August 01, 2022

How does Thermal Spray Powder Morphology Influence Coating Properties?

Thermal spray processes are a broad class of coating techniques in which a feedstock material is heated and sprayed onto a surface. When tiny droplets of the melted or partially-melted feedstock impact a substrate, they form lamellae (or “splats”) which accumulate and harden to create a uniform coating.1 Several thermal spraying techniques are commonly used to achieve this, including plasma spraying, wire arc spraying, flame spraying and cold spraying.

Thermal spray techniques are used to coat a wide range of structural and engineering components, creating durable coatings that protect them from corrosion, wear and high temperatures. Thermal spraying processes are extremely versatile: they can be used to deposit metals, ceramics, plastics and composites onto a range of different materials.

The impact of thermal spray powder morphology

Though the feedstock material for thermal spray processes can be supplied in the form of a wire (or, less commonly, liquid or suspension), most thermal spray processes utilize a powder feedstock.2

When using a thermal spray technique, your choice of thermal spray powder is crucial: not all thermal spray powders of a given material are equivalent. In fact, the precise shape and size distribution (the morphology) of individual powder particles has a significant effect on the quality of the coating produced.

Research shows that coatings made from powders with dissimilar particle morphologies can result in significant structural differences, even when considering powders with uniform chemical composition and size distribution.3,4 Powder morphology plays a fundamental role in the interaction between the feedstock and the thermal spray heat source, which in turn influences the microstructure of the coating, and hence its overall durability and performance.5,6

Fused and crushed (F&C)

To produce fused powders, components are mixed and melted at very high temperatures (and often high pressures) to fuse the material. After fusing, the material is cooled into a large block and crushed to form a powder with particles of a specific size suitable for thermal spray techniques.

Fusing and crushing is preferentially used for ceramics and other brittle materials. Fused and crushed powders are characterized by an angular morphology with smooth fracture surfaces. This can lead to lower flowability, so fused and crushed powders are sometimes post-processed by flame- or plasma-induced spheroidization. Typically, fused and crushed yttria-zirconia thermal spray powders produce coatings with higher thermal conductivity and a higher elastic modulus than agglomerated and sintered (A&S) or hollow spherical (HOSP) coatings of the same material.7

Agglomerated and sintered (A&S)

Agglomeration involves the “gluing” together of extremely fine particles of the same or differing compositions using a binder. The sintering process aims to remove this binder by heating it below the melting point of the coating material(s).

A&S powders tend to melt more easily than fused and crushed powders, but not as easily as HOSP powders. Since they’re more porous than F&C powders, A&S powders tend to have a lower thermal conductivity – but not as low as HOSP powders.

Hollow spherical (HOSP)

HOSP powders are commonly produced from ceramic materials: the process involves heating powder particles above their melting point and allowing surface tension to “pull” them into a spherical shape. This process results in a powder with a large cavity inside, leading to a highly porous coating which is ideally suited to thermal barrier applications. In addition to low thermal conductivity, HOSP powders offer high thermal spray deposit efficiency.

Ceramic Thermal Spray Powders from Saint-Gobain

Whatever your thermal spray process application, Saint-Gobain can provide a powder than meets or exceeds your requirements. To learn more about our thermal spray powder solutions, get in touch with a member of our team today.

References and Further Reading

  1. Li, C.-J. & Ohmori, A. Relationships between the microstructure and properties of thermally sprayed deposits. J Therm Spray Tech 11, 365–374 (2002).
  2. Killinger, A. et al. Review of New Developments in Suspension and Solution Precursor Thermal Spray Processes. J Therm Spray Tech 20, 677–695 (2011).
  3. From Powders to Thermally Sprayed Coatings | SpringerLink. https://link.springer.com/article/10.1007/s11666-009-9435-x.
  4. Azarmi, F. & Salimijazi, H. R. Grain growth and pore elimination in Inconel 625 deposited by APS. Surface & Coatings Technology Complete, 3–6 (2015).
  5. Nouri, A. & Sola, A. Powder morphology in thermal spraying. Jnl Adv Manuf & Process 1, (2019).
  6. Rojas, J., Cruchaga, M., Celentano, D., El Ganaoui, M. & Pateyron, B. Numerical Forecast of the Melting and Thermal Histories of Particles Injected in a Plasma Jet. (2010).
  7. Chi, W., Sampath, S. & Wang, H. Ambient and High-Temperature Thermal Conductivity of Thermal Sprayed Coatings. Journal of Thermal Spray Technology 15, 773–778 (2006).