Effects of modulation layer thickness on fracture toughness of a TiN/AlN-Ni multilayer film

A series of TiN/AlN-Ni multilayer films with different modulation layer thicknesses were prepared by magnetron sputtering. Their microstructures and mechanical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Therein, fracture toughness of multilayer films was determined by indentation method. The hardness and elastic modulus of all multilayers are greatly improved by introducing an AlN-Ni modulation layer. When the modulation-layer thickness is 2.4 nm, and the hardness and elastic modulus are the highest at the value of 39 GPa and 447 GPa, respectively, which is ascribed to Koehler strengthening and alternating stress hardening. Meanwhile the fracture toughness reaches the maximum, namely 5.49 MPa·m1/2. It is attributed to the indentation-induced phase transformation of c-AlN to w-AlN and the formation of a good coherent interface. With further increasing the thickness of the AlN-Ni modulation layer, the multilayer film gradually becomes an amorphous structure, resulting in the decrease of hardness and toughness.