The coating of particles and its importance for many applications

The coating of ceramic powder particles has recently gained importance and is therefore used more frequently.

During the coating process the properties of the powder get improved or changed. The change of the surface properties and / or the functionality of fine particles or powders is important for many different industrial purposes and it extends the possibilities to use coated ceramic powders.

Powder coating is of special importance for the additive manufacturing of ceramics. Very fine powders tend to agglomerate when exposed to air. This worsens the processability e.g. reducing the powders flowability. It is possible to influence properties like the flowability or wettability of the powder by treating the surface of it. This can be useful to customize a powder according to the use case.

There are various methods and processes to coat particle surfaces:

Silanization

The silanization is a common and widely used process for the coating of particles which is based on wet chemical sol-gel technology.

The target flowability and wettability can be customized by surface treatment of the particles with silanes or silane-based compounds.

Silanes are bifunctional compounds consisting of hydrolysable reactive OH groups and stable organofunctional R groups. The attachment to the particle surface is achieved by hydrolyzable groups. Meanwhile, the surface functionality of the powder is determined by the organofunctional R groups. Parameters that often have to be optimized are hydrophobic properties or the achievement of a certain wettability.

Hydrophobicity of the particle surface helps to avoid agglomeration of the powder or powder mixture. This improves the trickle of the powder, which ensures a constant layering during printing.

These properties are used by WZR for…

  • …3D printing of ceramic components using the binder jetting process. By using modified powder, an optimal powder layering is achieved, on which the print quality depends and affects the properties of the printed components, e.g. green strength, density and porosity of the final product after sintering.
  • …3D printing of ceramic-filled polymer systems according to Vat Photopolymerization  (VPP) process. For this application, the required wettability of ceramic particles in a polymer matrix is of great importance. This ensures optimum compatibility at the interface of the polymer matrix and the ceramic powder used as filler. Coated powder is easier to incorporate into a polymer or resin than uncoated powder. The optimum wetting of the ceramic powder with the polymer matrix can be specifically adjusted by certain organofunctional R groups of a silane. Silanization of a powder as a filler also makes it possible to increase the degree of filling of the powder in the polymer matrix. This increases the ceramic content in a composite material, resulting in increased green strength.

Binder coating

In this process, the particles are coated with a binder. A binder-containing system based on water or solvent serves as the coating solution. Powder coated in this way is also used in the binder jetting process. In this way, a higher green strength of the printed parts can be achieved.

Powder nanoparticle coating

Another application is the coating of a ceramic powder (large particles) with another powder (smaller particles). This gives the coated ceramic powder the properties of the coating powder.

The coarse particles are coated with the coating suspension by spraying. After evaporation of the liquid, the nano particles are attached to the surface of the large particles. This attachment can be achieved by two methods:

  • By covalent bonding, via wet chemical sol-gel process. The coating suspension contains nano- and microscale particles dispersed in a sol.
  • By direct binding by a binder. For this purpose, a binder-containing dispersion is prepared.

Application at WZR

One process for coating powders, which we dealt with last year, was coating by precipitation of SiO2 onto FeSi powder. Such powder is used, among other things, in magnetic field concentrators. The experiments were accompanied by an observation of the material in a scanning electron microscope, which allowed us to assess the quality and thickness of the coating.

BSE images of FeSi powder (left) in various stages of coating tests with SiO2 (center + right).

We are currently working internally, but also with cooperation partners, on further exciting powder coatings and look forward to sharing them with you soon.

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