroidization required.

Dr Maher Boulos, the author ofPowder Densification andSpheroidization Using InductionPlasma Technology, works for TeknaPlasma Systems Inc., Sherbrooke,Quebec, Canada.

The author