PACVD TiN/Ti–B–N multilayers: from micro- to nano-scale

Multilayer coatings with compositionally modulated structures in the nanometer range such as superlattices often show improved mechanical properties compared to homogenous materials. Various physical vapor deposition (PVD) methods have been utilized to deposit superlattice coatings, while chemical vapor deposition (CVD) is not suitable due to interdiffusion at high deposition temperatures. The aim of this work was to investigate the potential of plasma-assisted chemical vapor deposition (PACVD) for multilayers with extension to layer thicknesses in the nanometer range. TiN/Ti–B–N multilayers have been deposited in an industrially sized deposition chamber to optimize mechanical and tribological properties. Different layer numbers (4, 16, 32, 64, 128 and 300) of TiN and a 23 at.% B containing Ti–B–N coating have been alternately deposited by varying gas flow rates resulting in a total coating thickness of 2–3 μm. Electron energy-loss spectroscopy (EELS) measurements showed separated 5-nm thick layers for a total of 300 layers. A decrease in compressive stress has been observed for a decreasing individual layer thickness, which enhanced coating adhesion on tool steels. For the 4-layer coating, the friction behavior is determined by the higher chlorine containing low-friction TiN layer resulting in a constant steady-state friction coefficient of 0.2, whereas stable values of approximately 0.8 were reached for higher layer numbers. The optimum wear resistance was obtained for 128 layers, which is attributed to a combination of well-defined interfaces and moderate compressive stresses.