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Title Comparison of thermal and microwave-assisted plasma sintering of nickel-diamond
composites
Authors(s) Twomey, Barry; Breen, Aidan; Byrne, Greg; Hynes, Alan; Dowling, Denis P.
Publication date 2010-09-01
Publication information Powder Metallurgy, 53 (3): 188-190
Publisher Maney
Item record/more information http://hdl.handle.net/10197/5268
Publisher's version (DOI) 10.1179/174329010X12820493130451
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Comparison of thermal and microwave-assisted plasma
sintering of nickel-diamond composites
Barry Twomey, Aidan Breen, Greg Byrne, Alan Hynes and Denis P. Dowling*
School of Electrical, Electronic and Mechanical Engineering, University College
Dublin, Belfield, Dublin 4, Ireland
*(denis.dowling@ucd.ie)
Abstract
There is considerable interest in processing technologies which can lead to more
energy efficient sintering of metal powders. Microwave sintering has recently been
shown to reduce energy usage as the volumetric heating process is considerably more
efficient than resistance heating. RF plasma sintering meanwhile has been shown to
deliver heat via uniform excitation of the processing gas resulting in ion bombardment
of the workpiece. In this study the use of a rapid, novel microwave-assisted plasma
sintering (MaPS) technology for processing of nickel-diamond metal matrix
composites is evaluated. Nickel powder and polycrystalline diamond were mixed to
prepare 20 mm discs under uniaxial compaction pressures of 100, 200 and 300 MPa.
The discs were fired in a low pressure microwave plasma under a hydrogen
atmosphere. For comparison, discs were also sintered using conventional tube furnace
firing. The MaPS sintering is very rapid with full disc strength of >1000N, based on
3-point bend tests, being achieved within 10 minutes compared with 8 hours for
furnace treatment. This study demonstrates that the microwave-assisted plasma
sintered discs produced similar or superior performance to discs fired using furnace
firing conditions but with sintering cycle time reduced by up to 95%.
Introduction
Plasma sintering has been demonstrated as a rapid and uniform sintering technique
since its first reported studies by Bennett et al. [1]. Sintering has been carried out in
plasmas generated using microwave, rf or DC power supplies [1-3] and the dominant
heating mechanism is associated with ion and fast neutral bombardment. Diatomic
gases such as hydrogen or nitrogen are typically used for plasma sintering. Plasma
sintering is carried out under vacuum although unlike spark plasma sintering (SPS),
there is no external force exerted on the workpiece [4]. There have also been a large
number of reports in the literature on the microwave interaction with materials
including the type of processing microwave heating can offer [5, 6]. The ability to
penetrate the surface of the workpiece enables rapid volumetric heating in microwave
processing, reducing the need for external heat sources [6]. Advantages of non-plasma
microwave processing over furnace treatment include finer grain sizes, rounded
porosity and higher ductility and toughness [7]. Increased shrinkage rates and
decreased grain sizes can be achieved with microwave and plasma processing
compared with furnace sintering [1, 8, 9]. With these advantages and significantly
reduced cycle times, microwave-assisted plasma sintering (MaPS) offers an
alternative route to production and reduced energy consumption per process.
This study compares, for the first time, furnace and microwave assisted plasma
sintering of diamond metal matrix composites (MMCs). Nickel powder was chosen as
the test material as it is used in a wide range of engineering components due to its
corrosion resistance, wear resistance, mechanical strength, thermal expansion,
electrical conductivity and magnetic permeability [10]. Nickel powder is also used as
a metal binder for many bonding applications in diamond and carbide machining tools
[11].
Experimental Methods
Nickel-diamond MMCs were pressed in a uniaxial 20 mm diameter die at pressures of
100, 200 and 300 MPa (average green densities were 52, 58 and 62 % respectively).
INCO nickel (T110) and nickel coated diamond (6-12 µm) from Element Six were
mixed by weights of 80 and 20 % respectively in a T2F-Turbula for 100 minutes prior
to pressing. Density was determined using the Archimedes principle in mercury and
confirmed by measuring volume based on sample dimensions.
The microwave assisted plasma sintering process was carried out using a
Circumferencial Antenna Plasma (CAP) microwave system described in more detail
elsewhere [12]. The plasma was formed at a pressure of 20 mbar in a hydrogen
atmosphere. Three disc samples were rotated in a plasma located at the centre of the
chamber per run (see Figure 1). Input powers of 2.4 kW were supplied from a Mugge
microwave power supply operating at 2.45 GHz. Sample temperatures were measured
using a LASCON QP003 two-colour pyrometer from Dr. Merganthaler GmBH & Co.
