SiNx thickness dependent morphology and mechanical properties of CrAlN/SiNx nanomultilayers

CrAlN/SiNx nanomultilayers with different SiNx layer thicknesses were synthesized by reactive magnetron sputtering. The microstructure and mechanical properties were investigated by X-ray diffraction, high-resolution transmission electron microscopy and nano-indentation techniques. The average crystallite size, microstrain and average dislocation density of CrAlN/SiNx nanomultilayers with different SiNx layer thicknesses were evaluated by X-ray diffraction line profile analysis method. The results indicated that, when SiNx layer thickness was below 0.6 nm, SiNx was forced to crystallize and grew epitaxially with CrAlN layers, resulting in the decrease of average dislocation density and the enhancement in hardness and elastic modulus. As the SiNx layer thickness further increases, the epitaxial growth was firstly interrupted and then crystallized SiNx layers transformed back to amorphous state, leading to the increase of average dislocation density and the decrease of hardness and elastic modulus. An energy balance model was established to explain the microstructure evolution.

► Superhardness effect was found in reactively synthesized CrAlN/SiNx nanomultilayers.
► The maximum hardness and elastic modulus respectively reach 37.6 GPa and 437 GPa.
► Microstructural evolution was studied during disappearance of superhardness effect.
► An energy balance model was established to explain the microstructure evolution.