Spark Plasma Sintering And Densification Behavior Of Silicon Carbide

Spark plasma sintering (SPS) is a recent pressure assisted sintering techniquecharacterized by short sintering time and a relatively low sintering temperature.These characteristics make spark plasma sintering a promising technique inproducing highly dense materials with controlled microstructure. This recentlydeveloped sintering technique allows for the production of ceramic composites.Spark plasma sintering of silicon carbide nanoparticles has been used to productnanocrystalline structure ceramic composites. Moreover, reductions in siliconcarbide grain size have been shown to improve the fracture toughness of ceramiccomposites. However, significant gap exists between the technological andfabrication achievements to the fundamental understanding of the spark plasmasintering mechanisms. This gap is due to the complexity of the thermal, electricaland mechanical processes that may be involved during the spark plasma sintering,in addition to their dependence on the spark plasma sintering parameters. Althoughnot verified, several different mechanisms such as vaporization-condensation,plastic deformation, surface-, grain boundary-, and volume-diffusions were assumedfor the sintering and densification during the spark plasma sintering process.Only few researchers have developed continuum models to predict the thermal,electric, and displacement fields inside the sintering chamber. No mechanisms weredirectly observed during each stage of sintering consolidation by traditional finiteelement modeling method. However, we can get an atomic-and molecular-levelview of spark plasma sintering by using molecular dynamics simulation method.For now, molecular dynamics (MD) simulations have played an important role inelucidating the sintering mechanism on the atomic scale by tracking the neck andshrinkage between the sintered nanoparticles. The previously reported model of twonanoparticles did not reflect the properties of the porous structure or reproduce realistic sintering. We have used a multi-nanoparticle molecular dynamicssimulation method to investigate the sintering of silicon carbide nanoparticles.The contents are as follows:（1）Construct4spherical nano-crystals of3C-SiC stacked in a closed-packedarrangement. The neck growth and grain growth behavior of silicon carbidenanoparticles under periodic boundary conditions at different temperatures andpressures was investigated by molecular dynamics simulations. Themulti-nanoparticle simulation method here is effective for investigating the realisticdensification process. At the early stages of densification of the nanocrystallinepowder, the attraction between the two particles is responsible for neck growth.During the intermediate-and final-stages of spark plasma sintering simulations, weobserved plasticity mechanisms, such as twin, dislocation and viscous flow, as wellas amorphization near the contact neck regions of the nanoparticles.（2）The influence of temperature, particle size, pressure on the densificationand sintering mechanisms as well as that of heating rate and holding time on thestructural evolution has been investigated. It was found that higher temperature andhigher external pressure facilitate densification. An increase of sintering time leadsto greater final densities, but with the drawback of increasing grain size. There alsowas a major effect of pressure on grain size. Increased sintering rate contributednothing to improving the densification.（3）Effect of the particle size on the densification process during the sparkplasma sintering was examined. Construct nanoparticles with diameter ranging from1.5nm to6nm. The agglomeration of smaller particles can be observed apparentlyand sometimes large pores remain in the sintered product. On the other hand,smaller particle sizes facilitate densification along with higher temperature andexternal pressure. It is also found that drastic grain growth appears for nanoparticlesof very small sizes. For larger particles, the pores between particles becomedisconnected and the pore shrinkage is significant, but full densification is not reached. Silicon carbide nanoparticles are so sensitive to temperature that can besintered at a relatively low sintering temperature.We are performing classical molecular dynamics simulations of early-andintermediate-stage spark plasma sintering of nanocrystalline silicon carbide to betterunderstand the sintering process. This research will help lay the technicalfoundation for development of a lightweight structural super ceramic matrixcomposite. This of course has great significance on manufacturing highperformance materials.