The use of a two-colour pyrometer is expected to eliminate the interference effect of
the plasma on the emissivity of the sample. Typical heating and cooling rates of 7
o
C/s
were observed in the microwave plasma fired samples. Emission spectroscopy was
also carried out on the plasma using an Ocean Optics USB4000 spectrometer.
Measurements were carried out to investigate changes in the electron density and total
light emission of the plasma with increasing applied power. Furnace sintered samples
were treated in tube furnace at 850
o
C in a flowing argon atmosphere. Heating rates of
4
o
C/min were used with the same dwell time of 10 minutes at the maximum
temperature. The cooling rate was typically 2-3
o
C/min.
Figure 1: Samples (two visible) being sintered in a hydrogen microwave plasma
Flexural stress tests were carried out using a three point bend test. The span between
the bottom pins (11 mm) and the cross sectional area of the fractured samples were
used to calculate approximate flexural stress with an average value of three samples
taken. Hardness testing was carried out using a Rockwell diamond indenter (HR 15 N
scale), with the average value of 12 measurements taken. Rockwell indentation was
chosen as the large indent area (≈ 1 mm in diameter) should limit the effect of
porosity or diamond reinforcement (≤ 20 µm). Pin-on-disc testing was undertaken
using a TEER coatings POD-2 tester, with a 5 mm tungsten carbide ball as the pin, to
evaluate the wear and tribological behaviour of the sintered samples [13]. The pin was
loaded against the treated samples at 10 N at a linear speed of 5 cm/s for 1200
revolutions.
Results and discussion
Sintering conditions
Nickel-diamond samples pressed at 100, 200 and 300 MPa were sintered at 2.4 kW
applied power in the microwave assisted hydrogen plasma. Following a two minute
ramp up to an applied power of 2.4 kW, a maximum firing temperature of 850
o
C was
achieved. Samples were sintered in sets of three with a 10 minute dwell time. A total
cycle time of 20 minutes including pump down, firing and cooling was required. An
increase in applied power resulted in an increase in plasma emission intensity (Figure
2). The peak shift and full width half maximum (FWHM) of the H
α
and H
β
, at 656.3
and 486.1 nm respectively, indicate the electron density in the plasma [14]. As the
applied power increases from 1.2 to 2.4 kW, no increase in the normalised FWHM
was observed and the ratio of the two peaks (H
α
/ H
β
) remained almost constant at ≈
5.3. This indicates there is little or no increase in electron density with increasing
applied power although the overall emission doubled as a result of a volume increase
of the plasma. Furnace sintered samples were treated in a tube furnace at 850
o
C in
batches of three in a flowing argon atmosphere. A total cycle time of over 8 hours,
including heat up and cool down times, was observed.
Figure 2: Emission intensity observed in hydrogen plasma
Mechanical behaviour of sintered MMCs
Density measurements were carried out using the Archimedes principle and also
calculated based on the sample dimensions. As illustrated in Figure 3, there is little
difference in sample density from the two sintering techniques despite a significant
difference in total treatment times (20 minutes versus 8 hours). Sintering at these
temperatures resulted in a 15 % density increase for both plasma and furnace treated
samples.
Figure 3: Density of pressed and sintered samples
Flexural stress values (see Figure 4) were determined using a three-point bend test.
Due to the similarity in the cross-sectional area and densities achieved between the
both sintering techniques, it was not unexpected that there was little difference
observed between the two techniques. Samples pressed at higher pressures (300 MPa)
performed better when microwave plasma treated and samples pressed at lower
pressures (100 and 200 MPa) performing better when furnace treated.
As shown in Figure 4 however, microwave plasma fired samples exhibited
significantly better hardness (≥34 %) then the furnace fired samples. As previously
reported for microwave treated samples versus furnace treated samples, thermal
equilibrium occurs almost instantaneously enabling sintering to occur in minutes [15].
Figure 4: Flexural stress (left) and hardness values (right) of microwave assisted
plasma and furnace sintered samples pressed at 100, 200 and 300 MPa
Examining the cross-sectional area of the nickel-diamond composites (Figure 5), it
can be seen that the MaPS treated samples exhibit a much finer microstructure with
increased nickel bonding between diamond particles (black areas) rather then
increased flow into larger masses. The formation of larger masses of nickel may lead
to larger pores in the furnace treated samples resulting in reduced hardness. This is
most likely due to the heating rate of ≈ 420
o
C/min versus 4
o
C/min for MaPS over
furnace treatment. These heating and equivalent cooling rates can be achieved as the
high energy plasma discharge is isolated from the colder surroundings (≈ 30
o
C) of the
discharge chamber. The finer microstructure of the MaPS treated samples may
explain the increased hardness values observed [7].
