Most hard coatings for commercial production are grown under medium-vacuum environments which implies that residual gas atoms can be incorporated in the films in amounts ranging up to a few atomic percent depending on the growth rate and residual gas pressure. Such impurity contents have been found to affect the mechanical properties of bulk refractory compounds. Less than 1 at. % boron in TiC hinders dislocation slip through the precipitation ofTiB2 platelets on ( 111 ) planes.6o Also, noble gases such as Ar can be entrapped in the films during many PVD processes. For CVD processes, the substrate and the reactor walls are an additional source of impurities. Impurities such as C, N, and 0 from the residual gas are usually incorporated substitutionally in the lattice even if incorporation in interstitial positions and in grain boundaries may occur. The main interaction between substitutional incorporated atoms and dislocations is an electronic one. In ionic or partly ionic materials edge dislocations will terminate with the same type of atoms along its length. This implies that impurity atoms in the vicinity of the dislocation will have a strong electrostatic interaction with the dislocation that is either attractive or repulsive depending on the valence of the impurity atom.For covalently bonded materials, dangling bonds will be formed along the dislocation core. Because such dangling bonds are associated with specific electronic levels, any impurity atoms that change the electronic structure of the material by changing the Fermi level will interact with the levels associated with the dangling bonds along the dislocation core and thus influence the dislocation mobility. This interaction has been clearly demonstrated for doping elements in various semiconductors, but whether it has a strong effect on the dislocation mobility and the hardness of refractory materials is yet not clear.
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