Formation of defect structures in hard nanocomposites

Formation of structure defects and other microstructure phenomena in hard Cr-Al-(Si-)N, Ti-Al-(Si-)N and Zr-Al-N nanocomposite coatings with different aluminium and silicon contents deposited by using cathodic arc evaporation was investigated using a combination of X-ray diffraction and transmission electron microscopy. The subject of the microstructure studies was the analysis of the phase composition and, for the cubic phase, the determination of the stress-free lattice parameters, the crystallite size and the local disorientation of crystallites. It was found that the formation of structure defects starts with a fragmentation of the deposited clusters having the size of several tens of nanometers into nanocrystallites having the size below 12 nm. This fragmentation was driven by formation of dislocation networks. The formation of structure defects continued with the segregation of the excessive aluminium and silicon from the host structure of the transition metal nitrides that was followed by the growth of the wurtzitic AlN and an amorphous silicon nitride at higher aluminium and silicon concentrations. The microstructure of the coatings was correlated with their hardness. In all systems under study, an increase of the hardness with increasing density of the microstructure defects was observed. The maximum of the hardness was observed in the coatings containing both the cubic transition metal nitride (accommodating also aluminium and silicon) and the wurtzitic AlN. (C) 2008 Elsevier B.V. All rights reserved.