Thermal Spray Splat Formation

Impact Conditions

Impact velocity 80 m/s

Droplet diameter 60 µm

Substrate preheat 200 °C

Feedstock material

Impact Numbers

Reynolds Re

–

Weber We

–

Splash param K

–

Flattening ξ_max

–

— awaiting impact —

Splat diameter

–

Solidified

–

How it works

In plasma / HVOF thermal spraying, molten feedstock particles (10–100 µm) hit the substrate at 50–200 m/s and freeze in milliseconds into thin flattened disks ("splats"). Stacking millions of splats builds the coating. Two dimensionless groups set the outcome:

Re = ρ·v·D / μ    (inertia / viscosity)
We = ρ·v²·D / σ   (inertia / surface tension)

The final flattening ratio ξ_max = D_splat/D₀ follows the Pasandideh-Fard energy-balance correlation:

ξmax = sqrt( (We + 12) / (3(1 − cosθ) + 4·We/√Re) )

where θ is the droplet/substrate contact angle (better wetting → larger ξmax). Whether the splat spreads as a clean disk or breaks up into ejected satellite droplets ("splashing") is governed by Mundo's empirical parameter:

K = We^0.5 · Re^0.25
K > ~57  →  splashing (fingering + satellite ejection)
K < ~57  →  disk-type spreading

Impact velocity / diameter — raise both Re and We; higher K pushes the splat toward splashing.
Substrate preheat — a hotter substrate desorbs surface condensates and improves wetting (raises cosθ in the model), which is the standard industrial fix for suppressing splashing.
Material — sets density, molten viscosity and surface tension, so identical impact conditions give very different Re/We for a ceramic (Al₂O₃) vs. a metal (Ni-alloy, Mo).
After spreading, the splat cools and solidifies; the visualization sweeps a freezing front through the disk as it loses heat to the substrate.

Real-world relevance: this is the mechanism thermal-spray engineers control (particle velocity, size distribution, substrate temperature) to get dense, well-bonded thermal-barrier and wear coatings instead of porous, splash-riddled ones.

Molten splat Solidified splat Substrate

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