Understanding Particle Size Distribution for Thermal Spray Applications?
I often see coating failures that look like process problems, but the real issue starts much earlier, with the powder itself. Particle size distribution controls how powder behaves in the flame, and many teams underestimate its impact.
Particle size distribution is one of the most critical powder parameters in thermal spray. It controls how particles heat, accelerate, melt, and impact the substrate. A well-controlled PSD improves deposition efficiency, coating density, porosity control, hardness, and repeatability across spray runs.
If you understand PSD, you stop guessing. You start designing coatings with intent.
What PSD range is ideal for HVOF or plasma spray systems?
I have worked with customers who changed spray parameters for weeks, only to learn the powder PSD was outside the right window for their system. That mistake is costly and avoidable.
There is no single ideal PSD for all systems, but most powder-based plasma and HVOF processes operate best within controlled ranges. In practice, plasma and HVOF commonly use powders between 10 and 100 µm, with tighter cuts selected based on material type, gun power, and coating goals.
Typical PSD ranges by process
Different spray systems create very different thermal and kinetic environments. The PSD must match that environment.
| Spray Process | Common PSD Range (µm) | Notes |
|---|---|---|
| APS (Plasma) | 15–75 | High temperature, moderate velocity |
| HVOF | 10–45 | Lower temperature, very high velocity |
| HVAF | 10–35 | Lower heat, tight PSD preferred |
| Wire Arc | Not powder-based | PSD not applicable |
Why very fine particles are a problem
Particles smaller than about 10 µm cause several issues.
- They heat too fast
- They oxidize easily
- They may evaporate before impact
- They often fail to enter the hot core of the jet
Very fine particles also follow gas streamlines poorly. As a result, deposition efficiency drops even though powder consumption increases.
Why very coarse particles fail
Particles above 75–100 µm heat slowly. In many cases, only the surface melts while the core stays solid. These semi-molten particles do not flatten well on impact. They become defects inside the coating.
The key rule is simple:
Most particles must fully melt and reach optimal velocity at impact.
How does particle size affect coating thickness and porosity?
I once compared two coatings sprayed with the same gun, same parameters, and same material. The only difference was PSD width. The microstructure told the whole story.
Particle size directly controls melting behavior, splat formation, and pore creation in thermal spray coatings. Fine particles melt faster and spread well, while coarse or uneven particles create voids, poor bonding, and thickness variation.
Effect of small particles
Smaller particles have a high surface-to-volume ratio.
- They absorb heat quickly
- They melt fully with ease
- They flatten well on impact
However, when particles are too small, new risks appear.
- Oxidation increases
- Evaporation losses rise
- Deposition efficiency drops
This is especially critical for reactive materials like aluminum or zinc.
Effect of large particles
Large particles behave very differently.
- Heating is slower
- Partial melting is common
- Impact spreading is poor
These particles often embed as unmelted cores. They increase porosity and reduce bond strength.
PSD width matters as much as mean size
Two powders can have the same average size but very different performance.
| PSD Type | Coating Thickness | Porosity | Repeatability |
|---|---|---|---|
| Narrow PSD | Uniform | Low | High |
| Wide PSD | Variable | High | Low |
A narrow PSD reduces the spread in particle temperature and velocity at impact. This leads to more uniform splats and a denser coating.
Can I request customized PSD ranges from powder manufacturers?
I often hear buyers say they must accept standard grades. That is not always true, especially for serious applications.
Most professional powder manufacturers can supply customized PSD ranges when requirements are clear. Custom PSDs are common for aerospace, MRO, medical, and research coatings where consistency matters.
What can usually be customized
A capable manufacturer can adjust several PSD parameters.
- Lower and upper cut sizes
- PSD width (narrow vs wide)
- Modal shape (single vs multi-modal)
This is done through controlled sieving, air classification, or blending.
Information you should provide
Customization only works when communication is clear.
| Information Needed | Why It Matters |
|---|---|
| Spray process type | Determines thermal input |
| Gun model and power | Controls particle heating |
| Coating material | Affects melting behavior |
| Target coating properties | Guides PSD optimization |
When custom PSD makes sense
Custom PSD is most valuable when:
- Porosity must be tightly controlled
- Hardness consistency is critical
- Deposition efficiency affects cost
- Run-to-run repeatability matters
How do I read and interpret a powder particle size distribution chart?
Many teams receive PSD reports but do not truly read them. They look at one number and move on. That is a mistake.
A particle size distribution chart shows how powder mass is spread across particle sizes, not just an average value. Learning to read it helps you predict spray behavior before spraying begins.
Key PSD terms explained
- D10: 10% of particles are smaller than this size
- D50: Median particle size
- D90: 90% of particles are smaller than this size
Reading PSD width
| PSD Characteristic | What It Means |
|---|---|
| Small D90–D10 gap | Narrow PSD |
| Large D90–D10 gap | Wide PSD |
| Single peak | Uniform melting behavior |
| Multiple peaks | Mixed particle states |
Matching PSD charts to process limits
- Check fines below 10 µm
- Check coarse tails near system limits
- Confirm feeder compatibility
PSD charts are predictive tools, not paperwork.
Conclusion
Particle size distribution defines how powder behaves before it reaches the substrate. When PSD is controlled, coating quality becomes repeatable instead of accidental.
