Particle Rearrangement and Pore Space Coarsening During Solid‐State Sintering

Abstract

Coarsening of porosity during sintering has been observed in powder compacts of metallic, ceramic, and amorphous materials. Monitoring and modelling of the growth of individual (closed) pores in the late sintering stages are well established. Porosity is interconnected up to very high densities. Coarsening of the continuous pore space takes place during the initial and intermediate sintering stages. This coarsening is caused by localized transport of atoms or molecules (diffusion or viscous flow) as well as by bulk particle movement (rearrangement). Its quantitative exploration poses problems both experimentally and theoretically. Ways to characterize the geometry of the interconnected pore space and of closed pores are discussed with emphasis on stereological parameters. Recent and classical approaches, experimental findings with 2D model arrangements (as the formation and opening up of particle contacts, pore coarsening, and particle rearrangement) and some advances of computer simulations are discussed together with open questions.

Influence of Isothermal Heat Treatment on Porosity and Crystallite Size in Axial Suspension Plasma Sprayed Thermal Barrier Coatings for Gas Turbine Applications

“…The above porosity increase has been attributed to pore coarsening in case of SPS TBCs. Exner et al explained the pore coarsening effect in case of several powder compacts during solid state sintering by saying that the pore coarsening can be caused by localized transport of atoms/molecules due to diffusion and or bulk particle rearrangement [27]. Hence, apart from the anticipated microstructural change caused by sintering, the possibility of pore coarsening that might take place in certain conditions and be responsible for increase in the total porosity as noted in coatings Exp-2 and Exp-3, should also be borne in mind to explain the results observed.…”

“…At these temperatures, significant microstructural changes occur, which may affect the lifetime and functional performance of the TBCs [26,27]. Due to the presence of extremely fine as well as coarse pores in SPS sprayed YSZ layers [20], it is important to understand how these pores respond to sustain exposure to high temperatures and how the accompanying changes affect the thermal conductivity of the TBC.…”

Direct observation of grain rotations during coarsening of a semisolid Al–Cu alloy

“…Our current inadequate understanding of sintering kinetics can in part be traced to the experimental difficulty of tracking microstructural changes in three dimensions (8). With recent advances in 3D imaging, however, researchers can now perform time-resolved measurements of the true size, shape, and interconnectivity of particles and pores (9)(10)(11)(12), from which it may be possible to pin down the atomic-level mechanism(s) underlying each stage of sintering.…”

“…With recent advances in 3D imaging, however, researchers can now perform time-resolved measurements of the true size, shape, and interconnectivity of particles and pores (9)(10)(11)(12), from which it may be possible to pin down the atomic-level mechanism(s) underlying each stage of sintering. Of particular interest in this regard is the 3D rearrangement of particles that occurs during early-stage sintering (8,13), which can significantly affect subsequent densification but is hardly accessible from 2D cross-sections. In a recent experiment, Grupp et al (12) observed individual particle rotations as large as 8°during the sintering of loosely packed…”

Cooperative material transport during the early stage of sintering

“…After initial crystal misorientations at particle contacts have been introduced by the random orientation of each powder particle, the system will tend to reach a state with lower grain boundary energy by adjusting the misorientations [12][13][14][15] . Clear experimental evidence of particle motion during sintering was obtained in previous studies for one-dimensional rows of powder particles and for two-dimensional particle arrays as these situations can be easily observed with traditional methods [16][17][18] . A quantitative analysis of three-dimensional particle systems is just about to become feasible and is expected to yield a clearly different picture, because other than in 1D or 2D model systems, particle movements in 3D are highly frustrated due to the packing of the particles.…”