Nanostructured Hard Coatings - From Nanocomposites to Nanomultilayers

Nanomultilayers and nanocomposites provide a wide range of possibilities to promote hardness, oxidation resistance, improved wear behavior and other properties relevant for protective coatings. Most nanocomposite hard coatings show a maximum of the hardness, as the composition is changed from the pure crystalline phase to compositions dominated by the amorphous phase. At the hardness maximum the grain size is below 10 nm and the amorphous layer separating the nanocrystals is only a few atomic bond lengths thin. The critical dimensions necessary to obtain hardness enhancement in nanomultilayers are of the same order. The reason for this finding in both layer architectures is the absence of dislocation activity due to size restrictions in the single layers or the nanocrystals, respectively. Hindered dislocation movement in nanomultilayers cause enhanced hardness, while deformation of nanocomposites can occur only due to grain boundary sliding which requires more energy than deformation by dislocation movement, thus causing increased hardness.

Keywords
nanomultilayers nanocomposite hardness