Microstructure and superhardness effect of CrAlN/SiO2 nanomultilayered film synthesized by reactive magnetron sputtering

• The CrAlN/SiO2 nanomultilayered films were prepared by reactive magnetron sputtering.
• Effects of SiO2 layer thickness on structure and mechanical properties were studied.
• The amorphous SiO2 could be crystallized and grow epitaxially with CrAlN layers.
• The film shows an evident superhardness effect with maximal hardness of 38.9 GPa.
• The hardening can be explained by modulus-difference and alternating-stress theories.

The CrAlN/SiO2 nanomultilayered films with different SiO2 layers thickness were synthesized by reactive magnetron sputtering. The effects of SiO2 layer thickness on the microstructure and mechanical properties of CrAlN/SiO2 nanomultilayered films were studied. The results reveal that, when SiO2 layer thickness is less than 0.7 nm, originally amorphous SiO2 layers can be forced to crystallize under the “template effect” of NaCl-structured CrAlN layers and grow epitaxially with them, resulting in the preservation of columnar growth structure and remarkable superhardness effect with the maximum value of hardness of 38.9 GPa. As the SiO2 layer thickness exceeds 0.7 nm, however, SiO2 layers transform back into amorphous state, which destructs the epitaxial structure between CrAlN and SiO2 layers, leading to disappearance of columnar growth character and decrease of mechanical properties. The superhardness effect of CrAlN/SiO2 nanomultilayered film can be attributed to the joint effects of modulus-difference and alternating-stress strengthening mechanisms.