Understanding the deformation mechanisms of CVD alpha-alumina coatings

Giovannini, Tommaso

PhD Tesis (2018)

The mechanical performance of metal cutting tools has improved considerably since the industry started to adopt ceramic coatings deposited via physical (PVD) and chemical vapour deposition (CVD). Further improvements in cutting tool performance are therefore contingent on the improved mechanical performance of these coatings. These developments require a thorough understanding of the micromechanical mechanisms in operation during metal cutting operations, with specific emphasis on plastic deformation of the coatings, and how it affects overall tool performance. To investigate the plastic deformation behaviour of the coating materials, an experimental characterisation on commonly used CVD α-Al2O3 and Ti(C,N) coatings was performed. This investigation started with electron back scattered diffraction (EBSD) measurements of the different coatings to allow for measurement of their crystallographic texture. These measurements were complemented by high temperature (300°C) nanoindentation measurements. Micropillar compression experiments were also performed to probe the critical resolved shear stress of the active plastic deformation systems in the α-Al2O3 coatings. Combined, these investigations allowed to understand the effect of crystallographic orientation and high temperatures on the coating deformation behaviour. To complement the experimental characterization of the coating materials, development of a modelling framework, based on planar discrete dislocation plasticity theory, was also initiated. A fundamental input quantity of the modelling framework is known as the dislocation drag coefficient which is used to govern the mobility of plastic defects during simulations. An experimental framework, based on the micropillar compression of Zr-4 micropillars, was developed to evaluate the feasibility of rapid experimental measurement of the quantity.