Bulk MAX phases can be fabricated and simultaneously densified by using SPS.
SPS is capable of rapidly producing fully dense ceramics at temperatures up to over 2000 °C by using a spark plasma believed to be momentarily generated in the gaps between the powder particles by electrical discharge at the beginning of a DC pulse. Because plasma formation could not be validated, other terms were used for this technique, such as pulse
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discharge sintering (PDS) or the field-assisted sintering technique (FAST) (Munir et al., 2011).
The SPS process is a solid consolidation sintering process with simultaneous application of a low voltage, high density pulse current and uniaxial pressure. The key benefit of this sintering method include high density ceramics obtained in short times, and at lower sintering temperatures than with conventional sintering and hot pressing. The reported relative densities of SPS synthesized samples are all >99%.
However, the specific sintering mechanisms involved during SPS, either thermally, electrically or mechanically, remain relatively unclear (Munir et al., 2011). It is generally proposed that the dc current in the initial stage of the sintering process generates the spark discharge and rapid heating between the particles. It is believed, but not yet validated that the gas among the powder particles is ionised and transformed into newly-generated plasma. This process promotes elimination of absorbed gases and oxide layers on the surface of the powder particles and activates the sintering process. It has been suggested (Mamedov, 2002) that SPS proceeds through three stages: plasma heating, joule heating and plastic deformation. During plasma heating, electrical discharge between the powder particles gives rise to localized and momentary heating of the particles surfaces up to several thousand ºC. The purified and activated particle surfaces melt and fuse to each other forming “necks” between the particles. During joule heating, the joule heat generated by the DC electrical current increases atom diffusion in the necks regions enhancing grain growth. During plastic deformation, the uniaxial force combined with diffusion result in densification of the powder compact.
A conceptual model, which is useful to illustrate the mechanisms involved during SPS, of the sintering behaviour of porous alumina ceramics (Jayaseelan et al., 2002) has been determined from empirical results (Figure 6). The plateau from 900-1050 °C (Figure 6) corresponds to the rearrangement of particles with initial formation of necks. Necks form because of the geometric amplification of pressure on the interparticle point contacts; but as the neck grows the local pressure at the neck is substantially reduced. However, the pulsed current in SPS can make a major contribution to densification during the sintering process from 1050-1150 °C in Figure 6, and at these temperatures, surface diffusion is dominant resulting in neck growth and a little shrinkage. After the particles have been connected together, grain-boundary diffusion becomes the dominant densification mechanism. The
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highest temperature achieved in the necks provides the highest diffusion rate and thus enhances mass transport towards the neck area. The linear change in shrinkage can be monitored (from 1150-1300 °C in Figure 6) and the heating schedule can be interrupted here to avoid unwanted later stages of sintering (e.g. coarsening).
HP and SPS have been directly compared, particularly on sintering of submicron alumina (Langer et al., 2009). Some of the observations may apply when comparing HP and SPS of MAX phases:
(1) The densification behaviour and characterization of the microstructure revealed that SPS sintered samples reached a higher density compared with HP, in particular with finer starting powders. An increase in dwell time was required to reach the same final density by HP.
(2) Analysis of the sintering curves showed that the densification mechanism for both sintering methods was grain boundary diffusion.
(3) The sintering trajectory showed that the grain size was only dependent on density and was insensitive to the sintering method, in addition to showing a lack of preferential grain orientation.
The basic SPS technique uses pulsed electrical current combined with rapid heating and the application of pressure to achieve rapid sintering. The basic configuration of a SPS system is shown in Figure 7. The powder mixture is directly loaded into a graphite die and pressure is applied via graphite punches.
Zhou et al. (2005) used SPS to fabricate high-purity Ti2AlC samples from an Al-rich
powder mixture of Ti, Al and graphite (2Ti: 1.2Al: C, in molar ratio) heating at 1100 °C and 30 MPa for 1h. Excess Al was found to favour synthesis of Ti2AlC likely due to loss of Al
during the process due to its melting (at approximately 660 ºC) and evaporation. Zou et al. (2008) prepared high-purity polycrystalline Ti3AlC2 by SPS from Ti, Al and TiC powders
(2Ti: 2Al: 3TiC, in molar ratio) heating at 1300 °C and 50 MPa for 15 min. Yang et al. (2009) prepared Ti3AlC2 by mechanical alloying (MA) and SPS from elemental powder mixtures of
Ti, Al and graphite. High-purity bulk Ti3AlC2 (>99 wt. % Ti3AlC2 with <1 wt. % TiC) was
obtained at the relatively low temperature of 1050 °C for 10-20 min by SPS of MA powders from starting mixtures of 3Ti/1.1Al/2C (molar ratio).
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SPS of Ti2AlN has been rarely reported. In a short paper, high-purity bulk Ti2AlN was
fabricated by SPS at 1200 °C and 30 MPa for 10 min from Ti/Al/TiN powders in stoichiometric proportion (Yan et al., 2008).
The current project used SPS to generate Mn+1AXn ceramic and Mn+1AXn/metal composite
samples. SPS was carried out the assistance of Dr. Fawad Inam and Prof. Michael J Reece at Nanoforce Technology Limited, Queen Mary, University of London.
Figure 6. Densification behaviour of porous alumina ceramics as a function of sintering temperature. AZM: Al2O3/3 vol. % ZrO2/100 ppm MgO; AZTM: Al2O3/3 vol. % ZrO2/500
ppm TiO2/100 ppm MgO; AS: Al2O3/5 vol. % SiC. Courtesy of D.D. Jayaseelan (Jayaseelan
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Figure 7. (a) The SPS system (HPD 25/1, FCT Systeme, Rauenstein, Germany) at Nanoforce Technology Limited, Queen Mary, University of London. (b) A photo showing a hot sample being sintered in the sample chamber. (c) Basic SPS configuration. Courtesy of M. J. Reece.
a
c
b
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