Spark and plasma aided densification mechanisms during spark plasma sintering of ceramic powders

Spark plasma sintering (SPS) currently used for rapid and full densification of ceramic particles assisted by a pulsed dc current passed through the powder compact. Our investigations with different ceramic powders (LiF, NiO, and YAG) as model systems discovered local melting of the particle and nano-particle surfaces, confirming the formation of spark and plasma during the SPS. However, spark and plasma form at certain material and process conditions. The ceramic yield stress and its electrical conductivity, and their temperature dependence mainly determine the conditions at which spark and plasma will take place in a given non-conducting granular compact. We introduce the conditions for plasma formation in soft deformable and hard non-deformable ceramic powders as model systems, through the Plastic Deformation-Plasma Formation Temperature Windows Diagrams. The different behavior at different oxide systems depend on the material properties, and the pressure and its application regime. These conditions also determine the nano- micro-structure evolution during the sintering by grain growth via grain-rotation and grain sliding

Kinetics of liquid-assisted densification during flash sintering of ceramic nanoparticles

The recent liquid film capillary mechanism [1, 2] proposed for the rapid densification during the flash sintering was consistent energetically with the dissipated power during the process. Following this mechanism, melting of contacts with high electric resistance will induce high attractive capillary forces. Wetting of a solid substrate by its own melt is termed homologous wetting with zero wetting (dihedral) angle. Therefore, wetting and spreading of the melt on its own substrate is extremely fast. Here we will show that particle surface softening / melting and the following transient processes associated with it namely local melt wetting and spreading, particle rearrangement, melts solidification, are also kinetically compatible with the flash process and its duration. We analyzed the liquid-assisted densification kinetics of ceramic nanoparticles during flash sintering in terms of wetting and melt spreading, from the nanoparticle contacts, affected by the local electric field and capillary forces. Homologous wetting and spreading of the melt from the particle contacts reveal wetting velocities of 0.3×10-6 m×s-1 and 1 m×s-1 induced by the electric field and the capillary forces, respectively. The ultrafast densification kinetics by particle rearrangement is consistent with the enhanced diffusion and calculated wetting velocities. Epitaxial solidification of the melt after particle rearrangement is energetically favorable, and its tendency depends on the melt viscosity. 1. R. Chaim, Materials 9 (2016) 280. 2. R. Chaim, C. Estournes, J. Mater. Sci. 53 (2018) 6378-6389

Optically transparent ceramics by spark plasma sintering of oxide nanoparticles

Optical transparency in polycrystalline ceramic oxides can be achieved if the material is fully densified. Spark plasma sintering (SPS) of oxide nanoparticles leads to immediate densification with final-stage sintering. Further densification by annihilation of the isolated pores is associated with diffusional processes, regardless of the densification mechanism during the intermediate stage. Densification equations in conjunction with the concept of grain boundary free volume were used to derive the pore size–grain size–temperature map for designing the nanopowder and SPS process parameters to obtain transparent oxides

Effect of 1 wt% LiF additive on the densification of nanocrystalline Y2O3 ceramics by spark plasma sintering

Densification of nanocrystalline cubic yttria (nc-Y2O3) powder, with 18 nm crystal size and 1 wt% LiF as a sintering additive was investigated. Specimens were fabricated by spark plasma sintering at 100 MPa, within the temperature range of 700–1500 °C. Sintering at 700 °C for 5 and 20 min resulted in 95% and 99.7% dense specimens, with an average grain size of 84 and 130 nm, respectively. nc-Y2O3 without additive was only 65% dense at 700 °C for 5 min. The presence of LiF at low sintering temperatures facilitated rapid densification by particle sliding and jamming release. Sintering at high temperatures resulted in segregation of LiF to the grain boundaries and its entrapment as globular phase within the fast growing Y2O3 grains. The sintering enhancement advantage of LiF was lost at high SPS temperatures

Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering

Multiphase (MP) monoclinic and cubic Y2O3 nanoparticles, 40 nm in diameter, were densified by spark plasma sintering for 5–15 min and100 MPa at 1000 °C, 1100 °C, and 1500 °C. Densification started with pressure increase at room temperature. Densification stagnated during heating compared to the high shrinkage rate in cubic single-phase reference nanopowder. The limited densification of the MP nanopowder originated from the vermicular structure (skeleton) formed during the heating. Interface controlled monoclinic to cubic polymorphic transformation above 980 °C led to the formation of large spherical cubic grains within the vermicular matrix. This resulted in the loss of the nanocrystalline character and low final density

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y2O3 powders with 18nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600°C. Dense specimens were fabricated at 100MPa and 1400°C for 5min duration. A maximum in density was observed at 1400°C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400°C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150kJmol−1 and associated diffusion coefficients higher by 103 than expected for Y3+ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature

Grain growth stagnation in fully dense nanocrystalline Y2O3 by spark plasma sintering.

Nanocrystalline (nc) Y2O3 powders with 18nmcrystallite size were sintered using spark plasma sintering (SPS) at 1100 ◦C and 100MPa for different durations. Specimens with 98% density and 106±33nm mean grain size were formed after 20 min. The grain size at the final stage of sintering first increased and then tended to stagnation with the SPS duration. The nanostructure consisted of convex tetrahedron shaped nano-pores at part of the grain boundary junctions. Theoretical calculations were made for grain growth stagnation imposed by either drag from nano-pores at grain junctions or from dense triple junctions; the experimental results were in agreement with grain growth stagnation due to nano-pore drag in nc-Y2O3. The conditions for the stabilization of the nanostructure in Y2O3 were determined. Extended SPS duration up to 40 min led to sudden grain coarsening and loss of the nanocrystalline character

Plastic deformation of dense nanocrystalline yttrium oxide at elevated temperatures

Nanocrystalline yttrium oxide, Y2O3 with 110 nm average grain size was plastically deformed between 800 ◦C and 1100 ◦C by compression at different strain rates and by creep at different stresses. The onset temperature for plasticity was at 1000 ◦C. Yield stress was strongly temperature dependent and the strain hardening disappeared at 1100 ◦C. The polyhedral and equiaxed grain morphology were preserved in the deformed specimens. The experimentally measured and theoretically calculated stress exponent n = 2 was consistent with the plastic deformation by grain boundary sliding. Decrease in the grain size was consistent with decrease in the brittle to ductile transition temperature

Flash sintering of dielectric nanoparticles as a percolation phenomenon through a softened film

Recent work [Biesuz et al., J. Appl. Phys. 120, 145107 (2016)] showed analogies between the flash sintering and dielectric breakdown in α-aluminas pre-sintered to different densities. Here, we show that flash sintering of dielectric nanoparticles can be described as a universal behavior by the percolation model. The electrical system is composed of particles and their contact point resistances, the latter softened first due to preferred local Joule heating and thermal runaway during the flash. Local softening has a hierarchical and invasive nature and propagates between the electrodes. The flash event signals the percolation threshold by invasive nature of the softened layer at the particle surfaces. Rapid densification is associated with local particle rearrangements due to attractive capillary forces induced by the softened film at the particle contacts. Flash sintering is a critical phenomenon with a self-organizing character. The experimental electric conductivity results from flash sintering are in full agreement with those calculated from the percolation model