Published Paper Details:

This paper presents an integrated microstructure-tribology-performance analysis of CrN/AlTiN/TiB? multilayer Physical Vapour Deposition (PVD) coatings deposited on WC-Co solid carbide end mills for dry machining of Ti-6Al-4V titanium alloy. Drawing on XRD, AFM and SEM characterization data alongside machining trial results reported in the companion papers of this series, this study systematically correlates coating microstructural parameters specifically nanocrystalline grain size (D = 3.5-4.8 nm by Scherrer analysis), surface roughness (Ra = 4.21 nm by AFM), and morphological features (compact Zone T microstructure by SEM) with the observed tribological behaviour and machining performance (111.1% tool life improvement). The dominant tribological phenomena are identified as: (i) anti-adhesion provided by the TiB? outermost layer suppressing built-up edge (BUE) formation; (ii) adaptive tribo-oxidation of AlTiN generating Al?O? protective oxides that function as in-situ solid lubricants; (iii) crack deflection at CrN/AlTiN layer interfaces inhibiting coating delamination under cyclic milling loads; and (iv) Hall-Petch hardening from the sub-5 nm nanocrystalline grain structure maintaining coating hardness under extreme contact conditions. A mechanistic wear model is proposed that links these four phenomena in a sequential, self-reinforcing protective cascade. The study demonstrates that the CrN/AlTiN/TiB? system achieves its exceptional performance not through any single property advantage, but through the synergistic interaction of all three functional layers a principle that should guide the design of next-generation multilayer coatings for difficult-to-machine material applications.