It is already known that metal carbides, oxides and nitrides possess crystal structures which do not have sufficient slip systems for dislocation movement during plastic deformation. However, when we look at their crystal structures, its found that metal and carbon/nitrogen/oxide atoms have different lattice positions. So, is it possible by any any means that hardness comes from the fact that metal and Carbon/oxygen/nitrogen atoms possessing different magnitudes of burgers vector? or is it the only fact that the strains associated with the interstitial atoms pins dislocations from metal?
It is because of the bonding. The Burgers vector is related to the crystal lattice which is in fact a geometric description of the translation symmetry within the crystal. It is not related to any structure element. The definition of a Burgers vector presupposes some kind of deformability along lattice planes and directions. If the structural arrangement of atoms is too complex along low index planes you possibly do not observe any glidening. The sample fractures or cleaves easily., see cf. NaCl which has the same structure like some of your carbides and nitrides.
This should be the major reasons (or our current belief). I only wanted point out to understand that lattice and structure are totally different things and that the description of slip systems are per-se uses lattice for the sake of simplicity, but also because the model of dislocations was developed for materials where a lattice point is practically also a atomic position. For more complicated phases you need to introduce partial dislocations in order to explain that a translation period is simply too long for this model.
I agree with Gert Nolze bonding is the reason behind the brittleness and the hardness of ceramics. The strong bonds, complexity of the structure, and the differently changed neighbors (if any ) hinder the movement of the dislocations and the slipping systems; thus, brittleness is observed. The hardness is directly related to the bonds strength and the bonds density.
In metallic copper, the neighbour of a copper is a copper, and the metallic bond is only weakly oriented. In cementite Fe3C, a neighbouring atom of Fe is C. No interchange.
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