We know that the ceramics such as TiN and the intermetallic compounds such as TiAl are brittle. When these materials are used as coat, why they are not brittle?
[a] Ceramics are the materials having structures similar to those of ionic crystals (Rock Salt) where the cations and anions are present in various layers in such a way that the ions of the same charge do not lie exactly above and below one another.
But when stress is applied, the ions of the same charge get dislocated in such a way that the ions of the same charge are placed exactly above and below each other.This increases repulsion and structure breaks to make them brittle.
[b]Now IMC are nonstoichiometric compounds consisting of two metals held by ionic/ covalent bonds where the more electropositive metal becomes delta positive and the less electropositive becomes the delta negative .In a way, they can also be looked upon like those of ionic crystals and so are expected to be brittle on the application of stress.
Now the answer to the second part of the question is given as follows:
[a] Ceramics, generally, have a low THERMAL CONDUCTIVITY which means that the specimen is heated or cooled at the surface but not at the centre . The surface, therefore, expands or contracts, and is restrained by the centre, i.e. held in shape by the interior - surface tension.
[b] As regards to IMC, which being nonstoichiometric, are expected to have comparatively low mpts. The metal excess ions move into the interstial sites and attain some ductility like that of metals and so are not brittle.
Since the bonding is ionic therefore it renders "short of slip segments". One can also analyse this problem on the basis of fracture mechanics. In order to have catastrophic ;i.e, brittle fracture there required a certain crack length to propagate . Since, in case of thin coatings, the initiating crack puncture through the thickness even before it could grow to certain length required for propagation. (its just a thought)
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