I’m working on a ceramic water-based slurry composed of polymer-derived ceramic (PDC) SiC–Gd₂Zr₂O₇ powder, with HPMC as the binder and Dispex A4040 as the dispersant. The slurry is applied onto 25 mm × 25 mm × 1 mm silica and SiC substrates.
My goal is to avoid cracking entirely during the drying stage. Currently, I remove the sample from the humidity chamber and perform picosecond laser cutting before it fully dries, in an attempt to cut it before cracks begin to form. However, as soon as the sample is exposed to ambient lab conditions (air-conditioned, low humidity), surface drying accelerates, and cracking starts almost immediately, even before I can begin the laser process.
Ideally, I wouldn’t need to cut the coating in its wet or semi-dry state — I’m only doing that now because I haven't found a reliable way to dry it fully without cracking.
The water-to-powder ratio I'm using is 4:1, which helps maintain enough flowability for efficient ball milling and dispersion. I’ve tried reducing it to 3:1 or 2:1 to minimize water content, but the slurry becomes too thick and doesn't flow well enough for ball milling or proper application on the substrate.
Given that the environmental conditions (AC, low humidity) are out of my control, I’m looking for advice on:
- How to modify the formulation or drying method to prevent cracking entirely.
- Whether there are additives, drying protocols, or post-application treatments that have worked for similar systems.
- Any insights on improving flowability at lower water content ratios.
Any input or shared experience would be highly appreciated.
Thanks in advance!
It is a bit difficult to implement, but try. You have to increase the water vapour pressure in the essication chamber till it saturates, then you must increase the temperature a wait till your sample is at thermal equilibrium; then you must remove the water vapour from the chamber. The idea behind is that you must increase the energy of the water in the sample without that is evaporates (hence the high water vapour pressure in the chamber), and then remove it altogether. This might (might!) avoid cracking.
Thank Prof. @Valter Sergo for your suggestions, I really appreciate it.
Strategies to Prevent Cracking in Water-Based PDC Coatings
1. Controlled Drying Rate
- Why: Rapid water evaporation leads to surface skin formation and internal tensile stress.
- How: Dry under ambient or slightly elevated temperature (≤60 °C) with high relative humidity, or use stepwise drying (gradual temperature ramping).
2. Dilution of Precursor
- Why: High solid content causes excessive shrinkage during solvent loss.
- How: Dilute the PDC precursor (e.g., polysilazane) with a compatible solvent or water to reduce viscosity and shrinkage.
3. Additives to Reduce Stress
- Why: Additives can increase flexibility or control particle packing.
- How: Use plasticizers (e.g., PEG or PVP) to improve flexibility. Add nano-fillers (e.g., fumed silica) to reduce shrinkage.
4. Substrate Surface Treatment
- Why: Poor adhesion leads to delamination and cracking.
- How: Roughen or functionalize the substrate surface (e.g., oxygen plasma, HF etching, or silane coupling agents) to enhance bonding with the coating.
5. Multiple Thin Coats Instead of One Thick Layer
- Why: Thick coatings shrink more and crack easily.
- How: Apply the coating in thin layers (~10–20 μm) and dry each layer before applying the next.
6. Controlled Atmosphere Drying
- Why: Solvent gradients create stress differentials.
- How: Dry in a sealed chamber with controlled airflow and humidity.
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