Strategies for Improving the Tribotechnical Properties of TiN/a-C Coatings

Abstract

In the present work, the mechanical properties and tribological characteristics of TiN (monolayer) and TiN/a‑C (bilayer; two variants of combinations of elastoplastic properties of the layers) coatings were investigated using experimental and theoretical approaches. The microstructure of the synthesized coatings and the topography of the surface were characterized by scanning electron microscopy. Characterization of the a‑C coating was carried out by the diffuse total internal reflection (DTIR) method using a Fourier‑transform infrared (FT‑IR) spectrometer. Hardness and reduced modulus were obtained by continuous (instrumented) indentation with a Berkovich indenter. Wear rate and coefficient of friction were determined in sliding friction tests. The results of the comprehensive experimental study of the coatings served as the basis for developing a finite‑element model of indentation of a bilayer elastoplastic medium with various combinations of elastic and plastic properties and different relative layer thicknesses. A correlation between coating parameters H/E and H³/E² and the wear rate was established experimentally. Functional dependences of hardness and reduced modulus on the relative thickness of the surface metastable a‑C layer were determined theoretically. The optimal ratio of the thicknesses of the constituent layers in the TiN/a‑C coating for given elastoplastic material properties, yielding the best tribological performance, was identified.