In high-temperature coating applications, polysilazane has gained widespread attention due to its excellent heat resistance, oxidation resistance, and ceramic-forming capability. However, some users report that polysilazane coatings begin to crack when exposed to temperatures between 400°C and 500°C, leading to concerns about the material's thermal stability.

In many cases, the problem is not the polysilazane itself. One of the most common causes is excessive coating thickness.

The Ceramic Conversion Process Creates Internal Stress

When heated, polysilazane undergoes a transformation from an organic polymer into an inorganic ceramic-like structure, typically forming silicon oxide, silicon oxynitride, or related ceramic phases.

During this conversion process:

• Organic groups decompose and volatilize.
• The coating experiences volume shrinkage.
• The coating density increases significantly.
• Internal stresses gradually build up.

If the coating is applied too thickly in a single layer, these stresses become difficult to release uniformly throughout the film.

As the temperature reaches 400–500°C, the accumulated stress may exceed the coating's mechanical strength, resulting in visible cracking.

Why Thick Coatings Are More Likely to Crack

A thick polysilazane coating faces several challenges during heat treatment:

1. Uneven Temperature Distribution

The surface of the coating heats and cures faster than the inner layer. This difference creates thermal gradients and stress concentration.

2. Shrinkage Mismatch

The outer layer may have already converted into a ceramic structure while the inner portion is still undergoing chemical reactions. Different shrinkage rates generate tensile stress within the coating.

3. Poor Gas Release

During pyrolysis, decomposition products and residual solvents must escape from the coating. In thick films, gas diffusion becomes difficult, increasing the likelihood of internal defects, blistering, and cracking.

4. Coefficient of Thermal Expansion Differences

If the substrate and coating expand at different rates during heating, a thick coating will experience greater thermal stress than a thin coating, increasing the risk of crack formation.

Other Factors That May Contribute to Cracking

Although coating thickness is often the primary factor, several additional variables should also be considered:

• Excessively rapid heating rates
• Inadequate curing before high-temperature exposure
• Residual solvent trapped in the coating
• Poor substrate surface preparation
• Multiple thick layers applied without intermediate curing
• Improper formulation or filler selection
• Large thermal expansion mismatch between coating and substrate

How to Reduce the Risk of Cracking

To improve coating integrity at elevated temperatures, manufacturers typically recommend:

✔ Applying multiple thin layers rather than one thick layer

✔ Allowing sufficient curing between coats

✔ Using a gradual heating schedule

✔ Optimizing solvent content and coating viscosity

✔ Incorporating suitable ceramic fillers when necessary

✔ Matching coating design with the substrate's thermal expansion characteristics

Conclusion

When a polysilazane coating cracks at 400–500°C, it does not necessarily indicate insufficient heat resistance of the material. In many cases, excessive coating thickness leads to uneven ceramic conversion, volume shrinkage, and stress accumulation during heating.

By optimizing coating thickness, curing conditions, and heat-treatment procedures, polysilazane coatings can achieve significantly improved thermal stability and long-term performance in demanding high-temperature environments.

A well-designed thin coating often performs better than a single excessively thick layer.