Microstructure and mechanical properties of CVD TiN/TiB2 multilayer coatings

Chemical vapor deposited (CVD) TiN and TiB2 are both commonly used as wear-resistant hard coatings. The two materials exhibit pronounced differences in their properties, which can be exploited by combining them in a multilayer architecture. Thus, two multilayer coatings with different bilayer periodicities of ~80 and ~220 nm were synthesized. The multilayer architecture constrains the TiN grain size to dimensions comparable to the individual sublayer thickness, which are substantially smaller than those observed in the single-layer TiN reference coating. This grain refinement leads to significantly higher hardness of the TiN sublayers within the multilayer system compared to the single-layer coating. In contrast, the low grain size of the TiB2 coating appears unaffected, and the hardness of the TiB2 layers in the multilayer and corresponding bilayer reference coating is also comparable. The compressive residual stress in the TiB2 layers decreases with decreasing layer thickness, while the tensile residual stress in the TiN layers increases, resulting in a roughly constant stress difference between the sublayers, which is also comparable to the conventional TiN/TiB2 bilayer reference coating. However, while the tensile stress in the TiN sublayers is constant over coating thickness, TiB2 exhibits a pronounced gradient with only low compressive stress at the interface to the substrate, which increases significantly with increasing coating thickness. The fracture properties of the multilayers range between the values obtained for the corresponding reference coatings. Complementary finite element method simulations revealed that, for the multilayer coatings, the common assumption of a stress-free state of micro-cantilevers used for bending tests is not valid.