There are many theories on the critical thickness that can calculate the initiating cracks on coating. However, does anyone start applying the calculation on the actual samples with a maximum thickness of 20 microns?
Dear Magdi Hassn B. Mussa ,
The sol-gel technique is considered a fine chemistry synthesis, which involves the polymerization of the metallic isopropoxide on the substrate surface. However, I do not indicate the use of sol-gel for coatings with 20 mícrons, it is a technique that yield thin films of 20 nm, as reported before:
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The cracks will depend on the film structure, calcination temperature, substrate and moreover, it is very hard to determine for such unique synthetical procedure. However, you can perform investigations with AFM microscopy, DRS (it will change the optical properties), and with SEM.
Dear Magdi Hassn B. Mussa
If you based on that the steady state formation of cracks occurred when stresses are built-up during either drying or densification process practiced in a sol–gel coating steps, exceeds the film's capability to elastically respond. In this case theoretically one can estimate the critical thickness h critical, below which remains crack free. In [1] you can see the authors cited the most commonly used derivation leads to the equation:
h critical = E Gf/C π ϭ2
E is the elastic modulus (N/m2), Gf is the critical strain energy release rate (J/m2), C (-) is function of the difference in elastic properties between the coating layer and the substrates, and ϭ is residual stress (N/m2) in the coating layer.
Now, it is clear that the higher critical thickness can be achieved by minimizing the residual stresses in the coating layer because both E and Gf are coating layer material properties. Bearing in mind that the minimizing the residual stress in the coating layer is practical knowledge and should optimized during drying and densification processes of sol gel rout application by controlling the rate of both processes which are challenging issue in thin film but less with thick one.
Please any feedback is welcome
Best regards
[1] Article Formation and prevention of fractures in sol–gel-derived thin films
Absolutely, the equation is working fine in thin films but when I tried to calculate the critical thickness, I found it a little far from the equation result, maybe the 20 microns considered as thick film, or I this could return to the post-heating treatment which could change of the internal stresses to add to shrinkage stress as you know sol-gel need some heat treatment to finish the transfer to xerogel
What method do you use to deposit the sol-gel coatings? In case you use dip-coating, it was demonstrated that the roughness of the substrate, and the direction of the grinding lines in relation with the withdrawal direction of the substrate, play an important role in the thickness and the presence of cracks in the coatings.
The critical thickness depends on different parameters. Maybe you can decrese the presence of cracks trying to reduce the roughness of the substrates and performing a drying process at longer time and lower temperature.
Check this paper out for better calculations of the final thickness of sol-gel coatings deposited by dip-coating:
''Influence of roughness and grinding direction on the thickness and adhesion of sol-gel coatings deposited by dip-coating on AZ31 magnesium substrates. A Landau–Levich equation revision.''
I hope that helps.
Dear Juan Pablo Fernández-Hernán
Thank you for your suggestion for that paper I think it could help indeed.
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