Flash Spark Plasma Sintering (FSPS) of α and β SiC

Abstract

A novel processing methodology that allows combined preheating and Flash-SPS (FSPS) of silicon carbide-based materials has been developed. Beta-SiC (+10 wt% B 4 C) powders were densified (Ф 20 mm) up to 96% of their theoretical density in 17 s under an applied pressure of 16 MPa (5 kN). The flash event was attributed to the sharp positive temperature dependence of the electrical conductivity (thermal runaway) of SiC, and a sudden increase in electric power absorption (Joule heating) of the samples after a sufficient preheating temperature (>600°C) was reached. The microstructural evolution was analyzed by examining materials densified by FSPS in the range of 82%-96% theoretical densities. FEM modeling results suggest that the FSPS heating rate was of the order of 8800°C/min. A comparative analysis was done between FSPS and reference samples (sintered using conventional SPS in the temperature range of 1800°C-2300°C). This allowed for a better understanding of the temperatures generated during FSPS, and in turn the sintering mechanisms. We also demonstrated the scalability of the FSPS process by consolidating a large aSiC disk (Ф 60 mm) in about 60 s inside a hybrid SPS furnace equipped with an induction heater, which allowed us to achieve sufficient preheating (1600°C) of the material to achieve FSPS.

Review of flash sintering: materials, mechanisms and modelling

“…As shown in Figure 5, the transition from voltage to current control mode creates a power spike [13] which is not to be confused with NTC behaviour of the material. It has been clearly demonstrated that the sample temperature follows the powder dissipation [61]. Even if this control mode ( Figure 5(a)) results in a near instantaneous (5 s) consolidation of the materials [5], it might not be ideal to study or understand the mechanism involved in this process due to its extremely high rate.…”

“…In the case FS of semiconductors, such as SiC [61] and B 4 C [32], there is a metal-semiconductor junction, which produces Schottky barriers.…”

“…Such techniques have identified the formation of an apparent 'new' phase during FS of 3YSZ samples [39] and an apparent 'reversible' texturing formation only maintained while the field was applied to TiO 2 [40] samples. Even if these techniques are very powerful for in situ investigations, the interpretation of the results in terms of precise quantification (lattice parameters and crystal structure) of 'new' phase is difficult [39] mainly because of the narrow 2θ range, which is within 1°a nd the relatively slow acquisition time of 1.5 s. [56], silicon carbide (SiC) [43,61,80] and boron carbide (B 4 C) [32]; as well as several metalliclike non-oxide ceramics (ZrB 2 and MoSi 2 ) [35,80,81]. As illustrated in Figure 3, FSed materials can be grouped with respect to their type of electrical conductivity at T Onset ( Figure 9).…”

“…D > 2 cm), high heat loss fluxes by radiation would cause cracking. FEM modelling of experimental arrangements using insulting felt has been recently developed [61].…”

“…In order to avoid cracking by thermal shock during cooling, the thermal gradients within the samples should be minimised, and for large components thermal insulation becomes essential [61].…”