Cold compression of ceramic spray-dried granules: Role of the spatial distribution of the binder

Abstract

We report an experimental study of the compaction behaviour of zirconia granules formed by a spray drying process. Using two different binders that were fully characterized, we have investigated the influence of the binder spatial distribution in the granules on compaction of a granule packing. Spatial distributions were modified by tuning binder adsorption on the ceramic nanoparticles in the spray dried suspensions.Granule powders with binder either located between nanoparticles only, or partly concentrated within a segregated layer at the surfaces of granules were thus obtained.We have found the role of the segregated layer is fully negligible during granule compression, and we justify that result by providing estimates of the theoretical compression energies of segregated layers and polymer bridges. We discuss the role of the binder bridges between nanoparticles, and we suggest a picture accounting for the 2 experimental influence of binder mechanical properties and concentration on the compaction behaviour of spray-dried granules.

The breakdown of the Weibull behavior in dental zirconias

“…These are junction vertices of formerly spherical powder secondary particles (granulates), that failed to reach intimate contact during the pressing process. The stiff binder shell at the surface of granules assume polyhedral shapes during the first stages of uniaxial pressing, with the binder layer trapped particularly at the vertices being responsible for opposing the largest resistance to compaction, as modelled by Boursier et al 22 for spray-dried granules. Though the outer segregated binder shell resulting from such drying techniques [23][24][25] -implicated on formation of strength-limiting defects before and after sintering 23,26 -seems to contribute negligibly to the compaction energy when compared to the binder bridges between the nanoparticles within the granules.…”

“…Though the outer segregated binder shell resulting from such drying techniques [23][24][25] -implicated on formation of strength-limiting defects before and after sintering 23,26 -seems to contribute negligibly to the compaction energy when compared to the binder bridges between the nanoparticles within the granules. 22 With friction between primary nanoparticles governing the energy consumption at later stages, it is to expect that granulate size might play a significant role on densification, due simply to longer displacements necessary to fill large interstices. Engaging on such hypotheses is certainly bound to render fruitful insights concerning the origin of inhomogeneities in green bodies; still, the intricacies of the mechanics of defect formation is still unresolved in the most fundamental levels, and cannot be tackled solely by means of mechanical testing and fractographic interpretations.…”

“…The stiff binder shell at the surface of granules assume polyhedral shapes during the first stages of uniaxial pressing, with the binder layer trapped particularly at the vertices being responsible for opposing the largest resistance to compaction, as modelled by Boursier et al 22 for spray‐dried granules. Though the outer segregated binder shell resulting from such drying techniques 23–25 —implicated on formation of strength‐limiting defects before and after sintering 23,26 —seems to contribute negligibly to the compaction energy when compared to the binder bridges between the nanoparticles within the granules 22 . With friction between primary nanoparticles governing the energy consumption at later stages, it is to expect that granulate size might play a significant role on densification, due simply to longer displacements necessary to fill large interstices.…”

Granulation of Silicon Nitride Powders by Spray Drying: A Review

“…According to the present review, the addition of a polymer binder to ceramic powders before pressing them significantly improves the properties of the resulting green body. Particularly, the binder increases the cohesion of the powder and hence the strength of the green body [ 104 ]. However, it is also known that the addition of a binder can potentially lead to a large porosity of the ceramic after the sintering step, during which all organic content is removed.…”

“…Evidence for that layer has been provided [ 105 , 106 ] and it has been shown to form with binders that do not (or only poorly) adsorb onto the surfaces of the ceramic colloidal particle before spray drying. Segregated layers are suspected to have a role in green strength by enhancing the adhesion between granules [ 104 , 107 ]. However, they can also be detrimental to the quality of the final ceramics: their removal leaves coarse defects, and segregated layers are at the origin of defaults in the final sintered ceramics [ 108 ].…”